polymers

Review Tuning Power Ultrasound for Enhanced Performance of Thermoplastic Micro-Injection Molding: Principles, Methods, and Performances

Baishun Zhao 1,2 , Yuanbao Qiang 1,2, Wangqing Wu 1,2,* and Bingyan Jiang 1,2

1 State Key Laboratory of High-Performance Complex Manufacturing, Central South University, Lushan South Road 932, Changsha 410083, China; [email protected] (B.Z.); [email protected] (Y.Q.); [email protected] (B.J.) 2 School of Mechanical and Electrical Engineering, Central South University, Lushan South Road 932, Changsha 410083, China * Correspondence: [email protected]; Tel.: +86-158-7429-5500

Abstract: With the wide application of Micro-Electro-Mechanical Systems (MEMSs), especially the rapid development of wearable flexible electronics technology, the efficient production of micro-parts with thermoplastic polymers will be the core technology of the harvesting market. However, it is significantly restrained by the limitations of the traditional micro-injection-molding (MIM) process, such as replication fidelity, material utilization, and energy consumption. Currently, the increas-


ing investigation has been focused on the ultrasonic-assisted micro-injection molding (UAMIM)
 and ultrasonic plasticization micro-injection molding (UPMIM), which has the advantages of new

Citation: Zhao, B.; Qiang, Y.; Wu, W.; plasticization principle, high replication fidelity, and cost-effectiveness. The aim of this review is Jiang, B. Tuning Power Ultrasound to present the latest research activities on the action mechanism of power ultrasound in various for Enhanced Performance of polymer micro-molding processes. At the beginning of this review, the physical changes, chemical Thermoplastic Micro-Injection changes, and morphological evolution mechanism of various thermoplastic polymers under different Molding: Principles, Methods, and application modes of ultrasonic energy field are introduced. Subsequently, the process principles, Performances. Polymers 2021, 13, 2877. characteristics, and latest developments of UAMIM and UPMIM are scientifically summarized. https://doi.org/10.3390/ Particularly, some representative performance advantages of different polymers based on ultrasonic polym13172877 plasticization are further exemplified with a deeper understanding of polymer–MIM relationships. Finally, the challenges and opportunities of power ultrasound in MIM are prospected, such as the Academic Editors: Francesco Paolo mechanism understanding and commercial application. La Mantia, José António Covas and Sabu Thomas Keywords: micro-injection molding; ultrasonic molding; ultrasonic injection molding; ultrasonic

Received: 5 August 2021 micro-injection molding; power ultrasound; ultrasonic plasticization; ultrasonic vibration Accepted: 21 August 2021 Published: 27 August 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in In the last decade and even in the future, some electromechanical systems with published maps and institutional affil- special functions and personal electronic products will continue to maintain the trend iations. of miniaturization and precision. Therefore, the high-quality mass production of micro- parts has attracted significant attention. From the perspective of polymer processing technology, the advantages of thermoplastic micro-injection molding (MIM) are reflected in the short production cycle, large scale, good dimensional accuracy, and low restrictions Copyright: © 2021 by the authors. on complex shapes and details [1]. Compared with other molding technologies, MIM Licensee MDPI, Basel, Switzerland. is more suitable for low-cost mass production. Especially with the molding precision This article is an open access article reaching nano-scale, MIM is believed to be the technology that could meet the needs of distributed under the terms and most micro–nano products on the market [2]. For instance, the smallest part in the world conditions of the Creative Commons was fabricated by the MTD micro-molding company, weighing only 0.00313 mg, and then, Attribution (CC BY) license (https:// Holzer et al. [3] successfully fabricated a part with nano grooves of 18 nm width. Although creativecommons.org/licenses/by/ some researchers have shared their definitions of micro-molded products, which will be 4.0/).

Polymers 2021, 13, 2877. https://doi.org/10.3390/polym13172877 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2877 2 of 41

discussed later in this paper [1,4–8], the size of the micro-molded parts is expected to exceed their definitions due to the continuous development of MIM. In fact, micro-molded parts have been gradually extending to sub-micron [9–11] or even nano-scale [12–14]. Generally, micro-molded parts can be divided into two groups. One of them is macro- parts with cross-scale features such as micro/nano structures on functional surfaces [15–17]. The other is small parts with weight in milligram scale. Due to the limitations of the process characteristics and material properties, when the micro-molded parts comprise cross-scale features or break through a certain volume/size boundary, MIM could be quite challenging in terms of replication fidelity, materials utilization, and energy consumption. In this context, power ultrasound was introduced to enhance the MIM performance. Specifically, for the macro-parts with micro/nano features, the power ultrasound system has been integrated into the injection mold to facilitate the polymer melt filling and the replication of the micro structures [18]. For the small parts with milligram weight, power ultrasound has been employed as the only energy source during MIM, [19–22] where a small quantity of plastic raw materials can be injection molded directly after plasticization by ultrasonic vibration, without screw shearing and external heating. Many researchers have studied the application of power ultrasound in the MIM of thermoplastic polymers in various aspects such as tooling, process modeling and sim- ulation, and the micro-molded parts characterization. From the author’s point of view, these contributions followed different processing strategies and evolved into two typical MIM variants, i.e., ultrasound-assisted micro-injection molding (UAMIM) and ultrasonic plasticization micro-injection molding (UPMIM). Among their many differences, the main difference between them is whether to use the traditional screw unit or the power ultra- sound to plasticize the plastic raw materials. Nevertheless, what they still have in common is both of them focus on tuning power ultrasound for enhanced MIM performance. The purpose of this work is to share a detailed overview of the application status and development potential of tuning power ultrasound for the enhanced performance of thermoplastic MIM. The physical changes, chemical changes, and morphological evolu- tion mechanism of various thermoplastic polymers under different application modes of ultrasonic energy field are introduced. Subsequently, the process principles, characteristics, and latest developments of UAMIM and UPMIM are summarized, which have not been found in the literature. Particularly, some representative performance advantages of dif- ferent polymers based on ultrasonic plasticization are further exemplified with a deeper understanding of polymer–MIM relationships. Presently, both of the two MIM variants are in a critical period that requires rapid development and breakthrough results.

2. Ultrasound-Assisted Micro-Injection Molding 2.1. Scale Effect in MIM High surface-to-volume ratios generally are the main feature of micro/nano-parts; thus, the polymer melt solidifies particularly rapidly due to the significant heat diffusion effects in the filling stage [22,23]. A formed solid layer can be a resistance to the melt injection, which is the main reason why filling defects such as uncompleted features exist. In the experiment of Sha et al. [24], the micro-needle cavities with a diameter of 100 µm and 150 µm could not be completely filled with melt under the conditions of low temperature and low injection speed, but when these parameters were set higher, the cavities could be completely filled (Figure1a,d). The filling can also be complex and unstable in a smaller scale; it can be an increasingly difficult to fill into the cavity from the gate to the end of the mold insert. As shown in Figure1e,f, filling height quickly decreases as the distance from the gate increases [25]. Therefore, the size effect poses a challenge to the high-quality forming of micro-scale features, which cannot be solved perfectly by optimizing process parameters. Polymers 2021, 13, 2877 3 of 41 Polymers 2021, 13, x FOR PEER REVIEW 3 of 43

Figure 1. Microstructures molding by MIM. (a–d): Micro pins of PP [24]. (a and b): T = 225 ◦C, V = 100 mm/s, Figure 1. Microstructures molding by MIM. (a–d): Micro pins of PP [24]. (a and b): Tb = 225b °C, Vi = 100 mm/s,i (c,d): Tb = ◦ 225(c,d )T°C,b V=i 225= 200C, mm/s. Vi = 200 Tb is mm/s. the barrel Tb is thetemperature, barrel temperature, Vi is the injection Vi is the injectionspeed; Replicated speed; Replicated micro features micro features located locatedat (e) 1.5 at mm(e) 1.5 and mm (f) and 35 mm (f) 35 from mm the from gate the [25]. gate [25].

TheThe complex complex filling filling field field can affect the crystallization phase, which can be seen from thethe huge difference ofof thethe proportion proportion of of the the skin skin and and core core layer layer in in the the “skin–core” “skin–core” structure struc- turefor semi-crystalline for semi-crystalline materials materials [26]. [26]. For semi-crystallineFor semi-crystalline materials, materials, physical physical properties, properties, such suchas mechanical, as mechanical, optical, optical, electrical, electrical, and chemical and chemical properties, properties, mainly depend mainly on depend the crystalline on the crystallinemorphology morphology of the materials of the [27 ].materials By analyzing [27]. theBy crystallinityanalyzing the effect crystallinity of molded effect samples of with different sizes along the flow direction, Liu et al. [28] studied the difference between molded samples with different sizes along the flow direction, Liu et al. [28] studied the macroscopic and microscopic morphology of isotactic polypropylene (iPP). The results difference between macroscopic and microscopic morphology of isotactic polypropylene show that there is also a “skin–core” structure with through-thickness morphology in (iPP). The results show that there is also a “skin–core” structure with through-thickness micro-parts, which is similar to macro-parts, as shown in Figure2a. At the same time, as far morphology in micro-parts, which is similar to macro-parts, as shown in Figure 2a. At the as the orientation area including surface layer and shear layer is concerned, the micro-part same time, as far as the orientation area including surface layer and shear layer is con- (90%) is much larger than the macro-part (15%) [29]. Zhang et al. [30] also found that the cerned, the micro-part (90%) is much larger than the macro-part (15%) [29]. Zhang et al. volume ratio of the skin layer increased from 10% to 67% as the part thickness decreased [30] also found that the volume ratio of the skin layer increased from 10% to 67% as the from 500 to 100 µm, as shown in Figure2b. In addition, it was found that the Young’s part thickness decreased from 500 to 100 μm, as shown in Figure 2b. In addition, it was modulus, fracture strain, and yield stress usually increase with an increase of the skin found that the Young’s modulus, fracture strain, and yield stress usually increase with an ratio [31]. The different molding conditions of micro/nano scale and macro scale determine increasethe physical of the and skin chemical ratio [31]. properties The different of injection molding parts conditions by affecting of the micro/nano crystal morphology scale and macroand proportion. scale determine the physical and chemical properties of injection parts by affecting the crystalThe problems morphology that micro-injection-molding and proportion. technology is facing also have been discussed in some related literature [1,32,33]. Generally, to achieve the better molding filling process or morphology distribution, it is necessary to apply a faster injection speed [34,35], higher mold temperature [36], higher injection pressure [24], or surface coating technology [37] during the injection molding, as shown in Figure3. In the injection-molding process, the influence of operating conditions on the internal morphology distribution of semi-crystalline polymer mold parts has been well summarized by Pantani et al. [27]. However, the introduction of the ultrasonic energy field lowers the level of these parameters. The general choice of researchers is to improve the parameter levels of certain factors in an attempt to associate the factor with an easily available result. Most of the research work in the past focused on the effect of crystallinity on viscosity, especially the description of the flow-induced crystallization phenomenon. However, it is still a question of which variables or their combinations are the most suitable for describing the evolution of crystal morphology.

PolymersPolymers 20212021, 13, ,13 x, FOR 2877 PEER REVIEW 4 of4 of43 41

Polymers 2021, 13, x FOR PEER REVIEW 5 of 43 FigureFigure 2. Morphology 2. Morphology of “skin–core” of “skin–core” structure structure in different in different scale. (a scale.) Thickness (a) Thickness from 2000 from to 200 2000 μm to [28];200 (bµ)m Thickness [28]; (b) Thicknessfrom 100 to from 500 100μm to[31]. 500 µm [31].

The problems that micro-injection-molding technology is facing also have been discussed in some related literature [1,32,33]. Generally, to achieve the better molding filling process or morphology distribution, it is necessary to apply a faster injection speed [34,35], higher mold temperature [36], higher injection pressure [24], or surface coating technology [37] during the injection molding, as shown in Figure 3. In the injection- molding process, the influence of operating conditions on the internal morphology distribution of semi-crystalline polymer mold parts has been well summarized by Pantani et al. [27]. However, the introduction of the ultrasonic energy field lowers the level of these parameters. The general choice of researchers is to improve the parameter levels of certain factors in an attempt to associate the factor with an easily available result. Most of the research work in the past focused on the effect of crystallinity on viscosity, especially the description of the flow-induced crystallization phenomenon. However, it is still a question of which variables or their combinations are the most suitable for describing the evolution of crystal morphology.

FigureFigure 3. 3.“Skin–core” “Skin–core” structuresstructures atat differentdifferent conditions; conditions; TT represents represents temperature, temperature, P P represents represents pressure,pressure, t representst represents the the thickness thickness of of skin skin layers layers [27 [27].].

2.2.2.2. Principles Principles and and Methods Methods 2.2.1.2.2.1. Process Process Principle Principle DueDue toto the the filling filling problem problem that that reduced reduced scale scale brings, brings, the the ultrasonic ultrasonic vibration vibration was was introducedintroduced to to thethe MIMMIM systemsystem toto improve the filling filling capability capability by by improving improving the the fluidity fluid- ityof ofpolymer polymer melt. melt. Since Since the the application application of of ultrasound ultrasound technology in polymerpolymer micro-micro- injectioninjection molding molding has has not not been been systematically systematically reviewed, reviewed, the the names names of of similar similar technologies technologies havehave not not yet yet been been unified: unified: ultrasonic ultrasonic injection injection molding molding (UIM) (UIM) [ 18[18],], ultrasonic ultrasonic vibration vibration micro-injectionmicro-injection moldmold (UVMIM)(UVMIM) technologytechnology [38[38],], ultrasonic-assisted ultrasonic-assisted injectioninjection molding molding (UAIM)(UAIM) [ 39[39],], but but the the common common point point of of this this technology technology is is that that the the ultrasonic ultrasonic action action is is aimed aimed atat the the polymer polymer melt, melt, improving improving its its fluidity fluidity and and achieving achieving high high replication replication and and quality quality re- requirement of parts. Polymer plasticization, metering, injection, and other processes rely on existing micro-injection-molding machines (ARBURG [31,40], FANUC [14,41,42], BOY [43], Wittmann-Battenfeld [44]). Considering the difference between micro-injection molding and injection molding and the auxiliary effect of ultrasound, this kind of technology can be classified as ultrasound-assisted micro-injection molding (UAMIM). Although the internal morphology of micron-sized parts made of ordinary pure polymer materials has been widely studied and well understood, few papers have studied the morphology evolution of molded parts under the action of ultrasonic field: this problem has not been solved. UAMIM is a manufacturing process in which polymer pellets are plasticized by the conventional heating and screw shear heating in an MIM machine, and then, the polymer melt is injected into the mold cavity with the help of ultrasonic vibration, which is accompanied by energy exchange. Figure 4 shows the schematic diagram of UAMIM. The very high frequency of ultrasonic waves makes it possible to ensure the propagation direction in a very narrow space, which is not available for audible sound [45]. Ultrasonic energy exerts great acceleration on medium particles during transmission, which significantly affects the energy of medium materials [18]. Therefore, when the ultrasonic waves are applied to polymer melt, the fluidity of the polymer melt and molding quality of micro-parts are enhanced by the propagation of ultrasonic vibration energy in the polymer [45].

Polymers 2021, 13, 2877 5 of 41

quirement of parts. Polymer plasticization, metering, injection, and other processes rely on existing micro-injection-molding machines (ARBURG [31,40], FANUC [14,41,42], BOY [43], Wittmann-Battenfeld [44]). Considering the difference between micro-injection molding and injection molding and the auxiliary effect of ultrasound, this kind of technology can be classified as ultrasound-assisted micro-injection molding (UAMIM). Although the internal morphology of micron-sized parts made of ordinary pure polymer materials has been widely studied and well understood, few papers have studied the morphology evolution of molded parts under the action of ultrasonic field: this problem has not been solved. UAMIM is a manufacturing process in which polymer pellets are plasticized by the conventional heating and screw shear heating in an MIM machine, and then, the polymer melt is injected into the mold cavity with the help of ultrasonic vibration, which is accom- panied by energy exchange. Figure4 shows the schematic diagram of UAMIM. The very high frequency of ultrasonic waves makes it possible to ensure the propagation direction in a very narrow space, which is not available for audible sound [45]. Ultrasonic energy exerts great acceleration on medium particles during transmission, which significantly affects the energy of medium materials [18]. Therefore, when the ultrasonic waves are applied Polymers 2021, 13, x FOR PEER REVIEW 6 of 43 to polymer melt, the fluidity of the polymer melt and molding quality of micro-parts are enhanced by the propagation of ultrasonic vibration energy in the polymer [45].

Figure 4. Schematic diagram of UAMIM [[45].45]. 2.2.2. Configuration Design 2.2.2. Configuration Design As the auxiliary force field of ultrasound, the most important point is to transmit the As the auxiliary force field of ultrasound, the most important point is to transmit the vibration energy to the polymer melt during the filling process as much as possible. There vibration energy to the polymer melt during the filling process as much as possible. There are multiple options for the choice of the ultrasonic vibration point for this. Table1 lists are multiple options for the choice of the ultrasonic vibration point for this. Table 1 lists some common ultrasonic vibration point arrangements. Figure5 shows different setting some common ultrasonic vibration point arrangements. Figure 5 shows different setting points of ultrasonic vibration. In terms of contact with the melt, it can be divided into points of ultrasonic vibration. In terms of contact with the melt, it can be divided into direct ultrasonic vibration and indirect ultrasonic vibration. In indirect ultrasonic vibration, direct ultrasonic vibration and indirect ultrasonic vibration. In indirect ultrasonic the ultrasonic sonotrode is not in direct contact with the polymer melt, and the ultrasonic transmissionvibration, the is ultrasonic indirectly sonotr transmittedode is not to the in polymerdirect contact melt inwith the the micro-cavity polymer melt, through and thethe moldultrasonic itself. transmission As a result, ais high-powerindirectly transmitted ultrasonic energyto the polymer field is applied melt in tothe the micro-cavity mold, and highthrough energy the mold consumption itself. As and a result, low effective a high-power utilization ultrasonic of ultrasonic energy field energy is applied have become to the themold, biggest and challengeshigh energy for consumption the popularization and low of thiseffective process. utilization In direct of ultrasonic ultrasonic vibration energy (pointhave become c in Figure the5), biggest the ultrasonic challenges waves for directly the po contactpularization the polymer of this melt process. and apply In direct high frequencyultrasonic vibration vibration. (point This processc in Figure requires 5), the a ultrasonic special ultrasonic waves directly sonotrode contact as athe part polymer of the cavity,melt and which apply may high cause frequency problems vibration. such as its wearThis ofprocess and leakage requires of thea special rubber ultrasonic material. sonotrodeWhen as the a moltenpart of polymerthe cavity, flows which through may thecause ultrasonic problems vibration such as application its wear ofarea, and itleakage will follow of the the rubber vibration material. and absorb ultrasonic energy. However, when the melt flow directionWhen is the perpendicular molten polymer to the flows ultrasonic through vibration the ultrasonic direction, vibration there will application be slight flow area, re- it sistancewill follow in the the melt, vibration as shown and by absorb point bultras in Figureonic5 energy.. If the vibration However, point when is at the the melt entrance flow (pointdirection a), theis perpendicular resistance will to fill the the ultrasonic melt into thevibration cavity direction, from the vibration there will point; be slight therefore, flow resistance in the melt, as shown by point b in Figure 5. If the vibration point is at the entrance (point a), the resistance will fill the melt into the cavity from the vibration point; therefore, the weight of molded parts will increase [18], and the vibration part of the ultrasonic waves will produce a sinking mark. If the vibration point is at the mold cavity (point b) or in direct contact with the melt (point a), the shear flow resistance will be weakened by ultrasonic vibration, the melt flow length will increase [39], and the filling percentage will also be improved [18,46]. The way the vibration direction is parallel to the melt flow direction (point d) can force more of the melt to maintain a straight flow over a longer distance [43]. To change the direction of ultrasonic action, the directional converter of ultrasonic waves is often used by researchers [39,47]. Hence, the vibration point should be set according to the structural characteristics of the injection part. Finally, since the flow viscosity of the polymer melt is affected by the application direction and transmission mode of the ultrasonic energy field, the vibration point should be set according to the structural characteristics of the injection part. However, the instructive suggestions for setting vibration points still need to be further studied and standardized more systematically.

