Cold Chamber Diecasting Process:
In the cold chamber process (in which heating chamber is outside the machine unit) the molten metal is poured with ladle into the shot chamber for every shot.
This process reduces the contact time between the liquid metal and the shot chamber. For materials such as aluminium and brass their high melting temperatures make it difficult to cast them by hot chamber process, because the gooseneck of the hot chamber machine is continuously in contact with the molten metal. Also liquid aluminium would attack the gooseneck material and the hot chamber process is not used with aluminium alloys. The hot chamber process is used for most of the low melting temperature alloys such as zinc, load and tin.
Cold-chamber die casting: shot cylinder filled with a ladle for each cycle. Used for high melting temperature metals
The operation sequence in above figure is similar to that of the hot chamber process. The operation starts with the spraying of die lubricants throughout the dle cavity and closing of the die when molten metal is ladled into the shot chamber of the machine either manually by a hand ladle or by means of an auto ladle. An auto ladle is a form of a robotic device which automatically scoops molten aluminium from the holding furnace and pours it into the die at the exact instant required in the casting cycle.
The metal volume and pouring temperature can be precisely controlled with an auto ladle and hence the desired casting quality can be had. Then the plunger forces the metal into the die cavity and maintains the pressure till it solidifies. In the next step, the die opens. The casting is ejected.
At the same time, the plunger returns to its original position completing the operation.
The main disadvantage of the cold chamber process is the longer cycle time needed compared to the hot chamber process. Also, since the metal is ladled into the machine from the furnace. it may lose the super heat and sometimes may cause defects such a cold shuts.
Advantages of Cold Chamber Die-casting:
- Because of the use of the movable cores, it is possible to obtain fairly complex castings than that feasible by permanent mould casting.
- Very small thicknesses can be easily filled because the liquid metal is injected at high pressure.
- Very high production rates can be achieved. The typical rate could be 200 pieces per hour since the process is completely automated.
- Because of the metallic dies, very good surface finish of the order of 1 micron can be obtained.
- The surfaces generated by diecasting can be directly electroplated without any further processing.
- Closer dimensional tolerances of the order of+0.08 mm for small dimensional can be obtained compared to the sand castings.
- The die has a long life, which is of the order of 300000 places for zinc alloys and 150000 for aluminium alloys.
- Diecasting gives better mechanical properties compared to sand casting, because of the fine grained skin formed during solidification.
- Inserts can be readily cast in place.
- It is very economical for large-scale production
Limitations of Cold Chamber Die-casting:
- The maximum size of the casting is limited. The normal sizes are under 4 kg with maximum of the order of 15 kg because of the limitation on the machine capacity.
- This is not suitable for all materials because of the limitations on the die materials. Normally, zinc, aluminium, magnesium and copper alloys are diecast.
- The air in the die cavity gets trapped inside the casitng and is therefore a problem often with the diecastings.
- The dies and the machines are very expensive and therefore, economy in production is possible only when large quantities are produced.