lost wax casting

A modern variation on this method is called "ceramic shell"; instead of the cow-dung and clay, a silicaceous slurry is used to cover the model by dipping and/or pouring. Special dry aggregate is then applied to the wet pattern, either by hand or by using a "fluidized bed", which blows the dry particles around in a confined space, covering the wet areas until no more will adhere. The coated pattern is then left to dry a while and another layer of wet and dry material is applied. This is repeated, using coarser aggregate on the outermost layers, until a sufficient thickness has built up so the mold will hold together through the burn-out and pouring. lost wax casting, alloy steel casting

The other method commonly used is called "investment" casting. The pattern is set up in much the same way as for ceramic shell, except that more venting is necessary due to the decreased porosity of the mold material, which is made using gypsum plaster (plaster of Paris) as a binder for sand, silica flour, or another refractory aggregate. There are many proprietary investment mixtures available, or one may choose to mix one's own. The dry ingredients are mixed with water and poured into a container or "flask" surrounding the gated model, which is either waxed down to a board or attached to a commercially available rubber device which holds the pattern and flask. Jewelry flasks are generally placed in a vacuum chamber while this mixure is still fluid, where they are boiled at room temperature to remove air-bubbles clinging to the models. It is helpful to de-air the investment mixture before pouring it over the wax models, in order to reduce boiling-over when vacuuming.

Once the plaster mixture has set hard, the flasks are placed in a kiln, slowly heated to between 1000 and 1250 degrees F, and held at that temperature until all wax residues have disappeared. The place where the wax was is now a void- hence the "lost wax" designation for this process. The metal is then melted and the molds are filled with liquid bronze or whatever metal is being used. The flask is then allowed to cool, then the plaster mold is broken away, revealing the metal part, which faithfully reproduces every detail of the original wax, plus the gates and vents, which must be cut off. Silver, gold, aluminum, brass, or bronze may be used interchangably with this method. Special investment materials and melting equipment are necessary to deal with the higher temperatures of platinum, iron or stainless steel, but the process is much the same. Lower-melting metals such as tin, lead, and zinc may also be cast by this method, but there are other easier methods of casting them, so it is not often done. A variation on the lost-wax method is also used for casting objects in glass. lost wax casting, alloy steel casting