Die casting

Die casting is the process of forcing molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from nonferrous metals, specifically zinc, copper, aluminum, magnesium, lead, and tin based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where large quantities of small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.

This level of versatility has placed die castings among the highest volume products made in the metalworking industry.

In recent years, injection-molded plastic parts have replaced some die castings because they are cheaper and lighter. Plastic parts are a practical alternative if hardness is not required and little strength is needed.

For the casting of low melting point metals (such as pot metal, lead, aluminum, or magnesium) a multi-part die is used in a process called die casting. For automotive parts such as the cases of automatic transmissions these dies may be quite complex, as they must be disassembled in specific order to ensure that the work piece is released freely from the casting die. Parts or products produced by this method are referred to as die cast. Compared to lost wax casting the marginal production can be quite cheap, once the substantial investment in tooling and materials handling equipment is made. Compared to sand casting the die casting method can reproduce fine details on complex parts and yield a smooth surface, greatly reducing machining and polishing requirements. As some small portion of metal may leak between the mating seams of the die this can result in a sharp edge of metal called flash, which must be removed by grinding and buffing. For small metal toys the term die cast is generally considered a mark of quality, especially when compared to the cheaper stamping of lithographed sheet metal, or bare stamped metal possibly later painted.

History

Die casting was invented by Elisha K. Root, an inventor in the employ of Samuel W. Collins at the Collins ax-making factory in Canton, Connecticut in the 1830's.

Process:

There are four major steps in the die casting process. First, the mold is sprayed with lubricant and closed. The lubricant both helps control the temperature of the die and it also assists in the removal of the casting. Molten metal is then shot into the die under high pressure; between 10—175 MPa (1,500—25,000 psi). Once the die is filled the pressure is maintained until the casting has solidified. Finally, the die is opened and the shot (shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot) is ejected by the ejector pins. Finally, the scrap, which includes the gate, runners, sprues and flash, must be separated from the casting(s). This is often done using a special trim die in a power press or hydraulic press. An older method is separating by hand or by sawing, which case grinding may be necessary to smooth the scrap marks. A less labor-intensive method is to tumble shots if gates are thin and easily broken; separation of gates from finished parts must follow. This scrap is recycled by remelting it.

The high-pressure injection leads to a quick fill of the die, which is required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities are avoided even if the shape requires difficult-to-fill thin sections. This creates the problem of air entrapment, because when the mold is filled quickly there is little time for the air to escape. This problem is minimized by including vents along the parting lines, however, even in a highly refined process there will still be some porosity in the center of the casting.

Most die casters perform other secondary operations to produce features not readily castable, such as tapping a hole, polishing, plating, buffing, or painting.

Heated-manifold direct-injection die casting, also known as direct-injection die casting or runnerless die casting, is a zinc die casting process where molten zinc is forced through a heated manifold and then through heated mini-nozzles, which lead into the molding cavity. This process has the advantages of lower cost per part, through the reduction of scrap (by the elimination of sprues, gates and runners) and energy conservation, and better surface quality through slower cooling cycles.