LOST WAX CASTING TECHNOLOGY (Part 1)

This is the process where the mould is prepared around an expendable pattern. The various steps in the process are shown in next technology news. The first step in this process is the preparation of the pattern for every investment casting to be made. To do this,molern wax, which is used as the pattern material is injected under pressures of about 2.5 Mpa into a metallic die, which has the cavity of the casting to be made as shown in the next Step 1. The wax when allowed to solidify would produce the pattern. The pattern is ejected form the die as shown in Step 2. Then the cluster of wax patterns are attached to the gating system by applying heat as shown in Step3.

To make the mould, the prepared pattern is dipped into a slurry made by suspending fine ceramic materials in a liquid such as ethyl silicate or sodium silicate(Step 4). The excess liquid is allowed to drain off from the pattern. Dry refractory grains such as fused silica or zircon are stuccoed on this liquid ceramic coating(Step 5). Thus, a small sheel is formed around the lost wax casting pattern. The shell is cured and then the process of dipping and stuccoing is continued with ceramic slurries of graduallly increasing grain sizes. Finally, when a sheel thickness of 6 to 15 mm is reached, the mould is ready for further processing. The shell thickness required depends on the investment casting shpe and mass, typy of ceramic and the binder used.

The next step in the process is to remove the pattern from the mould, which is done by heating the mould to melt the pattern(Step 6). The melted lost wax casting is completely drained through the sprue by inverting the mould. Any wax remnants in the mould are dissolved with the help of the hot vapour fo a solvent, such as trichloro-ethylene.

The moulds are then pre-heated to temperature of 100 to 1000 degree, depending on the size, compplexity and the metal if the casting, This is done to reduce and last traces of wax left off and permit proper filling of all mould sections, which are too thin to be filled in a cold mould.

The molten metal is poured into the mould under gravity, under slight pressure, by evacuating the mould first(Step 7). The method chosen depends on the type of investment casting.

Other pattern materials used are plastics and mercury in place of wax. In the process called Marcast, the mercury is kept -57degree whre the mercury is frozen. The complete mould preparation is to be under taken at a temperature belov -38 degree. The main advantage of mercury as a pattern material is that it does not expand when changed from solid to liquid state as wax. But the main disavantage is keeping the pattern at such low temperature, which is responsible for its diminshing use.