Metal injection molding (MIM) DFM guidelines cover wall thickness (0.5–5 mm, uniform), draft angles (0.5–2° per side), rib/boss thickness (50–70% of wall), hole sizing (0.3–0.5 mm min), tolerances (±0.3%), gate location, and sintering shrinkage (15–20%), keeping tooling cost and unit price down.
Metal injection molding (MIM) can deliver small, three-dimensional, high-precision metal parts at production volumes that machining and casting cannot match economically. But MIM is a molding process first and a metallurgical process second — so a part that ignores molding fundamentals will fail before it ever reaches the sintering furnace. The rules below are the design-for-manufacturing (DFM) guidelines our engineers apply when reviewing every new MIM component, and they are the fastest way to keep tooling cost and unit price down.
In CNC machining, a difficult geometry usually just means a longer cycle time. In MIM, the same geometry can mean a tool that cannot be filled, a part that warps during debinding, or a mold that needs expensive side actions. Because the part shape is locked into a hardened steel mold and then shrinks by roughly 15–20% during sintering, design decisions made early carry a much higher cost multiplier. A 30-minute design review upstream routinely saves five figures of tooling rework downstream.
Nominal wall thickness for MIM typically falls between 0.5 mm and 5 mm, with 1–3 mm being the sweet spot for most consumer-electronics and medical parts. Walls should be as uniform as possible: because the feedstock shrinks isotropically during sintering, thick sections cool and densify more slowly than thin ones, which drives internal stress, warping and voids. Where a thick-to-thin transition is unavoidable, blend it gradually — keep the ratio of adjacent wall thicknesses below about 2:1 and add generous radii at the junction.
Every vertical wall that is formed by the mold should carry a draft angle so the green part can be ejected cleanly. A practical rule is 0.5–2° per side: 0.5° minimum on smooth walls, and 1–2° where a texture or fine detail must release. Insufficient draft causes scuffing, tearing and short shots that no amount of process tuning fully cures.
Ribs and bosses are useful for stiffness, but they are also where sink marks and internal voids originate. Keep rib thickness at roughly 50–70% of the adjacent wall, and boss walls in the same range. Where a boss must be thicker than the nominal wall, consider a cored or stepped design instead of a solid mass. Generous fillets at the rib root (≥ 0.25 mm) spread stress and improve fill.
MIM excels at small, deep holes that would be slow to drill. Core pins as small as 0.3–0.5 mm in diameter are feasible, and blind holes can run to depth-to-diameter ratios around 4:1. Add a small taper or a domed tip so the pin does not break during ejection, and avoid holes that intersect at sharp angles. Through-holes are preferred over blind holes because they fill and eject more reliably.
As-sintered MIM holds roughly ±0.3% of the dimension for most features — tighter than conventional powder metallurgy and competitive with investment casting. Where a critical feature needs more, design it so it can be touched up by a light secondary machining or coining operation, which typically reaches ±0.05 mm or better. Over-specifying tolerance on non-functional surfaces only adds cost.
Undercuts require side actions, lifters or complex tooling that raise mold cost and maintenance. Whenever the function allows, reorient the feature so it pulls in the main mold-opening direction. When an undercut is truly required, flag it early so it can be quoted with the correct tooling.
MIM parts are usually filled through a single gate, which leaves a small gate scar and a controlled weld line. Place the gate on a non-cosmetic, non-critical face, and orient the flow so it fills thick sections first and reaches thin ribs and far walls last. Gate position also influences the location of the knit line — keep it away from high-stress areas.
The green part is roughly 15–20% larger than the finished component in every dimension; that shrinkage is isotropic and is compensated in the mold cavity. The practical takeaway for designers is that the finished drawing is what matters — but dimensional relationships, wall uniformity and feature proportions should be designed for the shrunk state, not the as-molded state.
As-sintered surfaces reproduce the mold finish, typically Ra 1.0–1.6 µm. Smoother finishes are achieved by polishing the mold or by post-processing such as tumble polishing, blasting or plating. Specify the finish only where it is functionally needed — cosmetic over-specification is one of the most common cost drivers in MIM quotes.
Design rules and material selection go hand in hand. The most common MIM alloys are 316L and 304L stainless steel for corrosion resistance, 17-4PH for high strength and hardness, 430L and 440C for wear and magnetic applications, low-alloy steels such as 4605 and 4140 for structural parts, and specialty grades including Kovar, soft-magnetic alloys and titanium. Selecting the alloy early lets the tool and the sintering profile be designed around its shrinkage and dimensional behavior from the start.
| Design parameter | Typical recommendation | Notes |
|---|---|---|
| Wall thickness | 0.5–5 mm (1–3 mm ideal) | Keep uniform; transitions < 2:1 |
| Draft angle | 0.5–2° per side | More for textured surfaces |
| Rib / boss thickness | 50–70% of nominal wall | Prevents sink and voids |
| Minimum hole diameter | 0.3–0.5 mm | Blind holes up to ~4:1 depth:diameter |
| As-sintered tolerance | ±0.3% of dimension | Tighter after secondary machining |
| Linear sintering shrinkage | 15–20% (isotropic) | Compensated in the mold |
| As-sintered surface finish | Ra 1.0–1.6 µm | Improve by polishing / post-processing |
| Undercuts | Avoid if possible | Need side actions; raise tool cost |
The cheapest place to fix a MIM design problem is on the drawing, before the mold is machined. As an ISO 9001-certified MIM manufacturer in Ningbo, China, Precision-MIM runs a free design-for-manufacturing review on every new part, checking wall thickness, draft, tolerances and material selection against real tooling and sintering behavior — and returns a manufacturable design and a firm quotation. Send us your STEP file and target volume, and our engineers will tell you what will work, what will cost more than it should, and how to fix it.
Nominal wall thickness for MIM typically falls between 0.5 mm and 5 mm, with 1–3 mm being the sweet spot for most consumer-electronics and medical parts. Walls should be as uniform as possible, keeping adjacent thickness ratios below about 2:1.
The green part is roughly 15–20% larger than the finished component in every dimension, and that shrinkage is isotropic and compensated in the mold cavity. Design dimensional relationships and feature proportions for the shrunk state.
A practical rule is 0.5–2° per side: 0.5° minimum on smooth walls, and 1–2° where a texture or fine detail must release. Insufficient draft causes scuffing, tearing and short shots.
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