13.1 INTRODUCTION

Most commonly used plastics are either thermoplastics or thermosetting plastics. Thermo­plastics could be compared to wax. They are capable of remelting and changing shapes. Ther­mosetting plastics could be compared to an egg. When boiled, an egg becomes solid and sets, it can not be brought back to liquid condition and cannot be reshaped. Thermoplastics are weld­able thermosetting plastics are not weldable but can be joined by adhesive bonding processes.

A number of widely used plastics can be welded as they are thermoplastics. The most common of these are polyvinyl chloride (PVC), polyethylene, polypropylene, acrylonitrile budadiene styrene (ABS) and acrylics. Such plastics can be welded by melting the surfaces to be joined and allowing them to solidify while in contact. Plastics containing volatile compo­nents may form gas bubbles which cause the formation of defects in the welds made.

Friction welding machines can be used to produce excellent welds in circular cross­section components. The most common method of welding plastics uses hot gas as a source of heat and uses torches similar to an oxy-fuel torch.

Welding torches for plastics are designed to let a compressed gas flow through electri­cally heated coils which raise the gas temperature to between 175° and 315°C. This hot gas passes through an orifice forming a narrow gas, stream which can be directed to the surfaces to be joined. See Figs. 13.1 and 13.2.

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Joint preparation for welding Table 13.1. Manual welding force on filler rod (intermittent)

Power requirements rarely exceed 500 W for the heating element. Gas/Air flow is of the order of 280 l/min which can be supplied by 1/4 horsepower compressor motor. Some plastics (e. g. polyethylene) are easily oxidised. For such situations heated compressed nitrogen gives best results.

Fortunately there is a wide margin between the softening (melting) temperature and the burning or charring temperature for thermoplastics. It is still advisable to use a thermo­stat and maintain temperatures that give best results. As the filler material does not change shape significantly a good fused weld may appear incomplete. With little practice a welder can deposit excellent beads.

In the following paragraphs we shall discuss the practical aspects of the welding of PVC plastics.