Monday, January 25, 2010

Casting with Dead Moulds

In casting, the cavity of a mould is filled with the molten casting material. The work piece is in the primary form when it solidifies.

Molding and Casting Processes
1. With dead mould and permanent patterns: flask mould, template mould, shell mould.
2. With loss patterns: investment casting, male form casting
3. With permanent moulds: ingot molding, centrifugal casting, compound casting.

Make the Pattern
The model of the casting which is to produce the mold is known as the pattern. The pattern is made of wood, metal or plastic, following the technical drawing. A contraction rule is used in making the pattern, which is longer than the actual scale by an amount equal to the degree of shrinkage of the cast material between solidification and room temperature (cast iron 1%, zinc casting alloys 1.5%, cast steel 2%). The pattern is varnished in a color code representing the casting material (cast iron = red, malleable cast iron = grey steel = blue) to protect it from moisture.

Molding is carried out in sand or clay, mechanically or manually, in the furnace or in molding boxes.

Molding sand is used in molding with casting blocks. It consist of small group of quartz and a binding agent, a fine layer of clay surrounding each particle of quartz. The layer of clay become binding when the molding sand is moistened. Nowadays, synthetic resin adhesives are used instead of clay. The important properties of molding sand are plasticity, stability of form, heat resistance, and permeability of gases.

Hollow spaces are created in cast pieces by using a cone around which the molten metal flows. These core are produced in cone boxes. The core must be stronger than the box. This is why special sand core is used. The dried core is inserted in the drag (bottom box).

Finishing the mould for casting
Once the pattern has been molded in the bottom box, it is turned over and the cop (top box) placed on it. The pattern is rammed with an ingate funnel and a riser funnel). After piercing an air hole and extracting the wood piece for the ingate and the collector, the cope is lifted off. The pattern is removed and the mold lifted with a founding gate.

The cope is replaced and clammed to drag, or weighed down, so that it does not off during casting. Casting is carried out in the foundry. Casting has attained the shape of the pattern with the ingate and riser in addition, as well as the cavity formed by the core.

See more: Standardization of Metallica Material

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