Injection molding is one of the most efficient ways to turn raw material into millions of identical parts. But manufacturers face a fundamental choice early in every project: metal injection molding (MIM) or plastic injection molding? Both start with a mold and both excel at high-volume production of complex geometries – yet they serve very different jobs. This guide breaks down the practical differences to help you choose the right process for your parts.
1. How the processes work
Plastic injection molding melts thermoplastic pellets and injects the molten material into a precision mold under pressure. After filling, packing, and cooling, the plastic solidifies into the required shape and is ejected from the mold.
Plastic injection molding processes
Metal injection molding mixes fine metal powder (typically stainless steel, titanium, or cobalt-chrome) with a polymer binder, injects that feedstock into the mold, then removes the binder and sinters the part in a furnace. The result is a dense (95-99%+) metal component with the complex geometry only molding can achieve.
Metal injection molding processes
2. Material properties
Strength and temperature: MIM parts are real metal. They handle structural loads, high temperatures, and harsh chemicals. Plastic parts are lighter and corrosion-resistant by nature, but they creep under sustained load and soften with heat.
Weight: If weight reduction matters more than strength – enclosures, housings, consumer products – plastic wins. If the part must survive torque, pressure, or wear – gears, latches, surgical instruments – metal wins.
Tolerances: Both processes hold tight tolerances, typically in the +/-0.1 mm range as-sintered, with tighter control possible through secondary machining.
3. Cost structure
Tooling: Both need a custom mold, but MIM tooling is generally more expensive because of the abrasive feedstock and the stricter shrinkage control during sintering.
Part price: Plastic parts are almost always cheaper per unit – polymers cost less than metal powders, and cycle times are seconds instead of the multi-step MIM process.
Volume sweet spot: Plastic molding is economical from a few thousand units upward. MIM is usually justified from around 10,000 to millions of small parts, where the alternative (machining each part from bar stock) becomes far more expensive.
4. Size and geometry limits
MIM shines for small, complex parts – typically under 100 grams and under 100 mm, with walls under 5 mm. Think of a stainless steel latch, a titanium dental abutment, or a cobalt-chrome orthopedic component. Plastic molding handles a much wider size range, from tiny clips to large automotive panels.
5. Industry applications
Choose plastic injection molding for: enclosures, housings, fittings, consumer goods, packaging components, and anywhere weight, insulation, or cost per part dominates.
Choose metal injection molding for: medical devices and surgical tools, firearms components, aerospace fittings, automotive safety and drivetrain parts, watch cases, and small structural components that must be metal but are too complex to machine economically.
plastic injection molding applications vs Metal injection molding applications
6. A practical decision checklist
Ask these questions in order:
a) Does the part carry structural load or face wear/heat? → Consider MIM.
b) Is it a small, complex geometry (under ~100 g)? → MIM candidate. Larger part? → Consider plastic injection molding or another metal process.
c) What volume? Under ~10,000 units → Consider machining or plastic injection molding. Higher volumes → MIM may become more cost-effective.
d) Does the environment rule out polymers (steam sterilization, aggressive chemicals, fire risk)? → Consider metal.
e) Budget for tooling and timeline? → MIM adds binder removal and sintering days to lead times; plastic molds run faster.
7. When in doubt, prototype both
Many products combine the two: a plastic housing fastened to a MIM metal frame, or a plastic overmold gripping a sintered metal insert. The best answer is often a hybrid design that lets each material do what it does best.
About the authors
Emitech Metal Injection Molding manufactures precision MIM components in stainless steel, titanium, and cobalt-chrome alloys for medical, automotive, and industrial applications – from prototype to millions of parts.
