Water-Removable Sacrificial Photopolymers for Indirect Additive Manufacturing: Mechanistic Frameworks, Process Coupling and Emerging Applications

Abstract

Indirect additive manufacturing addresses a structural limitation of direct three-dimensional printing: enclosed channels, undercuts and intertwined cavities can be difficult or impossible to manufacture directly with some processes. The indirect route prints a temporary tool — mould, mandrel, core, template or support — and removes it after the final material has been cast, injected or otherwise formed around it. Water-removable sacrificial photopolymers are now documented across seven application areas considered in this review: microfluidics and lab-on-chip devices, biomedical phantoms and surgical training, investment casting patterns, ceramic injection moulding, metal injection moulding, thermoplastic injection moulding, and inkjet support materials. The published literature remains dominated by application-specific demonstrations rather than a unified mechanistic treatment. This review therefore proposes an operational framework distinguishing three aqueous-removal pathways — direct molecular dissolution, swelling-dominated removal and hydrolytic disintegration. It further advances a testable hypothesis that formulation architecture and cure conditions can jointly influence the realised removal pathway. General free-radical polymerisation literature supports the plausibility of cure-dependent branching, autoacceleration, diffusion-controlled termination and changes in network heterogeneity, but the magnitude of these effects has not been quantified for the commercial sacrificial systems reviewed here. The framework is illustrated with documented commercial examples and compared with academic systems including N,N-dimethylacrylamide/4-acryloylmorpholine chemistry. Peerreviewed demonstrations are available in microfluidics, vascular phantoms, investment casting, ceramic injection moulding, polymer micro-component injection moulding and inkjet support applications; evidence is substantially more limited for metal injection moulding with vat-photopolymerised water-removable sacrificial moulds. Vat-photopolymerisation and extruded water-soluble sacrificial materials occupy complementary regions of the application space, and selection should be driven by the complete process chain. Open challenges include higher thermal resistance with retained removability, faster and better-standardised removal kinetics, four-dimensional sacrificial behaviour, bioprinting integration, biodegradable formulations and controlled characterisation of storage stability.