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In extrusion welding of thermoplastics, how do you ensure good visual appearance of the weld?


Frequently Asked Questions

There are four main factors which ensure good visual appearance of the weld: temperature, angle of extruder to the workpiece, speed, and shoe design.

Pre-heat and extrudate temperature have to be set correctly in accordance with the manufacturer's recommendations. The pre-heat has to be sufficient to soften the surface of the material for fusion to take place. If it is too cold, the weld will appear to 'sit' on the surface; if it is too hot, the shoe of the extruder will bite into the surface. This will create an excessive amount of overspill and a build-up of material on the leading edge of the shoe, possibly blocking the pre heat, creating a cold weld part way through the run. The built up material is also the softened surface, removal of which will create a cold weld. The material will also take on a wet look and will start to degrade, creating a weak weld.

Weld surface finish is dependent on the temperature of both the extrudate and the PTFE shoe. With this in mind, the operator should make sure that the welder is put back on its stand, with the shield in place, between each run to ensure that the airflow is directed back onto the PTFE shoe, maintaining it at weld temperature.

When making a fillet weld, the extrusion welder has to be positioned at an angle of 45° to the workpiece. This will ensure an even pre-heat on both surfaces. Maintaining correct shoe to workpiece angle is also crucial, ensuring that the extruder moves in line with the direction of the weld run.

Welder travel speed should be consistent with production of the extrudate. If it is too slow, there will be a build up of material and excessive overspill; if it is too fast, the ribbon of weld will thin out and take on a round appearance, instead of having a smooth surface finish.

Shoe design is a very important factor in the eventual surface finish of the weld. The Standard DVS 2207-4: 1993 Schweissen von Thermoplastischen Kunststoffen - Exstrusionsschweissen, Tafeln und Rohre, Schweissparameter fur PE - HD und PP covers design of the PTFE shoe and correct methods associated with extrusion welding.  TWI offers training and certification in extrusion welding.

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