Polymers 2021, 13, 2877 6 of 41

the weight of molded parts will increase [18], and the vibration part of the ultrasonic waves will produce a sinking mark. If the vibration point is at the mold cavity (point b) or in direct contact with the melt (point a), the shear flow resistance will be weakened by ultrasonic vibration, the melt flow length will increase [39], and the filling percentage will also be improved [18,46]. The way the vibration direction is parallel to the melt flow direction (point d) can force more of the melt to maintain a straight flow over a longer distance [43]. To change the direction of ultrasonic action, the directional converter of ultrasonic waves is often used by researchers [39,47]. Hence, the vibration point should be set according to the structural characteristics of the injection part. Finally, since the flow viscosity of the poly- mer melt is affected by the application direction and transmission mode of the ultrasonic Polymers 2021, 13, x FOR PEER REVIEWenergy field, the vibration point should be set according to the structural characteristics7 of 43 of the injection part. However, the instructive suggestions for setting vibration points still need to be further studied and standardized more systematically.

FigureFigure 5. Different 5. Different setting setting points points of ultrasonic of ultrasonic vibration. vibration.

Table 1. List of the current schemes of ultrasound action. In this table, only the basic setting is listed; Table 1. List of the current schemes of ultrasound action. In this table, only the basic setting is listed; forfor detailed detailed configurations configurations and and process process parameters, parameters, please please refer refer to tothe the source. source.

VibrationVibration Point Point/Region/Region Direction Direction Indirect Indirect/Direct/Direct Reference Reference GateGate Vertical Vertical Direct Direct [18,48] [18,48 ] Indirect [45,49] Middle of mold insert Vertical Indirect [45,49] Middle of mold insert Vertical Direct [31,38,46] Direct [31,38,46] Integrated with mold Parallel Indirect [40,43,50] IntegratedWhole ofwith the mold mold Parallel Vertical Indirect Direct [40,43,50] [51] Whole of the mold Vertical Direct [51] 2.3. Engineering Characteristics 2.3.2.3.1. Engineering Improved Characteristics Flow Properties 2.3.1. ImprovedFor UAMIM Flow technology, Properties the plasticization is completed by a traditional micro-injection- moldingFor UAMIM machine, technology, and the ultrasonic the plasticization force field is is applied completed to the subsequentby a traditional injection micro- filling injection-moldingprocess. Numerous machine, experiments and theand ultrasonic simulations force havefield confirmedis applied theto the positive subsequent effect of injectionultrasound filling on process. the filling Numerous rate [52, 53experiments]. The introduction and simulations of an ultrasonic have confirmed field can the slow positivedown effect the cooling of ultrasound rate of molten on the polymer,filling rate prevent [52,53]. adhesion, The introduction reduce the of thicknessan ultrasonic of the fieldcondensate can slow layer,down andthe preventcooling rate excessive of molten shear polymer, forces and prevent flow resistance,adhesion, reduce which the leads thicknessto longer of flowthe condensate length and layer, molding and weightprevent [ 39excessive,45,54,55 shear]. The forces decrease and offlow flow resistance, resistance whichmakes leads injection to longer pressure flow length reach theand endmolding of the weight cavity [39,45,54,55]. successfully, thusThe decrease reducing of the flow pres- resistancesure loss makes between injection the proximal pressure and reach distal the en endsd of of the the cavity gate. successfully, The pressure thus loss reducing before and theafter pressure the vibration loss between area was the measuredproximal byand Yang distal et al.,ends and of itthe was gate. found The that pressure the pressure loss beforeloss and is more after obviousthe vibration in thinner area was parts, measured and the by pressure Yang et lossal., and value it was of UAMIMfound that is the close pressureto 27% loss compared is more withobvious MIM in [thinner31]. Later parts, research and the showed pressure that loss a shortvalue shotof UAMIM can also is be closereduced to 27% with compared an increased with MIM ultrasound [31]. Later power, research and ashowed higher moldthat a temperatureshort shot can is also good be for reducedslowing with the an cooling increased rate ultrasound and reducing power, the shearand a stresshigher [ 46mold]. More temperature intuitively, is good the filling for slowingprocess the can cooling be visualized rate and with reducing a high-speed the shear camera stress [[46].56–58 More]. In orderintuitively, to understand the filling the process can be visualized with a high-speed camera [56–58]. In order to understand the flow behavior of molten polymer in the ultrasonic vibration injection-molding process, Jiang et al. [43] developed a new visualization device for analyzing the filling velocity field in the injection-molding process. The results showed that when the ultrasonic power is 200 W, the filling front velocity can be increased by 27% by ultrasonic vibration. In addition, during ultrasonic vibration, the velocity gradient of the melt decreases, and the velocity distribution became more uniform. The simulation results of Gao et al. [59] show that the ultrasonic vibration can obtain higher speed, lower viscosity, a more uniform viscosity field as shown in Figure 6, and better mold-filling performance, which makes the mold-filling quality of micro-plastic parts better, which is consistent with the experimental result before Qiu [45].

Polymers 2021, 13, 2877 7 of 41

flow behavior of molten polymer in the ultrasonic vibration injection-molding process, Jiang et al. [43] developed a new visualization device for analyzing the filling velocity field in the injection-molding process. The results showed that when the ultrasonic power is 200 W, the filling front velocity can be increased by 27% by ultrasonic vibration. In addition, during ultrasonic vibration, the velocity gradient of the melt decreases, and the velocity distribution became more uniform. The simulation results of Gao et al. [59] show that the ultrasonic vibration can obtain higher speed, lower viscosity, a more uniform viscosity field as Polymers 2021, 13, x FOR PEER REVIEW 8 of 43 shown in Figure6, and better mold-filling performance, which makes the mold-filling quality of micro-plastic parts better, which is consistent with the experimental result before Qiu [45].

FigureFigure 6.6.( (aa)) Viscosity Viscosity and and (b ()b velocity) velocity distribution distribution of polymerof polymer melt melt along along the the vertical vertical centerline centerline of the of cross-sectionthe cross-section under under different different conditions conditions [59]; Condition [59]; Condition 1—without 1—without ultrasonic; ultrasonic; Condition Condition 2—with the2— flowwith frontier the flow vibration; frontier Condition vibration; 3—with Condition the flow 3—with frontier vibrationthe flow andfrontier changed vibration rheological and equation.changed rheological equation. 2.3.2. Improved Replication Fidelity 2.3.2.The Improved replication Replication degree ofFidelity molded plastic parts is an important criterion for judging the moldingThe replication quality. Thedegree improvement of molded ofplastic the melt parts quality is an important under the criterion effect of ultrasoundfor judging vibrationthe molding has quality. a positive The effect improvement on the improvement of the melt ofquality the replication under the ofeffect the of molded ultrasound part. Satovibration et al. [has18, 48a positive] studied effect the effect on the of improv ultrasonicement vibration of the replication on the quality of the of plasticmolded parts. part. TheSato results et al. [18,48] show thatstudied the replicationthe effect of performance ultrasonic vibration of lenses on is improved the quality from of plastic 84% to parts. 95% andThe theresults surface show accuracy that the isreplication improved performa by nearlynce 25%. of lenses The sameis improved result wasfrom obtained 84% to 95% in aand forming the surface experiment accuracy involving is improved a linear by microchannelnearly 25%. The product. same result Qiu etwas al. obtained [45] applied in a ultrasonicforming experiment vibration to involving the mold-filling a linear process microc ofhannel Fresnel product. lenses. The Qiu results et al. were [45] thatapplied the fillingultrasonic area vibration of the plastic to the parts mold-filling was improved process byof aboutFresnel 6.91% lenses. and The symmetric results were deviation that the wasfilling improved area of 15.62%the plastic on averageparts was because improved of ultrasonic by about vibration. 6.91% and Lateral symmetric experiment deviation and simulationwas improved proved 15.62% it again on average [49,59]. because The improvement of ultrasonic of vibration. the mass Lateral or the fillingexperiment length and of moldedsimulation parts proved is related it ag toain the [49,59]. temperature The improvement change under of the the action mass ofor ultrasonicthe filling vibrationlength of field.molded The parts temperature is related increases to the temperature of the oscillation change part under of the the ultrasonic action of waveultrasonic (∆T) isvibration shown as Equation (1). field. The temperature increases of the oscillation part of the ultrasonic wave (∆T) is shown as Equation (1). ∆T = It 1 − e−2αX /(XρH) (1)

where I denotes the sound intensity,Δ𝑇=𝐼𝑡t is the(1−𝑒 oscillation)/ time,(𝑋𝜌𝐻α) is the coefficient of ultrasonic(1) energy absorption, X is the distance, ρ is the density, and H is the heat capacity. where I denotes the sound intensity, t is the oscillation time, 𝛼 is the coefficient of Due to the absorption of ultrasonic energy, local heating occurs in the resin, which ultrasonic energy absorption, X is the distance, 𝜌 is the density, and H is the heat capacity. leads to the formation of oscillating flow in the packing and holding stages. The defor- Due to the absorption of ultrasonic energy, local heating occurs in the resin, which mation resistance of the skin layer is reduced by the local heating occurring between the leads to the formation of oscillating flow in the packing and holding stages. The molten and skin layer. Therefore, as shown in Figure7, surface replication, which mainly deformation resistance of the skin layer is reduced by the local heating occurring between occurs in the packing and holding stages, is enhanced in the UAMIM process. Therefore, the molten and skin layer. Therefore, as shown in Figure 7, surface replication, which mainly occurs in the packing and holding stages, is enhanced in the UAMIM process. Therefore, when producing micro-parts, especially with a functional surface microstructure, such as a microlens array, it is a feasible choice to apply an ultrasonic energy field to enhance the replication ability.

Polymers 2021, 13, 2877 8 of 41

Polymers 2021, 13, x FOR PEER REVIEWwhen producing micro-parts, especially with a functional surface microstructure, such9 ofas 43 a microlens array, it is a feasible choice to apply an ultrasonic energy field to enhance the replication ability.

FigureFigure 7. 7. ImprovingImproving surface surface replication replication mechanism mechanism by by ultrasonic ultrasonic vibration vibration [48]. [48].

2.3.3.2.3.3. Reduced Reduced Energy Energy Consumption Consumption InIn UAMIM UAMIM technology,technology, thethe enhancement enhancement of of fluidity fluidity and and the improvementthe improvement of repli- of replicationcation do do not not mean mean higher higher parameter parameter settings. settings. On On thethe contrary,contrary, the auxiliary effect effect of of ultrasoundultrasound can can reduce reduce the the parameter parameter level level required required for for material material molding. molding. ToTo achieve achieve a moldedmolded partpart of of higher higher quality, quality, other other assistive assistive variotherm variotherm molding molding tech- technologynology was was used, used, including including Rapid Rapid Heat CycleHeat Cycle Molding Molding (RHCM) (RHCM) [60], Induction [60], Induction Heating HeatingMolding Molding (IHM), and(IHM), Electricity and Electricity Heating MoldHeating (E-mold). Mold (E-mold). However, However, a higher process a higher pa- rameter setting, higher energy demand, and longer time are the cost. Chen et al. [61] process parameter setting, higher energy demand, and longer time are the cost. Chen et systematic studied the electromagnetic induction heating system with a power of 30 kW al. [61] systematic studied the electromagnetic induction heating system with a power of to heat the mold plate. On the premise of low cost and practicability, Chang et al. [62] 30 kW to heat the mold plate. On the premise of low cost and practicability, Chang et al. designed and investigated an infrared rapid surface heating system, which was used in the [62] designed and investigated an infrared rapid surface heating system, which was used injection-molding process. In this system, the surface heat of a mold insert is provided by in the injection-molding process. In this system, the surface heat of a mold insert is four 1 kW infrared halogen lamps as radiation sources. The former is almost 15 times the provided by four 1 kW infrared halogen lamps as radiation sources. The former is almost power of ultrasound and the latter is two times (the maximum power applied in UAMIM in 15 times the power of ultrasound and the latter is two times (the maximum power applied the experiment is 2 kW, and even it is not more than 1 kW in practical applications). Using in UAMIM in the experiment is 2 kW, and even it is not more than 1 kW in practical this kind of external heating technology, time to heat and cool was increased subsequently, applications). Using this kind of external heating technology, time to heat and cool was increasing the whole cycle time. Figure8 shows the time sequence of each cycle of the increased subsequently, increasing the whole cycle time. Figure 8 shows the time different processes. sequenceSince of ultrasonic each cycle energy of the dissipatesdifferent processes. energy more quickly than heat conduction, the cycle timeSince can beultrasonic reduced. energy Liu et al.dissipates [38] compared energy twomore technologies quickly than in moldingheat conduction, a high aspect the cycleratio time microstructured can be reduced. surface. Liu Oneet al. is [38] micro-injection compared two compression technologies molding in molding equipped a high with aspectRHCM ratio and vacuummicrostructured mold venting surface. (VMV), One and is anothermicro-injection is micro-injection compression molding molding with equippedultrasonic with technology RHCM (UAMIM).and vacuum The mold results venting showed (VMV), thatall and the another process is parameters micro-injection in the moldinglatter one with are lower,ultrasonic while technology a higher molding (UAMIM). height The was results obtained, showed as shownthat all inthe Table process2. In parametersUAMIM, ultrasonic in the latter vibration one are only lower, works while on the a higher microchannel, molding and height the heat was storage obtained, is little. as shownTherefore, in Table the cooling2. In UAMIM, time required ultrasonic is only vibration 10 s. Excluding only works injection on the time microchannel, and demolding and thetime, heat the storage production is little. cycle Therefore, in ultrasonic the coo vibrationling time micro-injection required is only molding 10 s. ( µExcludingUVIM i.e., injectionUAMIM) time is only and10 demolding + 1.86 = 11.86 time, s, the while production micro-injection cycle in compression ultrasonic vibration molding micro- (µICM) injection molding (μUVIM i.e., UAMIM) is only 10 + 1.86 = 11.86 s, while micro-injection compression molding (μICM) needs to consider vacuuming time, with the production cycle of 20 + 25 = 45 s. Therefore, using ultrasonic technology, the production efficiency has increased nearly three times. Additionally, there is no need to equip RHCM and VMV

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needs to consider vacuuming time, with the production cycle of 20 + 25 = 45 s. Therefore, using ultrasonic technology, the production efficiency has increased nearly three times. in the UAMIM process; hence, the manufacturing cost of the mold is relatively low. Additionally, there is no need to equip RHCM and VMV in the UAMIM process; hence, the Ultrasound in previous rheological tests has also shown that it can reduce the mold manufacturing cost of the mold is relatively low. Ultrasound in previous rheological tests temperature parameter settings during polymer processing as well as the obvious has also shown that it can reduce the mold temperature parameter settings during polymer viscosity drop and pressure reduction, which means that the molding cost is smaller. This processing as well as the obvious viscosity drop and pressure reduction, which means has been confirmed in the injection experiments of Yang et al. [39,46], Jiang et al. [43], and that the molding cost is smaller. This has been confirmed in the injection experiments of Masato et al. [44] Although the ultrasonic energy field can reduce the energy consumption Yang et al. [39,46], Jiang et al. [43], and Masato et al. [44] Although the ultrasonic energy of molding, it is difficult to quantitatively judge its advantages only by the reduced field can reduce the energy consumption of molding, it is difficult to quantitatively judge parameterits advantages level. only The byquantitative the reduced work parameter of related level. indicators The quantitativesuch as power work consumption of related andindicators its reduction such as rate power still consumptionneed to be clarified. and its reduction rate still need to be clarified. InIn the the process process of of injection injection molding, molding, low-frequency low-frequency vibration vibration is is also also used used to to improve improve thethe mechanical mechanical properties properties of of high-density high-density polyethylene polyethylene (HDPE) (HDPE) and and iPP, iPP, such such as as reducing reducing warpagewarpage [63[63,64],,64], reducing reducing residual residual stress stress [39], and [39], improving and improving welding strength welding[40 ,51strength,65,66]. [40,51,65,66].This technology This cantechnology also be can used also in be the used ejection in the ejection stage and stage reduce and reduce the friction the friction force forceduring during ejection ejection [44]. Table[44]. 3Table provides 3 provides some successful some successful cases in cases the ultrasound-assisted in the ultrasound- assistedmicroinjection-molding microinjection-molding process andprocess their and main their characteristics. main characteristics.

FigureFigure 8. 8. TimeTime and and production production sequence sequence required required for each production cycle of different processes [[62].62].

Table 2. Comparison of the best reproduction quality and process parameters between Μuvim (UAMIM) and μICM [38]. Table 2. Comparison of the best reproduction quality and process parameters between Muvim (UAMIM) and µICM [38]. Mold Temperature Injection Speed Packing Pressure Production Height Standard Mold Temperature Injection Speed Packing Production Height Standard Processes [[°C]◦C] [mm/s][mm/s] Pressure [MPa] CycleCycle [s] [s] [µm][µm] DeviationDeviation μUVIM µUVIM 7070 102102 123123 11.8611.86 36.4636.46 0.750.75 (UAMIM) μµICMICM (VMV(VMV + + 108108 131131 150150 4545 31.1231.12 0.730.73 RHCM)

Polymers 2021, 13, 2877 10 of 41 Polymers 2021, 13, x FOR PEER REVIEW 11 of 43 Polymers 2021, 13, x FOR PEER REVIEW 11 of 43 PolymersPolymers Polymers 2021 20212021, ,13, 1313, ,x, xx FOR FORFOR PEER PEERPEER REVIEW REVIEWREVIEW 111111 of ofof 43 4343 Polymers 2021, 13, x FOR PEER REVIEW 11 of 43

