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Manufacturing Processes > Plastic processing > Injection Moulding

 

Injection Molding

 

Process description

Granules of polymer material are heated and then forced under pressure using a screw into the die cavity. On cooling, a rigid part or tree of parts is produced.

Materials

Mostly thermoplastics, but thermosets, composites and elastomers can be processed.

Injection Molding

Process variations

  • Injection blow molding: allows small hollow parts with intricate neck detail to be produced.
  • Co-injection: for products with rigid cores pre-placed in the die before injection or simultaneous injection of different materials into same die.

Economic considerations

  • Production rates are high, 1–50/min, depending on size.
  • Thermoset parts usually have a longer cycle time.
  • Lead times can be several weeks due to manufacturing of complex dies.
  • Material utilization is good. Scrap generated in sprues and risers.
  • If material permits, gates and runners can be reused resulting in lower material losses.
  • Flexibility limited by dedicated dies, die changeover and machine setup times.
  • Economical for high production runs, typically 10 000+.
  • Full automation achievable. Robot machine loading and unloading common.
  • Tooling costs are very high. Dies are usually made from hardened tool steel.
  • Equipment costs are very high.
  • Direct labor costs are low to moderate.
  • Finishing costs are low. Trimming is required to remove gates and runners.

Typical applications

  • High precision, complex components
  • Automotive and aerospace components
  • Electrical parts
  • Fittings
  • Containers
  • Cups
  • Bottle tops
  • Housings
  • Tool handles

Design aspects

  • Very complex shapes and intricate detail possible.
  • Holes, inserts, threads, lettering, color, bosses and minor undercuts possible.
  • Uniform section thickness should be maintained.
  • Unsuitable for the production of narrow necked containers.
  • Variation in thickness should not exceed 2:1.
  • Marked section changes should be tapered sufficiently.
  • Living hinges and snap features allow part consolidation.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • The clamping force required proportional to the projected area of the molded part.
  • Radii should be as generous as possible. Minimum inside radii =1.5 mm.
  • Draft angle ranging from less than 0.25 to 4°, depending on section depth.
  • Maximum section, typically =13 mm.
  • Minimum section =0.4mm for thermoplastics, 0.9mm for thermosets.
  • Sizes ranging 10 g–25 kg in weight for thermoplastics, 6 kg maximum for thermosets.

Quality issues

  • Thick sections can be problematic.
  • Care must be taken in the design of the running and gating system, where multiple cavities used to ensure complete die fill.
  • Control of material and mold temperature critical, also injection pressure and speed, condition of resin, dwell and cooling times.
  • Adequate clamping force necessary to prevent the mold creating flash.
  • Thermoplastic molded parts usually require no de-flashing: thermoset parts often require this operation.
  • Excellent surface detail obtainable.
  • Surface roughness a function of the die condition. Typically, 0.2–0.8 µm Ra is obtainable.
  • Process capability charts showing the achievable dimensional tolerances using various materials are provided . Allowances of approximately ±0.1mm should be added for dimensions across the parting line. Note, that charts 1, 2 and 3 are to be used for components that have a major dimension, greater than 50mm, and typically large production volumes. The chart titled ‘Light Engineering’ is used for components with a major dimension, less than 150mm, and for small production volumes.

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