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Ceramic and Pottery Defects 3: Defects Generated During Forming Operations

Forming methods of ceramics are sometimes company and others supply a number of
classified as wet or dry. Dry forming aids to pressing.Plastic forming by
refers to pressing operations from dry or machine such as plate rollers and jigger
perhaps damp powders. Wet forming machines usually require the clay to be
includes slip casting and plastic forming in the flocculated state. Small amounts
methods. For a review of industrial of plaster added to the body when in
forming methods see Ceramics: Industrial slurry form before filter pressing and
Processing and Testing by John T. Jones extrusion will provide the desired
and M. F. Berard, Iowa State University plastic properties.Cast jiggering is an
Press.Dry pressing requires that a shape exception but even then the clay slip is
be dimensionally stable after firing. placed on a plaster mold providing what
That will occur if the pressing is needed to flocculate the slip as it
operations are in control and the firing builds up on the mold before
is specified. If a pressed part is jiggering.Clay for plastic forming must
oversize after firing, it can be ground have the correct moisture content or the
to size, but that is an extra operation dimensions of the dried product before
usually not included in the costing of firing will be incorrect. Firing will not
the part. If the part is undersize after improve this situation.Most other
firing, the part is scrap.Important problems occurring during plastic forming
factors in pressing are the die size, the can be resolved by keeping the clay
particle size distribution of the powder, flocculated.Nepheline syenite and alumina
the binder system, the pressing pressure tend to deflocculate plastic bodies
and pressing cycle. Problems are powder before forming so it may be essential not
sticking to the die, powder not flowing to delay forming in these cases
into the die freely, and incorrect fired only.Generally, aging of plastic clay
shrinkage.In isostatic pressing an improves forming operations especially
additional problem can occur due to the for bodies with low inherent plasticity
incompressibility of air. This can such as porcelain bodies.Slip casting
sometimes be relieved by flooding the requires that the clay slurry be
tooling cavity with propane which is dispersed. The slip specific gravity and
compressible and on release will viscosity must be controlled. The
evaporate into the air slowly rather than particle size distribution is critical
expand instantly like air expands which and must be controlled by formulation as
can crack the part. In isostatic pressing well as processing. See the referenced
the part usually must be machined after text for details of testing and slip
forming.Laminations in pressed parts can control.Whether from plastic forming or
be caused by the powder not flowing slip casting, leather hard ware must be
easily during die filling or during handled carefully during finishing
pressing. These laminations will not operations. Clay has a memory and
usually heal during firing and must be distortions in green ware can reappear
eliminated in the pressing during firing.See the reference for other
operations.Experiments with binders such forming methods and problems.John T.
as paraffin, gums, and starches, etc., Jones, Ph.D. (, a retired VP of R&D for
must be conducted to determine the Lenox China, is author of detective &
correct binder type and amount. Binders western novels, nonfiction (business,
with lubricating properties can scientific, engineering, humor), poetry,
help.Sometimes a lubricant or etc. Former editor of Ceramic Industry
anti-sticking agent is needed to prevent Magazine, Jones is Executive
sticking of parts to the die. See the Representative of International Wealth
reference for more information including Success. He calls himself "Taylor Jones,
spray drying of powders. Dow Chemical the hack writer.




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