Micro and Nano Engineering 3 (2019) 37–43 Contents lists available at ScienceDirect Micro and Nano Engineering journal homepage: www.journals.elsevier.com/micro-and-nano-engineering Accelerated publication Replication of nanoscale surface gratings via injection molding T a b b,c d Olga Muntada-López , Jordi Pina-Estany , Carles Colominas , Jordi Fraxedas , ⁎ Francesc Pérez-Muranoa, , Andres García-Granadab a Institute of Microelectronics of Barcelona (IMB-CNM, CSIC), C/Til·lers, Campus Universitat Autònoma de Barcelona. Cerdanyola del Vallès. Barcelona, Spain b GEPI-IQS Grup Enginyeria Producte Industrial, Universitat Ramon Llull, Via Augusta, 390. 08017 Barcelona, (Spain) c Flubetech SL, Carrer Montsià, 23, 08211 Castellar del Vallès, (Spain) d Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain A R T I C LE I N FO A B S T R A C T Keywords: Nanostructured gratings fabricated on silicon chips have been successfully transferred to polypropylene plastic Plastic injection molding parts by means of injection molding. Different sets of experiments were carried out along with a repeatability Nanostructured surfaces analysis in order to study the effect in the replication of process parameters such as maximum injection pressure, Computational fluid dynamics injection time, charge and polymer temperature, geometric factors such as width and separation between lines of Heat transfer the gratings and flow direction as well as demolding conditions. Among all factors, the one with a larger effect is the separation between consecutive trenches, which was studied in detail through Computational Fluid Dynamics simulations. In addition, a previously not reported sinking effect in the nanostructured area and a shrinking of the pattern period were characterized and simulated. 1. Introduction nanometer scale features in plastic parts, a precise knowledge of the injection process at nanometer scale is still lacking. This problem is The development of manufacturing methods for the production of addressed here through the comparison of the experimental results with plastic parts with micro/nano scale features is a relevant field of re- Finite Volume Method (FVM) simulations that allow the interpretation search both in the academic and in the industry realm due to its wide of the observed phenomenology. field of applications. These include surface hydrophobicity [1], antire- Sinking and shrinking effects were observed by comparing the final flectivity [2], antimicrobial properties [3], water treatment [4], lab-on- dimensions of the plastic parts with the original dimensions of the chip devices [5], etc. mold. The result is explained by the combined effect of pressure and Injection molding is a convenient manufacturing process due to its temperature at the nanostructured areas. low costs, high replication accuracy and its extended use in industry. However, the replication of nanometer scale features is not a straight- 2. Materials and methods forward procedure because the successful replication of the mold lar- gely depends on process and geometrical parameters such as type of 2.1. Mold manufacturing polymer [6], mold wall temperature [7,8], structure size [9,10], shear thinning [11], packing conditions [12] and coatings [13] among many Fig. 1(a) and (b) show, respectively, a cross section and a picture of others. This is especially relevant for the replication of nanoscale the mold assembly used in the present work and Fig. 1(c) an injected structures with high aspect ratio, since the polymer tends to solidify plastic piece with the replica of a grating in the top part. The injected before it has reached the bottom of the nanocavities. plastic, represented in red colour in Fig. 1(a), flows from right to left in In the present work, the plastic replication of nanometer scale the figure through the ca. 4 mm nozzle advancing radially to meet the gratings is studied in detail. The effect of process parameters such as nanostructured surface of the silicon chips represented in grey colour. maximum inlet pressure, filling time, charge and polymer temperature The thickness of the resulting plastic piece is 3 mm while the surface of in the replication is explored as well as the effect of grating geometry, the silicon chip is at about 1 mm above the mold support. The diameter relative orientation of the grating with regard to the plastic flow and of the plastic piece is about 50 mm. The mold shown in Fig. 1(b) was the demolding conditions. Despite the high interest in incorporating made out of