Permanent Mold Casting Process Review

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Permanent mold castings can be produced from all of the metals including iron and copper alloys, but are usually light metals such as zinc-base, magnesium and aluminum.

Gravity Permanent Mold -The flow of metal into a permanent mold using gravity only is referred to as a gravity permanent mold. There are two techniques in use: static pouring, where metal is introduced into the top of the mold through downsprues similar to sand casting; and tilt pouring, where metal is poured into a basin while the mold is in a horizontal position and flows into the cavity as the mold is gradually tilted to a vertical position.

Normally, gravity molding is used because it is more accurate than shell molding. It is preferred almost exclusively to shell molding for light alloy components.

Low-Pressure Permanent Mold - Low-pressure permanent mold is a method of producing a casting by using a minimal amount of pressure (usually 5-15 lb/sq in.) to fill the die. It is a casting process that helps to further bridge the gap between sand and pressure diecasting.

Advantages
Superior mechanical properties.
Produces dense, uniform castings with high dimensional accuracy.
Excellent surface finish and grain structure.
The process lends itself very well to the use of expendable cores and makes possible the production of parts that are not suitable for the pressure diecasting process.
Repeated use of molds.
Rapid production rate with low scrap loss.
Disadvantages

Higher cost of tooling requires a higher volume of castings.
The process is generally limited to the production of somewhat small castings of simple exterior design, although complex castings such as aluminum engine blocks and heads are now commonplace.

Permanent mold casting requires a set-up time on the order of weeks, after which production rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg. Hot molds are coated with refractory wash of acetylene soot before processing to allow easy removal of the workpiece.

Permanent molds have a life of 3000 castings after which they require redressing. Permanently cast metals generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.

The only necessary input is the coating applied before each casting. Typically, permanent mold casting is used in forming iron-, aluminium-, magnesium-, and copper-based alloys. The process is highly automated.