Single-screw extruder

A single-screw extruder is the standard machine used in polymer extrusion to continuously melt and shape thermoplastic materials. A rotating helical screw inside a heated cylindrical barrel picks up raw polymer pellets or powder from a hopper, melts them progressively along the barrel, and forces the resulting melt through a shaped die to produce a profile of fixed cross-section.[1][2]

Single-screw extruders are used to manufacture plastic pipe, tube, sheet, film, wire and cable insulation, fibres, and numerous other profiles. They also provide the plasticating stage in blown film extrusion and blow moulding lines, and form the final shaping step when raw resin is converted into pellets.[2]

Machine components

The main components of a single-screw extruder are:

Hopper
A gravity-fed or force-fed reservoir that supplies solid pellets or powder to the barrel inlet.
Barrel
A thick-walled steel cylinder surrounding the screw, fitted with electric resistance heater bands and cooling channels to maintain a set temperature profile along its length.
Screw
A helical flight wrapped around a central root, driven by a gearbox and motor at the rear of the machine. The ratio of screw length to diameter (L/D) is typically 20:1 to 36:1 in modern machines.[2]
Breaker plate and screen pack
A perforated steel disc fitted with stainless-steel mesh at the screw tip that filters contaminants and converts rotational melt flow into axial flow before the die.
Die
A precision-machined steel tool at the barrel exit whose internal channel cross-section determines the shape of the extrudate.

The screw and its zones

The screw is conventionally divided into three functional zones along its length:[1]

Feed zone
Has a deep flight channel. Its role is simply to drag cold pellets away from the hopper throat and convey them forward into the barrel.
Compression (melting) zone
The channel depth decreases progressively. Pellets are pressed against the hot barrel wall; a thin melt film forms at the wall and is continuously scraped off by the advancing flight into a growing melt pool. The unmelted solid bed shrinks until it is entirely consumed.[3][4]
Metering zone
A shallow channel of constant depth that delivers the now-uniform melt at a steady rate and pressure into the die.

In modern machines, the majority of the energy that melts the polymer is generated by viscous dissipation within the screw itself rather than conducted from the barrel heaters. The heaters primarily stabilise the wall temperature and assist during start-up.[1]

Performance and limits

The extruder itself is rarely the bottleneck in a production line. The practical upper limit on throughput is most often set by melt fracture, a family of viscoelastic instabilities that arise at the die above a critical wall shear stress and cause surface roughness or gross distortion of the extrudate.[5] Other limiting factors depending on the application include excessive viscous heating, thermal degradation of the polymer, and the pressure ratings of the breaker plate and die.[2]

History

The first patent for a screw extruder, granted to Mathew Gray in Britain in 1879, was for rubber rather than for thermoplastics; rubber screw extruders were running in cable factories by the 1880s.[6] Continuous extrusion of thermoplastics began in the 1930s with poly(vinyl chloride) and the early polyethylenes.[2]

The physical understanding of single-screw operation was largely established in the 1950s and 1960s. Bruce Maddock's screw-freezing experiments (1959), in which a running extruder was stopped, rapidly cooled, and the screw pulled out so the partly melted polymer plug could be sectioned and examined, gave the first direct visual picture of how melting proceeds along the screw.[4] Tadmor's 1966 analytical model, combining a heat balance for the melt film with a mass balance for the solid bed, made it possible to predict melting rates as a function of screw geometry and operating conditions, and remains the starting point for virtually all single-screw simulations today.[3]

See also

References

  1. ^ a b c Tadmor, Z.; Gogos, C. G. (2006). Principles of Polymer Processing (2nd ed.). Hoboken: Wiley. ISBN 978-0-471-38770-1.
  2. ^ a b c d e Rauwendaal, C. (2014). Polymer Extrusion (5th ed.). Munich: Hanser. ISBN 978-1-56990-516-6.
  3. ^ a b Tadmor, Z. (1966). "Fundamentals of plasticating extrusion. I. A theoretical model for melting". Polymer Engineering and Science. 6 (3): 185–190. Bibcode:1966PESci...6..185T. doi:10.1002/pen.760060303.
  4. ^ a b Maddock, B. H. (1959). "A visual analysis of flow and mixing in extruder screws". Society of Plastics Engineers Journal. 15: 383–389.
  5. ^ 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.
  6. ^ GB patent 5056, Gray, M., "Improvements in machines for forming articles of india-rubber and similar materials" 
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