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Manufacturing Processes > Plastic processing > Transfer Moulding

 

Transfer Molding

 

Process description

A heated mold is closed under low pressure and then a liquid resin and catalyst is loaded into an adjacent mixing head and forced via a plunger into the cavity where curing takes place. Full name is resin transfer molding.

Materials

  • Limited to only several thermosetting plastics and elastomers, with or without fillers.
  • Can use pre-pressed fiber-packs to fit the mold, called preforms. Fibers can be glass or carbon.
Transfer Molding

Process variations

  • Powder material placed in a heated melting pot and forced under pressure into a heated mold.
  • Vacuum assisted resin injection: additional vacuum can be used in mold cavity to assist resin filling of fiber preforms.

Economic considerations

  • Production rates 20–300/h. Fast curing speed.
  • ead time typically days, depending on complexity of tool.
  • Material utilization very good. Less than 3 per cent scrap typically.
  • Scrap material cannot be recycled directly.
  • High degree of automation possible.
  • Economical for production runs of 1000–10 000.
  • Tooling costs moderate to high.
  • Equipment costs generally moderate.
  • Direct labor costs low to moderate.
  • Some skilled labor required, but easily reduced with automation.
  • Finishing costs low, but no opportunity for in-mold trimming.

Typical applications

  • Electrical cabinets
  • Housings and panels
  • Car body panels
  • Wind turbine blades
  • Seating
  • Yacht hulls and decks
  • Plant growing trays
  • Garden ponds

Design aspects

  • Complex geometries possible and hollow shapes.
  • Cores possible for increased complexity.
  • Can mold around inserts and delicate cores easily.
  • Lettering, ribs, holes, inserts and threads possible.
  • Undercuts possible, but at added cost.
  • Thickness variation less than 2:1.
  • Draft angles ranging 2–3° preferred, but can be as low as 0.5°.
  • Minimum inside radius =6 mm.
  • Minimum section ranging 0.8–1.5 mm, depending on material used.
  • Maximum section =90 mm.
  • Maximum dimension =450 mm.
  • Minimum area =3mm².
  • Maximum size 16 kg in weight, but suited to smaller parts.

Quality issues

  • Differential stress distribution may occur due to flow characteristics of mold resulting in minor distortion.
  • High temperatures above resin melting temperatures must be maintained prior and during mold filling.
  • Improperly placed fiber preforms can cause dry spots or pools of resin on surface of finished part.
  • Fiber preforms can also move during injection mold filling without proper fixing arrangements within mold.
  • Variation in resin/fiber concentration is difficult to control in sharp corners.
  • It is not recommended for parts subjected to high loads in service.
  • Surface detail is excellent.
  • Surface roughness is a function of the die condition, with 0.8 µm Ra, readily obtainable.
  • Achievable dimensional tolerances are ±0.05 at 25 mm, ±0.15 at 150 mm. Wall thickness tolerances are typically ±0.25mm.

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