Why Pressure Diecasting ?

The decision to choose Pressure Diecasting as the preferred production method is generally driven by a requirement for high annual volume. 

As the annual demand increases, the lower piece part price offered by the pressure diecasting route results in a far cheaper "Total Project Cost" than other methods of casting Aluminium.

The diagram below indicates that should the volumes required be low, then the case for gravity diecasting or sand casting becomes increasingly strong, due to the low start up costs.

Some common Casting Processes used for aluminium are listed below.

Sand Casting

Sand casting is the simplest method of casting aluminium. Sand is made into a mould by forming around a wooden "pattern". The pattern is removed, the sand mould assembled and molten metal pored in. The process is chosen for small production runs, for complex shape castings requiring intricate cores or for very large castings.

Advantages
Low equipment costs
Largest size of castings possible by any casting method
suited to complex shapes and cores
Very low gas porosity is possible
It is a versatile casting process

Limitations
Low casting rate
3-5mm minimum wall thickness
Poor linear dimensional tolerances e.g. 4mm / m
Rough surface finish
Coarse grain size compared to diecasting
Casting weights in the range of 0.1 Kg - 100,000 Kg
Approximate economical quantity range 1 - 1000 castings.

Gravity Diecasting

Castings are produced by poring molten metal into permanent metal moulds. (Generally made from Cast Iron).
This process produces 'Chill Castings'

Advantages
Lower set up cost than Pressure Diecasting
Higher casting rate than sand casting
Low gas porosity levels are possible
Fine grain sizes may be obtained
The highest quality castings with regards to mechanical integrity can be produced by this method
Less finishing is required than for sand castings.

Limitations
Minimum wall thickness 3-5mm
Linear tolerance is approximately 3 mm/m
Surface finish better than sand casting
The complexity of possible casting shapes is limited
Casting weight range 0.1 Kg - 70 Kg
Approximate economical quantity range 500 - 2500 (This may increase where sand cores are used to produce shapes impossible with pressure diecasting.)

Low Pressure Diecasting

This is a repetitive process where identical parts are cast by injecting molten metal under low pressure into metal dies. This process requires complex machinery and is similar to high pressure diecasting.

Advantages
Fair production rates up to 30 / Hr
Thin wall thickness possible ( 2-3mm)
Better linear tolerances than gravity casting
surface finish improved on gravity casting, but not up to pressure diecasting standards
High Yields possible as runners and risers not required
Reduced finishing ir required
Pore free castings are obtainable
Sand cores may still be used to allow complex castings
die costs far lower than for pressure diecasting
Castings are heat treatable

Limitations
Size of casting limited by machine size
Production rates not up to pressure diecasting
Feeding thin sections through thick sections is not recommended
casting weight range 5 Kg - 25 Kg
Approximate economical quantity range >1000

High Pressure Diecasting

Pressure diecasting is a repetitive process casting identical parts by injecting Aluminium into metal moulds at pressures in the order of 1000psi. Complex machinery and expensive tooling is required for this process.

Advantages
Production rates may be in the order of 200 / Hr
Thin wall thickness at 1 - 2.5mm
The best surface finish is produced by this method
Very fine grain structure is obtained
The castings have high strength in the as-cast condition
Good linear tolerances and repeatable properties are obtained

Limitations
Size of castings limited by the machine
Sound, thick sections are difficult to cast
Core configuration may be complex to enable disassembly
Porosity may become a concern
High start up costs require long production runs to reduce the overall cost
Castings cannot be heat treated
casting weight range 0.01 Kg - 25 Kg
Approximate economical quantity range > 10,000 per annum.

Shell Moulding

A shell mould consists of a sand shell, varying in thickness between 4-10 mm. The sand particles are bonded together with phenolic resins giving a permeable mould. The production of shell moulds may be automated which lends itself to medium to high production runs. The resin coated sand is placed on a hot metal pattern; this is fired in an oven to harden the shell. After cooling, the shell is removed from the pattern and is ready for use. Molten metal is then poured into the shell mould cavity and allowed to cool. The mould material is broken off the casting. Better dimensioned tolerances are possible than with sand moulding, which reduces machining costs. Fine surface finishes equal to that of permanent moulds (12~130 rms) may be obtained. and consistently reproducible thin castings with fine detail may be made. The process is more costly than sand, permanent mould or die casting.

Plaster Mould Casting

Permeable plaster moulds give a smooth surface finish (80~125 rms) with a finer surface detail than is obtainable with shell moulds. Castings as thin as 0.5 mm are possible. Slow solidification rates reduce internal stresses so that any casting distortion is negligible.

Investment Casting

 This casting method involves producing a "wax pattern" by injecting wax or plastic into a pattern die. The pattern is attached to gating and runner systems and this assembly is dipped in a hard setting refractory slurry, which is then cured. The pattern is melted out of the mould to leave an exact cavity. The mould is heated to cure the refractory and to volatolize the remaining wax pattern material. The moulds are baked and molten metal is poured into the mould cavity. On solidification of the casting, the mould material is broken away from the castings.