Extrusion welding is used in the manufacture of thick-section fabrications, such as tanks and pipes, where it is necessary to produce large volume, homogeneous seams in a single pass, unlike hot gas welding where it would be necessary to produce a seam using multiple runs.
In tank fabrications, the main use of extrusion welding is to weld the bases and tops, although in certain applications, the body panels are also welded using this technique. In large section pipe fabrications, it can be used where manual welding techniques are required. Another use for extrusion welding is in environmental applications where it is used in the joining of lining material for the construction of landfill waste sites, lagoons and roof coverings. Itis also becoming increasingly used in agriculture and water engineering, for example, in the fabrication of land drainage systems, sump tanks and manways.
Extrusion welders are available in a variety of sizes, from compact units with an integral air supply, weighing a mere 2.8 kg, through to large 13kg machines used for welding lining material in landfill applications. They can be supplied in both 110v and 240v configurations.
Extrusion welding has historically been used to weld mainly PP and HDPE, although some modern types of extruders have also been engineered to weld PVC and PVDF. In the case of PVC, due to its narrow processing 'window', the extruder needs to be purged using PP or PE weld rod, to ensure that no PVC material is left to degrade in the barrel.
Extrusion welders are designed to ensure that certain parameters are accurately defined to maximise weld quality, these being:
- Temperature of the welding material - extrudate
- Mass flow rate of the welding material
- Temperature of the hot gas for substrate pre-heat
- Quantity of hot gas
The welding speed that can be achieved is dependent on the flow rate of the extrudate, the material thickness, the cross sectional area of the seam and the size and design of the PTFE welding shoe.
As for hot gas welding, good quality extrusion welded seams can only be achieved if the parts to be welded are prepared correctly. They must be scraped to remove any contamination and the oxide layer on the surface of the material, and then fully tacked together to ensure perfect alignment for the application of the initial root run of 3mm hot gas weld. The root run ensures full weld penetration which maximises weld strength and also ensures that the parts remain together during the extrusion process. Due to the large quantity of air used for the pre-heat (typically 300 ltrs/min to comply with the DVS guidelines) a standard hot gas tack weld would break due to expansion.
The angle of the extruder in relation to the work-piece is also extremely important, to ensure an even pre-heat of the substrate and an even flow of the extrudate (Fig.2a and 2b). If incorrect parameters are used, ie too cold, the surface of the weld will be very rough and irregular in appearance and the resulting weld will have low strength. If the weld is too hot, the surface will have a wet look and the weld will again have low strength.
The modern extrusion welder has the facility to control the melt temperature and the pre-heat air separately, with a display on the control box for easy operation and monitoring. The electronic control of the melting chamber does not allow the drive motor to operate until the material is at the correct temperature. This prevents strain being put on the motor and the screw drive.
The drive motor (1) is an electric drill with improved gearing, which drives a screw shaft in a heated barrel. This also feeds the welding rod (2) into the rod input point (3) via a pair of pinch rollers, then into the extruder (4). Modern extruders have a special welding rod feed, which prevents the welding rod from becoming twisted, and ensures constant rod input. This improves the homogeneous quality of the welding seam, because variations in input due to twists and kinks in the weld rod, can lead to deviations in output. The extruder screw grinds the welding rod into granules. The resulting granulate is then fed into the melting chamber (5) where it is melted. The mass then continues through the barrel to the pre-formed, interchangeable, PTFE shoe (6), where it is formed to the shape of the seam required.
Certain machines also have the facility to be fed with granulate directly, rather than welding rod. These extruders are mainly used in landfill applications where there is the potential for long seams and inclement weather conditions. The hoppers can be covered to reduce the possibility of moisture and contamination that can appear on weld rod.
The correct design of welding shoe required for the type of seam to be welded, is placed on the end of the extruder. The base material is heated (plasticised) by the heating nozzle (7) with air supplied (on this particular design) via an integral air heater unit (8).
Extrusion welding is a manual welding process and as such, is dependent on operator skill. Therefore to achieve a high quality seam, it is recommended that a quality training programme and certification is undertaken. As in hot gas welding, the European standard (BS EN 13067) sets out the requirements for plastics welder approval in extrusion welding. The CSWIP PW-6-96 certification scheme is available for the certification of plastics welding personnel, to undertake both a theory test and a practical examination in extrusion welding of PP and HDPE. Certification is valid for two years with a further two years prolongation, as long as the welder has continued to use the process on a regular basis for that period of time. After the four-year period, the welder will need to undergo a full retest.
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