Diecasting 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 total cycle
time is very short, typically between 2 seconds and 1 minute.
The Clamping
Process
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
diecasting 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.
The Injection
Procedure
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 diecasting 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 diecasting machine is being
used, the injection time must also include the time to manually ladle the
molten metal into the shot chamber.
Comments
Post a Comment