The core of MIM design is keeping walls between 0.5 and 6 mm and as uniform as possible, breaking sharp corners with generous radii, and planning for the predictable sintering shrinkage of roughly 15–20% by letting the mold run oversized rather than fighting it in secondary work. Undercuts and features that sit perpendicular to the parting line — a real cost driver in many processes — are a MIM strength, formed directly by side actions in the tool. Applied at the CAD stage, these rules turn a drawing into a part that molds, debinds, and sinters predictably.
This guide walks through the design rules that matter most for metal injection molding — wall thickness, draft and ejection, undercuts, holes and radii, and sintering shrinkage compensation — and points to the deeper DFM and undercut references where a given rule needs more detail. It is written for the design engineer who wants a part that is toolable on the first DFM pass.
Wall Thickness: Stay Uniform, Stay in Range
MIM injects a viscous metal-powder feedstock into a cavity under pressure, then removes the polymer binder in debinding and densifies the part in sintering. Both steps punish thick or uneven sections: a thick web takes longer to debind, is more prone to internal porosity and sink marks, and shrinks less predictably than a thin, uniform wall. The practical window is a wall thickness of 0.5 to 6 mm — below it the feedstock struggles to fill and the green part is fragile; above it debinding time climbs and the risk of trapped binder rises.
Uniformity matters as much as the absolute number. When a boss, rib, or flange must be heavier than the surrounding wall, core it out from the back or break it into ribs so the effective section stays in range. Transitions between thicknesses should be gradual and radiused — a step change in section is a favorite spot for debinding cracks and sink.
Part mass follows the same geometry. MIM covers parts from 0.1 to 200 g, and staying within that envelope keeps feedstock volume, cycle time, and furnace loading in balance. For the full picture of the size window, see the MIM size limitations guide.
Draft and Ejection: Less of a Constraint Than You Think
In plastic injection molding, draft angles are non-negotiable because the polymer shrinks onto the core and must slide free. MIM is more forgiving: the green part is ejected while still slightly compliant, and MIM parts are typically small, so a straight-walled part with modest depth will usually release without draft.
That said, a small draft — on the order of half a degree to a degree — still pays for itself on taller features, textured surfaces, and thin ribs, where it reduces ejection force, scuffing, and core wear. The bigger point is the direction of the draw: because MIM tooling can carry side actions and slides, features that would otherwise lock the part in a mold are not the deal-breaker they are elsewhere.
- Keep draw simple on the main parting line; leave perpendicular features to side actions rather than forcing them into the primary draw.
- Add roughly 0.5–1° draft on ribs and bosses taller than a few millimeters to protect surface finish and tool life.
- Avoid abrupt texture changes on vertical walls where ejection contact is highest.
Undercuts: A MIM Strength, Not a Wall
Undercuts — external snaps, internal threads, side ports, and cross-features — are where MIM pulls ahead of competing processes. Because the mold can carry sliding cores and collapsible mechanisms, a feature that sits perpendicular to the parting line is formed in the tool rather than machined afterward. There is no hard draft-direction lock the way there is in a simple two-plate die.
The cost lives in the tool: each slide adds a mechanism to cut, fit, and maintain, and slide travel and parting-line witness lines must be accounted for in the design. The geometry is routine, but the economics depend on how many slides a part needs. Emitech reviews undercut geometry in the DFM pass and returns a quotation within 24 hours of receiving the drawing.
For the full treatment of external and internal undercut design — snap-fit geometry, core travel limits, and thread-forming — see external and internal undercut design.
Holes, Slots, and Radii
Holes are formed by core pins in the mold, which makes them essentially free in cycle time but not free in tool cost. Through-holes are preferred: the pin is supported at both ends, stays straighter, and lasts longer than a cantilevered blind-hole core. Small through-holes are feasible well below the wall-thickness limit, though the exact minimum depends on the hole's length-to-diameter ratio and the material.
Slots and elongated openings follow the same logic but add a tooling caveat — a narrow slot needs a narrow, unsupported core, so very deep narrow slots should be reviewed early. Generous radii are the highest-value design change in MIM: a fillet at an internal corner reduces stress concentration, smooths feedstock flow, cuts sink and debinding-crack risk, and extends tool life, all for zero added part cost.
