Guide
What Is the As-Sintered Tolerance of MIM, and When Should You Machine or Size?
As-sintered tolerance is the dimensional accuracy a metal injection molded part has straight out of the furnace, before any coining, sizing, grinding or machining. The average per MIMA is within plus or minus 0.3% of the dimension, which is enough for many features and not enough for bores, sealing faces and threads, so the drawing should say which is which.
By Saola Manufacturing engineering team · Published · Updated
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What tolerance does MIM hold without secondary operations?
MIMA states that the average MIM tolerance falls within plus or minus 0.3% of the dimension, so many parts are used as sintered. It is an average for the process, not a guarantee for any single feature.
The statement comes from the association’s page on secondary operations, which adds that a secondary operation can be added when a feature needs tighter tolerances (MIMA, Secondary Operations with MIM). Read it for what it says. It is a percentage, so the absolute tolerance grows with the feature size. It is an average, so some features will be better and some worse. And it describes the process in general, not a specific alloy, part shape or tool.
This guide is for an engineer deciding which dimensions on a MIM drawing can stay as sintered and which need a second operation. It uses only the MIMA statements and does not add a table of supplier values, because the sources for such tables are rarely stated and the numbers vary by alloy and geometry. Material grades for MIM are specified separately, per ASTM B883, ISO 22068 or MPIF Standard 35-MIM.
Why is MIM tolerance a percentage of size?
Because the part shrinks by roughly a fifth in the furnace, so any variation in shrink is multiplied by the size of the feature. MIMA gives typical shrinkage of 18% to 22%, depending on binder volume, and says it is isotropic.
The molded green part is approximately 20% larger in all its dimensions than the finished part, and sintering shrinks it isotropically, typically by 18% to 22% depending on binder volume, to reach 96% to 99% of theoretical density (MIMA, Process Overview). A small variation in that shrink rate shifts every dimension by a proportional amount. As typical arithmetic, a 5 mm feature moves very little and a 50 mm feature moves ten times as much. That is why a flat tolerance of a few hundredths of a millimeter is easy on a small pin and hard on a long body.
The table shows the arithmetic. It is not a capability claim, because the average is a mean across features, shapes and tools; a stable, simple feature may do better, and a long thin feature, one far from the gate or one that sags in the furnace may do worse. Use it to see which of your dimensions are at risk, then ask the fabricator for the capability on those.
| Feature size (mm) | Tolerance band each side at 0.3% (mm) | Total band (mm) | ISO 2768-1 medium (m), each side (mm) |
|---|---|---|---|
| 5 | 0.015 | 0.03 | 0.1 |
| 10 | 0.03 | 0.06 | 0.2 |
| 20 | 0.06 | 0.12 | 0.2 |
| 30 | 0.09 | 0.18 | 0.2 |
| 50 | 0.15 | 0.30 | 0.3 |
Which features can safely stay as sintered?
Features whose function tolerates a band that scales with their size: external profiles, non-mating surfaces, clearance holes, decorative features and dimensions that a mating part adjusts for. Put the tight requirement only where function needs it.
Start from function. A latch body that sits loose in a housing, a bracket face that bolts to a surface through slotted holes, a cosmetic surface and a clearance hole for a fastener do not need to be tighter than the process holds. MIMA notes that many parts are sintered to final dimensions (MIMA, Secondary Operations with MIM), and that MIM typically limits interconnected porosity to under 0.2%, so plating or coloring can be applied without special surface preparation.
Mark the as-sintered group on the drawing with a general tolerance note, and mark everything else individually. Per ISO 2768-1, the medium class allows a deviation of 0.2 mm each side from over 6 up to 30 mm, so the 0.3% arithmetic band in that range sits well inside an ISO 2768-m note; the ISO 2768 guide has the full table. A good practice is to tighten only what the assembly requires and to say so. A drawing that carries the same tight tolerance on every dimension forces secondary operations that the function did not need and removes the cost advantage that justified molding in the first place.
When should you size, coin or grind a MIM feature?
When a feature needs tighter dimensional control or flatness than the as-sintered band gives, and the shape allows a die or mandrel to act on it. MIMA lists coining, machining, sizing and grinding among the operations MIM materials accept.
MIMA describes coining as forcing a sintered part against a rigid mandrel or substrate to improve dimensional control and flatness, and states that MIM materials can be machined, tapped, drilled, broached, sized, ground or welded (MIMA, Secondary Operations with MIM). Coining and sizing are cold-forming corrections: they work on a feature that has plastic margin to move, such as a bore, a flat face or an outside diameter, and they are suited to higher volumes where a die can be justified. Grinding removes material to size and fits features that are hard or that need a fine finish.
