Metal Injection Molding Process Guide: How MIM Makes Small Complex Metal Parts

# Metal Injection Molding Process Guide: How MIM Makes Small Complex Metal Parts

Metal injection molding sits in an interesting spot in manufacturing: it borrows the speed and shape freedom of plastic injection molding, then fires the plastic out and sinters what remains into a dense metal part.

For engineers and buyers who know conventional powder metallurgy or CNC machining, MIM can look like magic — or like an expensive extra step. Both impressions have some truth.

This guide walks through the full metal injection molding process step by step, compares MIM with conventional PM and machining, and gives you a practical checklist to decide whether MIM fits your part.

If you are comparing MIM with conventional press-and-sinter PM directly, see our detailed PM vs MIM comparison guide.

What Metal Injection Molding Is (In One Paragraph)

MIM starts by mixing fine metal powder — typically under 20 µm particle size — with a multi-component thermoplastic binder to form a pelletized feedstock. The feedstock is heated and injected into a mold cavity under moderate pressure, like a plastic part. The molded "green" part then goes through a two-stage furnace cycle: first, debinding removes most of the binder, and second, sintering at temperatures near the metal's melting point (often 1,200–1,350°C) bonds the particles and shrinks the part to 96–99% of theoretical density.

MIM Process: The Four Stages

Stage 1: Feedstock Preparation

The starting point is not dry powder in a hopper — it is a precise mixture:

• 60–65% by volume metal powder, typically gas- or water-atomized to under 20 µm
• 35–40% by volume thermoplastic binder, usually a multi-component system of wax, polyethylene, and stearic acid
• Mixed in a heated sigma-blade or twin-screw mixer, then pelletized for injection molding machines

Powder loading percentage is critical: too little metal and the part sags or distorts during sintering; too much and the feedstock won't flow. Most commercial MIM feedstocks target 60% ±2% by volume.

Stage 2: Injection Molding

The feedstock is fed into a standard reciprocating-screw injection molding machine running a heated barrel at roughly 150–220°C. Key differences from plastic injection:

• Mold design must factor in 15–20% linear shrinkage from green to sintered part
• Gates, runners, and weld-line placement affect not just fill but also local powder density — which translates to local shrinkage variation and potential distortion
• Multi-cavity tools can produce dozens of parts per shot, but cavity balancing is more demanding because density gradients in the green part carry through to the final dimensions

Stage 3: Debinding

This is the MIM-specific step that has no analog in conventional PM or plastic molding. The binder must be removed without disrupting the fragile powder skeleton.

Typical two-stage debinding:

1. Solvent debinding — the green part is immersed in a solvent bath (often hexane, acetone, or water depending on binder chemistry) that dissolves one binder component, creating interconnected pore channels.
2. Thermal debinding — the part is heated slowly (often 1–3°C/minute) to roughly 300–600°C, burning out or evaporating the remaining binder. Gas flow and temperature uniformity are critical to prevent cracking, blistering, or carbon residue.

The practical thickness limit. Debinding time scales with diffusion distance squared. A 3 mm wall section may debind cleanly in a few hours; a 10 mm section can take a day or more. This is the fundamental reason MIM is reserved for relatively small, thin-walled parts — typically under 100 g and under 10 mm cross-section.

Stage 4: Sintering

The debound "brown" part enters a continuous or batch sintering furnace at temperatures near the alloy's melting range — typically 1,200–1,350°C for stainless and low-alloy steels. Sintering simultaneously:

• Bonds metal particles through neck growth and grain boundary diffusion
• Shrinks the part 15–20% linearly (volume shrinks ~35–50%)
• Eliminates remaining porosity, reaching 96–99% of theoretical density
• Sets the final microstructure, carbon content, and mechanical properties

The atmosphere — often hydrogen, nitrogen-hydrogen mixes, vacuum, or argon — must match the alloy. For stainless steels, hydrogen or vacuum prevents oxidation and can help reduce surface oxides.

Shrinkage is isotropic when powder loading, molding density, and sintering conditions are uniform — but in practice, non-uniform sections, density gradients from injection, and gravitational sag on long spans can distort the part.

MIM vs Conventional PM vs CNC Machining

When MIM Wins

MIM is the right answer when a design hits several of these at once:

• Small part (under 100 g, often under 20 g)
• Complex 3D geometry with features in multiple axes — cross-drilled holes, internal threads, undercuts
• High density required — for strength, fatigue, or pressure-tightness that conventional PM cannot reach
• Medium-to-high volume — enough to pay back tooling, not so high that a dedicated automated machining cell is cheaper
• Hard-to-machine material — stainless steel, tool steel, or cobalt-chrome where chip-making is slow and tool wear is expensive

Classic MIM success stories: orthodontic brackets, watch bezels, firearm triggers and safeties, micro-gear trains for power tools, laparoscopic instrument jaws, smartphone hinges.

When MIM Loses (Or Costs Too Much)

• Large parts — beyond roughly 100–150 g, debinding times and furnace throughput make MIM uneconomical
• Very thin walls — below ~0.3–0.5 mm, filling becomes unreliable and green-part handling is fragile
• Extremely thick sections — above ~10–12 mm, debinding time becomes prohibitive
• Very low volume — under ~3,000–5,000 pieces per year, tooling amortization pushes piece price too high
• Simple 2D prismatic shapes — CNC or conventional PM is almost certainly cheaper

Design Guidelines for MIM Parts (Practical Checklist)

Send a part drawing to a MIM supplier with these questions already answered:

1. Section thickness: Are all cross-sections under ~8 mm? If not, can a thicker section be cored out or split?
2. Shrinkage allowance: Has the tool designer accounted for 15–20% tool oversizing?
3. Gate location: Is there a non-functional surface where the gate mark can be tolerated, or does it need post-machining?
4. Flatness after sintering: Large flat surfaces will distort; can stiffening ribs, a slight crown, or post-sinter sizing be part of the design?
5. Threads: External threads can often be molded; internal threads typically need secondary tapping unless the part volume justifies a collapsible core
6. Sintering support: During sintering the brown part is weak — long unsupported spans will sag; does the design include temporary support surfaces or sacrificial features?

SinterWorks and MIM: Where We Fit

SinterWorks specializes in conventional press-and-sinter powder metallurgy — not MIM. But we help engineers choose between processes honestly. If your part is in the MIM sweet spot, we will tell you and can refer you to qualified MIM foundries.

If your part is on the borderline — large enough for conventional PM, simple enough for machining, but with one or two complex features — a hybrid route (PM base + secondary machining) is often the most cost-effective answer. We do that every day.

To get a straight process recommendation, send your drawing through the quote form — our engineering team responds to feasibility questions within 24–48 hours.

Related Resources

• PM vs MIM: Detailed Process Comparison
• PM Material Selection Guide
• PM vs CNC Cost Comparison
• DFM Guide for Powder Metallurgy
• PM Tolerance Planning Guide