TableTable 3. 3. ExamplesExamples processed processed by by UAMIM. UAMIM. Table 3. Examples processed by UAMIM. TableTableTable 3. 3.3. Examples ExamplesExamples processed processedprocessed by byby UAMIM. UAMIM.UAMIM. MaterialMaterial MIM Machine Molded Parts Table 3. Examples processed by UltrasoundUAMIM. Parameter Results Ref. Molded Parts TradeMaterial Name andMIM Parameter Machine Ultrasound Parameter Results Ref. Molded Parts TradeMaterialMaterialMaterial Name andMIMMIMMIM Parameter Machine MachineMachine Ultrasound Parameter Results Ref. MoldedMoldedMolded Parts PartsParts TradeMaterial Name andMIM Parameter Machine UltrasoundUltrasoundUltrasound Parameter ParameterParameter ResultsResultsResults Ref.Ref.Ref. Molded Parts TradeTrade NameName andandAZ700 ParameterParameter Ultrasound Parameter Results Ref. Trade Name andAZ700 Parameter f =f 19= 19 kHz kHz PolycarbonatePolycarbonateTrade Name (PC) (PC) CompressionandAZ700 Parameter force: f = 19 kHz ReplicationReplication propertiesproperties↑ ↑ Concave Lens Lens Polycarbonate (PC) CompressionAZ700AZ700AZ700 force: 980 kN Amplitude:Amplitude:ff f= = =19 1919 0–11 kHz 0–11kHzkHzµ m μm Replication properties ↑ [18][18] Concave Lens PolycarbonateTARFLONPolycarbonatePolycarbonateTARFLON A2200 A2200(PC) (PC)(PC) Compression980AZ700 kN force: 980 kN Amplitude:f = 19 kHz0–11 μm ResidualResidualReplicationReplicationReplication opticaloptical properties propertiesproperties strainstrain↓ ↓↑ ↑ ↑ [18] ConcaveConcave LensLens CompressionCompressionCooling time: force:force: 120 980980 s kNkN Oscillation:Amplitude:Oscillation:Amplitude: 0–60 0–110–600–11 s μsμ mm ↓↑ [18][18] Concave Lens TARFLONPolycarbonateTARFLON A2200A2200 (PC) CompressionCooling time: force: 120 980 s kN Amplitude: 0–11 μm ResidualResidualReplication opticaloptical properties strainstrain ↓ ↓ [18] Concave Lens TARFLON A2200 CompressionCooling time: force: 120 980 s kN Amplitude:Oscillation: 0–11 0–60 μ sm Residual optical strain ↓ [18] TARFLON A2200 CoolingCooling time: time: 120 120 s s Oscillation:Oscillation:f: 20 kHz 0–60 0–60 s s Residual optical strain ↓ CoolingARBURG time: 120 s Oscillation: 0–60 s Micro-Tensile Polypropylene (PP) ARBURG Allrounder 320C f: 20f: kHz 20 kHz Micro-TensileMicro-Tensile PolypropylenePolypropylene (PP) (PP) ARBURGAllrounder Allrounder 320C 320C Max. f:power:f:f: 20 2020 kHz kHzkHz 800 W Weld line strength ↑ [40] Micro-TensileMicro-TensileMicro-TensileSample PolypropylenePolypropylenePPHPolypropylene 734-52 RNA (PP) (PP)(PP) ARBURGARBURGARBURGClamping Allrounder AllrounderAllrounder force:600 kN320C 320C320C Max.Max. power: power:f: 20 800 kHz 800 W W WeldWeld line line strength strength↑ ↑ [40[40]] Sample PPH 734-52 RNA Clamping force: 600 Max.Max.Max. amplitude: power:power: 800 80011 μ WWm WeldWeld lineline strengthstrength ↑↑ [40][40] Micro-TensileSampleSample PolypropylenePPHPPH 734-52734-52 RNARNA (PP) ARBURGClampingClamping Allrounder force:600force:600 kN kN320C Max. power: 800µ W Weld line strength ↑ [40] Sample PPH 734-52 RNA Clamping force:600 kN Max.Max. amplitude:Max. amplitude: power: 11 800 11m Wμm Weld line strength ↑ [40] Sample PPH 734-52 RNA ClampingkN force:600 kN Max.Max. amplitude: amplitude: 11 11 μ μmm Pressure loss ↓ Max. amplitude: 11 μm Pressure loss ↓↓ f: 19 kHz ResidualPressurePressurePressure stress loss loss loss↓ ↓ ↓ Polycarbonate (PC) ARBURGARBURG Allrounder 320C f: 19 kHz ResidualPressure stress loss ↓↓ [31,3 A Flat Sample Polycarbonate (PC) ARBURG Allrounder 320C Max.f: 19f: power:f:f: 19 kHz 1919 kHz kHzkHz 2 kW FillingResidualResidualResidualResidual efficiency stress stress stressstress↓ ↓↑ ↓ ↓ [31,3 A Flat Sample PolycarbonatePolycarbonateTeijinPolycarbonate AD5503 (PC)(PC) (PC) ARBURGARBURGClampingAllrounder AllrounderAllrounder force:600 320C kN 320C320C Max. f:power: 19 kHz 2 kW FillingResidual efficiency stress ↓↑ [31,3[9][31,331 , AAA Flat FlatFlat Sample SampleSample Polycarbonate (PC) ARBURG Allrounder 320C Max.Max.Max. amplitude: power: power:power: 2 kW 2152 kWkW μm ThicknessFillingFillingFilling efficiency ofefficiencyefficiency the frozen↑ ↑↑ [31,3 A Flat Sample PolycarbonateTeijinTeijinTeijin AD5503AD5503 AD5503 (PC) ARBURGClampingClamping Allrounder force:600 force:force:600 600 kN kN320C Max. power: 2 kW Filling efficiency ↑ 39[31,39]9]] A Flat Sample Teijin AD5503 Clamping force:600 kN Max.Max. amplitude:Max. amplitude: power: 15 2µ 15 mkW μm ThicknessThicknessFillinglayer of efficiencyof the ↓ the frozen frozen ↑ 9] Teijin AD5503 ClampingkN force:600 kN Max.Max. amplitude: amplitude: 15 15 μ μmm ThicknessThickness of of the the frozen frozen 9] Max. amplitude: 15 μm Thicknesslayerlayer of↓ the↓ frozen layerlayerlayer ↓ ↓ ↓ COC Filling performancelayer ↓ ↑ Fresnel Lenses FANUC ROBOT S-2000i 50B f: 27 kHz [45] COC Filling performance↑ ↑ Fresnel Lenses ZeonexCOCCOCCOC 480R FANUCFANUC ROBOT ROBOT S-2000i 50B f: 27 kHz FillingSymmetricFillingFilling performance performance performance deviation↑ ↑ ↑ [45] FresnelFresnelFresnel Lenses LensesLenses ZeonexCOC 480R FANUCFANUCFANUC ROBOT ROBOTROBOT S-2000i S-2000iS-2000i 50B 50B50B f: 27 f: f: f:27 kHz 2727 kHz kHzkHz SymmetricFilling performance deviation ↑ [45][[45]45[45]] Fresnel Lenses ZeonexZeonexZeonexZeonex 480R 480R480R 480R FANUCS-2000i ROBOT 50B S-2000i 50B f: 27 kHz SymmetricSymmetricSymmetricSymmetric deviation deviation deviationdeviation↑ ↑ ↑ ↑ [45] Zeonex 480R Symmetric deviation ↑

Average weight/height ↑ Microchannel PMMA DQ-1500T-A f: 28 kHz Average weight/height ↑ Microchannel PMMA DQ-1500T-A f: 28 kHz AverageAverageAveragestandard weight/height weight/heightweight/height deviation ↑ ↑ ↑ ↑ [38] MicrochannelMicrochannelMicrochannelArray HT50YPMMAPMMAPMMA ClampingDQ-1500T-ADQ-1500T-ADQ-1500T-ADQ-1500T-A force: 15,000 kN Max. amplitude:f:f:f: 28 2828 kHz kHzkHz 10 μm AverageAveragestandard weight/height weight/height deviation ↑ ↑ [38] MicrochannelMicrochannelArray HT50YPMMAPMMA ClampingDQ-1500T-A force: 15,000 kN Max.f: 28amplitude:f: kHz 28 kHz 10 μm standardstandardstandardreplication deviation deviationdeviation rate ↑ ↑ ↑ ↑ [38][38][38] ArrayArrayArray HT50YHT50YHT50Y ClampingClampingClampingClamping force: force:force: force: 15,000 15,00015,000 kN kNkN Max.Max.Max. amplitude: amplitude:amplitude: 10 1010 μ μμmmm standardstandardreplication deviation deviation rate ↑↑ ↑ [38[38]] ArrayArray HT50YHT50Y Clamping force: 15,000 kNMax. Max. amplitude: amplitude: 10 µ 10m μm replicationreplicationreplication rate raterate ↑ ↑ ↑ 15,000 kN replication rate ↑ ↑ replication rate

Polymethylmethacry Micro-Needle f: 55 kHz Filling rate ↑ PolymethylmethacryPolymethylmethacrylate (PMMA) FANUC ROBOT S-2000i 50B [49] Micro-Needle PolymethylmethacryPolymethylmethacrylate f: 55 kHz Filling rate ↑ ↑ Micro-NeedleMicro-NeedleArray Polymethylmethacrylate (PMMA) FANUCFANUC ROBOT ROBOT S-2000i 50B Max.f: amplitude: 55f:f: 55 kHz55 kHz kHz 28 μm MaterialFillingFillingFilling properties rate rate rate↑ ↑ ↑ [49] Micro-NeedleArray JPC lateNovateclatelate (PMMA) (PMMA) (PMMA)(PMMA) BC06N FANUCFANUCFANUC ROBOT ROBOTROBOT S-2000i S-2000iS-2000i 50B 50B50B Max. amplitude:f: 55 kHz 28 μm MaterialFilling properties rate ↑ ↑ [49][[49]49[49]] ArrayArrayArray JPC lateNovatec (PMMA) BC06N FANUCS-2000i ROBOT 50B S-2000i 50BMax.Max. Max.Max. amplitude: amplitude: amplitude:amplitude: 28 28 µ 2828m μ μμmmm MaterialMaterialMaterialMaterial properties properties propertiesproperties↑ ↑ ↑ ↑ [49] Array JPCJPCJPCJPC Novatec NovatecNovatec Novatec BC06N BC06NBC06N BC06N Max. amplitude: 28 μm Material properties ↑ JPC Novatec BC06N 2.4. Theoretical Interpretations 2.4. Theoretical Interpretations 2.4.2.4.2.4. Theoretical TheoreticalAs Theoretical an important Interpretations Interpretations Interpretations link in the so-called “chain of knowledge” reaching from the 2.4. TheoreticalAs an important Interpretations link in the so-called “chain of knowledge” reaching from the productionAsAsAsAs anan an of important importantimportant polymers link link linklinkto their in ininin the the theend-usethe so-called so-calledso-calledso-called prop “chain “certies,“c“chainhainhain of knowledge” rheology ofofof knowledge”knowledge”knowledge” plays reaching an reachingreaching reachingimportant from the fromfromfrom role produc- the thethein productionAs an ofimportant polymers link to their in the end-use so-called prop erties,“chain rheologyof knowledge” plays an reaching important from role the in polymerproductionproductionproductiontion of polymersresearch of ofof polymers polymerspolymers [67], to their which to toto end-use their theirtheir seems end-use end-useend-use properties, to be prop morepropprop rheologyerties,erties, erties,important rheology rheologyrheology plays in the an plays playsplays importantmicro/nano an anan important importantimportant role scale in polymerduring role rolerole in inin polymerproduction research of polymers [67], which to their seems end-use to be propmoreerties, important rheology in the plays micro/nano an important scale during role in MIMpolymerpolymerpolymerresearch [68]. research [ researchresearchThe67], whichimprovement [67], [67],[67], seems which whichwhich to of beseems seemsseems the more melt to importanttoto be beproperbe more moremoreties in important importantimportant the mentioned micro/nano in inin the the theabove scalemicro/nano micro/nanomicro/nano under during the MIMscale scalescale ultrasonic [during 68 duringduring]. The MIMpolymer [68]. research The improvement [67], which of seems the melt to be proper moreties important mentioned in the above micro/nano under the scale ultrasonic during fieldMIMMIMMIMimprovement is[68]. [68].[68]. ultimately The TheThe improvement ofimprovementimprovement the the melt improvement properties of ofof the thethe melt meltmelt mentioned of proper the properproper polymerties abovetiesties mentioned mentionedmentioned underrheological the above aboveabove ultrasonic properties. under underunder field the thethe It is ultrasonic ultrasonicultrasonicis ultimately worth fieldMIM is [68]. ultimately The improvement the improvement of the melt of properthe polymerties mentioned rheological above properties. under the It ultrasonic is worth mentioningfieldfieldfieldthe improvement is isis ultimately ultimatelyultimately that when of the thethe the theimprovement improvementimprovement polymer channel rheological size of ofof is the thethe less properties. polymer polymerpolymer than 10 rheological rheological Itrheologicalμm is, worth the micro mentioning properties. properties.properties. viscosity that It ItIt and is is whenis worth worth worthwall the mentioningfield is ultimately that when the theimprovement channel size of isthe less polymer than 10 rheological μm, the micro properties. viscosity It andis worth wall slipmentioningmentioningchannel play a size vital thatthat is lessrole whenwhen than[69]. thethe 10 µ chacham,nnel thennel micro sizesize is viscosityis lessless thanthan and 1010 wall μmμmμm slip,, thethe play micromicro a vital viscosityviscosity role [69 andand]. wallwall mentioningmentioning that that when when the the cha channelnnel size size is is less less than than 10 10 μm, the, the micro micro viscosity viscosity and and wall wall slipslip playplay aa vitalvital rolerole [69].[69]. slipslip play play a avital vital role role [69]. [69]. 2.4.1.2.4.1. Viscosity Viscosity in in Liquid Liquid 2.4.1. Viscosity in Liquid 2.4.1.2.4.1.The Viscosity TheViscosity index index thatin in that Liquid Liquid measures measures the the deformation deformation resist resistanceance of of fluid fluid at at a agiven given rate rate is is called called 2.4.1.The Viscosity index thatin Liquid measures the deformation resistance of fluid at a given rate is called thethe viscosityThe viscosityTheThe index indexindex of of thatfluid. thatthat fluid. measures measuresmeasures “The “The viscosity viscosity the thethe deformation deformationdeformation of of syrup syrup is resist isresistresisthigher higheranceanceance than than of ofof fluid fluidthatfluid that of at atat of water”a aa water” given givengiven is rate rate israte a acommon is commonisis called calledcalled the viscosityThe index of thatfluid. measures “The viscosity the deformation of syrup is resist higherance than of fluidthat ofat water”a given israte a common is called statement,thethethestatement, viscosity viscosityviscosity in in of ofofwhich which fluid. fluid.fluid. thethe “The “The“The conceptconcept viscosity viscosityviscosity of of viscosity of viscositofof syrup syrupsyrup correspondsy is isiscorresponds higher higherhigher than thanthan to the to that thatthat informalthe of ofof informal water” water”water” concept is is is concept a aa common commoncommon of “thick- of statement,the viscosity in ofwhich fluid. the “The concept viscosity of ofviscosit syrupy iscorresponds higher than tothat the of informalwater” is concepta common of “thickness”statement,statement,statement,ness” in terms ininin whichwhichtermswhich of liquid theofthethe liquid [ concept70conceptconcept]. Shear [70]. ofofof thinning Shear viscositviscositviscosit thinningy isyy corresponds thecorrespondscorresponds most is the common most tototo thethe thecommon type informalinformalinformal of non-Newtonian type conceptconceptconcept of non- ofofof “thickness”statement, inin whichterms ofthe liquid concept [70]. of Shearviscosit thinningy corresponds is the most to the common informal type concept of non- of Newtonian“thickness”“thickness”“thickness”behavior of behavior in fluids;inin termstermsterms as of aofofof kind fluids; liquidliquidliquid of non-Newtonian as [70].[70].[70]. a kind ShearShearShear of thinningthinning thinningnon-Newtonia behavior, isisis thethe the nmostviscositymostmost behavior, commoncommoncommon of polymerthe typetypetypeviscosity decreases ofofof non-non-non- of Newtonianwith“thickness” increasing behaviorin terms shear ofof rate fluids;liquid or shear as[70]. a stress.kindShear of Thethinning non-Newtonia time-independent is the nmost behavior, common relationship the typeviscosity between of non- of polymerNewtonianNewtonianNewtonian decreases behavior.behaviorbehavior with ofofof fluids;fluids;increasingfluids; asasas aa a shear kindkindkind ofofraof te non-Newtonianon-Newtonianon-Newtonia or shear stress.nnn behavior,behavior,behavior, The time-independent thethethe viscosityviscosityviscosity ofofof polymerNewtonian decreasesγ behavior with of fluids;increasingτ as a shearkind ofra tenon-Newtonia or shear stress.n behavior, The time-independent the viscosity of relationshippolymerpolymerpolymershear rate decreasesdecreasesdecreases ( between) and shear withwith withshear stress increasingincreasingincreasing rate ( ()𝛾) of and non-Newtonian shearshearshear shear rarara testresstete ororor shearshear( fluidsshearτ) of stress.non-Newtonian canstress.stress. be describedTheTheThe time-independenttime-independenttime-independent fluids by the can general be relationshipEquationpolymer (2)decreases between with shear increasing rate (𝛾) and shear shear ra testress or shear(τ) of stress.non-Newtonian The time-independent fluids can be describedrelationshiprelationshiprelationship by between thebetweenbetween general shear shearshear Equation rate raterate ( (𝛾( 𝛾(2))) ) and andand shear shear.shear stress stressstress ( (τ(ττ)) ) of ofof non-Newtonian non-Newtoniannon-Newtonian fluids fluidsfluids can cancan be bebe describedrelationship by betweenthe general shear Equation rate (𝛾 (2)) and γ shear= f ( τstress). (τ) of non-Newtonian fluids can (2)be describeddescribeddescribed by byby the thethe general generalgeneral Equation EquationEquation (2) (2)(2) described by the general Equation (2) 𝛾 = 𝑓(𝜏). (2) The behavior of fluids in the shear-thinning𝛾 = 𝑓(𝜏 regime). can be described with the power-(2) 𝛾𝛾𝛾 = ==𝑓𝑓𝑓(((𝜏𝜏𝜏)).) .. (2)(2)(2) lawThe equation behavior of Oswald of fluids and in the de shear-thinni Waele: 𝛾 =ng𝑓 regime(𝜏). can be described with the power-(2) The behavior of fluids in the shear-thinning regime can be described with the power- law equationTheTheThe behavior behaviorbehavior of Oswald of ofof fluids fluidsfluids and in in inde the thethe Waele: shear-thinni shear-thinnishear-thinni ngngng regime regimeregime can cancan be bebe described describeddescribed with withwith the thethe power- power-power- The behavior of fluids in the shear-thinni ngn regime can be described with the power- lawlaw equationequation ofof OswaldOswald andand dede Waele:Waele: dy . law equation of Oswald and de Waele: law equation of Oswald and deτ Waele:= K(T ) = K(T)γ. (3) 𝜏=𝐾(𝑇) dt =𝐾(𝑇)𝛾. (3) 𝜏=𝐾(𝑇) =𝐾(𝑇)𝛾. (3) 𝜏=𝐾𝜏=𝐾𝜏=𝐾(((𝑇𝑇𝑇))) =𝐾=𝐾=𝐾(((𝑇𝑇𝑇)))𝛾𝛾𝛾. .. (3)(3)(3) 𝜏=𝐾(𝑇) =𝐾(𝑇)𝛾. (3) ThisThis equation equation may may be be written written in in logarithmic logarithmic form: form: This equation may be written in logarithmic form: ThisThisThis equation equationequation may maymay be bebe written writtenwritten in inin logarithmic logarithmiclogarithmic form: form:form: This equation may be writtenlog( in𝜏) logarithmic=log(𝐾) +𝑛log form:(𝛾 .)
. (4) loglog(τ()𝜏)==loglog((K𝐾))++𝑛logn log (γ𝛾).. (4) (4) logloglog(((𝜏𝜏𝜏)))=log=log=log(((𝐾𝐾𝐾)))+𝑛log+𝑛log+𝑛log(((𝛾𝛾𝛾))).. (4)(4) log(𝜏) =log(𝐾) +𝑛log(𝛾.) . (4)(4)

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A log–log plot of shear stress (τ) versus shear strain (dy/dt) should yield a straight A log–log plot of shear stress (τ) versus shear strain (dy/dt) should yield a straight line line if the polymer solution or melt behaves as a pseudoplastic liquid. The apparent if the polymer solution or melt behaves as a pseudoplastic liquid. The apparent viscosity is viscosity is defined by the following Equation (5). defined by the following Equation (5). 𝜂= τ (5) η = . (5) A second power-law equationγ for the apparent viscosity is obtained by combining this expression with the Oswald Equation (6): A second power-law equation for the apparent viscosity is obtained by combining this expression with the Oswald Equation (6): 𝜂=𝐾(𝑇)𝛾 (6)

where K is the flow consistency index,. n− 1n is the flow behavior index, for shear thinning or η = K(T)γ (6) pseudoplastic flow, and n < 1. where K is the flowAs consistencyfar as viscosity index, measurementn is the flow behavior is conc index,erned, forcapillary shear thinning viscometer or and slit pseudoplasticviscometer flow, and aren < two 1. commonly used technologies for measuring melt viscosity [71]. By As far asusing viscosity a slit/capillary measurement die embedded is concerned, in a capillarynozzle or viscometer mold, some and work slit has viscome- been done to test ter are twothe commonly rheological used properties technologies of material for measuring with an meltinjection-molding viscosity [71]. machine By using or a an extruder slit/capillary[72–75]. die embedded in a nozzle or mold, some work has been done to test the rheo- logical propertiesThe of materialrheological with test an under injection-molding ultrasonic field machine is more or anfocused extruder on the [72– field75]. of extrusion The rheologicalmolding, testbut underits influence ultrasonic mechanism field is moreis also focused of great on significance the field of for extrusion the application of molding, butultrasound its influence in the mechanism injection-molding is also of process. great significance The mechanism for the and application influence of of ultrasonic ultrasound inenergy the injection-molding field on viscosity process. in the process The mechanism of micro-injection and influence molding of ultrasonic need to be further energy fieldstudied. on viscosity in the process of micro-injection molding need to be further studied. Under differentUnder ultrasonic different power ultrasonic levels, apower linear relationshiplevels, a linear between relationship log ηa (logarith- between log 𝜂 mic viscosity)(logarithmic and log γw viscosity)(logarithmic and shearlog 𝛾 rate) (logarithmic was detected shear in rate) the ultrasonic-assistedwas detected in the ultrasonic- extrusion process,assisted which extrusion obeys process, the equation. which obeys Based the on equation. the power-law Based on model, the power-law Guo et al. model, Guo have done aet lot al. of have work done about a lot the of effect work of about ultrasound the effect on rheologyof ultrasound improvement on rheology using improvement the speciallyusing designed the specially ultrasonic designed oscillation ultrasonic extrusion oscillation system, extrusion as shown system, in Figure as 9shown. The in Figure 9. results showedThe thatresults when showed ultrasonic that when vibration ultrasonic is introduced vibration into is PSintroduced melt, n increases into PS melt, with n increases the increasewith of ultrasonic the increase intensity, of ultrasonic which indicates intensity, that which ultrasonic indicates vibration that ultrasonic reduces vibration shear reduces sensitivity. Withshear the sensitivity. increase ofWith ultrasonic the increase intensity, of theultrasonic consistency intensity, index ofthe polystyrene consistency index of decreases, indicatingpolystyrene that decreases, the melt indicating viscosity ofthat polystyrene the melt viscosity decreases of underpolystyrene the action decreases under of ultrasonicthe oscillations action of ultrasonic [76]. The sameoscillations trend is[76]. obtained The same in LLDPE trend is [77 obtained], as shown in LLDPE in [77], as Figure 10, andshown HDPE in [Figure78]. The 10, most and intuitive HDPE [78]. conclusion The most is that intuitiv at differente conclusion temperatures, is that at different the apparenttemperatures, viscosity of thethe melt apparent decreases viscosity with increasingof the melt ultrasonic decreases density. with increasing In other ultrasonic words, whendensity. the apparent In other viscosity words, iswhen maintained the apparent at the sameviscosity level, is ultrasonic maintained vibration at the same level, will lower theultrasonic processing vibration temperature will lower of LLDPE. the processi Later studiesng temperature on other polymer of LLDPE. materials Later studies on show the similarother trendpolymer via materials ultrasonic show processing. the similar trend via ultrasonic processing. However, ultrasonicHowever, energy ultrasonic field energy can transform field can vibration transform energy vibration into energy heat energy into heat to energy to increase theincrease melt temperature. the melt temperature. It is difficult It to is ruledifficult out theto rule possibility out the that possibility the decreased that the decreased viscosity is causedviscosity by is the caused increased by the melt increased temperature. melt te Themperature. different The application different methodsapplication methods (direct or indirect)(direct ofor theindirect) ultrasonic of the energy ultrasonic field, energy the setting field, of the the setting vibration of the point vibration (parallel point (parallel or perpendicular),or perpendicular), and the energy and utilizationthe energy efficiency utilization are efficiency still unresolved. are still unresolved.

