The rapid metal solidification achieved in the die casting process imparts distinct characteristics to the metal. Grain structure is much finer and porosity distribution is different from gravity castings, which cool much more slowly. In addition, the chilled microstructures of the die-cast alloys possess mechanical properties that are equivalent to, or nearly equivalent to, those of heat-treated alloys.

Metal in the die casting die solidifies and cools by transferring heat through the surface of the die cavity into the die where it has been traditionally removed by cooling water. However, some die casters use closed-loop heat exchangers with oil. This promotes more even die temperatures, allows for localized heating where required, eliminates scale build-up in coolant passages, prolongs die life, and enhances casting quality.

Solidification begins at the surface of the casting and progresses to the center generating two distinct zones in each wall section. The skin, which has finer grain structure, begins at each surface and extends inward to a typical thickness of 0.015" to 0.020".

This area is usually free of porosity because the rapid initial solidification tends to drive porosity to the center of the section. The porosity is located between the skins in the core. The finer grain structure and absence of porosity give the skin superior mechanical properties.

Skin thickness of a die casting is relatively constant and is not a function of total wall thickness; therefore, thin wall sections can actually be stronger and more consistent than thick sections. This important point is not widely recognized by designers.

Parting lines usually appear on die castings as small ridges of flash wherever two die members meet. In the case of decorative components, these lines are usually removed to enhance appearance. Flash height and thickness depend upon the condition of the die and control of process variables. In certain applications, advanced casting processing can greatly minimize or eliminate flash.

The most pronounced parting line is formed at the interface of the ejector and the cover dies, where die surfaces are separated by the hydraulic shock loading (pressure intensification) associated with the injection of molten metal. This parting line is particularly significant in product design because in-gates and overflow gates (the orifices which conduct metal into and out of the die cavity) are usually located in the parting plane and appear as thick areas of the parting line. Gate location may affect the mechanical properties and/or appearance of the casting. Consequently, the product designer should work with the die caster to determine mutually satisfactory gate locations.

When the gate (metal that solidifies in the metal entry distribution system of the die) is trimmed from the casting, the surface exposed by trimming is not skin; it is interior or core metal, which may have some degree of porosity and appear pitted. If this area is subjected to cyclic tensile stresses, the porosity may act as stress risers, leading to premature failure of the casting. Possible porosity may also cause an unsatisfactory appearance.

In either case, the casting may have to be redesigned to move the parting line to a less critical surface. A handle subjected to cyclic bending developed tensile stress on the top surfaces and compression on the bottom. Surface porosity in the gates at the parting line created stress risers near to maximum tensile stress. The handle was redesigned to locate the parting line closer to the neutral axis in an area of lower stress.