Custom Search

Precision Casting

The processes of Precision casting differ from sand casting and shell molding in that the molds they employ consists of only one part i.e., are not assembled from two or more parts while the pattern itself is expendable each time a casting is made. Precision casting processes offer considerable freedom to the designer and produce castings of a superior surface finish and a high degree of dimensional accuracy. Among other purposes, they are used for the casting of metals and alloys that are difficult to machine, since the castings generally require little or no finishing treatment. Such castings are used in precision engineering, clock-making, the manufacture of metal ornaments, and other fields of industrial production.

The most widely used precision casting technique is the lost-wax, or investment molding, process. When the original metal pattern has been made, the first step is to make a master mold, which may consist of two or more parts and be provided with cores. It is usually made of low-melting metal alloy which is easily workable, though in certain cases, especially where very large numbers of castings are required, steel may be used for the master mold.

The wax pattern is now taken out of the paste bath and the coating is built up to a greater thickness by having strewn on grains of a coarser mold material. The coating, which closely envelops the wax pattern and reproduces every detail of its shape, is called the investment. The pattern thus invested is placed, with the pouring gate downwards, in a special box, which is filled up with more mold material. The complete mold is then heated, causing the wax to melt and run out. This is followed by baking the mold at about 1000 °C for several hours, so that it becomes hard and strong. The metal is cast in the mold while the latter is still hot. Filling the mold may be done by gravity, pressure, or centrifugal means. Finally, when the metal has solidified, the mold is broken up.

The latter is filled with molten wax, which is allowed to solidify and is then removed. Thus a wax pattern similar to the original is obtained. In some cases a number of wax patterns may be join together in a treelike assembly, so that a corresponding number of castings can be produced in one operation. This is represented in the accompanying illustrations. The wax pattern (or tree of patterns) is immersed in a wet slurry or paste consisting of a fine-grained refractory mold material and a bonding agent, so that the wax pattern becomes coated with this mixture.

There are many variants of the lost-wax process. Plastics may be used instead of wax, or the wax may be removed with solvents instead of being melted out. In the Mercast process used in United States, mercury is used for filling the master mold at normal temperature. Then this mold is cooled to – 40°C, causing the mercury to solidify. The frozen mercury pattern is removed from the master mold and dipped a number of times in a special investment mixture, so that it receives a multiplayer coating which forms the final mold. The temperature is allowed to rise, and the mercury liquefies and is retrieved from the mold, which is then baked.

The mold is preheated to a temperature above the maximum local casting temperature prior to casting of the melt and is also cooled to obtain a variation of temperatures throughout the mold. The resulting temperature gradient of the mold is intended to maintain the heat content per unit volume in the unsolidified melt portions greater than in the adjacent solidified melt portions to compensate for the latent heat of solidification in the melt and thus avoid shrinkholes and blowholes.