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Manufacturing Processes > Plastic processing > Blow Molding

 

Blow Molding

 

Process description

A hot hollow tube of plastic, called a parison, is extruded or injection molded downwards and then caught between two halves of a shaped mold which closes the top and bottom of the parison. Hot air is blown into the parison, expanding it until it uniformly contacts the inside contours of the cold mold. The part is allowed to cool and is then ejected.

Materials

Most thermoplastics.

Blow Molding

Process variations

  • Extrusion blow molding: more applicable to asymmetrical parts, integrated handles possible.
  • Injection blow molding: parison injection molded and then transferred to blow molding machine. For small parts with intricate neck detail.
  • Multiple parisons: can create multi-layered parts. This requires close control since uneven parisons produce waste.
  • Parisonless blowing: similar to dip-coating followed by expansion into the mold.
  • Stretch blow molding: the simultaneous axial and radial expansion of a parison, yielding a biaxially orientated container.

Economic considerations

  • Production rates between 100 and 2500/h, depending on size.
  • Lead times a few days.
  • Integration with extrusion process to produce parison provides continuous operation.
  • There is generally little material waste, but can increase with some complex geometries using extrusion blow molding.
  • Full automation readily achievable.
  • Flexibility limited since molds are dedicated.
  • Setup times and changeover times relatively short.
  • Production volumes of 1000, but better suited to very high volumes.
  • Tooling costs moderate to high.
  • Equipment costs moderate to high, especially for full automation.
  • Direct labor costs low. One operator can manage several machines.
  • Finishing costs low. Some trimming required.

Typical applications

  • Hollow plastic parts with relatively thin walls
  • Bottles
  • Bumpers
  • Ducting

Design aspects

  • Complexity limited to hollow, well rounded, thin walled parts with low degree of asymmetry.
  • Asymmetrical moldings, e.g. off-set necks possible with movable blowing spigots.
  • Undercuts, bosses, ribs, lettering, inserts and threads possible.
  • Corner radii should be as generous as possible (>3 mm).
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Holes cannot be molded-in.
  • Draft angles not required.
  • Maximum section =6 mm. Thick sections may need cooling aids (carbon dioxide or nitrogen gas).
  • Minimum section =0.25 mm.
  • Sizes ranging 12mm in length to volumes up to 3m³.

Quality issues

  • Poor control of wall thickness, typically ±50 per cent of nominal.
  • Creep and chemical stability of product important considerations.
  • Residual stresses, e.g. non-uniform deformation, may relax in time causing distortion of the part.
  • Good surface detail and finish possible.
  • The higher the pressure the better the surface finish of the product.
  • A process capability chart showing the achievable dimensional tolerances is provided. Allowances of approximately ±0.1mm should be added for dimensions across the parting line.

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