Die casting: Hot chamber process and cold chamber process

 Die casting

Die casting is a very commonly used type of permanent mold casting process. It is used for producing many components of home appliances (e.g rice cookers, stoves, fans, washing and drying machines, fridges), motors, toys and hand-tools – since Pearl river delta is a largest manufacturer of such products in the world, this technology is used by many HK-based companies. Surface finish and tolerance of die cast parts is so good that there is almost no post-processing required. Die casting molds are expensive, and require significant lead time to fabricate; they are commonly called dies. There are two common types of die casting: hot- and cold-chamber die casting.

In a hot chamber process (used for Zinc alloys, magnesium) the pressure chamber connected to the die cavity is filled permanently in the molten metal. 

The basic cycle of operation is as follows: 

(i) die is closed and gooseneck cylinder is filled with molten metal;

 (ii) plunger pushes molten metal through gooseneck passage and nozzle and into the die cavity; metal is held under pressure until it solidifies; 

(iii) die opens and cores, if any, are retracted; casting stays in ejector die; plunger returns, pulling molten metal back through nozzle and gooseneck; 

(iv) ejector pins push casting out of ejector die. As plunger uncovers inlet hole, molten metal refills gooseneck cylinder. 

The hot chamber process is used for metals that 

(a) have low melting points and 

(b) do not alloy with the die material, steel; common examples are tin, zinc, and lead.

In a cold chamber process, the molten metal is poured into the cold chamber in each cycle. 

The operating cycle is 

(i) Die is closed and molten metal is ladled into the cold chamber cylinder; 

(ii) plunger pushes molten metal into die cavity; the metal is held under high pressure until it solidifies; 

(iii) die opens and plunger follows to push the solidified slug from the cylinder, if there are cores, they are retracted away; 

(iv) ejector pins push casting off ejector die and plunger returns to original position. 

This process is particularly useful for high melting point metals such as Aluminum, and Copper (and its alloys).

(a) Hot chamber die casting (b) Cold chamber die casting
(a) Hot chamber die casting (b) Cold chamber die casting

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