The choice between them is a cost and geometry question for the fabricator. What you control is the requirement: state the dimension, the tolerance and the functional reason on the drawing, and let the supplier propose coining, sizing or grinding in the quote. Do not prescribe the operation unless the function depends on it, for example a surface that must be ground for a finish reason.
When is finish machining the right answer?
For threads that need a defined class of fit, sealing faces, bearing bores and any feature whose tolerance is a fixed value rather than a percentage. MIMA says all common machining operations can be applied to MIM components.
The same page says machining is often used to add threads, undercuts, grooves or other features that would be difficult or costly to build into the tooling (MIMA, Secondary Operations with MIM). In practice the economical pattern for many parts is hybrid: mold the shape, then finish-turn or finish-mill the few features with tight requirements. Because the part is near-net, the machining time is short and the material removed is small.
Add a machining allowance to the features you will finish, and tell the fabricator which. Saola places MIM through a partner plant it manages and finishes such features by CNC, so a hybrid part ships under one purchase order, with the production site and country of origin named on the quote. See the MIM and powder metal page and the CNC turning page. For machining tolerances in general, see standard machining tolerances.
How do you set up the datum scheme for a part that is part sintered and part machined?
Choose datums that are themselves stable, then tie each machined feature to them in a single setup. Datum features that are only as-sintered inherit the process variation, and tolerancing from them hides it.
The drawing should split the features into three groups: as-sintered, sized or coined, and finish-machined. Pick the primary datum on a feature that sits flat and supported through the furnace cycle, which is also the face that MIMA suggests as a common plane for support (MIMA, Complex Designs with MIM). Locate the machined features from that datum, not from each other, so that errors do not stack. If a machined bore must be concentric with an as-sintered outer diameter, say which one is the datum.
Tolerance stack-up matters more on a part with two kinds of feature. As-sintered dimensions carry a percentage band and machined dimensions carry a fixed one, so the worst-case sum can be dominated by the larger. The guide on worst-case versus RSS stack-up shows how to combine them.
How should you verify MIM dimensions in the RFQ and at receiving?
Ask for first-article measurements from each cavity, identify the cavity on the part, and tie the inspection plan to the three feature groups on the drawing. The as-sintered group is judged by process capability, the machined group by the fixed tolerance.
MIMA notes that cavity identification can be molded into the part at no added piece cost (MIMA, Complex Designs with MIM). Use it. When a multi-cavity tool makes the part, a cavity number lets you trace a dimensional shift to one cavity instead of rejecting a whole lot. Ask the fabricator for the sample plan, the gauging method and the measurement of critical features on a coordinate measuring machine for the first articles, and agree what happens when a sintered dimension drifts.
Do not accept a verbal assurance that the process will hold a tolerance tighter than the as-sintered band without a stated secondary operation. If the quote has no sizing, coining or machining step on a tight feature, ask how it will be held.
Frequently asked
What tolerance can MIM hold?
The average MIM tolerance falls within plus or minus 0.3% according to MIMA, which is about 0.03 mm each side on a 10 mm feature. Tighter requirements need coining, sizing, grinding or machining, and the achievable value depends on the alloy, shape and tool.
Is the MIM average tolerance a guarantee?
No. It is an average for the process, and individual features can be better or worse. Ask the fabricator for the capability on your critical dimensions rather than relying on the average.
Can MIM hold the same tolerance as CNC machining?
Not as sintered on most features. Features with a tight fixed tolerance are normally finish-machined after sintering, and MIMA states that all common machining operations can be applied to MIM parts.
Does MIM shrinkage affect my drawing?
The fabricator compensates for shrink in the tool, which is typically about 20% larger than the finished part. You design to the finished size; the shrink is why the tolerance is a percentage and why tight features are corrected afterwards.
How do I tolerance a thread on a MIM part?
MIMA says internal tapping is usually more precise and cost-effective than unscrewing cores, so plan to tap the thread after sintering and specify the class of fit on the drawing. External threads are best placed on the parting line.
Sources
- MIMA, Secondary Operations with MIM, Metal Injection Molding Association, accessed October 2026 (page copyright 2026)
- MIMA, Process Overview: MIM, Metal Injection Molding Association, accessed October 2026 (page copyright 2026)
- MIMA, Complex Designs with MIM, Metal Injection Molding Association, accessed October 2026 (page copyright 2026)
- MIMA, Designing with MIM, Metal Injection Molding Association, accessed October 2026 (page copyright 2026)
- ISO 22068:2012, sintered metal injection-molded materials, specifications, ISO, published June 2012 (Standards Council of Canada listing, accessed October 2026)
- ASTM B883, Standard Specification for Metal Injection Molded (MIM) Materials, ASTM International store page, B883-19 listed as historical, B883-24 as current (page updated 2024)