TWI Knowledge Summary
Extrusion welding of plastics
Extrusion welding is an established technique for the fabrication of assemblies made from thermoplastics. The process is typically used in the manufacture of large fabrications, such as chemical storage vessels. In these applications, where walls are thick, it is more economical than hot gas welding. Extrusion welding is a manual process. Weld quality is dependent upon the skill of the person operating the welding gun. Training and Certification are generally recommended in order to maintain high standards of fabrication. Extrusion welding is generally only used for polyethylene and polypropylene. Some applications of the process have used nylon, but care must be taken to dry the welding rod to prevent voids occurring in the weld. Other applications include the fabrication and repair of liners for refuse tips and lakes.
The process involves continuously extruding molten thermoplastic material into a weld preparation on the plastic component or structure which is being joined. The equipment is based upon an electric drill with a mini extrusion barrel attached to the front. The extrusion barrel is heated along its length, either by cartridge heaters or hot air. A thermoplastic rod or granule feedstock is fed into the rear of the extrusion barrel and the material is heated as it is drawn through the barrel by the rotating extruder screw. Molten thermoplastic is continuously ejected through a PTFE shoe attached to the front of the extrusion barrel. The PTFE shoe is shaped to match the profile being welded, and defines the shape and size of the final weld. At the leading edge of the PTFE shoe, hot gas is used to heat the substrate material in front of the area where the molten bead is to be laid. This ensures that there is sufficient heat in the substrate material to form the weld. Typical welding speeds are 0.5-1.0 m/min.
To apply extrusion welding successfully, operators need a practical understanding of fabricating thermoplastic assemblies using the hot gas welding process. Typically, the welding procedure consists of assembling the sheets of material being welded and tacking them together using a hot gas welding tacking nozzle. A single run of hot gas weld (using the same thermoplastic material as the substrate) is placed at the base of the weld preparation to ensure that there is adequate root penetration. An extrusion weld is then used to fill the remainder of the joint, again using the same thermoplastic material as the substrate. Generally, no further work is required to complete the joint unless a flush finish is required, for example, in a butt weld.
Further information on extrusion welding of plastics can be found via the Weldasearch literature database.
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