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Metal Casting Techniques - Ceramic Molding (Shaw Process)Ceramic molding can be accomplished through two diverse techniques:

1. True ceramic molding.

2. Ethyl silicate slurry molding (also known as the Shaw process, Avnet-Shaw, Osborn-Shaw and theDean process ).

Ceramics are materials which are made from a clay base and contain various oxides and ingredients other than

sand. The raw clays are calcined or fired at high temperatures and are then blended, mixed with water, formed

into mold components, and then fired.

In true ceramic molding, the refractory grain can be bonded with calcium or ammonium phosphates. The

preferred methods for producing ceramic molds is the dry pressing method in which molds are made by

pressing the clay mixture containing 4-9% moisture in dies under a pressure of 1-10 ton/sq in. After pressing,

molds are stripped from the dies and then fired at temperatures between 1650-2400F (899C and 1316C).

The ethyl silicate variation is accomplished in the following manner: a mixture of a graded refractory filler,

hydrolyzed ethyl silicate, and a liquid catalyst are blended together to form a slurry consistency. The slurry isthen poured over a pattern and allowed to jell. After gelation, the mold is stripped and torched with a high

pressure gas torch. The mold can then be cooled, assembled and fired prior to pouring.

The best known of these process variations is a development from the United Kingdom called the Shaw

process. The chief difference between the Shaw and other investment molding processes is that a jelling agent

is added to the refractory slurry-like mixture before it is poured over the pattern. When this mixture forms a

somewhat flexible gel, the mold can be stripped off the pattern.

Patterns can be made of various materials such as plaster, wood or metal and can be reused. In this manner,

this process differs from the expendable (wax or plastic) process. Molds are torched, then brought to a red heat

in a furnace. The molds are allowed to cool prior to assembly for pouring. Occasionally the Shaw process and

the lost wax process are combined to gain the advantages of each. The complex pattern configurations which

are difficult or impossible to remove from the mold can be made of wax and placed into the regular pattern.This provides for the regular pattern to be stripped off and the wax to be melted and burned out later.

When compared to investment castings the following apply:

Ceramic Mold ProcessCompared with Investment Casting

Casting

RequirementsInvestment

CastingCeramic Mold

Casting

Surface smoothness 40- 125 microinch 80-125 microinch

Intricacy Excellent

Excellent,

approaching but not

equalling precision

castings

Thinness of metal

sectionsExcellent Excellent

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Tolerances Excellent Good to excellent

Machining costsMinimum machining

required

Machining greatly

reduced, sometimes

but not always

eliminated

Lead time Longest lead time Very short lead time

Adaptability to various

sizesRestricted to small

castings

Casting size not

restricted except

above 100 lb for the

top size of the casting

Adaptability to various

metals and alloysNo limitations No limitations

Pattern costs Very high costVery low cost; job-

bin wood or metal

patterns may be used

Prototype adaptability High cost