MIM is the right process for small gears when part weight sits in the 0.1–200 g envelope, tooth modules fall in the roughly 0.3–1.5 range, and volumes are high enough to amortize tooling; as-sintered tolerances run about ±0.3–0.5 % of nominal, and secondary sizing or grinding takes critical teeth closer.
This page is a selection guide, not a process tour: it states what the process can hold, where the published capability bands really sit, and when a small gear should stay on a hobbing machine or a 5-axis mill instead. For the full production chain from feedstock to sintering, see the MIM gear manufacturing guide; for the general tolerance system, see MIM tolerances and accuracy.
What Counts as a Small Gear in MIM
In metal injection molding, “small” is defined less by diameter than by the combination of weight, wall thickness and tooth size the process can hold reliably. Emitech’s production envelope spans part weights from 0.1 g to 200 g with wall thicknesses from 0.5 mm to 6 mm, which covers most fine-pitch instrument, actuator and micro-drive gears. Published supplier guidance places typical MIM gears between about 5 mm and 50 mm in diameter, with modules of roughly 0.3 to 1.5 as the common sweet spot; some suppliers report micro-gear work below module 0.3, but that region is application-specific rather than a general capability.
The practical test is not whether a tooth can be formed, but whether the whole gear — hub, bore, web and teeth — sinters within tolerance without distortion. Thin webs under heavy rims, unsymmetrical spokes and very deep bores all push the part away from the process window, which is why a DFM review of the actual drawing says more than any catalog number.
Gear Types MIM Handles Well
Not every small gear benefits equally from MIM. The table below compares the common types when produced by metal injection molding.
| Gear type | MIM suitability | Notes |
|---|---|---|
| Spur | Well suited | Fine teeth around module 0.3–1.5 are a published MIM strength |
| Helical | Feasible | Lead error and distortion need part-by-part validation |
| Internal | Strong use case | Enclosed tooth geometry forms net-shape; costly to cut conventionally |
| Worm (steel) | Feasible | Small-module worms in 17-4 PH or 316L inside the standard envelope |
| Worm wheel (bronze) | Not a MIM material here | Bronze wheels are normally produced by PM or casting, not MIM |
Internal gears deserve a special note: the enclosed tooth profile is expensive to cut conventionally but forms naturally in a MIM mold, which makes internal rings and pinions one of the strongest MIM use cases. Helical teeth are feasible, but lead accuracy and shrinkage control have to be validated part by part rather than assumed from a spec sheet.
Small Worm Gears and Worm Wheels: The Pairing Rules
Worm drives pair a hardened steel worm with a wheel that is usually bronze or brass — tin bronze, phosphor bronze or aluminum bronze in published practice — because steel running directly on steel in a worm mesh risks seizure. That pairing rule defines what MIM can and cannot honestly do for small worm gear sets. For tooth geometry and ratio math, see the worm gear engineering guide.
On the worm side, MIM is a natural fit: small-module steel worms in 17-4 PH or 316L sit squarely inside the envelope described above. On the wheel side there is an honest limitation. Copper alloys are not part of Emitech’s MIM material family, so bronze worm wheels are normally produced by conventional powder metallurgy or casting, not MIM. Where a design tolerates an all-steel pairing — low speed, well lubricated, modest load — MIM can supply both members of the set; where the application needs a bronze wheel, MIM supplies the worm and the wheel is sourced separately.
Tolerances and Gear Accuracy
As-sintered, Emitech holds about ±0.3–0.5 % of nominal dimension, roughly ±0.025 mm on features under 3 mm; secondary operations such as sizing, grinding or CNC finish machining bring critical dimensions to about ±0.01 mm. For gear accuracy specifically, public benchmarks cluster around ISO 1328 grade 8–9 as-sintered and roughly grade 6–7 after post-processing — treat those as application-specific bands, not guarantees.
| Stage | Typical capability |
|---|---|
| As-sintered | ±0.3–0.5 % of nominal (about ±0.025 mm on features under 3 mm) |
| After sizing or finish machining | About ±0.01 mm on critical dimensions |
| Gear accuracy, public benchmarks | ISO 1328 grade 8–9 as-sintered; roughly 6–7 after post-processing (application-specific) |
Materials for Small MIM Gears
Emitech’s MIM material list for gears includes 316L and 304 stainless, 17-4 PH and 420 martensitic stainless, 4605, 8620 and 4340 low-alloy steels, Ti-6Al-4V, soft-magnetic Fe-Ni alloys, Kovar ASTM F15 and CoCr ASTM F75. Sintered density reaches 95–99 % of theoretical depending on alloy — the number that ultimately drives tooth strength and wear life. The full equipment and capacity picture is on the MIM capabilities page.
For hardened teeth, 8620-type carburizing grades and 17-4 PH with post-sinter heat treatment are the usual starting points; for corrosion service, 316L. Material choice should follow the loads, the environment and any magnetic requirements in the drive — not habit.
When MIM Is the Wrong Choice for a Small Gear
MIM is the wrong choice when volumes are low, the schedule cannot absorb tooling time, or the tooth geometry sits outside the module envelope. Tooling takes about 15–20 days from approved drawings and production runs 4–6 weeks after tool approval; a one-off prototype gear is almost always faster from a hobbing shop or a 5-axis CNC cell. Very large modules, extreme aspect-ratio rims, or requirements for as-cut DIN 5-level accuracy also point away from MIM. A supplier who quotes MIM for every gear without asking about volume or accuracy class is not doing selection work.
Frequently Asked Questions
Q: What is the minimum module for a MIM spur gear?
Published supplier guidance puts the common sweet spot at module 0.3 to 1.5; some suppliers report production below module 0.3 for micro-gears, but that region is application-specific. Submit the drawing for a DFM verdict rather than assuming the lower bound.
Q: Can MIM produce the worm and the worm wheel as one set?
Only when the design tolerates an all-steel pairing at low speed with good lubrication. Bronze worm wheels — the standard wear partner for a steel worm — are outside Emitech’s MIM material family and are normally made by powder metallurgy or casting. In bronze-wheel applications, MIM supplies the worm only.
Q: What gear accuracy grade can I expect from MIM?
Public benchmarks cluster around ISO 1328 grade 8–9 as-sintered and roughly grade 6–7 after post-processing. The exact grade depends on module, part size and whether teeth receive secondary operations, so specify the functional requirement rather than a bare grade number.
Q: When is hobbing or CNC better than MIM for a small gear?
At low volumes, when lead time dominates, at very large modules, or when as-cut accuracy near DIN 5 is mandatory. A one-off gear is faster from a hobbing shop; MIM earns its tooling cost back at production volumes.
Q: What are typical lead times for a small MIM gear program?
Quotes are answered within 24 hours; tooling takes about 15–20 days from approved drawings, and production runs 4–6 weeks after tool approval. Part weights from 0.1 g to 200 g fit the standard envelope.
If your drawing sits inside these envelopes — 0.1–200 g, modules around 0.3–1.5, volumes that amortize tooling — send it through the contact page for a DFM review. Emitech answers quote requests within 24 hours.
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