green

In powder metallurgy, a green part is a component made from compacted metal powder that has been pressed into shape but not yet heated to fuse the particles together. At this stage, the part holds its form through friction and mechanical interlocking alone, with almost no metallic bonding between the powder particles. A green ferrous compact might have a relative density of 85 to 92 percent, meaning the rest is porosity. The material is fragile and can crumble or delaminate if handled roughly or exposed to vibration before sintering.

The term "green" comes from ceramics and has no connection to environmental concerns. In that field, a green ceramic is one that has been shaped but not yet fired. Powder metallurgists adopted the language because the processes are structurally similar: both involve compacting powder, both require a subsequent thermal step to develop final strength, and both involve progressive densification. The green state in both trades is essentially intermediate: the part exists as a discrete object but lacks the properties of the finished material.

Green parts are ejected from the die cavity while still warm from the compaction process. Their surfaces are often dusty and can shed fine particles. Dies must be designed with draft angles and ejection pins to release the part without fracture. Pressing pressures vary widely depending on material and geometry, from around 20 MPa for soft metals like copper to 600 MPa or higher for iron-based powders. The relationship between green density and final sintered density is critical: higher green density generally yields higher sintered strength, but excessive pressure can cause uneven compaction or die wear.

Green parts are often stacked or nested during storage and transport to the sintering furnace. They must be kept dry, since absorbed moisture can cause steam explosions during heating. Exposure to humidity over hours can soften some compacted powders. Most facilities store green parts in climate-controlled areas or move them to the furnace within hours of pressing. Some designs incorporate green strength additives, typically small amounts of organic binder, which improve handling safety but must burn off cleanly during the preheat phase of sintering without generating gases that cause defects.

Defects can appear at the green stage: cracks from uneven compaction, laminations from incomplete filling of the die, or voids from air entrapment. Some defects, such as minor cracks, may close during sintering; others will persist or grow. Density variation within a single green part is common and difficult to detect without sophisticated equipment like computed tomography. Quality control often relies on weight sampling and careful process monitoring rather than non-destructive testing of individual parts.