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