Cold Chamber Die Casting Process
The
hot chamber process is used for most of the low melting temperature alloys such
as zinc, lead and tin. For materials such as aluminum and brass, their high
melting temperatures make it difficult to cast them by hot chamber process,
because gooseneck of the hot chamber machine is continuously in contact with
the molten metal. Also liquid aluminum would attack the gooseneck material and
thus hot chamber process is not used with aluminum alloys. In the cold chamber
process, the molten metal is poured with a ladle into the hot chamber for every
shot. This process reduces the contact time between the liquid metal and the
hot chamber.
The
operation starts with the spraying of die lubricants throughout the die cavity
and closing the die when molten metal is ladled into the hot chamber of the
machine either manually or by means of an auto ladle. An auto ladle is a form
of robotic device, which automatically scoops molten aluminum from the holding
furnace and pours it into the die at the exact instance when required in the
casting cycle. The pouring temperature can be precisely controlled with the
help of 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, plunger returns to its
position completing the operation.
Cold
chamber and hot chamber die casting differs from each other in the following
respects :
Melting
unit is not an integral part of the cold chamber die casting machine. Molten
metal is brought and poured into the die casting machine with the help of
ladles.
In
case of cold chamber process high pressures tend to increase the fluidity of
molten metal possessing relatively lower temperature and hence castings
produced are denser, dimensionally accurate and free from blowholes.
In
case of cold chamber process die components experience less thermal stresses
due to lower temperature of the molten metal. However, dies are required to be
made stronger in order to bear high pressures.
Cold
chamber process has a longer cycle time compared to hot chamber process.
In
case of cold chamber process as metal is ladled from a furnace, it may loose
superheat and may cause defects such as cold shuts.
