Extrusion welding

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Introduction [1][2][edit]

Extrusion welding is one of the processes used to weld thermoplastics and composites, developed in the 1960's.

The process involves a machine that heats a plastic filler material (in the form of a rod or pellets) and at the same time heats the base material surface with hot air or halogen lamps. The molten filler material is extruded thru a die (PTFE shoe) on the molten surface of the base material (with some pressure applied) and allowed to cool.

Before the welding process starts, it is necessary to preheat the joint surfaces. This done by moving the hot air nozzle back and forth above the joint surface.

The steps of this welding process are generally similar to other plastic welding processes and involves:

  • Surface preparation of base material- machining the required groove as per joint design, cleaning dirty or oxidized surfaces by scraping. Cleaning chemicals (if present from former use).
  • Fit-up of weld joint- tacking the base material components (by hot gas welding process) or using a clamping system.
  • Welding.
  • Finishing of the final weld appearance- if necessary, remove flash with a scraper in order to avoid stress notches.

The welding step consist inter-molecular diffusion that connects the welded materials.

This process can be manual or automated that is mainly used to weld polypropylene and high density polyethylene.

Joint configuration[1][edit]

Most welds are designed to be a single pass, but plates over 30 mm thickness will require multiple passes.

Joint design may vary between butt joints, T joints and overlap joints.

Normally, butt joints will be single or double V groove with 45 to 90 degrees opening and 0-2 mm root gap.

T joints will normally have a single bevel of 45-60 degrees and 0-2 mm root gap.

Applications[2][edit]

The main use of this process is where large welds are required, made with a single run. This is mainly needed in the environmental, agricultural, and water engineering fields.

The main products are large thermoplastic tanks and pipes, and products made from Polyethylene sheet, such as pontoons, storage container or boxes .

Materials[2][edit]

The common materials welded with this process are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and Polyvinylidene fluoride.

Equipment[2][1][edit]

A Hot Air gun is used to tac weld and pre-weld the material, to prevent movement during extrusion welding with a machine.

Equipment may vary between: stationary extruder with a movable welding head, manual extrusion welding machine (hand held), film welding system, welding machine with manual transfer of filler material or movable welding machine with a melt chamber.

The manual welding machine usually weights about 3-12 kg and can be used to weld in all positions.

The manual welding machine is built from: Extruder drive motor, Welding rod holder, Pinch rollers, Extruder, Die (PTFE welding shoe), Hot air unit and nozzle [3].

Welding parameters[2][edit]

The main welding parameters are:

  • Extrusion rate.
  • Temperature of extrudate.
  • Preheat air flow rate and temperature.
  • Welding speed.
  • Position of welding gun.

Advantages and disadvantages[1][2][edit]

The main advantage of this process (compared with hot gas welding) is in its speed of application, which can be about five times faster than hot gas speed welding. this is achieved by applying high deposition rates with a single pass. Another advantage of the process is a higher weld strength that may be achieved due to stricter control on welding parameters.

The main disadvantage of this process is in the size of the machine, a manual extrusion welding machine can reach to a weight of 12kg which may be difficult to use (for example in the overhead position). Another disadvantage is that trained and qualified personnel are required in order to produce good welds [4].

References[edit]

  1. ^ a b c d Plastics and Composites Welding Handbook. HANSER. 2003. pp. 91–108. ISBN 1-56990-313-1.
  2. ^ a b c d e f Troughton (2008). Handbook of plastics joining : a practical guide. Norwich, NY: William Andrew Inc. pp. 73–79. ISBN 978-0-8155-1581-4.
  3. ^ "Extrusion Welding Guns for HDPE Welding | Acu-Tech Piping Systems". Acu-Tech Piping Systems. Retrieved 2018-11-15.
  4. ^ "Extrusion Welding Training - Polyweld Tech Academy". Polyweld Tech Academy. Retrieved 2018-11-15.