Polymer extrusion

Polymer extrusion is a continuous shaping process used in the plastics industry. A thermoplastic, usually supplied as small pellets or as powder, is poured into a hopper, picked up by a rotating screw inside a heated barrel, and melted by the combined action of barrel heaters and the heat generated by its own viscous flow. The melt is then forced through a die that gives it a fixed cross-section, and the strand is cooled, pulled away, and cut or wound up.

Most plastic pipes, tubes, sheets, films, fibres, and the insulation on electrical cable are made this way. Extrusion is also the first step in blown film and blow moulding lines, and it is how almost every batch of plastic pellets is produced from raw resin in the first place.

Overview

The standard machine is the single-screw extruder. The screw is conventionally divided into three regions along its length, each doing a different job. The feed zone has a deep channel and simply drags the cold pellets away from the hopper into the barrel. In the compression (or melting) zone the channel gets shallower, the pellets are pressed against the hot barrel wall, and a thin film of melt forms between the solid bed and the wall. The relative motion of the screw flight scrapes this melt off and pushes it sideways into a growing pool, while the unmelted core shrinks until it is gone.[1][2] The metering zone is shallow again, and its job is to deliver the now-uniform melt at a steady rate and pressure into the die.[3]

In modern machines, most of the energy that melts the polymer comes from the screw itself, not from the barrel heaters. The heaters mainly stabilise wall temperature and help with start-up. Bruce Maddock's "screw-freezing" experiments in the 1950s, in which an extruder was stopped, cooled, and the screw pulled out so the partly melted plug could be examined, were the first clear picture of how this melting really happens.[2] Tadmor's 1966 model, which combined a heat balance for the melt film with a mass balance for the solid bed, made it possible to predict melting rates along the screw and is still the starting point for almost all single-screw simulations today.[1]

Twin-screw extruders, with two screws meshing inside a figure-of-eight barrel, are used where mixing matters more than simple melt pumping: compounding additives or fillers into a base resin, processing fibre-loaded polymers, or running reactive extrusion reactions during the pass through the machine.[4]

History

Manual ram extruders were used for lead pipe and macaroni in the early nineteenth century, but they did not melt anything; they just pushed soft material through a hole. The first patent for a screw extruder, by Mathew Gray in Britain in 1879, was for rubber rather than plastics, and rubber screw extruders were running in cable factories by the 1880s.[5]

Continuous extrusion of thermoplastics came in the 1930s with poly(vinyl chloride) and the early polyethylenes. Theory caught up only later. Solids conveying, melting, and melt flow inside the screw were largely figured out in the 1950s and 1960s, with Maddock's experiments and Tadmor's modelling work doing most of the heavy lifting.[2][1]

Limits

The extruder itself is rarely the bottleneck. What usually sets the practical maximum throughput of a line is melt fracture, a family of viscoelastic instabilities that appear at the die above a critical wall shear stress and ruin the surface or shape of the extrudate.[6][7] Other limits, depending on the line, are excessive viscous heating, thermal degradation of the polymer, and the pressure ratings of the breaker plate and die.[4]

See also

References

  1. ^ a b c Tadmor, Z. (1966). "Fundamentals of plasticating extrusion. I. A theoretical model for melting". Polymer Engineering and Science. 6 (3): 185–190. doi:10.1002/pen.760060303.
  2. ^ a b c Maddock, B. H. (1959). "A visual analysis of flow and mixing in extruder screws". Society of Plastics Engineers Journal. 15: 383–389.
  3. ^ Tadmor, Z.; Gogos, C. G. (2006). Principles of Polymer Processing (2nd ed.). Hoboken: Wiley. ISBN 978-0-471-38770-1.
  4. ^ a b Rauwendaal, C. (2014). Polymer Extrusion (5th ed.). Munich: Hanser. ISBN 978-1-56990-516-6.
  5. ^ GB patent 5056, Gray, M., "Improvements in machines for forming articles of india-rubber and similar materials" 
  6. ^ Denn, M. M. (2001). "Extrusion instabilities and wall slip". Annual Review of Fluid Mechanics. 33: 265–287. Bibcode:2001AnRFM..33..265D. doi:10.1146/annurev.fluid.33.1.265.
  7. ^ Vergnes, B. (2015). "Extrusion defects and flow instabilities of molten polymers". International Polymer Processing. 30 (1): 3–28. doi:10.3139/217.3011.
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