Manufacturing Signatures of Injection Molding and Injection Compression Molding for Micro-Structured Polymer Fresnel Lens Production

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Article Manufacturing Signatures of Injection Molding and Injection Compression Molding for Micro-Structured Polymer Fresnel Lens Production

Dario Loaldi 1,* , Danilo Quagliotti 1 , Matteo Calaon 1, Paolo Parenti 2, Massimiliano Annoni 2 and Guido Tosello 1

1 Department of Mechanical Engineering, Technical University of Denmark (DTU), 2800 Kgs. Lyngby, Denmark; [email protected] (D.Q.); [email protected] (M.C.); [email protected] (G.T.) 2 Politecnico di Milano, Department of Mechanical Engineering, 20156 Milan, Italy; [email protected] (P.P.); [email protected] (M.A.) * Correspondence: [email protected]; Tel.: +45-4525-4847

Received: 18 September 2018; Accepted: 7 December 2018; Published: 10 December 2018

Abstract: Injection compression molding (ICM) provides enhanced optical performances of molded polymer optics in terms of birefringence and transmission of light compared to Injection molding (IM). Nevertheless, ICM requires case-dedicated process optimization to ensure that the required high accuracy geometrical replication is achieved, particularly especially in the case of surface micro-features. In this study, two factorial designs of experiments (DOE) were carried out to investigate the replication capability of IM and ICM on a micro structured Fresnel lens. A laser scanning confocal microscope was employed for the quality control of the optical components. Thus, a detailed uncertainty budget was established for the dimensional measurements of the replicated Fresnel lenses, considering specifically peak-to-valley (PV) step height and the pitch of the grooves. Additional monitoring of injection pressure allowed for the definition of a manufacturing signature, namely, the process fingerprint for the evaluation of the replication fidelity under different process conditions. Moreover, considerations on the warpage of parts were related to a manufacturing signature of the molding processes. At last, the global part mass average and standard deviation were measured to correlate local geometrical replication performances with global part quality trends.

Keywords: manufacturing signature; process ﬁngerprint; Fresnel lenses; injection compression molding; injection molding; micro structures replication; confocal microscopy; optical quality control; uncertainty budget; optimization

1. Introduction Fresnel lenses are well-known optical devices with enhanced illumination properties combined with a compact and lightweight design. They are plano-convex optics, where the lens proﬁle curvature is collapsed into a series of discontinuous frusto-conical grooves of reduced thickness. For mobile communication and electronic devices, as well as automotive and medical applications, the dimensions of the grooves lie in the micrometer scale and deﬁne the Fresnel lens optical performances [1–4]. Replication technologies represent the state-of-the-art solution to enable mass-manufacturing of polymer optics. The most cost-effective replication processes implemented currently in the industry are molding-based solutions such as injection molding (IM) and injection compression molding (ICM). Even though IM and ICM are established processes, it appears that they are still not clearly understood in the literature with regard to which process conditions provide the optimal results in terms of micro-geometrical replication for complex polymer optical systems.

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Despite IM and ICM being interrelated processes, an additional phase in the operations sequence of ICM, i.e., compression, may have a substantial impact on the final product. In fact, while in IM, the polymer melt is injected in a closed mold cavity with almost the same dimensions and geometry of the final part, depending on the material shrinkage (i.e., the ones achieved when the two halves of the mold are forced against each other by the clamping force provided by the IM machine), in ICM, the melt is injected into an “open” cavity with the two mold halves initially being separated from each other. The mold is successively closed during a compression phase at the end of the operations sequence or during the injection phase [5]. The additional gap between the molds is called the compression gap, and it provides the necessary stroke to perform the compressing action. The compression gap is achieved in different ways. One of them consists of the design of molds with a so-called “vertical flash” area (see Figure1a), in which the entire mold halves are kept separated. Another option employs a “compression frame” into the mold. The frame is built with spring systems, and an additional compression plate is mounted into the movable mold side (see Figure1b). A more accurate but also more cost-intensive solution consists of the adoption of an independent “compression core” (see Figure1c). In this configuration, the insert in the movable cavity is directly actuated for cavity closure, and it performs the compression. The selection of the most suitable solution depends on cost, and partly on geometry and target accuracy. The compression action is seen as an additional holding phase that is applied to the material inside the cavity. In IM, holding starts at the so-called switch/over point, i.e., the moment when part filling is considered complete, and the machine control switch from a filling control criterion (injection velocity, screw position, injection pressure, etc.) to a holding control criterion, generally the holding pressure. Holding ensures that the final part volume is equal to the one of the cavity, since the polymer material shrinks during cooling. In IM, holding is consequently a crucial quality step; however, for parts showing long flow length and/or small wall thickness, the required holding pressure to compensate for the pressure drop can be significantly high, as are the resulting residual stresses inside the part. The major advantage of ICM consists of the opportunity to reduce stresses in the part, as compression action provides an in-thickness holding effect on the cavity, ensuring a uniform distribution of stresses inside the cavity, while the part solidifies [6–8]. For this reason, ICM has been extensively favored to IM in the production of optical components. It is proven that it provides enhanced optical functionalities by reducing the birefringence and increasing the transmission efficiency, which are both correlated to the part internal stresses distribution. Birefringence effect is an optical property, which causes preferential light multiple refractions within the optics [9–11]. At the same time, the light transmission measures the effective spectral and power transmittance of the optics, i.e., a combined result of the material absorption of specific wavelengths, reflection, and surface loss. All of these aspects are heavily dependent on the geometrical and dimensional optical design, and on local surface defects such as roundings or drafts that are natural outcomes of the molding process [12,13]. Even though ICM leads to functional benefits, the compression phase increases IM complexity, because additional parameters must be taken into consideration for optimizing the process. Suzuki et al. [14] presented the importance of increasing the compression stroke in order to improve surface replication. On the contrary, Rohde et al. [15] discouraged the increment of the compression gap, as it reduces the transcription ratio of micro-structures. These controversial results in relation to the compression gap require further consideration, and they have received attention in recent research. Masato et al. [16] observed a significant interaction between the compression gap and the injection velocity, confirming that the optimization of the compression gap must take into account the overall polymer flow conditions. The study highlighted the negative impact of a large compression gap with respect to the replication homogeneity. A similar result was achieved by Chen et al. [17], who proved that a smaller compression gap induces a more uniform part shrinkage, and in another study, Chen et al. [18] validated that a larger compression gap increases part birefringence. The different theories regarding the compression gap selection can be justified by the results obtained by Shen et al. [19]. In their work, replication and birefringence improvements were initially noticed by increasing the compression gap. Such a gain was due to a larger compression energy provided to the polymer melt that increased the shear rate and reduced the viscosity. However, the advantage arose from a delay in the compression phase, due to either a large compression gap, or a slow compression Micromachines 2018, 9, 653 3 of 21 speed (or both), that increased the material viscosity as the polymer cooled down before being compressed, generating heat dissipation with the mold and shear rate reduction. In these conditions, the polymer melt front formed a thick solid layer, so-called “skin layer” on the mold wall, limiting the melt flow more than what the compression could favor. This theory was supported by Ho et al. [20] observing an injection pressure and shear rate reduction with increased compression strokes. An additional result on compression parameters was given by Ito et al. [21], who found that the compression starting point and the compression gap are relevant factors affecting both optical performance and internal stresses. The optimal compression start was identified when the cavity injection was completed. The work of Kuo et al. [22] is one of the few studies mentioningMicromachines 2018 the, 9,importance x of the compression force. In the study, it was3 of 22 found that both

IM and ICM parameterscompression affect gap, or thea slow replication compression ofspeed micro-features (or both), that increased less when the material the compressionviscosity as the force exceeds a certain threshold.polymer Sortino cooled down et al. before [23] being verified compressed, the influence generating heat on dissipation the transcription with the mold ratio and shear of IM factors such rate reduction. In these conditions, the polymer melt front formed a thick solid layer, so-called “skin as holding pressure,layer” on injection the mold wall, velocity, limiting and the melt mold flow temperature more than what the in compression ICM. Their could study favor. demonstratedThis that the statistical effecttheory of was the supported IM factors by Ho waset al. [20] reduced observing in an ICM. injection A pressure confirmation and shear rate was reduction given with by Han et al. [24], who discovered thatincreased the compression holding pressure strokes. An could additional be result reduced on compression in ICM upparameters to 50% was with given respect by Ito et to IM, thanks to al. [21], who found that the compression starting point and the compression gap are relevant factors a more uniformaffecting cavity both pressure optical performance distribution and internal that is stresses. achieved The optimal with compression the compression start was identified phase. In the case of micro- or nano-structureswhen the cavity replicated injection bywas ICM,completed. a significant The work of effect Kuo et on al. the [22] replication is one of the isfew also studies given by the mold mentioning the importance of the compression force. In the study, it was found that both IM and ICM temperature, asparameters proved byaffect Rytka the replication et al., Nagatoof micro-features et al., less and when Chuan-Zhen the compression et force al. exceeds [25–27 a ].certain The effective local mold temperaturethreshold. also supports Sortino et al. different [23] verified replication the influence on quality the transcription conditions ratio of in IM dependence factors such as holding on the cavity design pressure, injection velocity, and mold temperature in ICM. Their study demonstrated that the statistical and the polymereffect melt of flowthe IM [28 factors–30 ].was Understanding reduced in ICM. A the confirmation effective was replication given by Han behavior et al. [24], of who surface grooves is of paramount importance,discovered that totheensure holding pressure the designed could be reduced functionality in ICM up to of 50% polymer with respect optics, to IM, suchthanks asto Fresnel lenses. a more uniform cavity pressure distribution that is achieved with the compression phase. In the case of Moreover, the complexitymicro- or nano-structures of the lenses’ replicated features by ICM, demands a significant for effect dedicated on the replicat qualityion is controlalso givencriteria. by the For example, the so-called “interferencemold temperature, by adjacent as proved by step” Rytka is et a al., Fresnel Nagato et lens al., and efficiency, Chuan-Zhen loss et al. due [25–27]. to its The stepped effective discontinuous profile [31]. In somelocal mold cases, temperature it is possible also supports to reduce different the rep efficiencylication quality loss conditions by designing in dependence total on internalthe reflection cavity design and the polymer melt flow [28–30]. Understanding the effective replication behavior of (TIR) lenses [32–surface34]. However, grooves is of molding-basedparamount importance, processes to ensure the are designed not always functionality capable of polymer of reproducing optics, the ideal design, e.g., becausesuch as of Fresnel minimum lenses. Moreover, draft angles the complexity required of the for lenses’ de-molding features demands [1]. for In dedicated addition, quality the sharp edges of control criteria. For example, the so-called “interference by adjacent step” is a Fresnel lens efficiency, the micro-steppedloss grooves due to its stepped cannot discontinuous be fully replicated, profile [31]. In producing some cases, it roundedis possible to featuresreduce the thatefficiency reduce the overall optical performanceloss by [designing35–37]. total In internal general, reflection functional (TIR) lenses optical [32–34]. testsHowever, based molding-based on photogrammetry processes are ensure the not always capable of reproducing the ideal design, e.g., because of minimum draft angles required for correct optical functionality.de-molding [1]. In Such addition, tests the are sharp robust edges of and the micro-stepped investigate grooves whether cannot optical be fully aberrationsreplicated, occur while operating the lens.producing From rounded those features results, that redu it isce possiblethe overall optical to reconstruct performance [35–37]. the lens In general, geometry functional when an optical model is available.optical Nevertheless, tests based on photogrammetry such tests do ensure not the distinguish correct optical whether functionality. aberration Such tests are occurs robust due to material and investigate whether optical aberrations occur while operating the lens. From those results, it is dependent degradationpossible to orreconstruct geometrical/dimensional the lens geometry when an imperfections optical model is available. occurring Nevertheless, in the manufacturingsuch tests process. The identificationdo ofnot manufacturing distinguish whether signatures, aberration i.e.,occurs the due link to between material adependent measurable degradation feature or of the final part geometrical/dimensional imperfections occurring in the manufacturing process. The identification of geometry and themanufacturing individual signatures, manufacturing i.e., the link processbetween a conditions,measurable feature allows of the forfinal apart comprehensive geometry and the understanding of the productionindividual steps, andmanufacturing ensures process effective conditions, andefficient allows foroptimization a comprehensive solutions understanding [38 of,39 the]. production steps, and ensures effective and efficient optimization solutions [38,39].

