Powder Compaction Glossary
Terms used in powder compaction and press specification, defined as they are used on the shop floor and in machine enquiries.
The powder
Apparent density — the density of loose, un-tapped powder. It determines how much cavity depth is needed to hold the charge for a given finished part.
Tap density — density after controlled settling. The gap between apparent and tap density indicates how much the powder will consolidate on its own during fill.
Flow rate — how freely the powder runs into the cavity. Poor flow produces weight variation between parts, and weight variation becomes dimensional variation after sintering.
Particle size distribution — the spread of particle sizes in the blend. Affects packing, flow and the pressure needed to reach a given density.
Binder — an additive giving the green compact enough strength to be handled. Burned out later, which introduces its own dimensional change.
Lubricant — added to reduce die-wall friction and ejection force. Too little scores the part on ejection; too much reduces sintered density.
The press cycle
Fill — the powder charge entering the die cavity, normally volumetric via a fill shoe.
Compaction — applying pressure to consolidate the powder into a coherent compact.
Dwell — holding at full pressure so the powder settles and entrapped air escapes.
Ejection — pushing the compact out of the die. Ejection force can be a significant fraction of compaction force, particularly for metal powders that spring back against the die wall.
Single action — pressure applied from one direction only. Simple, but produces a density gradient through the height of the part.
Double action — top and bottom punches moving together, halving the distance pressure must travel through the powder and so halving the density gradient. Required for tall parts.
Floating die — a die that moves with the powder during compaction, producing an effect similar to double action without a second driven punch.
Multi-level tooling — independently controlled punches for parts with steps, so each level reaches its intended density.
Sizing and control
Compaction pressure — the pressure required to reach target density, taken from the material datasheet or established by trial. Expressed in MPa or kg/cm².
Projected area — the area of the part seen along the pressing axis. Multiplied by compaction pressure it gives required force: F = P × A.
Tonnage — press capacity, derived from the force calculation above, normally with a safety factor.
Density gradient — variation in density through the compact caused by die-wall friction. The central quality problem in powder compaction.
Springback — elastic recovery when pressure is released. Must be allowed for in tooling dimensions.
Green strength — the strength of the compact before sintering. Enough to handle, never enough to machine.
Hi-lo circuit — a dual-pump hydraulic arrangement where both pumps run together for fast approach and only the small high-pressure pump runs during compaction, keeping installed motor power down.
Where these terms are applied
The force calculation behind compaction pressure and projected area is worked through with numbers on the press tonnage reference chart.
Material behaviour differs enough that each powder family has its own page: PTFE powder, metal powder where density gradient dominates, and graphite powder where platelet alignment does.
For the machines themselves, see powder compaction presses. The PTFE-specific vocabulary is in the PTFE glossary.