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Principle No. 2

Understanding the Fundamentals of the Casting Process

Most modifications to the "ideal" casting shape, aimed at optimum production economy, arise from the understanding of the casting process. This required knowledge may be summarized as follows:

The first step in the casting process is to produce the pattern, a nearly exact reproduction of the whole casting, usually in wood or metal. The pattern is then sectioned (a) usually in one or more parallel planes. The sections, one after the other, are placed into molding boxes or flasks and the box is tightly filled with bonded, firm sand (b) (c). The pattern creates a cavity after withdrawal (d). This cavity will be filled later with liquid iron to form the casting.

 

Cavities in the mold other than that of the casting cavity are needed for two purposes; 1, to deliver the liquid iron from the top of the assembled mold to the casting cavity, 2, to replace the volume lost due to the shrinkage of the liquid iron from the pouring temperature to the freezing temperature. The first set of cavities or channels is called the gating system while the "feeding" cavities are called risers. As a rule, both the gating system and the risers are integral parts of the pattern.

The liquid iron first passes through the gating system, then, through one or more openings (gates) into the mold cavity, and finally, the entire mold cavity including the risers fills with liquid iron. While passing through all the channels and filling the mold, the temperature of the liquid iron decreases. The thinner the casting walls, the greater the temperature loss. This increases markedly the danger of defects due to improper fusion.

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