stainless steel coated with a DLC film [14]. The ⁎ Corresponding author. E-mail addresses: francesc.perez@imb-cnm.csic.es (F. Pérez-Murano), andres.garcia@iqs.edu (A. García-Granada). https://doi.org/10.1016/j.mne.2019.03.003 Received 20 December 2018; Received in revised form 14 March 2019; Accepted 19 March 2019 2590-0072/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). O. Muntada-López, et al. Micro and Nano Engineering 3 (2019) 37–43 plastic injection using a Zeiss Auriga scanning electron microscope to determine the s and w parameters, as shown in Fig. 1(c). The accurate dimensions of the trenches were determined by means of a Bruker ICON atomic force microscope (AFM) operated in tapping mode using OTESPA cantilevers. Four combinations of geometric parameters of the trench arrays were selected, with nominal values s = 500 and 1000 nm and w = 500 and 1000 nm, respectively, in order to analyze the dependence of the replication on the dimensions of the trenches (see Table 1). These va- lues were used in the simulations for the sake of clarity. It can be ob- served that the measured values are quite close to the nominal one. The trench depth dm shows some dispersion from the target 400 nm value; for trenches b and d (s = 500 nm), it is lower than for trenches a and c (s = 1000 nm), differences that can be attributed to the etching process. 2.4. Injection molding The injections were performed in a Babyplast 6/10P, a micro in- jection molding machine, with raw material SABIC© PP 621P, a PP random copolymer for cast film with a melt flow rate of 8.0 dg/min at 230 °C and 2.16 kg according to ISO 1133 standards and to the manu- facturer. Previous experiments using polycarbonate (PC) were used as starting conditions to design a set of experiments in order to analyze the Fig. 1. (a) CAD section of the mold assembly with the critical dimensions (scale influence of first and second injection pressures (Pi and Ph, respec- in mm). The injected plastic and the silicon mold are represented by red and tively), injection (ti) and holding times (th), polymer temperature (TP) grey colours, respectively. The plastic injection flows from right to left. (b) DLC- and new parameters such as the stroke of pressure piston or charge (C) coated stainless steel mold with glued silicon chips. (c) Injected red-colored [14]. The injection pressure is applied from zero to Pi during ti seconds, polypropylene part showing light diffraction in the nanostructured area. (For interpretation of the references to colour in this figure legend, the reader is with a fraction of this time at constant Pi. After ti the pressure is reduced referred to the web version of this article.) to Ph and applied during th seconds. With the aim of stabilizing and ensuring the optimal machine per- formance, tens of test injections were done before every injection set. nanostructured patterns were fabricated in silicon chips, with After that, a first injection was performed with the selected conditions 15 mm × 15 mm dimensions, and glued with an epoxy adhesive on the in order to confirm the viability of the process (complete mold filling, injection side of the mold, as can be seen in Fig. 1(b). A red-colored easy detachment from the mold, no polymer burning, etc.) and then, a polypropylene (PP) plastic piece replicating the nanometer-size pat- minimum of two additional parts were injected. In order to avoid terns is shown in Fig. 1(c). The replicated area can be easily identified possible interferences with the previous parameters the first injected by the colored squares as a consequence of the diffraction of white light part was always discarded. The injections were done on the same day in induced by the pattern structure that will be introduced below. order to minimize the influence of external factors. Additionally, a specific analysis was carried out to confirm repeatability (see 2.2. Manufacturing of the nanostructured inserts Supplemental Information I). Thermal imaging was used during the injection to monitor the temperature of the polymer and mold walls The chips were fabricated by standard micro/nano fabrication (See Supplemental Information II). The velocity of the polymer melt methods. First, a photolithography process was performed: Fujifilm ranged from 60 mm/s near the inlet to 40 mm/s at the end of nanos- OiR620-09 photoresist was spun on a standard 4-in. silicon wafer with a tructured zone where the thickness of the plastic part is reduced from final thickness of 0.6 μm, then, selected areas of the