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pressure (70 to 5000 kg/cm2) is maintained while the casting solidifies after which the dies are separated, cores are withdrawn and the casting is ejected.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).
Common metals in Die Casting:
Alloys of aluminum
less than 30 grams upto 7 kg
As thin as 0.75mm to 13 mm
varies with metal being cast
typically 0.1 mm for the first 2.5 cm and 0.02 mm for each additional centimeter
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.