Casting
or founding, shaping of metal by melting and
pouring into a mold. Most castings, especially large ones, are made in
sand molds. Sand, mixed with a binder to hold it together, is pressed
around a wooden pattern that leaves a cavity in the sand. Molten metal
is poured into the cavity and allowed to solidify. Permanent metal molds
are used to make many small, simple parts; shell molding gives greater
accuracy for a large volume of semiprecision parts. A two-step process,
investment casting, produces small, complex shapes. Wax or plastic
replicas of the parts are molded in accurate metal molds. These replicas
are covered with sand in a box to make the final mold. When the whole
mold is heated, the replica melts, leaving behind a cavity into which
metal is poured. Large numbers of small, precise parts of metals that
have a low melting point, such as zinc, are made by die-casting. In an
automatic process, molten metal is forced under pressure into metal
molds. Cast iron and cast steel are more brittle than forged iron and
forged steel
Die casting
is a metal casting process that is characterized by forcing molten metal
under high pressure into a mold cavity. The mold cavity is created using
two hardened tool steel dies which have been machined into shape and
work similarly to molds during the process. Most die castings are made
from non-ferrous metals, specifically zinc, copper, aluminium,
magnesium, lead, pewter and tin based alloys. Depending on the type of
metal being cast, a hot- or cold-chamber machine is used.
Manufacture of parts using die
casting is relatively simple, involving only four main steps, which
keeps the incremental cost per item low. It is especially suited for a
large quantity of small to medium sized castings, which is why die
casting produces more castings than any other casting process. Die
castings are characterized by a very good surface finish (by casting
standards) and dimensional consistency.
Two variants are pore-free die
casting, which is used to eliminate gas porosity defects; and direct
injection die casting, which is used with zinc castings to reduce scrap
and increase yield.
The main die casting alloys
are: zinc, aluminium, magnesium, copper, lead, and tin; although
uncommon, ferrous die casting is possible.
Die casting
Equipment
There are two basic types of
die casting machines: hot-chamber machines and cold-chamber
machines. These are rated by how much clamping force they can apply.
Typical ratings are between 400 and 4,000 st (2,500 and 25,000 kg).
Hot-chamber
Machines
Schematic of a hot-chamber
|
Hot-chamber machines, also
known as gooseneck machines, rely upon a pool of molten metal to
feed the die. At the beginning of the cycle the piston of the machine is
retracted, which allows the molten metal to fill the "gooseneck". The
pneumatic or hydraulic powered piston then forces this metal out of the
gooseneck into the die. The advantages of this system include fast cycle
times (approximately 15 cycles a minute) and the convenience of melting
the metal in the casting machine. The disadvantages of this system are
that high-melting point metals cannot be utilized and aluminium cannot
be used because it picks up some of the iron while in the molten pool.
Due to this, hot-chamber machines are primarily used with zinc, tin, and
lead based alloys. |
Cold-chamber
machines
These are
used when the casting alloy cannot be used in hot-chamber machines;
these include aluminium, zinc alloys with a large composition of
aluminium, magnesium and copper. The process for these machines start
with melting the metal in a separate furnace. Then a precise amount of
molten metal is transported to the cold-chamber machine where it is fed
into an unheated shot chamber (or injection cylinder). This shot is then
driven into the die by a hydraulic or mechanical piston. This biggest
disadvantage of this system is the slower cycle time due to the need to
transfer the molten metal from the furnace to the cold-chamber machine. |
Schematic of a cold-chamber die
casting machine |
Dies
The ejector die half |
The cover die hal |
Two dies are used in die
casting; one is called the "cover die half" and the other the "ejector
die half". Where they meet is called the parting line. The cover die
contains the sprue (for hot-chamber machines) or shot hole (for
cold-chamber machines), which allows the molten metal to flow into the
dies; this feature matches up with the injector nozzle on the
hot-chamber machines or the shot chamber in the cold-chamber machines.
The ejector die contains the ejector pins and usually the runner, which
is the path from the sprue or shot hole to the mold cavity. The cover
die is secured to the stationary, or front, platen of the casting
machine, while the ejector die is attached to the movable platen. The
mold cavity is cut into two cavity inserts, which are separate
pieces that can be replaced relatively easily and bolt into the die
halves.