wafer were exposed 3.0 down to 2.5 mm. After > 500 injections no significant changes were with a NSR-2205-i12D i-line stepper of Nikon with a resolution of observed on the silicon patterned area of the mold. This validates the 0.35 μm and developed with a OPD-4262 developer (Fujifilm). Second, stability of silicon inserts in small batches used for the prototyping of the resist pattern was transferred into the silicon via reactive ion plastic parts. etching using an Alcatel 601E Deep Reactive Ion Etcher (RIE) with a modified Bosch process to a final depth around 400 nm. Then, the re- 3. Results and discussion maining resist on the wafer was removed by over-etching in oxygen plasma (PVA TePla system) and finally, 15 mm × 15 mm chips were cut 3.1. Influence of mold geometric parameters on the replication fidelity from the wafer. The nanostructured patterns consisted on arrays of trenches carved in silicon with different dimensions w, s and dm, as We analyze first the influence of the w and s parameters on the schematized in Fig. 2 (a) and (b), respectively. After injection molding, efficiency of the replication process when the direction of the plastic the cavities in the mold appear as raised features in the plastic part with flow is perpendicular to the direction of the lines of the gratings [see heights represented by dPP. The chips were glued on the mold in two Fig. 2(c)]. We define the fidelity factor (f) as the ratio between dPP and different configurations, namely with the lines of the gratings both dm; f = dPP/dm [see Fig. 2(b)]. The different injection conditions used, parallel and perpendicular to the direction of the plastic flow, respec- A to E, are described in Fig. 3 and the replication depth dPP of the tively. Fig. 2(c) shows a picture of a silicon chip where the gratings can trenches is determined by AFM for four combinations of w and s, 0.5 be easily identified as colored squares as well as SEM images of one and 1 μm. grating with details of the patterns at different scales. From Fig. 3 it becomes obvious that the geometrical factor with a major effect on the height of the replicated plastic nanostructures is the 2.3. AFM and SEM characterization distance between consecutive trenches (w); the smaller this parameter is, the deeper the polymer penetrates into the cavities achieving a The manufactured silicon patterns were characterized prior to the maximum fidelity factor f = 0.7. To our knowledge, this observation is 38 O. Muntada-López, et al. Micro and Nano Engineering 3 (2019) 37–43 Fig. 2. (a) Scheme of a silicon chip showing the relevant dimensions. (b) Cartoon of the cross section of the micro/nanostructures on the silicon (grey) and polypropylene (red) parts defining the dimensional parameters: w and s (horizontal) and dm and dPP (vertical). (c) Picture of a silicon chip with the patterns grating revealed by diffraction of white light and SEM images of one grating showing details of the patterns at different scales. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 Silicon chip nominal and measured values for s, w and dm parameters. Trench w nom. s nom. w real s real dm AFM dm SEM array (nm) (nm) (nm) (nm) (nm) (nm) 1000 1000 995 1020 413 428 a 1000 500 995 525 363 377 b 500 1000 500 1020 417 432 c 500 500 470 540 367 381 d Fig. 4. Model used for analyzing the impact of w in the replication of nano- cavities. The control point is located 40 nm above the mold surface. not previously reported in the literature and it deserves a detailed analysis. This effect can be explained by the fact that a smaller w value leads to a warming of the mold walls between consecutive cavities and, hence, the replicated depth increases as if the polymer was effectively injected at a higher mold temperature. Mold temperature has been re- ported as a factor with a large positive effect in the replication depth by many authors including for simulations and injections of micro- cantilever structures [15], for microcantilevers with different aspect ratio [16] and for nanocavities with a submodeling simulation ap- proach [14], among others. In order to prove this hypothesis, a 2D CAD model was created with adjustable w dimension and a temperature control point was set at the mold surface, as depicted in Fig. 4. In order to analyze the mold temperature increase the model takes into con- sideration two consecutive trenches. Fig. 3. Set of experiments for quantifying the effect of four factors in the na- The submodeling approach described in [14,17] was used in the nocavities