Figure 9. SpeciallyFigure designed9. Specially ultrasonic designed oscillation ultrasonic extrusion oscillation system. extrusion system.

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Figure 10. Figure(a) Apparent 10. (a )flow Apparent curves flowof PS curves for differe of PSnt ultrasound for different intens ultrasoundities at 200 intensities °C; (b) Apparent at 200 ◦C; flow (b) curves Apparent of LLDPE for differentflow ultrasound curves of intensities LLDPE for at different180 °C; (c) ultrasound Correlation intensities between K at and 180 n◦ valuesC; (c) Correlation of LLDPE under between ultrasonic K and intensity n at 180 °C; (d) Apparent viscosity of LLDPE vs. ultrasonic intensity at different temperatures (𝛾 = 80 s−1) [77]. values of LLDPE under ultrasonic intensity at 180 ◦C; (d) Apparent viscosity of LLDPE vs. ultrasonic −1 intensity at different temperaturesThe orientation (γω of= 80molecular s )[77]. chain flow is the main reason for the elasticity and non- Newtonian property of polymer melts. It is said that entanglement of molecular chains The orientation of molecular chain flow is the main reason for the elasticity and decreases the possibility of molecular chain orientation and slows down relaxation, while non-Newtonian property of polymer melts. It is said that entanglement of molecular ultrasonic vibration can shorten relaxation time, obviously [79]. Chen et al. [79] found that chains decreases the possibility of molecular chain orientation and slows down relaxation, the apparent relaxation time dropped sharply by increasing the ultrasonic intensity from while ultrasonic vibration can shorten relaxation time, obviously [79]. Chen et al. [79] 0 to 50 W. Molecular chain disentanglement, which orients molecules, can be attributed found that the apparent relaxation time dropped sharply by increasing the ultrasonic to the strong vibration, crushing, and cavitation of power ultrasound, which is helpful to intensity fromexcite 0 to 50 and W. activate Molecular molecular chain disentanglement, chains [80,81]. In fact, which at a orientssufficiently molecules, high shear can rate, highly be attributed toanisotropic the strong polymer vibration, chains crushing, can be and disentangled cavitation and of poweraligned ultrasound, along the shear which direction [82]. is helpful to exciteTherefore, and activatethe viscosity molecular of polymer chains melt [80,81 can]. Inbe fact, reduced at a sufficientlyby fewer molecule/particle high shear rate, highlyinteractions anisotropic and polymerlarger free chains space can[83]. be Additionally, disentangled the and influence aligned of power along theultrasound on shear directionthe [82 physical]. Therefore, and chemical the viscosity properties of polymer of polymer melt can melts be reducedis also one by of fewer the important molecule/particleresearch interactions contents. andThe largerresearch free shows space that [83 unde]. Additionally,r most research the intensity, influence the of influence of power ultrasoundchemical on the effect physical on the and apparent chemical viscosity properties of polypropylene of polymer a meltsccounts is alsofor 35–40% one of of the total the important researchultrasonic contents. effect [84]. The research shows that under most research intensity, the influence of chemicalThe above effect reports on the sufficiently apparent viscosityexplain why of polypropylene ultrasonic vibration accounts can for improve the 35–40% of the totalquality ultrasonic of molded effect parts, [84 ].increase the replication fidelity, and even reduce the viscosity. The aboveInterpretation reports sufficiently at the explain molecular why level ultrasonic is co vibrationnducive to can the improve design theand quality optimization of of molded parts,process increase parameters. the replication At the fidelity, same andtime, even this reducemay be the the viscosity. most significant Interpretation advantage of the at the molecularUAMIM level is conduciveprocess. to the design and optimization of process parameters. At the same time, this may be the most significant advantage of the UAMIM process. 2.4.2. Wall Slip in Cavity 2.4.2. Wall Slip in CavityGenerally, viscous fluid means adhering to the boundary and reaching the boundary Generally,velocity viscous in fluid the meansprocess adhering of flow. However, to the boundary the so-called and reaching “wall slip” the here boundary means that there velocity in the process of flow. However, the so-called “wall slip” here means that there is a relative velocity on the contact line between the fluid and the solid boundary during the flow process [85].

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Polymers 2021, 13, 2877 13 of 41

is a relative velocity on the contact line between the fluid and the solid boundary during the flow process [85]. ThereThere are two two slip slip regions regions in in the the flow flow of ofmolten molten polymer. polymer. When When the wall the shear wall shearstress stressexceeds exceeds a critical a critical value, value, molten molten polymers polymers slip macroscopically slip macroscopically over wall over surfaces, wall surfaces, which whichis known is known as weak as weak slip slipor oradhesion-detachment adhesion-detachment wall wall slip slip [86–89]. [86–89]. Then, Then, for thethe characteristicscharacteristics of of linear linear polymers, polymers, especially especially those those with relativelywith relatively narrow narrow molecular molecular weight distributionweight distribution [90], when [90], a secondwhen a critical second value critical is exceeded,value is exceeded, another slip another will occur,slip will which occur, is calledwhich strongis called slip strong or entanglement-disentanglement slip or entanglement-disentanglement wall slip wall [42,90 slip–92 [42,90–92].]. Deng et Deng al. [93 et] foundal. [93] that found with that the decreasewith the ofdecrease channel of size channel (the increase size (the of shearincrease stress), of shear the destructionstress), the speeddestruction of the speed entanglement of the entanglement points was fasterpoints thanwas thefaster reconstruction than the reconstruction speed, as shown speed, inas Figureshown 11 in. Figure When the11. When diameter the ofdiameter the channel of th decreasese channel furtherdecreases and further the shear and ratethe shear increases rate toincreases a certain to value, a certain there value, is not enoughthere is timenot toenough reconstruct time to the reconstruct entanglement the points.entanglement Hence, thepoints. free Hence, chains canthe befree easily chains oriented can be alongeasily theoriented velocity along field, the but velocity those infield, the but attachment those in areathe attachment remain entangled area remain and attached entangled to theand wall. attached to the wall.

Figure 11. Polymer flows through microchannel with size less than Ds (0.2–0.5 mm); (a) laminar Figure 11. Polymer flows through microchannel with size less than Ds (0.2–0.5 mm); (a) laminar flow; flow; (b) plug flow and laminar flow; (c) Newtonian fluid [93]. (b) plug flow and laminar flow; (c) Newtonian fluid [93].

SlipSlip playsplays anan importantimportant rolerole inin correctlycorrectly determiningdetermining thethe rheologicalrheological propertiesproperties ofof polymerspolymers [94[94],], whichwhich can can correct correct the the slip slip effect effect data data and and explain explain the reasonsthe reasons for the for mis- the matchmismatch of rheological of rheological data data obtained obtained from from various various rheometers rheometers with with different different geometries geometries [95]. In[95]. micro-injection In micro-injection molding, molding, the no-slip the no-slip boundary boundary condition condition is not is valid not valid due to due the to high the velocity,high velocity, high pressure, high pressure, and micro-scale and micro-scale flow conditions; flow conditions; hence, thehence, melt the flow melt will flow exhibit will unevenexhibit uneven and complex and complex changes changes in the micro-cavity. in the micro-cavity. Especially Especially in the micro-sized in the micro-sized cavity, becausecavity, because the shear the stress shear is greater stress thanis greater that of thethan conventional that of the cavity,conventional the occurrence cavity,rate the ofoccurrence wall slip israte higher of wall in theslip filling is higher process in the [96 filling,97]. process [96,97]. TheThe moldingmolding quality quality of of micro-polymer micro-polymer parts parts is largelyis largely controlled controlled by the by melt the flowmelt fieldflow infield the in micro-cavity, the micro-cavity, in which in the which influence the influence of wall slip of is wall complex slip andis complex cannot beand ignored cannot [98 be]. Theignored influence [98]. The includes influence significantly includes reducing significantly the wall reducing shear stressthe wall and shear the meltstress apparent and the viscosity,melt apparent reducing viscosity, the velocity reducing gradient, the velocity improving gradient, the uniformity improving of the flow uniformity rate distribution of flow andrate viscositydistribution distribution, and viscosity and distribution, thus promoting and the thus mold promoting filling in the micro-cavity, mold filling or in further micro- transformingcavity, or further the flowtransforming field into athe piston flow flow,field whichinto a piston is good flow, for filling which [99 is, 100good]. for filling [99,100].Although UAMIM has been proved the ability to improve the rheological properties of polymerAlthough melt UAMIM and promote has been melt proved flow, the there abilit arey to few improve related the research rheological studies properties on the mechanismof polymer ofmelt wall and slip promote under an melt ultrasound flow, ther field.e Gaoare few et al. related [42] established research bothstudies adhesion– on the detachmentmechanism andof wall entanglement–disentanglement slip under an ultrasound field. wall slipGao models et al. by[42] combing established the effect both ofadhesion–detachment ultrasonic vibration. and The entanglement–disentanglement results measured in the home-made wall slip equipment models by show combing that boththe effect weak of and ultrasonic strong wall vibration. slip of meltThe inresults a micro-cavity measured canin the be enhancedhome-made by equipment ultrasonic vibration,show that which both weak agrees and with strong the built wall theoretical slip of melt models, in a micro-cavity as shown in Figure can be 12 enhanced. Ultrasonic by vibrationultrasonic can vibration, reduce which the apparent agrees with viscosity the built of polymer theoretical melt, models, release as shear shown stress, in Figure and improve12. Ultrasonic the filling vibration ability can of meltreduce in the micro-cavity,apparent viscosity thus improving of polymer the melt, molding release quality shear ofstress, micro-polymer and improve parts the [45 filling]. With ability the introduction of melt in ofthe a micro-cavity, new physical field,thus improving it is necessary the tomolding customize quality viscosity of micro-polymer testing equipment parts for [45]. polymer With the rheology, introduction and there of area new few physical reports in this field. At the same time, the test standards and test conditions of viscosity and wall slip should not be excluded from the discussion.

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Polymers 2021, 13, 2877 field, it is necessary to customize viscosity testing equipment for polymer rheology,14 ofand 41 there are few reports in this field. At the same time, the test standards and test conditions of viscosity and wall slip should not be excluded from the discussion.

FigureFigure 12.12. The schematicschematic diagram diagram of of the the weak weak slip slip (a) ( anda) and strong strong slip slip (b) under(b) under ultrasonic ultrasonic vibration vibration [42]. 3.[42]. Ultrasonic Plasticization Micro-Injection Molding 3.1. Size Effect in MIM 3. Ultrasonic Plasticization Micro-Injection Molding The size effect is mainly reflected in the situation in which the volume and the injection 3.1. Size Effect in MIM volume of the final molded part are seriously mismatched. Almost all micro-injection- moldingThe machinessize effect cannot is mainly avoid reflected material wastein the causedsituation by miniaturization.in which the volume and the injectionThree-section volume of screws the final areused molded to plasticize part are polymers seriously commonly mismatched. in the Almost processing. all micro- The screwinjection-molding diameter is machines limited due cannot to the avoid size material of standard waste granules caused andby miniaturization. large shear force in processing,Three-section making screws the smallest are used diameter to plasticize about 14polymers mm, and commonly the screw in moves the processing. just 1 mm. AboutThe screw 0.185 diameter g of plastic is limited material due are to injected the size [ 19of], standard while a part granules can only and be large 0.024 shear g or force less, whichin processing, means 0.185–0.024 making the g ofsmallest material diameter is wasted. about In the 14 production mm, and the of medical screw moves devices, just it is1 extremelymm. About important 0.185 g of to plastic avoid material wasting are raw injected materials [19], that while directly a part affect can theonly price be 0.024 of parts, g or becauseless, which the costmeans per 0.185–0.024 gram of bio-absorbable g of material polymers is wasted. may In be the 5–10 production dollars [21 ].of medical devices,In order it is extremely to achieve aimport smallant injection to avoid volume wasting (<1 mg),raw materials IKV (Institute that fürdirectly KunststoffVer- affect the arbeitung,price of parts, Aachen, because Germany) the cost has per prepared gram of abio-absorbable small desktop-level polymers micro-injection-molding may be 5–10 dollars machine[21]. that is only the size of a shoe box, in which 2 mm injection plungers, 5 mm me- teringIn plungers, order to TOOLVAC achieve technologya small injectionTM-have beenvolume used (<1 [101 ].mg), However, IKV plasticization(Institute für timeKunststoffVerarbeitung, has become a large proportionAachen, Germany) of the entire has molding prepared cycle a small due todesktop-level the electric heating micro- plasticization;injection-molding hence, machine ultrasonic that plasticization is only the size of traceof a polymersshoe box, wasin which investigated 2 mm injection in order toplungers, improve 5 themm efficiency metering by plungers, Michaeli etTOOLVAC al. [19], and technology the moldingTM- have process been based used on [101]. this kindHowever, of plasticization plasticization method time ishas called become UPMIM. a large It is proportion reported that of the injectionentire molding volume cycle can bedue distributed to the electric between heating 5 andplasticization; 300 mg. The hence, optimization ultrasonic ofplasticization the structure, of althoughtrace polymers fully electrifiedwas investigated to some in extent, order to alleviates improve the the problems efficiency caused by Michaeli by miniaturization, et al. [19], and butthe molding it is still farprocess from based enough. on UPMIMthis kind technology of plasticization with a method simple plasticizationis called UPMIM. and moldingIt is reported principle that hasthe generallyinjection volume demonstrated can be its distributed characteristics betw andeen advantages 5 and 300 mg. since The its introductionoptimization in of 2002. the Obviously,structure, although this technology fully electrified provides ato new some choice extent, for thealleviates injection the molding problems of small caused batch by micro-parts.miniaturization, but it is still far from enough. UPMIM technology with a simple plasticization and molding principle has generally demonstrated its characteristics and 3.2. Principles and Methods advantages since its introduction in 2002. Obviously, this technology provides a new 3.2.1.choice Process for the Principleinjection molding of small batch micro-parts. Similar to conventional micro-injection molding, UPMIM has the same molding stage: feeding,3.2. Principles plasticization, and Methods injection and holding, and cooling and ejection. The schematic of the UPMIM process is shown in Figure 13. Since UPMIM is designed to plasticize a small 3.2.1. Process Principle amount of polymer in a single shot, only one molding cycle of raw materials is needed at the feedingSimilar stage. to conventional During feeding, micro-injection the sonotrode molding, is displaced UPMIM upward has tothe leave same the molding feeding space.stage: Then,feeding, it is plasticization, displaced downward injection to provideand holding, sufficient and pre-compression cooling and ejection. to plasticize The theschematic polymer of particles the UPMIM at high process frequency. is shown During in theFigure injection 13. Since and UPMIM holding stage,is designed with the to downwardplasticize a movementsmall amount of the of sonotrode, polymer thein a filling single of shot, the polymer only one melt molding is completed. cycle of After raw thematerials part is cooledis needed down, at the rodfeeding ejects stage. the part, Du andring afeeding, molding the cycle sonotrode is completed, is displaced and the secondupward molding to leave canthe befeeding performed space. byThen, feeding. it is displaced There are downward various names to provide for this sufficient process, such as ultrasonic molding [102,103], ultrasonic injection molding [21], ultrasonic micro molding [104], ultrasound injection molding [105], and ultrasonic micro-injection mold- ing [106]. In order to avoid confusion with ultrasound-assisted micro-injection molding (UAMIM) and ultrasonic compression molding (UCM), ultrasonic plasticization micro- Polymers 2021, 13, x FOR PEER REVIEW 16 of 43

pre-compression to plasticize the polymer particles at high frequency. During the injection and holding stage, with the downward movement of the sonotrode, the filling of the polymer melt is completed. After the part is cooled down, the rod ejects the part, and a molding cycle is completed, and the second molding can be performed by feeding. There are various names for this process, such as ultrasonic molding [102,103], ultrasonic injection molding [21], ultrasonic micro molding [104], ultrasound injection molding Polymers 2021, 13, 2877 [105], and ultrasonic micro-injection molding [106]. In order to avoid confusion15 with of 41 ultrasound-assisted micro-injection molding (UAMIM) and ultrasonic compression molding (UCM), ultrasonic plasticization micro-injection molding (UPMIM) is named after its most important process feature, that is, power ultrasound is the only energy sourceinjection for molding plasticizing (UPMIM) polymers. is named after its most important process feature, that is, powerIn ultrasoundterms of molding is the only process, energy only source a small forplasticizing amount of polymers.raw materials are plasticized and injectedIn terms in of a molding single cycle process, of UPMIM. only a small Therefore, amount it seems of raw that materials reducing are plasticizedcycle time and materialinjected inwaste, a single especially cycle of UPMIM. medical Therefore, materials, it seemsis the that main reducing advantages cycle time of andUPMIM. mate- Furthermore,rial waste, especially the process medical can save materials, production is the costs main when advantages producing of UPMIM. small batches, Furthermore, and it isthe especially process can suitable save productionfor the initial costs stage when of product producing development. small batches, However, and it is the especially micro- injection-moldingsuitable for the initial process stage ofstill product has irreplaceable development. advantages However, the in micro-injection-molding mass production and process still has irreplaceable advantages in mass production and automated production. automated production.

Figure 13. Schematic representation of the UPMIM process.

