Permanent mold casting using gravity provides much more detail than sand casting. Holes can be precisely placed and sized. Overall dimensional stability can be maintained. Machining allowances are smaller because the finished casting is much closer to the final spec.
Compared to aluminum sand castings, those done in permanent molds have less shrinkage, lower gas porosity, and finer dendrite arm spacing (DAS).
Compared to die casting, permanent mold aluminum casting is less likely to contain entrapped gas. The result is a denser, stronger, tighter aluminum casting that requires less finishing work. These qualities are consistent throughout the run - up to 150,000 pieces - and regardless of the casting size. Permanent mold cast parts can be as small as a few ounces or as large as 100 pounds or more.
Maximizing the benefits of permanent mold casting is hardly child's play, however. It requires extensive knowledge and understanding of the permanent mold process.
During the permanent mold casting process, permanent metal molds are filled with molten metal using gravity or tilt pouring. The process involves five steps:
Permanent mold castings usually have better mechanical properties than sand castings because solidification is quicker and the fill is more laminar. The basic difference between permanent mold and die-casting is that permanent mold is a gravity feed process whereas die-casting uses pressurized injection. Gravity feed yields a denser casting.
Metal molds (or dies) usually are made of high-alloy iron or steel and may have a production life of 120,000 castings or more.
Because the permanent mold itself tends to chill the molten metal, castings produced in a permanent mold are sounder than sand castings. For pressure-tight and nonporous castings, the permanent mold process is excellent. Permanent mold castings can be stronger than die and sand castings and less porous than die castings.
Permanent mold castings typically require less finish work and polishing than sand castings. If the parting lines on the outside of a casting are held to a minimum, high quality as-cast finishes of 100 to 200 RMS can be achieved. As-cast finish quality is so high, in fact, that many aluminum castings are used without additional finishing even for such applications as cooking utensils, hardware items, automotive parts, and ornamental work.