The two principal types of die casting machines are: 1. Hot-Chamber Die Casting Machines 2. Cold-Chamber Die Casting Machines.
A hot-chamber die casting machine is shown in Fig. 4.13 (a) and (b). The main Components of the machine includes a steel pot filled with the molten metal to be cast and a pumping system that consists of a pressure cylinder, a plunger, a gooseneck passage, and a nozzle.
With the plunger in the up position (Fig. a), the molten metal flows by gravity through the intake ports into the submerged hot chamber. When the plunger is pushed downward by the power cylinder, it shuts off the intake port.
Then, with further downward movement of plunger, the molten metal is forced through the gooseneck passage and the nozzle into the die cavity as shown in Fig. (b). The pressure of molten metal coming out from the nozzle is about 50 to 150 atmospheres per square inch.
The pressure is maintained after the cavity is full of molten metal, for a specific time to solidify the casting completely. Next, the two halves of the die are separated and before the cycle are repeated.
The advantages of hot-chamber die casting are numerous, some important to write here are:
(i) High production rates, especially when multi-cavity dies are used.
(ii) Improved productivity and surface finish.
(iii) Very close dimensional tolerances.
(iv) Ability to produce intricate shapes with thin walls.
Nevertheless, the hot-chamber die casting has some limitations, these are:
(i) Only low-melting-point alloys (such as zinc, tin, lead, aluminum and like) can be cast because the components of the pumping system are in direct contact with the molten metal throughout the process.
(ii) Also, it is usually only suitable for producing small castings that weigh less than 4.5kg.
A Cold-chamber die casting machine is shown in Fig. 4.14. (a). the molten metal is first ladled through the pouring hole of the shot chamber. The two halves of the die are closed and locked together. Next, the plunger moves forward to close off the pouring hole and forces the molten metal into the die cavity.
The pressure in shot chamber may go over 2000 atmospheric per square inch. After the castings have solidified, the two halves of the die are separated, and the casting, together with the gate and slag of excess metal, are ejected from the die, by means of ejector pins.
In cold-chamber die casting machines, the molten-metal reservoir is separate from casting machines, unlike the hot-chamber die casting machines. One shot of molten metal is ladled every stroke. The steel chamber (shot chamber) is too little to have any reaction with hot molten metal to be casted.
(i) Large parts weighing 20 kg can be produced by cold-chamber die casting.
(ii) The process is very successful for casting aluminum and alloys, copper and alloys, and high- temperature zinc-aluminum alloys.
(iii) Intricate shapes are easily made.
(i) A longer cycle time when compared with hot-chamber die casting.
(ii) An auxiliary system for pouring the molten metal is needed.
Owing to the above limitations, Vertical cold-chamber machines were developed. A typical vertical cold- chamber machine is shown in Fig. 4.14 (b). It has a transfer tube that is submerged into the molten metal. It is fed into the shot chamber by connecting the die cavity to a vacuum tank by means of a special valve. Then the molten metal is forced into the die cavity when the plunger moves upward.