The Setup Design for Selective Laser Sintering of High-Temperature Polymer Materials with the Alignment Control System of Layer Deposition

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Article The Setup Design for Selective Laser Sintering of High-Temperature Polymer Materials with the Alignment Control System of Layer Deposition

Alexey Nazarov 1, Innokentiy Skornyakov 1 and Igor Shishkovsky 1,2,* ID

1 Moscow State University of Technology “STANKIN”, Vadkovsky Per. 1, Moscow 127055, Russia; [email protected] (A.N.); [email protected] (I.S.) 2 Lebedev Physics Institute of Russian Academy of Sciences, Samara Branch, Samara 443011, Russia * Correspondence: [email protected]; Tel.: +7-846-334-4220; Fax: +7-846-335-5600

Received: 21 January 2018; Accepted: 5 March 2018; Published: 5 March 2018

Abstract: This paper presents the design of an additive setup for the selective laser sintering (SLS) of high-temperature polymeric materials, which is distinguished by an original control system for aligning the device for depositing layers of polyether ether ketone (PEEK) powder. The kinematic and laser-optical schemes are given. The main cooling circuits are described. The proposed technical and design solutions enable conducting the SLS process in different types of high-temperature polymer powders. The principles of the device adjustment for depositing powder layers based on an integral thermal analysis are disclosed. The PEEK sinterability was shown on the designed installation. The physic-mechanical properties of the tested 3D parts were evaluated in comparison with the known data and showed an acceptable quality.

Keywords: selective laser sintering (SLS); setup design; polyetheretherketone (PEEK)

1. Introduction Selective laser sintering (SLS) is one of the most famous and commercially successful methods of additive technologies [1–5]. The SLS makes it possible to manufacture products with a unique internal and/or external surface shape and perspective physical and mechanical properties [1–5]. At the same time, there are a number of promising polymer powder materials that differ from those traditionally and long used in the technology of SLS—nylon [6,7], polycarbonate [8–12], polyamides [13–17], polyethylenes [18,19], or ferrielectric polyvinylidene ﬂuorides [20] and so on—by a combination of high heat resistance, durability, and a wide range of interesting physic-mechanical and chemical-biological properties [21–26]. Particularly interesting are the parts manufactured by the SLS technology from some types of powders based on polyetheretherketone (PEEK). They have high strength values of up to 95 MPa (vs. 45 MPa for conventional polyamides) and Young’s modulus of up to 4400 MPa (vs. 1500 MPa for conventional polyamides) [3,27–29]), have high heat resistance (maintenance of physical and mechanical properties during short-term exposure to temperatures up to 310 ◦C and long-term exposure to temperatures of 260 ◦C), as well as excellent biocompatibility and insulating (dielectric) properties [29–32]. A set of these properties in combination with the capabilities of the SLS method allows creating unique parts. These parts are increasingly used in the aerospace industry, medicine, and motorsport [29–32]. A set of these properties in combination with the SLS capabilities allows creating unique details. These parts are increasingly used in aerospace, medicine, and auto- and motorsport [29–32]. The authors know a limited number of additive setups are operating in the framework of the SLS technology with different types of polymer powders. In general, the world-famous additive technology giants—3D Systems and EOS GmbH

Machines 2018, 6, 11; doi:10.3390/machines6010011 www.mdpi.com/journal/machines Machines 2018, 6, 11 2 of 17

(Krailling, Germany) [33,34] produce, however, only a few EOS system pieces of equipment to operate with heat-resistant polymers. The small number of the SLS setups applying the promising heat-resistant polymer powders limits the business and industry opportunities involved in the development and production of science-intensive products. This paper’s goal is to present the design of the SLS facility (hereinafter referred to as SLS-machine), which uses high-temperature polymers. We propose original solutions for the alignment control system of the powder delivery layers under layerwise SLS materials based on PEEK.

2. Equipment for Plastic Additive Manufacturing

2.1. Analysis of Analogue Equipment The literature analysis of analogue installations of the world’s leading manufacturers [33,34] showed that there is only one producer of additive equipment in the world—Electro Optical Systems (EOS, Krailling, Germany), which successfully implemented the high-temperature PEKE into the SLS process [27]. This is a well-known family equipment of the type Eosint P, from which the P500 and P800 systems [35,36] used the PEEK polymer, and the parameters of which are presented at the indicated links on the manufacturer’s website. The policy of this company boils down to the fact that, together with the SLS machine, the customer is supplied with the proprietary software which makes it impossible to control part of the manufacturing process. Besides, the machine can only operate with one type of the PEEK powder (grade EOS PEEK HP3), which is only tested and supplied by the machine manufacturer. In the scientific and technical literature, there is no information on the Eosint P500/P800 technical characteristics (except for those listed in the brochures and on the official website), nor on the constructive decisions taken in its development. It is only known that the Eosint P800 is a result of a serious modification of the Eosint P760 machine, taking into account the heating of the workspace up to 385 ◦C. In the Eosint P500 machine, the operating temperature is already ~300 ◦C. However, the creators note that the Eosint P500 is operated simultaneously by two 70 W lasers, and a new design of the recoater and the preheating of the changeable frame are applied. These innovations, according to the creators, significantly increased the productivity of the process of manufacturing parts from high-temperature polymers [35]. According to the designers, this brought the rate of the application of the layers to 0.6 m/s.

