Lesson 9.1ShapeChapter 9 · Powders and ceramics

Powder metallurgy (press and sinter)

Press powder in a die, then sinter in a belt furnace

Powder metallurgy makes metal parts without ever melting the metal: you press fine powder into a die, then heat it until the particles bond. Small gears in cordless drills and oil-filled bronze bushings in desk fans are made this way. It is one of the cheapest ways to make small, accurate metal parts by the million.

Photo: Alchemist-hp, CC BY-SA 3.0 de, via Wikimedia Commons

Press-and-sinter powder metallurgy · fill, press, eject, sinterCircled numbers match the steps below: the top row happens in a press, the bottom row in a long furnace.

How it works

Blend. Fine metal powder, with grains mostly under 0.15 mm (about like flour), is mixed with alloy powders such as graphite or copper. About 0.5–1% wax lubricant is added so the powder slides in the die.
Fill. A feed shoe slides over the die and drops powder into the cavity (the hollow that gives the shape). The lower punch sits at a set depth, so each fill holds the same amount.
Press. Upper and lower punches squeeze the powder from both ends at about 400–800 MPa (4–8 tonnes per square centimeter). The result is a “green” part: full shape, but held only by interlocked grains, so it is about as fragile as chalk.
Eject. The lower punch rises and pushes the part out. The feed shoe sweeps it aside as it returns to refill, so the press makes a part every few seconds.
Sinter. A mesh belt carries the parts through a long furnace filled with protective gas (gas that prevents oxidation, the reaction behind rust). The lubricant burns off first. Then, at about 1,120 °C for iron and steel, atoms drift across each contact point (diffusion) and grow solid “necks” between particles.
Finish if needed. Many parts are ready as they leave the furnace. Others get a sizing press for tighter dimensions, oil soaking for self-lubricating bearings, heat treatment for hardness, or machining for side holes and threads.

Key facts

Materials: Mostly iron and steel; also stainless steel, bronze, brass, copper, aluminum, magnet alloys and tungsten carbide
Typical parts: Gears, sprockets, cams, bushings, lock parts, magnets, filters
Part size: Typically a few grams to about 2 kg, mostly palm-sized; press force limits the face area
Precision: About ±0.05–0.1 mm as sintered; tighter after a sizing press
Surface finish: Fine matte gray, roughly Ra 1–3 µm (Ra is the average height of surface bumps)
Batch size it suits: High volume: typically from about 10,000 to millions of parts a year
Tooling and machine cost: Die set roughly $10,000–100,000; presses and furnaces cost hundreds of thousands to millions of dollars
Speed: About 10–30 parts per minute per press; the furnace trip takes roughly 1–3 hours but runs nonstop

Strengths and limits

Strengths

Near-net shape: little or no machining and very little scrap
Low cost per part at high volume
Makes mixes that melting cannot, like copper-tungsten or metal with built-in pores
Very repeatable from part to part

Limits

Leftover pores (often 5–15% of the volume) make parts weaker than solid steel
Die sets are expensive, so small batches rarely pay off
Shapes must come straight out of the die: no undercuts or side holes
Size and height are limited by press force and by how evenly powder packs

Where it's used

AutomotiveOil-pump gears, timing sprockets, valve-timing rotors, gearbox synchronizer hubs
Power tools and appliancesPlanetary gears; self-lubricating bushings in drills, fans and mixers
Locks and hardwareLock cams, levers, latch parts
ElectricalSoft magnetic motor cores, ferrite and rare-earth magnets
FiltersPorous bronze and stainless steel filters, mufflers on air valves
Cutting toolsTungsten carbide inserts and drill tips

How to recognize it

A uniform matte gray surface; under a magnifier you may spot tiny pores.
Prism-like shapes: the outline stays the same from top to bottom, with flat faces and steps only along the pressing direction.
Bronze bushings that feel slightly oily, because their pores hold oil.
Chamfers that end in a tiny flat at the edge, because a knife-sharp punch edge would break.

Compare with

Metal injection molding9.2molds a powder-and-plastic paste, so it makes smaller, more complex parts that end up almost fully dense.Hot isostatic pressing9.3squeezes with hot gas from all sides to close the last pores, at a much higher cost per part.Die casting1.4melts aluminum or zinc and injects it into a steel mold; it suits housings, not steel gears.

Watch it

MPIF Videos · The whole route from powder to finished part, from the industry association

Simeko AG · What happens between the particles inside the furnace

Comtec Mfg., Inc. · Real compaction presses, a sintering furnace and finished parts

GKN Powder Metallurgy · How the metal powder itself is made, and modern production lines

See real photos

Wikipedia: Powder metallurgy — overview of the process, with photos of sintered parts
Wikimedia Commons photo search

Remember

Press powder into shape, then heat it below its melting point so the particles bond.
Best for small parts with a simple outline, made by the hundreds of thousands with almost no waste.
Leftover pores cost some strength, but they make self-lubricating bearings and filters possible.