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Manufacturing Processes > Plastic processing > Reaction Injection Moulding

 

Reaction Injection Molding

 

Process description

Two components of a thermosetting resin are injected into a mixing chamber and then injected into the mold at high speed where polymerization and subsequent solidification takes place.

Materials

  • Mostly thermosets.
  • Foamed materials possessing a solid skin can be created by setting up a pressure differential between mixing chamber and mold.
  • Can add chopped fiber material (glass, carbon) for added stiffness to mixing to produce composites.
Rotation Injection Moulding

Process variations

  • Mold material is usually aluminum. Can also use resin for low production runs or hardened tool steel for very high volumes.
  • Further heating of resin components before mixing is dependent on material used.

Economic considerations

  • Production rates from 1 to 10/h.
  • Lead times can be several weeks.
  • Material utilization good. Less than 1 per cent lost in scrap.
  • Scrap cannot be recycled.
  • Flexibility limited by dedicated dies, die changeover and machine setup times.
  • Economical for low to medium production volumes (10– 10 000).
  • Can be used for one-offs, e.g. prototyping.
  • Tooling costs low.
  • Equipment costs high.
  • Direct labor costs moderate to high.
  • Finishing costs low. A little trimming required.Production rates from 1 to 10/h.
  • Lead times can be several weeks.
  • Material utilization good. Less than 1 per cent lost in scrap.
  • Scrap cannot be recycled.
  • Flexibility limited by dedicated dies, die changeover and machine setup times.
  • Economical for low to medium production volumes (10– 10 000).
  • Can be used for one-offs, e.g. prototyping.
  • Tooling costs low.
  • Equipment costs high.
  • Direct labor costs moderate to high.
  • Finishing costs low. A little trimming required.

Typical applications

  • Car bumpers
  • Cups
  • Containers
  • Panels
  • Housings
  • Footwear
  • Garden furniture

Design aspects

  • Very complex shapes and intricate detail possible.
  • Ribs, holes, bosses and inserts possible.
  • Small re-entrant features possible.
  • Radii should be as generous as possible.
  • Uniform section thickness should be maintained.
  • Marked section changes should be tapered sufficiently.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Draft angles ranging 0.5–3°, depending on section depth.
  • Maximum section =10 mm.
  • Minimum section =1.5 mm; foamed material =3 mm.
  • Maximum dimension =1.5 m.
  • Sizes ranging 100 g–10 kg in weight.

Quality issues

  • Thick sections can be problematic.
  • Care must be taken in the design of the running and gating system, where multiple cavities are used to ensure complete die fill.
  • Problems can be created by premature reaction before complete filling of mold.
  • Excellent surface detail is obtainable.
  • Surface roughness is variable, but mainly dependent on mold finish.
  • Achievable dimensional tolerances are approximately ±0.05 at 25mm, ±0.3 at 150 mm. Allowances of approximately ±0.2mm should be added for dimensions across the parting line.

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