Die casting dies are made from specially developed steel alloys. They incorporate one or more die cavities, which are as intricate in configuration as the castings they produce. Some dies are further complicated by components such as core slides. The high initial cost of die casting dies is justified when production rates are high enough to keep the amortized cost per casting at a reasonable level, or, in the case of lower-volume production, when casting a complex component to machined tolerances can eliminate costly secondary operations.

Die casting dies are subjected to very severe operating conditions in normal service. Although dies are made from high-quality, engineered tool steel, the injection of molten metal and subsequent rapid cooling induce thermal shock and cyclic thermal stresses which cause deterioration and ultimately failure of the die steel. The rate of deterioration is a function of the quality of the die steel, the total amount of heat transferred from the molten metal to the die and the associated temperature increase. Deterioration is also affected by the tendency of the molten metal to attack the die steel.

The high-speed flow of molten metal through the die also causes die steel erosion, called die wash. The amount of erosion on one shot is infinitesimal, but the accumulation over many shots must be recognized. The most severe erosion occurs at locations of high metal flow velocity, sudden change in the direction of metal flow, and at irregular die features and die sections, where these overflows are utilized for support as much as possible, but the casting design must reflect the requirements imposed by the trimming operations. It is usually advantageous to consult with the die caster early in the design process to ensure that proper allowance has been made for trimming.

Flash occurs at the die parting; thus, trim die cost can be reduced if the parting surface can be kept in a single plane. Further economies in trim die cost can be realized by eliminating or avoiding irregular features on the parting line. The cost of more complex, multistage trim dies can be justified when costly secondary operations can be eliminated, lowering final piece-part costs.

Vibratory Finishing--As mentioned previously, flash can also occur at the interface of moving die menbers such as core slides. When the flash is not accessible to trim dies, vibratory finishing is often employed. The castings are placed in a container with abrasive media and subjected to high-rate oscillations. The abrasive media preferentially attacks protruding features such as flash.

Polishing--In most decorative and some functional components, the visible lines that remain on the casting where flash has been removed must be eliminated. It is often necessary to polish the surface of the casting by buffing prior to decorative finishing.

Belt sanding and grinding, which are often hand operations, are occasionally employed to remove a heavy flash line. However, the amount of metal removed may exceed product design specifications, and lead to field failures. The product designer must identify critical structural areas to ensure that the casting will not be weakened by grinding and belt sanding.