Guide
Why Do MIM Parts Distort in Sintering, and How Should a Design Allow for It?
MIM parts are small metal components molded from powder and binder, then debound and sintered, and they shrink by roughly a fifth in the furnace, so flatness depends on how the part is supported while it shrinks. The rules below follow MIMA design guidance: a large flat face or common plane, uniform walls, and caution with long spans and cantilevers.
By Saola Manufacturing engineering team · Published · Updated
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Why do MIM parts distort during sintering at all?
A molded MIM part shrinks by about 20% while the binder leaves and the powder densifies, so any feature the furnace support cannot hold in place moves as the part shrinks. The Metal Injection Molding Association (MIMA) says the parts must be adequately supported at this stage to minimize the possibility of distortion.
After molding and debinding, the part is staged on a ceramic or graphite setter and sintered in a controlled-atmosphere batch or continuous furnace. It is heated along a monitored profile to approximately 85% of the metal’s melting temperature, and it shrinks isotropically, typically 18% to 22% depending on binder volume (MIMA, Process Overview). Isotropic shrinkage means the part scales evenly in every direction when it is uniform. It does not mean the part stays flat. A plate resting on a setter drags against it as it shrinks, and an arm with nothing under it sags under its own weight at temperature.
Uneven geometry adds a second cause. MIMA states that variations in wall thickness lead to distortion, internal stresses, voids, cracking and sink marks, and that they cause non-uniform shrinkage that interferes with dimensional and tolerance control (MIMA, Complex Designs with MIM). So there are two separate mechanisms to keep apart: the part is not supported where it needs to be, or the part shrinks unevenly because its sections differ.
This guide is for an engineer holding MIM samples that are bowed or out of flat on a long span, or one drawing a new thin plate, cover, ring or arm for MIM. It covers what the furnace support can and cannot hold, what to write on the drawing, what to ask the supplier, and when a different process or a machined finish is the better answer. It gives no flatness number, because the sources behind it do not publish one.
Which shapes does the furnace support hold, and which need special setters?
Shapes with a large flat face, or several features that share a common plane, can be supported by standard fixtures. MIMA says that parts with long spans, cantilevers or delicate points may need part-specific fixtures or setters, which can be expensive to produce.
The table turns that guidance into a design review. Read the “Basis” column first: only the rows marked MIMA come from the published guidance, and the rows marked Inference are engineering judgment that the supplier should confirm for the actual alloy and part. The practical point is cost. A part that fits a standard setter uses support the plant already has. A part that needs its own setter adds tooling, and the setter is a second shape that has to be designed, made and proven.
This is also why the same envelope can be cheap or costly depending on one feature. A flat cover with a small boss may sit happily on a plate. The same cover with a long overhanging tab may not. When you convert a machined part, check each overhang and each long thin region against this table before you accept the geometry as drawn.
| Geometry | What MIMA says | Basis | Design response |
|---|---|---|---|
| Large flat face | Parts typically sit on flat ceramic plates or trays, so a large flat surface is ideal | MIMA | Mark one large face as the support face and keep gate and mark zones off it |
| Several features in a common plane | A common plane lets standard fixtures be used | MIMA | Keep supporting features coplanar in the model and on the print |
| Long span | May need part-specific fixtures or setters, which can be expensive to produce | MIMA | Shorten the span, or accept that a custom setter is part of the program |
| Cantilever arm | May need part-specific fixtures or setters | MIMA | Review the arm length and thickness with the supplier before tool release |
| Delicate point or thin tip | May need part-specific fixtures or setters | MIMA | Add section at the tip, or move the tip into a supported plane |
| Thin ring or tube | Not addressed on the MIMA page | Inference | Ask the supplier how the ring sits in the furnace and which face carries the load |
| Thick-to-thin transitions | Thickness variation causes distortion and non-uniform shrinkage | MIMA | Core out thick regions and keep the wall near constant |
What flatness can you ask for on a MIM part?
The sources behind this guide give no flatness value for MIM parts, and MIMA publishes none, so any number you see in a brochure is that supplier’s offer, not a design rule. Put a flatness callout only on the faces that need it, and agree the value with the supplier before the tool is cut.
Flatness is a form control, and ASME Y14.5-2018 is the standard that defines the symbol and the rules for reading it. The callout limits how far every point of one surface may stray from a perfect plane. It does not by itself say whether the requirement applies to the part as sintered or after a later step, so say so on the print. The behavior of the symbol, and the common mistakes with it, are covered in the guides on flatness on die castings and parallelism versus flatness.
