Low pressure die casting
Low pressure die casting is a well-known casting process, especially for aluminium wheels
in passenger cars. The process is also used in the production of big V-engine blocks in
hypereutectic aluminium alloys and in the casting of air-cooled cylinder heads for motor
cycles.
The casting unit of a low-pressure machine consists of a pressure-tight holding furnace, the
pressure control unit, the hydraulic die manipulators and usually one die for the casting.
High pressure die castings
Filling speed and intensification pressure differentiates high-pressure die casting
from most other casting processes.
After liquid metal is transferred to the shot sleeve (s. fig. below), the plunger slowly
closes to shut-off the filling port.
The plunger is then moved towards the die at a controlled but high speed in order to
fill the die cavity.
With the potential for highly automated operation, the high-pressure die casting
(HPDC) process is capable of extremely high levels of productivity.
HPDC machines are size rated by the closing force (s. figs.)
The pressure cycle in a modern HPDC machine is closely controlled in all stages.
Filling times are typically on the order of 10-25 milliseconds.
Once the die is filled, pressures, which may exceed 70 MPa, are maintained on the
casting, using an accumulator, until solidification is complete (third stage, at right)
With the potential for highly automated operation, the high-pressure die
casting process is capable of extremely high levels of productivity.
Multiple hydraulic circuits are used depending on the plunger speed needed at each
stage as shown below.
To be die cast, engine blocks must designed with an open deck
The common size range and close tolerances have been used to good effect in defining the
hydraulic network in large transmission cases shown above. The Belt Tensioners (below) are
typical of small die cast parts:
Physical design considerations:
Only straight steel core pulls can be used to define internal passages.
Core pulls increase tooling costs.
Sand coring is generally not possible.
Inserts can frequently be cast into the part. Hollow tubes, threaded inserts to fill out
bosses, engine bore liners, and wear resistant inserts are common.
Feeding of shrinkage is via the gates alone; parts should be designed with as uniform
a thickness as possible.
Metallurgical design considerations:
Part ductility is limited by process considerations to <3% on average.
Die castings should not be used for load bearing safety critical parts.
Die castings are not heat treatable.
Die casting are generally produced using secondary (recycled) alloys; this reduces
cost.
Fatigue properties of die castings are good so long as the very smooth as-cast
surface is not machined away.
http://www.alueurope.eu/wp-content/uploads/2012/01/AAM-Manufacturing-1-Casting-methods.pdf
