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Manufacturing Processes > Casting Processes > Centrifugal Casting

 

Centrifugal Casting

 

Molten metal is poured into a high-speed rotating mold (300–3000 rpm depending on diameter) until solidification takes place. The axis of rotation is usually horizontal, but may be vertical for short work pieces

Materials

  • Most metals suitable for static casting are suitable for centrifugal casting: all steels, iron, copper, aluminum and nickel alloys.
  • Also, glass, thermoplastics, composites and ceramics (metal molds sprayed with a refractory material) can be molded by this method
Centrifugal Casting

Process variations

  • Semi-permanent or expendable molds.
  • Semi-centrifugal casting: used to cast parts with radial symmetry in a vertical axis of rotation at low speeds.
  • Centrifuge casting: a number of molds are arranged radially around a central sprue. Molten metal is poured into the sprue and is forced into the mold cavities by centrifugal force due to high-speed rotation. Used for small gears mainly and parts of intricate detail.

Economic considerations

  • Production rates of up to 50/h possible, but dependent on size.
  • Lead time may be several weeks.
  • Material utilization high (90–100 per cent). No runners or risers.
  • Economic when the mechanical properties of thick-walled tubes are important and high alloy grades of steel are required.
  • In large quantities, production of other than circular external shapes becomes more economical.
  • Small diameter steel tubes made by this method not competitive with welded or rolled tubes.
  • Selection of mold type (permanent or sand) determined by shape of casting, quality and number to be produced.
  • Production volumes low, typically 100+.Can be used for one-offs.
  • Tooling costs moderate.
  • Equipment costs low to moderate.
  • Direct labor costs low to moderate.
  • Finishing costs low to moderate.Normally, machining of internal dimension necessary.

Typical applications

  • Pipes
  • Brake drums
  • Pulley wheels
  • Train wheels
  • Flywheels
  • Gun barrels
  • Gear blanks
  • Large bearing liners
  • Engine-cylinder liners
  • Pressure vessels
  • Nozzles

Design aspects

  • Shape complexity limited by nature of process, i.e. suited to parts with rotational symmetry.
  • Contoured surfaces possible.
  • Circular bore remains in the finished part.
  • Dual metal tubes that combine the properties of two metals in one application possible.
  • Inserts and bosses possible, but undercuts are not.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Cored holes greater than Ø25 mm.
  • Machining allowances ranging 0.75–6 mm.
  • Draft angle approximately 1°.
  • Maximum section thickness approximately 25 mm.
  • Minimum section ranging 2.5–8 mm, depending on material cast.
  • Maximum length = 15 m.
  • Sizes ranging Ø25mm–Ø2m.
  • Sizes up to 5 t in weight have been cast.

Quality issues

  • Properties of castings vary by distance from the axis of rotation.
  • Due to density differences in the molten material, dross, impurities and pieces of the refractory lining tend to collect on the inner surface of the casting. This is usually machined away.
  • Tubular castings have higher structural strengths and more distinct cast impressions than gravity die cast or sand cast parts.
  • Castings are free of shrinkage due to one-directional cooling.
  • The mechanical properties of dense castings are comparable with that of forgings. Fine grain castings and low porosity is an advantage.
  • Good mechanical properties and fine grain structure. . Surface detail is fair to good.
  • Surface roughness ranging 1.6–12.5 µm Ra.
  • A process capability chart showing the achievable dimensional tolerances is provided . Allowances of approximately ±0.25–±0.75mm should be added for dimensions across the parting line. Note, the chart applies to outside dimensions only. Internal dimensions are approximately 50 per cent greater.

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