replication (maximum inlet pressure, injection time, charge and simulations. This algorithm overcomes the computational impossibility polymer melt temperature). Legend depicts nominal values, real values are of carrying a simulation of a macroscopic part with a nanoscale mesh. found in Table 1. First, a global simulation of injection molding was performed with the Solidworks Plastics software without considering nanometer features and, second, the results of this simulation were used as boundary conditions in a second nanoscale computational fluid dynamics 39 O. Muntada-López, et al. Micro and Nano Engineering 3 (2019) 37–43 In order to investigate the precision of the tests, a repeatability analysis was performed. A total of 12 parts, 6 at 220 °C and 6 more at 270 °C were injected for the equivalent four combinations of w and s. The average standard deviation (1σ) of the height measurements was of 4.4 nm confirming that the replication process is repeatable and that the observed behavior is significant (See Supplemental Information I). Additional experiments were carried out to prove that the de- molding time did not affect the replication of the cavities either when the two sides of the mold were separated or in contact. The results can be found in the Supplemental Information III. 3.2. Analysis of the replication fidelity Fig. 7 shows cross sections of AFM images of the mold (black con- Fig. 5. Simulated results of the impact of w for s = 1 μm in the replication of nanocavities. For w < 1 μm a clear tendency can be observed: smaller w leads tinuous lines) and of the PP part (red continuous lines). The direction of to higher temperatures at the control point (red) and higher replications (blue). the plastic flow is indicated in the figure, which is perpendicular On the other hand, this tendency was not observed for w > 1 μm. Inset: [Fig. 7(a)] and parallel [Fig. 7(b)] to the direction of the lines of the Simulation of the depth reached by the polymer depending on the temperature. gratings, respectively. The injections for both directions were per- (For interpretation of the references to colour in this figure legend, the reader is formed in separate experiments using different silicon molds. The referred to the web version of this article.) nominal w and s parameters are w = 0.5 μm and s = 1 μm in Fig. 7(a) and w = 0.75 μm and s = 0.5 μm in Fig. 7(b), respectively. In both simulation performed using the ANSYS Fluent software. The simula- cases Pi and ti were set to 80 bar and 1.5 s, respectively, and the tions used the following parameters: Pi = 80 bar, ti = 1.5 s, C = 44 mm, polymer temperature to 200 and 220 °C in Fig. 7(a) and (b), respec- TP = 220 °C and mold temperature of 50 °C, and w was varied from tively, and the polymer charge 48 and 19 mm in Fig. 7(a) and (b), re- 0.01 μm up to 6 μm in order to study its effect on the replication and in spectively. the control point temperature. The results are shown in Fig. 5. In each figure the left and right part correspond to the plastic flow The simulation results are consistent with the hypothesis of larger entering and exiting the mold pattern, respectively. The scale of the replication due to the warming of the mold wall between nanotrenches. mold curve has been inverted in order to facilitate the comparison. Two Simulations showed a negligible influence of the s parameter in the salient effects are observed: (i) the PP never reaches the bottom of the replication. Indeed, the simulation shows an increase of the control trenches and the base line of the replicated region lies below the base point temperature with smaller w dimensions (right axis of Fig. 5) and line corresponding to the plastic region in contact with the flat silicon this leads to a larger replication of the nanocavity (left axis of Fig. 5). It surface, which is represented by Δh (sinking effect) and (ii) the period is also observed that there is a threshold w dimension, around 1 μm, of the replicated pattern is smaller than that corresponding to the mold above which the effect of w becomes negligible. Fig. 6 depicts the si- (shrinking effect). Both effects are studied next. mulated temperature profile once the polymer reaches the no flow temperature (NFT) for w = 1 μm [Fig. 6(a)] and w = 0.1 μm [Fig. 6(b)] 3.2.1. Sinking effect confirming that the mold between nanotrenches reaches larger tem- As mentioned above the replicated grating appears below the peratures for smaller w dimensions. The NFT was set to 60 °C in the ground level of the region