plastic extrusion


  • [3] Process In the extrusion of plastics, the raw compound material is commonly in the form of nurdles (small beads, often called resin) that are gravity fed from a top mounted
    hopper into the barrel of the extruder.

  • Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be “tweaked” by varying screen pack composition (the
    number of screens, their wire weave size, and other parameters).

  • There are two different types of die tooling used for coating over a wire, tubing (or jacketing) and pressure.

  • In fact, if an extrusion line is running certain materials fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside
    the barrel.

  • The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product’s profile shape.

  • The process has much in common with plastic injection molding from the point of the extruder technology, although it differs in that it is usually a continuous process.

  • While pultrusion can offer many similar profiles in continuous lengths, usually with added reinforcing, this is achieved by pulling the finished product out of a die instead
    of extruding the polymer melt through a die.

  • [10] The screw design is commonly modular in that various conveying and mixing elements are arranged on the shafts to allow for rapid reconfiguration for a process change
    or replacement of individual components due to wear or corrosive damage.

  • The extruded layer can also be used as an adhesive to bring two other materials together.

  • This technology is used on any of the processes described above (blown film, overjacketing, tubing, sheet).

  • For example, this process can be used to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water.

  • A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated until soft (plastic), and formed via a mold into a new shape.

  • [7] Often co-extrusion is used to apply one or more layers on top of a base material to obtain specific properties such as UV-absorption, texture, oxygen permeation resistance,
    or energy reflection.

  • In most processes, a heating profile is set for the barrel in which three or more independent PID-controlled heater zones gradually increase the temperature of the barrel
    from the rear (where the plastic enters) to the front.

  • A simple plastic extrusion screw Extruder screws From Boston Matthews Most screws have these three zones: • Feed zone (also called the solids conveying zone): this zone feeds
    the resin into the extruder, and the channel depth is usually the same throughout the zone.

  • ability/ available density of the sheet to be drawn to the mold which can vary in depths from 1 to 36 inches typically) is highly important and greatly affects forming cycle
    times for most plastics.

  • This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different viscous plastics to a single extrusion head (die) which
    will extrude the materials in the desired form.

  • • Melting zone (also called the transition or compression zone): most of the polymer is melted in this section, and the channel depth gets progressively smaller.

  • While either single- or double-screw extruders may be used in traditional extrusion, the necessity of adequate mixing in compounding extrusion makes twin-screw extruders all
    but mandatory.

  • If intimate contact or adhesion is required between the new layer and existing wire, pressure tooling is used.

  • Positive pressure can be applied to the internal cavities through the pin, or negative pressure can be applied to the outside diameter using a vacuum sizer to ensure correct
    final dimensions.

  • Compared to steel, plastic conducts its heat away 2,000 times more slowly.

  • Plastic extruder cut in half to show the components At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants
    in the melt.

  • This breaker plate and screen pack combination also eliminates the “rotational memory” of the molten plastic and creates instead, “longitudinal memory”.

  • If the film is drawn more than it is blown (the final tube diameter is close to the extruded diameter) the polymer molecules will be highly aligned with the draw direction,
    making a film that is strong in that direction, but weak in the transverse direction.

  • The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath.

  • Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile.

  • The purpose of these dies is to reorient and guide the flow of polymer melt from a single round output from the extruder to a thin, flat planar flow.

  • The drawing and blowing cause the film to be thinner than the extruded tube, and also preferentially aligns the polymer molecular chains in the direction that sees the most
    plastic strain.

  • The material is gradually melted by the mechanical energy generated by turning screws and by heaters arranged along the barrel.

  • For films and very thin sheeting, air cooling can be effective as an initial cooling stage, as in blown film extrusion.

  • Compound extrusion allows a blended material to be extruded, but coextrusion retains the separate materials as different layers in the extruded product, allowing appropriate
    placement of materials with differing properties such as oxygen permeability, strength, stiffness, and wear resistance.

  • In pressure tooling, the melt contacts the inner wire long before it reaches the die lips; this is done at a high pressure to ensure good adhesion of the melt.

  • The machine sizes range from as small as 12 mm to as large as 380mm [12- Polymer Mixing by James White, pages 129-140] Advantages A great advantage of extrusion is that profiles
    such as pipes can be made to any length.


Works Cited

[‘TEPPFA, The European Plastic Pipes and Fittings Association. “Production Processes”.
2. ^ Tadmor and Gogos (2006). ‘’Principles of Polymer Processing’’. John Wiley and Sons. ISBN 978-0-471-38770-1
3. ^ Rauwendaal, Chris (2001), Polymer Extrusion,
4th ed, Hanser, ISBN 3-446-21774-6.
4. ^ Todd, Allen & Alting 1994, pp. 223–227.
5. ^ “HOW TO SOLVE BLOWN FILM PROBLEMS” (PDF). Lyondell Chemical Company. Retrieved 31 August 2012.
6. ^ John Vogler (1984). Small Scale Recycling of Plastics.
Intermediate Technology Publication. pp. 6–7.
7. ^ Process, Methods and Features of plastic extrusion technology, archived from the original on 2013-02-02, retrieved 2012-08-01
8. ^ Rosato, Marlene G. (2000), Concise encyclopedia of plastics,
Springer, p. 245, ISBN 978-0-7923-8496-0.
9. ^ Giles, Harold F.; Wagner, John R.; Mount, Eldridge M. (2005), Extrusion: the definitive processing guide and handbook, William Andrew, p. 151, ISBN 978-0-8155-1473-2.
10. ^ Shah, A and Gupta, M (2004).
“Comparison of the flow in co-rotating and counter-rotating twin-screw extruders”. ANTEC,
11. ^ TEPPFA, The European Plastic Pipes and Fittings Association. “Production Processes”.
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