The dies are designed so that
the finished casting will slide off the cover half of the die and stay
in the ejector half as the dies are opened. This assures that the
casting will be ejected every cycle because the ejector half contains
the ejector pins to push the casting out of that die half. The
ejector pins are driven by an ejector pin plate, which accurately
drives all of the pins at the same time and with the same force, so that
the casting is not damaged. The ejector pin plate also retracts the pins
after ejecting the casting to prepare for the next shot. There must be
enough ejector pins to keep the overall force on each pin low, because
the casting is still hot and can be damaged by excessive force. The pins
still leave a mark, so they must be located in places where these marks
will not hamper the castings purpose.
Other die components include
cores and slides. Cores are components that usually
produce holes or opening, but they can be used to create other details
as well. There are three types of cores: fixed, movable, and loose.
Fixed cores are ones that are oriented parallel to the pull direction of
the dies (i.e. the direction the dies open), therefore they are fixed,
or permanently attached to the die. Movable cores are ones that are
oriented in any other way than parallel to the pull direction. These
cores must be removed from the die cavity after the shot solidifies, but
before the dies open, using a separate mechanism. Slides are similar to
movable cores, except they are used to form undercut surfaces. The use
of movable cores and slides greatly increases the cost of the dies.
Loose cores, also called pick-outs, are used to cast intricate
features, such as threaded holes. These loose cores are inserted into
the die by hand before each cycle and then ejected with the part at the
end of the cycle. The core then must be removed by hand. Loose cores are
the most expensive type of core, because of the extra labor and
increased cycle time. Other features in the dies include water-cooling
passages and vents along the parting lines. These vents are usually wide
and thin (approximately 0.13 mm or 0.005 in) so that when the molten
metal starts filling them the metal quickly solidifies and minimizes
scrap. No risers are used because the high pressure ensures a continuous
feed of metal from the gate.
The most important material
properties for the dies are thermal shock resistance and softening at
elevated temperature; other important properties include hardenability,
machinability, heat checking resistance, weldability, availability
(especially for larger dies), and cost. The longevity of a die is
directly dependent on the temperature of the molten metal and the cycle
time. The dies used in die casting are usually made out of hardened tool
steels, because cast iron cannot withstand the high pressures involved,
therefore the dies are very expensive, resulting in high start-up costs.
Metals that are cast at higher temperatures require dies made from
higher alloy steels.
The main failure mode for die
casting dies is wear or erosion. Other failure modes are heat
checking and thermal fatigue. Heat checking is when surface
cracks occur on the die due to a large temperature change on every
cycle. Thermal fatigue is when surface cracks occur on the die due to a
large number of cycles.
Die casting
Process
The following are the four
steps in traditional die casting, also known as high-pressure
die casting, these are also the basis for any of the die casting
variations: die preparation, filling, ejection, and shakeout. The dies
are prepared by spraying the mold cavity with lubricant. The lubricant
both helps control the temperature of the die and it also assists in the
removal of the casting. The dies are then closed and molten metal is
injected into the dies under high pressure; between 10 and 175
megapascals (1,500 and 25,400 psi). Once the mold cavity is filled, the
pressure is maintained until the casting solidifies. The dies are then
opened and the shot (shots are different from castings because there can
be multiple cavities in a die, yielding multiple castings per shot) is
ejected by the ejector pins. Finally, the shakeout involves separating
the scrap, which includes the gate, runners, sprues and flash, from the
shot. This is often done using a special trim die in a power press or
hydraulic press. Other methods of shaking out include sawing and
grinding. A less labor-intensive method is to tumble shots if gates are
thin and easily broken; separation of gates from finished parts must
follow. This scrap is recycled by remelting it. The yield is
approximately 67%.
The high-pressure injection
leads to a quick fill of the die, which is required so the entire cavity
fills before any part of the casting solidifies. In this way,
discontinuities are avoided, even if the shape requires
difficult-to-fill thin sections. This creates the problem of air
entrapment, because when the mold is filled quickly there is little time
for the air to escape. This problem is minimized by including vents
along the parting lines, however, even in a highly refined process there
will still be some porosity in the center of the casting.
Most die casters perform other
secondary operations to produce features not readily castable, such as
tapping a hole, polishing, plating, buffing, or painting.
Inspection
for Casting defect
After the shakeout of the
casting it is inspected for defects. The most common defects are misruns
and cold shuts. These defects can be caused by bold dies, low metal
temperature, dirty metal, lack of venting, or too much lubricant. Other
possible defects are gas porosity, shrinkage porosity, hot tears, and
flow marks. Flow marks are marks left on the surface of the
casting due to poor gating, sharp corners, or excessive lubricant. |