without grating. Fig. 8 shows the measured simulation. heights due to sinking both at the entry (Δhentry) and exit (Δhexit) in the case of the polymer flow perpendicular to the direction of the trenches for the experimental conditions depicted in Table 1 and Fig. 3. From the figure it can be concluded that no clear difference between Δhentry and Δhexit is observed but that the values are clearly higher for smaller trench separations (w). In this case the polymer is at higher tempera- ture, as discussed in Section 3.1, and as a consequence the polymer fills more fraction of the cavity as compared to the case of larger w and, therefore the pressure increases more. In addition, at higher tempera- tures the Young's modulus of the polymers decreases becoming more easily deformed (see Discussion below). An analogous analysis for the case of the plastic flowing parallel to the trenches leads to similar results, as evidenced in Fig. 7(b). For such analysis a mold with w = 0.7 μm (nominal 0.75 μm) and s = 0.55 μm (nominal 0.5 μm) was used with the following injection parameters: Pi = 80 bar, ti = 1.5 s, C = 19 mm and TP = 220 °C. In this case we obtained a sinking depth of 60 nm and a penetration dpp of 190 nm. We attribute the sinking of the replicated pattern to the trapping of air in the cavities of the mold. The polymer completely covers the full cavity of the mold in < 0.6 s, since the velocity of the polymer melt at the end of the mold is about 40 mm/s and the radius of the mold is 25 mm [see Fig. 1(a)]. Moreover, the polymer melt flows over the si- licon mold in < 0.4 s, since the lateral dimensions are 15 mm × 15 mm and for an individual pattern (1.5 mm × 1.5 mm) this time is < 0.04 s. However, the injection time is above 1 s, so that the patterns of the Fig. 6. Temperature distribution once the polymer reaches the no flow tem- mold are already covered when the polymer is still in the phase of perature for wall width w = 1 μm (top) and w = 0.1 μm (bottom). Trench width filling the cavities. The advance of the melted polymer in the cavities is s = 1 μm in both cases. compresses the trapped air, thus increasing its pressure following the 40 O. Muntada-López, et al. Micro and Nano Engineering 3 (2019) 37–43 Fig. 7. Superposed AFM cross sections of the polymer (continuous red line) and mold (continuous black line) at the flow entry (left) and exit (right) of the pattern. (a) w = 0.5 μm and s = 1 μm (nominal), Pi = 80 bar, ti = 1.5 s, TP = 200 °C and C = 48 mm. (b) w = 0.75 μm and s = 0.5 μm (nominal), Pi = 80 bar, ti = 1.5 s, TP = 220 °C and C = 19 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web ver- sion of this article.) Boyle's law [18] and, therefore, hindering the polymer progress. applied to plastic is the summation of number of marks multiplied by Therefore, the sinking should appear regardless of the orientation, as the surface area of each mark. Note that the colour scale in the figure has been experimentally demonstrated. reflects displacements referred to unperturbed regions sufficiently away Fig. 9 shows the prediction of the sinking effect arising from si- from mold. mulations for w and s values of 0.5 and 1 μm and at two different From the simulations we can conclude that the sinking effect de- pressures of 0.2 and 0.4 MPa corresponding to trapped air in the cav- pends on the trench density, obtaining a displacement Δh of 81.8 nm for ities. The simulations considered a 2D layer of 1 μm thickness. The w = 0.5 and s = 1 μm. However, a very small influence of w on Δh is polymer Young's modulus was reduced from 800 MPa at room tem- observed, as evidenced when comparing Fig. 9 (b) and (c). The reason is perature down to 20 MPa for the simulation of air trapped at high that the reduction of the Young's modulus with temperature has to be temperature following previously reported results [19]. Air is initially considered. If the Young's modulus decreases, the polymer penetrates set to atmospheric pressure of approximately 0.1 MPa. When air is more into the trenches, increasing the effective pressure (less volume compressed to one fourth of its original volume the pressure increases and more temperature of trapped air), and so, enhancing the sinking up to 0.4 MPa. When the replication height is only reduced to half of the effect. The sinking is very sensitive to pressure, as it is shown by trench height the pressure is only 0.2 MPa. Then, the resultant force comparing Fig. 9(a) and (b) with Fig. 9(e) and (f), when the pressure of Fig. 8. Study of the sinking effect at the entry (left) and at the exit (right) of the silicon chip for different process conditions and plastic flow perpendicular to the lines of the gratings. The legend depicts nominal values. The actual values are found in Table 1. 