- Prefer through-holes over blind holes; the core pin is supported at both ends.
- Add radii at every internal corner and sharp edge — they cost nothing and protect the tool.
- Keep hole and slot aspect ratios within DFM limits; Emitech flags borderline geometry in the quotation review.
For specific hole and slot geometry guidance, see MIM hole and slot design.
Sintering Shrinkage: Design for the Scale-Up
Every MIM part shrinks as it sinters to 95–99% of theoretical density. The shrinkage is large in absolute terms — a part typically contracts about 15–20% linearly — but it is predictable, and that predictability is the whole point: the mold cavity is cut oversized by the material's shrinkage factor, and the part sinters down to its target size.
The catch is that shrinkage is not perfectly uniform. It drifts slightly with wall thickness, part orientation in the furnace, and geometry, which is why as-sintered tolerance lands at ±0.3–0.5% of nominal dimension (about ±0.025 mm on features under 3 mm) rather than at a fixed number. Features that must hold tighter are finished with sizing or CNC secondary operations.
The practical rule: design the part to its intended dimensions, send the drawing, and let the tool shop apply the shrinkage factor. For dimension control in detail, see MIM tolerances and accuracy.
The DFM Pass: A Quick Reference
| Feature | Guideline | Why it matters |
|---|---|---|
| Wall thickness | 0.5–6 mm, kept uniform | Filling, debinding time, sink and shrinkage consistency |
| Draft | Not strictly required; 0.5–1° aids release | Tool life, surface finish, ejection force |
| Undercuts | Formed by side actions and collapsible cores | MIM signature advantage; no draft-direction lock |
| Holes | Through-holes preferred, small cores feasible | Core support, tool cost, aspect ratio |
| Radii | Fillet every internal corner | Stress concentration, feedstock flow, tool wear |
| Shrinkage | Compensate in tool scale-up | ±0.3–0.5% as-sintered tolerance, dimensional accuracy |
Most of these rules are checkable in CAD before a single email is sent, and catching them early is what keeps a MIM project on its 15–20 day tooling and 4–6 week production schedule. For the full MIM-plus-CNC design-for-manufacturing treatment, see DFM guidelines for MIM and CNC, and for what the factory actually runs, the MIM manufacturing capabilities page.
Frequently Asked Questions
Q: What wall thickness should I design MIM parts at?
Keep walls between 0.5 and 6 mm and as uniform as possible. Below 0.5 mm the feedstock struggles to fill and the green part is fragile; above 6 mm debinding slows and sink and internal porosity become more likely. Where a feature must be heavier, core it out or rib it to stay in range.
Q: Do MIM parts need draft angles?
Not strictly — MIM green parts release more easily than plastic and most parts are small, so simple parts can run without draft. A small draft of about 0.5–1° still helps on tall ribs and textured surfaces, and features perpendicular to the draw are handled by side actions rather than draft.
Q: Can MIM produce undercuts?
Yes — external snaps, internal threads, and side ports are a MIM strength. The mold carries sliding cores or collapsible mechanisms to form them, so there is no draft-direction lock. The cost sits in the tooling, which adds one mechanism per slide, so Emitech reviews the geometry in the DFM pass.
Q: How is sintering shrinkage handled in design?
The part is designed to its target dimensions and the mold cavity is cut oversized by the material's shrinkage factor — a typical MIM part contracts about 15–20% linearly during sintering to 95–99% density. Because shrinkage drifts slightly with geometry, as-sintered tolerance is ±0.3–0.5% of nominal, tightened by sizing or CNC on critical features.
Q: What is the smallest hole MIM can mold?
The limit is set by aspect ratio and material rather than an absolute size: through-holes are supported at both ends and can be quite small, while blind holes need a cantilevered core that limits depth. Send the drawing for a DFM review — Emitech flags borderline hole geometry in the quotation.
Good MIM design is mostly about respecting the process: uniform walls, radiused corners, and a mold that runs oversized to absorb shrinkage. Applied at the CAD stage, these rules keep a part toolable, sinterable, and consistent — and they are the difference between a quotation that comes back clean and one that comes back with a list of changes. Send your drawing for a DFM review and a quotation through the contact page, and Emitech will return process feedback within 24 hours.
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