Permanent Mold

The die coatings used in permanent mold casting of aluminum establish a barrier between the molten aluminum and the mold, influence the solidification rate of the metal, promote venting and filling, aid casting ejection and provide the desired casting surface finish. Because mold coatings play such a critical role in aluminum permanent mold casting, the coating must be properly applied and maintained, or casting quality will be adversely affected even when other casting parameters are in order.   

permanent mold castings, while not as flexible as sand castings in allowing the use of different patterns, lower the cost of producing a part. At a production run of 1000 or more parts, permanent mold castings produce a lower piece cost part. Of course, the break-even point depends on the complexity of the part. More complex parts being favored by the use of permanent molds. Instead of using sand as the mold material, a metal is used as a mold. Typically cast iron or Meehanite (a dense cast iron) is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate.

Permanent Mold or gravity die-casting is mainly used for nonferrous metals and alloys. The mold  is usually iron, steel, or graphite, and is cooled by water channels or by air jets on the outer surfaces. Cavity surfaces in metal dies are coated with a thin layer of heat-resistant material. The mold or die design is usually in two halves, although many multiple-part molds are in use, with loose sand or metal cores to form "undercut" surfaces.
   
The cast metal is simply poured into the bowls on the side of the mold.
During the period after solidification and before ejection, cooling continues but shrinkage of the casting is restricted by the die. The alloy being cast must be sufficiently ductile to accommodate these restrictions without fracturing. Dimensions of the casting at shop temperatures will be related to the die temperature and the dimensions at ejection.

Tilting mechanisms are used to control the passage of metal into (and emergence of air from) the remote portions of die cavities.
Because the die temperature varies during the casting cycle, its dimensions vary correspondingly. The die is opened and ejectors push the casting(s) out as soon as their temperature is low enough for sufficient strength to build up.
   
Mixing and application of mold coatings contribute significantly to ...