Die
casting utilities two blocks of heat resistant metal machined to meet along the
plane of the parting line and having cavities machined accurately and smoothly
into each to form opposite halves of the shape to be cast around the edges of
the mould. Fine vents are cut to allow
air to escape as the metal enters. If
air is not allowed to escape it would be trapped and produce blow holes in the
casting.
In
die casting, molten or semi-molten metal
is either poured under gravity or is forced under high pressure into a die and
pressure is maintained t8ill solidification stage. As a result, a very smooth and accurate part
is obtained. Die casting is used for
mass production and is most suitable for non-ferrous metals and alloys of low
fusion temperature. By this method
castings of nearly all sizes can be produced economically. The process is rapid
and the surface is smooth, consequently little or no finishing operation is
required. Size can be controlled, so
that no machining is required. The material
is dense and homogeneous and has no possibility of sand inclusions or other
impurities. Thin wall can be produced
and uniform thickness can be maintained.
In
gravity die casting, no external force is applied other than the head of the
liquid. A feeder is incorporated so that
when the level of the metal in the feeder is above that at the highest point of
the casting, head of liquid in feeder forces the metal into all parts of the
mould cavity. The feeder is cut off
after solidification. It is usually used
for shouter production runs.
As
metal flows under high pressure in pressure in pressure die casting, within a
fraction of second the fluid alloy fills
the entire die including all minute cavities, therefore, the intricate casting
can be production is possible. Further
close dimensional tolerances with good surface finish can be obtained. Due to high initial cost of die casting
machine, this process will be advantageous in cases where 10000 or more pieces
are to be produced, where complex shapes are desired and where tolerances
matter and sections are thin. Surface
quality is excellent and features such as screw threads locating flanges etc.
Can be readily incorporated, as well inserts of other metals.
The
high initial cost of the casting process is easily made up by the reduction or
total elimination of machining and finishing costs for a component required in
large numbers. The principal base metals
most commonly employed in die casting re zinc, aluminium, copper, magnesium,
lead and tin. Depending upon the melting
temperature of alloys and their suitability for die casting, they are
classified as high melting point and low melting point alloys.
Advantages of DIE-CASTING:
·
Requires less floor space than is
required by other casting processes.
·
Die casting provides for precision
manufacture with a subsequent reduction in machining cost.
·
Thin sections of complex shape are
possible by die casting. Under favourable conditions, minimum thickness that
can be cast is 0.5mm for zinc and 0.9mm for aluminium.
·
Die casting provides for greatly
improved surface finish when compared to other casting methods. Finishing
operations are thus minimum.
·
True shape as that of original shape of
die cavity can be obtained. Details can be reproduced successfully with a high
degree of precision.
·
Castings produced by die casting are
usually less defective, owing to increased casting soundness. These are strong
with dense metal structure.
·
The increased soundness and reduction of
defects provides increased yield.
·
The labour cost involved in die casting
is less with the minimum of job training.
·
The rate of production of castings is
very high, which can be as mush as 800 castings per hour.
Limitations of DIE CASTING:
- · The cost of die and equipment used is high. The life of die decreases rapidly if metal temperature is high.
- · There is a limited scope of non-ferrous alloys that can be used for die casting.
- · There are many complex features which limit the application of die casting.
- · The size of castings produced for maintenance and supervision of die.
- · The minimum economic quantity for die casting is around 20,000.
- · Die casting usually contain some porosity due to the entrapping of air.
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