(a) (b) (c)

Figure 1. Different injection compression molding (ICM) mold closures before (upper picture) and during compression (lower picture), and their schematic architectures: (a) generating a “vertical ﬂash” area; (b) using a spring-connected “compression-frame”; (c) adopting an actuated compression die. Micromachines 2018, 9, 653 4 of 21

Dedicated geometrical metrology is needed for the assessment of a manufacturing signature. Tactile measuring equipment is still extensively exploited, even though they can generate scratches on the lenses surface and generally require long set-up time and suffer accuracy loss in PV measurements [40,41]. Alternatively, non-contact optical solutions such as 3D optical microscopes can also be adopted for the scope [42]. Nevertheless, the high transparency of the material prevents the possibility of using focus variation systems or contrast-based microscopes. Similar limitations of these techniques are observed when optical or near-optical surface roughness (i.e., down to single digits to tens of nanometers, respectively) is measured [43,44]. In addition, setting up a scatterometry-based inspection is challenging, as the tips and roots of the Fresnel surface severely manipulate the scattering properties of the specimen [45]. In this study, the low aspect ratio surface micro-grooves of a Fresnel lens were investigated using a confocal microscope. The microscope principle is well known for its ﬂexibility and the possibility to have both lateral and vertical resolutions in the sub-micrometer level. In this work, the identiﬁcation of different manufacturing signatures in the production of Fresnel lenses is tackled. To do so, an initial metrological procedure with a detailed uncertainty budget is proposed, to evaluate the lens surface micro-feature replication. The methodology is proposed for two different materials, providing robust applicability for the procedure. The objective of this work is to provide a comprehensive methodology for the quantitative evaluation of IM and ICM performances, based on manufacturing signatures that address micro-replication quality. The four different manufacturing signatures (micro-replication accuracy, warpage, injection pressures, and part mass) were applied, providing a methodology for the optimization of IM and ICM. These four manufacturing signatures are employed, and their respective results are compared simultaneously as drivers of the optimization process for micro-structured optical parts manufacturing. The methodology, based on a metrological approach, provides a robust guideline for the effective molding of high precision polymer optics.

2. Materials and Methods

2.1. Device under Investigation The component under investigation is the aspheric-corrected square Fresnel lens shown in Figure2. Tolerance specifications are allocated to surface groove peak-to-valley (PV), as presented in a previous study [46]. The materials employed for the experimentation were a cyclo-olefin polymer (COP) commercially available under the trade name Zeonex® E48R, produced by Zeon© company,Tokyo, Japan, and a polymethyl methacrylate (PMMA), traded under Altuglas® V825T, produced by Arkema©, Colombes, France. Their viscosities (a) and pressure-specific volume-Temperature (pvT) curves (b) are reported in Figure3. The data were collected from the Moldflow® software database, version 2018, by Autodesk® (San Rafael, CA, United States).

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(b) Aspheric properties Value Unit r, Radius of curvature 40.01 mm c, Curvature 0.0246 1/mm k, Conic constant -1.261 1 αi, Higher order terms 0 -

(a) (c)

Figure 2. Representation of the studied Fresnel lens: (a) 3D view of the flat, squared, 40 mm × 40 mm Figure 2. RepresentationFresnel of the surface studied aperture, with Fresnel a section view lens: of the axis-symmetric (a) 3D view radial profile, of which the follows flat, a squared, 40 mm × 40 mm stepped aspheric profile as described by the conic equation; (b) a highlight of the section profile in the Fresnel surface aperture, withz, r plane a with section indication view of the peak-to-valley of the axis-symmetric(PV) nominal specifications in radial comparison profile, with the which follows a stepped equivalent continuous aspheric curvature; (c) summary table of the aspheric properties of the aspheric profile as describedconsidered by the lens. conic equation; (b) a highlight of the section profile in the z, r plane with indication of the peak-to-valley (PV) nominal specifications in comparison with the equivalent continuous aspheric curvature; (c) summary table of the aspheric properties of the considered lens.

(a) Viscosity (b) pvT

Figure 3. Cyclo-olefin polymer (COP) and polymethyl methacrylate (PMMA) material properties: (a) shear rate-dependent viscosity at different temperature values; (b) temperature-dependent specific volumes at different pressure values. Data collected from the Moldflow® software database, version 2018, by Autodesk® (San Rafael, CA, USA).

2.2. Injection Molding and Injection Compression Molding Machine Experiments were performed on an injection molding machine (Negri Bossi©, Cologno Monzese, Milano, Italy) equipped with a reciprocating injection screw having a diameter of 32 mm, capable of a maximum clamping force of 600 kN. The clamping unit was equipped with the toggle clamp system represented in Figure 4a. The unit had a control on the position of the toggle with a repositioning error of 0.1 mm. The closure between the fixed and movable mold plates was measured at the two different positions of the toggle. In Figure 4c, a strong linear correlation (R2 = 99.8%) was found between the toggle positions and the mold halves measured the closure with a resulting precision on the gap between the plates corresponding to 0.01 mm. In this way, the compression gap was controlled with the same level of precision, which is of central importance since the closing action of the mold compresses the melt inside the cavity. Furthermore, the compression speed is limited by

Micromachines 2018, 9, x 5 of 22

(b) Aspheric properties Value Unit r, Radius of curvature 40.01 mm c, Curvature 0.0246 1/mm k, Conic constant -1.261 1 αi, Higher order terms 0 -

(a) (c)

Figure 2. Representation of the studied Fresnel lens: (a) 3D view of the flat, squared, 40 mm × 40 mm Fresnel surface aperture, with a section view of the axis-symmetric radial profile, which follows a stepped aspheric profile as described by the conic equation; (b) a highlight of the section profile in the z, r plane with indication of the peak-to-valley (PV) nominal specifications in comparison with the Micromachinesequivalent2018, 9, 653continuous aspheric curvature; (c) summary table of the aspheric properties of the 5 of 21 considered lens.

(a) Viscosity (b) pvT

FigureFigure 3. Cyclo-olefin 3. Cyclo-olefin polymer polymer (COP)(COP) and and polymethyl polymethyl methacrylate methacrylate (PMMA) (PMMA) material material properties: properties: (a) (a) shearshear rate-dependentrate-dependent viscosity viscosity at different at different temperature temperature values; ( values;b) temperature-dependent (b) temperature-dependent specific ® specificvolumes volumes at different at different pressure pressure values. values. Data collected Data collected from the fromMoldflow the Moldflow software database,® software version database, 2018, by Autodesk® (San Rafael, CA, USA). version 2018, by Autodesk® (San Rafael, CA, USA).

2.2. Injection2.2. Injection Molding Molding and and Injection Injection Compression Compression MoldingMolding Machine Machine ExperimentsExperiments were were performed performed on anon injection an injection molding molding machine machine (Negri (Negri Bossi ©Bossi©,, Cologno Cologno Monzese, Monzese, Milano, Italy) equipped with a reciprocating injection screw having a diameter of 32 mm, Milano, Italy) equipped with a reciprocating injection screw having a diameter of 32 mm, capable of capable of a maximum clamping force of 600 kN. The clamping unit was equipped with the toggle a maximumclamp system clamping represented force of in 600 Figure kN. The4a. The clamping unit had unit a control was equipped on the position with theof the toggle toggle clamp withsystem a representedrepositioning in Figure error4 a.of The0.1 mm. unit The had closure a control between on the the position fixed and of movable the toggle mold with plates a repositioning was measured error of 0.1at mm. the two The different closure betweenpositions theof the ﬁxed toggle. and In movable Figure 4c, mold a strong plates linear was correlation measured (R at2 = the 99.8%) two was different positionsfound of between the toggle. the toggle In Figure positions4c, a and strong the linearmold halves correlation measured (R 2the= 99.8%)closure waswith founda resulting between the toggleprecision positions on the gap and between the mold the halves plates corresponding measured the to closure 0.01 mm. with In this a resulting way, the compression precision on gap the gap betweenwas thecontrolled plates correspondingwith the same level to 0.01of precision, mm. In which this way, is of the central compression importance gap since was the controlled closing action with the sameof level the mold of precision, compresses which the melt is of inside central the importance cavity. Furthermore, since the the closing compression action of speed the mold is limited compresses by the meltMicromachines inside 2018 the, 9 cavity., x Furthermore, the compression speed is limited by the machine6 dynamics,of 22 and different average compression velocities and times for different starting compression gaps are the machine dynamics, and different average compression velocities and times for different starting reported in Figure4d. compression gaps are reported in Figure 4d.

