Guide
What Are the Wall Thickness, Rib and Radius Rules for Metal Injection Molded Parts?
Metal injection molded parts are small steel and stainless components formed in a mold from powder and binder, then debound and sintered, so their design rules come from molding and furnace shrinkage, not from cutter access. The rules below follow the Metal Injection Molding Association’s design guidance: even walls, ribs no thicker than the wall, generous radii and a supportable shape.
By Saola Manufacturing engineering team · Published · Updated
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Why do MIM parts follow molding rules, not machining rules?
Because the shape is formed in a mold cavity and then shrinks a typical 20% in a furnace, so every rule is about filling the cavity evenly, ejecting the part and keeping it flat while it shrinks. MIMA notes that the cavity sets the part’s dimensional limits and that changes after ejection are costly.
The simplest MIM mold has two sections that meet at a parting plane, with a core forming internal features and an impression forming external ones, and slides or cores add complexity and cost (MIMA, Complex Designs with MIM). The molded green part is approximately 20% larger in every dimension than the finished part, and it shrinks isotropically, typically by 18% to 22% depending on binder volume (MIMA, Process Overview). A design that works in plastic molding is a good starting point; a design that works only because a cutter can reach every face is not.
This guide is for a design engineer drawing a new part for MIM or converting a machined one. It follows the MIMA design rules in the order a drawing review goes: walls, ribs, holes, radii and draft, gate position, sintering support, threads. It gives no minimum wall, because MIMA publishes none and a number copied from a supplier’s brochure is not a design rule. It ends with a checklist for the drawing and the RFQ.
How thick and how uniform should the walls be?
MIMA prefers walls between 1.3 and 6.3 mm (0.05 to 0.25 in.) and prefers them uniform, with exceptions possible in both directions. It publishes no numeric minimum, so a thin wall is a conversation with the fabricator, not a table lookup.
The 1.3 to 6.3 mm range is the association’s stated preference, and it explicitly allows exceptions (MIMA, Complex Designs with MIM). Uniformity matters more than the absolute value: uneven walls shrink unevenly and cause distortion, voids and sink marks. Where a thick section is unavoidable, MIMA recommends coring it, and where the thickness must change, providing a gradual transition between the two.
For a part converted from machining, this is usually the first change. A machined bracket often has a thick root and a thin tip because that is how it was cut from bar. In MIM, the same shape should be cored out from the thick region so the wall stays near-constant. Ask the fabricator to mark up the section thickness on a copy of your drawing before tool release; it is the cheapest review in the whole program.
| Feature | Published guidance | Why it matters |
|---|---|---|
| Wall thickness | 1.3 to 6.3 mm (0.05 to 0.25 in.) preferred; exceptions possible both ways | Uniform walls reduce distortion, voids and sink marks |
| Rib thickness | Not thicker than the adjoining wall; use multiple ribs instead of one thick rib | A thick rib acts like a thick section and sinks or distorts |
| Thickness transitions | Gradual where uniform thickness is not possible | Abrupt changes cause uneven shrink |
| Fillets and radii | 0.4 to 0.8 mm (0.015 to 0.030 in.) generally preferred | Lower stress concentration, easier flow and ejection |
| Draft | 0.5 to 2.0 degrees normal; more for long or textured features | Allows ejection without drag |
| Core holes | Parallel to the mold-opening direction (perpendicular to the parting line) preferred; through holes preferred over blind when length/diameter exceeds 4:1 | A blind hole uses a cantilevered core pin; a through hole supports it at both ends |
| Gate | Usually on the parting line at the thickest section | Material flows from thick to thin |
| Sintering support | Large flat surfaces or a common plane suit standard fixtures | Reduces distortion while the part shrinks |
| Thread flats | Narrow flats of about 0.005 in. typical at the parting line for external threads | Helps hold thread tolerance and seal the mold |
| Material callout | A grade and condition under one standard, such as MIM-17-4 PH per ASTM B883 | Property values come from the material standard, not from the design rules |
How should ribs, webs and radii be designed?
Keep every rib no thicker than the wall it joins, add more ribs instead of one thick one, and put a fillet at the base. The generally preferred fillet and radius range per MIMA is 0.4 to 0.8 mm (0.015 to 0.030 in.).
MIMA states that rib thickness should not exceed that of the adjoining wall, and that where more stiffness is required, multiple ribs are better than a thicker one (MIMA, Complex Designs with MIM). A rib as thick as the wall it joins creates a local heavy section that fills and shrinks differently, which is where sink and distortion start. The preferred fillet and radius range helps the feedstock flow and the part release, and it lowers stress concentrations in service.
On a drawing, avoid sharp internal corners as a default. Specify the radius on the print, or use a general note that all unspecified internal radii are within the preferred range, and mark any corner that must stay sharp for function. A machined part often has sharp internal corners because a cutter can leave them; a molded part does not need them.
Which holes, undercuts and threads can the mold form?
