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Shell molding

 

Process description

A heated metal pattern is placed over a box of thermosetting resin-coated sand. The box is inverted for a fixed time to cure the sand. The box is re-inverted, and the excess sand falls out. The shell is then removed from the pattern and joined with the other half (previously made). They are supported in a flask by an inert material ready for casting.

Materials

Most metals, except: lead, zinc, magnesium and titanium alloys, also beryllium, refractory and zirconia alloys.

Shell Casting

Process variations

  • Molds produced from other casting processes may be joined with shell molds.
  • Patterns are generally made of iron or steel giving good dimensional accuracy.
  • Aluminum patterns may be used for low-volume production.
  • Other pattern materials used are plaster, and graphite for reactive materials.

Economic considerations

  • Production rates of 5–200/h, but dependent on size.
  • Lead time several days to weeks depending on complexity and size.
  • Material utilization high; little scrap generated.Potential for automation high.
  • With use of gating systems several castings in a single mold possible.
  • Resin binders cost more, but only 5 per cent as much sand used as compared to sand casting.
  • Difficult to change design during production.
  • More suited to moderate to high volume production, but production volumes of 100–500 may be economical.
  • Considered the best of low cost casting methods for large quantities.
  • Tooling costs low to moderate.
  • Equipment costs moderate to high.
  • Labor costs low to moderate.
  • Low finishing costs. Often no finishing required.

Typical applications

  • Small mechanical parts requiring high precision
  • Gear housings
  • Cylinder heads
  • Connecting rods
  • Transmission components

Design aspects

  • Good for molding complex shapes, especially when using composite molds.
  • Great variations in cross section possible.
  • Sharper corners, thinner sections, smaller projections than possible with sand casting.
  • Bosses and inserts possible.
  • Undercuts difficult.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Cored holes greater than Ø3 mm.
  • Draft angle ranging 0.25–1°, depending on section depth.
  • Maximum section =50 mm.
  • Minimum section =1.5 mm.
  • Sizes ranging 10 g–100 kg in weight. Better for small parts less than 20 kg.

Quality issues

  • Blowing sand onto pattern makes depositing more uniform, especially good for intricate forms.
  • Few castings scrapped due to blowholes or pockets. Gases are able to escape through thin shells or venting.
  • Composite cores may include chills and cores to control solidification rate in critical areas.
  • Moderate porosity and inclusions.
  • Mechanical properties better than sand casting.
  • Uniform grain structure.
  • Surface detail good.
  • Surface roughness ranging 0.8–12.5 µm Ra.
  • Process capability charts showing the achievable dimensional tolerances using various materials provided. Allowances of ±0.25–±0.5mm should be added for dimensions across the parting line.

costing

shellcasting