| Casting processes - High Pressure Die
The pressure die casting
process has its origins in typecasting machines, which had reached a high
level of automation and mechanical efficiency by the mid-1800's. By the end
of the 19th century variants of the typecasting machines were being used to
produce components for cash registers and gramophones, and by the beginning
of the current century die cast bearings were being produced for automotive
applications. Although the technology of the process continued to evolve
during the early part of the 20th century, it was not until the 1920's that
the forerunners to the modern hot chamber and cold chamber machines were
developed. These processes have been refined to the extent that tiny zinc
alloy castings can be produced in a one-second cycle on fully automated hot
chamber machines. The cold chamber process is used predominantly for the
production of aluminium alloy components, which include complex castings
weighing in the region of 15kg (30lb), for the automotive industry. For such
components the cycle time would be about two minutes.
Process outline
Hot chamber process. A
schematic diagram of a hot chamber pressure die casting machine is shown in
fig. 8.3. The metal for casting is maintained at an appropriate temperature
in a holding furnace adjacent to, if not part of, the machine. The injection
mechanism is located within the holding furnace and a substantial part of it
is therefore in constant contact with the molten metal. Pressure is
transmitted to the metal by the injection piston, which forces it through
the gooseneck and into the die. On the return stroke metal is drawn into the
gooseneck for the next shot. In this process there is minimum contact
between air and the metal to be injected, thus minimising the tendency for
turbulent entrainment of air in the metal during injection. However, there
is prolonged contact between the metal and parts of the injection system,
which effectively restricts this process to zinc-base alloys.
Cold chamber
process
The essential feature
of this process is the independent holding and injection units. In the cold
chamber process metal is transferred by ladle, manually or automatically, to
the shot sleeve. Actuation of the injection piston forces the metal into the
die. This is a single-shot operation. This procedure minimises the contact
time between the hot metal and the injector components, thus extending their
operating life. However, the turbulence associated with high-speed injection
is likely to entrain air in the metal, which can cause gas porosity in the
castings. The cold chamber process is used for the production of aluminium
and copper base alloys and has been extended to the production of steel
castings.
The advantages of the
pressure die casting processes may be summarised as follows:
- the ability to
produce castings with close dimensional control
- the ability to
produce castings with a good surface finish
- the ability to
produce castings with thin walls, and therefore of reduced weight
- the ability to
produce castings at a high rate of production
Against these
advantages the following disadvantages should be weighed:
- high capital plant
costs
- high tooling costs
- restrictions on the
range of alloys which can be cast
- restrictions on the
maximum size of casting that can be cast
From Precision
Casting Processes by A J Clegg. Reprinted by permission of Butterworth
Heinmann
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