This page has
been designed as a brief description of the basic process of
die casting. The description is in the most basic form in
layman's terms, so it can be understood even by those not
familiar with die casting.
INTRODUCTION TO DIE CASTING:
In the die casting process a mold (called the die) is
required. The mold is constructed from very high
quality steel specially made for molds (dies). The
molds (dies) are
reusable and are made of two halves. During the process of
die casting, molten metal is injected into the closed die
under very high pressure through a relatively small hole in
one half of the die. When the molten metal has solidified
sufficiently, the two parts of the die are opened to expose
the actual casting inside. The casting is then removed
from the die, cooled further and then trimmed. The
trimming separates the gates and runners from the casting.
The gates and runners are incorporated into the mold design
as a way to distribute molten metal to the various parts of
the mold. The trimmed gates and runners are then
usually re-melted with new metal also being added. After
trimming, further operations can be performed such as
drilling and tapping, powder coating and assembly of
finished components, etc.
Die casting is used to make various cast alloy parts in
different sizes for a wide variety of applications. The die
casting process is very efficient and can produce large
volumes of castings in a short period of time and has
precise repeatability. The flexibility of product types,
casting sizes, a virtually infinite possibility of shapes
makes die casting the preferred process for a wide variety
of industries. While die casting molds (dies) are more
expensive than molds for other forms of casting, such as sand
casting and permanent mold casting or gravity casting, die
cast parts offer many advantages such as very high
tolerances, precise repeatability, less porosity, sharply defined parts and
a possibility of textured or smooth surfaces.
With the development of CAD programs and the development
of sophisticated shot monitoring and shot control systems of
die casting machines, most limitations have been overcome.
There are basically only two limitations left. The
first is size and weight of the casting. The size of
the physical dimensions of the casting will determine the
size of the machine that is required. However, obviously
there is a size limit to this. The item being cast must fit
into the mold (die) with sufficient clearance all around the
casting and then mold (die) itself then must fit into the
die casting machine. The mold (die) clearance determining
factor is the "distance between tie bars" on the die casting
machine. The mold (die) must be able to fit between these
dimensions with reasonable clearance. So for example,
a 600 ton die casting machine usually will have a distance
between tie bars of 737 mm H x 737 mm W (29"H x 29"W).
If the mold (die) is bigger than that you will have to move
up to the next size of machine. The largest size of
machine ever produced has been a 4000 ton machine with a
"distance between tie bars" of 1780 mm H x 1780 mm W (70" H
x 70" W) and a max shot weight of 65kg (143 lbs).
The other limitation is the weight of the casting. For
example, a 600 ton die casting machine usually has a maximum
shot weight of about 7 kg (15 lbs). The shot weight is
defined as the total weight of the metal that is injected.
The shot weight therefore includes the actual casting plus
any runners, gates and the biscuit. The reason for the
weight limitation is that the die casting machine must have
sufficient power and velocity to inject molten metal through
the shot sleeve into the die and to fill the die cavity
completely with the metal before the metal starts to
solidify. The whole process must be done within milliseconds
and at tremendous pressure otherwise the extreme ends of the
mold (die) will not get filled or will be partially filled.
The result will be a scrap casting and wasted effort. While
the calculation of "weight to velocity to time" is extremely
complicated, die casting machine manufactures have already
done most of the work for you by "standardizing" this ratio.
As such, each machine manufacturer builds a relatively
standardize range of machines with relatively standardized
shot weight ranges and relatively standardized distances
between tie bars. For a comparison chart please see our