As to limitations of thickness of section, the only general rule for die castings is this: If the section is too heavy, exceeding 1" or so, the structural uniformity of the section suffers. There is a rather a steep chilling gradient from the outside to the center of the casting, thus causing a considerable variation in grain size and mechanical strength. Also, the danger of internal porosity increases with the thickness of the section. Unduly heavy sections should be avoided; and when mechanical strength or stiffness must be high, thinner sections with judiciously placed strenthening and stiffening ribs should be used instead.
The average grain size of the die cast structures is on the order of .0005", which is compared with grain size of .020"-.050" for permanent-mold castings and from .050" and up for sand castings. Because of the fine gain structure obtained by the rapid chilling of the metal in the die, die castings have an "as-cast" strength that is greater than the castings produced by any other method.
Keeping sections thinner to allow for this rapid cooling only helps in creating grain structure that is complementary to die cast strength.
Doehler, H.H. (1951) Die Casting. New York, New York. McGraw-Hill Book Company, Inc.