What is High Pressure Die Casting (HPDC)?
In simplest terms, it is a metal casting process that is accomplished by injecting liquid metal under high pressure into a die (mold) cavity. The mold cavity is created using two hardened tool steel sections which have been machined into the net shape of the desired part(s). Most parts are made from non-ferrous metals, specifically zinc, copper, aluminum, magnesium, lead, pewter and tin based alloys. Depending on the type of metal being cast, a hot or cold chamber machine is used.
Casting equipment and the dies represent large capital expenditure, and this tends to limit the process to high volume production. The manufacture of parts is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small to medium sized castings, which is why die casting produces more castings than any other casting process. Parts feature a very good surface finish (by casting standards), near net shape and dimensional consistency.
Fundamentals of Die Casting
Die casting is an adaptable process for producing engineered metal parts by injecting molten metal under high pressure into reusable steel molds. These molds, called dies, can be designed to produce intricate shapes with a high level of repeatability and accuracy. Parts can be sharply defined, with smooth or textured surfaces, and are suitable for a broad variety of appealing and serviceable finishes.
The die casting process has evolved from the original low-pressure injection method to techniques including high-pressure casting — at forces exceeding 4500 pounds per square inch — squeeze casting and semi-solid die casting. These modern processes are capable of producing high integrity, near net-shape castings with excellent surface finishes.
The fundamental die casting process is comprised of injecting molten metal under high pressure into a steel mold called a die. Die casting machines are typically rated in clamping tons equal to the total amount of pressure they can exert on the die. Machine sizes range from 400 tons to 4000 tons. Irrespective of their size, the only essential difference in die casting machines is the technique used to inject molten metal into a die. The two approaches are hot chamber or cold chamber. A complete die casting cycle can vary from less than one second for small components weighing less than an ounce, to two-to-three minutes for a casting of a number of pounds, making die casting the fastest technique existing for producing precise non-ferrous metal parts.
Hot Chamber Die Casting
Hot chamber machines are used primarily for zinc, copper, magnesium, lead and other low melting point alloys that do not easily attack and erode metal pots, cylinders and plungers. The injection mechanism of a hot chamber machine is immersed in the molten metal bath of a metal holding furnace. The furnace is fastened to the machine by a metal feed system called a gooseneck. As the injection cylinder plunger rises, a port in the injection cylinder opens, allowing molten metal to fill the cylinder. As the plunger moves downward, it shuts the port and forces molten metal through the gooseneck and nozzle into the die cavity. After the metal has cooled to hardness in the die cavity, the plunger is withdrawn, the die opens and the casting is expelled.
Cold Chamber Die Casting
Cold chamber machines are used for alloys such as aluminum and other alloys with high melting points. The molten metal is poured into a "cold chamber," or cylindrical sleeve, individually by a hand ladle or by an automatic ladle. A hydraulically operated plunger seals the cold chamber port and forces metal into the locked die at high pressures.