Centering Elements Ejector

Hydraulic Brass Insert Toggle

µ-structured Nickel Insert Movable Movable Fixed Mold - Back Mold - Front Mold

(a) (b) 1.10 1.00 0.90 0.80 Compression Gap Compression Compression 0.70 0.60 time velocity 0.50 0.40 0.40 ± 0.01 mm 0.53 ± 0.01 s 0.75 ± 0.01 mm/s 0.30 0.20 y = 0.1322 x - 5.0166 mm 0.70 ± 0.01 mm 0.57 ± 0.01 s 1.12 ± 0.01 mm/s 0.10 R² = 0.998 0.00 1.00 ± 0.01 mm 0.70 ± 0.01 s 1.43 ± 0.01 mm/s Compression Gap / mm Compression 39 40 41 42 43 44 45 46 Screw axial position / mm (c) (d)

FigureFigure 4. Injection4. Injection compression compression molding molding solution: solution: (a) toggle (a) toggle clamp clamp unit with unit its with hydraulic its hydraulic circuit; (b) circuit; (b) movablemovable (left) and and fixed ﬁxed (right) (right) mold mold plates plates in open in open configuration; conﬁguration; (c) linear (c) linear regression regression of the oftoggle the toggle unitunit position position against against the the mold mold plates’ plates’ closure closureto to controlcontrol thethe compression gap; gap; (d (d) summary) summary table table of of the the compression gap, compression time, and average compression velocity. compression gap, compression time, and average compression velocity. 2.3. Metrology and Uncertainty The employed confocal instrument was a commercially available microscope with the trade name Lext OLS4000, manufactured by Olympus, Tokyo, Japan. Measurements were performed in the central location of the lens, as shown in Figure 5, using the microscope 20× standard magnification objective (measurement parameters summarized in Table 1). The lens showed a global squared dimension of 40 mm × 40 mm. The center of the lens was selected, as its near contour allowed for the evaluation of warpage and replication differences, depending on the features geometry, symmetry, and location with respect to the lens center (where the optical axis lies). Stitching of five different images was executed with a 3.712 mm × 0.640 mm sample area of the lens center. In this area, the first groove of the Fresnel lens was assessed. The groove had the lowest aspect ratio in the lens (17.3 µm/748.1 µm); nonetheless, the feature is the shortest in the lens design. The stitching overlapping factor between each image was 20% of the area of a single scan. An image size of 5328 × 913 pixels was achieved, with a resolution below the diffraction limit for the X-axis and above it for the Y one. The overall expanded uncertainty related to measurements was guided by ISO 15530-3:2011 [47] and ISO 14253-2:2007 [48]. The individual uncertainty contributors are reported in Table 2 for step height, and in Table 3 for pitch measurements, and they were calculated as described in a previous study [46]. The traceability was established through calibrated gauge blocks. A step height of (14.45 ± 0.26 µm) was obtained wringing together two gauge blocks, and it was related to the height of the lenses’ grooves, while a gauge block with a value of 1500 ± 0.08 µm was related to the pitch measurements. In this last case, the measurements were made by stitching three fields of view in the X direction, both for the measured lenses and the reference standards.

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2.3. Metrology and Uncertainty The employed confocal instrument was a commercially available microscope with the trade name Lext OLS4000, manufactured by Olympus, Tokyo, Japan. Measurements were performed in the central location of the lens, as shown in Figure5, using the microscope 20 × standard magnification objective (measurement parameters summarized in Table1). The lens showed a global squared dimension of 40 mm × 40 mm. The center of the lens was selected, as its near contour allowed for the evaluation of warpage and replication differences, depending on the features geometry, symmetry, and location with respect to the lens center (where the optical axis lies). Stitching of five different images was executed with a 3.712 mm × 0.640 mm sample area of the lens center. In this area, the first groove of the Fresnel lens was assessed. The groove had the lowest aspect ratio in the lens (17.3 µm/748.1 µm); nonetheless, the feature is the shortest in the lens design. The stitching overlapping factor between each image was 20% of the area of a single scan. An image size of Micromachines 2018, 9, x 7 of 22 5328 × 913 pixels was achieved, with a resolution below the diffraction limit for the X-axis and above it for theTable Y one. 1. Measurement The overall set-up expanded of the laser uncertainty scanning confocal related microscope to measurements for the studied wasFresnel guided lens. by ISO 15530-3:2011 [47] and ISO 14253-2:2007 [48]. The individual uncertainty contributors are reported in Objective Image Properties Table2 for step height, and in Table3 for pitch measurements, and they were calculated as described Field of view 640 × 640 µm2 Dimensions 3712 × 640 µm2 in a previous study [46]. The traceability was established through calibrated gauge blocks. A step Numerical aperture 0.6 Image size 5328 × 913 pixels2 height of (14.45XY diffraction± 0.26 µlimitm) was obtained 0.412 µm wringing together X pixel two size gauge blocks, 0.120 and µm it was related to the height ofStitching the lenses’ overlap grooves, while 20% a gauge block with Y pixel a value size of 1500 ± 0.7010.08 µµmm was related to the pitch measurements.Z spatial resolution In this last 0.030 case, µm the measurements were made by stitching three fields of view in the X direction, both for the measured lenses and the reference standards.

(a) (b)

FigureFigure 5. 3D 5.measurements 3D measurements performed performed with with the the laser laser scanning scanning confocal confocal microscope: microscope: (a ()a Measurement) Measurement set-up usingset-up the 20 ×usingobjective; the 20× (b objective;) Resulting (b) 3D Resulting view of 3D the view sampled of the image. sampled image.

TableTable 1. Measurement 2. Step height measurement set-up of the uncertainty laser scanning budget confocal for COP microscope and PMMAfor in injection the studied molding Fresnel (IM) lens. and ICM, of a nominal grooves’ height of 17.3 µm. Measurements are calibrated with a gauge step of 14.45 µm. Objective Image Properties Field of view 640 × 640 µm2 Dimensions 3712 × 640 µm2 UncertaintyNumerical Contributor aperture IM—COP 0.6 ICM—COP Image size IM—PMMA5328 × 913 ICM—PMMA pixels2 ucal,z (calibrationXY diffraction artefact) limit 0.4120.26 µµmm 0.26 X µm pixel size 0.26 µm 0.120 µm 0.26 µm up,z (instrumentStitching repeatability) overlap 0.03 20% µm 0.03 Y µm pixel size 0.03 µm 0.701 µm 0.03 µm Z spatial resolution 0.030 µm ub,z (instrument thermal) 0.00 µm 0.00 µm 0.00 µm 0.00 µm uwp,z (part repeatability) 0.13 µm 0.13 µm 0.11 µm 0.11 µm Table 2. Stepuwt,z (part height thermal) measurement uncertainty0.01 µm budget for 0.01 COP µm and PMMA 0.04 in µm injection molding 0.04 µm (IM) and ICM, of au nominalform,z (form grooves’ error) height of 17.30.17µm. µm Measurements 0.17 µm are calibrated0.03 with µm a gauge step0.05 of 14.45µm µm. k (coverage factor) 2 2 2 2 UUncertainty (exp. Uncertainty) Contributor 0.7IM—COP µm ICM—COP0.7 µm IM—PMMA0.6 µm ICM—PMMA 0.6 µm ucal,z (calibration artefact) 0.26 µm 0.26 µm 0.26 µm 0.26 µm µ µ µ µ Tableup,z (instrument3. Pitch measurements’ repeatability) uncertaint 0.03y budgetm for COP 0.03 andm PMMA in 0.03 IM andm ICM, of 0.03 a nominalm u (instrument thermal) 0.00 µm 0.00 µm 0.00 µm 0.00 µm grooves’b,z width of 748.1 µm. Measurements are calibrated with a gauge block thickness of 1500 µm. uwp,z (part repeatability) 0.13 µm 0.13 µm 0.11 µm 0.11 µm Uncertaintyuwt,z (part Contributor thermal) IM—COP 0.01 µm ICM—COP 0.01 µm IM—PMMA 0.04 µm ICM—PMMA 0.04 µm u (form error) 0.17 µm 0.17 µm 0.03 µm 0.05 µm form,z ucal,z (calibrationk (coverage artefact) factor) 0.08 µm 2 0.08 µm 2 0.08 2µm 0.08 2 µm up,z (instrumentU (exp. Uncertainty) repeatability) 1.09 0.7 µmµm 1.09 0.7 µmµm 1.09 0.6 µmµm 0.6 1.09µm µm ub,z (instrument thermal) 0.01 µm 0.01 µm 0.01 µm 0.01 µm uwp,z (part repeatability) 1.12 µm 1.12 µm 1.43 µm 1.43 µm uwt,z (part thermal) 0.45 µm 0.45 µm 0.83 µm 0.84 µm uform,z (form error) 0.99 µm 1.45 µm 0.73 µm 1.27 µm k (coverage factor) 2 2 2 2 U (exp. Uncertainty) 3.8 µm 4.4 µm 4.2 µm 4.7 µm

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Table 3. Pitch measurements’ uncertainty budget for COP and PMMA in IM and ICM, of a nominal grooves’ width of 748.1 µm. Measurements are calibrated with a gauge block thickness of 1500 µm.

