Excerpt from the May 2012 issue of Design World by Adam Scichitano and Mike Guilfoyle
What’s the best way to rapid prototype a die cast component? Engineers with some die casting experience may reflexively answer “spin casting” or “machining.” A better answer, however, would be “die casting.”
That’s right. It turns out that the ideal way to prototype a die cast part is actually to die cast it.
While spin casting and machining can produce great-looking prototypes, neither process produces parts whose mechanical properties truly replicate those of a production die casting. The reason why comes down to process-dependent microstructure differences.
Even when they employ the same or very similar metal alloys, each of these three prototyping processes produces parts whose grain structure and density differ from one another. With die casting in particular, the part’s final microstructure depends heavily on the interaction of the individual casting alloy with the process conditions, the tooling and the part geometry.
The microstructure differences that matter most from a strength standpoint can be found on the part surface. Die cast components derive a significant amount of their tensile strength and fatigue life from the finely grained, non-porous “skin” that forms as the pressurized molten metal fills the die and cools. Skin thickness is typically about 0.5 mm, though the exact transition point between the skin and part interior can be somewhat hazy from a metallurgical standpoint.
Machined prototypes lack a skin altogether and are the weaker for it. Strength studies of both zinc and aluminum die casting alloys have quantified skin’s contribution to strength and effects of removing that skin through machining:
A study conducted by the U.S. National Energy Technology Laboratory, Determination of Mechanical Properties of Die Cast Zinc Alloys for Automotive Applications, examined the effects of machining away all or part of the skin from die cast zinc specimens. Removing the skin from the specimens lowered the values for offset yield strength by nearly 10% and also reduced other key mechanical properties (See Table 1.)