The die is most important part in die casting machine which will be discussed here in detail. Dies for all the machines are similar in construction as there is little difference in the method of holding and operating them. Generally it is made into two halves so that the casting can be easily removed from it after solidification. One part of die is stationary (ejector die) and other is movable.
The two halves, when closed have a vertical parting surface and form a cavity similar to the casting desired. The die is so designed that after pouring in of casting desired. The die is so designed that after pouring in of metal and its solidification, the casting will always cling to the ejector die.
In order to keep the two halves in proper alignment, dies are equipped with heavy dowel pins. Initially the dies are locked and metal enters from the stationary side. The die is then opened and casting from the moveable die is removed by advancing the ejector plate in the moveable half of the die, so that the ejector pins project through the die half and force the casting out. A separate mechanism, which synchronizes the movements of ejector plate, moveable die and moveable cores, is also provided with the dies.
Die surfaces should be as free from tool marks as possible. Tool marks act as notches and may result in fatigue failure. Die material should be properly heat treated and be of correct composition.
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The die is made from a suitable alloy steel, selection of which depends upon the fusion temperature of the metal to be cast. In addition the metal selected should have low expansion characteristics, dimensionally stable, be wear resistant against erosion, stable against high temperature and pressure, and should not have any tendency of welding with the castings.
In cold chamber die casting machine, the material of die is nickel-chrome steel and in hot chamber die casting machine the material of the die is hardened and tempered chrome-vanadium or chrome-tungsten steel.
It is usual to incorporate provision of vents for escape of air into the parting surface. The metal from these vents is afterwards removed by trimming or filing. Provision is also made for catching surplus metal in the overflow wells and the gating system.
Depending upon the size of casting either single-cavity or multiple-cavity die can be employed. Single cavity die is used for small and complex castings and multiple- cavity for large size, and large number of castings at a time in a single operation. Most dies are provided with channels for water cooling to keep the die at correct temperature for rapid production.
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The die materials for various casting alloys are:
1. Tin and lead alloy ― Carbon steel without heat treatment.
2. Zinc, aluminium ― Heat treated low alloy steel.
3. Copper base alloys ― Heat treated special alloy steel.