2.2. Known Features of the SLS Process for Heat-Resistant PEEK From the literature sources [27,30,31], it is known that the best physical and mechanical characteristics are represented by the PEEK parts manufactured using special SLS technology. This technology has a number of fundamental differences from the SLS technology for conventional polymers. The main stages of the SLS technology for manufacturing the PEEK parts are described below (Figure1): (a) The heaters built into the platform and the changeable frame are heated up to a temperature of ~360 ◦C; (b) The protective gas (nitrogen) is pumped through the airtight chamber to obtain the required purity, and the entire SLS machine is held for two hours till uniform heating of all its elements in order to avoid their thermal deformation during operation; (c) The platform is lowered to the thickness of the ﬁrst powder layer to be applied (usually ~120 µm); (d) The recoater (in the form of a double knife) moves to the left limit position under the splined shaft and the required powder batch is dosed to the recoater with the help of the splined shaft; (e) The recoater moves to the right limit position (shown in phantom) applying and leveling the ﬁrst layer of powder on the platform during the movement; (f) The applied powder layer is heated up to ~385 ◦C with the help of upper heaters; Machines 2018, 6, 11 3 of 17

(g) The platform is lowered to the thickness of the second powder layer to be applied (usually ~120 µm); (h) TheMachines recoater 2018 moves, 6, x FOR PEER to the REVIEW left limit position and with the help of the splined shaft and3 of 17 the powder delivery hopper applies in a similar way the second powder layer over the ﬁrst layer on the (e) The recoater moves to the right limit position (shown in phantom) applying and leveling the platform,first dropping layer of powder excess on powderthe platform into during the powderthe movement; collecting hopper; (i) The(f) process The applied of applying powder layer preliminary is heated up powderto ~385 °C with layers the (abouthelp of upper 50 layers heaters;in total) is carried out according(g) The toplatform the c–h is lowered points to without the thickness laser of the treatment. second powder It is layer necessary to be applied for uniform (usually ~120 heating of the μm); machine together with the powder, and also for checking the correctness of the recoater knife (h) The recoater moves to the left limit position and with the help of the splined shaft and the positionpowder relative delivery to the hopper platform. applies During in a similar this way stage, the second the heaters powder maintain layer over thethe first constant layer on temperature ◦ (about 360the platform,C) of the dropping whole excess powder powder volume; into the powder collecting hopper; (j) The(i) part The manufacture process of applying begins preliminary when the powder recoater layers knife (about is set 50correctly layers in total) relative is carried to the out platform; according to the с–h points without laser treatment. It is necessary for uniform heating of the (k) The 51stmachine layer oftogether powder with is the applied powder, over and also the for 50 checking heated powderthe correctness layers; of the recoater knife (l) The 51stposition powder relative layer to is heatedthe platform. to 385 During◦C with this the stage, help the of heaters upper heaters;maintain the constant (m) Powdertemperature is sintered (about with 360 a laser°C) of beamthe whole in thepowder individual volume; zones of the applied layer depending on (j) The part manufacture begins when the recoater knife is set correctly relative to the platform; the(k) shape The of 51st the layer part of beingpowder manufactured; is applied over the 50 heated powder layers; (n) Then,(l) aThe new 51st powder layer layer of powder is heated to is 385 applied °C with the and help the of upper process heaters; is repeated until the part is completely(m) Powder manufactured; is sintered with a laser beam in the individual zones of the applied layer depending on the shape of the part being manufactured; (o) After(n) the Then, whole a new part layer construction, of powder is applied it is cooled and the down process very is repeated slowly until (the the cooling part is completely time is usually twice as longmanufactured; as the time of the part construction) together with the volume of the unsintered powder in the(o) SLSAfter machine the whole in part the construction, changeable it frameis cooled by down means very of slowly heater (the control; cooling time is usually (p) After completetwice as long cooling as the of time the of part the part together constructi withon) thetogether unsintered with the powder,volume of thethe unsintered changeable frame is powder in the SLS machine in the changeable frame by means of heater control; removed(p) After from complete the SLS cooling machine of the part and together moved with to thethe unsintered cleaning powder, station the where changeable the part frame is cleanedis from the unsinteredremoved powder.from the SLS machine and moved to the cleaning station where the part is cleaned from the unsintered powder.

Figure 1. Schematic view of the SLS machine for manufacturing parts made of the PEEK powder: Figure 1. Schematic view of the SLS machine for manufacturing parts made of the PEEK 1—platform, 2—heater, 3—changeable frame, 4 and 5—heaters, 6—airtight chamber, powder:7—double-knife 1—platform, recoater, 2—heater, 8—splined 3—changeable shaft, 9—powder frame, delivery 4 and hopper, 5—heaters, 10—splined 6—airtight shaft, chamber, 7—double-knife11—powder recoater, collection 8—splined hopper, 12—top shaft, heaters, 9—powder 13—powder delivery delivery hopper, hopper, 10—splined 14—powder collection shaft, 11—powder collectionhopper, hopper, and 15—laser-optical 12—top heaters, unit. 13—powder delivery hopper, 14—powder collection hopper, and 15—laser-optical unit. 3. Main Technical Characteristics and Requirements for the Developing Equipment

3. Main TechnicalFor manufacturing Characteristics various and large Requirements capacity parts for(see theSection Developing 2.2), the following Equipment technical requirements (the most complicated characteristics) were imposed on the SLS machine: Fori. manufacturingthe possibility of various using the SLS large technology capacity for various parts types (see of Section the PEEK-based 2.2), the powders; following technical requirements (the most complicated characteristics) were imposed on the SLS machine:

Machines 2018, 6, 11 4 of 17 i. the possibility of using the SLS technology for various types of the PEEK-based powders; ii. the workspace of 500 × 500 × 300 mm3 (length × width × height); iii. preheating of the applied powder layer to 385 ◦C with an accuracy ±2 ◦C (over the entire area of 500 × 500 mm2); iv. nitrogen protective atmosphere (nitrogen purity: up to 99.8%); v. the accuracy of the applied powder layer ±10 µm; vi. the capability to maintain controllable slow cooling of the entire volume of the manufactured part together with the unsintered powder from 385 ◦C to 20 ◦C; vii. the capability of automated control of the powder recoater alignment; viii. the possibility of changing the intensity distribution into the spot of laser radiation from “gauss” to “reverse gauss” or “top hat”, which in the opinion of some researchers, can improve the quality of the components produced by the SLS method [36–38].

