Permanent Mold Casting 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:
Mold Preperation:A refractory wash or mold coating is applied to those surfaces of the preheated mold that will be in direct contact with the molten metal alloy. The proper operating mold temperature is set
for each casting (typically 250º-500ºF or 121º-260ºC).
Mold Assembly / Core Insertion:Cores, if applicable, are inserted, and the mold is closed mechanically.
Pouring:The alloy, heated above its melting point, is poured into the mold through a gating system. This gating system is a precision-engineered part of the mold assembly that controls the feed and flow of molten metal into the part cavity. The gating system also serves as a reservoir of the molten metal needed to supply the thick sections of the part as they shrink during solidification.
Cooling / Opening:After the casting has been allowed to solidify, cores and other loose mold members are withdrawn, the mold is opened, and the casting is removed.
Trimming / Finish:The usual foundry practice is followed for trimming gates and risers from the castings. Typically this excess metal is cut from the part while the casting is still hot using a band saw. The excess metal is then returned to the melting furnace, recapturing both material and energy.
Benefits of Permanent Mold Castings
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.
Forming cavities in a permanent mold casting is best done with permanent steel cores. When the design is such that permanent cores cannot be removed, destructive cores are used. This variation is called the semi-permanent mold method. Sectional steel cores are sometimes used.
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 125 to 300 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.