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Manufacturing Processes > Plastic processing > Rotational Moulding

 

Rotational Molding

 

Process description

Raw material is placed in the mold and simultaneously heated and rotated forcing the particles to deform and melt on the walls of a female mold without the application of external pressure or centrifugal forces. The part is cooled whilst rotating. The mold is designed to rotate about two perpendicular axes.

Materials

  • Several common thermoplastics, including difficult fluoropolymers.
  • Raw material supplied as finely ground powder.
Rotational Molding

Process variations

  • Slush molding: uses liquid polymers (plastisols) for small hollow parts.
  • Air, water or mist can be used for mold cooling.

Economic considerations

  • Production rates of 3–50/h, but dependent on size.
  • To increase production rates, three-arm carousels often used with one mold each in the loadunload, heat and cool positions. Lead time several days.
  • Material utilization very high. Little waste material.
  • Production volumes in the range of 100–1000 typically.
  • Tooling costs low.
  • Equipment costs low to moderate.
  • Labor costs moderate.
  • Finishing costs low. Little finishing required.

Typical applications

  • Water tanks
  • Storage vessels
  • Dust bins
  • Buckets
  • Housings
  • Drums
  • Prototypes

Design aspects

  • Complexity limited to large, hollow parts of uniform wall thickness.
  • Long, thin projections not possible.
  • Large flat surfaces should be avoided due to distortion and difficulty to form. Use stiffening ribs.
  • Internal walls need to be well spaced.
  • Molded-in holes, bosses, finishes and lettering all possible at added cost and limited accuracy.
  • With rotation speed variation, can build up thicker layers at key points in the mold.
  • Integral handles possible.
  • Large threads can be molded-in.
  • Undercuts should be avoided.
  • Sharp corners difficult to fill in the mold. Radii should be as generous as possible (greater than five times the wall thickness) and tend to become thicker than the wall thickness on molding.
  • Metal or higher-melting point plastic inserts can be molded-in.
  • Can clad the inside of the finished part using another polymer.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Holes cannot be molded although open-ended articles possible.
  • Thickness variation should be less than 2:1.
  • Draft angles generally greater than 1°, typically 3°.
  • Maximum section =13 mm.
  • Minimum section typically 2 mm, but can be as low as 0.5mm for certain applications.
  • Sizes up to 4m³.

Quality issues

  • The part is practically free from residual stresses.
  • Surface detail is fair.
  • Outer surface finish of the part is a replica of the inside finish of the mold walls.
  • Control of inside surface finish is not possible.
  • Wall thickness is determined by the close control of the amount of raw material used.
  • Dimensional variations can be large if sufficient setting time is not allowed before removal of the part.
  • A process capability chart showing the achievable dimensional tolerances is provided. Allowances of approximately ±0.5mm should be added for dimensions across the parting line.
  • Wall thickness tolerances are generally between ±5 and ±20 per cent of the nominal.

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