The process of powder pressing
How a green compact is made
The process workflow for the production of powder metallurgical components can be roughly subdivided into:
1. Powder production and mixing: Production and subsequent mixing of one or more powders with a pressing additive (lubricant).
2. Pressing: The powder is pressed into shape using moulding pressures of 150 - 900 N/mm². By cold welding the powder particles together, this is where the compact attains its form stability. Here it should be noted that at this point in time, this compact, which is known as a “green compact” has just enough firmness to allow for it to be shaped and manipulated. Depending on the complexity and requirements made of the component, pressing then takes place with mould movements that are adapted for the product, and with more or less regulated punch axles. Following this, the compact created in this way is demoulded from the die and transported to the further process steps.
3. Sintering: In the sintering process (heat treatment), the compact is heated to a temperature below the melting point of the main component, usually in an inert gas atmosphere but also under vacuum, causing the individual powder particles to coalesce by means of diffusion. This is how the component obtains its final firmness. Where necessary, various types of follow-up treatment enable compliance with the specific product requirements regarding the mechanical properties and the geometry. This includes calibration (compaction), surface densification, impregnation, hardening, coating and other procedures.
4. Follow-up treatment: Various types of follow-up treatment enable compliance with the specific product requirements regarding the mechanical properties, the geometry and further technical properties. This includes calibration (compaction), surface densification, impregnation, hardening, coating and other procedures.
5. Calibrating press: The compacts created by sintering have a surface quality and dimensional accuracy that is sufficient for most application purposes. If the purpose of use has particularly close tolerances, the compacts can be pressed once more using special tools. This re-pressing or compaction uses work hardening to achieve the necessary quality of the surface and a high dimensional accuracy. This re-pressing procedure is known as calibration.