3.2.2. Configuration Configuration Development The UPMIMUPMIM equipment equipment currently currently used used for moldingfor molding can be can divided be intodivided the following:into the (1)following: an independently (1) an independently developed developed ultrasonic plasticizingultrasonic plasticizing experimental experimental platform, includingplatform, includingan upgrade an by upgrade an ultrasonic by an welding ultrasonic machine welding [19 ,machine102,107– 109[19,102,107–109],], completely home-made completely home-madeequipment [ 20equipment,110,111]; (2)[20,110,111]; commercial (2) equipment: commercial Sonorus equipment: 1 G has Sonorus a maximum 1 G singlehas a maximuminjection weight single of injection 2 g, but weight for the needsof 2 g, of but the for industry, the needs this of machine the industry, can accommodate this machine a canslightly accommodate larger shot sizea slightly [106,112 larger–115]. shot Subsequently, size [106,112–115]. the second Subsequently, version of the the equipment second (theversion Sonorus of the S210 equipment machine) (the came Sonorus out in S210 2016, ma withchine) a maximum came out injectionin 2016, with weight a maximum of 5 g. As injectionfor the driving weight method, of 5 g. As the for ultrasonic the driving plasticizing method, the experimental ultrasonic plasticizing device was drivenexperimental by the previouslydevice was unstabledriven by air the pressure previously [108 unstable,112] and air replaced pressure with [108,112] a servo and drive replaced with precise with a servodisplacement drive with and precise pressure displacement control [20]. and pressure control [20]. In terms of UPMIM processing modes, they cancan bebe divideddivided intointo twotwo categories.categories. TheThe firstfirst type is “injection while plasticizing”. An Anotherother type is “injection after plasticization”. For thethe former, former, the plasticizingthe plasticizing cavity cavity and the and cavity the are connected.cavity are Ultrasonicconnected. plasticization Ultrasonic is accompanied by injection filling of the melt. At present, almost all related equipment plasticization is accompanied by injection filling of the melt. At present, almost all related operations adopt “injection while plasticizing” mode. Under this mode, the structure equipment operations adopt “injection while plasticizing” mode. Under this mode, the of the equipment can be divided into two configurations, as shown in Figure 14. In structure of the equipment can be divided into two configurations, as shown in Figure 14. Configuration 1, the lower plunger is fixed during the molding process, and the ultrasonic In Configuration 1, the lower plunger is fixed during the molding process, and the sonotrode performs ultrasonic vibration while completing the injection-filling operation. ultrasonic sonotrode performs ultrasonic vibration while completing the injection-filling In Configuration 2, the ultrasonic sonotrode has no displacement during the molding operation. In Configuration 2, the ultrasonic sonotrode has no displacement during the process, and it is only responsible for ultrasonically plasticizing the polymer. The injection and filling behavior are completed from the bottom to the plunger. The most intuitive performance of the two frame structures is that the end face of the sonotrode of the latter is always flush with the surface of the cavity, and the position of the sonotrode of the former has been dynamically changed, which is one of the reasons for the surface wear of the sonotrode. The polymer on the surface of the sonotrode in Configuration 1 is plasticized first, and the energy is transferred from the top to the bottom. During the injection filling, the unplasticized impurities easily enter the cavity, and Configuration 2 can reduce the wear and uneven filling to a certain extent. Reports in recent years have shown that Configuration 1 is being replaced by Configuration 2 with obvious advantages. However, Polymers 2021, 13, x FOR PEER REVIEW 17 of 43

molding process, and it is only responsible for ultrasonically plasticizing the polymer. The injection and filling behavior are completed from the bottom to the plunger. The most intuitive performance of the two frame structures is that the end face of the sonotrode of the latter is always flush with the surface of the cavity, and the position of the sonotrode of the former has been dynamically changed, which is one of the reasons for the surface wear of the sonotrode. The polymer on the surface of the sonotrode in Configuration 1 is Polymers 2021, 13, 2877 plasticized first, and the energy is transferred from the top to the bottom. During16 of the 41 injection filling, the unplasticized impurities easily enter the cavity, and Configuration 2 can reduce the wear and uneven filling to a certain extent. Reports in recent years have shown that Configuration 1 is being replaced by Configuration 2 with obvious no matter which configuration it is, more profound basic theoretical research is needed. advantages. However, no matter which configuration it is, more profound basic Only in this way can the process stability be improved. On the other hand, the “injection theoretical research is needed. Only in this way can the process stability be improved. On after plasticization” mode is superior in process stability. Compared with the former, the the other hand, the “injection after plasticization” mode is superior in process stability. latter not only uses an ultrasonic energy field to plasticize raw materials but also includes Compared with the former, the latter not only uses an ultrasonic energy field to plasticize metering and injection devices. In fact, almost no current equipment adopts this mode. raw materials but also includes metering and injection devices. In fact, almost no current However, well-designed equipment according to this model still has a chance to be one of equipment adopts this mode. However, well-designed equipment according to this model the potential development directions. still has a chance to be one of the potential development directions.

FigureFigure 14. 14.Schematic Schematic representation representation of of “injection “injection while while plasticizing” plasticizing” processing processing mode mode in in UPMIM. UPMIM.

3.3.3.3. EngineeringEngineering Characteristics Characteristics 3.3.1.3.3.1. IncreasedIncreased Material Material Utilization Utilization ThereThere is is still still a a runner runner and and sprue sprue in in molded molded parts, parts, which which made made it it appear appear particularly particularly largelarge in in micro micro molding. molding. However, However, compared compared with with traditional traditional MIM, MIM, UPMIM UPMIM can can save save 40% 40% toto 70% 70% of of the the equivalent equivalent cold cold runner. runner. The The volume volume of of the the cylindrical cylindrical gate gate depends depends on on the the diameterdiameter ofof thethe sonotrodesonotrode andand plunger,plunger, and and the the commonly commonly used used sizesize is is 8 8 mm mm [ 108[108,112],112] andand 1010 mmmm [20,111]. [20,111]. In In the the experiment experiment of ofSacris Sacristtáná etn al. et al.[112], [112 polylactide], polylactide (PLA) (PLA) was wasused usedto mold to mold eight eight test specimens test specimens of small of small dimensions, dimensions, each each weighting weighting 10 mg, 10 mg,where where a shot a shotweight weight of 250 of mg 250 is mg required. is required. Finally, Finally, nearly nearly 70% of 70% the of loaded the loaded materials materials became became waste waste(170 mg), (170 mg),which which were werewasted wasted on the on sprue the sprue and andrunners, runners, saving saving 20% 20% of the ofthe materials mate- rials compared with the traditional injection-molding system [19]. In another UPMIM compared with the traditional injection-molding system [19]. In another UPMIM equipment, Grabalosa et al. [108] reduced the waste to 45% according to calculations of equipment, Grabalosa et al. [108] reduced the waste to 45% according to calculations of Heredia et al. [21]. Figure 15 shows the quantitative comparison of waste materials Heredia et al. [21]. Figure 15 shows the quantitative comparison of waste materials among among the feeding subsystems in different micro-part production systems. For some the feeding subsystems in different micro-part production systems. For some high- high-performance polymer, such as polyetheretherketone (PEEK), the polymer itself is not performance polymer, such as polyetheretherketone (PEEK), the polymer itself is not only only expensive but also requires processing at high temperatures and employing addi- expensive but also requires processing at high temperatures and employing additional tional special equipment which, in turn, further increases the cost of production, which renders the PEEK injection micro-molding process uneconomic for low volume series and customized micro-parts [116]. Dorf et al. [117] analyzed the influence that the main process parameters have when processing the PEEK polymer via UPMIM successfully; the results demonstrated the fact that UPMIM technology is capable of producing parts with competitive properties. In terms of a material utilization ratio, the smaller the molded part, the more obvious the advantages of UPMIM, especially when the performance of some medical materials will be affected after secondary plasticization. Polymers 2021, 13, x FOR PEER REVIEW 18 of 43

Polymers 2021, 13, x FOR PEER REVIEW 18 of 43

special equipment which, in turn, further increases the cost of production, which renders the PEEK injection micro-molding process uneconomic for low volume series and special equipment which, in turn, further increases the cost of production, which renders customized micro-partsthe PEEK [116].injection Dorf micro-molding et al. [117] analyzedprocess uneconomicthe influence for thatlow thevolume main series and process parameterscustomized have whenmicro-parts processing [116]. the Dorf PEEK et al. polymer [117] analyzed via UPMIM the influencesuccessfully; that the main the results demonstratedprocess parameters the fact thathave UPMI whenM processing technology the is PEEK capable polymer of producing via UPMIM parts successfully; with competitivethe properties. results demonstrated In terms of thea material fact that utilization UPMIM technology ratio, the smaller is capable the moldedof producing parts Polymers 2021, 13, 2877 part, the more withobvious competitive the advantages properties. of In UP termsMIM, of aespecially material utilization when the ratio, performance the smaller17 of of 41 the molded some medical materialspart, the morewill be obvious affected the after advantages secondary of UPplasticization.MIM, especially when the performance of some medical materials will be affected after secondary plasticization.

FigureFigure 15. 15.QualitativeQualitativeFigure comparison 15. comparison Qualitative among comparison among different different among feeding feeding different systems systems feeding [21]. [21 systems]. [21].

3.3.2. Reduced Reduced 3.3.2.Energy Energy Reduced Consumption Energy Consumption An all-electricall-electric moldingAn molding all-electric machine machine molding (Battenfeld (Battenfeld machine Microsystem Microsystem(Battenfeld 50) Microsystem was 50) designed was designed 50) for precise,was fordesigned for precise,small micro-parts small micro-partsprecise, by splitting small by micro-parts splitting plasticizing, plasticizing, by splitting dosing, dosing, andplasticizing, injecting and injectingdosing, unit. Its and unit. injection injecting Its injection system unit. Its injection systemconsists consists of a screwsystem of plasticizinga screwconsists plasticizing barrel,of a screw a plunger baplasticizingrrel, injection a plunger ba system,rrel, injection a plunger and asystem, melt injection dosage and system, controla melt and a melt dosagebarrel, ascontrol shown dosagebarrel, in Figure controlas shown 16 a.barrel, in Figure as shown 16a. in Figure 16a. InIn the MIM system, In the nearly MIM system, 20% of nearly the total 20% en energy ofergy the in total the en injection ergy in the process injection is used process to is used to heat the the plasticizing plasticizingheat the unit unit plasticizing [6]. [6]. According According unit [6]. to toAccordingthe the research research to ofthe ofSpiering research Spiering et of al., etSpiering al.,about about et40% al., 40% ofabout the of 40% of the totalthe total energy energy consumptiontotal consumption energy consumptionis isconcentrated concentrated is concentratedin in the the injection-molding injection-molding in the injection-molding process, process, including process, including mold heating, mold mold moving, heating, moldand melt moving, injection and melt [7]. [7]. injection In different different [7]. types In different of energy types generators of energy generators such as electric, hydraulic, or hybrid, in fact, the most effective one should be the electric. such as electric, hydraulic, hydraulic, or or hybrid, hybrid, in in fact, fact, the the most most effective effective one one should should be be the the electric. electric. The power requirements of hybrid and all-electric machines are shown in Figure 16b, both The power power requirements requirements of of hybrid hybrid and and all-elect all-electricric machines machines are shown are shown in Figure in Figure 16b, both16b, of which run the same components with a cycle time of 14 s. The results show that using ofboth which of which run the run same the samecomponents components with a with cycle a time cycle of time 14 s. of The 14s. results The results show showthat using that all-electric hybrid technology can save substantial energy when the efficiency of the all-electricusing all-electric hybrid hybrid technology technology can cansave save substa substantialntial energy energy when when the the efficiency efficiency of of the hydraulic machine is even lower than that of hybrid technology [118]. hydraulic machine is even lower than that of hybrid technology [[118].118].

Figure 16. (a) Injection unit of the Battenfeld Microsystem 50 and the sensor installation positions [119]; (b) Comparison of energy consumption in injection-molding cycle between hybrid power (electric screw drive) and all-electric machine [118].

The plasticizing stage is the main energy-consuming stage in the UPMIM process. Com- pared with the relatively energy-saving full-electric micro-injection molding, the plasticizing stage also has greater energy savings. The plasticizing phase is performed by the sonotrode, and energy is provided by an ultrasonic power source. The rate power of most ultrasonic generators used now in UPMIM are 1 kW [112] and 1.5 kW [108]. In fact, under normal conditions, the required power is between 400 and 500 W. If the pressure of the end face of the sonotrode is overloaded, the power will exceed 1000 W, and the whole system will be Polymers 2021, 13, x FOR PEER REVIEW 19 of 43

Figure 16. (a) Injection unit of the Battenfeld Microsystem 50 and the sensor installation positions [119]; (b) Comparison of energy consumption in injection-molding cycle between hybrid power (electric screw drive) and all-electric machine [118].

The plasticizing stage is the main energy-consuming stage in the UPMIM process. Compared with the relatively energy-saving full-electric micro-injection molding, the plasticizing stage also has greater energy savings. The plasticizing phase is performed by the sonotrode, and energy is provided by an ultrasonic power source. The rate power of

Polymers 2021, 13, 2877 most ultrasonic generators used now in UPMIM are 1 kW [112] and 1.5 kW [108]. In18 fact, of 41 under normal conditions, the required power is between 400 and 500 W. If the pressure of the end face of the sonotrode is overloaded, the power will exceed 1000 W, and the whole system will be unstable. Jiang et al. [120] showed that the ultrasonic power increasesunstable. with Jiang the et al.pres [120sure] showed at the first that thetwo ultrasonic minutes. powerThe maximum increases plasticizing with the pressure power at doesthe firstnot exceed two minutes. 290 W, The as shown maximum in Figure plasticizing 17a. Grabalosa power does et notal. [108] exceed pointed 290 W, out as shownthat the in higherFigure the 17a. pressure, Grabalosa the et better al. [108 the] pointed energy out utilization that the higher ratio for the melting pressure, the the polymer better the (under energy theutilization condition ratio that for the melting ultrasonic the generator polymer (under is not overloaded). the condition Therefore, that the ultrasonic with the generatorincrease ofis pressure, not overloaded). the efficiency Therefore, of the with ultrasonic the increase sonotrode of pressure, plasticizing the efficiency process of will the ultrasonicincrease fromsonotrode about plasticizing10% to 50%. process In any will case, increase the maximum from about power 10% to of 50%. the Ingenerator any case, can the reach maximum 150 W,power as shown of the in generator Figure 17b. can reach 150 W, as shown in Figure 17b.

FigureFigure 17. 17. (a()a )Curve Curve of of ultrasonic ultrasonic power power under under different different pressure pressure [120]; [120]; (b (b) )Comparison Comparison between between generator, generator, dissipated dissipated power,power, and and required required average average power power [108]. [108].

OnOn the the other other hand, hand, molding molding a amicro-part micro-part or or structures structures requires requires less less pressure pressure and and lowerlower energy consumptionconsumption in UPMIM.in UPMIM. In traditional In traditional precision precision injection-molding injection-molding machines, machines,typically, 1600–3500typically, 1600–3500 bar was used bar [1 was,108 ].used However, [1,108]. in However, the UPMIM in process, the UPMIM these pressuresprocess, thesedrop pressures to the range drop of to 300 the torange 500 of bar. 300 For to 500 example, bar. For Sonorus example, 1 G Sonorus is technically 1 G is technically rated with rateda clamping with a clamping force of 3 force m.t., whileof 3 m.t., 1.5 while to 2.2 1. m.t.5 to of 2.2 clamping m.t. of clamping force is needed. force is Inneeded. addition, In addition,the energy the consumption energy consumption of Sonorus of 1 GSonorus is directly 1 G reduced is directly by 85% reduced to 90% by compared 85% to 90% with comparedthat of the with standard that of injection-molding the standard injection-molding machine due to machine the elimination due to ofthe the elimination heater bands, of thehydraulic heater bands, pumps, hydraulic and motors, pumps, which and are motors, usually which used are to keepusually the used clamp to shutkeep underthe clamp high shutpressure. under Grabalosa high pressure. et al. [Grabalosa108] adopted et al. an [1 electro-pneumatic08] adopted an electro-pneumatic ultrasonic molding ultrasonic machine, moldingwhich reduces machine, the which waste reduces of micro-parts the waste by of nearly micro-parts 10%. Forby nearly the UPMIM 10%. For equipment, the UPMIM all- equipment,electricalization all-electricalization is also a developmental is also a developmental trend. trend. ManyMany reports reports have have proved proved that that the the UPMI UPMIMM process process has has lower lower energy energy consumption consumption thanthan the the traditional traditional injection injection process. process. Howeve However,r, in in the the process process of of mass mass production, production, the the advantagesadvantages of of UPMIM UPMIM are are limited limited by by its its production production capacity. capacity. It It is is difficult difficult to to explain explain how how obviousobvious this this advantage advantage is is only only through through one one or or several several cycles. cycles. The The cost cost of of manpower manpower and and timetime needs needs to to be be considered, considered, and and more more research research needs needs to to be be carried carried out. out. 3.3.3. Reduced Residence Time In the field of precision molding, material degradation may be the key issue that plastic parts manufacturers pay the most attention to. The residence time directly affects the degradation of materials, which is an inevitable problem in the configuration of a screw barrel and heater band in all traditional molding techniques. The innovative plasticizing unit, a so-called “inverse screw”, was designed by the German research institute IKV and Arburg company, which is applied in a new electric Allrounder 270A injection moulding machine, as shown in Figure 18[ 5]. Therefore, the appearance of an inverse screw is an improvement to the processing of thermally sensitive and medically relevant materials such as polylactic acid. Polymers 2021, 13, x FOR PEER REVIEW 20 of 43

3.3.3. Reduced Residence Time In the field of precision molding, material degradation may be the key issue that plastic parts manufacturers pay the most attention to. The residence time directly affects the degradation of materials, which is an inevitable problem in the configuration of a screw barrel and heater band in all traditional molding techniques. The innovative plasticizing unit, a so-called “inverse screw”, was designed by the German research institute IKV and Arburg company, which is applied in a new electric Allrounder 270A injection moulding machine, as shown in Figure 18 [5]. Therefore, the Polymers 2021, 13, 2877 19 of 41 appearance of an inverse screw is an improvement to the processing of thermally sensitive and medically relevant materials such as polylactic acid.

FigureFigure 18. 18. (a(a) )Schematic Schematic diagram diagram of of inverse inverse screw screw design; design; (b (b) )Engineering Engineering case case of of injectio injection-moldingn-molding machine machine with with inverse inverse screw structure [5]. screw structure [5].