Named technical characteristics and requirements for the machine under development (in combination with the peculiar features of the SLS technology for the PEEK-based powders) complicate the task of the design development because of the following problems: a. the thermal deformations in the installation mechanical elements (for example, at the peak of heating, the upper heaters can reach 2600 ◦C[39,40], and most of the remaining elements of the machine are at room temperature); b. use of special materials for parts and assemblies under heating. For example, for springs whose alloys lose their mechanical properties during heating (spring alloys A 29 1060 ASTM, A 576 1060 ASTM, 5147 ASTM, 6150 ASTM, 9262 ASTM, etc., which are designed for operation at temperatures up to 400 ◦C); c. thermal deformation of the laser-optical unit of the installation, thermal aberrations, up to the destruction of protective glasses, lenses, mirrors, etc., at high temperatures; d. the property change in the optical elements of the laser-optical assembly of the installation due to heating. For example, it is changing the refractive index of the protective glass in a complex scanner lens (f-theta lens); e. the chamber sealing (most standard silicone seals and sealants really work at temperatures up to 300 ◦C); f. thermal insulation of the working chamber (space where the part is made from the remaining elements of the installation, for example, the precise drive of the vertical movement and alignment, the device for depositing the powder layers, the laser-optical unit, etc.); g. an interactive system for the thermal picture monitoring during the SLS process; h. the protection of electrical components (induction position sensors, contact sensors of emergency situations, electric wires, electrical connectors, etc.), used up to a temperature of 50–70 ◦C.

4. Design and Technological Solutions for the High-Temperature Plastic SLS Machine The appearance of the SLS machine is shown in Figure2. The unit is equipped with the separate nitrogen generator and the cooling station, which is conditionally removed in Figure2. The longitudinal and cross section of the SLS machine with the main nodes are shown in a drawing in Figure3. The kinematic scheme of the SLS machine is shown in Figure4. The laser-optical scheme of the SLS machine is shown in Figure5. Machines 2018, 6, 11 5 of 17

Machines 2018, 6, x FOR PEER REVIEW 5 of 17

Figure 2. General view of the SLS machine (the outer panels are conditionally shown as transparent). Figure 2. GeneralDimensions view 2470 of× 1570 the × SLS 2480 machine mm (length (the × width outer × height). panels are conditionally shown as transparent). Dimensions 2470 × 1570 × 2480 mm (length × width × height). During the equipment design, the two conflicting tasks had to be constantly addressed:

During(1) thein the equipment nitrogen protective design, atmosphere the two conflicting (99.8% purity), tasks it was had necessary to be constantly to pre-heat addressed:the applied powder layer to 385 °C, (due to the peak heating of the upper heaters to 2600 °С [38,39]), and (1) in the nitrogenalso to support protective the entire atmosphere volume of (99.8% the manu purity),factured it part was together necessary with to the pre-heat unsintered the applied powder at a temperature◦ of 360 °C, with subsequent slow controllable cooling to 20◦ °C; powder(2) layerit was toalso 385 necessaryC, (due to maintain to the peak the accuracy heating of ofthe the deposited upper layer heaters of powder to 2600 at 120C[ ±38 10, 39μm]), and also to supportover the the entire entire working volume area of theof 500 manufactured × 500 mm, and also part to together keep the focused with the laser unsintered spot and the powder at a temperatureaccuracy of of 360 its positioning◦C, with subsequentduring laser processing. slow controllable cooling to 20 ◦C; (2) it was alsoSo necessarycomplex is it tothat maintain at the same the time accuracy it was necessary of the to deposited apply even layer and uniform of powder layers atover 120 a ± 10 µm overlarge the entirearea of working500 × 500 mm, area but of all 500 the× main500 elements mm, and of the also installation to keep were the focusedlocated in laserthe direct spot and the vicinity of the working area, subject to significant alternating heating. We are talking about (check by accuracyFigure of 3): its 7—recoater; positioning 8—double during knife; laser processing.9—recoater drive; 1—lower transition table precision vertical drive, 11—laser-optical unit; 12—laser-optical unit frame; 13—ring of laser-optical unit; So complex14—ZnSe-glass; is it that 17—main at the same frame time of the it SLS was machine necessary and toall. apply These evendifficulties and required uniform creation layers of over a large area of 500the× thermal500 mm, protection but all circuits the main for the elements main elements of the of installation the installation. were located in the direct vicinity of the working area,Problem subject 1 was tosolved significant thanks to alternatingthe following heating.technical solutions We are (Figure talking 3). aboutWe used: (check by Figure3): 7—recoater;1. 8—doublequartz halogen knife; heaters—19 9—recoater [39,40], installed drive; 1—lowerin the upper transitionpart of the sealed table inner precision chamber vertical and drive, 11—laser-opticalcombined unit; in 12—laser-optical the heating system of unit the applied frame; layer 13—ring of powder; of laser-optical unit; 14—ZnSe-glass; 17—main2. frame flat ceramic of the heaters SLS machine [41,42], built and into all. the work These table difficulties and combined required into the heating creation system of theof thermal the platform—6; protection circuits for the main elements of the installation.

Problem 1 was solved thanks to the following technical solutions (Figure3). We used:

1. quartz halogen heaters—19 [39,40], installed in the upper part of the sealed inner chamber and combined in the heating system of the applied layer of powder; 2. ﬂat ceramic heaters [41,42], built into the work table and combined into the heating system of the platform—6; 3. ﬂat ceramic heaters—25 [41,42], built into the interchangeable production bunker and integrated into the heating system of the replacement manufacturing hopper. Machines 2018, 6, x FOR PEER REVIEW 6 of 17

3.Machines flat2018 ceramic, 6, 11 heaters—25 [41,42], built into the interchangeable production bunker 6and of 17 integrated into the heating system of the replacement manufacturing hopper.

Figure 3. Structural plan (described in the text). Zone A is highlighted in further detail in below Figure 3. Structural plan (described in the text). Zone A is highlighted in further detail in below Figure 8: (a)—longitudinal section, (b)—cross-section. Figure 8: (a)—longitudinal section, (b)—cross-section.

