Exploring Permanent Molding
Permanent mold casting is a manufacturing process using a metal or graphite mold instead of sand, ceramics or other media that can be broken up to remove the solidified part. The mold halves, or dies, are closed when molten metal is introduced to create a casting and opened to remove the casting. The molds are themselves cast to shape and
machine finished or cut entirely from billet.
Permanent mold castings typically are poured in one of four ways. Static pouring involves hand pouring the metal into an opening at the center of the mold. Test bars and chemistry slugs often are poured this way, along with simple shape castings. Molds also can be poured horizontally and then hydraulically tilted to a vertical position. This “tilt-pour” method utilizes a pouring cup and engineered system to introduce metal into the mold in a more controlled, calm manner. Preventing metal splashing in the mold cavity increases the casting’s mechanical properties. The third method involves using low pressure to slowly fill the mold and reduce turbulence even further. Finally, high pressure diecasting involves forcing molten metal into the mold cavity quickly using a hydraulic mechanism. Aluminum alloys are well suited to the permanent mold casting process, particularly diecasting.
The thickness of the mold area affects the molten metal’s solidification rate. Due to the mold’s high thermal conductivity, molds can be heated to prevent premature solidification. Specific areas of the mold also can be coated to insulate the molten metal, as well as protect the mold surface. Another method of controlling solidification is implementing cooled metal pieces, or inserts, in the mold. Due to the complete lack of permeability of a permanent mold, vents must be considered during the engineering phase and machined into the dies. Sand cores can be used to produce complex castings in the permanent mold process.
Despite the size and shape limitations of the process, permanent mold castings offer good repeatability, good surface finish and fine grain structure. The process is best when used for medium to high volume production runs.
--Cast Metals Institute, Schaumburg, Ill.