Quick Facts

The permanent mold casting process, also called gravity die casting, involves the production of castings by pouring molten metal into permanent metal molds under gravity, low pressure, or centrifugal pressure.  Permanent mold castings, like die castings, are chill castings.  When the casting has re-entrant surfaces or cavities from which one piece permanent metal cores cannot be withdrawn, destructible cores made of sand, resin-sand (shell), plaster and other materials, or sectional steel cores, are used.  Use of these cores is called the semi-permanent mold process.

The Process

To make permanent mold castings, a refractory wash or mold coating is brushed or sprayed onto those surfaces of the preheated mold which will be in direct contact with the molten aluminum.  The proper casting temperature for each mold and alloy being cast can best be determined by experience.  Cores, if applicable, are inserted and the mold is closed either manually or mechanically,  The aluminum alloy, heated to the proper temperature, is poured into the mold through the sprue opening.  After the casting has been allowed to solidify, cores and other loose mold members are withdrawn, the mold is opened, and the casting is removed.  The usual foundry practice is followed for trimming tates and risers from the castings.

Permanent Mold Casting Advantages

Castings produced in permanent molds generally have fine grain structure and better strength properties than those cast from a similar alloy in sand.  Therefore, the casting can often be designed with thinner walls and lower weight.  In addition, the surfaces are generally smoother than sand castings and closer dimensional tolerances can be maintained.  Permanent mold castings do not ordinarily contain entrapped gas found in die castings and therefore are superior to die castings in soundness and pressure tightness.  Complex internal shapes can be achieved by using destructible or sectional steel cores.

Permanent Mold Casting Disadvantages

If part complexity, surface finish, and tolerance requirements are exceptionally critical, the part might be produced better by investment casting.  There are limitations to the size of castings that can be produced in permanent molds.  Most permanent mold castings weigh under 20 lbs, although castings of 50 to 100 lbs and more are produced.  Tooling costs are greater than sand castings.

Alloys

In general, only non-ferrous materials are cast using the permanent mold process.

Tooling

Permanent molds generally have a life of 10,000 to 150,000 castings or more.  Mold material is generally iron or steel, or graphite, and is cooled by water channels or by the air jets on the outer surfaces.

Design and Technical Information