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Permanent Mold Casting

The foundry term “permanent mold casting” is used to describe a mold that can be used repeatedly. In contrast to a sand mold which must be destroyed to remove the casting, the permanent mold is designed so that it can be separated to eject and remove the solidified casting. In the current practice, the molds are usually made of cast iron or steel. 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 which is by far the most common alloy.

Advantages

The Permanent Mold process produces a sound dense casting with superior mechanical properties. Since the mold is made of metal and is relatively stable, the castings produced are quite uniform in shape and often have higher a degree of dimensional accuracy than castings produced in sand, which reduces or eliminates some of the machining that might be required on the part. The permanent mold process is also capable of producing a consistent quality of finish on the castings. The process also lends itself very well to the use of expendable cores and makes possible the production of parts that for one reason or another are not suitable for pressure die casting process.

Disadvantages

The cost of tooling is usually higher than for sand castings. The process is generally limited to the production of somewhat small casings of simple exterior design.

Precision Molding

Permanent Mold Casting

Typical dimensional tolerances, inches± .010″, ± .050″
Relative cost in quantityLow
Relative cost for small numberHigh
Permissible weight of casting100 lbs.
Thinnest section castable, inches1/8″
Relative surface finishGood
Relative ease of casting complex designFair
Relative ease of changing design in productionPoor
Range of alloys that can be castAluminum and copper base preferable

Parting Line Influence
When Parting lines are considered, very close tolerances are difficult to obtain. A parting line absorbs fractions of inches per inch. A foundry is doing well to hold a parting line to 0.015 inch. Additional measurement is added to the casting tolerance.