Uncertainty Contributor IM—COP ICM—COP IM—PMMA ICM—PMMA

ucal,z (calibration artefact) 0.08 µm 0.08 µm 0.08 µm 0.08 µm up,z (instrument repeatability) 1.09 µm 1.09 µm 1.09 µm 1.09 µm ub,z (instrument thermal) 0.01 µm 0.01 µm 0.01 µm 0.01 µm uwp,z (part repeatability) 1.12 µm 1.12 µm 1.43 µm 1.43 µm uwt,z (part thermal) 0.45 µm 0.45 µm 0.83 µm 0.84 µm uform,z (form error) 0.99 µm 1.45 µm 0.73 µm 1.27 µm k (coverage factor) 2 2 2 2 U (exp. Uncertainty) 3.8 µm 4.4 µm 4.2 µm 4.7 µm

2.4. Design of Experiments The injected polymer volume and dosage were calibrated with preliminary injection molding short shots experiments, considering a screw injection velocity of 40 mm/s. An optimal switchover was defined for an injection screw position of 10 mm from the end stroke position. The switchover point was kept constant for both COP and PMMA in all process conditions. For all the experiments, the compression phase started after the injection, i.e., when the reciprocating screw reached the optimal switchover position of 10 mm. An initial factorial experimental campaign was performed to investigate the interaction between compression and holding phase, with the aim of analyzing which of the two process phases had a larger influence on the manufacturing signature, for the two analyzed materials: a. IM without holding pressure; b. IM with holding pressure; c. ICM without holding pressure; d. ICM with holding pressure.

Compression and post-ﬁlling holding phases were alternately switched on and off. Table4a presents the factorial design. In this case, the compression gap was kept constant at 0.7 mm. All of the other process parameters are presented in Table4b. A further optimization design was performed for the case of COP material based on the initial experimental campaign results. Aiming to validate the possibility to use the methodology as optimization tool of process parameters settings in ICM, only COP was chosen, as the two materials had different viscosity and processing requirements. Compression gaps and holding pressure levels were varied from 0.4 mm to 1.0 mm, and from 250 bar to 450 bar respectively, as shown in Table5a. The compression starting point, switchover, and injection velocity were not changed, and kept as in the previous experimentation (see Table5b). Thermal conditions such as melt and mold temperatures were not varied through the two experimental sessions. The melt temperature for the COP and PMMA materials was set respectively at 280 ◦C and 260 ◦C, being the viscosity of the material that was comparable at low shear rates for those temperature values. The mold’s temperature controller was set according to previous experimentation and the material manufacturer suggested values. The measured mold temperature was constant on both ﬁxed and movable sides during the whole experimentations, being (105 ± 3) ◦C for COP and (93 ± 3) ◦C for PMMA. The temperature of the mold was not changed, and considered an experimental constant parameter, as it strongly affects the process cycle time. Micromachines 2018, 9, 653 8 of 21

Table 4. Screening experiments investigating the effect of compression and holding phases as independent process stages of IM and ICM for PMMA and COP materials: (a) with three factors on two levels; (b) constant parameters.

(a) (b) Factors Low Level High Level Parameters Value Compression OFF ON Injection Velocity 40 mm/s Holding OFF ON Switch/over 10 mm Material PMMA COP Compression gap 0.7 mm Holding pressure 450 bar T melt COP 280 ◦C T melt PMMA 260 ◦C T mold COP 105 ± 3 ◦C T mold PMMA 93 ± 3 ◦C

Table 5. Optimization experiment investigating the effect of compression gap and holding pressure in the ICM of the COP material: (a) with two factors on two levels; (b) constant parameters.

(a) (b) Factors Low Level High Level Parameters Value Compression gap 0.4 mm 1.0 mm Injection velocity 40 mm/s Holding pressure 250 bar 450 bar Switchover 10 mm T melt COP 280 ◦C T mold COP 105 ± 3 ◦C

3.1. Absolute Dimensions Average absolute dimension measurements and uncertainties are reported in the form of interaction plots. When analyzing the step height (Figure6a), it is possible to observe that for the case of IM without holding, the use of compression provided a higher step height replication for the COP material. On the contrary, the holding phase increased the step height replication in ICM for PMMA, as shown in Figure6b. The other shown conditions are statistically equivalent, due to the relatively high measurement uncertainty with respect to the process deviations. In the case of the COP step height, the absolute deviations from nominal specifications ranged from a maximum value of (1.5 ± 0.7) µm, in the case of IM without holding, to a minimum of (0.1 ± 0.7) µm in the case of ICM. For PMMA, absolute deviations from nominal specifications are at a maximum value of (0.8 ± 0.6) µm in the case of ICM without holding, and at a minimum of (0.1 ± 0.6) µm in the case of IM. Considering the measured values, the compression improved the pitch replication for both COP and PMMA material, as shown in Figure7a,b. The holding effect was significant and increased the pitch average only for COP in ICM. When employing PMMA, an opposite effect of holding was observed, as shown in Figure7b. It reduced the average pitch in ICM with statistical significance. In this case, the reduction of pitch was attributed to the higher part of the shrinkage that the PMMA samples underwent in comparison to COP for the considered processing conditions. For all these conditions, the COP maximum absolute deviations from the target values were (13.9 ± 3.8) µm in case of IM without holding, which reduced to a minimum value of (3.3 ± 4.4) µm for ICM. Analyzing PMMA, the absolute maximum deviation from the nominal value was (11.1 ± 4.2) µm for IM, and the minimum Micromachines 2018, 9, x 9 of 22

3. Results Ten different runs were performed, and three samples were extracted, measured, and averaged for each condition. The quality control was performed on the absolute dimension measurements of the pitch and peak-to-valley (PV) step height of the Fresnel lens’ central grooves. The warpage was investigated, considering the central profile of the sampled images. Injection pressure results were analyzed separately for the filling and holding phases. Average and mass standard deviations are presented as part of global quality features. Micromachines 2018, 9, 653 9 of 21 3.1. Absolute Dimensions Average absolute dimension measurements and uncertainties are reported in the form of one (3.7 ± 4.2) µm for IM without holding. The experimental factors’ effects are plotted into the main interaction plots. When analyzing the step height (Figure 6a), it is possible to observe that for the case effect plotsof IM inwithout Figure holding,8a for the step use height, of compression and Figure provided8b for a higher pitch. step The height addition replication of compression for the COP to IM led to improvementsmaterial. On the contrary, of both the pitch holding and phase step heightincreased replications. the step height PMMA replication showed in ICM anfor averagePMMA, higher replicationas shown than in COP, Figure which 6b. The can other be justiﬁedshown conditions by its lower are statistically viscosity equivalent, at the same due processing to the relatively temperature and pressure.high measurement The holding uncertainty phase waswith responsiblerespect to the for process a higher deviations. step height In the case replication of the COP of thestep features, while theheight, pitch the of absolute the grooves deviations was from more nominal dependent specifications on the ranged material from and a maximum compression. value of (1.5 ± 0.7) µm, in the case of IM without holding, to a minimum of (0.1 ± 0.7) µm in the case of ICM.

18.25 18.25 17.95 17.95 17.65 17.65 17.35 17.35 17.05 17.05 16.75 16.75 16.45 16.45 16.15 16.15

Step Height COP / µm / COP Step Height ICM 15.85 ICM 15.85 µm / PMMA Step Height IM IM 15.55 15.55 OFF ON OFF ON Holding Holding (a) COP (b) PMMA

FigureFigure 6. Interaction 6. Interaction plots plots showing showing the the replication replication of of ab absolutesolute dimensions dimensions of the of Fresnel the Fresnel lens’ grooves lens’ grooves with a nominal step height of 17.3 µm. A measurement uncertainty is added to the results obtained withMicromachines a nominal 2018 step, 9, x height of 17.3 µm. A measurement uncertainty is added to the results obtained10 of for 22 for IM without the holding pressure, IM with holding pressure, ICM without holding pressure, and IM without the holding pressure, IM with holding pressure, ICM without holding pressure, and ICM ICM with holding pressure: (a) COP material; (b) PMMA material. withtemperature holding and pressure: pressure. (a) COP The material;holding phase (b) PMMA was resp material.onsible for a higher step height replication of the features,MicromachinesFor PMMA, while 2018 absolutethe, 9, xpitch deviations of the grooves from nominalwas more specifications dependent onare the at amaterial maximum and value compression. of10 (0.8of 22 ± 0.6) µm in the case of ICM without holding, and at a minimum of (0.1 ± 0.6) µm in the case of IM. temperature757 and pressure. The holding phase was responsible757 for a higher step height replication of Considering the measured values, the compressionICM improved the pitch replication for both COP and the features, while the pitch of the grooves was more dependent on the material and compression.ICM PMMA754 material, as shown in Figure 7a,b. TheIM holding effect754 was significant and increased theIM pitch 751 average only757 for COP in ICM. When employing PMMA, 751757an opposite effect of holding was observed, ICM ICM as shown748 in754 Figure 7b. It reduced the averageIM pitch in ICM748754 with statistical significance. InIM this case, the reduction745 751 of pitch was attributed to the higher part745751 of the shrinkage that the PMMA samples underwent742 in748 comparison to COP for the considered processing742748 conditions. For all these conditions, the COP739 maximum745 absolute deviations from the target739745 values were (13.9 ± 3.8) µm in case of IM without µm Pitch / COP 736 holding,742 which reduced to a minimum value of742 (3.3 ± 4.4) µm for ICM. Analyzing PMMA, Pitch µm / PMMA 736 739 the absolute733 maximum deviation from the nominal value739 was (11.1 ± 4.2) µm for IM, and the

