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.
- Drying and firing
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".
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.
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.