China precision machining Casting is used to make metal products of almost any desired shape by the pouring of molten metal into a reshaped hollow mold. As the metal freezes, the mold is removed. This technique was learned thousands of years ago when it was discovered that damp sand could be packed by hand into almost any shape.
Generally speaking, clean, fine sand is placed in a wooden or steel box and packed around a preformed wooden pattern or actual object. When the pattern is
removed, its imprint remains in the sand. China precision machining Molten metal is then poured into the hollow mold. Castings made from sand molds have a rough surface. They must be cleaned,
trimmed and, at times, machined. Sand molds must be repacked after each casting to obtain additional parts. Therefore, the casting of many pieces using sand molds requires much time and labor.
Permanent Mould Casting
The mould material is selected on the consideration of the pouring temperature, size of the casting and frequency of the casting cycle. They determine the total heat to be borne by the die. Fine grained grey cast iron is the most generally used die material. Alloy cast iron, C20 steel and alloy steel (H11 and H14) are also used for very large volumes and large parts. Graphite mould may be used for small volume production from aluminum and magnesium. The die life is less for higher melting temperature alloys such as copper or grey cast iron.
For making any hollow portions, cores are also used in permanent mould casting. The cores can be made out of metal, or sand. When sand cores are used, the process is called semi-permanent moulding. The metallic core cannot be complex with under-cuts and the like. Also, the metallic core is to be withdrawn immediately after solidification, otherwise, its extraction becomes difficult because of shrinkage. For complicated shapes, collapsible metal cores (multiple-piece cores) are sometimes used in permanent moulds. Their use is not extensive because of the fact that it is difficult to securely position the core as a single piece as also due to the dimensional variations that are likely to occur. Hence, with collapsible cores, China precision machining the designer has to provide coarse tolerance on these dimensions.