Pitch COP / µm Pitch / COP 733 736 Pitch µm / PMMA 736 minimum730 one (3.7 ± 4.2) µm for IM without holding. The experimental factors’ effects are plotted into 733 730 the main effect plotsOFF in Figure 8a for stepON height, and Figure733 8b for pitch.OFF The addition of compressionON 730 Holding 730 Holding to IM led to improvementsOFF of both pitchON and step height replications.OFF PMMA showedON an average (a) COPHolding (b) PMMA higher replication than COP, which can be justifie d by its lower viscosityHolding at the same processing (a) COP (b) PMMA FigureFigure 7. Interaction 7. Interaction plots plots showing showing the the replication replication of the the absolute absolute dimensions dimensions of Fresnel of Fresnel lens’ lens’ grooves grooves withwith a nominal a Figurenominal width 7. Interaction width of 748.1of plots748.1µ showingm. µm. The The the measurement measurement replication of theun uncertainty certaintyabsolute dimensions is isadded added toof Fresnelthe to theresults lens’ results obtainedgrooves obtained for for with a nominal width of 748.1 µm. The measurement uncertainty is added to the results obtained for IM withoutIM without holding holding pressure, pressure, IM withIM with holding holding pressure, pressure, ICM ICM without without holding holding pressure, pressure, andand ICMICM with with holdingIM without pressure: holding (a pressure,) COP material; IM with ( bholding) PMMA pressure, material. ICM without holding pressure, and ICM holding pressure:with holding (a) COPpressure: material; (a) COP (material;b) PMMA (b) PMMA material. material. Compression Holding Material Compression Holding Material 18.25 Compression Holding Material 757 Compression Holding Material 18.25 757 17.95 17.95 754 17.65 17.65 751 17.35 17.35 748 17.05 17.05 745 16.75 16.75 742 Pitch / µm Pitch / 16.45 16.45 µm Pitch / 739

Step Height / µm / Height Step 16.15 736 Step Height / µm / Height Step 16.15 736 15.85 733 15.85 733 15.55 730 15.55 ON OFF OFF ON COP PMMA 730 ON OFF OFF ON COP PMMA ON OFF OFF ON COP PMMA ON OFF (aOFF) Step heightON COP PMMA (b) Pitch (a) Step height (b) Pitch Figure 8. MainFigure effects8. Main ploteffects of plot compression, of compression, holding, holding, and material material on the on average the average replication replication of the of the Figureabsolute 8. Main dimensions effects plot of aof Fresnel compression, lens’ groove holding, with a nominaland material height onof 17.3 the µmaverage and width replication 748.1 µm: of the absolute dimensions of a Fresnel lens’ groove with a nominal height of 17.3 µm and width 748.1 µm: absolute(a) dimensionseffects on step of height; a Fresnel (b) effects lens’ ongroove pitch. with a nominal height of 17.3 µm and width 748.1 µm: (a) effects(a) effects on step on step height; height; (b) (b effects) effects on on pitch. pitch. A further analysis regarding the interaction between the holding pressure and the compression Agap further was carriedanalysis out regarding in the optimization the interaction experiment betweenal campaign the holding for the pressure COP material. and the The compression related results are reported in Figure 9a for step height and Figure 9b for pitch. At a high holding pressure gap was carried out in the optimization experimental campaign for the COP material. The related (450 bar), the replication of the step height and pitch was not affected by the compression gap level. results are reported in Figure 9a for step height and Figure 9b for pitch. At a high holding pressure (450 bar), the18.25 replication of the step height and pitch was 757not affected by the compression gap level. 17.95 754

18.25 µm / 17.65 751757 17.95 17.35 748754

µm 17.05 745 / 17.65 751 17.35 16.75 742748 17.05 16.45 739 Pitch COP/ µm Pitch COP/ 745 Compression 16.15 Compression 736 16.75 Gap / mm 742 Gap / mm Step Height COP COP Height Step 15.85 0.4 733 0.4 1.0 16.45 15.55 1.0 730739 250 Compression450 µm Pitch COP/ 250 450Compression 16.15 Gap / mm 736 Gap / mm Holding Pressure / bar Holding Pressure / bar Step Height COP COP Height Step 15.85 0.4 733 0.4 1.0 1.0 15.55 (a) Step height (COP) 730 (b) Pitch (COP) 250 450 250 450

Holding Pressure / bar Holding Pressure / bar (a) Step height (COP) (b) Pitch (COP)

Micromachines 2018, 9, x 10 of 22

temperature and pressure. The holding phase was responsible for a higher step height replication of the features, while the pitch of the grooves was more dependent on the material and compression.

757 757 ICM ICM 754 IM 754 IM 751 751 748 748 745 745 742 742 739 739 Pitch COP / µm Pitch / COP 736 Pitch µm / PMMA 736 733 733 730 730 OFF ON OFF ON Holding Holding (a) COP (b) PMMA

Figure 7. Interaction plots showing the replication of the absolute dimensions of Fresnel lens’ grooves with a nominal width of 748.1 µm. The measurement uncertainty is added to the results obtained for IM without holding pressure, IM with holding pressure, ICM without holding pressure, and ICM with holding pressure: (a) COP material; (b) PMMA material.

Compression Holding Material Compression Holding Material 18.25 757 17.95 754 17.65 751 17.35 748 17.05 745 16.75 742 16.45 µm Pitch / 739

Step Height / µm / Height Step 16.15 736 15.85 733 15.55 730 ON OFF OFF ON COP PMMA ON OFF OFF ON COP PMMA (a) Step height (b) Pitch

MicromachinesFigure2018 ,8.9 ,Main 653 effects plot of compression, holding, and material on the average replication of the 10 of 21 absolute dimensions of a Fresnel lens’ groove with a nominal height of 17.3 µm and width 748.1 µm: (a) effects on step height; (b) effects on pitch. A further analysis regarding the interaction between the holding pressure and the compression A further analysis regarding the interaction between the holding pressure and the compression gap was carried out in the optimization experimental campaign for the COP material. The related gap was carried out in the optimization experimental campaign for the COP material. The related results are reported in Figure9a for step height and Figure9b for pitch. At a high holding pressure results are reported in Figure 9a for step height and Figure 9b for pitch. At a high holding pressure (450 bar),(450 bar), the replicationthe replication of theof the step step height height and and pitchpitch was not not affected affected by by the the compression compression gap gaplevel. level.

18.25 757 17.95 754 µm

/ 17.65 751 17.35 748 17.05 745 16.75 742 16.45 739

Compression µm Pitch COP/ Compression 16.15 Gap / mm 736 Gap / mm

Step Height COP COP Height Step 15.85 0.4 733 0.4 1.0 15.55 1.0 730 250 450 250 450 Holding Pressure / bar Holding Pressure / bar (a) Step height (COP) (b) Pitch (COP)

Figure 9. Interaction plots showing the replication of absolute dimensions of the Fresnel lens’ groove with a nominal height of 17.30 µm and a width of 748.10 µm. The measurement uncertainty is added for the optimized ICM on compression gaps and holding pressures levels for the COP material: (a) to the step height; (b) to the pitch.

As shown in Figure9a, the absolute deviation from the nominal step height is equal to (0.1 ± 0.7) µm for both compression gap levels. For the replicated pitch (see Figure9b), the absolute deviation was (6.2 ± 4.4) µm with a 0.4 mm compression gap, and (4.1 ± 4.4) µm with a 1.0 mm compression gap. When the holding pressure was lower (250 bar), the effect of the compression gap was higher for both pitch and step height replication. The results also show another trend, that when compression gap was kept at a low level (0.4 mm), the replication difference related to the holding pressure levels was negligible. The reduction of pitch was attributed to the lower shrinkage occurring at high holding pressure level. These combined results indicated that the compression gap and the holding pressure levels should not be considered as independent factors, and that they should be optimized simultaneously to ensure higher replication quality.

3.2. Warpage Form deviations, named as warpage, are generally associated with uneven cooling and differential shrinkage of the part, as well as deformations induced by the demolding of the part from the mold. In the present case, warpage attributed to unbalanced shrinkage was caused by different process settings, and was investigated along the orthogonal direction with respect to the main symmetry axis of the part, i.e., orthogonally to the melt flow direction (left/right). Warpage as a manufacturing signature on the produced Fresnel lenses was analyzed on the residuals from the nominal profile geometries (Figures A1–A3 in AppendixA). Residuals were filtered using a median filter, and fitted with second-order polynomials. The selection of the degree of the polynomials was made on the joint inspection of the coefficients of determination (R2) and the significant coefficients in the regressions of several-order polynomials, eventually choosing the best behavior. Warpage individual results are reported in Table6. Micromachines 2018, 9, 653 11 of 21

Table 6. Results summary of part warpage as the maximum absolute deviation in the evaluated area, and as a quadratic coefﬁcient of regression of the evaluated residuals.

Quadratic Quadratic Max Warpage Max Warpage Factors Regression Factors Regression µm µm Coefﬁcient µm/mm2 Coefﬁcient µm/mm2 IM without holding; COP 2.0 −0.6 ICM; 250 bar, 0.4 mm 1.1 0.3 IM; COP 0.7 0.2 ICM; 250 bar, 1.0 mm 2.0 0.4 ICM without holding; COP 4.3 0.9 ICM; 450 bar, 0.4 mm 1.0 0.2 ICM; COP 2.0 0.4 ICM; 450 bar, 1.0 mm 1.1 0.2 IM without holding; PMMA 1.6 0.4 IM; PMMA 1.9 0.5 ICM without holding; PMMA 106.9 39.5 ICM; PMMA 2.2 0.6

In the first analysis, for the COP material, a maximum absolute deviation of 4.3 µm was observed for the case of ICM without holding (Figure A1f); for the case of PMMA, the maximum observed deviation was 106.9 µm again for the case of ICM without holding (Figure A2f). In this last case, the form error was not the only defect associated with the part. A further analysis showed that the combined effect of warpage and of an air trap occurred in the central region of the lens. ICM without holding produced the lower form of replication and favored air traps during the part filling. In this case, a higher initial cavity volume ensured the possibility of performing compression; nonetheless, this fact resulted in a higher cavity pressure drop to produce the same filling conditions (as shown in Section 3.3. The shape of the Fresnel lens’ grooves promoted the air trap, starting from the lens’ center, Micromachineswhere the slopes 2018, 9,of x the grooves changed with respect to the melt front direction, and the air stagnation12 of 22 area was favored (Figure 10).

(a) (b)

FigureFigure 10. 10. CombinationCombination of of warpage warpage and and air air trap trap in in the the replication replication of of PMMA PMMA in in ICM ICM without without holding: holding: ((aa)) Top Top view view of of the the defect defect in in the the central central location location of of the the lens; lens; ( (bb)) schematic schematic visualization visualization of of air air trap trap formationformation inside inside the the mold mold cavi cavityty during during the the filling ﬁlling phase. phase.

