Die Casting
Die casting is a manufacturing process that can produce
geometrically complex metal parts through the use of reusable
molds, called dies. The die casting process involves the use of
a furnace, metal, die 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 die
casting machine. There are two main types of die 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 steps in this process are described in greater
detail in the next section.
Die casting cold chamber machine overview
Die casting hot chamber machine overview
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.
Process Cycle
The process cycle for die casting consists of five main stages,
which are explained below. The total cycle time is very short,
typically between 2 seconds and 1 minute.
Clamping - The first step is the preparation and clamping
of the two halves of the die. Each die half is first cleaned
from the previous injection and then lubricated to facilitate
the ejection of the next part. The lubrication time increases
with part size, as well as the number of cavities and
side-cores. Also, lubrication may not be required after each
cycle, but after 2 or 3 cycles, depending upon the material.
After lubrication, the two die halves, which are attached inside
the die casting machine, are closed and securely clamped
together. Sufficient force must be applied to the die to keep it
securely closed while the metal is injected. The time required
to close and clamp the die is dependent upon the machine -
larger machines (those with greater clamping forces) will
require more time. This time can be estimated from the dry cycle
time of the machine.
Injection - The molten metal, which is maintained at a
set temperature in the furnace, is next transferred into a
chamber where it can be injected into the die. The method of
transferring the molten metal is dependent upon the type of die
casting machine, whether a hot chamber or cold chamber machine
is being used. The difference in this equipment will be detailed
in the next section. Once transferred, the molten metal is
injected at high pressures into the die. Typical injection
pressure ranges from 1,000 to 20,000 psi. This pressure holds
the molten metal in the dies during solidification. The amount
of metal that is injected into the die is referred to as the
shot. The injection time is the time required for the molten
metal to fill all of the channels and cavities in the die. This
time is very short, typically less than 0.1 seconds, in order to
prevent early solidification of any one part of the metal. The
proper injection time can be determined by the thermodynamic
properties of the material, as well as the wall thickness of the
casting. A greater wall thickness will require a longer
injection time. In the case where a cold chamber die casting
machine is being used, the injection time must also include the
time to manually ladle the molten metal into the shot chamber.
Cooling - The molten metal that is injected into the die
will begin to cool and solidify once it enters the die cavity.
When the entire cavity is filled and the molten metal
solidifies, the final shape of the casting is formed. The die
can not be opened until the cooling time has elapsed and the
casting is solidified. The cooling time can be estimated from
several thermodynamic properties of the metal, the maximum wall
thickness of the casting, and the complexity of the die. A
greater wall thickness will require a longer cooling time. The
geometric complexity of the die also requires a longer cooling
time because the additional resistance to the flow of heat.
Ejection - After the predetermined cooling time has
passed, the die halves can be opened and an ejection mechanism
can push the casting out of the die cavity. The time to open the
die can be estimated from the dry cycle time of the machine and
the ejection time is determined by the size of the casting's
envelope and should include time for the casting to fall free of
the die. The ejection mechanism must apply some force to eject
the part because during cooling the part shrinks and adheres to
the die. Once the casting is ejected, the die can be clamped
shut for the next injection.
Trimming - During cooling, the material in the channels
of the die will solidify attached to the casting. This excess
material, along with any flash that has occurred, must be
trimmed from the casting either manually via cutting or sawing,
or using a trimming press. The time required to trim the excess
material can be estimated from the size of the casting's
envelope. The scrap material that results from this trimming is
either discarded or can be reused in the die casting process.
Recycled material may need to be reconditioned to the proper
chemical composition before it can be combined with non-recycled
metal and reused in the die casting process.
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