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Figure3 shows the setup structural plan, which includes the following items: a—longitudinal section (axonometry), b—cross-section (axonometry). The SLS machine includes the following main units, systems and parts: 1—lower transition table vertical drive, 2—lower transition table bracket; 3—lower transition table cooled rod; 4—lower transition table (has the ability to automatically engage with and disengage from the building platform); 5—building platform; 6—building platform heating system; 7—recoater; 8—double knife (only Figure3a); 9—recoater drive (only Figure3b); 10—airtight casing of recoater (only Figure3b); 11—laser-optical unit; 12—laser-optical unit frame; 13—ring of laser-optical unit; 14—ZnSe-glass; 15—powder depositing main plate; 16—main plate frame; 17—main frame of the SLS machine; 18—airtight inner chamber; 19—top heaters; 20—pyrometer (only Figure3b); 21—illumination lamp; 22—protective external chamber; 23—external chamber frame; 24—changeable frame; 25—changeable frame heating system; 26—changeable frame clamping device; 27—double-protective door (only Figure3b); 28—left powder delivery hopper (only Figure3a); 29—left powder delivery hopper clamping device (only Figure3a); 30—right powder delivery hopper (only Figure3a); 31—right powder delivery hopper clamping device (only Figure3a); 32—left powder collection hopper (only Figure3a); 33—left powder collection hopper clamping device (only Figure3a); 34—right powder collection hopper (only Figure3a); 35—right powder collection hopper clamping device (only Figure3a); 36—air-gas system; 37—electric control device; 38—thermocouple. Control of the accuracy of preheating the applied powder layer to 385 ◦C was carried out by means of the pyrometer—20 (Figure3b), installed in the upper part of the protective chamber. Maintenance control of the entire volume of the workpiece together with the unsintered powder at a temperature of 360 ◦C in a closed loop followed by a slow controlled cooling to 20 ◦C is achieved thanks to the thermocouples—38, installed in the central cut-out in each ﬂat ceramic heater—25 and in each ﬂat ceramic heater in the heating system of the working platform—6. The closed loop problem was solved through the introduction of additional water, gas, and air cooling circuits, which prevent the SLS machine elements from thermal deformation and protect the electrical connectors, sensors, optical, and other SLS machine elements from overheating as the machine elements cannot operate under heating by more than 40–70 ◦C. In other words, it was possible to isolate the working space heated up to 385 ◦C together with the volume of the manufactured part and the powder not yet processed from all the units and devices responsible for the exact deposition of the powder layers and material fusion with a laser. In the kinematic scheme of the SLS machine (Figure4), we proposed the following most interesting technical solutions:

- vertical movement of the desktop is realized thanks to a ball screw rotated from the servomotor, which is a traditional solution for such nodes in a variety of SLS installations; - the horizontal movement of the powder layer’s delivering device is realized by means of a ball screw rotated from the servomotor. This fundamentally differentiates our equipment from those of other manufacturers, which often use a belt drive stretched between the two rollers to realize horizontal movement. However, the experience of operating the belt drive stretched between the rollers has shown its unreliability due to periodic clogging of the tension rollers with powder, which leads to frequent drive failures; - dosing of the necessary powder portions is realized thanks to splined shafts, rotated by stepping motors, which is a classic solution for modern SLS machines working with polymer powders; - for the prevention of emergency movements of the nodes, end position sensors are provided; - for precise tracking of the movement, the sensors of the coordinate control are applied. Machines 2018, 6, 11 8 of 17 Machines 2018, 6, x FOR PEER REVIEW 8 of 17

FigureFigure 4.4. Kinematic scheme scheme of of the the SLS SLS machine: machine: М M1,1, М M2,2, М3, M3, М4—electric M4—electric motors; motors; Х, Y—axes X, Y—axes of the of themain main movements; movements; Z, Z,Х1, X1, В, B,В1, B1, Х2—axes X2—axes of of auxiliary auxiliary movements; movements; SQ1, SQ1, SQ2, SQ3, SQ4—endpointSQ4—endpoint sensors;sensors; BQ1,BQ1, BQ2,BQ2, BQ3,BQ3, BQ4—positionBQ4—position controlcontrol sensors.sensors.

TheThe mainmain cooling cooling circuits circuits of of the the machine machine are are shown shown in Figure in Figure5. They 5. includeThey include the following the following items: 1—airitems: cooling1—air cooling of the protective of the protective ZnSe-glass ZnSe-glass from the scannerfrom the side; scanner 2—nitrogen side; 2—nitrogen cooling of protectivecooling of ZnSe-glassprotective ZnSe-glass from the sidefrom of the the side ring of ofthe laser-optical ring of laser-optical unit; 3—nitrogen unit; 3—nitrogen cooling cooling of recoater of recoater drive; 4—waterdrive; 4—water cooling cooling of the main of the plate main applying plate theapplying layers ofthe powder; layers of 5—water powder; cooling 5—water of the cooling flange of thethe ringflange of of the the laser-optical ring of the unit, laser-optical on which unit, the protectiveon which the ZnSe protective glass is mounted; ZnSe glass 6—water is mounted; cooling 6—water of the cooledcooling rod of the for movingcooled rod the for lower movi transitionng the lower table; transition 7—water coolingtable; 7— ofwater the plate cooling of the of clamping the plate device of the ofclamping the changeable device of frame; the changeable 8—nitrogen frame; cooling 8—nitrog of the opticalen cooling part of of the the optical pyrometer; part 9—waterof the pyrometer; cooling of9—water the pyrometer cooling case;of the 10—air pyrometer cooling case; of electrical10—air c connectorsooling of electrical of the lower connectors transition of the table lower and buildingtransition platform. table and Allbuilding the inductive platform.position All the induct sensorsive in position the tightening sensors in device the tightening of the changeable device of framethe changeable are provided frame with are the provided local airflow with to preventthe local overheating airflow to (not prevent shown). overheating Nitrogen cooling(not shown). of the illuminationNitrogen cooling lamp of has the been illumina implementedtion lamp (nothas been shown). implemented Temperature (not control shown). points Temperature using a manual control pyrometerpoints using for a metals manual are: pyrometer 11—head for bearing metals of are: ballscrew; 11—head 12—lower bearing transitionof ballscrew; table 12—lower bracket; 13—lowertransition transitiontable bracket; table 13—lower of cooled rod;transition 14—plate table of laser-opticalof cooled rod; unit 14—plate (rear edge); of laser-optical 15—plate of laser-opticalunit (rear edge); unit (middle);15—plate 16—plateof laser-optical of laser-optical unit (middle); unit (first 16—plate line); 17—ballscrew; of laser-optical 18—pulley; unit (first 19—plateline); 17—ballscrew; of recoater drive;18—pulley; 20—ring 19—plate of laser-optical of recoater unit; drive; 21—plate 20—ring ofof double-protectivelaser-optical unit; 21—plate door; 22—point of double-protective on the plate neardoor; connector. 22—point on the plate near connector.

