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Manufacturing Processes > Joining processes > Plastic(Thermoplastic)Weldings

 

Thermoplastic welding

 

Process description

Joint edges are heated using hot gas from a hand held torch causing the thermoplastic material to soften. A consumable thermoplastic filler rod of the same composition as the base material is used to fill the joint and create the bond with additional pressure from the filler rod at the joint area.

Materials

Only thermoplastic materials.

Thermo Plastic

Process variations

  • Hot gas can be either nitrogen or air, depending on thermoplastic to be joined. Nitrogen minimizes oxidation of some thermoplastic materials.
  • Various nozzle types for normal welding, speed welding and tacking.
  • Other thermoplastic welding techniques available:
  • Spin welding: similar to Friction Welding (FRW), where the two parts to be joined, one stationary and one rotating at speed, have their joint surfaces brought into contact. Axial pressure and frictional heat at the interface create a solid state weld on discontinuation of rotation and on cooling .
  • Ultrasonic Welding (USW): hardened probe introduces a small static pressure and oscillating vibrations at the joint face disrupting surface oxides and raising the temperature through friction and pressure to create a bond. Can also perform spot welding using similar equipment.
  • Hot plate welding: electrically heated platens are used to soften base material at the joint and a bond is created with additional pressure giving good joint strength.

Economic considerations

  • Production rates very low.
  • Weld rates typically less than 1.5 m/min.
  • Lead time typically hours.
  • Manual operation typically using transportable equipment.
  • Automation possible using a trolley system traversing over joint.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low.
  • Equipment costs generally low.
  • Direct labor costs moderate to high. Some skill needed by operator.
  • Finishing costs low. Scraping the joint flush may be required for aesthetic reasons.
  • Other thermoplastic welding techniques have a moderate to high production rate, are applicable to large volumes, have a moderate to high equipment cost and are more readily automated.

Typical applications

  • Joining plastic pipes
  • Ducts
  • Containers
  • Repair work

Design aspects

  • Moderate levels of complexity possible.
  • Typical joint designs possible using hot gas welding: butt, lap and fillet, in thin sheet.
  • Horizontal welding position only.
  • Parts to be joined must be in contact.
  • Minimum overlap for lap joints =13 mm.
  • Minimum sheet thickness =2 mm.
  • Maximum sheet thickness =8 mm.
  • Multiple weld runs required on sheet thicknesses ≥5 mm .

Quality issues

  • Filler rods must be same thermoplastic as base material.
  • The force from the filler rod is applied to encourage mixing of softened material and must be consistent through the operation.
  • Joints are weakened by incomplete softening, oxidation and thermal degradation of plastic material.
  • Process variables are hot gas temperature, pressure (either from filler rod or fixtures) and speed of welding.
  • Hot gas needs excess moisture and contaminants removed using filters.
  • Weld strength is between 50 and 100 per cent of base material.
  • Recast plastic filler at the joint can be made flush with base material using a scraper.
  • Tack welding of parts to be joined should be performed before welding commences.
  • Use of additional fixtures is advised for large parts, also to provide additional pressure to aid joint formation.
  • Surface finish of weld is fair to good.
  • Fabrication tolerances are typically ±0.5 mm.

 

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