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Manufacturing Processes > Plastic processing > Continuous Extrusion

 

Continuous Extrusion

 

Process description

The raw material is fed from a hopper into a heated barrel and pushed along a screw-type feeder where it is compressed and melts. The melt is then forced through a die of the required profile where it cools on exiting the die.

Materials

  • Most plastics, especially thermoplastics, but also some thermosets and elastomers.
  • Raw material in pellet, granular or powder form.
Continuous Extrusion

Process variations

  • Most extruders are equipped with a single screw, but two-screw or more extruders are available. These are able to produce coaxial fibers or tubes and multi-component sheets.
  • Metal wire, strips and sections can be combined with the extrusion process using an offset die to produce plastic coatings.
  • Pultrusion: for fiber-reinforced rods, tubes and sections.

Economic considerations

  • Production rates are high but are dependent on size. Continuous lengths up to 60 m/min for some tube sections and profiles, up to 5 m/min for sheet and rod sections.
  • Extruders are often run below their maximum speed for trouble free production.
  • It can have multiple holes in die for increased production rates.
  • Extruder costs increase steeply at the higher range of output.
  • Lead times are dependent on the complexity of the 2-dimensional die, but normally weeks.
  • Material utilization is good. Waste is only produced when cutting continuous section to length.
  • Process flexibility is moderate. Tooling is dedicated, but changeover and setup times are short.
  • Production of 1000 kg of profile extrusion is economical, 5000 kg for sheet extrusions (equates to about 10 000 items).
  • Tooling costs are generally moderate.
  • Equipment costs are high.
  • Some materials give off toxic or volatile gases during extrusion. Possible need for air extraction and washing plant which adds to equipment cost.
  • Direct labor costs are low.
  • Finishing costs are low. Cutting to length only real cost.

Typical applications

  • Complex profiles. All types of thin walled, open or closed profiles
  • Rods, bar, tubing and sheet
  • Small diameter extruded bar which is cut into pellets and used for other plastic processing methods
  • Fibers for carpets, tyre reinforcement, clothes and ropes
  • Cling-film
  • Plastic pipe for plumbing
  • Plastic-coated wire, cable or strips for electrical applications
  • Window frames
  • Trim and sections for decorative work

Design aspects

  • Dedicated to long products with uniform cross-sections.
  • Cross-sections may be extremely intricate.
  • Solid forms including re-entrant angles, closed or open sections.
  • Section profile designed to increase assembly efficiency by integrating part consolidation features.
  • Grooves, holes and inserts not parallel to the axis of extrusion must be produced by secondary operations.
  • No draft angle required.
  • Maximum section =150 mm.
  • Minimum section =0.4mm for profiles (0.02mm for sheet).
  • Sizes ranging 6mm²–1800mm wide sheet, and Ø1–Ø150mm for tubes and rods.

Quality issues

  • The rate and uniformity of cooling are important for dimensional control because of shrinkage and distortion.
  • Extrusion causes the alignment of molecules in solids.
  • Die swell, where the extruded product increases in size as it leaves the die, may be compensated for by:
    • Increasing haul-off rate compared with extrusion rate
    • Decreasing extrusion rate
    • Increasing the length of the die land
    • Decreasing the melt temperature.
  • There is a tendency for powdered materials to carry air into the extruder barrel: trapped gases have a detrimental effect on both the output and the quality of the extrusion.
  • Surface roughness is good to excellent.
  • Process capability charts showing the achievable dimensional tolerances for various materials are provided

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