Plastics are finding surprisingly new and diversified applications replacing metals and ceramics. From ordinary toys and utensils to the complicated precision heart valves, the plastics have proved not only to make life more comfortable but also to extend it.

Plastics have a combination of desirable properties. They have high strength to weight ratio, corrosion resistance against most of the corrosive media, low cost and ability to take good finish.

Plastic structures can be fabricated by welding. Thermo-plastics are the only weldable plastics as they maintain their molecular structure even after repeated heating. Among the common thermo-plastics are: acrylics, fluorocarbons, shellac, asphalt, nylon, polyethylenes, polyvinyles and protein substances. Among the above the rigid polyvinyle chloride has sufficient resistance against corrosion, strong acids alkalies and organic solvents. It is, therefore, the most common thermoplastic in use these days. The term ‘welding of plastics’ is still rarely known amongst the engineers because of the fact that the use of plastics is still not very common in many industries and the plastics which are used can normally be joined by organic solvents like carbon tetrachloride and adhesives like areldite. PVC, however, is almost insoluble in most of the organic solvents. Though, it is slightly soluble in carbon tetrachloride but, the action is very slow. There are certain other limitations too, in the way of joining plastics by the methods other than welding. With the help of welding adequate strength at the joint is achieved in minimum time. Within a few minutes after welding, any welded joint can be handled with reasonable care, facilitating rapid and economic fabrication of plastic structures.

13.2 WELDING ACTION

Unlike metals, the welding action in plastics takes place due to the adhesive bonding at high temperatures, between the parent material and the filler rod. There is no mixing or puddling action as is common in the metallic weld pools. The melt of plastic is quite viscous and has poor flow properties, while good flow properties are essential for obtaining homogeneous welds. The surfaces of the parent material and the filler rod are heated and brought near to the melting temperature and by the application of pressure the filler rod gets adhered to the ad­joining (weld bead) surfaces to be joined. Thus a homogeneous weld bead is not obtained but the filler rod gets adhered to the material in its neighbourhood and thus, gives a defect free non porous joint.

13.3 EQUIPMENT

The tool used for hot gas welding resembles in appearance with the ordinary welding torch (Fig. 13.1). Direct flame chars the material (PVC) and, therefore, hot gas is used for welding purposes. The torch consists of a main body which contains a heating element. At one end of the body there is an inlet hose connector for the gas and a handle for gripping the torch while the other end has a nozzle through which the hot gas is available for use. The welding gas (usually air) enters the torch at some pressure and gets heated while passing over heating element and comes out of the exit nozzle at a desired temperature. The gas temperature is

controlled by providing in the heating element circuit, a thermostat valve which controls the ‘on’ and ‘off period of the current fed to the element, thus regulating the temperature of the gas to a desired value depending upon the parent plate thickness. The torch may also be heated by using a fuel gas. A sectioned view of the torch used is shown in Fig. 13.1.