MIM Design Guide: Part Geometry, Wall Thickness, Tolerances, and Tooling Rules

# MIM Design Guide: Part Geometry, Wall Thickness, Tolerances, and Tooling Rules

Metal injection molding is generous about shape and ruthless about process physics. You can mold nearly any geometry that releases from a two-part mold — but whether that geometry survives debinding without cracking and shrinks to tolerance during sintering is a different question.

This MIM design guide walks through the geometry rules that determine whether your part is a straightforward MIM candidate, a borderline case that needs DFM compromise, or better served by another process. It complements our MIM process overview and MIM material guide.

For designers comparing processes, see also our DFM guide for conventional PM and PM tolerance planning guide.

The Three Physical Constraints

Every MIM design rule traces back to one of three physical realities:

1. Fill. The feedstock — metal powder suspended in molten polymer — must reach every corner of the cavity before it freezes. Density gradients during fill become shrinkage gradients during sintering.
2. Debind. The binder must escape through interconnected channels before the brown part enters the sintering zone. Section thickness controls debinding time, squared.
3. Shrink uniformly. A 15–20% linear shrinkage from cavity to final part is not a problem if it is the same everywhere. If it varies across the part — because of thickness differences, fill orientation effects, or gravity — the part distorts.

Wall Thickness: The Most Important Number

The coring strategy. If your part has a thick boss or hub, core it from the back side to bring the effective wall section below ~6 mm. A hollow boss with 2–3 mm walls debinds cleanly; a solid 12 mm boss does not.

Thickness Transitions: The Distortion Source

Abrupt thickness changes are the most common cause of sintered-part distortion and the most common reason for MIM tool rework. The physics:

• During injection, a thick section holds heat longer and the feedstock continues to flow and pack while adjacent thin sections are already frozen. This creates a density gradient.
• During sintering, the denser region shrinks less and the less-dense region shrinks more. The result is warpage, sink marks, or dimensional scatter.

Rule of thumb: Keep the ratio of thickest to thinnest section below about 3:1. A 6 mm rib next to a 2 mm wall is reasonable (3:1). A 10 mm boss next to a 1 mm wall (10:1) will distort.

Design fixes for thickness transitions:

• Taper or radius the transition — a 30–45° ramp distributes the density gradient over a longer distance
• Add a generous fillet at the junction to reduce stress concentration in the green part
• If the thick section is unavoidable, add a thin web or stiffener to constrain it during sintering

Gating: Where the Feedstock Enters

In MIM, gate placement is not just a molding convenience — it determines the flow orientation and density distribution through the entire part.

Good gate locations:

• On the thickest section — so the gate freezes last and packing pressure reaches the entire cavity
• On a non-functional surface — because the gate vestige will be visible after sintering unless post-machined
• Aligned so the melt front advances uniformly — avoid jetting, race-tracking, or weld lines at thin sections

Bad gate locations:

• On a critical sealing or cosmetic surface
• At the thin end of a part where the gate freezes before the thick sections are packed
• Creating a long weld line through a mechanically stressed region — MIM weld lines have lower local density and reduced fatigue strength

Discuss gate strategy with your MIM supplier before locking the part geometry, not after the mold is cut.

Sintering Support and Distortion Control

A MIM brown part is fragile — roughly the strength of chalk. During the initial stage of sintering, before neck growth provides structural integrity, the part must support its own weight without sagging. This is most critical for parts with long spans, cantilevered features, or asymmetry.

Design strategies:

• Flat-on-flat setter contact. If the part has a flat face, design it to sit flat on the sintering tray in the orientation that minimizes unsupported overhang.
• Ribs and gussets. Add stiffening features early in the design — they are nearly free in MIM tooling and pay for themselves in yield.
• Sacrificial support tabs. If no natural stable orientation exists, temporary support features can be added and then ground off after sintering. Budget for the secondary operation.
• Post-sinter sizing. For flatness-critical faces, plan a coining or sizing step after sintering. It is standard in the MIM cost model and far cheaper than scrapping sintered parts for flatness.

Tolerances: What Is Achievable from As-Sintered MIM

The golden rule: If you need IT7 or tighter on a feature, plan for secondary machining. The MIM shop can quote the as-sintered near-net shape plus a quick machining pass, and the combined cost is usually lower than trying to force the sintering process to hold machining-class tolerances.

Features That Work Well in MIM

• External threads: Coarse-pitch, generous root radii, oriented in the mold-opening direction
• Cross-holes: Feasible with retractable side cores; adds tooling cost but saves secondary drilling
• Undercuts: Possible with side actions, collapsible cores, or split-cavity tooling — each adds complexity and cost
• Knurling and texturing: Can be molded directly into the cavity surface
• Logos and markings: Easily molded; raised is better than recessed for tool life

Features to Avoid or Approach With Caution

• Sharp internal corners: Stress concentrations in the green part; minimum 0.3–0.5 mm internal radius recommended
• Deep blind holes: Core pins deflect during injection and can break during ejection; depth-to-diameter ratio above ~3:1 is risky
• Zero-draft walls: Possible but reduce process window; 0.5–1° draft is cheap insurance
• Very long, slender cantilevers: Will sag during sintering unless supported

The MIM DFM Checklist

Before sending a drawing to a MIM supplier, answer these:

• [ ] All wall sections between 1–8 mm?
• [ ] Thickest-to-thinnest section ratio under ~3:1?
• [ ] Gate location identified on a non-functional surface?
• [ ] Part has a stable sintering orientation (flat face down)?
• [ ] Critical dimensions flagged for potential secondary sizing?
• [ ] Internal radii ≥0.3 mm on all corners?
• [ ] Draft angles included on all mold-opening surfaces?
• [ ] Annual volume high enough to amortize $15K–$50K tooling (typically 10K+)?

Related Resources

• MIM Process Guide — Full Step-by-Step
• MIM Material Selection Guide
• PM vs MIM Comparison
• DFM for Conventional PM
• PM Tolerance Planning