The permanent mold casting process produces engineered components with tight tolerances, good surface finishes and high mechanical properties. Its ability to achieve these details is based on the fundamental principle of this process—the pouring of molten metal into reusable metal molds. This metal mold applies chill characteristics to the metal during solidification for a finer grain structure, reduced porosity and higher mechanical properties of the solidified cast component.

In high volume production, permanent mold castings typically range in weight from 1 oz. to more than 100 lbs.; however, castings up to 400 lbs. are produced commercially. If a casting requires a core to form an internal cavity, either a reusable permanent metal core or a collapsible sand core can be used. This choice is based on casting design and complexity.

The advantages of permanent mold include:
• Superior dimensional accuracy.
• The ability to achieve high-quality as-cast surface finishes of 100 root mean squared (RMS).
• Dimensional consistency from part to part with cored holes, bosses, pads and other points.
• The ability to cast-in ferrous and nonferrous inserts to combine properties for improved strength, wear resistance or fatigue life.
• A finer grain structure, less porosity and better mechanical properties due to the increased chill of the metal mold.

Permanent mold casting uses zero to minimal levels of pressure or vacuum to pour metal into the mold while other processes use in excess of 15,000 psi. Generally suited for aluminum and copper-base component manufacturing, the type of permanent mold casting process selected depends upon the type of cast component being produced.

The four main types of permanent mold casting are:
• Gravity.
• Tilt pour.
• Low-pressure.
• Vacuum.

The selection of the proper permanent mold process for specific components depends on several factors, including quantity, size, cost restrictions and mechanical property requirements. In general, the more a permanent mold process involves pressures and/or vacuums, the greater the mechanical properties that can be achieved, as well as the higher the manufacturing costs. In addition, as the complexities of the casting process increase, so does the complexity and cost of the tooling.

Gravity pouring is the oldest, simplest and most traditional form of permanent mold casting. In the process, two metal mold halves are joined together to form the mold cavity.

Molten metal is poured down into the pouring basin (using gravity) and travels through the gating system into the mold cavity where it solidifies into the cast component.

Tilt pour permanent mold casting takes traditional gravity pour and kicks it up a notch by turning/rotating the metal mold during or after the metal is poured into a pouring basin to fill the mold cavity. The molds rotate up to 90 degrees during the tilting process, with the goal of reducing the turbulence the metal encounters as it travels from the pouring basin, through the gating system and into the mold cavity. By reducing the turbulence, the mechanical properties of permanent mold castings are increased.

Low-pressure permanent mold casting turns the mold upside down and places it in a casting device above a sealed airtight chamber that contains a crucible holding molten metal. A fill tube extends from the mold down into the molten metal. The casting is made by pressurizing the chamber containing the molten metal and forcing the metal into the mold. The metal in the fill tube acts as the riser, feeding the casting during solidification, allowing this process to achieve good yield. In addition, by controlling the rate of mold fill with pressure, the metalcaster ensures the mold is filled smoothly, without trapping air and other inclusions in the casting—increasing casting quality and mechanical properties. This method also lends itself to automation, and usually runs at lower mold temperatures with shorter cycle times-08-20 than conventional permanent mold. The lower temperature mold reduces casting solidification time, which increases casting mechanical properties.