ComparedCompared with with traditional MIM,MIM, UPMIM UPMIM has has no no plasticizing plasticizing screw, screw, screw screw barrel, barrel, heater heaterband (notband necessarily), (not necess etc.arily), In terms etc. ofIn process, terms of it onlyprocess, plasticizes it only the plasticizes amount of the material amount needed of materialfor each injection,needed for and each it melts injection, in situ inand the it mold melt nears in thesitu gate. in the This mold reduces near the the thermal gate. history This reducesof plastics the to thermal several history milliseconds, of plastics reduces to se theveral waste milliseconds, of raw materials, reduces avoids the degradationwaste of raw as materials,much as possible, avoids anddegradation even eliminates as much the needas possible, for material and purging. even eliminates UPMIM has the been need already for materialproved ablepurging. to mold UPMIM parts made has ofbeen PLA alread[112,121y proved]. However, able into the mold traditional parts made micro-molding of PLA [112,121].process, when However, the injection in the traditional amount of amicro-molding micro-part may process, only be 0.1when g, the the machine injection has amount a 100 g ofcapacity a micro-part barrel, may which only indicates be 0.1 g, that the themachine barrel has must a 100 be emptied g capacity after barrel, nearly which 1000 indicates injections. thatTherefore, the barrel UPMIM must providesbe emptied a feasible after nearly solution 1000 for injections. heat-sensitive Therefore, and poorly UPMIM stable provides materials. aOn feasible the other solution hand, for short heat-sensitive residence timeand poor bringsly stable challenges materials. to plasticization On the other uniformity hand, short and residencestability, time which brings is related challenges to process to plasticization maturity and uniformity stability. and The stability, synergistic which principle is related and tomechanism process maturity of some processand stability. parameters The suchsynergistic as injection principle speed, and ultrasonic mechanism amplitude, of some and processpower areparameters conducive such to enhancing as injection process sp stability.eed, ultrasonic amplitude, and power are conducive to enhancing process stability. 3.3.4. Improved Filling and Molding Ability 3.3.4. ImprovedThe fact that Filling the viscosity and Molding of the Ability polymer melt will decrease under the action of ultra- soundThe has fact been that confirmed the viscosity by many of the scholars polymer [76 melt,84,122 will–124 decrease], which under means the that action UPMIM of ultrasoundcan apply lowerhas been pressure confirmed at the same by many melting sc temperaturesholars [76,84,122–124], and can make which the materialmeans that flow UPMIMinto thinner, can apply tinier lower geometries, pressure which at the could same not melting be filled temperatures previously. Althoughand can make there the are materialalmost noflow reports into aboutthinner, the UPMIMtinier geomet processries, used which to produce could not industrial be filled microfluidic previously. de- Althoughvices, many there researchers are almost have no explored reports its about feasibility. the InUPMIM terms ofprocess filling capacityused to in produce UPMIM, Jiang et al. [125] used a spiral flow test based on an Archimedes spiral mold with mi- industrial microfluidic devices, many researchers have explored its feasibility. In terms of crochannels (depth from 250 to 750 µm) to test the fluidity of molten polymer plasticized filling capacity in UPMIM, Jiang et al. [125] used a spiral flow test based on an Archimedes by ultrasonic vibration. The results show that with an increase of the ultrasonic amplitude, spiral mold with microchannels (depth from 250 to 750 μm) to test the fluidity of molten ultrasonic action time, plasticizing pressure, and mold temperature, the fluidity and filling polymer plasticized by ultrasonic vibration. The results show that with an increase of the length of polymer melt (PA66, PP, PMMA) can also be effectively increased. On the other ultrasonic amplitude, ultrasonic action time, plasticizing pressure, and mold temperature, hand, Ferrer et al. [126] proved the repeatability and reproducibility of processing a mi- the fluidity and filling length of polymer melt (PA66, PP, PMMA) can also be effectively crochannel thin-walled plate in polystyrene polymer. The results show that the thickness increased. On the other hand, Ferrer et al. [126] proved the repeatability and deviation of the final part is less than 7%, and the reproduction depth of the microchan- reproducibility of processing a microchannel thin-walled plate in polystyrene polymer. nel is greater than the width, with average deviation of 4% and 11%, respectively. The The results show that the thickness deviation of the final part is less than 7%, and the authors also demonstrated the process feasibility to ratio parts of polylactide acid (PLA) by reproduction depth of the microchannel is greater than the width, with average deviation an ultrasonic molding process and discussed the feasibility of producing PLA products with a low aspect ratio [127]. Additionally, UPMIM was proved to mold another kind of micro-piece that requires high precision to replicate its details [112]. Among them, parts with small size details such as guitar strings, the width of which is 70 µm, can be molded well, which is difficult to produce by conventional micro-injection due to the high pressure requirements of machines [128]. The reduced melt viscosity and injection pressure seem to illustrate the improved injection-molding ability. However, due to the different plasticizing process, the UPMIM process can hardly control the melt temperature strictly at present. Hence, the same process parameter level cannot be strictly guaranteed, and the performance improvement caused by the miniaturization of equipment cannot be excluded. Furthermore, rapid thermal cycling technology can also improve the molding ability of micro-injection molding. In the Polymers 2021, 13, 2877 20 of 41

end, UPMIM may have certain advantages in molding ability, but it is not enough to be a reason to completely replace MIM.

3.3.5. Wear of Sonotrode Regardless of whether it is Configuration 1 or Configuration 2, it is necessary to consider the axial positioning accuracy of the sonotrode and the plasticizing chamber as well as the tolerance (clearance fit) and other issues during assembly. Although the sonotrode is subject to longitudinal high-frequency vibration, lateral vibration will occur during the work process. Once the amplitude is greater than the amount of the fit gap, it will dynamically contact and wear with the side wall of the plasticizing cavity. This phenomenon is particularly obvious in Configuration 1. During the packing phase of Configuration 1, the sonotrode is displaced upward to leave space for particle feeding. After adding the required particles, it is downwardly displaced to pre-press the particles. Ultrasonic vibration is turned on for high-frequency vibration while accompanying with slight radial vibration. Pressing down the polymer particles will cause a sharp frictional heat generation in the plasticization chamber, as shown in Figure 19a, and the ultrasonic power will increase sharply, which will cause the frequency of the ultrasonic generator to exceed the vibration frequency range of the power supply. Wear also occurs after working, which can be classified into two types: longitudinal and lateral/diametrical wear, as shown in Figure 19b [102]. The mass variation at the tip of the sonotrode is reflected in the uneven wear, which will lead to the change of longitudinal vibration mode frequency. In addition, due to the inhomogeneity at the worn surface, stress concentration regions will appear, which will Polymers 2021, 13, x FOR PEER REVIEWthreaten the stability of the system and plasticizing process. In some experiments, the22 tipof 43 of

the horn should be cleaned before action to ensure uniform contact with the sample [129].

FigureFigure 19. 19. ((aa)) Intense Intense flash; flash; ( (bb)) Wear Wear observed in in different sonotrod sonotrodeses used in UPMIM machines machines [102]; [102]; ( (cc)) A A nodal nodal point point sonotrode (left) and a conventional one (right); (d) Nodal point ultrasonic micro-injection-molding configuration [130]. sonotrode (left) and a conventional one (right); (d) Nodal point ultrasonic micro-injection-molding configuration [130].

3.3.6. Instability of System Configuration 1 and Configuration 2 have one thing in common: that is, a single direct gate is used to connect the mold cavity and plasticizing chamber. Communication between the plasticizing unit and the injection unit also directly causes the ultrasonic sonotrode to bear excess injection pressure during the plasticizing process, which in turn causes the process to be unstable. In the case of pressure applied in the experiment designed by Grabalosa et al. [108], when the injection pressure is higher than 2 bar (one bar measured by the manometer represents 311 N of force at the tip of the sonotrode), the forming rate of the part is 90%, while the injection pressure lower than 2 bar cannot even mold a complete part. However, the injection pressure higher than 5 bar will overload the ultrasonic equipment, because the excessive interaction force between the ultrasonic sonotrode and the material will make the vibration frequency exceed the rated value, thus interrupting the molding cycle. In reference to the molding force, it takes more than 300 N injection pressure to fully mold eight samples. However, the above situation does not mean that there is no upper limit on the applied pressure, because in some cases, the force of about 500–600 N means that the ultrasonic electrode will be overloaded (unable to vibrate) due to the high compression of PLA, that is, the system will be unstable [129]. In this regard, the author has done a test with a small lifting force (<300 N), loading on an ultrasonic sonotrode of 10 mm diameter and 40 kHz vibration frequency. A short glass fiber-reinforced PA6 plate can be easily plasticized and perforated, as shown in Figure 20. In other words, from the perspective of protecting the sonotrode and the power supply system, the working load of the ultrasonic sonotrode should be minimized while ensuring the plasticized state of the polymer. In the patent of Wu et al. [131], a solenoid valve was used to isolate the plasticizing cavity from the cavity. When the solenoid valve is open, the plasticizing cavity is separated from the cavity. After a certain period of ultrasonic action, the solenoid valve is closed, the plasticizing cavity and the cavity are connected, and the displacement of the sonotrode fills the polymer melt into the micro-

Polymers 2021, 13, 2877 21 of 41

According to the working principle and characteristics of ultrasonic probe, Janer et al. [130] solved the problems of sonotrode wear and flash by introducing the concept of a “nodal point” (position of the sonotrode for which its vibration is “zero”) and improving the structure and cooperation of ultrasonic sonotrodes, as shown in Figure 19c,d. At the same time, the quality of injection parts and the stability of quality have been im- proved to varying degrees. However, the utilization rate of materials decreased from 20% to 9% compared with that before upgrading. At present, it may be the best choice by sacrificing some other capabilities to extend the service life of the sonotrode.

3.3.6. Instability of System Configuration 1 and Configuration 2 have one thing in common: that is, a single direct gate is used to connect the mold cavity and plasticizing chamber. Communication between the plasticizing unit and the injection unit also directly causes the ultrasonic sonotrode to bear excess injection pressure during the plasticizing process, which in turn causes the process to be unstable. In the case of pressure applied in the experiment designed by Grabalosa et al. [108], when the injection pressure is higher than 2 bar (one bar measured by the manometer represents 311 N of force at the tip of the sonotrode), the forming rate of the part is 90%, while the injection pressure lower than 2 bar cannot even mold a complete part. However, the injection pressure higher than 5 bar will overload the ultrasonic equipment, because the excessive interaction force between the ultrasonic sonotrode and the material will make the vibration frequency exceed the rated value, thus interrupting the molding cycle. In reference to the molding force, it takes more than 300 N injection pressure to fully mold eight samples. However, the above situation does not mean that there is no upper limit on the applied pressure, because in some cases, the force of about 500–600 N means that the ultrasonic electrode will be overloaded (unable to vibrate) due to the high compression of PLA, that is, the system will be unstable [129]. In this regard, the author has done a test with a small lifting force (<300 N), loading on an ultrasonic sonotrode of 10 mm diameter and 40 kHz vibration frequency. A short glass fiber-reinforced PA6 plate can be easily plasticized and perforated, as shown in Figure 20. In other words, from the perspective of protecting the sonotrode and the power supply system, the working load of the ultrasonic sonotrode should be minimized while ensuring the plasticized state of the polymer. In the patent of Wu et al. [131], a solenoid valve was used to isolate the plasticizing cavity from the cavity. When the solenoid valve is open, the plasticizing cavity is separated from the cavity. After a certain period of ultrasonic action, the solenoid valve is closed, the plasticizing cavity and the cavity are connected, and the displacement of the sonotrode fills the polymer melt into the micro- cavity. Wu further imitated the three-stage injection-molding model of Microsystem 50 to form a three-stage ultrasonic plasticization molding process of plasticization, metering, and injection [132,133]. The instability of the plasticizing stage will directly affect the subsequent injection filling. Through tracking of the flow front, Masato et al. [105] found that compared with the MIM process, the filling time of the UPMIM process is longer and more dispersed, as shown in Figure 21, which indicates that the process is less consistent and stable. The filling time in UPMIM is mainly controlled by the melting rate, which depends on the ultrasound vibration characteristics, while are difficult to control now. In the experiment of Dorf et al. [113], from the 196 combinations of parameters setting, only 47 sets allowed the cavity to be completely filled, which indicates the unstable parameter setting. Polymers 2021, 13, x FOR PEER REVIEW 23 of 43

cavity. Wu further imitated the three-stage injection-molding model of Microsystem 50 to form a three-stage ultrasonic plasticization molding process of plasticization, metering, and injection [132,133].

Polymers 2021, 13, x FOR PEER REVIEW 23 of 43

Polymers 2021, 13, 2877 cavity. Wu further imitated the three-stage injection-molding model of Microsystem22 50 of 41to form a three-stage ultrasonic plasticization molding process of plasticization, metering, and injection [132,133].

Figure 20. Ultrasonic plasticization test under low pressure.

The instability of the plasticizing stage will directly affect the subsequent injection filling. Through tracking of the flow front, Masato et al. [105] found that compared with the MIM process, the filling time of the UPMIM process is longer and more dispersed, as shown in Figure 21, which indicates that the process is less consistent and stable. The filling time in UPMIM is mainly controlled by the melting rate, which depends on the ultrasound vibration characteristics, while are difficult to control now. In the experiment

of Dorf et al. [113], from the 196 combinations of parameters setting, only 47 sets allowed Figure 20. Ultrasonic plasticization test under low pressure. theFigure cavity 20. Ultrasonicto be completely plasticization filled, test which under indicates low pressure. the unstable parameter setting.

The instability of the plasticizing stage will directly affect the subsequent injection filling. Through tracking of the flow front, Masato et al. [105] found that compared with the MIM process, the filling time of the UPMIM process is longer and more dispersed, as shown in Figure 21, which indicates that the process is less consistent and stable. The filling time in UPMIM is mainly controlled by the melting rate, which depends on the ultrasound vibration characteristics, while are difficult to control now. In the experiment of Dorf et al. [113], from the 196 combinations of parameters setting, only 47 sets allowed the cavity to be completely filled, which indicates the unstable parameter setting.

Figure 21. Flow front trackingtracking for for five five repeated repeated MIM MIM (a) ( anda) and UPMIM UPMIM (b) experiments(b) experiments when when the set the flow set rateflow is rate 5000 is mm 50003/s mm [1053/s]. [105]. Gülçür et al. [134] evaluated the stability of UPMIM by an in-line monitoring method, whichGülçür consists et al. of [134] a series evaluated of sensor the technologies, stability of UPMIM including by an data in-line recorded monitoring by the machinemethod, whichcontroller, consists a high-speed of a series thermal of sensor camera, technologies, and a cavityincluding pressure data recorded sensor. The by resultsthe machine show controller,that the data a high-speed obtained fromthermal machine camera, sensors and a iscavity essential pressure for understanding sensor. The results each show stage thatof the the ultrasonic data obtained micro-molding from machine cycle, sensors as shown is essential in Figure for understanding22. The plunger each position stage of is thehighly ultrasonic correlated micro-molding to the characteristics cycle, as shown of each in Figure molding 22. stage,The plunger as shown position in Figure is highly 22a.

correlatedTherefore, to the the different characteristics stages of of the each process moldin cang bestage, easily as trackedshown byin Figure analyzing 22a. the Therefore, position Figure 21. Flow front trackingtheof the fordifferent five plunger repeated stages in detail.MIM of the (a)Figure andprocess UPMIM 22 canb also ( bbe) experiments easi showsly tracked that when although by the analyzing seteach flow rate channel the is position 5000 of mm machine 3of/s the [105]. plungerdata may in change detail. Figure significantly 22b also with shows different that although cycles, the each main channel features of ofmachine the process data plotsmay changeare still significantly displayed in with each different molding cycles, cycle. Whenthe main the dynamicfeatures of process the process environment plots are of still the Gülçür et al. [134] evaluated the stability of UPMIM by an in-line monitoring method, displayedUPMIM process in each ismolding clearly cycle. described, When the the mechanical dynamic process data captured environment in-line of havethe UPMIM a high accuracy.which consists This technologyof a series of can sensor not onlytechnologies, directly reflect including the relationshipdata recorded between by the machine process parameterscontroller, a and high-speed the quality thermal of molded camera, parts and but a also cavity meet pressure the indicators sensor. of The stabilizing results show part qualitythat the and data tracing obtained part from defects. machine Wu et sensors al. [135 ]is proposed essential afor new understanding method to quantitatively each stage of

characterizethe ultrasonic the micro-molding efficiency of cycle, simultaneous as shown plasticization in Figure 22. The and plunger filling by position redefining is highly the injectioncorrelated rate to asthe the characteristics mass flow during of each melt moldin filling.g stage, The results as shown show in that Figure without 22a. damagingTherefore, thethe mechanicaldifferent stages properties of the ofprocess the micro-molded can be easily samples,tracked by increasing analyzing the the injection position rate of the is beneficialplunger in to detail. simultaneously Figure 22b increasingalso shows the that efficiency although of each plasticization channel of andmachine filling. data At may the samechange time, significantly Janer et al. with [130 different] improved cycles, the the quality main of features injection of partsthe process and the plots stability are still of qualitydisplayed to varyingin each molding degrees bycycle. introducing When the the dynamic concept process of a “nodal environment point” and of the improving UPMIM the structure and cooperation of an ultrasonic sonotrode. In view of the results obtained in the above reports, deepening the understanding of UPMIM principles and forming

Polymers 2021, 13, x FOR PEER REVIEW 24 of 43

process is clearly described, the mechanical data captured in-line have a high accuracy. This technology can not only directly reflect the relationship between process parameters and the quality of molded parts but also meet the indicators of stabilizing part quality and tracing part defects. Wu et al. [135] proposed a new method to quantitatively characterize the efficiency of simultaneous plasticization and filling by redefining the injection rate as the mass flow during melt filling. The results show that without damaging the mechanical properties of the micro-molded samples, increasing the injection rate is beneficial to simultaneously increasing the efficiency of plasticization and filling. At the same time, Polymers 2021, 13, 2877 Janer et al. [130] improved the quality of injection parts and the stability of quality23 of 41 to varying degrees by introducing the concept of a “nodal point” and improving the structure and cooperation of an ultrasonic sonotrode. In view of the results obtained in the above reports, deepening the understanding of UPMIM principles and forming mechanisms,mechanisms, especially especially the the synergistic synergistic effects effects of of various various parameters, parameters, is is the the best best choice choice to to improveimprove process process stability, stability, and and there there is is still still much much work work to to be be done. done.

FigureFigure 22. 22.(a ()a Machine) Machine data data captured captured during during ultrasonic ultrasonic micro-molding micro-molding of of PP; PP; (b ()b Plunger) Plunger position position recorded recorded from from three three consecutiveconsecutive ultrasonic ultrasonic micro-molding micro-molding cycles cycles while while the the sonication sonication was was applied applied [134 [134].].

3.3.7. Uniformity of Molded Part Whether it is MIM or UPMIM, the uniformity of polymer melt in the plasticizing process is an issue that attracts much attention. Molding quality and system stability are still the biggest challenges facing UPMIM. Unlike MIM processing, there is no screw in the UPMIM technology, meaning that it is necessary to test the mixing effects in UPMIM. Michaeli et al. [107] firstly tested the mixing effect of UPMIM; blue and yellow PP-powder was plasticized under ultrasonic vibration. Under the same pressure, a small amplitude (29.4 µm) will cause uneven plasticizaed morphology, and a large amplitude (49.0 µm) setting can obtain a more uniform melt, as shown in Figure 23. Michaeli et al. [136] have recorded and evaluated the homogenization and plasticization results of molten materials Polymers 2021, 13, x FOR PEER REVIEW 25 of 43

3.3.7. Uniformity of Molded Part Whether it is MIM or UPMIM, the uniformity of polymer melt in the plasticizing process is an issue that attracts much attention. Molding quality and system stability are still the biggest challenges facing UPMIM. Unlike MIM processing, there is no screw in the UPMIM technology, meaning that it is necessary to test the mixing effects in UPMIM. Michaeli et al. [107] firstly tested the mixing effect of UPMIM; blue and yellow PP-powder Polymers 2021, 13, 2877 was plasticized under ultrasonic vibration. Under the same pressure, a small amplitude24 of 41 (29.4 μm) will cause uneven plasticizaed morphology, and a large amplitude (49.0 μm) setting can obtain a more uniform melt, as shown in Figure 23. Michaeli et al. [136] have recorded and evaluated the homogenization and plasticization results of molten materials with a microscope, which showed that the materials had a regular and homogeneous with a microscope, which showed that the materials had a regular and homogeneous crystalline structure. crystalline structure. It can be seen that the ultrasonic amplitude, as one of the important process parameters, has a significantIt can be impactseen that on thethe plasticizing ultrasonic abilityamplitude, and quality. as one However, of the important due to the lackprocess of strongparameters, shear forcehas a fieldsignificant caused impact by the on traditional the plasticizing screw plasticizing ability and process,quality. UPMIMHowever, does due notto the have lack good of strong material shear mixing force ability. field caused by the traditional screw plasticizing process, UPMIM does not have good material mixing ability.

FigureFigure 23. 23.Effect Effect of of the the mixing mixing of of colored colored powder powder (blue/yellow) (blue/yellow) during during the the plasticization: plasticization: different different microscopicmicroscopic appearance appearance after after changing changing the the amplitude amplitude [107 [107].].

