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Manufacturing Processes > Plastic processing > Contact Molding

 

Contact Molding

 

Process description

Glass fiber reinforced material (30–45 per cent by volume) and a liquid thermosetting resin are simultaneously formed into a male or female mold and cured at room temperature or with the application of heat to accelerate the process.

Materials

  • Glass reinforced fiber in woven, continuous and chopped roving, mat and cloth forms.
  • Can use pre-impregnated sheets of uncured resin and fiber, called SMC.
  • Thermosetting liquid resin: commonly catalyzed polyester or epoxy.
Contact Molding

Process variations

  • Hand lay-up: manual laying of fiber reinforced material and application of resin to mold to build up the thickness. Hand or roller pressure removes any trapped air.
  • Variations on hand lay-up are:
  • . Vacuum bag molding: uses a rubber bag clamped over the mold. A vacuum is applied between the mold and the bag to squeeze the resin/reinforcement together removing any trapped air. Curing performed in an oven.
  • Pressure bag molding: as vacuum bag molding, but pressure is applied above the bag. Can be used for thicker section parts.
  • Hand lay-up using SMC: cured by heat and clamped if necessary to further reduce air pockets.
  • Spray lay-up: use of an air spray gun incorporating a cutter that chops continuous rovings to a controlled length before being blown into the mold simultaneously with the resin.
  • Molds can be made of wood, plaster, concrete, metal or glass fiber reinforced plastic.
  • Cutting of composites can be performed using knives, disc cutters, lasers and water jets.

Economic considerations

  • Production rates low. Long curing cycle typically.
  • Production rates increased using SMC materials.
  • Lead times usually short, depending on size and material used for the mold.
  • Mold life approximately 1000 parts.
  • Multiple molds incorporating heating elements should be used for higher production rates.
  • Material utilization moderate. Scrap material cannot be recycled.
  • Limited amount of automation possible.
  • Economical for low production runs, 10–1000. Can be used for one-offs.
  • Tooling costs low.
  • Equipment costs generally low.
  • Direct labor costs high. Can be very labor intensive, but not skilled.
  • Finishing costs moderate. Some part trim is required.

Typical applications

  • Hulls for boats and dinghies
  • Large containers
  • Swimming pools and garden pond moldings
  • Bath tubs
  • Small cabins and buildings
  • Machine covers
  • Car body panels
  • Sports equipment
  • Wind turbine blades
  • Prototypes and mock-ups
  • Architectural work

Design aspects

  • High degree of shape complexity possible, limited only by ability to produce mold.
  • Produces only one finished surface.
  • Fibers should be placed in the expected direction of loading, if any. Random layering gives less strength. . Avoid compressive stresses and buckling loads.
  • Used for parts with a high surface area to thickness ratio.
  • Molded-in inserts, ribs, holes, lettering and bosses are possible.
  • Draft angles are not required.
  • Undercuts are possible with flexible molds.
  • Minimum inside radius =6 mm.
  • Minimum section =1.5 mm.
  • Maximum economic section =30 mm, but can be unlimited.
  • Sizes ranging 0.01–500m² in area.
  • Maximum size depends on ability to produce the mold and the transport difficulties of finished part.

Quality issues

  • Air entrapment and gas evolution can create a weak matrix and low strength parts.
  • Non-reinforcing gel coat helps to create smoother mold surface and protects the molding from moisture.
  • Resin and catalyst should be accurately metered and thoroughly mixed for correct cure times.
  • Excessive thickness variation can be eliminated by sufficient clamping and adequate lay-up procedures.
  • Toxicity and flammability of resin is an important safety issue, especially because of high degree of manually handling and application.
  • Surface roughness and surface detail can be good on molded surface, but poor on opposite surface.
  • Shrinkage increases with higher resin volume fraction.
  • A process capability chart showing the achievable dimensional tolerances for hand/spray lay-up is provided. Wall thickness tolerances are typically ±0.5 mm.

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