The production of a ceramic article by injection molding involves the following steps: selection of the powder and the binder; mixing the powder with the binder, production of a homogeneous feed material in the form of granules, injection molding of the green body, removal of the binder (debinding) at lower temperatures and, finally, sintering at higher temperatures to produce a dense, final article.
Powder Characteristics
When the forming step is considered in combination with other processing steps such as debinding and sintering, A small particle size is beneficial for shape retention during debinding and for ease of sintering, but the viscosity of the powder/binder mixture is higher and binder removal from the molded article is slower.
Binder System
Although the binder plays a transient role in the overall fabrication route, careful selection of a binder is vital to the success of the injection molding operation. The binder must provide the desired rheological properties to the feed material so that the powder can be formed into the desired shape and then must be removed completely from the shaped article prior to firing, without the disruption of the particle packing or any chemical reaction with the powder.
Powder–Binder Mixture
The ratio of powder to binder is a key parameter for successful injection molding. Too little binder in the mixture leads to a high viscosity and to the formation of trapped air pockets, both of which make molding difficult. On the other hand, too much binder leads to microstructural heterogeneities in the molded article and to slumping during binder burnout.
Molding
When compared to injection molding of plastics, control of the molding process in ceramics is more difficult because of the higher density, viscosity, thermal conductivity, and elastic modulus associated with the particle-filled polymer.