Die casting

Die casting is the process of forcing molten metal under high pressure into the cavities of steel molds. The molds are called dies. Dies range in complexity to produce any non-ferrous metal parts (that need not be as strong, hard or heat-resistant as steel) from sink faucets to engine blocks (including hardware, component parts of machinery, toy cars, etc). In fact, the process lends itself to making any metal part that:

• must be precise (dimensions plus or minus as little as 50 µm--over short distances),
• must have a very smooth surface that can be bright plated without prior polishing and buffing,
• has very thin sections (like sheet metal--as little as 1.2 mm),
• must be produced much more economically than parts primarily machined (multicavity die casting moulds operating at high speed are much more productive than machine tools or even stamping presses),
• must be very flexible in design; a single die casting may have all the features of a complex assembly.

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If several machining operations would be required or assembly of several parts would be required (to make a finished part), die casting is probably far more economical. This level of versatility has placed die castings among the highest volume products made in the metalworking industry.

Common metals used in die casting include zinc and aluminum. These are usually not pure metals; rather are alloys which have better physical characteristics.

In recent years, injection-molded plastic parts have replaced some die castings because they are usually cheaper (and lighter--important especially for automotive parts since the fuel-economy standards). Plastic parts are practical (particularly now that plating of plastics has become possible) if hardness is not required and if parts can be redesigned to have the necessary strength.

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 injected into the die under high pressure. The high pressure assures a casting as precise and as smooth as the mold. Typically it is around 100 MPa (1000 bar). Once the cavity is filled then the pressure is maintained until the casting has become solid (though this period is usually made short as possible by water cooling the mold). Finally, the die is opened and the casting is ejected.

Equally important as high-pressure injection is high-speed injection--required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities (spoiling the finish and even weakening the casting) are avoided even if the design requires difficult-to-fill very thin sections.

Before the cycle can be started the die must be installed in the die casting machine (set up) and brought to operating temperature. This set-up requires 1-2 hours after which a cycle can take anywhere between a few seconds to a few minutes depending on the size of the casting. Maximum mass limits for magnesium, zinc, and aluminium parts are roughly 4.5 kg, 18 kg, and 45 kg, respectively. A typical die set will last 500,000 shots during its lifetime with lifetime being heavily influenced by the melting temperature of the metal or alloy being used. Aluminum and its alloys typically shorten die life due to the high temperature of the liquid metal resulting in deterioration of the steel mold cavities. Molds for die casting zinc last almost indefinitely due to the lower temperature of the zinc. Molds for die casting brass are the shortest-lived of all. This is despite, in all cases, making the mold cavities out of the finest "hot work" alloy steel available.

A shot occurs every time the die is filled with metal. Shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot. Also the shot consists not only of the individual castings but also the "scrap" (which, unlike in the case of scrap from machining, is not sold cheaply; it is remelted) that consists of the metal that has hardened in the channels leading into and out of the cavities. This includes, for example, the sprue, runners and overflows. Also there is usually some unplanned-for thin scrap called flash, the result of wear in the molds allowing metal to leak into the space outside the casting, between the dies.

Equipment

The die must fulfill four primary purposes. First, it must hold molten metal in the shape of the final casting. The die must also provide a path for the molten metal to reach the casting cavity. Third, the die is designed to remove heat from the casting. Finally, a die must be able to eject the solidified casting.

Because die sets open and shut along a parting line of the casting, design features such as undercuts cannot be cast without the addition of movable slides in the die set. Otherwise these features must be added (more expensively) by secondary machining operations.

Die casting machines are rated by how much clamping force they can apply. Typical sizes range from 100 to 4,000 tons. Along with size there are two main categories that die casting machines fall into. They are hot chamber machines for zinc, magnesium and lower melting-point metals. Or cold chamber machines for aluminum and higher melting-point metals. A die casting machine automatically opens and closes the mold and injects the liquid metal, all under high pressure and as rapidly as possible, in the case of zinc up to several hundred times an hour. (However the very smallest zinc machines may cycle thousands of time an hour.) Sometimes means are provided to automatically remove the shot and re-cycle the machine. The largest machines are as big as a house.

Often there is a secondary operation to separate the castings from the scrap; this is often done using a 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 gate mark where molten metal entered or left the cavity. Finally, a less labor-intensive method is to tumble shots if gates are thin and easily broken. Separation of gates from finished parts must follow.

Most die casters perform other secondary operations to produce features not readily castable. Most common is tapping a hole (to receive a screw). Finally, the surface may be improved by polishing, plating, buffing or painting.