polymers Article Analysing Powder Injection Moulding of a Helix Geometry Using Soft Tooling Alberto Basso * , Yang Zhang, Jacob Kjeldahl Pløger, Jon Spangenberg and Hans Nørgaard Hansen   Citation: Basso, A.; Zhang, Y.; Kjeldahl Pløger, J.; Spangenberg, J.; Hansen, H.N. Analysing Powder Injection Moulding of a Helix Geometry Using Soft Tooling. Polymers 2021, 13, 4183. https:// doi.org/10.3390/polym13234183 Academic Editor: Andrea Sorrentino Received: 13 October 2021 Accepted: 26 November 2021 Published: 29 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Mechanical Engineering, Technical University of Denmark, 2800 Copenhagen, Denmark; yazh@mek.dtu.dk (Y.Z.); s145012@student.dtu.dk (J.K.P.); josp@mek.dtu.dk (J.S.); hnha@mek.dtu.dk (H.N.H.) * Correspondence: albass@mek.dtu.dk; Tel.:+45-45-254884 Abstract: Freeform injection moulding is a novel technology for powder injection moulding where a sacrificial 3D printed mould (i.e., a soft tooling) is used as an insert in the injection process. The use of 3D printed moulds enable a higher geometrical design flexibility as compared to the conventional injection moulding process. However, there is still very limited knowledge on how the sacrificial soft tooling material and powder suspension handles the increased geometrical complexity during the process. In this study, a stainless steel powder suspension is injected into a geometrically challenging sacrificial mould (viz. a helix structure) that is produced by vat photopolymerization additive manufacturing. Computed tomography is used to quantify the geometrical precision of the mould both before and after injection. In addition, a new numerical model that considers the suspension feedstock is developed to investigate the powder injection moulding process. The numerical results are found to be in qualitative good agreement with the experimental findings in terms of pinpointing critical areas of the structure, thereby highlighting a new pathway for evaluating sacrificial inserts for powder injection moulding with a high geometrical complexity. Keywords: powder injection moulding; additive manufacturing; soft tooling; low pressure injection moulding; simulation; freeform injection moulding 1. Introduction Powder Injection Moulding (PIM) is a well-established technology for mass manu- facturing of metal parts. Nowadays the market demands new and highly customized products, forcing the producers to continuously update the moulds in their manufacturing processes. Applications of PIM include dental implants [1,2], artificial joints [3,4], med- ical hand tools for special surgical operations [5,6] and steel cutting tools [7,8]. PIM is a profitable process for mass production, but in the case of prototyping or small batch production, the cost of the metal tools has an influential weight on the overall cost of the final parts [9]. An efficient way to reduce the manufacturing cost of PIM parts, when only a low number of units need to be produced, is to integrate Additive Manufacturing (AM) with PIM. In addition to being more affordable, the use of AM in a PIM process chain (also called soft tooling) opens new avenues for the possible geometry of the final part, which overcomes some limitation of the traditional PIM process [10]. Moulds for soft tooling in PIM are generally produced via Material Extrusion Additive Manufacturing [11] or Vat-Photopolymerization Additive Manufacturing (VPAM) [12,13]. In the case of VPAM, it is possible to manufacture sacrificial moulds with special resins that can be dissolved chemically [14] or thermally [15]. This soft tooling technique is also known as freeform injection moulding or lost form injection moulding. The advantage of freeform injection moulding is its ability to produce complex geometries like a helix structure; however, there is a lack of knowledge for powder injection moulding with such complex structures, since they are not possible to produce with conventional injection moulding. One of the challenges of using a sacrificial mould, or a soft tool in general, is that the strength of this material is lower than that of traditional tool steel [16], which hinders the possibility to use Polymers 2021, 13, 4183. https://doi.org/10.3390/polym13234183 https://www.mdpi.com/journal/polymers