Ceramic and Pottery Defects 3: Defects Generated During Forming Operations

John T Jones, Ph.D.
 


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Forming methods of ceramics are sometimes classified as wet or dry. Dry forming refers to pressing operations from dry or perhaps damp powders. Wet forming includes slip casting and plastic forming methods. For a review of industrial forming methods see Ceramics: Industrial Processing and Testing by John T. Jones and M. F. Berard, Iowa State University Press.

Dry pressing requires that a shape be dimensionally stable after firing. That will occur if the pressing operations are in control and the firing is specified. If a pressed part is oversize after firing, it can be ground to size, but that is an extra operation usually not included in the costing of the part. If the part is undersize after firing, the part is scrap.

Important factors in pressing are the die size, the particle size distribution of the powder, the binder system, the pressing pressure and pressing cycle. Problems are powder sticking to the die, powder not flowing into the die freely, and incorrect fired shrinkage.

In isostatic pressing an additional problem can occur due to the incompressibility of air. This can sometimes be relieved by flooding the tooling cavity with propane which is compressible and on release will evaporate into the air slowly rather than expand instantly like air expands which can crack the part. In isostatic pressing the part usually must be machined after forming.

Laminations in pressed parts can be caused by the powder not flowing easily during die filling or during pressing. These laminations will not usually heal during firing and must be eliminated in the pressing operations.

Experiments with binders such as paraffin, gums, and starches, etc. , must be conducted to determine the correct binder type and amount. Binders with lubricating properties can help.

Sometimes a lubricant or anti-sticking agent is needed to prevent sticking of parts to the die. See the reference for more information including spray drying of powders. Dow Chemical company and others supply a number of aids to pressing.

Plastic forming by machine such as plate rollers and jigger machines usually require the clay to be in the flocculated state. Small amounts of plaster added to the body when in slurry form before filter pressing and extrusion will provide the desired plastic properties.

Cast jiggering is an exception but even then the clay slip is placed on a plaster mold providing what is needed to flocculate the slip as it builds up on the mold before jiggering.

Clay for plastic forming must have the correct moisture content or the dimensions of the dried product before firing will be incorrect. Firing will not improve this situation.

Most other problems occurring during plastic forming can be resolved by keeping the clay flocculated.

Nepheline syenite and alumina tend to deflocculate plastic bodies before forming so it may be essential not to delay forming in these cases only.

Generally, aging of plastic clay improves forming operations especially for bodies with low inherent plasticity such as porcelain bodies.

Slip casting requires that the clay slurry be dispersed. The slip specific gravity and viscosity must be controlled. The particle size distribution is critical and must be controlled by formulation as well as processing. See the referenced text for details of testing and slip control.

Whether from plastic forming or slip casting, leather hard ware must be handled carefully during finishing operations. Clay has a memory and distortions in green ware can reappear during firing.

See the reference for other forming methods and problems.

John T. Jones, Ph. D. (tjbooks@hotmail.com, a retired VP of R&D for Lenox China, is author of detective & western novels, nonfiction (business, scientific, engineering, humor), poetry, etc. Former editor of Ceramic Industry Magazine, Jones is Executive Representative of International Wealth Success. He calls himself “Taylor Jones, the hack writer. "

More info: http://www.tjbooks.com

Business web site: http://www.bookfindhelp.com (IWS wealth-success books and kits and business newsletters / TopFlight flagpoles)

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