Casting Process
Hot
chamber Die casting, and
Cold
chamber Die casting.
The
main difference between these two types is that in hot chamber, the holding
furnace for the liquid metal is integral with the diecasting machine, whereas
in the cold chamber machine, the metal is melted in a separate furnace and then
poured into the diecasting machine with a laddle for each casting cycle which
is also called ‘shot’.
Hot Chamber Process
In
this process, a gooseneck is used for pumping the liquid metal into the die
cavity. The gooseneck is submerged into the holding furnace containing the
molten metal. The gooseneck is made of grey, alloy or ductile iron or of cast
steel. A plunger made of alloy cast iron, which is hydraulically operated moves
up in the gooseneck to uncover the entry port for the entry of liquid metal
into the gooseneck. The plunger can then develop the necessary pressure for
forcing the metal into the die cavity. A nozzle at the end of the gooseneck is
kept in close contact with the sprue located in the cover die.
The
cycle starts with the closing of the die when the plunger is in the highest
position in the gooseneck, thus facilitating the filling of the gooseneck by
the liquid metal. The plunger then starts moving down to force the metal in the
gooseneck to be injected into the die cavity. The metal is then held at the
same pressure till it is solidified. The die is opened, and any cores if
present, are also retracted. The plunger then moves back returning the unused
liquid metal to the gooseneck. The casting, which is in the ejector die, is now
ejected and at the same time the plunger uncovers the filling hole, letting the
liquid metal from the furnace to enter the gooseneck.
Air
pressure required for injecting the metal into the die is that of the order of
30-45 kg/cm2. Depending upon its size, this hot chamber die casting
machine can produce about 60 or more castings upto 20 kg each per hour and
several hundred castings per hour for single impression castings weighing a few
grams.
Advantages of Die Casting Process
1 Very high rate of production can be achieved.
2 close dimension of tolerance of the order of ± 0.025 mm is possible.
3 Surface finish of 0.8 micron is achievable.
4 Very thin sections of the order of 0.50 mm can be cast.
5 Fine details may be produced.
6 Less floor space is required.
7 Longer die life is obtained.
1 Not economical for small runs.
2 Only economical for non-ferrous alloys.
3 Cost of die and die casting equipment is high.
4 Heavy castings cannot be cast. In fact, the size of the dies and the capacity of the die casting machines available limit the maximum size.
5 Die castings usually contain some porosity due to entrapped air
Applications
The typical products made by die casting
are carburetors, crank cases, magnetos, handle bar housings, parts of scooters
and motor cycles, zip fasteners, head lamp bezels, and other decorative
automobile items.
