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Manufacturing Processes - Ceramic Molding (Shaw Process)

 

Manufacturing: Metal Casting


Casting Methods Online Resources

 

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 the Dean 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 is then 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 Process
Compared with Investment Casting
Casting
Requirements
Investment
Casting
Ceramic Mold Casting
Surface smoothness 40- 125 microinch 80-125 microinch
Intricacy Excellent Excellent, approaching but not equalling precision castings
Thinness of metal sections Excellent Excellent
Tolerances Excellent Good to excellent
Machining costs Minimum machining required Machining greatly reduced, sometimes but not always eliminated
Lead time Longest lead time Very short lead time
Adaptability to various sizes Restricted to small castings Casting size not restricted except above 100 lb for the top size of the casting
Adaptability to various metals and alloys No limitations No limitations
Pattern costs Very high cost Very low cost; job-bin wood or metal patterns may be used
Prototype adaptability High cost  




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