Metal Injection Molding vs Die Casting

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  Metal injection molding vs die casting at a glance
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  What each process actually does to the metal
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  Wall thickness and feature size decide most cases
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  Alloy choice narrows the field fast
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  Tooling cost and run size: where the lines cross
• 6
  Post-processing and CNC finishing

Metal injection molding starts with fine metal powder, usually under 20 μm, mixed with a polymer binder. The mix is heated and injected into a small steel mold, the same way plastic injection works. The green part that comes out is oversized. Binder is removed in a solvent or thermal step, then the part is sintered near 1,300 °C. During sintering the part shrinks roughly 15–20 percent and densifies to about 96–99 percent of theoretical density.

Die casting melts metal and forces it into a hardened steel die under high pressure. Aluminium, zinc and magnesium all flow well and freeze fast, so cycle times are short. The part comes out near net shape with a thin skin and a cored interior. Porosity is normal and comes from trapped gas and shrinkage. That porosity is the single biggest difference from MIM when you look at a cross section.

This is why the two processes rarely compete on the same part. MIM is a powder route that gives fine detail on small parts. Die casting is a liquid route that gives large, stiff parts at high output. Where they overlap, the deciding factors are wall thickness, alloy family and annual volume.

One more difference matters for engineers: MIM parts are isotropic after sintering, so strength is similar in every direction. Die cast parts carry a skin and a cored center, so fatigue behavior depends on where you machine and where the gate was.

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  MIMPowder + binder, sintered to 96–99% density
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  Die castingMolten metal, high pressure, fast freeze
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  Same part, two answersOnly small thin-wall parts justify a real comparison

Wall thickness is the fastest filter. MIM handles walls from 0.3 mm to 3 mm and keeps them uniform without much trouble. Die casting prefers 1.5 mm and up for aluminium. Go thinner than that and the melt freezes before it reaches the end of the cavity, so you get cold shuts and short fills.

Corner radii follow the same logic. MIM can hold a 0.1 mm radius on a small feature and keep draft near zero, because the green part is slightly flexible before sintering. Die cast parts need 1–2 degrees of draft on walls and generous fillets, or the part drags on ejection and tears.

Weight is the other hard boundary. MIM is a small-part process in practice. A 5 g stainless part with a 0.5 mm wall and a fine internal thread is a good MIM candidate. A 900 g aluminium housing with 4 mm walls and cooling ribs is a die casting part, and no amount of MIM tooling work changes that.

If your part has both a thin cosmetic wall and a heavy structural boss, split it. We often see designs where a MIM insert or a machined detail is assembled into a die cast body. That hybrid is usually cheaper than forcing one process to do both jobs.

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  Choose MIMWall under 1 mm, tight radii, internal features
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  Choose die castingWall over 2 mm, large flat faces, ribs
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  Watch outMixed thin and heavy sections in one part

MIM works with alloys that can be atomized into powder and sintered. That means stainless steels such as 316L and 17-4PH, tool steels, low-alloy steels, and titanium including Ti-6Al-4V. These are the grades where hardness, corrosion resistance or fatigue life matter. A sintered 17-4PH part can be heat treated to a high hardness after sintering, which is why it shows up in medical and aerospace hardware.

Die casting covers a different alloy family. Aluminium grades such as ADC12 and A380, zinc alloys, and magnesium AZ91D flow well and cast cleanly. They are light and stiff, but they are not high-strength alloys in the same sense. Tensile strength for a cast aluminium part can be raised with T6 heat treatment, yet it stays below a wrought or sintered steel of similar cost.

So the material question usually settles the process before geometry does. If the part must be stainless, magnetic, or heat treated to a hardness spec, MIM is the path. If the part must be light and the loads are moderate, die casting wins.

There is a middle ground worth knowing. Die cast aluminium parts can be machined on critical faces to hold tolerances that the casting itself cannot. That is a normal production route, not a workaround, and it belongs in the cost model from the start.

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  MIM alloys316L, 17-4PH, tool steel, Ti-6Al-4V
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  Die casting alloysADC12, A380, zinc, AZ91D magnesium
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  Hybrid routeCast near net shape, then CNC the critical faces

Both processes need a steel tool, and both tools cost real money. The difference is scale. A MIM mold is small because the parts are small, so the tool is cheaper and easier to change. A die casting die is large, heavy and often needs its own thermal management, so the upfront cost is higher.

Unit cost then falls with volume, but along different curves. MIM has a higher per-part material and sintering cost, so it benefits less from volume. Die casting has a low per-part cost once the die is paid off, so it benefits more. The crossover is usually somewhere between 2,000 and 10,000 parts per year, depending on part weight and how much machining follows.

Below the crossover, neither process is the cheap answer. For 50 to 500 parts, CNC machining from bar stock is often faster and cheaper than building either tool, especially when the design is still moving. We quote all three routes when a part is near the boundary.

Above the crossover, the question flips. If you need 200,000 small stainless hooks, MIM absorbs the tooling cost quickly and gives you a finished part with almost no secondary work. If you need 200,000 aluminium brackets, die casting plus T6 and a light CNC skim will beat MIM on unit cost.

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  Under ~1,000 partsCNC from solid or vacuum casting usually wins
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  1,000–10,000 partsCompare MIM and die casting line by line
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  Over 50,000 partsTooling is amortized; unit cost rules

MIM parts come out of the furnace ready to use on many features, but not all. Sintering leaves a matte surface around Ra 1.6–3.2 μm. Critical bores, threads, sealing faces and flatness callouts usually get a light CNC pass. Bead blasting, tumbling and polishing take MIM parts down to Ra 0.8–1.6 μm, and finer polishing can reach Ra 0.2–0.8 μm on small areas.

Die cast parts need trimming first, then often heat treatment. T6 raises strength on aluminium but moves the part slightly, so critical faces are machined after heat treat, not before. Coating comes last: anodizing, powder coating, zinc or nickel plating, or a conversion coating.

The trap in both processes is sequencing. If you machine a die cast part before heat treatment, the heat treat will move it out of tolerance. If you polish a MIM part before the final sintering check, you may be polishing a part that is already out of density spec. Fix the sequence, then fix the tolerance.

This is where a shop that runs both processes helps. At GreatLight we machine MIM and die cast parts on the same 127 CNC machines, with ±0.005 mm capability on the finishing passes and 100% inspection before shipment. The fixture and the datum scheme get settled once, not twice.

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  MIM postCNC critical faces, bead blast, polish to Ra 0.8–1.6 μm
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  Die cast postTrim, T6 heat treat, then CNC, then coating
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  Sequence ruleHeat treat before final machining, always