InIn the the developed developed Configuration Configuration 2 later,2 later, the th sonotrodee sonotrode is flush is flush with with the gate, the thegate, con- the centratedconcentrated energy energy is used is to used plasticize to plasticize the polymer, the andpolymer, then injection and then filling injection is performed. filling Ais newperformed. application A modenew ofapplication ultrasonic waves,mode includingof ultrasonic continuous waves, ultrasound including and continuous intermit- tentultrasound ultrasound, and is intermittent considered asultrasound, one of the is parameters considered [106 as]. one Optimal of the plasticizing parameters results [106]. areOptimal achieved plasticizing using a medium results setting.are achieved However, using results a medium showed setting. the different However, molecular results weightshowed distribution the different in three molecular regions. weight In the distri first fewbution seconds in three of ultrasonic regions. processing,In the first thefew materialseconds closerof ultrasonic to the sonotrode processing, is exposed the materi to ultrasonical closer energy to the and sonotrode generates is heatexposed earlier, to whichultrasonic is the energy first and and most generates easily heat degradable. earlier, wh However,ich is the thefirst polymer and most far easily away degradable. from the probeHowever, can only the receivepolymer attenuated far away ultrasonic from the energyprobe can during only the receive same exposureattenuated time, ultrasonic which isenergy a challenge during to thethe plasticizationsame exposure uniformity. time, wh Atich the is same a challenge time, the to limited the plasticization plasticizing capacityuniformity. has becomeAt the same one of time, the factors the limited restricting plasticizing its large-scale capacity mass has production. become one of the factorsGrabalosa restricting et al.its [large-scale108] found mass that production. due to the application of ultrasonic energy, the polymerGrabalosa sample et realized al. [108] linear found flow that from due the to middlethe application to the end, of ultrasonic resulting inenergy, a better the appearance,polymer sample as shown realized in Figurelinear 24flowa. Thefrom short the middle distance to (2 the mm) end, from resulting the plasticizing in a better chamberappearance, to the as moldshown cavity in Figure is insufficient 24a. The toshort make distance the melt (2 flowmm) uniform.from the plasticizing The chain arrangementchamber to isthe mainly mold duecavity to the is formabilityinsufficient ofto polymer make the at themelt exit flow of theuniform. mold during The chain the injection.arrangement The arrangementis mainly due of PA12to the chains formability causes of the polymer end sample at the region exit toof bethe different mold during from thethe injection injection. and The center arrangement regions, of and PA12 the SWAXSchains causes measurements the end sample show differentregion to absorption, be different asfrom shown the ininjection Figure 24 andb. center regions, and the SWAXS measurements show different absorption,It can be as seen shown that in the Figure molecular 24b. weight distribution is not uniform along the radial direction centered on the plasticizing chamber; i.e., there are differences in the plasticizing effect in the whole molding process. An uneven distribution of molecular weight may lead to residual stress and then affect the precision of injection parts. Therefore, this defect can be avoided by limiting the size of the molded part. Polymers 2021, 13, x FOR PEER REVIEW 26 of 43

It can be seen that the molecular weight distribution is not uniform along the radial direction centered on the plasticizing chamber; i.e., there are differences in the plasticizing

Polymers 2021, 13, 2877 effect in the whole molding process. An uneven distribution of molecular weight25 may of 41 lead to residual stress and then affect the precision of injection parts. Therefore, this defect can be avoided by limiting the size of the molded part.

FigureFigure 24. 24. (a(a) )The The molecular molecular weights weights of ofthree three different different regions regions of the of themolded molded sample sample under under different different ultrasonic ultrasonic amplitudes ampli- [106];tudes ( [b106) SWAXS]; (b) SWAXS measurements measurements collected collected in PA12 in sample PA12 sampleat the in atjection, the injection, center, and center, end andregions end by regions considering by considering patterns atpatterns 0° and at90° 0◦ [108].and 90◦ [108].

3.3.8.3.3.8. Degradation Degradation Problem Problem MostMost polymerpolymer materialsmaterials are are suitable suitable for maturefor mature conventional conventional micro-injection-molding micro-injection- moldingprocessing, processing, but there but are nothere relevant are no standards relevant forstandards UPMIM. for High-quality UPMIM. High-quality parts can only parts be canobtained only underbe obtained the coordination under the of coordination various parameters. of various If the parameters. ultrasonic energy If the isultrasonic relatively energylow, only is relatively partial interfaces low, only are partial welded interfaces together [are137 welded], and excessive together ultrasonic [137], and energy excessive can ultrasoniccause polymer energy degradation; can cause thepolymer most intuitivedegradation; is the the change most in intuitive topography. is the change in topography.The morphology of specimens was evaluated by scanning electron microscope (SEM) in manyThe morphology studies. Sacrist of áspecimensn et al. [112 was] evaluated evaluated PLA by scanning samples electron under different microscope processing (SEM) inconditions. many studies. The SEM Sacristán micrographs et al. [112] of the eval processeduated PLA PLA samples sample under under different optimal parametersprocessing conditions.are shown The in Figure SEM micrographs 25a, in which of the no processed obvious physical PLA sample defects under can optimal be detected parameters [115]. µ areNumerous shown in holes Figure from 25a, 50 inm which to 1 mm no willobvious occur physical in the samples defects when can be a higherdetected molding [115]. µ Numerouspressure (3 holes bar) from and a50 lower μm to ultrasonic 1 mm will amplitude occur in the (28.4 samplesm) are when applied, a higher as shown molding in Figure 25b, which can be explained by the cavitation process. The rough morphology pressure (3 bar) and a lower ultrasonic amplitude (28.4 μm) are applied, as shown in of the material adhered to the surface of the sonotrode also indicates the occurrence of Figure 25b, which can be explained by the cavitation process. The rough morphology of degradation during plasticization, as shown in Figure 25c. The above reports illustrate that the material adhered to the surface of the sonotrode also indicates the occurrence of various process parameters should be set according to different material characteristics in degradation during plasticization, as shown in Figure 25c. The above reports illustrate UPMIM, because the incompatible parameters with material properties not only affect the that various process parameters should be set according to different material molding quality but also the physical and chemical properties of injection parts. characteristics in UPMIM, because the incompatible parameters with material properties The experimental results of Dorf et al. [113] proved that the samples without pores not only affect the molding quality but also the physical and chemical properties of and visual dark marks (without degradation) show the highest tensile strength, while the injection parts. highly degraded samples have very low tensile strength in other sets of parameters, such The experimental results of Dorf et al. [113] proved that the samples without pores as low plunger speed and long ultrasonic exposure time. A high degradation of polymer and visual dark marks (without degradation) show the highest tensile strength, while the material can be observed on the sample shown in Figure 25d. Another manifestation of highlydegradation degraded is a decreasesamples have in molecular very low weight. tensile Forstrength example, in other under sets the of conditionparameters, that such the asaverage low plunger molecular speed weight and long is only ultrasonic reduced exposure by less than time. 6%, A PLA high and degradation PBS can be of molded polymer in materialpowder formcan be [54 observed]. The main on the processing sample show parametersn in Figure in UPMIM 25d. Another are the amplitudemanifestation of the of degradationvibration, the is sonicatinga decrease time,in molecular and the appliedweight. forceFor example, [105]. Therefore, under the there condition are significant that the differences between MIM and UPMIM processes in parameter control, process control, and parameter scale, which indicates that it is necessary to improve the understanding of the process [138]. Table4 lists some of the best parameters for ultra-high molecular weight polyethylene (UPMIM) without degradation. Note that degradation always happened in parts molded Polymers 2021, 13, x FOR PEER REVIEW 27 of 43

average molecular weight is only reduced by less than 6%, PLA and PBS can be molded in powder form [54]. The main processing parameters in UPMIM are the amplitude of the vibration, the sonicating time, and the applied force [105]. Therefore, there are significant differences between MIM and UPMIM processes in parameter control, process control, and parameter scale, which indicates that it is necessary to improve the understanding of Polymers 2021, 13, 2877 the process [138]. 26 of 41 Table 4 lists some of the best parameters for ultra-high molecular weight polyethylene (UPMIM) without degradation. Note that degradation always happened in parts molded by UHMWPE via ultrasonic processing, but the final thermal stability was bynot UHMWPE significantly via influenced ultrasonic processing,by a decrease but in the the final molecular thermal weight stability [106], was notand significantlythe thermal influencedstability of by all a the decrease UHMWPE/graphite in the molecular composites weight [106 was], and considerably the thermal better stability than of that all the of UHMWPE/graphite composites was considerably better than that of pure UHMWPE [114]. The pure UHMWPE [114]. The prepared specimens showed considerably better mechanical prepared specimens showed considerably better mechanical properties than pure UHMWPE. properties than pure UHMWPE.

FigureFigure 25.25. ((aa)) AA PLAPLA specimenspecimen moldedmolded atat 24-300-1.224-300-1.2 conditioncondition (please(please referrefer toto thethe originaloriginal texttext forfor detailed explanation) without any physical defects [115]; (b) Sprue with numerous holes from 50 detailed explanation) without any physical defects [115]; (b) Sprue with numerous holes from 50 µm μm to 1 mm; (c) Molten material remained adhered to the sonotrode sufface [112]; (d) Highly to 1 mm; (c) Molten material remained adhered to the sonotrode sufface [112]; (d) Highly degraded degraded sample with very low tensile strength [113]. sample with very low tensile strength [113].

TableTable 4.4.Optimized Optimized parametersparameters forfor fillingfilling withoutwithout degradation.degradation.

GeometryGeometry Tensile Tensile AmplitudeAmplitude MaterialMaterial StandardStandard TimeTime [s] [s] Pressure/Velocity Pressure/Velocity Ref. Ref. TestTest [cm [cm3] 3] [µm][µm] IRAM-IAS-IRAM-IAS- PLAPLA 1.5 1.5× 0.1 × 0.1× 0.1 × 0.1 48.148.1 33 3 bar 3 bar [112[112]] U500-102/3U500-102/3 PLAPLA Poly Poly IRAM-IAS- (nonamethylene 1.5 × 0.1 × 0.1 IRAM-IAS- 24 1.2 300 N [121] (nonamethylene azelate) 1.5 × 0.1 × 0.1 U500-102/3 24 1.2 300 N [121] azelate) U500-102/3 (PE99) PA12 33 × 2.5 × 1.25 ASTMD638 35 5 2 bar [108] Poly (ε-caprolactone) IRAM-IAS- Graphistrength® C10 1.5 × 0.1 × 0.1 37 7 or 8 2500 N [139] U500-102/3 carbon nanotubes PEEK 30 × 2 × 2 ASTMD638 52.2/58 8/5 5/6 [117] PPSU 30 × 2 × 2 ASTMD638 58/40.6 1.4/2.8 11/5 [113] UHMWPE ——[106] 30 × 2 × 1 ISO-527-4 50.6/56.2 UHMWPE + graphite — — [114] Polymers 2021, 13, 2877 27 of 41

3.4. Theoretical Interpretations Theoretical research about UPMIM is currently lacking, mainly focusing on ultrasonic energy balance and heat generation mechanisms, including frictional heating [110,111,140] and viscoelastic heating [20,55]. Up to now, although the heat mechanism of ultrasonic welding has been reported [141–144], the research on the thermal mechanism of ultrasonic plasticization of polymer still needs further improvement.

3.4.1. Ultrasonic Energy Balance Grabalosa et al. [108] proposed a mathematical modeling method based on acous- tic/ultrasonic energy balance, including theoretical dissipated energy, energy provided by the process, and energy required for melting materials. According to Rienstra and Hirschberg [145], Equation (7) describes the acoustic energy of a homentropic flow, which is given as:

∂  1    1  ρe + ρv2 + ∇ · v ρe + ρv2 + p = −∇ · q + ∇ · (τ · v) + f · v (7) ∂t 2 2

where ρ is the density of the material, e is the internal energy per unit mass, q is the heat flux resulting from the heat conduction, v is the material’s flow velocity, f is the external force density, p is the pressure, τ is the viscous stress tensor, and ∇ is the symbol representing the gradient operator. In terms of the process, it is considered the dissipated energy resulting from oscillation movement and the movement of the sonotrode. The dissipated heat flux during the ultrasound injection process could be found from the following expression (8):

. Qavg = 4paω (8)

where ω is the ultrasonic frequency, and a is the oscillatory amplitude of the sonotrode tip. Whereas, in terms of the material, the theoretical melting energy required is also included. Considering the amount of material that is melted in each cycle (the heat capacity of the material and the fusion heat) as well as the temperature increase required to reach the material’s melting temperature, an approach of the minimum energy required can be obtained using the following Equation (9):

Qm = mCp∆T + m∆Hf (9)

where Cp is a heat constant, ∆T is the temperature difference, and ∆Hf represents the enthalpy fusion. Cp = 2.10 J/gK and ∆Hf = 245 J/g were chosen when processing 300 mg of polyamide with different processing parameters. Results from the theoretical approach indicate that the power delivered by the sonotrode is lower than the power required to melt the material in 1 s, which explains why it was not possible to obtain completed parts with such vibration time, in accordance with the experimental observation.

3.4.2. Heating in Solids Friction Heating One of the main heat sources in the initial stage of ultrasonic plasticizing polymer particles is interfacial friction heating. According to our previous research referring to ultrasonic weld- ing [146,147], interfacial friction heating has a significant influence on subsequent viscoelastic heating [20,110,111]. Interfacial friction between polymer granulates is dry friction type, the interfacial friction heating is mainly contributed by sliding friction [111], and the heating rate at the interface between two granulates can be described as:

→ → Q(t) = τ(t) × v(t) (10) Polymers 2021, 13, x FOR PEER REVIEW 29 of 43

3.4.2. Heating in Solids Friction Heating One of the main heat sources in the initial stage of ultrasonic plasticizing polymer particles is interfacial friction heating. According to our previous research referring to ultrasonic welding [146,147], interfacial friction heating has a significant influence on subsequent viscoelastic heating [20,110,111]. Interfacial friction between polymer granulates is dry friction type, the interfacial friction heating is mainly contributed by sliding friction [111], and the heating rate at the interface between two granulates can be described as: → → Polymers 2021, 13, 2877 𝑄(𝑡) =𝜏(𝑡) 𝑣(𝑡) 28(10) of 41 where 𝜏(𝑡)⃗ is the equivalent friction stress, and 𝑣(𝑡)⃗ is the relative sliding velocity. The relative movement between polymer granulates causes frictional heating. The → → relativewhere τ sliding(t) is the rate equivalent and equivalent friction stress,friction and stress,v(t) whichis the relativeare closely sliding related velocity. to the heat flow Therate, relativeincrease movement with the increased between polymerultrasonic granulates amplitude causes [110]. frictionalThe main heating. parameters The affectingrelative sliding the friction rate and properties equivalent of friction polymers stress, include which arecontact closely pressure, related tovelocity, the heat flowand temperaturerate, increase [148]. with the increased ultrasonic amplitude [110]. The main parameters affecting the frictionMost research properties papers of polymers are about include the contact effect pressure, of parameters velocity, including and temperature processing [148]. parametersMost research and structural papers are parameters about the effecton the of parametersheating mechanism including processingduring ultrasonic parame- plasticizing.ters and structural In terms parameters of processing on the parame heatingters, mechanism studies duringof the ultrasonicultrasonic plasticizing.amplitude, frequency,In terms of and processing pressure parameters, on the heating studies ofrate the of ultrasonic polymers amplitude, have been frequency, carried andout [20,55,107,110,111,149].pressure on the heating rate of polymers have been carried out [20,55,107,110,111,149]. TheThe process process of of the the temperature temperature curve curve is is the the typical typical process process of of all all tested polymers up up toto now, now, which which is is the the same as as the method proposed proposed by by Michael Michael et et al. al. [107], [107], as shown in FigureFigure 26a,b.26a,b. The The first first tenth tenth of of a a second second of of the the process process cycle cycle is is the the stage stage of of rapid rapid heating, heating, whichwhich can bebe explainedexplained as as the the effect effect of of rapid rapi frictiond friction heating. heating. It has It has been been demonstrated demonstrated that thatthere there is friction is friction heating heating only only at the at the initial initial stage stage of theof the plasticizing plasticizing process process in thein the study study of ofWu Wu et al.et [110al. [110],], where where the interface the interface could coul haved a have steep a temperature steep temperature increase upincrease to polymers up to polymersflow temperature flow temperature in 0.078 s in 0.078 the case s in of the PMMA case of granulates. PMMA granulates. The similar The curve similar trends curve in trendsFigure in26 Figurec,e show 26c,e that show the heatingthat the rateheating decreases rate decreases from a certain from a point certain until point the until melting the meltingtemperature temperature level is reached. level is In Figurereached. 26 e,In the Figure ultrasonic 26e, amplitudethe ultrasonic was confirmed amplitude to havewas confirmedmore significant to have impact more significant than the plasticizing impact than pressure the plasticizing on theinterfacial pressure on friction the interfacial heating. frictionSince the heating. energy Since of the the ultrasonic energy waveof the is ultras proportionalonic wave to theis proportional square of the to amplitude, the square it of is thenecessary amplitude, to amplify it is necessary the amplitude to amplify through the theamplitude booster through in order tothe obtain booster the in ultrasonic order to obtainwave withthe ultrasonic a large energy. wave Whenwith a ultrasonic large ener amplitudegy. When isultrasonic increased amplitude from 10 to is 30increasedµm, the ◦ fromaverage 10 to heating 30 μm, rate the is average increased heating from 460.4rate is to increased 1687.5 C/s, from which 460.4 leadsto 1687.5 to the °C/s, ultrasonic which ◦ leadsplasticization to the ultrasonic of polymer plasticization particles from of polymer 30 to 160 particlesC. from 30 to 160 °C.

Figure 26. ((aa)) Viscoelastic Viscoelastic heating heating and and ( (bb)) friction friction heating heating in in UPMIM; UPMIM; ( (cc)) Example Example for for temperature courses during plasticizing for three runs at equal parameter settings [107]; [107]; ( d) Schematic and experimental se setuptup for interfacial friction heating; (e) Interfacial friction heating under different plasticization pressure and amplitude [110]; (f) Longitudinal vibration

transducer structure; L1—rear cover length, L2—piezoelectric ceramic thickness, L3—front cover length, L4—horn length, L5—ultrasonic sonotrode length, S1—large cross-section area of horn, S2—small cross-section area of horn, F1—rear cover force, Vb—rear cover vibration velocity, F2—front cover force, Vf—front cover vibration velocity, amplification ratio N = −2 (S1/S2) ;(g) Influence of the structural parameters on the heating rate [149].

For granular materials, ultrasonic amplitude is a more significant factor than plasticiz- ing pressure on interfacial friction heating. However, many uncertain factors are introduced due to the compressibility of granular materials, which make the plasticizing quality and injection speed unstable: that is, it is not conducive to the process stability. However, it seems that conventional rod materials can improve the process stability; hence, the heating principle and mechanism need to be further developed. However, in actual experiments, the temperature sensor may only be damaged after several tests, resulting in no measurement curve in this case [110]. Based on the repeatability Polymers 2021, 13, 2877 29 of 41

of the experiment, the heat generation rate detection can be determined using sapphire windows [105,129,150] or high-speed infrared cameras [55,105]. Janer et al. [55] studied the polypropylene heating when applying high-power mechanical ultrasound with a high- velocity infrared camera. The results show that the heating of a polypropylene cylinder caused by ultrasonic vibration is highly uneven, and there are different heating steps in this process. The development of new thermal detection technology plays an important role in understanding the plasticization process. Coordinating different plasticizing heating stages and injection speeds will become an important step in process development. In the aspect of structural parameters, the ultrasonic plasticization of polymer particles was studied by introducing different combinations of structural parameters of transducers and the interaction mechanism of friction heating and plasticization of polymer particles under the longitudinal vibration excitation. The friction plasticizing heating equations of the polymer granulates under the longitudinal vibration excitation were established by Li et al. [149]. Figure 26f shows the longitudinal vibration transducer structure used in UPMIM. The analysis results show that in the initial stage of ultrasonic plasticization, among the structural parameters of the longitudinal vibration transducer, the magnification of the horn has the greatest effect on the heating rate of frictional plasticization. In addition, the front cover length, ultrasonic sonotrode length, and the horn length have little influence on the heating rate, while the piezoelectric ceramic thickness of the piezoelectric ceramic and the length of the rear cover have the least influence on the heating rate. Additionally, Jiang et al. [140] characterized the contact angle of some plastic polymer pellets (PMMA, PP, and PA66) with a super-high magnification lens zoom 3D microscope, taking into account the random stack of materials and extremely short interfacial friction heating time compared with the certain contact area of ultrasonic welding [151]. With the increasing parameter level, the proportion of interfacial friction angle in the range of 0◦–10◦ and 80◦–90◦ increased, while the proportion in the range of 30◦–60◦ decreased accordingly, as shown in Figure 27. In the actual production process, the distribution of the contact angle is affected by factors such as particle size and shape. Therefore, the uncertain factors introduced by the particle material are not conducive to the stability of the UPMIM. The Polymers 2021, 13, x FOR PEER REVIEW 31 of 43 regularization of production materials may become one of the important development directions of this process.

