Introduction
Metal injection molding rewards the engineer who understands its constraints as thoroughly as its capabilities. The process offers geometric freedom, material density, and production economics that few alternatives can match within its optimal window. But that window has boundaries, and the programmes that run into difficulty are almost always those that treated the boundaries as negotiable rather than as fixed conditions of the process physics. Design rules in metal injection moulding are not arbitrary preferences. They are the accumulated product of understanding what happens to a metal powder and binder system as it flows through a mould, loses its binder through debinding, and densifies under sintering temperatures approaching the melting point of the alloy. Knowing those rules, and knowing where the process reaches its production limits, is the foundation of a programme that delivers what it promises.
Wall Thickness: The Most Consequential Design Variable
Wall thickness governs more downstream outcomes in metal injection moulding than any other single design parameter. Sections that are too thin risk incomplete cavity fill during injection as the feedstock cools and loses flow before packing pressure is fully transmitted. Sections that are too thick create extended binder diffusion paths during debinding, raising the risk of residual binder, internal cracking, and carbon contamination in thermally processed parts.
The practical working range for wall thickness in most metal injection moulding feedstock systems falls between 0.5 and 10 millimetres, though specialist processes can extend modestly beyond these bounds. More consequential than absolute thickness is thickness uniformity. Abrupt transitions between thick and thin sections generate differential shrinkage gradients during sintering that produce internal stress, warpage, and in severe cases, fracture. Gradual tapering between sections of different thickness, designed in from the outset, eliminates much of this risk without adding production cost.
Draft Angles, Corners, and Feature Geometry
Draft angles are as important in metal injection moulding as in any injection moulding process. A minimum of one degree on vertical walls facilitates clean part ejection and reduces surface damage during demoulding. For textured or detailed surfaces, the required draft increases accordingly. Overlooking draft requirements is among the most common errors in first-time metal injection moulded component designs.
Sharp internal corners concentrate stress during sintering in ways that blunt corners do not. A minimum internal radius of 0.5 millimetres is a widely applied guideline, though the appropriate value depends on wall thickness, alloy, and the stress state generated by the specific geometry during densification. External corners carry less risk but benefit from modest radii that reduce tool wear and improve feedstock flow through the cavity.
Holes and internal channels are achievable in metal injection moulding but carry their own geometric constraints. Core pins forming internal features must withstand injection pressure without deflection. As core diameter decreases, the risk of deflection and dimensional inconsistency increases. Internal diameters below approximately 0.8 millimetres become difficult to achieve reliably without specialist tooling and tightly controlled process parameters.
Shrinkage: Anticipating and Compensating
Sintering shrinkage in metal injection moulding typically falls between 15 and 20 percent in linear dimensions, depending on powder loading, binder system, and sintering conditions. This shrinkage must be precisely characterised and compensated for in mould design. A target sintered dimension of 10 millimetres with 17 percent linear shrinkage requires a mould cavity machined to approximately 12.05 millimetres. Small errors in shrinkage prediction translate directly into dimensional non-conformance.
The more demanding challenge is anisotropic shrinkage, where the component contracts at different rates in different directions due to powder particle alignment during injection. Anisotropy is influenced by gate location, flow path geometry, and feedstock rheology, and cannot be fully eliminated through mould design alone. Singapore’s metal injection moulding sector addresses this through systematic shrinkage characterisation using coordinate measurement data from first-article builds, applying empirical corrections to mould dimensions before committing to production tooling.
Production Limitations
Metal injection moulding is not a universal solution, and its production limitations are as important to understand as its capabilities.
Component size is the most clearly defined constraint. The process is most economically and technically suited to components in the one gram to one hundred gram range. Larger components introduce debinding challenges, extended sintering cycle times, and distortion risks that erode the near-net-shape economics that make Metal injection moulding attractive.
Tolerance capability after sintering is inherently less precise than machining. Dimensional tolerances of approximately 0.3 to 0.5 percent of nominal dimension are achievable under well-controlled conditions. Features requiring tighter tolerances, typically below 0.1 percent of nominal, will require post-sintering machining, which adds cost and partially offsets the near-net-shape advantage.
Material options, while broad, are constrained by sintering compatibility. Alloys requiring sintering temperatures or atmospheres that are difficult to achieve in industrial furnace systems, or that exhibit problematic phase transformations during thermal processing, are not well suited to metal injection moulding. Aluminium alloys, for instance, present significant processing challenges that have limited their adoption relative to steels, titanium, and cobalt-chrome systems.
Production volume requirements impose an economic floor. The upfront tooling investment in metal injection moulding is substantial, and the process validation required for regulated industries adds further fixed cost before the first conforming part is shipped. Annual volumes below several thousand parts rarely justify that investment for components of moderate complexity, making machining or alternative forming processes more rational choices at low volumes.
Conclusion
The design rules and production limitations of metal injection molding define a specific and well-characterised process window. Components that fall within it, combining geometric complexity, structural alloy requirements, and volume production, are natural candidates for a process that has demonstrated its capability across medical device, aerospace, firearms, and electronics applications over several decades. Those that fall outside it are better served elsewhere, and recognising that boundary clearly, rather than discovering it through first-article failures or production yield problems, is precisely the kind of engineering discipline that Metal injection molding demands and rewards.
