Die Casting vs Metal Injection Molding

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  Die casting vs metal injection molding at a glance
• 2
  How the two processes actually fill a mold
• 3
  Which alloys each process can actually run
• 4
  Wall thickness, draft and feature size
• 5
  Tool cost, volume and cost per part
• 6
  What tolerance each process can hold

Die casting melts an alloy and pushes it into a hardened steel die at high speed. The metal freezes in seconds, the die opens, and the part drops out. Cycle times of 30 to 90 seconds are normal, which is why the process dominates housings, brackets and covers in aluminium and zinc.

Metal injection molding, usually shortened to MIM, starts with fine metal powder blended into a thermoplastic binder. That feedstock is injection molded exactly like plastic, giving a green part. Binder is then removed and the part is sintered near the melting point, so it shrinks roughly 15 to 20 percent and densifies to about 96 to 99 percent of theoretical density.

The shrink is uniform, which is what makes MIM viable for tight features. A 10 mm boss and a 40 mm span both shrink by the same percentage, so a well-built tool holds the geometry. Die casting does not shrink uniformly, because thick sections cool slower than thin ones and pull differently.

That single difference explains most of the selection rules below. Where geometry is uniform and thin, MIM wins. Where the part is large and walls are chunky, casting wins.

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  Filling is fast in bothThe difference is what happens during cooling, not during fill.
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  Sintering drives MIM toleranceShrinkage is predictable, but it is not zero.
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  Casting needs draft and thick wallsMIM needs neither.

Aluminium and zinc dominate casting. ADC12 and A380 flow well and take a good as-cast surface. Magnesium AZ91D is common where weight matters. High-melting alloys are a different story: the die would not survive repeated contact with molten stainless or tool steel at 1,400 °C or above.

MIM runs the alloys casting cannot. 17-4PH stainless, 316L, 420, 430 and low-alloy steels are routine. Titanium and copper feedstocks exist too. If your part needs corrosion resistance, wear resistance, or a magnetic response, casting usually cannot deliver it in one piece.

Mechanical properties follow the alloy. Sintered 17-4PH reaches roughly 90 percent of wrought strength after proper heat treatment, and 316L gives good ductility with corrosion resistance. Cast aluminium is softer and more porous, so it is a poor choice for a load-bearing latch or a wear surface.

Porous castings also trap plating solutions. If the part will be anodized or electroplated to a cosmetic standard, porosity shows up as blistering or pitting after finishing.

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  Casting wins on volume of metalCheap per kilogram, limited to low-melting alloys.
• 2
  MIM wins on alloy rangeSteel, stainless and titanium are all available.
• 3
  Porosity matters after finishingCast surfaces can blister under plating.

Casting needs walls thick enough to flow before the metal freezes. Below about 2 mm on a large part, short shots and cold laps appear. Draft of 1 to 3 degrees per side is normal so the part releases. Sharp internal corners concentrate stress and can crack during ejection.

MIM holds 0.5 to 3 mm walls without draft, and it fills blind holes, slots and cross-drilled passages that casting cannot form. Small internal features down to about 0.1 mm are achievable in the tool, though very fine features add tool cost and may need discussion before the design is frozen.

The trade-off is size. A MIM part is usually under 100 g, because the binder has to escape through the part during debinding. Thicker sections debind slower and can crack or blister. A 500 g stainless bracket is not a MIM part, no matter how complex the geometry.

Casting handles the opposite case well. A 4 kg aluminium housing with 6 mm walls and a few ribs is a single-shot casting, cheap per part, and can be machined afterward on the sealing faces.

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  Thin and complexMIM, no draft needed.
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  Large and chunkyCasting, with draft and generous radii.
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  Mixed geometryCast the body, machine the critical faces.

Tooling is the first real decision. A casting die is expensive and take longer to build, because it must survive thermal cycling and high injection pressure. A MIM tool sits closer to a plastic injection mold in cost and lead time. Neither number should be quoted without a drawing.

Cost per part splits the two processes apart. Casting has a low material cost and a fast cycle, so at 10,000 pieces or more it is usually the cheaper route for a simple part. MIM has higher feedstock cost, but it removes secondary machining and assembly. When a casting needs four drilling operations and a tapping step, the gap narrows fast.

There is a break-even band. Below roughly 5,000 pieces a year, neither process justifies a hard tool and you should look at CNC machining or vacuum casting for the pilot run. Between 5,000 and 50,000, MIM and casting are genuinely competitive, and geometry decides. Above that, both are strong; pick on alloy and tolerance, not on price.

One more line item is often forgotten: finishing. Castings frequently need trimming, deburring and machining on sealing surfaces. Sintered parts may need a sizing operation or a tumbling pass. Budget both before comparing quotes.

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  Low volumeCNC or vacuum casting for the pilot.
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  Mid volumeGeometry picks the winner.
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  High volumeAlloy and tolerance pick the winner.

As-cast tolerance on a die casting is around ±0.05 mm on a short dimension, and it drifts over long spans because of thermal contraction. Critical bores, bearing seats and sealing faces are normally machined after casting. That is standard practice, not a defect.

MIM holds about ±0.3 percent of the dimension after sintering, which on a 20 mm feature is roughly ±0.06 mm. Sizing or coining can tighten specific features. As with casting, the tightest features are usually finished by machining afterward.

Neither process competes with CNC on tolerance. If a drawing calls for ±0.005 mm or a Ra 0.2–0.8 μm surface, the last operation will be a machining pass. The practical question is how much of the part must be machined, and that is where MIM's near-net shape saves the most money.

A useful rule: if more than about 30 percent of the surface needs machining, the near-net advantage is gone. Redesign so the critical faces are accessible, or switch to a machined part.

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  As-cast±0.05 mm on short dimensions.
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  As-sintered±0.3% of the dimension.
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  Critical facesMachine them, in both processes.