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 non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, and tin based alloys, although ferrous metal die castings are possible. The die casting method is especially suited for applications where a large quantity 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.
Pressure Die Casting was invented by Elisha K. Root, an inventor in the employ of Samuel W. Collins at the Collins axe-making factory in Canton, Connecticut in the 1830s.
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. The die is then 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. Approximately 15% of the metal used is wasted or lost due to a variety of factors.
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.
Pore-free casting process
When no porosity is required for a casting then the pore-free casting process is used. It is identical to the standard process except oxygen is injected into the die before each shot. This causes small dispersed oxides to form when the molten metal fills the dies, which virtually eliminates gas porosity. An added advantage to this is greater strength. These castings can still be heat treated and welded. This process can be performed on aluminium, zinc, and lead alloys.
Heated-manifold direct-injection die casting
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.
There are two basic types of die casting machines: hot-chamber machines (a.k.a. gooseneck machines) and cold-chamber machines. These are rated by how much clamping force they can apply. Typical ratings are between 400 and 4,000 short
Hot-chamber machines rely upon a pool of molten metal to feed the die. At the beginning of the cycle the piston of the machine is retracted, which allows the molten metal to fill the "gooseneck". The gas or oil powered piston then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times (approximately 15 cycles a minute) and the convenience of melting the metal in the casting machine. The disadvantages of this system are that high-melting point metals cannot be utilized and aluminium cannot be used because it picks up some of the iron while in the molten pool. Due to this, hot-chamber machines are primarily used with zinc, tin, and lead based alloys.
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