Dies are made of alloy tool steels and must have at lea […]
Dies are made of alloy tool steels and must have at least two sections. The two sections are called the fixed or cover half, and the release or ejector half. Among them, the ejector half is designed to permit removal of die casting.
The fixed die may have sprue holes to allow molten metal to enter and fill the cavity. Movable slides, cores or other sections are use to produce holes, threads and other desired shapes. The ejector half may contain runners and gates to route molten metal into the cavity. Locking pins are used to keep the two halves together once the molten metal has been poured.
Hydraulic pressure is used to hold the two halves together. The surface where the two halves meet and lock is called the "die parting line." The projected surface of the area being cast and the pressure required to inject the molten metal into the die are both used to determine the hydraulic pressure required to hold the two halves together or the clamping force.
The wall thickness of an injection-mold part is generally in the range from .080 to .160", while the wall thickness of a thin wall mold can be as low as .020". Shorter cycle times are accomplished by ensuring the minimum wall thickness required for part function and mold filling be considered during the mold design process.
The other part to remember is that during cooling, the thin section will solidify first, and the thicker sections will take longer. As the thicker sections cool, shrinkage will occur and the material for the shrinkage must come from the un-solidified areas. This builds stresses between the thin and thick sections and this often leads to warping, twisting, or cracking. Uniform wall thickness will reduce or eliminate this problem. However, when uniform wall is not possible, the change in section should be as gradual as possible.
It is not good practice to go beyond the pressure and temperature recommendations to compensate for other defects in the mold. If runners need to be sized differently to allow for a proper fill, or the gate sizes need to be changed, then those changes need to happen and should happen during the design stage.
Otherwise, the finished parts will have too much built in stresses, could crack in service or warp, leading to more severe problems, such as customer returns or field service issues.