Postmolding Shrinkage Evaluation of 316L Feedstock Micromolded Parts D. Annicchiarico,* ,† U. M. Attia,* ,‡ and J. R. Alcock* ,§ † Weber Saint Gobain Ltd., Dickens House, Enterprise Way, Flitwick, Bedford MK45 5BY, United Kingdom ‡ Manufacturing Technology Centre Ltd., Ansty Park, Pilot Way, Coventry CV7 9JU, United Kingdom § Cranfield University, Building 61, Wharley End, Cranfield, Bedfordshire MK43 0AL, United Kingdom ABSTRACT: The purpose of this paper was to evaluate the shrinkage behavior of a 316L molding feedstock. The methodology adopted a statistical approach (design of experiment) and a standard microshrinkage measurement approach. The statistical approach identified the mold temperatureparallel to the flow directionand the combined effect of the holding and injection pressurenormal to the flow directionas critical factors. In comparison with the polymer on which the feedstock was based, lower shrinkage values and fewer critical factors were observed. In conclusion, the lower shrinkage values were a consequence of the powder loading. The critical factors identified in the present work have found confirmation in the literature, except the absence of melt temperature between feedstock critical factors. 1. INTRODUCTION Powder injection molding (PIM) is a near-net-shape manufactur- ing technique for ceramic or metals. 1 It typically consists of four steps: feedstock (binder mixed with powder) formulation, molding, debinding, and sintering. 2 The potential of net-shape manufacturingin particular at the microscale (μ-PIM) 3 is key to the future development of several technologies, notably the nonsilicon microelectromechanical system. 4 The μ-PIM process still faces several technical challenges 3 such as the mechanical stability of the microstructures (especially those with high aspect ratio) and the accuracy and reproducibility of final dimensions. Both of these aspects are affected by shrinkage. 5 Total shrinkage in μ-PIM components can be thought of as comprised of three parts: postmolding shrinkage, postdebinding shrinkage, and postsintering shrink- age. However, in comparison to the microshrinkage of nonfilled polymers, postmolding shrinkage has been little studied in the μ-PIM literature, and it is likely that this behavior will be different from that observed for macroscale PIM. 6 Because shrinkage will form a part of the overall replication quality at the microscale, 7 some indications with regard to factors affecting shrinkage may be taken from microscale replication studies. Tay et al. 8 considered the replication quality of micropillar arrays molded using 316L powder mixed with a low-density polyethylene binder base: the mold temperature, melt temper- ature, and packing pressure were shown to affect the replication quality. Fu et al. 9 investigated the influence of the same processing parameters as those adopted in the present paper on micropillar molded parts by injecting a 316L feedstock. The study was not performed using a statistical approach, but the final results showed that high pressures (injection and holding) and molding temperature values improved and affected the filling of microcavities. The injection pressure and mold temperature also were considered to be critical parameters by Huang and Chiu 10 and Tay et al., 8 respectively. Feedstocks with different percentages of nanoceramic powders were used by Huang and Chiu 10 for filling square geometries. The work did not adopt a statistical approach, but the results showed that shrinkage can be signifi- cantly reduced by increasing the powder loading, and the injection pressure and mold temperature had to be increased to improve the cavity filling. The 316L molded shrinkage was compared with respect to its pure binder shrinkage [poly(oxymethylene) (POM)]. 11 The comparison between 316L feedstock and POM was performed in terms of different shrinkage values as a consequence of the powder loading and in terms of different critical factors. Previous studies investigated the powder loading effect on the morphology 12 and thermal properties: 13,14 the latter aspect is important also because of the influence the crystalline arrange- ment of molded parts. 15 With regard to the shrinkage value, there is little prior work in the literature to compare with this trend. However, Huang and Chiu 10 demonstrated that high powder loading has been shown to lead to lower shrinkage at the microscale (although without any estimate of statistical significance). The comparison between 316L feedstock and POM shrinkage considered also the processing parameters: mold temperature, holding pressure, injection pressure, holding time, and melt temperature. Previous studies demonstrated that the mold temperature is a critical factor for pure polymers 16,17 and for feedstocks. 8,10 As a trend, an increase of the mold temperature leads to a decrease in shrinkage. The holding pressure is a critical factor both for POM 11,18 and for feedstock: 8 as a general trend, high holding values lead to reduced shrinkage. Received: November 26, 2013 Revised: September 21, 2014 Accepted: September 22, 2014 Published: September 22, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 16559 dx.doi.org/10.1021/ie4040048 | Ind. Eng. Chem. Res. 2014, 53, 16559-16567