ness of 27 mm (1.06 in.) were chosen so that the pipe could withstand bending stresses during deployment in the sea and due to sea currents. Thermoplastic Welding Processes Plastics can be joined by gluing using certain solvents or adhesives. They are also joined using mechanical means like bolting and screwing. However, strong and leaktight joints can be obtained only by welding. There are a number of processes for welding plastics (Refs. 6–8); for most of them, both heat and pressure are applied to make a sound joint. Among these, the most appropriate and widely used process for HDPE is the hot plate/ heater mirror welding process (Ref. 9). This process consists of three main steps: heating, pressing, and cooling. 52 WELDING JOURNAL / NOVEMBER 2016 Fig. 2 — Various possible weld joint defects that may occur during HDPE fusion buttjoint welding. Table 1 — Results of the Parameter Optimization Process Parameter Operation Step Optimized Value Observation Made on the Quality of Weld when the Value Is More than Optimum Value Less than Optimum Value Formation 300 Excessive molten ring is Molten rings formed may not of bead (t1) formed and may result in fuse and there may be excess of bead height. incomplete fusion. Continual 420 The melting takes place to a The melting takes place to a heating/ larger depth, which may not smaller depth, resulting in soaking become a part of the fusion small bead height. period (t2) zone but can deform while welding. Time Changeover 7 Joining edges may cool and No effect on the quality of the duration (s) process (t3) will not fuse properly. weld. Pressure 12 Joining edges may cool and Not effect on the quality of building step (t4) will not fuse properly. the weld. Cooling 1440 No effect on the quality of Can cause distortion or period (t5) weld but can be wastage of misalignment or crack in the time. weld joint. Hydraulic Formation 30 Unevenly formed molten ring Molten ring formation may pressure in of bead (P1) around the pipe. not take place or may take a the machine long time. (kg/cm2) Continual 4 May result in excess bead Soaking may not be proper heating (P2) heigh. hence melting will be restricted to lesser depth, which may result in lesser bead height. (add drag Pressure 30 May result in narrow and Voids can be formed and also pressure of building step excess of bead height. may result in lesser bead the (P1) height. equipment) Temperature Throughout 210 ± 5 Results in a large and distorted incomplete fusion or cracks. (°C) the welding profile of the weld bead. process
Welding Journal | November 2016
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