Machines 2018, 6, 11 9 of 17 Machines 2018, 6, x FOR PEER REVIEW 9 of 17

Figure 5. Cooling circuits of the SLS machine (cross-section of the Figure 3b). Different indicator Figure 5. Cooling circuits of the SLS machine (cross-section of the Figure3b). Different indicator colors correspond to the selected cooling circuit: water—blue, nitrogen—yellow, air—green, control colors correspond to the selected cooling circuit: water—blue, nitrogen—yellow, air—green, control points—red color.color.

5. Adjustment Control System for Powdered Layer Delivery 5. Adjustment Control System for Powdered Layer Delivery The following most interesting technical solutions are visible from the developed scheme of The following most interesting technical solutions are visible from the developed scheme of the the laser-optical node (Figure 6): laser-optical node (Figure6): - CO2—Firestar ti-Series 80 laser (Synrad, Mukilteo, WA, USA) [43]. This laser provides a power - CO —Firestar ti-Series 80 laser (Synrad, Mukilteo, WA, USA) [43]. This laser provides a power control2 range from 5 W to 80 W management to the frequency modulation of laser influence; control range from 5 W to 80 W management to the frequency modulation of laser inﬂuence; - laser Beam Expander x5 (Synrad, Mukilteo, WA, USA) [44] is necessary for matching the optical - characteristicslaser Beam Expander between x5 (Synrad,the scanner Mukilteo, and WA,the USA)laser [beam44] is necessarydue to changing for matching the the intensity optical distributioncharacteristics device between of laser the scannerradiation; and the laser beam due to changing the intensity distribution device of laser radiation; - the intensity distribution device—Focal-piShaper-12 CO2 10.6 (Ald Optics, Berlin, Germany) - [45].the intensity This technical distribution solution device—Focal-piShaper-12 fundamentally distinguishes CO2 10.6 the (Ald laser-opti Optics,cal Berlin, node Germany)developed [45 by]. us,This from technical other solutionmanufacturers fundamentally of the SLS distinguishes equipment.the The laser-optical Focal-piShaper node device developed allows by changing us, from theother laser manufacturers intensity distribution of the SLS from equipment. “gauss” to The “reverse Focal-piShaper gauss” or device “top hat” allows shape, changing which theby somelaser intensityresults [36–38], distribution can impr fromove “gauss” the component’s to “reverse quality gauss” produced or “top hat” by the shape, SLS whichmethod; by some - theresults AXIALSCAN-50 [36–38], can improve scanner the (Raylase, component’s Wessling, quality Germany) produced [46]. by the A SLSstriking method; feature of this scanner is the constant diameter of the focused laser beam along the entire large 500 × 500 mm field of scanning. It is maintained due to the joint interaction of the fixed focusing lens and the

Machines 2018, 6, 11 10 of 17

- the AXIALSCAN-50 scanner (Raylase, Wessling, Germany) [46]. A striking feature of this scanner is the constant diameter of the focused laser beam along the entire large 500 × 500 mm ﬁeld of scanning. It is maintained due to the joint interaction of the ﬁxed focusing lens and the rapidly movingMachines defocusing 2018, 6, x FOR lens.PEER REVIEW This technical solution is fundamentally different from the10 of classical17 use of a dual-axisrapidlyscanner moving defocusing with an f-theta lens. This objective technical in solution the SLS is fundamentally equipment; different from the ◦ - two CO2classicalmirrors use are of a necessary dual-axis scanner to turn with the an direction f-theta objective of laser in the beam SLS propagationequipment; by 180 in order to achieve- the two maximumCO2 mirrors are compactness necessary to turn of the the laserdirection scanning of laser beam system. propagation by 180° in order to achieve the maximum compactness of the laser scanning system.

Figure 6. The laser-optical scheme of the SLS machine. Figure 6. The laser-optical scheme of the SLS machine. The correct thickness of each powder layer of ±10 μm applied over the whole 500 × 500 mm The correctarea under thickness considerable of each and powder variable layer heating of ±up10 toµ m385 applied °C much over depends the whole on the 500 correct× 500 mm area under considerablehigh-precision and alignment variable of the heating recoater up relative to 385 to ◦theC muchplatform depends on which onthe thelayers correct are applied. high-precision Until recently, the correct recoater alignment was carried out manually by the operator before alignmentstarting of the the recoater sintering relativeprocess. Then, to the the platform process of onthe whichpart manufacturing the layers was are carried applied. out in Until the recently, the correctautomatic recoater regime alignment without further was re carriedcoater control. out In manually the case of byimproper the operatorinitial alignment before and/or starting the sinteringsubsequent process. recoater Then, thedeflection, process layers of of the various part thicknesses manufacturing appear. This was can carried cause quality out in loss the or automatic regime withoutmanufactured further part recoater failure. control.The designed In the SLS case machine of improper is equipped initial with alignmentthe automated and/or recoater subsequent alignment control system based on the integral thermal pattern evaluation of the applied powder recoater deflection,layer. Functioning layers of of this various monitoring thicknesses system is based appear. on the This following can cause principles quality (Figure loss 7): or manufactured part failure. The designed SLS machine is equipped with the automated recoater alignment control (a) leveling of the double-knife recoater is set by the operator manually before starting the system basedoperation on the integral on the upper thermal surface pattern of the platform; evaluation of the applied powder layer. Functioning of this monitoring(b) leveling system of the is baseddouble-knife on the recoater following is always principles skewed (Figurerelative to7): the platform top surface. While applying the powder layer from the left limit position (Figure 7a), the applied layer (a) levelingthickness of the double-knife is specified by recoaterknife 4; is set by the operator manually before starting the operation on the(c) upperwhile applying surface the of thepowder platform; layer from the right limit position (Figure 7b), the thickness of the (b) levelingapplied of the layer double-knife is specified by recoater knife 5, and is alwaystherefore skewedthe difference relative of the tothickness the platform of the layers top surface. applied from the left limit position and the right limit position will be δ μm; While applying the powder layer from the left limit position (Figure7a), the applied layer (d) in case of proper adjustment of the heaters installed above the applied powder layer, they thicknesstransfer is specified an equal by heat knife amount 4; to each applied powder layer; (c) while applying the powder layer from the right limit position (Figure7b), the thickness of the applied layer is specified by knife 5, and therefore the difference of the thickness of the layers applied from the left limit position and the right limit position will be δ µm; (d) in case of proper adjustment of the heaters installed above the applied powder layer, they transfer an equal heat amount to each applied powder layer; Machines 2018, 6, 11 11 of 17

