Lesson 9.2ShapeChapter 9 · Powders and ceramics

Metal injection molding (MIM)

Mold a powder-and-binder paste, remove the binder, sinter

Metal injection molding (MIM) shapes metal like plastic: a hot paste of metal powder and plastic binder is injected into a mold. Then the plastic is removed and the metal is sintered, so the part shrinks into nearly solid metal. It makes tiny, complex parts such as watch-case pieces, phone hinge parts and surgical tool jaws.

Photo: INDO-MIM, CC BY-SA 4.0, via Wikimedia Commons

Metal injection molding · mix, mold, debind, sinterThe top row turns pellets into a molded “green” part; the bottom row follows that part as it loses its binder and shrinks. Circled numbers match the steps.

How it works

Mix the feedstock. Very fine metal powder, with grains typically under 20 µm (a quarter of a hair's width), is kneaded with a binder of wax and plastic. The paste is about 60% metal by volume and is chopped into pellets.
Mold. An injection molding machine (a heated barrel with a turning screw) melts the binder and pushes the paste into a steel mold. Out comes a “green” part: full shape, but 15–20% bigger than the final part. The runners (feed channels) are ground up and reused.
Debind. Most of the binder is removed, usually with a solvent or a chemical vapor first, then with gentle heat. What is left is a fragile “brown” part: metal grains held by a trace of binder, full of open pores.
Sinter. The part goes into a vacuum or hydrogen furnace at roughly 1,200–1,400 °C for steels, close to (but below) melting. The grains fuse and the pores close, so the part shrinks evenly by about 15–20% and ends up 95–99% dense. Heat treatment, plating or light machining can follow.

Key facts

Materials: Mostly stainless steels (such as 17-4PH and 316L) and low-alloy steels; also titanium, tungsten alloys, cobalt-chrome and magnetic alloys
Typical parts: Watch-case parts, hinge parts, small gears, orthodontic brackets, surgical tool jaws, lock and firearm parts
Part size: Small: typically 0.1–100 g, from fingertip to palm size
Precision: About ±0.3–0.5% of a dimension (about ±0.05 mm on a 10 mm feature)
Surface finish: Smooth, roughly Ra 0.8–1.6 µm (Ra is the average height of surface bumps), copying the mold
Batch size it suits: High volume: typically 10,000 to millions of parts a year
Tooling and machine cost: Mold roughly $10,000–100,000; molding machines, debinding units and furnaces cost hundreds of thousands of dollars each
Speed: Molding: one shot every 10–60 s, often several parts per shot; debinding and sintering take many hours, often a day or more per batch

Strengths and limits

Strengths

Complex 3D shapes, thin walls and fine details that would be costly to machine
95–99% dense, so strength comes close to that of solid metal
Handles alloys that are hard to machine, like stainless steel and titanium
Little waste: runners are recycled into new feedstock

Limits

Only small parts: thick sections debind slowly and can crack or sag
The 15–20% shrinkage must be predicted exactly when the mold is designed
Expensive mold and a long debind-and-sinter cycle; needs large volumes
Fine powders cost more than ordinary press-and-sinter powders

Where it's used

Consumer electronicsPhone hinge parts, SIM-card trays, camera and connector parts
Medical and dentalOrthodontic brackets, surgical tool jaws and handles
Watches and jewelryWatch cases, buckles, bracelet links
AutomotiveTurbocharger vanes, fuel-injector parts, lock and sensor parts
FirearmsTriggers, hammers, safeties and other small action parts
Tools and locksLock parts, small gears, power-tool parts

How to recognize it

Small, intricate steel parts with smooth, satin surfaces and crisp details, like a molded plastic part made of metal.
Tiny round marks from ejector pins and a small gate scar where the paste entered the mold.
A faint parting line where the two mold halves met.
Even wall thickness, ribs and hollowed-out pockets, typical of molded parts rather than machined ones.

Compare with

Powder metallurgy9.1presses powder in a die; cheaper for simple, flat shapes, but leaves more pores.Injection molding2.1the same machine and mold idea, but the plastic stays in the part.Investment casting1.2also makes complex steel parts, usually bigger ones, by pouring molten metal into a ceramic shell.

Watch it

AMT Pte. Ltd. · The four steps and the kinds of parts MIM makes

GKN Powder Metallurgy · How the binder leaves and how much the part shrinks

MPP · Real molding machines, furnaces and finished parts

Zimmer Group · A manufacturer's full process chain, from molding cell to part

See real photos

Wikipedia: Metal injection molding — overview of the steps, materials and uses
Wikimedia Commons photo search

Remember

MIM is plastic injection molding with a metal-filled paste, followed by binder removal and sintering.
The part shrinks 15–20% in the furnace, so the mold is made bigger by exactly that much.
It is the go-to process for small, complex metal parts in large numbers.