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