41 O. Muntada-López, et al. Micro and Nano Engineering 3 (2019) 37–43 Fig. 9. Simulated displacements of a replicated mold using a colour code using the following parameters: (a) w = 0.5 μm, s = 1 μm, 0.4 MPa, (b) w = 0.5 μm, s = 0.5 μm, 0.4 MPa, (c) w = 1 μm, s = 1 μm, 0.4 MPa, (d) w = 1 μm, s = 0.5 μm, 0.4 MPa, (e) w = 1 μm, s = 1 μm, 0.2 MPa and (f) w = 1 μm, s = 0.5 μm, 0.2 MPa. trapped air is reduced from 0.4 MPa to 0.2 MPa. Note that at higher 1.6 · 10−4 K−1 and to 1.6 · 10−5 K−1 for the silicon chip, which is an pressures the polymer separates from the mold in the regions defined by order of magnitude smaller (the thermal expansion of steel, 2 · the w parameter [see Fig. 2(b)]. Thus, simulations of sinking effect are 10−5 K−1, is similar to that of silicon). Fig. 10(b) shows a clear decrease useful if the larger temperatures achieved on thinner walls, where a of the polymer period, with a total shrinking of 420 nm, after cooling major replication is observed, are taken into account. down to 20 °C after separation of the plastic part and mold and over- lapping with the mold as it is shown experimentally in Fig. 7. 3.2.2. Shrinking effect As pointed out when describing Fig. 7, the period of the replicated 4. Conclusions grating is smaller than that corresponding to the silicon mold and we attribute this result to the thermal contraction of the plastic part once Plastic injection is an inexpensive, fast and established technique separated from the mold. In order to explore the effect of thermal suitable for replicating micro/nanostructures at industrial level. Silicon contraction we have performed a conventional plastic injection simu- chips glued onto conventional injection molds provide a fast and simple lation to estimate the size of the volumetric shrinking in the whole method for experimental investigation of the injection process applied plastic part [see Fig. 10(a)]. From the figure we observe that the pre- to the replication of nanostructures. The replication of nanoscale grat- dicted volumetric shrinking varies from about 10% in the area close to ings has been investigated and different phenomena have been identi- the center of the plastic disc to about 5% in the radial direction. An fied that will be relevant for the future development of plastic injection additional simulation of the effect of thermal expansion considering the as a method for high volume fabrication of nanostructured surfaces. gratings was carried out and the results are shown in Fig. 10(b). The From the experimental analysis of dimensional parameters, it is initial temperature was set to 220 °C for the polymer and 50 °C for the concluded that the width of the trenches in the mold is a factor of minor mold. The thermal expansion coefficient for polypropylene was set to significance in the investigated dimensions range (between 0.5 and Fig. 10. (a) Volume shrinking simulation with conventional plastic injection simulation and (b) thermal contraction of steel mold with silicon chip on top compared to plastic part bottom to justify different period in trenches at 20 °C after cool down. 42 O. Muntada-López, et al. Micro and Nano Engineering 3 (2019) 37–43 1 μm). However, the trench separation plays a key role in the dimen- org/10.1016/j.mee.2014.03.027. sions of the features in the plastic part: thinner walls lead to a more [3] S. Kim, U.T. Jung, S.K. Kim, J.H. Lee, H.S. Choi, C.S. Kim, M.Y. Jeong, Nanostructured multifunctional surface with antireflective and antimicrobial efficient heat transfer and thus to a more accurate polymer replication. characteristics, ACS Appl. Mater. Interfaces 7 (2015) 326–331, https://doi.org/10. The trench separation effect was found to be significant for w values 1021/am506254r. below 1 μm, as corroborated by Computational Fluid Dynamics simu- [4] S. Baruah, S. K. Pal, J. Dutta, Nanostructured zinc oxide for water treatment, Nanosci. Technol. 2 (2012) 90–102, https://doi.org/10.2174/ lations. 