At least three families of molding and casting processes can be categorized as metal mold processes. These include diecasting (high-pressure diecasting), low-pressure permanent mold casting and permanent mold casting. Unlike sand casting processes, in which a mold is destroyed after pouring to remove the casting, permanent mold casting uses the mold repeatedly.
Diecasting is used to produce small- to medium-sized castings at high production rates. The metal molds are coated with a mold surface coating and preheated before molten metal is injected into it. Premeasured amounts of molten metal are forced from a shot chamber into the permanent mold or die under extreme pressure (greater than 15,000 psi). This allows for high production rates.
Castings of varying weights and sizes can be produced. Nearly all die castings are produced in nonferrous alloys with limited amounts of cast iron and steel castings produced in special applications.
The die casting process is suitable for a wide variety of applications in which high part volumes are needed. Benefits include:
- excellent mechanical properties and surface finish;
- dimensional tolerances of 0.005-0.01 in.;
- recommended machining allowances of 0.01-0.03 in.;
- thin-section castings.
Permanent Mold Casting (Gravity Diecasting)
Another form of permanent mold casting is when the molten metal is poured into the mold, either directly or by tilting the mold into a vertical position. In this process, the mold is made in two halves from cast iron or steel. If cores are to be used, they can be metal inserts, which operate mechanically in the mold, or sand cores, which are placed in the molds before closing (semi-permanent molding).
The mold halves are preheated and the internal surfaces are coated with a refractory. If static pouring is to be used, the molds are closed and set into the vertical position for pouring; thus, the parting line is in the vertical position. In tilt pouring, the mold is closed and placed in the horizontal position at which point molten metal is poured into a cup(s) attached to the mold. The mold then is tilted to the vertical position, allowing the molten metal to flow out of the cup(s) into the mold cavity.
The various permanent mold techniques—static pour and tilt pour—offer a variety of advantages for a variety of metalforming applications. Benefits include:
- castings with superior mechanical properties because the metal mold acts as a chill;
- castings are uniform in shape and have excellent dimensional tolerances because molds are made of metal;
- excellent surface finishes;
- high-production runs;
- sections of the mold that can be selectively insulated or cooled, which helps control the solidification and improves overall casting properties.
Low-Pressure and/or Vacuum Permanent Mold Casting (LPPM)
In this process, low pressure is used to push the molten metal (and/or a vacuum is used to draw the metal) into the mold through a riser tube, as the furnace is below the mold cavity. The amount of pressure, from 3-15 psi, is dependent on the casting configuration and the quality of the casting desired. When internal passageways are required, they can be made by either mechanically actuated metal inserts or sand cores. The goal of this process is to control the molten metal flow as much as possible to ensure a tranquil fill of the mold cavity.
Nearly all of the LPPM castings produced are made of aluminum, other light alloys and, to a lesser extent, some copper-base alloys. Because it is a highly controllable process, LPPM offers the following advantages:
- when molten metal is fed directly into the casting, excellent yields are realized, and the need for additional handwork is reduced;
- odd casting configurations and tooling points for machining can be placed in areas where gates and risers normally would be placed;
- the solidification rate in various sections of the casting can be controlled through selective heating or cooling of the mold sections, thus offering excellent casting properties;
- surface finish of castings is good to excellent.