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Slip casting, also called drain casting, is one of the most common casting techniques developed to process ceramics into useful products. Processing ceramics is done in several steps:

• Crushing or grinding the raw materials into very fine particles.
• Mixing them with additives to impart certain desirable characteristics.
• Shaping.
• Drying and firing the material.

The three basic shaping processes for ceramics are:

• Casting: slip casting, cope-and-drag investment casting, etc.
• Plastic forming: extrusion, injection molding, jiggering, etc.
• Pressing: dry pressing, wet pressing, isostatic pressing, etc.

The discussion on this web page is restricted to slip casting, but click here for ceramic injection molding. For further information, we refer the reader to Kalpakjian, "Manufacturing Processes for Engineering Materials".

Slip Casting

The slip is poured into a porous mold made of plaster of paris. It is important that the slip has sufficient fluidity and low viscosity to flow easily into the mold. Some of the water from the outer layers of the suspension is absorbed by the mold. Then, the mold is inverted and the remaining suspension is poured out for making hollow objects, as in slush casting of metals. The top of the part is then trimmed, the mold is opened, and the part is removed.

Sequence of operations in slip casting a ceramic part. After the slip has been poured, the part is dried and fired in an oven to give it strength and hardness.

For solid ceramic parts, the slip is supplied continuously into the mold to replenish the absorbed water. The suspension is not drained from the mold. At this stage the part is a soft solid or semi rigid The higher the concentration of solids in the slip, the less water has to be removed. The part, called the green, as in powder metallurgy, is then fired.

Drying and Firing

Now, we need to dry the part and fire it to give it the proper strength. Drying is a critical stage because of the tendency of the part to warp or crack from variations in moisture content and thickness within the part and the complexity of its shape.
In a humid environment, the evaporation rate is low. Therefore, the moisture gradient across the thickness of the part is lower than in a dry environment. This prevents a large, uneven gradient in shrinkage from the surface to the interior during drying.

Firing involves heating the part to an elevated temperature in a controlled environment, similar to sintering in powder metallurgy. Firing gives the ceramic part its strength and hardness. This improvement in properties is the result of:
1- development of a strong bond between the complex oxide particles in the ceramic.
2- reduced porosity.