Powder Metallurgy (Press and Sinter)
Press-and-sinter powder metallurgy compacts metal powder in a rigid die and sinters it below its melting point into a finished net-shape part.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Metal
- Typical tolerances
- Roughly ±0.001–0.002 in per inch on dimensions perpendicular to the pressing direction, controlled by die and core rod steel; dimensions along the pressing axis hold considerably looser unless the part is sized after sintering
- Typical volumes
- 25,000–5,000,000+ parts per year; below about 5,000–10,000 machining is usually cheaper
- Lead time
- 12–20 weeks for die set design, build, and qualification; 3–6 weeks per production run thereafter
- Materials
- Metal
What it is
Press-and-sinter powder metallurgy compacts metal powder in a rigid die at 20–50 tons per square inch, then heats the resulting green compact below its melting point until the particles bond into a solid part. Iron and steel parts sinter at roughly 2,050–2,100°F (1,120–1,150°C) in a controlled atmosphere.
The parts come out net shape with essentially no scrap: material utilization approaches 100%, against 50% or worse when the same part is machined from bar. That, plus cycle times of seconds, is why PM makes automotive gears, sprockets, cams, bearing caps, synchronizer hubs, and hundreds of millions of self-lubricating bronze bushings a year.
The trade is porosity. A conventional pressed-and-sintered ferrous part typically finishes at 6.6–7.2 g/cm³ against 7.87 g/cm³ for solid iron, so 10–15% of the volume is pores, and strength and fatigue performance are lower than wrought material accordingly. On bearings that porosity is the product — the pores are filled with oil.
How it works
- Powder blending. Elemental or pre-alloyed powder is blended with graphite, alloying additions, and a solid lubricant that lets the compact eject from the die without galling.
- Compaction. The blend is fed into a rigid die and pressed by upper and lower punches at roughly 20–50 tsi (275–690 MPa). Multi-level parts use multiple independently controlled punches so each level reaches similar density. The green compact holds together by mechanical interlocking alone and is fragile.
- Sintering. The compact travels through a continuous belt or pusher furnace at roughly 2,050–2,100°F (1,120–1,150°C) for ferrous materials, held at temperature for something on the order of 20–45 minutes, in an endothermic or nitrogen-hydrogen atmosphere. The lubricant burns off in a delube zone first. Particles bond by diffusion; the part shrinks or grows slightly depending on the alloy system, and that dimensional change is a controlled process variable, not an accident. See metal melting points for how far below melting this sits.
- Sizing (optional). A repressing operation in a sizing die brings critical dimensions into a tighter band and improves surface finish.
- Secondary operations. Oil impregnation for self-lubricating bearings (typically filling 20–25% of the volume), resin impregnation to seal porosity for plating or pressure tightness, copper infiltration for higher strength, steam treatment for corrosion and wear resistance, heat treatment, and machining of features the press cannot form.
Density is the master variable. Everything about a PM part's strength, fatigue life, machinability, and corrosion behavior tracks its density, and density is set by compaction pressure, powder characteristics, and how uniformly the punches can pack a given geometry. Tall thin sections and multi-level shapes are hard to press to uniform density, which is why PM design rules focus on shapes that pack evenly.
Design guidelines
Think in one pressing direction
The die is rigid and the punches move along a single axis. That means no undercuts, no cross holes, no threads, no reverse tapers, and no features on the side wall — anything perpendicular to the pressing direction has to be machined afterward. Design the part as a profile extruded along the press axis.
No draft required
Unlike casting or molding, PM walls parallel to the pressing direction need no draft at all — straight walls eject cleanly from a rigid die. Draft actually complicates tooling.
Length-to-diameter ratio
Keep the pressed height under roughly 2.5–3× the minimum section width. Powder does not transmit pressure like a fluid; friction against the die wall means the middle of a tall part is pressed less than its ends, giving a density gradient and weak zones.
Minimum wall and hole
Hold walls above about 0.060 in (1.5 mm) and holes above about 0.060–0.080 in (1.5–2 mm) diameter. Thin die sections and slender core rods break under repeated 50 tsi compaction.
Radius, chamfer, and flat land
Sharp corners on the punch face wear and chip. Use radii wherever possible. Where a chamfer meets an outside diameter, leave a small flat land — typically about 0.010 in (0.25 mm) — so the punch does not end in a feather edge.
