MIM can mold threads directly — internal and external — and for most volume applications that is the right call: a molded thread needs no secondary operation and costs nothing extra per part once the tool is cut. The decision that actually matters is tolerance. As-sintered threads hold commercial classes on sizes around M2 and above; when a drawing calls for a tight class of fit, a tapped or single-pointed thread after sintering is the reliable route, at the price of a secondary operation. Threads under about M2 are generally better tapped than molded.
This page covers the thread capability of metal injection molding end to end: what the molding route delivers, where tapping wins, how sintered thread strength compares to machined and rolled threads, and the tool wear economics that decide the route at volume.
Two Routes to a Thread: Molded or Tapped
Every MIM thread arrives by one of two routes. Route one: the thread is molded — a rotating thread core in the tool forms an internal thread (and unscrews on withdrawal), or a threaded insert forms an external one. The green part carries the thread through debinding and sintering, and it emerges at as-sintered tolerance. Route two: the part is molded with a plain hole or boss, and the thread is cut after sintering — tapped with form or cut taps, single-pointed on a lathe, or milled with a thread mill.
| Route | How it forms | Tolerance outcome | Per-part cost | Tooling cost |
|---|---|---|---|---|
| Molded thread | Thread core / insert in the MIM tool | Commercial class, as-sintered (±0.3–0.5% discipline) | Near zero at volume | Higher — precision cores, maintenance |
| Tapped / cut after sintering | Plain hole molded, tap or thread mill finishes | Cut-thread classes on treated feature (≈±0.01 mm class) | Constant per part | Lower — plain core |
| Rolled after sintering | Plain boss molded, thread rolled | Rolled-class accuracy with work-hardened root | Constant per part | Lower — plain core |
The routes are not exclusive on one part: a common design molds the load-bearing thread where commercial tolerance serves, and taps only the thread that carries a sealing or precision-locating duty.
What Tolerance a Sintered Thread Holds
As-sintered MIM tolerance is ±0.3–0.5% of nominal dimension, and thread pitch diameter inherits that discipline. In practice: commercial internal thread classes on sizes M2 and coarser are routine; fine pitches and close classes are where sintering variation — throat and die influences, load placement in the furnace — starts to matter. Sizing operations can improve thread quality, and a tapped or single-pointed thread after sintering reaches conventional cut-thread tolerance classes on the treated feature, in the same family as any other CNC-finished MIM feature at approximately ±0.01 mm.
- M2 and coarser, commercial class — molded route, no second operation needed.
- Under M2 — tap after sintering; molded fine threads do not sinter reliably and tool cores become fragile.
- Close fit class or pitch-critical duty — tap or single-point after sintering.
- External threads — moldable where the profile is not class-critical; roll the thread after sintering when strength or fatigue life is the design driver (see the strength section below).
Thread Strength: Sintered Versus Machined and Rolled
A cut thread and a rolled thread in wrought material are the references engineers know. A rolled thread is the strongest — cold rolling work-hardens the root and lays the fiber structure along the flank. A cut thread in wrought material is the baseline. A MIM thread, molded or cut, operates on material at 95–99% of theoretical density: static strength is competitive with the wrought cut thread in the same alloy and heat-treat condition, with the honest caveat that remaining porosity costs some fatigue performance and ductility — the same caveat that applies to any sintered feature. For a fatigue-critical or safety thread, the conservative engineering answer is a MIM boss finished by thread rolling or single-point turning after sintering, which puts wrought-quality thread flanks exactly where the load path needs them.
Practical field report from the engineering community: titanium MIM parts with critical threads are often produced as turned blanks plus cut threads, because titanium thread cores wear MIM tooling quickly and fine titanium threads are hard to hold — a real trade-off a designer should hear before specifying.
Tool Wear and the Volume Decision
A thread core is a precision tool component doing rotating work in abrasive feedstock, and fine threads wear faster than coarse ones. Tool life enters the per-part cost as maintenance and core replacement. At volume, a molded thread amortizes tooling cost away and a tapped thread pays a per-part operation cost forever — which is why volume parts mold their threads. At low volume the calculus flips: the tool saving on a plain hole is immediate, and the tapping cost applies to few parts. The crossover is part-specific; the MIM tooling cost and part cost reduction pages cover the general structure of both calculations.
Design Rules for Specifying MIM Threads
- Prefer coarse pitches: they mold more reliably, sinter more uniformly and wear tools slower than fine pitches.
- Give internal threads an unthreaded lead-in and a relief at the blind end so the core can withdraw and sintering can do its work.
- Do not specify a thread class tighter than the function needs — every class step buys a secondary operation.
- For blind holes, leave tap depth margin or specify a thread mill route; cut taps in sintered material behave like taps in any tough stainless.
- State the route on the drawing when it matters (molded / tapped / rolled) so the supplier quotes the process the design assumes.
- Keep external thread crests away from parting lines and gate scars — sintered witness lines on flanks are the first quality complaint.
Frequently Asked Questions
Q: Can MIM make internal threads without tapping?
Yes — a rotating thread core in the mold forms the internal thread and unscrews during ejection, and the thread survives debinding and sintering. Commercial thread classes on M2-and-coarser sizes are routine. Tight classes, fine pitches or sizes under M2 are better tapped or milled after sintering.
Q: Are MIM threads as strong as machined threads?
For static loading, a MIM thread in the same alloy and heat-treat condition is competitive with a cut wrought thread. For fatigue-critical duty the honest answer is to finish the thread after sintering — rolled threads work-harden the root and are the strongest option; single-point cutting gives cut-thread reliability on the critical feature.
Q: What thread sizes can be molded in MIM?
M2 and coarser is the practical molding range for internal threads; below that, cores become fragile and sintering variation dominates the pitch diameter. External threads mold across the MIM size range where the profile is not class-critical.
Q: Do molded threads need any post-processing?
Usually no — that is their economic point. Degating or removing a witness line on external crests may appear depending on tool design, and tight-class features get a sizing, tapping or rolling pass on exactly the features that need it, not the whole part.
Q: Why would I tap a MIM part instead of molding the thread?
Three reasons dominate: the class of fit is tighter than sintered tolerance delivers, the pitch is too fine to mold reliably, or the volume is too low to justify thread cores. Tapping costs a per-part operation but removes precision core maintenance from the tooling bill.
Q: How do I specify a MIM thread on a drawing?
Give size, pitch and class as usual, then state the route when it matters — molded, tapped or rolled — and mark which threads are function-critical. A supplier can then quote the process your tolerance assumes instead of discovering the intent after tooling starts.
MIM threads come down to a tolerance decision: molded threads win at volume where commercial classes serve, and post-sintered threads — tapped, milled or rolled — win wherever the drawing gets serious about fit or fatigue. Specify the route on the drawing and the economics follow. The secondary operations and tolerance pages carry the general framework; the contact page starts a part-specific conversation.
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