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 quantity  
Low
Relative cost for small number  
High
Permissible weight
of casting
 
100 lbs.
Thinnest section castable, inches  
1/8″
Relative surface finish  
Good
Relative ease of casting complex design  
Fair
 
Relative ease of changing design in production  
Poor
 
Range of alloys that can be cast    
  Aluminum 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.