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Manufacturing Processes > Casting Processes > Gravity die casting

 

Gravity die casting

 

Process description

Molten metal is poured under gravity into a pre-heated die, where it solidifies. The die is then opened and the casting ejected. Also known as permanent mold casting.

Materials

Usually non-ferrous metals, for example: copper, aluminum, magnesium, but sometimes iron, lead, nickel, tin and zinc alloys. Carbon steel can be cast with graphite dies.

Gravity

Process variations

  • Dies typically cast iron, graphite or refractory material.
  • Metal or sand cores can be used although surface finish can be poor.
  • Low pressure die casting: uses low-pressure (1 bar) air to force the molten metal into the die cavity. Less popular than gravity die casting, and tends to be used purely for the production of car wheels. Gives lower production rates.
  • Slush casting: for creating hollow parts without cores in low melting point metals such as lead, zinc and copper alloys.

Economic considerations

  • Production rates of 5–50/h, but dependent on size.
  • Lead times can be many weeks.
  • Material utilization moderate to high (10–40 per cent lost in scrap, but can be recycled).
  • If accuracy and surface finish not an issue, can use sand cores instead of metallic or graphite for greater economy.Production volumes of 500–1000 may be viable, but suited to higher volume production.
  • Tooling costs moderate.
  • Equipment costs moderate.
  • Labor costs low to moderate.
  • Finishing costs low to moderate. Gates need to be removed.

Typical applications

  • Cylinder heads
  • Engine connecting rods
  • Pistons
  • Gear and die blanks
  • Kitchen utensils
  • Gear blanks
  • Gear housings
  • Pipe fittings
  • Wheels

Design aspects

  • Shape complexity limited by that obtained in die halves.
  • Undercuts are possible with large added cost.
  • Inserts possible with small added cost.
  • Machining allowances usually in the range 0.8–1.5 mm.
  • Vertical parting lines commonly used.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Cored holes greater than Ø5 mm.
  • Draft angle ranging 2–3.
  • Maximum section =50 mm.
  • Minimum section =2 mm.
  • Sizes ranging from 50 g to 300 kg in weight.
  • Commonly used for castings less than 5 kg.

Quality issues

  • Little porosity and inclusions: can be minimized by slow die filling to reduce turbulence.
  • Redressing of the dies may be required after several thousand castings.
  • Collapsible cores improve extraction difficulties on cooling.
  • ‘Chilling’ effect of cold metallic dies on the surface of the solidifying metals needs to be controlled by pre-heating at correct temperature.
  • Large castings sometimes require that the die is tilted as molten metal is being poured in to reduce turbulence.
  • Mechanical properties fair to good.
  • Surface detail is good.
  • Surface roughness ranging 0.8–6.3 µm Ra.
  • Process capability charts showing the achievable dimensional tolerances using various materials provided. Allowances of ±0.25–±0.75mm should be added for dimensions across the parting line.

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