Hot Chamber Die Casting
Hot chamber die casting is a metal casting process that involves (1) injecting molten
metal into a steel die, (2) allowing the metal to cool and solidify, (3) ejecting the
casting, (4) spraying the die cavity with a release agent and then closing the mould in
preparation for the next casting cycle.
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Hot chamber die casting machine parts
The hot chamber casting machine consists of:
a heated crucible that holds the molten metal that will be forced into the die
cavity. The molten metal is called the charge. The crucible is heated by a
gas or electric furnace continuously to a temperature that melts the charge
and keeps it molten.
Aluminium, magnesium and zinc are the most commonly used alloys but
other alloys with fairly low melting points can also be used in hot chamber die
casting.
a gooseneck and nozzle. The gooseneck transports molten metal from the
shot chamber to the die. A nozzle fixed to the end of the gooseneck connects
the gooseneck to the die. The gooseneck and the nozzle are heated to
ensure that the charge stays molten inside them.
The plunger. A hydraulic cylinder forces the plunger down the shot chamber
in the gooseneck. The plunger moves past the shot chamber inlet port, covers
it and seals it.
The plunger forces molten metal through the heated gooseneck and nozzle
with great pressure, into the die cavity, then it holds the charge under
pressure until the casting has cooled and solidified.
On the return stroke, the plunger draws molten metal back out of the nozzle
and opens the inlet port allowing the shot chamber to be recharged.
The mould / die. The die is a steel mould into which molten metal is injected
under high pressure into the die cavity. It has at least two parts, the cover die
and the ejector die. The cover die is a fixed part that has an inlet port through
which molten metal is injected under high pressure. The other half is the
movable ejector die which has ejector pins that eject the solidified casting.
Hydraulic cylinders move the ejector die to open and close the die assembly.
Hydraulic cylinders exert great pressure on the ejector die when the die
assembly is closed to prevent molten metal from escaping from the die cavity.
The die cavity is full of air, prior to molten metal being injected into it.
For molten metal to be able to fill the die cavity,
o air in the die cavity must be pushed out, through vents, by the molten
metal as it moves up the gooseneck and into the die cavity. These
vents are large enough for air to pass through but not the molten metal.
o or air in the die cavity must be evacuated using a vacuum system
before, or as, molten metal is injected into it.
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Conventional hot chamber die casting
The problem with conventional die casting, i.e. casting that does not utilise a vacuum
system, is that air can be trapped in parts of the die cavity or within the molten metal.
This trapped air causes casting defects called porosity.
Casting porosity is voids or pores in the casting resulting from trapped gas preventing
the die cavity from filling fully.
Finishing
The ejected casting needs to have the sprue removed.
If flashing is present, it is also removed and any defects like porosity or marks left by
the ejector pins are repaired. The casting is now ready for use.
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Vacuum assisted hot chamber die casting
A solution to the problem of trapped air in the die is vacuum assisted die casting.
In vacuum assisted die casting, a vacuum pump is used to draw air out of the
gooseneck and die cavity. This can happen as molten metal is being injected into the
die, or just before injection starts.
The vacuum draws the molten metal into all parts of the die cavity, eliminating the
possibility of casting defects caused by trapped air in the die. It also reduces the
force needed to inject the molten metal into the die cavity.
As soon as the die cavity is filled and the molten metal has reached the shut-off
valve, a sensor sends a signal that instantly closes the valve and stops the vacuum
pump. Holding pressure is maintained until the metal in the die cavity has cooled and
solidified.
Hot chamber die casting magnesium alloys
Molten magnesium alloy oxidises rapidly in air and can burn explosively. When
magnesium is cast using the hot chamber die casting process, it is heated in a
sealed crucible, covered with a protective layer of gas.
The covering gas used has been sulphur hexafluoride (SF6), a very powerful
greenhouse gas. SF6 has a global warming potential approximately 24,000 times
that of CO2. ( Ref: https://www.sciencedirect.com/science/article/pii/S2213956713000030? )
SF6 has been banned for some applications in the EU since 2014.
3M's Novec 612 is an alternative that’s claimed to be a safe, sustainable cover gas
for magnesium casting operations which has minimal impact on the environment.
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Hot chamber die casting process
Preparation for casting
1. Metal ingots are loaded into the hot chamber die casting machine crucible and
heated continuously to a temperature that melts the metal and keeps it molten.
2. The gooseneck and nozzle are preheated.
3. The die is preheated to prevent cracking in the casting.
If molten metal would be injected into a cold steel die, the outside of the casting
would chill and shrink suddenly while the inside of the casting would still be very hot.
This would result in cracks forming on the casting.
Stage 1.
A hydraulic cylinder rams the plunger with great pressure and high speed down the
shot chamber past the inlet port.
The plunger shuts off the inlet port and forces molten metal through the heated
gooseneck into the die.
Air in the die is forced out through vents, or is evacuated through vents, chill blocks
or vacuum systems.
Stage 2.
The die is kept closed, under great pressure, until the molten metal in it has cooled
and solidified. The metal stays molten in the heated gooseneck and nozzle.
To prevent the casting from shrinking and forming defects, the plunger maintains a
holding pressure on the metal charge until the casting has cooled and solidified.
Stage 3.
When the casting has cooled and solidified, the plunger is raised drawing molten
metal back through the nozzle and gooseneck. The plunger no longer covers the
inlet port so molten metal flows into the shot chamber.
The die is opened and the casting is ejected.
Stage 4.
The open die is lubricated; after lubrication, the die is closed.
The die is ready for the next die casting cycle.
More metal ingots are loaded into the crucible when needed.
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