Technical Data - Casting Types

Pressure diecasting

Cold Chamber Diecasting Machines +

Cold Chamber Diecasting Machines

Cold chamber machines differ from hot chamber machines primarily in one respect; the injection plunger and cylinder are not submerged in molten metal. The molten metal is poured into a "cold chamber" through a port or pouring slot by a hand or automatic ladle. A hydraulically operated plunger, advancing forward, seals the port forcing metal into the locked die at high pressures. Injection pressures range from 3,000 to over 10,000 psi for aluminium alloys.

Figure 1: Cold Chamber Machine. Diagram illustrates die, cold chamber and horizontal ram or plunger (in charging position).In a cold chamber machine, more molten metal is poured into the chamber than is needed to fill the die cavity. This helps sustain sufficient pressure to pack the cavity solidly with casting alloy. Excess metal is ejected along with the casting and is part of the complete sho

Hot Chamber Diecasting Machines +

Hot Chamber Diecasting Machines

Hot chamber machines (Fig.1) are used primarily for zinc, and low melting point alloys which do not readily attack and erode metal pots, cylinders and plungers.

Figure 2: Hot Chamber Machine. Diagram illustrates the plunger mechanism which is submerged in molten metal. Modern machines are hydraulically operated and equipped with automatic cycling controls and safety devices. In the hot chamber machine, the injection mechanism is immersed in molten metal in a furnace attached to the machine. As the plunger is raised, a port opens allowing molten metal to fill the cylinder. As the plunger moves downward sealing the port, it forces molten metal through the gooseneck and nozzle into the die. After the metal has solidified, the plunger is withdrawn, the die opens, and the resulting casting is ejected.

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 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.

Sand Casting +

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 poured 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 0.1Kg - 100,000 Kg
  • Approximate economical quantity range 1 - 1000 castings
Gravity Casting +

Gravity Casting

Castings are produced by pouring molten metal into permanent metal moulds. (Generally made from Cast Iron). This process produces 'Chill 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 0.1Kg - 100,000 Kg
  • Approximate economical quantity range 1 - 1000 castings
Low 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 if required
  • Pour 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 5Kg - 25Kg
  • Approximate economical quantity range>1000
RPM 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 0.5mm are possible. Slow solidification rates reduce internal stresses so that any casting distortion is negligible.

Machining and finishing operations may be eliminated by the use of plaster moulds. Small holes may be cast to size ready for tapping. Surface finish and dimensional accuracy equates to die casting qualities. LM25 alloys are commonly cast by this process.

Investment Casting +

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 volatilize 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.

Shell Moulding +

Shell Moulding

A shell mould consists of a sand shell, varying in thickness between 4-10mm. 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 (12~130rms) 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.