FigureFigure 27.27. TheThe measurementmeasurement processprocess ((aa,,bb)) andand resultsresults ((cc)) ofof frictionfriction angleangle [[140].140].

Subsequently,Subsequently, Wu Wu et al.et [ 152al.] performed[152] perfor united-atommed united-atom molecular dynamicsmolecular simulations dynamics tosimulations reveal the interfacialto reveal frictionthe interfacial heating mechanismfriction heating of amorphous mechanism polyethylene of amorphous under singlepolyethylene sliding under friction single (SSF) sliding and reciprocating friction (SSF) sliding and reciprocating friction (RSF) sliding modes. friction The results (RSF) showmodes. that The RSF results is a moreshow efficientthat RSF wayis a more of generating efficient way heat of than generati SSF inng terms heat than of heating SSF in asterms shown of heating in Figure as shown28a; that in is,Figure ultrasonic 28a; that plasticization is, ultrasonic is theplasticization one with higheris the one heating with efficiency.higher heating Figure efficiency. 28b shows Figure the simulation28b shows resultsthe simulation of the molecular results of chainthe molecular orientation chain in orientation in the two friction models at the same time point. In terms of orientation, the molecular chain in the RSF model is disordered due to the frictional form of high- frequency vibration restricted in its region, as shown in Figure 28d. Therefore, the concentrated high-frequency chain motion related to molecular rearrangement is considered as the main mechanism for enhancing frictional heating at the RSF interface. Secondly, as far as process parameters are concerned, the effect of sliding rate on temperature rise is more critical than that of loading pressure, as shown in Figure 29. This work illustrates the advantages of the ultrasonic plasticization principle compared to screw plasticization. As a potential tool, molecular dynamics simulation is indispensable for deepening process understanding.

Polymers 2021, 13, 2877 30 of 41

the two friction models at the same time point. In terms of orientation, the molecular chain in the RSF model is disordered due to the frictional form of high-frequency vibration restricted in its region, as shown in Figure 28d. Therefore, the concentrated high-frequency chain motion related to molecular rearrangement is considered as the main mechanism for enhancing frictional heating at the RSF interface. Secondly, as far as process parameters are concerned, the effect of sliding rate on temperature rise is more critical than that of loading pressure, as shown in Figure 29. This work illustrates the advantages of the ultrasonic plasticization principle compared to screw Polymers 2021, 13, x FOR PEER REVIEWplasticization. As a potential tool, molecular dynamics simulation is indispensable32 forof 43

deepening process understanding.

FigureFigure 28. 28.(a )(a Changes) Changes in in temperature temperature as as a functiona function of of time time in in SSF SSF and and RSF RSF modes; modes; (b )(b Friction) Friction simulation simulation results results of of PE PE internalinternal structure structure under under two two friction friction modes modes at at the the same same simulation simulation time time point; point; Variation Variation of of chain chain orientation orientation parameters parameters withwith sliding sliding time time in in the the SSF SSF process process (c) ( andc) and the the RSF RSF process process (d)[ (d152) [152].].

Figure 29. Variation of PE bulk temperature with time in RSF mode under (a) various loading pressures and (b) various frequencies [152].

Viscoelastic Heating The viscoelastic heat generation effect is a “self-heating process”. It is well known that polymeric materials are sensitive to strain rate and temperature. When polymer particles are subjected to a vibration pressure load, due to the hindrance of the internal

Polymers 2021, 13, x FOR PEER REVIEW 32 of 43

PolymersFigure2021, 1328., 2877(a) Changes in temperature as a function of time in SSF and RSF modes; (b) Friction simulation results of PE31 of 41 internal structure under two friction modes at the same simulation time point; Variation of chain orientation parameters with sliding time in the SSF process (c) and the RSF process (d) [152].

FigureFigure 29.29. VariationVariation ofof PEPE bulkbulk temperaturetemperature withwith timetime inin RSFRSF modemode underunder ((aa)) variousvarious loadingloading pressurespressures andand ((bb)) variousvarious frequenciesfrequencies [[152].152].

Viscoelastic HeatingHeating The viscoelasticviscoelastic heat heat generation generation effect effect is ais “self-heatinga “self-heating process”. process”. It is It well is well known known that polymericthat polymeric materials materials are sensitive are sensitive to strain to ratestrain and rate temperature. and temperature. When polymer When particlespolymer areparticles subjected are subjected to a vibration to a vibration pressure pressure load, due load, to the due hindrance to the hindrance of the internal of the internal macro- molecular segments, the segments generate “internal friction” during the deformation and recovery process. The mechanical work performed by external forces is converted into the heat of the polymer itself, causing an increase in the local temperature of the polymer [153], which is both strain and strain-rate dependent [154]; the compression and unloading curves do not coincide, as shown in Figure 30d, forming a “hysteresis loop” whose area is equal to the thermal energy increase of the polymer in a single vibration cycle. During continuous loading, the polymer consumes mechanical energy in each cycle and is converted into the thermal energy of the polymer. The thermal energy increase of the polymer in unit time under vibration load is: I I dε(t) Q = f σ(t)dε(t) = f σ(t) dt. (11) dt When the vibration load is a sinusoidal alternating load, the expression of stress and strain is:

σ(t) = σ0 sin(ωt)

ε(t) = ε0 sin(ωt − δ)

where σ0 is the stress amplitude, ε0 is the corresponding strain amplitude, δ is the phase angle of the strain hysteretic stress, and ω is the angular frequency of the vibration load.

2π/ω I Z Q = f σ(t)dε(t) = f σ0ε0ω sin ωt cos(ωt − δ)dt = f πσ0ε0 sin δ (12) 0

For the ultrasonic plasticizing polymer process, the alternating load frequency of polymer particles is in the kilo hertz range, and the amplitude is in the micron order. The stress and strain of the polymer can reach a larger order of magnitude with a smaller amount of plasticization. The viscoelastic heat generation effect is also an important heat generation effect in the ultrasonic plasticization heat generation process. Additionally, the irregular shape and random stacking of polymer materials are causes of the uneven and complex stress field in UPMIM. Hence, in order to solve this problem, as shown in Figure 30a, the loading conditions of micro-units in polymer pellets are simplified. It is assumed that the micro-unit cell is loaded with ideal uniaxial normal stress σ(t), which Polymers 2021, 13, 2877 32 of 41

is a sine function with the same frequency as the ultrasonic vibration. The diameter of the Polymers 2021, 13, x FOR PEER REVIEWpolymer body cylinder is 10 mm, the height is 5 mm, and it is periodically loaded34 by of the 43

sonotrode, as shown in Figure 30a [20].

FigureFigure 30.30. ((a)) SimplifiedSimplified loadingloading conditionsconditions forfor polymerpolymer pellet;pellet; ((bb)) SimplifiedSimplified viscoelasticviscoelastic heatingheating modelmodel inin ultrasonicultrasonic plasticizing; (c) Typical stress–strain curve of polymer material in a vibration cycle [20]; (d,e) Stress–strain curves obtained plasticizing; (c) Typical stress–strain curve of polymer material in a vibration cycle [20]; (d,e) Stress–strain curves obtained from the cyclic compression tests of three cylindrical specimens (SP1, SP2, and SP3) of PP. The tests were performed by from the cyclic compression tests of three cylindrical specimens (SP1, SP2, and SP3) of PP. The tests were performed by the the Janer et al. with different processing conditions [155]. Janer et al. with different processing conditions [155]. In order to illustrate the complex thermomechanical behavior of PP, Janer et al. [155] In order to illustrate the complex thermomechanical behavior of PP, Janer et al. [155] applied cycles of loading and unloading in uniaxial compression and incremental levels applied cycles of loading and unloading in uniaxial compression and incremental levels of loadingof loading on on cylindrical cylindrical specimens specimens (diameter (diameter = = 12 12 mm; mm; height height = = 2020 mm)mm) of polypropylene. ® The obtained curves, as shown in Figure 3030d,e,d,e, were testedtested byby aa 55 kNkN MTSMTS LandmarkLandmark® servohydraulic machine, with severalseveral combinationscombinations ofof temperaturetemperature andand strainstrain rate.rate. Based on thethe generalizedgeneralized MaxwellMaxwell model,model, ArrheniusArrhenius model,model, andand semi-empiricalsemi-empirical Williams LandelLandel Ferry Ferry (WLF) (WLF) model, model, the literaturethe literature has involved has involved the study the of study the viscoelas- of the ticviscoelastic heat generation heat generation mechanism mechanism in the process in the of process ultrasonic of weldingultrasonic through welding theoretical through modelingtheoreticaland modeling experimental and experimental research [141 resear,156–ch158 [141,156–158].]. In UPMIM, In processing UPMIM, parametersprocessing suchparameters as ultrasonic such frequency,as ultrasonic amplitude, frequency, and amplitude, initial temperature and initial were temperature considered inwere the viscoelasticconsidered in heating the viscoelastic study by Wuheating et al. study [20], asby shown Wu et inal. Figure[20], as 31 shown. The resultsin Figure show 31. thatThe ultrasonicresults show amplitude that ultrasonic is a more amplitude effective is factora more than effective ultrasonic factor frequencythan ultrasonic in affecting frequency the heatin affecting generation the rate.heat Asgeneration far as the rate. initial As temperature far as the ofinitial the materialtemperature is concerned, of the material the initial is temperatureconcerned, the of theinitial PMMA temperature cylinder of has the no PMMA significant cylinder effect has on theno viscoelasticsignificant effect heating on rate the beforeviscoelastic reaching heating 105 ◦rateC. before reaching 105 °C. In ultrasonic machining, the hammering phenomenon caused by periodic contact loss caused by high-frequency vibration between an ultrasonic sonotrode and adherents directly affects heating efficiency, but there is no relevant research in UPMIM.

Polymers 2021, 13, 2877 33 of 41 Polymers 2021, 13, x FOR PEER REVIEW 35 of 43

FigureFigure 31. 31.(a )( Viscoelastica) Viscoelastic heat generationheat generation rates under rates different under ultrasonicdifferent frequencies;ultrasonic frequencies; (b) Viscoelastic (b) Viscoelastic heating curve of PMMA at various ultrasonic amplitudes, when the initial temperature heating curve of PMMA at various ultrasonic amplitudes, when the initial temperature is 25 ◦C(c) or is 25 °C (c) or 80 °C (d) [20]. 80 ◦C(d)[20].

ViscoelasticIn ultrasonic heating, machining, which the is maximizedhammering around phenomenon the glass caused transition by periodic temperature contact of theloss thermoplastic caused by high-frequency polymer, can bevibration quantified betw basedeen an on ultrasonic the work bysonotrode Tolunay and et al. adherents [159]. directly affects heating efficiency, but there is no relevant research in UPMIM. Viscoelastic heating, which is maximized. ωε ar2oundE00 the glass transition temperature of Q = e (13) the thermoplastic polymer, can be quantifiedbulk based2 on the work by Tolunay et al. [159] where e is called the hammering efficiency, i.e., the𝜔𝜀 ratio𝐸 between actual heat generation 𝒬 =𝑒 (13) (with hammering) and ideal heat generation (without2 hammering), ω = 2πf is the pulsation ofwhere vibration, e is calledE00 is the losshammering modulus efficiency, of the material, i.e., the and ratioε is between the amplitude actual strainheat generation tensor. (withIn hammering) ultrasonic vibration and ideal processing, heat generation the sonotrode (without hammering), tip has a displacement ω = 2πf is the that pulsation is sinu- soidalof vibration, as shown 𝐸 in: is the loss modulus of the material, and ε is the amplitude strain tensor. In ultrasonic vibration processing,usono = athesono sonotrodecos(ωt) tip has a displacement that(14) is

wheresinusoidalαsono asis theshown amplitude, in: ω = 2πf is the pulsation, and f is the frequency. As a result of the hammering effect and the loss of contact between the sonotrode and 𝑢 =𝑎 𝑐𝑜𝑠(𝜔𝑡) (14) the composite, the imposed displacement µimp on the top surface of the top adherend is notwhere equal αsono to isµ thesono amplitude,. Rather, it isω a= 2 truncatedπf is the pulsation, sine, as illustrated and f is the in frequency. Figure 32a. The contact time ratioAs a canresult be definedof the hammering as: effect and the loss of contact between the sonotrode and the composite, the imposed displacement μtimpc on the top surface of the top adherend αt = 1 − (15) is not equal to μsono. Rather, it is a truncated sine,T as illustrated in Figure 32a. The contact timeThe ratiotc canis the be lossdefined of contact as: time during an ultrasonic period T = 2π/ω. αt ranges between 0 and 1 and reaches 1 for a perfect contact with no hammering. Thus, the 𝑡 viscoelastic efficiency e of the process is: 𝛼 =1− (15) 𝑇

The tc is the loss of contact time duringsin( 2anπα ultrasonic) period T = 2π/ω. αt ranges e = α + t (16) between 0 and 1 and reaches 1 for a t perfect2 πcontact with no hammering. Thus, the viscoelastic efficiency e of the process is:

𝑠𝑖𝑛(2𝜋𝛼) 𝑒=𝛼 + (16) 2𝜋

Polymers 2021, 13, x FOR PEER REVIEW 36 of 43 Polymers 2021, 13, 2877 34 of 41

which can be expressed as a function of the amplitude transfer ratio αh using the following which can be expressed as a function of the amplitude transfer ratio α using the following Equation (15). Figure 32b shows the dependency of the efficiency e versush the amplitude Equation (15). Figure 32b shows the dependency of the efficiency e versus the amplitude transfer ratio αh. transfer ratio αh. e 𝑒≈𝛼≈ αh ℎ (17)(17) InIn the UPMIM, PengPeng etet al. al. [ 111[111]] found found that that temperature temperature increases increases occurred occurred only only in the in theloading loading stage stage (NT~(2 (NT~(2nn + 1)T/2, 1)T/2, N = N 0, 1, = 2,…). 0, 1, In 2,the... unloading). In stage the ((2N unloading + 1)T/2~(N stage + 1)T,((2N N + = 1)T/20, 1, 2,…),~(N +the 1)T, temperatureN = 0, 1, of 2, the... fric), thetion temperaturesurface will be of transferred the friction with surface the form will ofbe heat transferred conduction with thein the form polymer of heat conductionfriction interface, in the polymer as shown friction in Figure interface, 32c,d. as shownIn the unloadingin Figure 32 stage,c,d. In the the friction unloading surface stage, temperatur the frictione is surface almost temperatureconstant due is to almost the short constant heat transferdue to the time short and heat low transfer heat transfer time and coefficient, low heat which transfer means coefficient, that the which ultrasonic means hammer that the effectultrasonic directly hammer has a effectsignificant directly impact has aon significant the heating impact rate. on the heating rate.

Figure 32. (a) Displacement of of the the sonotrode an andd upper adherend versus time; ( b) Viscoelastic efficiency efficiency e vs. amplitude transfer ratio αh [144]; (c) The nephogram of temperature change of analysis results during T/8–T; (d) Transient transfer ratio αh [144]; (c) The nephogram of temperature change of analysis results during T/8–T; (d) Transient temperature temperaturetrend curve oftrend the nodecurve with of the the node highest with temperature the highest intemper the threeature regions in the three of the regions friction of interface the friction [111 interface]. [111].

4.4. Conclusions Conclusions and and Remarks Remarks InIn orderorder toto address address the the challenges challenges confronted confronted by the by micro-injection-molding the micro-injection-molding process, process,the application the application of power of ultrasound power ultrasound has proved has toproved be a successful to be a successful and promising and promising attempt attemptfor various for various polymeric polymeric micro-molded micro-molded parts. In parts. the caseIn the of case the macroof the componentsmacro components with a withsurface a surface with micro/nano with micro/nano functional functional structures, structures, ultrasonic-assisted ultrasonic-assisted micro-injection micro-injection molding molding(UAMIM) (UAMIM) has been has developed been developed to facilitate to facilitate the polymer the polymer melt flow melt in micro/nano flow in micro/nano cavities. cavities.This could This improve could theimprove replication the fidelity,replication avoid fidelity, the use avoid of high the injection use of speed/pressure, high injection speed/pressure,and accordingly and reduce accordingly the energy reduce consumption. the energy In addition, consumption. the ultrasonic In addition, vibration the is ultrasonicalso beneficial vibration for the is movement also beneficial of the macromoleculesfor the movement and thereforeof the macromolecules has the possibility and to thereforetailor the microhas the morphologies possibility to (orientation, tailor the micro crystallization, morphologies etc.) (orientation, and molding crystallization, defects such as etc.)weld and line molding for improved defects molding such as quality. weld line On thefor otherimproved hand, molding for the small quality. components On the other with hand,weight for in the milligram small components scale, ultrasonic with plasticization weight in milligram micro-injection scale, ultrasonic molding (UPMIM)plasticization has micro-injectionbeen developed molding to address (UPMIM) the challenge has been regarding developed the excessiveto address plasticization. the challenge Ultrasonicregarding thevibration excessive energy plasticization. could be used Ultrasonic as the onlyvibrat sourceion energy for the could plasticization be used as of the the only plastic source raw

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materials with just the amount needed for the successive molding. Furthermore, with the unique simultaneous plasticizing and injection, the polymer melt could be immediately injected into the mold cavity after plasticization, which is essential to reduce the residence time for thermally sensitive plastics. Therefore, the material utilization could be improved via UPMIM besides the benefits of the power ultrasound demonstrated in the UAMIM. This review provided a general and introductory overview of the application status of power ultrasound in both UAMIM and UPMIM from the aspects of scale and size effect, process principles, configuration design, engineering characteristics, and theoretical interpretations. In the case of UAMIM, the power ultrasound is used as an external auxiliary energy field. The research is focusing on the influence mechanism of the power ultrasound on the polymer melt flow properties and the micro morphological evolution. However, in UPMIM, the power ultrasound becomes the dominant energy source for the plasticization and injection. The research focus is extended to the new plasticization concept via ultrasonic vibration and to the mechanism of the possible change of the material properties. So far, the tuning power ultrasound for enhanced MIM performance of thermoplastic polymers is still challenged by several issues such as the melt flow behavior in the micro-cavity in the presence of the ultrasonic vibration in UAMIM, the stability of the power ultrasound system under the coupled loading conditions during the unique simultaneous plasticization and injection in UPMIM, and the reproducibility of the molding quality in both technologies. In summary, the instructive suggestions for ultrasonic energy field still need to be further studied and standardized more systematically for UAMIM, including but not limited to the different application methods (direct or indirect), the setting of the vibration point (parallel or perpendicular), and the energy utilization efficiency. For UPMIM, the prerequisites for reduced energy consumption and the significance of improved filling and molding ability need to be further clarified. In addition, the stability improvement of the system, process, and parts quality is still facing challenges. What may be predicted is that the application of power ultrasound in the MIM will become even more reliable, accurate, and versatile in the future.

Funding: National Natural Science Foundation of China (No. 51875582 and 51575540). National Key Research and Development Program of China (No. 2020YFB2008200). Huxiang Young Talents Program of Hunan Province (No. 2019RS2003). Graduate Research and Innovation Project of Central South University (No. 2020zzts497). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge financial support given by the National Natural Science Foundation of China (No. 51875582 and 51575540), the National Key Research and Develop- ment Program of China (No. 2020YFB2008200), and the Huxiang Young Talents Program of Hunan Province (No. 2019RS2003). Yuanbao Qiang acknowledges financial support from the Graduate Research and Innovation Project of Central South University (No. 2020zzts497). Conflicts of Interest: The authors declare no conflict of interest.

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