For the welding of PVC sheets, hot-air technique is commonly used. Air is easily available and gives good results with PVC. Air flow needed for the process can be obtained by using a small air compressor, with automatic tripping device to obtain constant pressure. Supply air pressure can be measured by a mercury manometer shown in Fig. 13.3.

Rigid PVC sheets in common use are of 3 mm thickness and can be welded by using 3 mm filler rods. Air temperature was controlled by using a simmer-stat that controls the amount of current in the heating coil (Fig. 13.3). Edge preparation for different plate thick­nesses is given in Table 13.1.

Welding traverse speed. It depends upon air, temperature, nozzle distance from plate and filler rod. It is manipulated by the experienced welder to obtain quality welds. Rod is fed to the plate at an angle of 90°. A fixture can be made if required to guide the filler rod at 90° and keep the torch nozzle at an angle of 45° with the joint line (Fig. 13.3). Milling machine table could be used to obtain uniform traverse speed. A large number of traverse speeds are possible with this arrangement.

Gap Distance. There is a slight variation of temperature with change of gap distance. This could be noticed from Fig. 13.4. Thus a slight variation of gap distance between the torch nozzle and plate due to hand welding will not appreciably affect the weld quality.

Welded joints. Two types of welded joints in general use are:

(а) Butt joints.

(б) Double strap fillet joints (see Fig. 13.6).

To obtain a butt joint, the plates to be joined are bevelled (60° V groove angle), cleaned, assembled over a backing plate and clamped to the machine table-vice. The compressor is started, torch is switched on, air pressure is regulated to about 100 mm of mercury. When a constant temperature of the system is achieved, a tack weld is made at the starting end by simultaneously heating the base plate and filler rod. As the mating surfaces fuse, it will be possible to slightly rotate the filler wire in-place, slight pressure is applied to the filler rod to affect proper adhesion. The table is then moved away from the torch. The pressure on the rod is maintained with slight rotary motion on the filler wire as shown in Fig. 13.2. This manoeuvre is a matter of practice on the part of the welder. After completion of one pass, the table is stopped and filler rod is cut. The process is repeat for subsequent passes as needed to fill the joint groove completely. Satisfactory welds have been obtained at a traverse speed of 50 mm/min.

Similar procedure is adopted for obtaining double strap fillet joints except that the assembly of the piece to be welded is tilted through an angle of 45° to facilitate the heating of fillet properly. The fillet in this position served as a 90° V-groove angle and heat is equally distributed to the plates to be joined.