Casting
Die
•
Metal is injected into the mould under high pressure of 10-210Mpa.Generally good surface finish
and good dimensional accuracy.
•
Aluminium, Zinc and Copper alloys are the materials predominantly used in die-casting.
•
Pure Aluminium is rarely cast due to high shrinkage, and susceptibility to hot cracking. It is alloyed
with Silicon, which increases melt fluidity, reduces machinability.
•
Copper is another alloying element, which increases hardness, reduces ductility, and reduces
corrosion resistance.
•
E.g. Aluminium is cast at a temperature of 650 ºC (1200 ºF). It is alloyed with Silicon 9% and
Copper about 3.5% to form the Aluminium Association 380 alloy (UNS A03800). Silicon increases
the melt fluidity, reduces machinability, Copper increases hardness and reduces the ductility. By
greatly reducing the amount of Copper (less than 0.6%) the chemical resistance is improved. A
high silicon alloy is used in automotive engines for cylinder castings, with 17% Silicon for high
wear resistance
•
Cost Die casting moulds (called dies in the industry) tend to be expensive as they are made from
hardened steel-also the cycle time for building these tend to be long. Also the stronger and harder
metals such as iron and steel cannot be die-cast
Shapes
Typical
Thin-walled: Complex
Solid: Cylindrical
Solid: Cubic
Solid: Complex
Feasible
Flat
Thin-walled: Cylindrical
Thin-walled: Cubic
Manufacture Process
Cold Chamber process, In this
process molten metal is poured
into the injection cylinder manually
by a hand ladle or by an automatic
ladle. The metal is forced into the
die cavity at high pressures. High
melting point alloys of aluminium
and copper are normally cast using
cold chamber die casting.
Hot Chamber process, this process involves
the use of a plunger which traps a certain
volume of molten metal and forces it into the
die cavity through a gooseneck and nozzle.
After the metal has solidified in the die cavity,
the plunger is withdrawn, the die opens and
the casting is ejected. Metals having low
melting points such as Zinc, copper,
magnesium and lead are cast using Hot
chamber die casting.
Advantages & Disadvantages
Advantages
Can produce large parts
Can form complex shapes
High strength parts
Very good surface finish and
accuracy
High production rate
Low labour cost
Scrap can be recycled
Disadvantages
Trimming is required
High tooling and equipment cost
Limited die life
Long lead time
Gravity Die Casting
• This is a low-tech process is well suited for the batch production of simple forms without undercuts.
• This process is where the liquid metal is poured into metallic moulds without application of any
external pressure.
• The liquid metal enters the cavity by gravity. Gravity die casting (GDC) is different from High
Pressure Die Casting (HPDC), where the liquid metal is injected into the metal mould under
very high pressures for production of thin walled smaller castings with better dimensional
accuracy and surface finish
Manufacture process
Mazak, an alloy of zinc and aluminium, is first melted in a crucible furnace. Molten metal is
then poured by ladle into a open steel mould where it is allowed to cool and solidify. The
mould is then turned upside down and tapped with a hammer to release the finished casting.
Advantages & Disadvantages
Advantages
Minimal Finishing required
Accurately dimensioned
sharply defined
smooth or textured-surface metal parts
The process is suitable for mass production
Castings ranging from few grams to 100 Kgs of
Aluminium alloy can be cast
Disadvantages
Limitation of geometry /size
Beyond a particular shape and size the process
becomes uneconomical
Difficult to attach gates and risers
Pressure Casting
•
Pressure is usually required because of viscosity related flow limitations.
•
This process, similar to injection moulding, is a variant of porous mould
casting in which the ceramic suspension is injected into the mould under
high pressure.
• The moulds may be fabricated from plastic, plaster or ceramics.
• The higher the applied pressure, the shorter the casting time.
• Aluminium alloy
High Pressure Casting
• This equipment consists of two vertical platens on which bolsters are located which hold
the die halves.
• One platen is fixed and the other can move so that the die can be opened and closed.
• A measured amount of metal is poured into the shot sleeve and then introduced into the
mould cavity using a hydraulically-driven piston.
• Once the metal has solidified, the die is opened and the casting removed.
In this process, special precautions must be taken to
avoid too many gas inclusions which cause blistering
during subsequent heat-treatment or welding of the
casting product.
Low Pressure Casting
• The die is filled from a pressurised crucible below, and pressures of up to 0.7
bar are usual.
• Low-pressure die casting is especially suited to the production of components
that are symmetric about an axis of rotation.
• Light automotive wheels are normally manufactured by this technique.
Advantages
Complicated shapes can be made
Disadvantages
Expensive die and machine
Sand Casting
•
Produced by forming a mould from a sand mixture and pouring molten liquid
metal into the cavity in the mould. The mould is then cooled until the metal
has solidified. In the last stage the casting is separated from the mould.
•
Green sand is a mixture of silica sand, clay, moisture and other additives.
This must be mixed carefully as if too much water is added - when molten
aluminium is poured into the mould an explosion can result
•
The'air set method uses dry sand bonded to materials other than clay,
using a fast curing adhesive.
• Typically iron, steel, bronze, brass, aluminium, magnesium alloys
or various pot metal alloys, which often include lead, tin, and zinc.
Process of manufacture
There are six steps in this process:
1. Place a pattern in sand to create a mould.
2. Incorporate a gating system.
3. Remove the pattern.
4. Fill the mould cavity with molten metal.
5. Allow the metal to cool.
6. Break away the sand mould and remove the casting.
Industrial casting
•
Brass, Bronze, Aluminium, Stainless steel, and Nickel-boron alloys
• castings may range in size from a few ounces to 15 pounds.
Major Casting Processes:
Sand Casting
Shell Moulding
Investment Casting
Lost Foam Casting
Die Casting
Pressure Die Casting
Gravity Die Casting
The Hitchiner Process
Vacuum (“V”) Process
Centrifugal Casting
Cosworth Process