Holes perpendicular to the parting line are cheapest to mold, holes parallel to it need slides, and internal undercuts are usually avoided. MIMA says external undercuts can be formed with a split mold and that internal tapping is usually more precise than unscrewing cores.
Cross holes, internal threads, undercuts and cantilevers are the geometries MIMA cites as well suited to MIM (MIMA, Designing with MIM), but each has a tooling consequence. Holes in the mold-opening direction are formed by fixed cores. Holes parallel to the parting line need slides or hydraulic cylinders, which add cost. For deep holes, MIMA prefers through holes to blind holes when the length-to-diameter ratio is above 4:1 because the core pin can be supported at both ends.
For threads, MIMA says internal tapping is usually more precise and cost-effective than unscrewing cores, and recommends external threads on the parting line with narrow flats, typically .005 inch. If a thread needs a defined class of fit, plan for a tapping or thread-rolling step after sintering and note it on the drawing. MIMA also lists the option of machining features into the green part, such as undercuts or cross holes (MIMA, Process Overview).
Where should the parting line, gate and draft go?
Put the parting line on a single flat plane where you can, locate the gate at the thickest section so material flows from thick to thin, and allow 0.5 to 2.0 degrees of draft. More draft is needed for long or textured features.
MIMA says features should be perpendicular to the parting line where possible and that a single flat parting plane is preferred. Gates are usually on the parting line at the thickest section (MIMA, Complex Designs with MIM). Because the gate leaves a witness mark, the drawing should say which surfaces may carry it. Decorative features such as logos, knurls, part numbers and cavity identification can be molded in, per the same page.
Cavity identification is worth asking for. When a multi-cavity tool makes a part, a number molded in lets the quality team trace a dimensional problem back to one cavity. The draft range is a general rule; a feature that is long or textured needs more, and a short feature may work with less if the fabricator agrees.
How do you design for sintering support and shrink?
Give the part a flat face or a common plane that a standard setter can support, and avoid long unsupported spans. The part shrinks a typical 20% in the furnace, so unsupported features sag or warp if the geometry cannot be held.
MIMA says that green parts shrink by about 20%, depending on binder volume, and that large flat surfaces or a common plane help standard fixtures support them (MIMA, Complex Designs with MIM). A part that sits on its flat face through the cycle shrinks without dragging; a part resting on a point or a thin edge may distort. When you design a hook, a long arm or a thin plate, ask yourself how it will sit in the furnace, not only how it will function.
If a part has to be flat or straight to a tight requirement, coining after sintering is one fix; MIMA describes coining as forcing a sintered part against a rigid mandrel or substrate to improve dimensional control and flatness (MIMA, Secondary Operations with MIM). Tolerances and which features to machine are covered in the companion guide on as-sintered tolerances.
What belongs on the MIM drawing and in the RFQ?
The material grade and standard, the critical features and their datums, the surfaces that may carry gate or parting-line marks, the annual volume, and which features may be machined after sintering. A drawing that says only “MIM” invites guesswork.
A workable RFQ package contains the following. State the grade, for example a 316L or 17-4 PH designation under ASTM B883 or ISO 22068 (ISO 22068:2012), plus the heat-treatment condition. Mark critical-to-function dimensions and the datum structure. Say which surfaces are cosmetic. Give annual volume and expected program life. Ask the fabricator for a design review that marks section thickness, gate location and support face before the tool is cut.
Saola places MIM through a partner plant it manages, finishes faces and threads by CNC where the drawing asks for it, and names the production site and country of origin on each quote. See the MIM and powder metal capability page, and for the CNC side the guide on design for machining.
Frequently asked
What is the minimum wall thickness for a MIM part?
MIMA publishes no numeric minimum; it states a preferred range of 1.3 to 6.3 mm (0.05 to 0.25 in.) and allows exceptions in both directions. Ask the fabricator what they will hold for your alloy and part size.
How much draft does a MIM part need?
MIMA gives 0.5 to 2.0 degrees as the normal range, with more for longer features or textured surfaces. Confirm with the fabricator for your geometry.
Can MIM make internal threads?
MIMA lists internal threads among the geometries suited to MIM, and says internal tapping after molding is usually more precise and cost-effective than unscrewing cores. Plan a tap or thread-forming step and say so on the drawing.
Do I need to design for shrinkage myself?
No. The fabricator builds shrink into the tool. You design to the finished dimension and ask the fabricator to confirm which features are likely to move, since MIMA notes the green part is typically about 20% larger than the finished part.
Which standard do I cite for the material?
ISO 22068:2012 covers MIM materials and ASTM B883 covers ferrous MIM materials; MPIF Standard 35-MIM is the third common reference. These standards are sold, so confirm the edition with your fabricator.
Sources
- 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)
- MIMA, Process Overview: MIM, Metal Injection Molding Association, accessed October 2026 (page copyright 2026)
- MIMA, Secondary Operations with MIM, Metal Injection Molding Association, accessed October 2026 (page copyright 2026)
- MPIF, Metal Injection Molding (Intro to PM), Metal Powder Industries Federation, accessed October 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)