Tolerances differ by axis
Dimensions perpendicular to the pressing direction — diameters, hole sizes — are controlled by rigid die and core rod steel and hold well, on the order of ±0.001–0.002 in per inch. Dimensions along the pressing axis depend on fill weight and press stroke and hold considerably looser. Never put the tightest tolerance on the press-axis dimension. For bearing and housing fits, see ISO 286 fits and tolerances; for property comparison against wrought material, the material properties chart.
Design around the porosity
A conventional PM part is 10–15% porous. It cannot be leak-tight without resin impregnation, it does not electroplate without sealing, and its fatigue strength sits well below a wrought equivalent. If those matter, either specify a higher-density route or choose another process.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Undercuts and cross holes | None | Machine them afterward | Rigid die moves along one axis only |
| Draft | 0° | — | Straight walls eject cleanly from steel dies |
| Height ÷ minimum width | ≤ 2.5 | 3 | Die-wall friction creates a density gradient |
| Minimum wall | 0.080 in (2 mm) | 0.060 in (1.5 mm) | Thin die sections fail under compaction load |
| Minimum hole diameter | 0.080 in (2 mm) | 0.060 in (1.5 mm) | Slender core rods break |
| Flat land at chamfer | 0.010 in (0.25 mm) | — | Punches cannot end in a feather edge |
| Tolerance across press axis | ±0.001–0.002 in/in | — | Controlled by rigid die and core rod steel |
| Tolerance along press axis | Looser | — | Depends on fill weight and press stroke |
Cost drivers
PM's economics rest on two things: near-total material utilization and very short cycle times. A compaction press makes several parts per minute with essentially no scrap, so per-part cost at volume is dominated by powder price and furnace throughput rather than labor. Powder costs more per pound than bar stock, but the part uses almost all of it, which flips the comparison against machining as soon as the machined version would generate significant chips.
Tooling is the barrier. A multi-level die set with independently controlled punches and core rods is a precision assembly and takes months. Every secondary operation — sizing, machining a cross hole, impregnation, heat treatment — adds a handling step that erodes the process's core advantage.
Volume breakpoints: below roughly 5,000–10,000 parts a year the die set will not amortize and machining wins. From 25,000 to several million a year is the classic PM window.
- Eliminate the secondary machining. A cross hole or a thread turns a net-shape process into a machined part with extra steps. Redesign it out if you can.
- Reduce the number of levels. Each pressed level needs its own punch and its own control axis. Two-level parts are dramatically cheaper to tool than four-level ones.
- Keep the press-axis dimension loose. Tight axial tolerance forces a sizing operation on every part.
- Only specify high density where it is needed. Higher density means higher compaction pressure, bigger presses, and sometimes double pressing and double sintering.
- Use the porosity. Self-lubricating bearings, oil-retaining bushings, and filters exist because PM porosity is a feature, not a defect.
Questions
6 questionsHow dense are pressed-and-sintered PM parts?
Typically 6.6–7.2 g/cm³ for ferrous parts, against 7.87 g/cm³ for solid iron — so 10–15% of the volume is porosity. Strength and fatigue performance scale with density and sit below wrought equivalents. Double pressing, copper infiltration, or a switch to metal injection molding gets closer to full density.
Why can't PM parts have undercuts or cross holes?
Because the die is rigid steel and the punches only move along one axis. Any feature perpendicular to the pressing direction cannot be formed and must be machined after sintering, which adds a handling operation and gives back much of the net-shape advantage.
Do powder metal parts need draft?
No. Walls parallel to the pressing direction eject cleanly from a rigid steel die, so zero draft is standard and draft actually complicates the tooling. This is a significant difference from casting and molding, where draft is mandatory.
What tolerance can press-and-sinter hold?
Roughly ±0.001–0.002 in per inch on diameters and hole sizes, which are set by rigid die and core rod steel. Dimensions along the pressing axis depend on powder fill weight and press stroke and hold looser. A sizing operation after sintering tightens the axial dimensions at additional cost.
Why are PM bearings self-lubricating?
The 20–25% interconnected porosity in a bronze bushing is deliberately vacuum-impregnated with oil. As the shaft rotates and the bearing warms, oil is drawn out to the running surface and reabsorbed when it cools. The porosity that limits structural PM parts is the entire point of the bearing.
Powder metallurgy or metal injection molding?
Press-and-sinter for simple shapes that can be pressed along one axis, at high volume, with 10–15% porosity acceptable — gears, bushings, bearing caps. MIM for small complex geometry with undercuts and cross features, at 96–99% density, where the part could not be pressed in a rigid die at all.