Complex debinding and sintering processes
a vital role in the powder metallurgy industry
The first debinding step removes most of the binder, creating open pores in the component. The residual binder is removed in a second step by means of thermal degradation. As this generally takes place during the sintering cycle, it is essential to ensure that no binder remains before sintering closes the pores. Components with defects incurred during sintering cannot be recycled, resulting in complete loss of the material. Consequently, precise control of conditions is essential.
Traditional powder metallurgy process
In a first step the powder metal parts are pressed in a mould to obtain their basic shape. For a homogeneous green body so called binders are added to the powder. These binders also work as lubricant in the forming process. However, the binders have to be removed before the products can be sintered. This debinding process is done in the first kiln zone (debinding or delubing zone). In the following high temperature zone the powder metal parts are sinterd at temperatures shortly below the melting point. Metal Oxides are being reduced and the carbon content is controlled and adjusted. All process steps are run under protective atmosphere, a combination of Nitrogen and Hydrogen or endogas.
At least the powder metal parts are cooled down. This can be either done via a direct rapid cooling system to achieve high degrees of hardness or via an indirect slow cooling system. An optional and additional tempering process is normally done in a tempering furnace.
Metal injection moulding process
The current state-of-the-art is a combined process, comprising secondary debinding and high-temperature sintering. Key challenges include the complete removal of utiliresidual binder (e.g. via an additional, upstream debinding step), carbon control, oxygen reduction, precisely controlled shrinkage and optimum diffusion.
Injecting the feedstock (a mixture of metal and additives heated to a liquid state) into a closed mold produces a green part that consists of approximately 60 % metal powder and 40 % binder. During the first debinding step, thermal treatment and a catalytic reaction, or a solvent, reduces binder content to 5 %. The resulting brown part has a porosity of 35 %.
Secondary debinding at temperatures between 200–600 °C, and sintering at 1200–1360 °C in various atmospheres (vacuum, nitrogen, hydrogen and/or argon), converts the brown part into the finished component with a porosity of less than 2 %.
