Casting

Die• Metal is injected into the mould under high pressure of 10-210Mpa.Generally good surface finish

and good dimensional accuracy.

Typical

Thin-walled: ComplexSolid: CylindricalSolid: CubicSolid: Complex

Feasible

FlatThin-walled: CylindricalThin-walled: Cubic

Shapes

• 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

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 After the metal has solidified in the die cavity, the plunger is withdrawn, the die opens and the casting is ejected.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 partsCan form complex shapesHigh strength partsVery good surface finish and accuracyHigh production rateLow labour costScrap can be recycled

Disadvantages

Trimming is requiredHigh tooling and equipment costLimited die lifeLong 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 processMazak, 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 uneconomicalDifficult 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 manufactureThere 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