7.2.2 Influence
of the Forming Process
Similarly to the manufacture of metallic
or polymeric materials, the manufacturing process for ceramic
parts possesses not only certain advantages but also has its
limitations.
Thus for example, using slip casting, parts
with constant wall thickness can be manufactured, but only
up to a maximum of a few millimetres (depending on the material).
It is more difficult to manufacture parallel surfaces, since
the removal of the cast part from the mould is more difficult.
Undercuts are also difficult and only possible through the
use of complex, multi-part casting moulds.
Tubular or bar-shaped parts are usually manufactured by extrusion.
Here, the manufacturing skill lies in retaining the cross
section of the body and in avoiding deformation (sagging,
ovalising) both axially and radially.
Injection moulding permits the production
of very complex forms very close to their final dimensions.
The tooling costs and batch sizes represent a considerable
factor in the cost calculations.
The isostatic pressing technique is used
mainly for the production of blanks with a particularly uniform
density. These are subsequently processed further using chip-forming
operations such as turning, milling, boring, grinding or cutting
(using diamond tools).
Dry pressing is a process often used for
the Mass Prod.s production of smaller parts. This process allows
the economical production of relatively flat parts with a
height to wall thickness ratio < 4 (in special cases <
8) and with no excessive height differences (blind holes,
steps, etc.). The pressing tools are subject to high wear
due to the high hardness of the ceramic powders. The lifetime
of the tools is therefore limited if sufficiently accurate
dimensions are to be maintained in the parts.
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