(e) pyrometer mounted above the applied powder layer measures the heating of the applied powder layer and registers slight differences between the values of heating the powder layers applied

fromMachines the left 2018 limit, 6, x FOR position PEER REVIEW and those applied from the right limit position due to the11 of difference17 of thickness layers of the δ ~20 µm;

(f) the sum(e) pyrometer of the heating mounted values above (integralthe applied heating) powder layer for themeasures 25 layers the heating of powder of the appliedapplied from the powder layer and registers slight differences between the values of heating the powder layers leftmostapplied position from will the differ left limit from position the sum and those of the ap heatplied valuesfrom the for right the limit 25 layersposition of due powder to the deposited from thedifference extreme of right thickness position layers of due the toδ ~ the 20 μ differencem; in layer thickness by the δ value; (g) Based(f) onthe the sum difference of the heating in thevalues integral (integral heating heating) forfor the 25 powder layers of layers powder applied applied from from the the extreme leftmost position will differ from the sum of the heat values for the 25 layers of powder left anddeposited rightmost from positions, the extreme the right skewing position degree due to the of difference the leveling in layer double thickness knife by relativethe δ value; to the upper surface(g) ofBased the on platform the difference can bein the unambiguously integral heating for concluded; the powder layers applied from the extreme (h) the alignmentleft and controlrightmost of positions, the double the skewing knife accordingdegree of the to leveling the principles double knife described relative to above the must be carried outupper before surface the of the 3D platform part building can be unambiguou for the correctnesssly concluded; veriﬁcation of the operator adjustment, (h) the alignment control of the double knife according to the principles described above must be and alsocarried directly out during before the 3D construction part building process. for the correctness verification of the operator adjustment, and also directly during the construction process.

Figure 7. Schematic drawing of the recoater alignment control system of the SLS machine: (a) Figure 7.applicationSchematic of powder drawing layer from of thethe left recoater limit position, alignment (b) application control of systempowder layer of thefrom SLSthe machine: (a) applicationright limit of powderposition. Designation layer from of thenumbers: left limit 1—double-knife position, recoater; (b) application 2—upper surface of powder of the layer from the rightplatform; limit position. 3—platform; Designation 4—left knife; of 5—rig numbers:ht knife; 1—double-knife6—top heaters; 7—pyrometer. recoater; 2—upper surface of the platform; 3—platform; 4—left knife; 5—right knife; 6—top heaters; 7—pyrometer. The above-described principles allow controlling the powder delivery alignment device in the developed SLS machine in the automatic regime. We still have questions that remain unresolved: the The above-describedduration and frequency principles of the heater’s allow switching controlling on; and the thepowder summation delivery sequence alignment for the integral device in the developedheating SLS machineand allowance in thefor possible automatic skewing regime. at the Weplatform still haveedges. questionsBeyond this thatpaper, remain there are unresolved: the durationremaining and frequency questions in of relation the heater’s to determining switching the technological on; and the regimes summation of the SLS sequence process for for the the integral heating andselected allowance PEEK, which for will possible be decided skewing during the at commissioning the platform stage. edges. Beyond this paper, there are remaining6. questionsExperimental in Implementation relation to determining and Discussion the technological regimes of the SLS process for the selected PEEK,During which the willstartup-setup be decided operat duringions, experiments the commissioning on the inclusion stage. of heaters built into the working table—6 (Figure 3) and heaters built into the replaceable hopper—25 to a temperature of 6. Experimental360 °C were Implementation conducted to measure and Discussion how fast the stable heated state of the installation is being established. The temperature readings were taken from the thermocouples 38 (see Figures 3 and 8) During the startup-setup operations, experiments on the inclusion of heaters built into the and in the specially installed points 101–108 for additional thermocouples (Figure 8). The workingobservation table—6 (Figurepoints, which3) and were heaters specially built installed into thermocouples, the replaceable started hopper—25 with the number to 101. a temperature It of 360 ◦Cwas were found conducted that temperature to measure fluctuations how in thermoco fast theuples stable 38 (Figures heated 3 and state 8) do of not the exceed installation ±5 °C. is being established.These The fluctuations temperature can be considered readings as were a “stable taken heated from state the of the thermocouples installation” and 38 are (see achieved Figures in 3 and8) no more than 25 min. However, temperature variations that do not exceed ±5 °C in thermocouples and in the specially installed points 101–108 for additional thermocouples (Figure8). The observation established at points 101–108 (Figure 8) are achieved in a longer time, about 90–120 min (depending points, whichon the were heating specially rate setting installed and ambient thermocouples, temperature). Based started on this, with we therecommend number accepting 101. It a was time found that temperatureof about ﬂuctuations two hours infor thermocouples the stable heated state 38 (Figures for the setup.3 and 8) do not exceed ±5 ◦C. These ﬂuctuations can be considered as a “stable heated state of the installation” and are achieved in no more than 25 min. However, temperature variations that do not exceed ±5 ◦C in thermocouples established at points 101–108 (Figure 8) are achieved in a longer time, about 90–120 min (depending on the heating rate setting and ambient temperature). Based on this, we recommend accepting a time of about two hours for the stable heated state for the setup. Machines 2018, 6, x FOR PEER REVIEW 12 of 17