Three decisions belong on the drawing or in the RFQ, because they change what the supplier can promise. First, which face needs the flatness, and how it is used in the assembly. A sealing face and a cosmetic face are different requirements. Second, whether the flatness is required as sintered, or after coining or machining. Third, how it will be measured, for example on a coordinate measuring machine with a stated number of points, or against a surface plate. A flatness value with no stated method invites a dispute at first article.
How should you change the design to cut distortion?
Keep walls uniform, give the part a large flat face or a common plane to sit on, and shorten long spans or add section to cantilevers. The first two come directly from MIMA, and the rest are design moves to confirm with the supplier.
- Make the wall uniform. Core out heavy regions so thickness stays near constant, and make any change of thickness gradual. This addresses the non-uniform shrinkage that MIMA links to distortion.
- Give the part a base. A large flat surface, or several features in one plane, lets the plant use standard fixtures. If the natural base is curved, ask whether a flat pad can be added and machined away later.
- Shorten or support spans. A long thin region is the first place to look. Reducing its length or adding section is a conversation to have before tool release, because MIMA does not give a limit.
- Plan a finishing step for the face that matters. MIMA lists coining, in which a sintered part is forced to conform to a rigid mandrel or substrate for additional control of dimensional tolerances, and notes that all common machining operations can be applied to MIM components (MIMA, Secondary Operations with MIM). Either can bring a critical face into the tolerance that the sintering step does not hold.
None of these changes the function of a part, but each changes its cost. A pad that is machined off adds an operation. Coining adds a step and a tool. The cheapest fix is almost always the geometry change made before the mold is built.
What should the drawing, the RFQ and the first-article report say?
State the flatness face and its function, whether the requirement is as sintered or after a finishing step, and the measurement method. Then ask the supplier how the part is supported in the furnace and whether the quote includes a part-specific setter.
A short RFQ checklist covers the cases above.
- Which face is the flatness face, and what does it seal, locate or carry.
- As sintered, or after coining or machining, and who owns the finishing step.
- Standard fixture or part-specific setter, and whether the setter cost is inside the tooling quote.
- Which face rests on the setter, so cosmetic and sealing faces are not the contact face.
- A first-article flatness map of each critical face, not a single number, with the measurement method stated. The first-article inspection guide covers how to ask for it.
If the first samples are bowed, ask the supplier whether the molded (green) part was checked before sintering, so that the cause is separated into molding, support or shrinkage. That is a reasonable first diagnostic, not a step from the sources, and the answer decides whether the fix is in the mold, the setter or the design.
When is a long flat feature the wrong job for MIM?
When a face must be flat to a tight value over a long span and nothing in the geometry can give the furnace a base, plan on a machined finish or a different process for that feature. MIM is strongest on small, complex three-dimensional shapes, and a long thin plate is not one of them.
The options are to machine the critical face after sintering, to split the part so the flat feature becomes a separate component, or to move the whole part to another route. Saola places MIM work through a partner plant it manages, and faces and threads that need machined tolerances are finished by CNC. The MIM and powder metal capability page describes how that is organized, and CNC milling covers the machined-finish side.
Which route is cheaper depends on volume, geometry and tolerance, and a drawing review answers it faster than a rule. Send the drawing with the annual volume and the flatness requirement marked on the face that matters.
Frequently asked
How much does a MIM part shrink in sintering?
MIMA gives about 20% for green parts, depending on binder volume, and a typical range of 18% to 22% in its process overview. The supplier builds that shrinkage into the mold, so you design to the finished dimension.
Can distortion be corrected after sintering?
Sometimes. MIMA lists coining, which forces a sintered part against a rigid mandrel for additional dimensional control, and machining as secondary operations (MIMA). Whether either brings a bowed face into tolerance depends on how far it moved and the material.
Does a bowed sample mean the mold is wrong?
Not necessarily. Distortion can come from molding, from how the part sat in the furnace, or from uneven wall thickness. Ask the supplier which one it is before paying for a tool change.
Who pays for a part-specific setter?
The sources do not say. MIMA only notes that part-specific setters can be expensive to produce, so ask in the RFQ whether the setter is inside the tooling quote or a separate item.
Is flatness the same as parallelism?
No. Flatness limits one surface against a perfect plane, while parallelism relates one feature to a datum; ASME Y14.5-2018 defines both, and the parallelism versus flatness guide explains the difference.
Sources
- MIMA, Complex Designs with MIM, Metal Injection Molding Association, accessed October 2026
- MIMA, Process Overview: MIM, Metal Injection Molding Association, accessed October 2026
- MIMA, Secondary Operations with MIM, Metal Injection Molding Association, accessed October 2026
- ASME Y14.5-2018 (R2024), Dimensioning and Tolerancing, ASME, standard page accessed October 2026