WarpageWarpage waswas alsoalso inspected inspected by by observing observing the quadraticthe quadratic regression regression coefﬁcients. coefficients. Since they Since reduced they reducedwhen considering when considering IM instead IM instead of ICM, of in ICM, the ﬁrstin the experiments, first experiments, compression compression favored favored a less a stable less stableshrinkage shrinkage of the of parts. the parts. This behavior This behavior can be can recognized be recognized in Figure in Figure A1d,h A1d for COP, and Figure and Figure A1h A2ford,h COP, for andPMMA, Figure respectively. A2d and Figure A2h for PMMA, respectively. When ICM was optimized using a high holding pressure level (450 bar), for both compression gap levels (0.4 mm–1.0 mm), the quadratic regression coefficients were at a minimum value of 0.2 µm/mm2 (Figure A3f,h), indicating that warpage could be minimized in ICM with respect to IM. In the optimization case, the effect of compression gap was recognizable only for a low holding pressure (250 bar), as the coefficient increased from 0.3 µm/mm2 in the case of a low compression gap (0.4 mm) (Figure A3b), and to 0.4 µm/mm2 in the case of a long compression gap (1.0 mm) (Figure A3d). The interaction of the two factors supports the previous observations, and is in accordance with the step height results described in Section 3.1 by (Figure 9a). Finally, the case of a larger compression gap resulted in a greater pressure drop in the cavity (Section 3.3), not only reducing the absolute replication of the features, but also increasing the warpage of the parts.

3.3. Injection Pressure Injection pressure profiles over time were investigated as fingerprints of the different process settings [49]. Plastic pressure was measured in the injection chamber before the nozzle with a pressure transducer (Dynisco® Europe GmbH, Heilbronn, Germany, model MDT465C) and a sampling rate of 1 kHz. This pressure was equivalent to the pressure of the hydraulic circuit that moved the ram and was displayed in the machine control user display, and it is shown in Figure 11. The pressure was integrated over time (Pwork) on a defined time period, according to Equation (1):

Pwork = ∑i=1:n−1 · (Pi + Pi+1) · Δt · 1/2 (1) where n is the total number of sampled pressure points, Pi and Pi+1 are consecutive injection pressure values, and Δt is the pressure sampling time.

Micromachines 2018, 9, 653 12 of 21

When ICM was optimized using a high holding pressure level (450 bar), for both compression gap levels (0.4 mm–1.0 mm), the quadratic regression coefficients were at a minimum value of 0.2 µm/mm2 (Figure A3f,h), indicating that warpage could be minimized in ICM with respect to IM. In the optimization case, the effect of compression gap was recognizable only for a low holding pressure (250 bar), as the coefficient increased from 0.3 µm/mm2 in the case of a low compression gap (0.4 mm) (Figure A3b), and to 0.4 µm/mm2 in the case of a long compression gap (1.0 mm) (Figure A3d). The interaction of the two factors supports the previous observations, and is in accordance with the step heightMicromachines results 2018 described, 9, x in Section 3.1 by (Figure9a). Finally, the case of a larger13 of 22 compression gap resulted in aPwork greater was calculated pressure in two drop parts, in theone before cavity (Figure (Section 12a) and3.3 ),one not after only (Figure reducing 12b) thethe absolute replication ofswitch/over the features, point, butwhich also corresponded increasing to thean injectio warpagen time of of the0.57 parts.s. Only an initial part of the holding phase was considered, up to an injection time of 0.95 s. The results are presented only for the 3.3. Injectionoptimization Pressure experiments in ICM using the COP material. Different compression gap levels modified the shapes of the pressure curves before the switchover. During the initial part of the filling, the Injectionpressure pressure reached profiles the maximum over time value were depending investigated on the compression as fingerprints gap level. of theA longer different process compression gap (1.0 mm) induced a greater pressure drop, considering the constant injection speed. settings [49].The Plastic maximum pressure injection was pressure measured rose from in the (398.2 injection ± 3.7) bar chamber and (408.5 before ± 3.8) the bar nozzlefor the low with a pressure transducer (Dyniscocompression® gapEurope (0.4 mm), GmbH, to (444.4 Heilbronn, ± 3.4) bar and Germany,(440.7 ± 3.5) bar model for the MDT465C)high compression and gap a (1.0 sampling rate of 1 kHz. Thismm), pressure respectively, was for equivalent low (250 bar) to and the high pressure holding ofpressures the hydraulic (450 bar). After circuit this thatpoint, movedthe the ram pressure started to decrease as the cavity was further filled. In this case, a higher compression gap and was displayedalso sustained in the a greater machine pressure control drop. userThe pressure display, reached and ait value is shown at the switchover in Figure point 11 of. The(351.4 pressure was integrated over± 3.7) time bar and (P work(355.2) on± 3.8) a bar defined for the low time compression period, gap according (0.4 mm), towhich Equation further reduced (1): to (326.7 ± 3.4) bar and (321.0 ± 3.5) bar for a high compression gap in conditions of low and high holding pressures, respectively. Pwork = i=1:n−1 · (Pi + Pi+1) · ∆t · 1/2 (1) The integral of the curves over∑ time (Pwork) decreased for the low compression gap from (188.0 ± 0.1) bar·s and (187.1 ± 1.3) bar·s to (185.5 ± 1.4) bar·s and (185.2 ± 1.3) bar·s for low and high holding where n is thepressure total numberrespectively. of The sampled overall pressureintegration points,of pressure P ioverand time Pi+ 1demonstratedare consecutive that the injectionmajor pressure values, and ∆contributort is the pressure of the pressure sampling during filling time. was given by the compression gap before the switch/over point as cavity size was changed. The interaction plot in Figure 12a shows this result.

Figure 11. Injection pressure over time for a two-level compression gap (0.4–1.0 mm) and holding Figure 11. pressureInjection (250 bar–450 pressure bar). Compression over time gap for of 1.0 a mm two-level and holding compression pressure of 250 bar; gap compression (0.4–1.0 mm) and holding pressuregap of 0.4 (250 mm bar–450 and holding bar). pressure Compression of 450 bar; compression gap of gap 1.0 of mm 1.0 mm and and holding holding pressure pressure of of 250 bar; compression450 gap bar: of before 0.4 mm and after and switch/over. holding pressure of 450 bar; compression gap of 1.0 mm and holding pressure of 450 bar: before and after switch/over.

Pwork was calculated in two parts, one before (Figure 12a) and one after (Figure 12b) the switch/over point, which corresponded to an injection time of 0.57 s. Only an initial part of the holding phase was considered, up to an injection time of 0.95 s. The results are presented only for the optimization experiments in ICM using the COP material. Different compression gap levels modiﬁed the shapes of the pressure curves before the switchover. During the initial part of the ﬁlling, the pressure reached the maximum value depending on the compression gap level. A longer compression gap Micromachines 2018, 9, 653 13 of 21

(1.0 mm) induced a greater pressure drop, considering the constant injection speed. The maximum injection pressure rose from (398.2 ± 3.7) bar and (408.5 ± 3.8) bar for the low compression gap (0.4 mm), to (444.4 ± 3.4) bar and (440.7 ± 3.5) bar for the high compression gap (1.0 mm), respectively, for low (250 bar) and high holding pressures (450 bar). After this point, the pressure started to decrease as the cavity was further ﬁlled. In this case, a higher compression gap also sustained a greater pressure drop. The pressure reached a value at the switchover point of (351.4 ± 3.7) bar and (355.2 ± 3.8) bar for the low compression gap (0.4 mm), which further reduced to (326.7 ± 3.4) bar and (321.0 ± 3.5) bar for a high compression gap in conditions of low and high holding pressures, respectively. Micromachines 2018, 9, x 14 of 22

200 120 190 110 180 100 170 90 160 80 150 70 140 Compression 60 Compression Gap / mm Gap / mm 130 0.4 50 0.4 1.0 120 40 1.0 250 450 250 450 Pwork after switch/over / bar * /s bar switch/over after Pwork Pwork before switch/over / bar * s / bar switch/over before Pwork Holding Pressure / bar Holding Pressure / bar (a) (b)

FigureFigure 12. 12. InteractionInteraction plots plots of the of theintegral integral of pressure of pressure over overtime timefor the for different the different compression compression gap and gap holdingand holding pressure pressure levels: levels:(a) calculated (a) calculated on a time on interval a time intervalthat goes that from goes zero from to end zero of tofilling end of(before ﬁlling switch/over);(before switch/over); (b) calculated (b) calculated on a time on interval a time interval that goes that from goes fromthe end the endof injection of injection until until end end of of compressioncompression (after (after switch/over). switch/over).

