MIM Tolerances and Design Guidelines for Precision Metal Parts

Why MIM tolerances differ from CNC tolerances

CNC machining achieves tolerances by removing material from a solid blank with a cutting tool. MIM, by contrast, forms the part from metal powder and binder, then sinters it to near-full density. During sintering, the part shrinks predictably, typically 15 to 20 percent from its molded green state. This shrinkage is controlled and repeatable, but it means that MIM tolerances are influenced by factors that do not apply to machining: powder particle size distribution, binder content, sintering temperature uniformity, and part geometry.

The practical result is that MIM can hold tolerances that are excellent for a net-shape process but not as tight as precision machining on every dimension. For critical features, secondary machining or sizing after sintering can achieve tighter tolerances where needed.

Typical MIM tolerance ranges

The following table summarizes typical as-sintered MIM tolerances. These are general guidelines, not guarantees. Actual achievable tolerances depend on part geometry, material, size, and process conditions. SINTS confirms specific tolerances during drawing review.

For dimensions that require tighter control, SINTS can apply secondary sizing, machining, or grinding after sintering. This hybrid approach, combining MIM's shape freedom with machining's precision, is common for parts that need both complex geometry and critical tolerances on specific features.

Wall thickness guidelines

Wall thickness is one of the most important design parameters for MIM. Consistent wall thickness helps ensure uniform shrinkage, reduces distortion during sintering, and improves dimensional stability.

• Recommended minimum: 0.5 mm for structural reliability.
• Absolute minimum: 0.3 mm for short sections or non-structural features.
• Recommended maximum: 15 mm. Very thick sections increase debinding time and risk internal defects.
• Uniform thickness: Transition between thick and thin sections gradually to avoid stress concentrations and shrinkage distortion.
• Coring out thick sections: If a part has a thick area, consider coring it out to reduce mass and improve sintering uniformity.

Holes and bores

MIM can mold through-holes and blind holes directly into the part. The following guidelines help ensure successful molding and sintering:

• Minimum hole diameter: 0.5 mm for through-holes; 1.0 mm for blind holes.
• Aspect ratio: Keep length-to-diameter ratio below 10:1 for through-holes and below 4:1 for blind holes.
• Hole spacing: Maintain at least 2 material thicknesses between adjacent holes.
• Draft on blind holes: Apply 0.5 to 1 degree draft to core pins for blind holes.
• Counterbores and countersinks: These can be molded directly, reducing secondary machining.

Threads in MIM parts

Threads require careful consideration in MIM design.

• External threads: Can be molded directly in the tool. Half-round or trapezoidal thread forms are preferred. Precision threads may need secondary rolling or machining.
• Internal threads: Typically added as a secondary tapping operation after sintering. Molding internal threads requires unscrewing cores, which increase tooling cost.
• Thread class: For most applications, 6H/6g is achievable with secondary operations. Tighter classes require machining.
• Chamfers: Add a chamfer at the thread start to prevent flash and improve assembly.

Undercuts and complex features

One of MIM's greatest advantages is its ability to form complex geometries that would be difficult or expensive to machine. However, undercuts require careful design:

• External undercuts: Can often be accommodated with slider or lifter mechanisms in the tool.
• Internal undercuts: More challenging and may require collapsible cores. If possible, redesign to avoid internal undercuts.
• Side holes: Can be molded using side action cores. Position them perpendicular to the parting line for simpler tooling.
• Gear teeth and splines: MIM is excellent for small gears. Helical and bevel gears can be molded, though tooling complexity increases.

Draft angles

Draft angles help the green part eject from the mold without damage. While MIM feedstock is less rigid than plastic, draft is still recommended:

• External surfaces: 0.5 to 1 degree minimum.
• Internal surfaces and cores: 1 to 2 degrees.
• Textured surfaces: Add 1 degree extra for every 0.02 mm of texture depth.
• Parting line: Design draft so both halves of the tool have draft in the direction of opening.

Radii and fillets

Sharp corners are problematic in MIM for two reasons: they concentrate stress in the molded part during handling and sintering, and they increase tool wear. Generous radii improve both part quality and tool life.

• Internal corners: Minimum radius of 0.3 mm. Sharp internal corners cause cracking during sintering.
• External corners: Minimum radius of 0.2 mm. A slight break on external edges prevents edge chipping.
• Fillet transitions: Use fillets to transition between sections of different thickness. The fillet radius should be at least 0.5 times the thickness difference.

Material impact on tolerances

Different materials behave differently during sintering, which affects achievable tolerances:

How to specify tolerances in an RFQ

When submitting a drawing for quotation, the way tolerances are specified directly affects the cost and feasibility of the part. Here are best practices SINTS recommends:

• Mark critical dimensions: Use GD&T or explicit tolerance callouts only on dimensions that matter for function. Over-tolerancing increases cost.
• Provide a 3D model: A STEP or IGES file alongside the 2D drawing helps the engineer understand design intent and identify potential issues early.
• Indicate the application: Knowing how the part functions helps the supplier recommend the right process and tolerance strategy.
• Specify inspection requirements: If certain dimensions require CMM verification or specific gages, note them on the drawing.
• Allow process flexibility: If you can accept secondary machining on critical features, say so. This opens up more manufacturing options and can reduce cost.

Common design mistakes to avoid

In our experience reviewing customer drawings, several design issues appear repeatedly. Avoiding these will save time and cost:

• Over-tolerancing non-critical dimensions: Applying tight tolerances to every dimension forces unnecessary secondary operations.
• Inconsistent wall thickness without transition: Abrupt thickness changes cause distortion and internal stress.
• Sharp internal corners: These lead to cracking during sintering. Always add a fillet radius.
• Ignores parting line: If cosmetic surfaces cross the parting line, visible flash lines may appear. Design with the parting line in mind.
• Very thin, long features: Features like thin blades or pins are prone to distortion during debinding and sintering.
• Missing draft on blind holes: Without draft, the core pin can be difficult to extract, causing green part damage.

When to combine MIM with secondary machining

For many parts, the best approach is not pure MIM or pure machining, but a hybrid. MIM forms the overall geometry efficiently, and secondary machining achieves tight tolerances on critical features such as mating bores, threaded holes, or sealing surfaces.

This hybrid strategy is particularly effective for:

• Parts with complex overall geometry but a few critical precision features
• Components that need both shape complexity and tight flatness on a mounting surface
• Parts where a bored hole must hold H7 tolerance
• Assemblies where mating features need machining-level precision

SINTS reviews each drawing to identify which features can be held as-sintered and which need secondary operations, then recommends the most cost-effective combination.

Conclusion

MIM is a powerful process for precision metal parts, but getting the most out of it requires designing within the process's natural capabilities. By following these tolerance guidelines, wall thickness rules, and feature design practices, engineers can create parts that are manufacturable, repeatable, and cost-effective.

The most valuable step is early collaboration. If you send your drawing or 3D model to SINTS during the design phase, our engineering team can review manufacturability, recommend design adjustments, and confirm achievable tolerances before tooling begins. This reduces development time, avoids costly tool modifications, and ensures the final part meets your specification.