Permanent mold castings, sometimes known as gravity castings, are, as the name implies, made in molds that are designed for repetitive use. Molds are made in two or more parts and are hinged and clamped for easy removal of the finished castings,which like plastic mold. Parts made as permanent mold castings must be of relatively simple design although some degree of complexity can be attained by the use of sand core in conjunction with the iron or steel molds,and the plastic injection molding is also a good way for complexity molding
When sand cores are used, the process is known as semipermanent molding. Parts made as permanent mold castings have good mechanical properties, due largely to the chilling effect of the metal molds. Quality can be controlled by heating the molds prior to pouring and by the use of plaster mold materials in certain areas to achieve different cooling rates.
Permanent molds are used to produce slush castings, which are made by emptying the mold of liquid metal after a shell has been formed by the solidification of metal immediately adjacent to the mold surfaces. Permanent mold castings are usually dense and fine-grained and can be made with better surfaces and closer tolerances than sand castings. Tolerances must be wider than for die castings, shell mold castings and plaster mold castings. Tool costs are lower than for die castings, higher than for sand, plaster and shell mold castings.
Permanent mold castings offer good possibilities where moderation, but not the ultimate in complexity, dimensional tolerances or high speed production are desired. Design features Dimensional tolerances of permanent mold castings can usually be assumed to be within the range of = 0.025 in. on average sized pieces. Closer tolerances can be held when the occasion demands, but the original tolerances provided by the molds are likely to grow larger in time due to wear on the mold. Required draft angles of 2 to 3 deg to permit removal of pieces from the molds add to drawing dimensions on some parts.
Tolerances are affected by ceramic mold washes which are used to prevent mold damage and assist in casting removal. The average size of permanent mold castings varies from about lb up to about 12 lb although there is now a trend toward larger and heavier castings of this type. As to size, the majority of castings have a minimum size of 2 in. dia by ‘/i in. thick up to 10 in. dia and 1 in. thick. Materials Materials which can be used in permanent mold casting form are generally limited to a relatively few low melting point alloys. Some gray iron castings are made by this method with considerable success; the high pouring temperatures are likely to damage the mold materials and reduce service life considerably.
Copper-base alloys are also cast in permanent molds, but here too the pouring temperatures required are such as to be harmful to the molds. Thus, the majority of permanent mold casting are made of aluminum and magnesium alloys. When the process is used to make slush castings the materials most frequently used are zinc and lead alloys.