TWI Frequently asked questions
By Andy Knight
The temperature of the torch is a very important factor for plastics welding. There is a narrow 'window' to achieve optimum weld strength. The temperature should be sufficient to soften both the parent material and the welding rod to allow fusion to take place. The required temperature will vary from plastic to plastic. If the temperature is too low, a 'cold' weld will be achieved that will fracture easily as a result of poor fusion. If the temperature is toohigh, then degradation of the thermoplastic will occur, producing a weak weld. Therefore recommended temperatures should be used at all times.
There are some makes of welding torch on the market which have a digital readout on the handle. The indicated temperature is measured within the torch, and is not necessarily the same as the hot gas stream. Therefore the welding torch should always be calibrated, using a digital thermometer and probe, with the tip typically 5mm from the end of the nozzle, within the gas stream. This check should be made with all torches at regular intervals (and at least once a day). This procedure can be added to a company's QA system.
To establish the correct torch temperature, test welds should be made on material off-cuts. A competent welder will use visual inspection followed by a manual bend test in a vice, to assess the weld quality, and hence confirm that the torch temperature is set correctly.