Machines 2018, 6, 11 12 of 17

Machines 2018, 6, x FOR PEER REVIEW 12 of 17

Figure 8. Details of the “Zone A” area from Figure 3a: points (101–108) are the locations of specially Figure 8. Detailsinstalled of thermocouples the “Zone A” for area the from experiment, Figure poin 33a:a:ts points points38 are (101–108) (101–108)stationary arearethermocouples thethe locationslocations of the ofof speciallyspecially installedinstalled thermocouplesthermocouplesinstallation (repeat for positionfor the the experiment, with experiment, Figure 3a), points 4—low poin 38er arets transition 38 stationary are table, stationary 5—building thermocouples thermocouplesplatform of (repeat the installation of the installation(repeat positionposition (repeat withwith position Figure Figure 3a). with3 a), Figure 4—lower 3a), transition 4—lower table,transition 5—building table, 5—building platform (repeatplatform position (repeat positionwith Figure with3a). Figure 3a). The correctness of the design, execution, and functioning of the cooling circuits set was confirmed by us experimentally. In particular, at points 11–22 (Figure 5), the temperature was The correctnesscorrectnesscontrolled repeatedly of of the the design, duringdesign, execution,the execution,commissioning and functioningand wo rkfunctioning with a of manual the coolingof pyrometer the circuitscooling for metals setcircuits was (the conﬁrmed set was confirmedby us experimentally. pyrometerby us experimentally. accuracy In is particular, ±1 °C). In particular, at points 11–22at points (Figure 11–225), the(Figure temperature 5), the temperature was controlled was controlledrepeatedly repeatedly duringFigure the9 showsduring commissioning the thetime commissioning course work of temperat with woure a manual rkchanges with at pyrometera mentionedmanual pyrometer forpoints. metals Temperature (thefor metals pyrometer (the accuracy isdependence±1 ◦C). testifies to the operation reliability of all the protective cooling circuits during the SLS pyrometerprocess accuracy of the is designed±1 °C). unit. As it follows from Figure 9, pyrometric control showed that in points Figure11–19, 99 showsshows the heating the the timetemperature time course course does of nottemperat exceed of temperature theure ambient changes temperature changesat mentioned by atmore mentioned points.than 5 °C. Temperature At points. dependenceTemperaturepoints testifies dependence 20–22 toduring the testiﬁes theoperation unit to operation, the reliability operation the temp of reliability eratureall the ranges protective of all from the 40 protective cooling to 50 °C, circuits which cooling does during circuits not the during SLS processthe SLS of process exceedthe designed ofthe thepermissible designed unit. values.As unit.it followsIn the As point it followsfrom 13 setup Figure from was 9,water Figure pyrometric cooled9, pyrometric by meanscontrol of control ashowed chiller set showed that (~20 in thatpoints in °C) and therefore it had a temperature below that of the environment. Figure 10 shows the ◦ 11–19,points 11–19,the heating the heating temperature temperature does not does exceed not exceed the ambient the ambient temperature temperature by more by more than than 5 °C. 5 AtC. appearance of the unit which we designed and assembled. ◦ pointsAt points 20–22 20–22 during during the the unit unit operation, operation, the the temp temperatureerature ranges ranges from from 40 40 to to 50 50 °C,C, which which does does not ◦ exceed the permissiblepermissible values.values. InIn the the point point 13 13 setup setup was was water water cooled cooled by by means means of aof chiller a chiller set (~20set (~20C) °C)and and therefore therefore it had it a temperaturehad a temperature below that below of the th environment.at of the environment. Figure 10 shows Figure the 10 appearance shows the of appearancethe unit which of the we unit designed which and we assembled.designed and assembled.

Figure 9. Dependence between the temperature in control points 11–22, and the work time of the installation.

Figure 9. Dependence between the temperature in control points 11–22, and the work time of the Figure 9. Dependence between the temperature in control points 11–22, and the work time of installation. the installation.

Machines 2018, 6, 11 13 of 17

Machines 2018, 6, x FOR PEER REVIEW 13 of 17

Figure 10. The created installation with the removed protective covers and nodes for access to points Figure 10.11–22The createdfrom Figure installation 5 in order to with maintain the temperature removed protectivecontrol. covers and nodes for access to points 11–22 from Figure5 in order to maintain temperature control. The correctness of the installation design is indirectly confirmed by the movement accuracy The correctnessmeasured in ofnode the 4 installation(lower transition design table, isFigures indirectly 3 and conﬁrmed8), which determines by the movementthe vertical accuracy movement accuracy of node 5 (the building platform) through the system of rigid clamps. measuredMeasurements in node 4 (lower of the transitionvertical displacement table, Figures in node3 4 and were8 ),carried which out determines under stabilized the heating vertical of the movement accuracy ofinstallation node 5 (theusing building a laser interferometer platform) throughXL-80 (Renishaw, the system Wotton-under-Edge, of rigid clamps. UK, Measurementsaccuracy ±0.5 of the vertical displacementppm [47]). After in node node 41 weresetting carried (lower outtransition under table stabilized vertical heatingdrive, Figure of the 3) installation by means of using a a laser interferometerprogram XL-80 correction (Renishaw, application, Wotton-under-Edge, node 4 repeatability was UK,±8μm. accuracy The moving± accuracy0.5 ppm of [node47]). 4 was After node 1 ±5 μm. The given values of the fourth node displacement accuracy with the recoater correct setting (loweroperation transition ensure the table uniform vertical powder drive, layers Figure are deli3)vered by meansover the of entire a program working area correction of 500 × 500 application, node 4 repeatabilitymm with a thickness was ± 8ofµ 120m. ± The 10 μ movingm. accuracy of node 4 was ±5 µm. The given values of the fourth node displacementThe developed installation accuracy has with its own the software recoater open correct to modernization operation and ensure enhancement, the uniform with powder layers arethe delivered ability to over manage the a entirelarge set working of technological area of and 500 control× 500 parameters mm with (the a thicknessLI scan velocity of 120 and± 10 µm. power, hatch distance, layer thickness, scanning strategy type, delay times on scanner mirrors, The developedheater operation installation regimes, and has speed its own moving software drives, open etc.). toThis modernization is very important and for carrying enhancement, out an with the ability to manageexperimental a large study set of searching of technological for optimal and technological control parametersregimes. (the LI scan velocity and power, hatch distance,Installation layer thickness, operability scanning and correctness strategy of the type, adopted delay technical times on solution scanners are mirrors,confirmed heater by the operation regimes, andmanufactured speed moving test samples drives, shown etc.). in ThisFigure is 11. very In Figure important 11, the for3D test carrying samples out synthesized an experimental by us study are shown; they have complex forms of various air nozzles with cooling channels. The material of of searchingthe for3D optimalsamples was technological PEEK, courtesy regimes. of the manufacturer (Kabardino-Balkarian State University Installation[48,49]). operabilityThe next technological and correctness regime was of used the for adopted the successful technical fabrication solutions of parts are (Figure conﬁrmed 11): by the manufacturedthe platform test samples heaters and shown the walls in Figure of the changeable 11. In Figure frame 11 are, theset to 3D a temperature test samples of 360 synthesized °C; the by us are shown;layer they is havepreheated complex up to 385 forms °C; after of various 2 s, laser air sintering nozzles begins with (with cooling upper channels. heaters being The cooled); material of the CO2 laser radiation power was 12 W; the laser beam diameter was 300 μm; the scanning speed was 3D samples was PEEK, courtesy of the manufacturer (Kabardino-Balkarian State University [48,49]). The next technological regime was used for the successful fabrication of parts (Figure 11): the platform heaters and the walls of the changeable frame are set to a temperature of 360 ◦C; the layer is preheated ◦ up to 385 C; after 2 s, laser sintering begins (with upper heaters being cooled); CO2 laser radiation power was 12 W; the laser beam diameter was 300 µm; the scanning speed was 200 mm/s; the thickness of the delivered layer of the powder was 120 µm; and the hatching step was 250 µm. The laser scanning strategy in the single layer was organized by the following route. First, we outlined the outer contours, and then the inner part of the sample’s cross-section was hatched, usually with parallel lines, Machines 2018, 6, x FOR PEER REVIEW 14 of 17

