MIM steel part
- Strength and hardness close to wrought metal
- Handles wear, impact, and heat that plastics cannot
- Needs feedstock, furnace capacity, and usually heat treatment
Process guide
Metal Injection Molding
A MIM part leaves the press looking like a dull gray plastic part. By the time it leaves the furnace it is smaller, much heavier, and made of stainless steel or another alloy.
Why it exists
Too intricate to machine, too small to cast well
MIM borrows the shape making of injection molding and hands the result to a furnace.
Some small metal parts are awkward to machine and too detailed for casting. Lock parts, surgical instrument jaws, watch cases, firearm components, and phone hinges are the classic examples. MIM molds them the way plastic parts are molded, then turns the molded shape into dense metal.
It pays off on small, detailed parts in large volumes. Above a certain size or below a certain quantity, machining and casting usually come out ahead. The powder is expensive, furnace cycles are long, and the mold only makes sense when its cost is spread over a lot of parts.
The route
From powder to finished metal
Molding is one of five stages, and not the longest.
- 1
Feedstock
Fine metal powder is blended with a wax and polymer binder and pelletized. It handles like a very heavy, very abrasive plastic.
- 2
Green part
The feedstock is injection molded into an oversized copy of the final part. The green part is fragile and is picked and placed gently.
- 3
Debinding
Most of the binder comes out by solvent, catalytic, or thermal treatment. What is left is a porous brown part held together by the last of the binder.
- 4
Sintering
In a controlled atmosphere furnace the powder fuses into dense metal. The part shrinks a great deal as the pores close, and it only shrinks evenly if it was designed and set down with that in mind.
- 5
Secondary work
Heat treatment, sizing, machining of the few features that need it, and plating or passivation.
Design
Designing for the furnace, not just the mold
A MIM part has to survive being soft and unsupported at high heat.
- Keep sections even. Heavy areas hold binder longer and shrink differently from thin ones
- Give the part a flat face to sit on in the furnace, since it softens while sintering and sags wherever it hangs
- Avoid long overhangs and cantilevers that droop, or plan a custom setter to hold them
- Save the tightest tolerances for features that will be machined or sized after sintering
- Expect gate and ejector marks to stay visible on the finished metal
Options
MIM next to the other ways to make a small metal part
Each method has a range where it wins outright.
| Method | Strong at | Weak at |
|---|---|---|
| MIM | Intricate small shapes in high volume with near wrought strength | Large parts, low volumes, and very tight as sintered tolerances |
| Machining from bar | Low volumes, tight tolerances, almost any alloy | Complex internal shapes, and material waste at high volume |
| Investment casting | Larger parts and a wide range of alloys | Fine detail and very thin sections |
| Pressed powder metal | Simple prismatic shapes like bushings and flat gears | Undercuts, cross holes, and side features |
| Molded plastic | Light, corrosion free, and often the least costly part when loads allow | The strength, stiffness, and heat tolerance of steel |
The first question
Stay metal, or convert to plastic
Some MIM candidates would work fine as molded plastic. Nobody checked.
Filled or high heat plastic part
- Molded to final shape with no sintering stage
- Weighs a fraction of the steel part and does not rust
- Works where the loads and temperatures suit PPA, PEEK, or glass filled nylon
FAQ
Questions about metal injection molding
How dense are MIM parts?
Close to wrought metal, and dense enough for most structural and wear uses. The remaining porosity is small and closed, which matters when a part will be plated or has to seal.
Which metals can be MIM molded?
Stainless steels are the most common, followed by low alloy steels, tool steels, titanium, tungsten heavy alloys, and soft magnetic alloys.
Can a MIM part go inside a plastic part?
Yes. Small MIM or machined metal pieces are often insert molded into plastic housings. That keeps metal where the strength is needed and plastic everywhere else.
Does NetProto make MIM parts?
No. NetProto's production path is thermoplastic injection molding, insert molding, and overmolding. This page helps you judge whether a part really has to be metal. A plastic version, or a plastic housing around a metal part, can be quoted from CAD.
Keep reading
Related guides
Converting metal parts, combining metal with plastic, and the resins that replace steel.
Metal to plastic conversion
What has to change when steel becomes resin.
Metal to plastic conversionInsert molding
Metal parts molded into plastic.
Insert moldingMolded in inserts
Holding metal securely in a plastic part.
Molded in insertsGears
When a plastic gear can replace a metal one.
GearsPPA
Stiff and hot enough for many metal jobs.
PPAGlass filled nylon
The usual first try at replacing metal.
Glass filled nylonCeramic injection molding
The same route with a ceramic powder.
Ceramic injection moldingProcesses hub
Every molding process and secondary operation.
Processes hubNext step
Check whether the part could be plastic
Upload CAD with the loads, temperatures, and wear the metal part sees. The review looks at filled and high heat resins and what the geometry would need to change. NetProto manufacturing is ISO 9001 certified.