Die Casting:
The
die casting process uses steel dies into which metal is forced under
pressure through a runner and gate to fill the dies. The pressure (70 to
5000 kg/cm2) is maintained while the casting solidifies after which the dies are separated, cores are withdrawn and the casting is ejected.
Metals and alloys that are die cast include zinc, aluminium, and magnesium, copper, lead and tin.
Typical
applications of die casting process include automobile components,
household appliances, railway and aircraft fittings, bath room hardware,
business machines, locks, pullers and many other similar parts.
Die Casting Dies:
The
dies used for die casting resemble a permanent mould. They are
generally made in two parts arranged to open and close with a vertical
parting. When mounted on a die casting machine one of the die halves
remains stationary during operation and is called a cover die.
The
other half moves for opening and closing and is called the ejector die.
Die casting dies are made of special die steels which are resistant to
heat checking, hammering and mechanical wear and are also dimensionally
stable. Die cavities are machined to very close accuracies. Vents and
overflow wells are provided in the dies for escape of air. The dies may
be water cooled to speed up cooling of the casting.
Die casting machines:
A die casting cycle consists of the following steps:
(i) Closing the die halves
(ii) Clamping the die halves securely together
(iii) Forcing the liquid metal into the dies
(iv) Opening the die halves and
(v) Ejecting the casting.
Die
casting machines are designed to perform all these functions. To be
effective these machines should be strong and rigidly built to take up
die weights and provide holding pressures against the pressure of the
molten metal. The machine frame should hold the die halves rigidly in
correct alignment. The die holding forces should be in excess of the
maximum force developed by the molten metal to ensure leak proof joint
in the dies. The die closing and locking arrangements generally used in
the die casting machines include hydraulic, hydraulic mechanical or
mechanical devices depending on the capacity of the machine.
Modern die casting machines are of two basic types namely
(i) hot chamber or submerged plunger die casting machines ( 7 to 35 MPa ) and
1. Goose Neck type or Air injection type
2. Submerged plunger type
(ii) Cold chamber die casting machines (14 to 140 MPa).
Process parameters:
Common metals:
- Alloys of aluminum
- Zinc
- Magnesium
- Lead
- Copper
- Tin
Size limits:
- less than 30 grams upto 7 kg
Thickness limits:
- As thin as 0.75mm to 13 mm
Typical tolerances:
- varies with metal being cast
- typically 0.1 mm for the first 2.5 cm and 0.02 mm for each additional centimeter
Draft allowances:
- 2 degree
Surface finish:
- 1 –25 micrometer
Hot Chamber Die Casting Machine:
It
is also called a gooseneck machine because of the shape of the metal
passage way. In this machine the melting pot, usually made of cast iron,
is a part of the machine. The gooseneck containing a cylinder and metal
passage way is kept immersed in the metal pot. The plunger in the
gooseneck cylinder is actuated either hydraulically or pneumatically. In
operation the plunger is withdrawn letting the liquid metal into the
gooseneck cylinder through the port provided.
When
the die halves are closed and ready for casting the plunger forces the
liquid metal entrapped in the cylinder into the die through the
gooseneck passage and a nozzle. After a predetermined time interval the
plunger is retracted allowing the liquid metal in the gooseneck channel
and nozzle to fall back into cylinder.
The die
halves are opened and the solidified casting is ejected from the die.
Hot chamber machines are designed to operate almost automatically and
fast. A press button operation will make the machine go through a
complete cycle of activities including closing the die halves, forcing
the metal into the die, holding the pressure for a predetermined time,
withdrawing the plunger, opening the die, ejecting the casting and stop
ready for the next cycle. The operator then removes the casting,
inspects the dies, gives spray lubrication to the dies and starts the
next cycle. Metal injection speeds and pressures can be controlled to
suit different metals and castings.
Since
the melting pot plunger and cylinder of a hot chamber die casting
machine are made of cast iron and cast iron reacts with metals like
aluminium at elevated temperatures, only low melting-point metals can be
cast by this method. There is also a limit on the maximum pressure
which can be applied. Hot chamber machines are mostly operated below 14
kPa. Alloys of lead, tin and zinc are the most common metals cast by
this process.
Cold chamber die casting machine:
The
metal in this case is melted in a separate furnace and the required
quantity of metal is ladled to the machine. A plunger operated
hydraulically forces the metal into the die. Injection pressures of 28
kPa to 250 kPa are possible in cold chamber machines. The machine is
semiautomatic in that after the metal is ladled into the cold chamber
the rest of the operation is automatic. Hot chamber machines are made in
capacities varying from 0.25 to 7.5 MN and cold chamber ones from 1 to
10 MN.
Advantages of Die Casting Process:
•
Advantages of die casting include excellent die life, high production
rates, close dimensional tolerances, good details, and excellent surface
finish of the castings.
• Die casting dies retain their accuracies for long production runs
•
Production rates vary- from 5 to 6 castings per minute with hot chamber
machines to 2 to 3 castings per minute when cold chamber machine are
used.
• Dimensional tolerances can be held to ± 0.075 mm.
• Very thin sections can be cast and good surface finish obtained with excellent details.
Disadvantages of Die Casting Process:
(1) High cost of dies and machines.
(2) Restriction on the size of the casting to about 100 kg for zinc alloys and 30 kg for aluminium alloys:
(3) Only certain non-ferrous metals can be economically die cast.
(4) Die casting products also are mechanically weaker because of the air entrapped during casting.
(5)
The entrapped air makes die casting unsuitable for heat treatment. When
these castings are heated for heat treatment the entrapped air expands
causing blisters to be formed on the surface of the castings.
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