200 mm/s; the thickness of the delivered layer of the powder was 120 μm; and the hatching step was Machines250 μm.2018 The, 6 laser, 11 scanning strategy in the single layer was organized by the following route.14 First, of 17 we outlined the outer contours, and then the inner part of the sample’s cross-section was hatched, usually with parallel lines, but sometimes with overlapping ones. On the next layer, everything was butrepeated, sometimes with a with clockwise overlapping 45 degrees ones. turn On therelative next to layer, the everythingprevious layer. was The repeated, temperature with a clockwisecontrol in 45thedegrees laser sintering turn relative zone can to the be previousrealized as layer. described The temperature earlier in [50]. control Table in 1 theshows laser the sintering comparative zone canresults be realizedof the mechanical as described properties earlier in of [50 the]. Table obta1ined shows 3D thesamples comparative and properties, results of and the the mechanical samples propertiesclaimed by of the the EOS obtained [51]. 3D samples and properties, and the samples claimed by the EOS [51].

Figure 11. Produced test 3D parts from the PEEK. Figure 11. Produced test 3D parts from the PEEK.

Table 1.Comparison of mechanical properties. Table 1. Comparison of mechanical properties. Name of Item Manufactured Samples EOS Data [51] TensileName modulus of Item ASTM D638 Manufactured 3150 ± Samples150 MPa 4250 EOS ± Data150 MPa [51] TensileTensile modulus strength ASTM ASTM D638 D638 3150 ± 70150 ± MPa8 MPa 4250 90 ± 5150 MPa MPa TensileElongation strength at ASTMbreak D638ASTM D638 70 ± 8 2.5 MPa ± 0.2% 902.8± ± 50.2% MPa Elongation at break ASTM D638 2.5 ± 0.2% 2.8 ± 0.2% It can be seen from Table 1 that the 3D part properties manufactured by the EOS company are higher,It can which be seenis due from to the Table study1 that incompleteness the 3D part properties of our approach. manufactured However, by the the EOS high company mechanical are higher,characteristics which isof duethe obtained to the study 3D parts incompleteness (the second ofcolumn our approach. of Table 1) However, indicate the the correctness high mechanical of the characteristicstechnical solutions of the discussed obtained and 3D partsreceived (the in second our presented column of installation. Table1) indicate The search the correctness for optimal of thetechnological technical solutionsregimes that discussed provide and improved received physical in our presented and mechanical installation. properties The search of 3D forparts optimal from technologicalvarious high-temperature regimes that polymers provide improvedhas not yet physical been completed. and mechanical properties of 3D parts from various high-temperature polymers has not yet been completed. 7. Conclusions 7. Conclusions In this paper, for the first time in open access, we present the original design of SLS equipment for high-temperatureIn this paper, for polymers the ﬁrst time with in an open alignment access, wecontrolling present thesystem original for depositing design of SLSpowder equipment layers. forThe high-temperature kinematic and laser-optical polymers with schemes an alignment of the inst controllingallation systemare discussed. for depositing It is shown powder that layers. for Thesuccessful kinematic realization and laser-optical of the SLS schemes process of for the high-temperature installation are discussed. polymers, It is it shown is necessary that for to successful achieve realizationmulti-circuit of heating the SLS systems process forand high-temperature to protect all the polymers, exact elements it is necessary and devices to achieve of the multi-circuit setup from heatingthermal systemseffects at and the to same protect time. all theThe exact types elements of protective and devices cooling of circuits the setup are from described, thermal and effects the atoperation the same principles time. The of types the alignment of protective control cooling system circuits for the are powder described, layer and delivering the operation are disclosed. principles It ofhas the been alignment shown that control the distortion system for degree the powder can be de layertermined delivering in an areautomatic disclosed. regime It has for been the powder shown thatlayer the device, distortion and degreeby comparison, can be determined the integral in anheating automatic for the regime powder for thelayers powder applied layer from device, the and by comparison, the integral heating for the powder layers applied from the extreme left and rightmost positions. The operation capacity of the designed unit with the PEEK polymer was shown. Machines 2018, 6, 11 15 of 17

The test 3D parts were successfully synthesized. The presented results of comparative tests of physical and mechanical properties of the manufactured parts showed their good quality. The solutions offered by us expand the business and industry opportunities involved in the development of science-intensive production from high-temperature PEEK polymers.

Acknowledgments: The study was conducted within the frameworks of government contract N14.Z56.17.21.49- MK from 22 February 2017. Author Contributions: A.N. conceived and designed the installation, he performed the experiments and wrote parts 2–6. I.Sk. prepared Introduction Section and was translator. I.Sh. analyzed the results, formulated the conclusions, was translator and proofreader, took part in writing parts 1, 2, 6, 7. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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