Recent advances in low-pressure powder injection moulding: A short review of wax-based, backbone-free systems

Low-pressure powder injection moulding (LPIM) has advanced through developments in feedstock formulation, mould-filling simulation, thermal wick-debinding, and sintering of irregular metallic powders. This review focuses specifically on the wax-based, backbone-free branch of LPIM, in which thermal wick-debinding is the distinctive binder-removal route, as this is where the most recent quantitative progress has occurred. It synthesizes these developments and identifies the gaps that define the next phase of LPIM research. Wax-based binder formulations have been developed for stainless steels, iron, titanium, and Inconel 718, with solid loadings up to 62 vol.% and viscosities below 20 Pa·s. Numerical simulation has evolved from qualitative flow-pattern matching to quantitative in-cavity pressure validation, and current models reproduce experimental pressures with errors of 30%–64% across complex geometries. Thermal wick-debinding, the distinctive debinding route for backbone-free LPIM feedstocks, has been placed on a quantitative footing through a coupled experimental and COMSOL-based study. Sintering studies of irregular water-atomized iron powders, matched to optimized debinding and sintering schedules, have achieved densities and tensile properties comparable to those obtained with spherical gas-atomized feedstocks, supporting a lower-cost route to structural LPIM components. The review concludes with five research opportunities: closing the simulation–experiment gap; systematically studying the green-density–sintered-density relationship that distinguishes LPIM from HPIM, including dimensional accuracy and shrinkage anisotropy; extending to reactive and oxidation-sensitive alloys; integrating with material extrusion additive manufacturing, including SLA-printed and sacrificial mould strategies; and developing sustainable wax recovery, biosourced binders, and reduced-energy debinding routes.