Die casting is a manufacturing process
that can produce geometrically complex metal parts through the use of reusable
molds, called dies. The casting process involves the use of a furnace, metal,
casting machine, and die. The metal, typically a non-ferrous alloy such as
aluminum or zinc, is melted in the furnace and then injected into the dies in
the casting machine. There are two main types of casting machines - hot chamber
machines (used for alloys with low melting temperatures, such as zinc) and cold
chamber machines (used for alloys with high melting temperatures, such as
aluminum). The differences between these machines will be detailed in the
sections on equipment and tooling. However, in both machines, after the molten
metal is injected into the dies, it rapidly cools and solidifies into the final
part, called the casting.
The
creation of casts
The castings that are created in this
process can vary greatly in size and weight, ranging from a couple ounces to
100 pounds. One common application of die cast parts are housings - thin-walled
enclosures, often requiring many ribs and bosses on the interior. Metal
housings for a variety of appliances and equipment are often die cast. Several
automobile components are also manufactured using die casting, including
pistons, cylinder heads, and engine blocks. Other common die cast parts include
propellers, gears, bushings, pumps, and valves.
The dies into which the molten metal is
injected are the custom tooling used in this process. The dies are typically
composed of two halves - the cover die, which is mounted onto a stationary
platen, and the ejector die, which is mounted onto a movable platen. This
design allows the die to open and close along its parting line. Once closed,
the two die halves form an internal part cavity which is filled with the molten
metal to form the die casting. This cavity is formed by two inserts, the cavity
insert and the core insert, which are inserted into the cover die and ejector
die, respectively. The cover die allows the molten metal to flow from the
injection system, through an opening, and into the part cavity. The ejector die
includes a support plate and the ejector box, which is mounted onto the platen
and inside contains the ejection system. When the clamping unit separates the
die halves, the clamping bar pushes the ejector plate forward inside the
ejector box which pushes the ejector pins into the molded part, ejecting it
from the core insert. Multiple-cavity dies are sometimes used, in which the two
die halves form several identical part cavities.
The
chambers in die casting
The flow of molten metal into the part
cavity requires several channels that are integrated into the die and differs
slightly for a hot chamber machine and a cold chamber machine. In a hot chamber
machine, the molten metal enters the die through a piece called a sprue bushing
(in the cover die) and flows around the sprue spreader. The sprue refers to
this primary channel of molten metal entering the die. In a cold chamber
machine, the molten metal enters through an injection sleeve. After entering
the die, in either type of machine, the molten metal flows through a series of
runners and enters the part cavities through gates, which direct the flow.
Often, the cavities will contain extra space called overflow wells, which
provide an additional source of molten metal during solidification.
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