Usg Plasters and Gypsum Cements for the Ceramic Indust Bedienungsanleitung PDF herunterladen

Plaster molds are a vital to producing many types of ceramicware. Along with a few other materials, plaster

is the basic modeling material for tooling in the ceramic industry. Consequently, a review of recommended

procedures and the relationship between plaster tooling and hand and mechanized clay fabrication may

prove helpful.

Following are deﬁnitions of common technical terms mentioned in this discussion.

Block Mold. Original mold made from a ﬁnished model. Used to make check casts before case molds are

fabricated.

ase Mold.

Made from the block mold, it becomes the die for fabricating working molds.

orking Mold.

Sometimes called a production mold, it can be one of three types: casting mold, jigger mold, or

press mold.

Consistency. The water-to-plaster ratio or number of parts of water, by weight, mixed with 100 parts of plaster,

by weight.

Alpha Hemihydrate. Gypsum calcined under pressure, producing a lower-consistency material with different

crystalline structure than ordinary plaster. It produces harder and stronger casts with limited absorptive power.

Alpha hemihydrate can be obtained in different formulations to control expansion, consistency and strength.

Also known as gypsum cement.

Beta Hemihydrate (also known as plaster of paris, kettle plaster, kettle stucco). Gypsum calcined in a kettle at

atmospheric pressure. The individual particles are highly irregular in shape, which causes high water demand to

make a workable slurry.

Snap-Set of Plaster Slurry. The condition when plaster sets before it is sufﬁciently soaked and completely mixed.

Slow Set of Plaster. Plaster unusually slow in setting.

Absorption: The absorptive property of plaster molds makes slip casting possible. The mold can be likened to a rigid sponge,

Key to Slip Casting drawing water from the slip, retaining it and gradually passing it through the mold to the back, where it evaporates.

Maintaining a desirable rate of casting depends upon establishing equilibrium between water absorption from

the slip and drying out of the mold. The factors governing this relationship are:

1. The water-to-plaster ratio (varying from 68 to 90 parts of water to 100 parts of plaster, by weight) governs the

initial absorptive power of the mold. In large, multi-piece sanitaryware molds, some parts of the mold require a

higher water-to-plaster ratio than others to give greater absorption to selected portions of the cast.

2. Homogeneity (freedom from pinholes and bubbles) governs to some extent the even capillary movement of water

through the plaster mold. Uniformity is a function of mold-making technique and attention to detail in the mixing

operation.

3. The rate of absorption over a broad range of water content in the mold governs the long-term casting quantities of

the mold. At what minimum moisture content is the high absorptive rate too great? At what maximum moisture

content is the low absorptive rate enough to do the job?

Best results generally are achieved when the driving power of a mold is “in balance” with the layer of slip that

has built up on its working face. The ability of a mold to transport water from the ware surface should have some

relation to the rate at which water reaches the interface between ware and mold.

If the mold absorbs water from the slip too quickly, the result is surface shrinkage of the body immediately next to

the mold. This produces a reduced rate of casting, surface cracks, case hardening or premature release of the

ware. If the mold removes water too slowly, the cast does not build up at a satisfactory rate, is likely to be soft or

“mucky,” and the ware tends to stick to the mold. Once a state of approximate equilibrium is reached, continuous

capillary columns exist from the slip-mold interface entirely through the mold to its exterior.

Plasters and

Gypsum Cements

Product

Application

Plasters for the Ceramic Industry

1 2 3 4 5 6

Inhaltsverzeichnis

Seite 1 - Gypsum Cements

Plaster molds are a vital to producing many types of ceramicware. Along with a few other materials, plasteris the basic modeling material for tooling

Seite 2

The data in Fig. 1 indicates how fast water may be taken up by a casting mold in various shop conditions.The maximum rate of absorption varies from ap

Seite 3

To hold down costs, use as dense a mold as shop conditions permit. Denser molds are stronger, more resistant todeterioration, and longer lived.Many sh

Seite 4

In mechanical jiggering, a clay bat of measured size is cut off a column of pugged clay so as to fall on the moldface. A stamping or rotary pressing o

Seite 5 - Case Mold Tooling

5. The resistance of plaster molds to shearing stresses is very low, approximately 450 psi.6. The compressive strength and abrasion resistance of a we

Seite 6 - Printed in U.S.A

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Usg Plasters and Gypsum Cements for the Ceramic Indust Bedienungsanleitung

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