Several techniques (slip casting, extrusion, dry pressing, wet pressing, hot pressing, isostatic pressing, jiggering and inspection molding) are available for processing ceramics into useful products (shown in below table 1), depending on the type of ceramics involved and their shapes. Production of some ceramic parts (such as pottery, floor tiles) generally doesn’t involve the same level of control of materials and processes as do high-tech components (made of such structural ceramics as silicon nitride (Si3N4) and silicon carbide (SiC) and cutting tools.
Generally, however, the procedure involves the following steps
- Crushing or grinding raw materials into very fine particles.
- Mixing them with additives to impact certain desirable characteristics.
- Shaping, drying and firing the material.
Table 1: General Characteristics of Ceramic Processing
|1||Slip casting||Large parts, complex shapes, low equipment cost.||Low production rate, limited dimensional accuracy.|
|2||Extrusion||Hollow shapes and small diameters, high production rate.||Parts have constant cross section, limited thickness.|
|3||Dry pressing||Close tolerances, high production rates with automation.||Density variation in parts with high length to diameter ratios, dies require abrasive wear resistance, equipment can be costly.|
|4||Wet pressing||Complex shapes, high production rate.||Limited parts size and dimensional accuracy, tooling cost can be high.|
|5||Hot pressing||Strong, high density parts.||Protective atmospheres required, die life can be short.|
|6||Isostatic pressing||Uniform density distribution.||Equipment can be costly.|
|7||Jiggering||Low tooling cost, high production rate with automation.||Limited to axisymmetric parts, limited dimensional accuracy.|
|8||Inspection molding||Complex shapes, high production rate.||Tooling can be costly.|
Ceramics may be subjected to additional processing, such as machining and grinding, for better control of dimensions and surface finish.
The first step in processing ceramics is the crushing of the raw materials. Generally crushing is done in a ball mill, either dry or wet. Wet crushing is more effective because it keeps the particles together and also prevents the suspension of fine particles in the air. The particles then may be sized filtered and washed.
The ground particles are then mixed with additives the functions of which are one or more of the following:
- Binder: Binder is used for holding ceramic particles together.
- Lubricant: Lubricants are used to reduce internal friction between particles during molding and to help remove the part from the mold.
- Wetting agent: the wetting agent is used to improve mixing.
- Plasticizer: Plasticizer is used to make the mix more plastic and formable.
- Agents: Agents are used to controlling foaming and sintering.
- Deflocculant: deflocculant are used to make the ceramic-water suspension more uniform by changing the electrical charges on the particles of clay (so that the particles repel rather than attract each other). Water is added to make the mixture some pourable and less viscous. Commonly used deflocculants are Sodium carbonate (Na2CO3) and Sodium silicate (Na2SiO3) in amounts of less than 1%.
The three basic shaping processes for ceramics shaping are casting, plastic forming and pressing.