2210681211202020090. Remarkably, a sinking in the nanostructured area of the polymer [5] H.J. Oh, J.H. Park, S.J. Lee, B. Il Kim, Y.S. Song, J.R. Youn, Sustainable fabrication part is found out, ranging from 25 to 180 nm depending on the injection of micro-structured lab-on-a-chip, Lab Chip 11 (2011) 3999–4005, https://doi.org/ 10.1039/c1lc20441f. molding conditions. This sinking effect is attributed to the combined [6] K. Monkkonen, T.T. Pakkanen, J. Hietala, E.J. Pakkonen, P. Pakkonen, T. Jskelinen, effect of the pressure effectuated by the trapped air in the pattern area T. Kaikuranta, Replication of sub-micron features using amorphous thermoplastics, and to the local lowering of polymer Young's modulus in this region. Polym. Eng. Sci. 42 (2002) 1600–1608, https://doi.org/10.1002/pen.11055. [7] S. Kuhn, A. Burr, M. Kübler, M. Deckert, C. Bleesen, Study on the replication quality In addition, a shrinking of the pattern period is observed which is of micro-structures in the injection molding process with dynamical tool tempering due to the thermal contraction of the plastic part once it is removed systems, Microsyst. Technol. 16 (2010) 1787–1801, https://doi.org/10.1007/ from the mold. Simulations predict a shrinking ranging from about s00542-010-1104-y. 4.4% to 10%. [8] S. Hattori, K. Nagato, T. Hamaguchi, M. Nakao, Rapid injection molding of high- aspect-ratio nanostructures, Microelectron. Eng. 87 (2010) 1546–1549, https://doi. Finally, no difference in the replication accuracy was observed for org/10.1016/j.mee.2009.11.028. different demolding conditions, including different cooling times and [9] C. Rytka, P.M. Kristiansen, A. Neyer, Iso- and Variothermal Injection Compression temperatures. Moulding of Polymer Micro- and Nanostructures for Optical and Medical Applications, 25 (2015), https://doi.org/10.1088/0960-1317/25/6/065008. [10] H. Schift, C. David, M. Gabriel, J. Gobrecht, L.J. Heyderman, W. Kaiser, S. Köppel, Acknowledgements L. Scandella, Nanoreplication in polymers using hot embossing and injection molding, Microelectron. Eng. 53 (2000) 171–174, https://doi.org/10.1016/S0167- 9317(00)00289-6. This work has been performed within the aim4np project [11] T.R. Tofteberg, Injection Molding of Microfeatured Polymer Components, Doctoral (Automated in-line Metrology for Nanoscale Production), supported by Dissertation by (2009), p. 118. the EC [grant number 309558] within the 7th Frame-work Program [12] M. Zhou, B. Jiang, C. Weng, L. Zhang, Experimental study on the replication quality of micro–nano cross-shaped structure arrays in injection molding, Microsyst. NMP on Nanoscale mechanical metrology for industrial processes and Technol. 23 (2017) 983–989. products. The ICN2 is funded by the CERCA Program/Generalitat de [13] M. Matschuk, H. Bruus, N.B. Larsen, Nanostructures for all-polymer microfluidic Catalunya. The ICN2 is supported by the Severo Ochoa program of the systems, Microelectron. Eng. 87 (2010) 1379–1382, https://doi.org/10.1016/j. mee.2009.11.167. Spanish Ministry of Economy, Industry and Competitiveness MINECO [14] J. Pina Estany, C. Colominas, J. Fraxedas, J. Llobet, F. Perez Murano, J.M. Puigoriol [grant number SEV-2017-0706]. Forcada, D. Ruso, A.A. García Granada, A statistical analysis of nanocavities re- plication applied to injection moulding, Int. Commun. Heat Mass Transf. 81 (2017) Appendix A. Supplementary data 131–140. [15] B. Sha, S. Dimov, C. Griffiths, M.S. Packianather, Micro-injection moulding: factors affecting the achievable aspect ratios, Int. J. Adv. Manuf. Technol. 33 (2007) Supplementary data to this article can be found online at https:// 147–156, https://doi.org/10.1007/s00170-006-0579-2. doi.org/10.1016/j.mne.2019.03.003. [16] W.W. Kim, M.G. Gang, B.K. Min, W.B. Kim, Experimental and numerical in- vestigations of cavity filling process in injection moulding for microcantilever structures, Int. J. Adv. Manuf. Technol. (2014) 293–304, https://doi.org/10.1007/ References s00170-014-6104-0. [17] T. Tofteberg, E. Andreassen, Injection moulding of microfeatured components, Proc. Polym. Process. Soc. 24th Annu. Meet. PPS-24, 2008 http://www.sintef- [1] Y.E. Yoo, T.H. Kim, D.S. Choi, S.M. Hyun, H.J. Lee, K.H. Lee, S.K. Kim, B.H. Kim, norge.com/project/mpc/Tofteberg2-PPS-2008.pdf. Y.H. Seo, H.G. Lee, J.S. Lee, Injection molding of a nanostructured plate and [18] P.a. Tipler, G. Mosca, Physics for Scientists and Engineers, (2009) measurement of its surface properties, Curr. Appl. Phys. 9 (2009) e12–e18, https:// (doi:013613923X). doi.org/10.1016/j.cap.2008.12.023. [19] C. Stern, On the Performance of Polypropylene: Between Synthesis and End-Use [2] A.B. Christiansen, J.S. Clausen, N.A. Mortensen, A. Kristensen, Injection moulding Properties, University of Twente, 2005. antireflective nanostructures, Microelectron. Eng. 121 (2014) 47–50, https://doi. 43