Among weldable thermoplastics, polyvinyl chloride and polyethylene are the most frequently joined. Thermoplastics or plastics that soften when heated can be

welded using a filler rod of the same composition as the base material. Applications of welded plastics have been used in the chemical and food industries, in laundries, breweries, in home and industrial plumbing, and other industries.

The process requires a torch using either electricity or an oxyfuel gas flame to heat air or an inert gas, which will then be used to heat the filler rod and the base material. Temperature of the compressed gas stream may be regulated by gas flow rate and by varying the torch-to-work spacing. Although the filler rod should preferably be of the same chemistry as the base material, a rod which is slightly more plasticized than

the base material will give greater control over weld contour. However, welds made with plasticized filler rod show a slight reduction in resistance to chemicals.

Welds in plastic resemble types produced by electric arc welding. For a fillet weld, no joint preparation is necessary. For a butt weld, the two pieces are beveled by sawing, filing, or grinding.

The prepared surfaces should be clean and roughened with a scraper to improve the weld bond. A sealing pass on the reverse side of the butt weld will ensure higher tensile strength.

One essential difference between metal welding and plastic welding lies in the joining of filler rod and parent material. Complete fusion of the two is characteristic of arc welding. With plastics, however, a simple bonding process takes place, since only the actual meeting surfaces melt. The other parts remain relatively unaffected and rigid. The slight pressure required to force the filler rod into the joint combines melted surfaces into one homogeneous mass. In this manner, a bonded, integral weld is produced.

The torch should be used to pre-heat surfaces or edges to be welded, as well as the filler rod, to produce uniform coalescence. The filler rod cannot adhere properly if surfaces have not been sufficiently preheated and melted. Torch-to-work spacing must be

controlled to avoid overheating, which causes darkening of the material, and too much sub-surface melting.

The process can be used in the downhand, vertical and overhead positions. The tensile strength of butt welds in plastic should approach 90% of the tensile strength of the base material. Figures for fillet welds are somewhat lower than this. A weld will attain full strength in two to six hours, depending on the size of the weldment and the type of weld. To determine the soundness of a weld and the thoroughness of surface bonding, the operator may attempt to pull the end of the welding rod from the welded piece after the weld has completely cooled. In a good weld, the filler rod will tear off at the end of the weld; a poorly made weld will allow the rod to be pulled out of the joint.

 

 

 

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