Metal Injection Molding takes the geometric freedom of plastic injection molding and applies it to steel, stainless, titanium, and tungsten parts. The process bridges a gap that conventional powder metallurgy (press-and-sinter) and CNC machining both leave open: small, intricate metal parts produced in the tens of thousands at a price machining can't touch.

The four-stage process:

• Feedstock — Fine metal powder (typically 5–20 μm, much finer than press-and-sinter powder) is blended with a thermoplastic binder system (waxes, polymers, surfactants) at roughly 60% metal / 40% binder by volume. The mix is granulated into pellets.
• Injection molding — Pellets are melted and injected into a mold cavity at 150–200°C and 1,000+ bar, just like plastic. The "green part" looks like the final geometry but is ~20% oversized to account for shrinkage.
• Debinding — The binder is removed via solvent bath, catalytic decomposition (nitric acid vapor for POM-based binders), or thermal evaporation. The "brown part" is fragile and porous — about as strong as a sandcastle.
• Sintering — Heated to 1,200–1,400°C (just below the metal's melting point) in a controlled atmosphere. Particles diffuse into each other, the part shrinks ~15–20% linearly, and density reaches 96–99% of wrought material.

Shrinkage rule of thumb: Linear shrinkage from green to sintered is typically 17–20%. So for a final part dimension of 10.0 mm, the mold cavity must be sized to ~12.2 mm (10.0 / 0.82). Tooling designers reference a "Z-factor" — the mold-to-sintered ratio — and shrinkage anisotropy means parts often need iterative tooling tweaks for tight tolerances.

Real-world example: The trigger components and hammer in many polymer-framed pistols (SIG, Glock aftermarket, S&W) are MIM. They have undercuts, internal features, and crisp text/logos that would require 5-axis machining or EDM to produce conventionally — at maybe $40/part. MIM delivers them at $2–4 each at volume. Surgical instrument jaws, orthodontic brackets, watch cases, and smartphone hinge components are also overwhelmingly MIM.

Where MIM wins and loses: Sweet spot is parts under 100 g with complex geometry at annual volumes above ~10,000. Below that, tooling cost (often $30,000–$100,000) doesn't amortize. Wall thickness should be 0.3–10 mm — thicker sections trap binder and crack during debinding. Tolerances of ±0.3% are typical; tighter requires post-sinter machining or coining.

MIM parts are not "powder metal parts" in the porous, oil-impregnated sense — they're nearly fully dense, with mechanical properties matching wrought equivalents within 5–10%.