AfterThe the integral switch/over, of the P curveswork decreased over time from (P (107.1work) decreased± 2.2) bar·s forand the (111.4 low ± 2.2) compression bar·s in the gap case from of high(188.0 holding± 0.1) pressurebar·s and to (187.1 (75.7 ± 1.7)1.3) bar·s, bar·s and to (185.5 (71.7 ± 1.5)1.4) bar·s bar·s for and the (185.2 low holding± 1.3) bar pressure·s for low level, and respectively,high holding for pressure a high and respectively. low compression The overall gap. integrationThe transient of from pressure velocity over to time pressure demonstrated control can that bethe seen major in Figure contributor 11, as ofthe the molding pressure machine during increases ﬁlling was the givenpressure by theto the compression desired holding gap before control the levelswitch/over with a certain point lag. as cavity size was changed. The interaction plot in Figure 12a shows this result. InAfter Figure the 12b, switch/over, the interactionPwork betweendecreased holding from (107.1 pressure± 2.2) and bar compression·s and (111.4 gap± levels2.2) bar showed·s in the that case theof compression high holding had pressure a contribution, to (75.7 ± 1.7) which bar ·dependeds, and (71.7 on± the1.5) pressure bar·s for thecondition low holding at the pressureswitchover, level, andrespectively, on the energy for a that high is andintroduced low compression by the compre gap.ssion The transient itself. Nonetheless, from velocity this to contribution pressure control was 10 can timesbe seen lower in Figure than the11, asone the was molding given machineby the holding increases pr theessure pressure control, to the as desiredshown holdingin the last control part level of Figurewith a11. certain As Pwork lag. could be considered as a direct indicator of the energy stored in the polymer during processingIn Figure [49]. 12Theb, themaximum interaction energy between was holdingachieved pressure when the and holding compression pressure gap was levels set showed at a high that levelthe (450 compression bar) and hadthe compression a contribution, gap which was set depended to a low level on the of pressure0.4 mm. condition at the switchover, and on the energy that is introduced by the compression itself. Nonetheless, this contribution was 3.4.10 Part times Mass lower than the one was given by the holding pressure control, as shown in the last part of Figure 11. As P could be considered as a direct indicator of the energy stored in the polymer during The parts’ masswork was measured on all the 10 specimens per process condition, including the processing [49]. The maximum energy was achieved when the holding pressure was set at a high level sprue. The interaction plots of the average part mass highlighted that a major effect on the average (450 bar) and the compression gap was set to a low level of 0.4 mm. part mass was given by the holding phase for both COP (Figure 13a) and PMMA (Figure 13b). For COP,3.4. Partin case Mass of IM, the average part mass increased from (13.463 ± 0.024) g to (14.283 ± 0.003) g when holding was performed, and from (13.493 ± 0.014) g to (14.298 ± 0.011) g in the same case for ICM. ConsideringThe parts’ PMMA, mass the was increment measured was on allfrom the (15.808 10 specimens ± 0.033) per g to process (17.168 condition, ± 0.021) g including in IM, and the from sprue. (15.803The interaction ± 0.036) g to plots (17.187 of the ± 0.050) average g for part ICM. mass highlighted that a major effect on the average part massThe was effect given of compression by the holding on phasethe average for both part COP mass (Figure was lower 13a) andthan PMMA 30 mg, which (Figure was 13b). negligible For COP, within case respect of IM, to the the effect average of the part holding mass increasedpressure. In from the (13.463case, the± standard0.024) g todeviation (14.283 was± 0.003) considered g when asholding a quantitative was performed, way to characterize and from the (13.493 process± 0.014) precision; g to (14.298 for both± the0.011) materials, g in the precision same case increased for ICM. as holding was performed. An interaction between compression and holding was observed. The lowest precision was observed when performing ICM for PMMA (Figure 14b). Meanwhile, the most favorable condition for COP was observed for IM (Figure 14a). When the process was optimized, the weight variation of ICM parts was reduced to 1 mg when a long compression gap and a low holding pressure were selected. This meant an increment in the precision of three times the previous IM result.

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Considering PMMA, the increment was from (15.808 ± 0.033) g to (17.168 ± 0.021) g in IM, and from (15.803 ± 0.036) g to (17.187 ± 0.050) g for ICM. The effect of compression on the average part mass was lower than 30 mg, which was negligible with respect to the effect of the holding pressure. In the case, the standard deviation was considered as a quantitative way to characterize the process precision; for both the materials, precision increased as holding was performed. An interaction between compression and holding was observed. The lowest precision was observed when performing ICM for PMMA (Figure 14b). Meanwhile, the most favorable Micromachines 2018, 9, x 15 of 22 Micromachinescondition for 2018 COP, 9, x was observed for IM (Figure 14a). 15 of 22 17.3 17.3 17.017.3 17.017.3 16.717.0 16.717.0 16.416.7 16.416.7 16.116.4 16.116.4 15.816.1 15.816.1 15.515.8 15.515.8 15.215.5 15.215.5 14.915.2 14.915.2 14.614.9 14.614.9 14.314.6 14.314.6 14.3 14.3 14.0 14.0 ICM Average Mass COP / g COP / Mass Average 14.0 ICM 14.0 13.7 Average Mass PMMA / g ICM Average Mass COP / g COP / Mass Average ICM 13.7 IM

13.7 IM Average Mass PMMA / g 13.4 IM 13.413.7 IM 13.4 OFF ON 13.4 OFF ON OFF ON OFF ON Holding Holding Holding Holding (a) COP (b) PMMA (a) COP (b) PMMA Figure 13. Interaction plots showing the average part mass obtained for IM without the holding Figure 13 13.. InteractionInteraction plots plots showing showing the the average average part part mass mass obtained obtained for for IM IM without the holding pressure, IM and ICM without the holding pressure, and ICM for: (a) COP material; (b) PMMA pressure, IMIM andand ICM ICM without without the the holding holding pressure, pressure, and ICMand for:ICM (a )for: COP (a material;) COP material; (b) PMMA (b) material.PMMA material. material.

54 54 54 ICM 54 48 IMICM 48 48 IM 48 42 42 42 42 36 36 36 36 30 30 30 30 24 24 24 24 18 18 18 18 12 12 12 12 6 6 ICM 6 6 IMICM 0 0 IM 0 OFF ON 0 OFF ON OFF ON OFF ON Std. Deviation of Mass COPmg / Mass of Std. Deviation Holding Holding Std. Deviation of Mass PMMA / mg PMMA Mass of Std. Deviation Std. Deviation of Mass COPmg / Mass of Std. Deviation Holding Holding (a) COP / mg PMMA Mass of Std. Deviation (b) PMMA (a) COP (b) PMMA FigureFigure 14. 14. InteractionsInteractions plots plots showing showing the the part part mass’ mass’ standard standard deviation deviation for for different different compression compression and and Figure 14. Interactions plots showing the part mass’ standard deviation for different compression and holdingholding combinations: combinations: IM IM without without the the holding holding pre pressure,ssure, IM IM with with the the holding holding pressure, pressure, ICM ICM without without holding combinations: IM without the holding pressure, IM with the holding pressure, ICM without thethe holding holding pressure, pressure, and and ICM ICM with with the the holding holding pressure: pressure: ( (aa)) COP COP material; material; ( (bb)) PMMA PMMA material. material. the holding pressure, and ICM with the holding pressure: (a) COP material; (b) PMMA material. IncreasingWhen the the process holding was pressure optimized, with the a short weight co variationmpression of gap ICM resulted parts was in a reducedmass increment to 1 mg whenfrom Increasing the holding pressure with a short compression gap resulted in a mass increment from (14.067a long compression± 0.008) g to (14.282 gap and ± a0.004) low holdingg (i.e., +1.5% pressure increase), were selected.and from This (14.088 meant ± 0.001) an increment g to (14.298 in the ± (14.067 ± 0.008) g to (14.282 ± 0.004) g (i.e., +1.5% increase), and from (14.088 ± 0.001) g to (14.298 ± 0.015)precision g (i.e., of three+1.5% times increase) the previous in the case IM result.of long compression gap, see (Figure 15a). The interaction 0.015) g (i.e., +1.5% increase) in the case of long compression gap, see (Figure 15a). The interaction betweenIncreasing the compression the holding gap pressure and the with holding a short pressure compression (Figure gap 15b) resulted was not in anegligible mass increment when between the compression gap and the holding pressure (Figure 15b) was not negligible when consideringfrom (14.067 the± mass0.008) standard g to (14.282 deviation.± 0.004) An increm g (i.e.,ent +1.5% of the increase), holding andpressure from caused (14.088 a ±reduction0.001) g considering the mass standard deviation. An increment of the holding pressure caused a reduction ofto the (14.298 mass ±standard0.015) deviation g (i.e., +1.5% when increase) the compressi in theon case gap of was long kept compression short. The opposite gap, see phenomenon (Figure 15a). of the mass standard deviation when the compression gap was kept short. The opposite phenomenon wasThe observed interaction with between the long the compression gapgap. and the holding pressure (Figure 15b) was not negligible was observed with the long compression gap. whenDuring considering compression, the mass the action standard on the deviation. polymer insi Ande increment the cavity of works the holding as an in-thickness pressure causedforce. During compression, the action on the polymer inside the cavity works as an in-thickness force. Ona reduction the other of hand, the mass during standard holding, deviation the polymer when theis pushed compression from the gap injection was kept nozzle short. Thethrough opposite the On the other hand, during holding, the polymer is pushed from the injection nozzle through the injectionphenomenon point. was The observedtwo actions with on thethe longpolymer compression are of different gap. natures, and the final filling conditions injection point. The two actions on the polymer are of different natures, and the final filling conditions depend on the combined action of the two phases. The significance of the interaction of holding and depend on the combined action of the two phases. The significance of the interaction of holding and compression was verified on both at the micro-level and at the global part level with their compression was verified on both at the micro-level and at the global part level with their corresponding quality features. corresponding quality features.

Micromachines 2018, 9, 653 15 of 21 Micromachines 2018, 9, x 16 of 22

14.5 30 14.4 Compression 27 Gap / mm mg

14.3 / 24 0.4 14.2 1.0 14.1 21 14.0 18 13.9 15 13.8 12 13.7 9 13.6 Compression 6 13.5 Gap / mm Average Mass COP / g 13.4 0.4 3 1.0 13.3 0 250 450 250 450 Holding Pressure / bar COP Mass of St. Deviation Holding Pressure / bar (a) Average part mass (b) Mass standard deviation

FigureFigure 15. 15. InteractionInteraction plots plots showing showing for for the the COP COP materi material,al, and and different different compression compression gap gap and and holding holding pressurepressure levels levels combinations: combinations: ( (aa)) average average part part mass; mass; ( (bb)) mass mass standard standard deviation. deviation.

4. ConclusionsDuring compression, the action on the polymer inside the cavity works as an in-thickness force. On the other hand, during holding, the polymer is pushed from the injection nozzle through the The evaluation of the manufacturing signature of IM and ICM for different quality features in injection point. The two actions on the polymer are of different natures, and the final filling conditions Fresnel lenses manufacturing was performed, investigating the replication of absolute dimensions, depend on the combined action of the two phases. The significance of the interaction of holding part warpage, injection pressure, and part mass in order to identify (both locally, i.e., at the optical and compression was verified on both at the micro-level and at the global part level with their micro feature level, and globally, i.e., at the part level). The most suitable quality criteria and corresponding quality features. manufacturing fingerprints for the optimization of the production have been determined and validated.4. Conclusions The methodology is based on a quantitative metrological approach, and it allows for the comparison of the performance of IM and ICM on the experimental case of micro-structured optics. As a result,The evaluation the adequate of the selection manufacturing and setting signature of process of IM parameters and ICM forwas different found, enabling quality features specified in qualityFresnel features lenses manufacturingto be achieved, wasin terms performed, of both investigatingaccuracy and theprecision. replication The main of absolute conclusions dimensions, of the researchpart warpage, can be injectionsummarized pressure, as follows: and part mass in order to identify (both locally, i.e., at the optical micro feature level, and globally, i.e., at the part level). The most suitable quality criteria and • manufacturing The replication fingerprints of absolute for the dimensions optimization in terms of the of production groove step have height been an determinedd pitch improved and validated. from TheIM methodology to ICM. A is basedhigher on replication a quantitative fidelity metrological was achieved approach, using and the it allows PMMA for thematerial. comparison The of thecompression performance phase of IM had and a larger ICM on influence the experimental on pitch values. case of An micro-structured optimal condition optics. for As ICM a result, was the adequateachieved selectionwhen a higher and setting holding of process pressure parameters and a lower was compression found, enabling gap specifiedwere selected. quality features • to beThe achieved, holding in phase terms was of both of paramount accuracy and importance precision. in The both main IM and conclusions ICM for ofthe the reduction research of can the be summarizedwarpage. asThe follows: parts’ warpage was described with second-order polynomials and it was related to differential shrinkages of the parts, due to different process conditions. The absence of the • holdingThe replication phase in of ICM absolute was dimensionsdetrimental. in It terms increased of groove the warpage step height and and favored pitch improved the formation from IMof airto ICM.traps, A as higher shown replication in the case fidelity when was proc achievedessing using PMMA. the PMMA The optimal material. condition The compression of ICM promotingphase had less a larger warpage influence occurred on pitch when values. a high An holding optimal pressure condition level for was ICM selected. was achieved when • Thea higher compression holding pressurephase led and to a a pressure lower compression cavity variation gap were over selected. time, both during and after the • fillingThe holding phase. phaseHowever, was the of paramountmain driver importance of pressure in variation both IM during and ICM the for filling the reduction phase was of the the compressionwarpage. The gap parts’ while warpage its effect was during described the with holding second-order phase was polynomials overcome and by itthe was holding related pressure.to differential Pwork was shrinkages used as ofan the indicator parts, dueof the to differentenergy transferred process conditions. to the polymer The absence part during of the processing,holding phase and in its ICM monitoring was detrimental. served as It a increasedproduction the manufacturing warpage and favored signature. the formation of air • Thetraps, holding as shown phase in was the casethe major when contributor processing to PMMA. variations The in optimal the average condition part ofmass. ICM In promoting this case, IMless and warpage ICM showed occurred similar when a process high holding precision. pressure However, level was process selected. precision, measured as a • globalThe compression part mass standard phase led deviation, to a pressure can be cavity minimized variation in the over optimized time, both ICM during case andwith after a high the compressionfilling phase. gap However, and low the holding main phase driver levels. of pressure variation during the filling phase was the compression gap while its effect during the holding phase was overcome by the holding pressure. From these conclusions, it was shown that ICM leads to advancements in terms of surface micro- P was used as an indicator of the energy transferred to the polymer part during processing, replication,work part warpage and process precision with respect to IM. However, particular attention has and its monitoring served as a production manufacturing signature. to be paid in setting the holding phase and the compression gap when carrying out the ICM process, in order to achieve high replication fidelity, as well as the required geometrical accuracy and precision.

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• The holding phase was the major contributor to variations in the average part mass. In this case, IM and ICM showed similar process precision. However, process precision, measured as a global part mass standard deviation, can be minimized in the optimized ICM case with a high compression gap and low holding phase levels.

From these conclusions, it was shown that ICM leads to advancements in terms of surface micro-replication, part warpage and process precision with respect to IM. However, particular attention has to be paid in setting the holding phase and the compression gap when carrying out the ICM process, in order to achieve high replication ﬁdelity, as well as the required geometrical accuracy and precision.

Author Contributions: D.L. and G.T. conceptualized the work; performed the investigation, and managed the visualization. D.L., D.Q., M.C., P.P., M.A. and G.T. were all involved in data curation; methodology; writing—review & editing. D.L., D.Q. and G.T. completed the formal analysis. D.L. worked on writing—original draft. D.Q., M.C., P.P., M.A. and G.T. conducted supervision. G.T. was responsible for funding acquisition and project administration. Micromachines 2018, 9, x 17 of 22 Funding: This research work was undertaken in the context of the research projects PROSURF and MADE DIGITAL. The PROSURFAuthor project Contributions: (“Surface D.L. and G.T. Specifications conceptualized the work; and pe Processrformed the investigation, Chains for and Functionalmanaged the Surfaces”, visualization. D.L., D.Q., M.C., P.P., M.A. and G.T. were all involved in data curation; methodology; writing – http://www.prosurf-project.eu/review & editing.) D.L., is fundedD.Q. and G.T. by completed the HORIZON the formal analysis. 2020 D.L. programworked on writing (Project – original ID: draft. 767589) of the European Commission. MADE DIGITAL,D.Q., M.C., P.P., ManufacturingM.A. and G.T. conducted Academysupervision. G.T. of was Denmark responsible for( fundinghttp://en.made.dk/ acquisition and project ), Work Package WP3 “Digital manufacturingadministration. processes”, is funded by Innovation Fund Denmark (https://innovationsfonden.dk/en). Funding: This research work was undertaken in the context of the research projects PROSURF and MADE Acknowledgments: TheDIGITAL. collaboration The PROSURF fromproject (“Surface Eng. Igor Specificatio Di Vorans and atProcess Automotive Chains for Functional Lighting Surfaces”, Italia S.p.A. in connection http://www.prosurf-project.eu/) is funded by the HORIZON 2020 program (Project ID: 767589) of the European with the injection andCommission. compression MADE DIGITAL, molding Manufacturing experiments Academy of is Denmark greatly (http://en.made.dk/), acknowledged. Work Package WP3 “Digital manufacturing processes”, is funded by Innovation Fund Denmark (https://innovationsfonden.dk/en). Conflicts of Interest: The authors declare no conflicts of interest. Acknowledgments: The collaboration from Eng. Igor Di Vora at Automotive Lighting Italia S.p.A. in connection with the injection and compression molding experiments is greatly acknowledged. Appendix A Conflicts of Interest: The authors declare no conflicts of interest.

Appendix A

(a) (b)

(e) (f)

Figure A1. On the left, 3D view of the lens’ central location, sampled as shown in Figure5. On the right, the residuals of the respective x proﬁles in comparison with nominal geometry, sampled in the image

center. The following process conditions are considered for the COP material results: (a,b) IM without holding; (c,d) IM; (e,f) ICM without holding; (g,h) ICM. Micromachines 2018, 9, x 18 of 22

Micromachines 2018, 9, 653 17 of 21

Micromachines 2018, 9, x 18 of 22

(g) (h)

Figure A1. On the left, 3D view of the lens’ central location, sampled as shown in Figure 5. On the (g) (h) right, the residuals of the respective x profiles in comparison with nominal geometry, sampled in the image center.Figure The A1. following On the left, process 3D view ofconditions the lens’ central are considered location, sampled for theas shown COP inmaterial Figure 5. results:On the (a,b) IM right, the residuals of the respective xFigure profiles in comparison A1. Cont with. nominal geometry, sampled in the without holding; (c,d) IM; (e,f) ICM without holding; (g,h) ICM. image center. The following process conditions are considered for the COP material results: (a,b) IM without holding; (c,d) IM; (e,f) ICM without holding; (g,h) ICM.

(a) (b)

(e) (f)

Micromachines 2018, 9, x (e) (f) 19 of 22

(g) (h)

Figure A2. On the left, 3D view of the lens’ central location, sampled as in Figure 5. On the right, the Figure A2. Onresiduals the of left, the 3Drespective view x ofprofiles, the lens’in compar centralison with location, nominal sampledgeometry sampled as in Figurein the image5. On the right, the residualscenter. of the The respective following process x proﬁles, conditions in are comparison considered for with PMMA nominal material geometryresults: (a,b) IM sampled without in the image center. The followingholding; (c,d) process IM; (e,f) ICM conditions without holding; are considered (g,h) ICM. for PMMA material results: (a,b) IM without holding; (c,d) IM; (e,f) ICM without holding; (g,h) ICM.

(a) (b)

(e) (f)

Micromachines 2018, 9, x 19 of 22

(g) (h)

Figure A2. On the left, 3D view of the lens’ central location, sampled as in Figure 5. On the right, the residuals of the respective x profiles, in comparison with nominal geometry sampled in the image Micromachinescenter.2018 The, 9, 653following process conditions are considered for PMMA material results: (a,b) IM without 18 of 21 holding; (c,d) IM; (e,f) ICM without holding; (g,h) ICM.

(a) (b)

Micromachines 2018, 9, x (e) (f) 20 of 22

(g) (h)

FigureFigure A3. A3.On On the the left, left, 3D 3D view view of the lens’ lens’ central central locati locationon sampled sampled as in asFigure in Figure 5. On the5. Onright, the the right, the residualsresiduals ofof thethe respectiverespective x x profiles proﬁles in in comparis comparisonon with with nominal nominal geometry, geometry, sampled sampled in the in image the image center.center. The The following following process process conditions conditions are are consideredconsidered for for COP COP material material results: results: (a,b ()a Compression,b) Compression gap 0.4 mm and holding pressure 250 bar; (c,d) Compression gap 1.0 mm and holding pressure 250 gap 0.4 mm and holding pressure 250 bar; (c,d) Compression gap 1.0 mm and holding pressure 250 bar bar (e,f) Compression gap 0.4 mm and holding pressure 450 bar; (g,h) Compression gap 1.0 mm and (e,f) Compression gap 0.4 mm and holding pressure 450 bar; (g,h) Compression gap 1.0 mm and holding pressure 450 bar. holding pressure 450 bar. References

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