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Optimizing hot plate welding of thermoplastics.

Optimizing Hot Plate Welding of Thermoplastics

Heated tool welding, commonly called hot plate welding, is perhaps the most basic welding technique for joining plastics. It is widely used in mass production and for large structures such as pipelines. Equipment for shop-floor and for onsite use is available that is capable of welding components from a few centimeters to 1.5 m in diameter. Hot plate welding is a relatively slow process; cycle times range from about 15 sec for small components to many minutes for thick-walled assemblies. However, its simplicity and ability to weld almost all thermoplastics make it attractive where time is not a restriction. The process is widely used in applications where high-integrity joints are required, such as fluid containment in automotive header tanks and hydraulic reservoirs.

Although the hot plate welding technique is widely used, there is little understanding of the effect of operating parameters on welded joint integrity and process optimization. This article considers the application of the hot welding process to the joining of three thermoplastics: polypropylene (PP), high-impact polystyrene (HIPS), and polyphenylene oxide (PPO). PP was chosen as it is widely used in the automotive and domestic appliance industries, HIPS for its use in electronic enclosures and microcomputer housing, and PPO as it is a high-temperature engineering resin.


Trials were conducted on a small-part welding machine with a resistance-heated, 150 X 100-mm aluminum bronze hot plate covered in a 0.25-mm-thick polytetrafluoroethylene (PTFE)-impregnated woven glass cloth. Instrumentation was provided to continously monitor hot plate temperature, displacement, and forces.

Welds were made between injection molded circular cups, 65 mm in diameter and of 3 m m wall thickness. For tensile testing, a circular metal disk with a threaded hole was placed inside each specimen before welding. A typical sample undergoing testing is shown in Fig. 1. Weld beads were left on in the test and a cross-head movement of 25 mm/min was used.

The parameters investigated were hot plate temperature, heating pressure, heating time, displacement stop position in the heating phase, consolidation pressure, and consolidation time. The effect of individual parameters was assessed by varying only the parameter under study. Welding conditions studied were: Hot plate 190-270[deg.]C (HIPS) temperature 185-270[deg.]C (PP) 230-280[deg.]C (PPO) Heating pressure 0.3- 1.2N/mm.sup.2 Heating time 5-40 sec Consolidation time 2-30 sec Consolidation 0.3-1.2 N/mm.sup.2 pressure Displacement stop 0.2-1.0 mm position

Tensile strengths from suppliers' data were 34, 17, and 50 N/mm.sup.2 for PP, HIPS, and PPO, respectively.


Figure 2 shows that the hot plate temperature range in which satisfactory tensile strengths were achieved was extensive for both PP and HIPS, but not for PPO. It should be noted that the maximum temperature was limited to 280[deg.]C by fume emission from the PTFE sleeve on the hot plate. The amount of material displaced increased with hot plate temperature for all three materials and was greater for HIPS and lower for PPO, compared with PP. Constant welding conditions were: heating pressure--0.5 N/mm.sup.2 (PP and HIPS), 0.34 N/mm.sup.2 (PPO); heating time--15 sec (PP), 20 sec (PPO and HIPS); displacement stops--0.4 mm; consolidation pressure--0.50 N/mm.sup.2; and consolidation time--20 sec.

Figure 3 shows that when material displacement was limited to 0.4 mm in the heating stage, increasing the heating pressure up to 1.0 N/mm.sup.2 gave a small increase in the tensile strength for PP but slight decreases for both HIPS and PPO. With displacement stops omitted, the strengths achieved were lower. Other constant conditions were: hot plate temperature--203[deg.]C (HIPS), 222[deg.]C (PP), 268[deg.]C (PPO); heating time--20 sec (HIPS and PPO), 8 sec (PP); consolidation pressure--0.5 N/mm.sup.2; and consolidation time--20 sec.

For PP the optimum heating time was 15 sec (Fig. 4). Some degradation of material occurred at longer times. For HIPS, a tensile strength of 16 N/mm.sup.2 was achieved at heating times of 20 sec and greater, whereas for PPO, tensile strength was still increasing at the maximum heating time investigated. Conditions were: hot plate temperature--205[deg.]C (HIPS), 221[deg.]C (PP), 272[deg.]C (PPO); heating pressure--0.50 N/mm.sup.2; displacement stops--0.4 mm; consolidation pressure--0.5 N/mm.sup.2; and consolidation time--20 sec.

In Fig. 5, the decline in tensile strength at higher displacement limit positions recorded for PP was caused by a reduction in available heat soak time. At stops positioned 0.2 mm and 0.4 mm in the heating stage, 2.7 sec and 0.8 sec of heat soak time were given, respectively. For stops positioned at greater than 0.4 mm, no heat soak material was available; thus operating conditions remained constant. For HIPS and PPO, the rise in tensile strength with greater displacement limit positions was in line with increasing material displacement. Conditions were: hot plate temperature--205[deg.]C (HIPS), 222[deg.]C (PP), 268[deg.]C (PPO); heating pressure--0.27 N/mm.sup.2 (HIPS), 0.33 N/mm.sup.2 (PPO and PP); heating time--8 sec (PP), 20 sec (HIPS and PPO); consolidation pressure--0.5 N/mm.sup.2; and consolidation time--20 sec.

Applied pressure had little effect on material displacement with the semi-crystalline PP, but had a greater influence on the amorphous HIPS and PPO. Figure 6 shows that the tensile strength of HIPS remained constant over the consolidated pressure range investigated. The highest tensile strength for PP was achieved at 0.53 N/mm.sup.2. The tensile strength of PPO decreased with increasing consolidation pressure. Welding conditions were the same as in the displacement limit trials except that heating pressure was 0.34 N/mm.sup.2 for PPO and PP and displacement stops were at 0.4 mm.

The tensile strength values remained nominally constant (within the range for the individual materials), for all weld consolidation times greater than 5 sec.


Tensile strengths approaching that of the parent material can be achieved with displacements of about 0.2 mm. An important influence in obtaining high joint strengths was the heat soak time. Ideally, the stops should be as close to the joint interface as possible, consistent with bringing the joint interface into intimate contact with the heating element. When the displacement stops were reached, the heating time controlled the amount of heat soak. The optimum range found for heating time was 10-20 sec. Without the application of displacement limit stops, the heat soaked material was squeezed out during the heating stage, thus resulting in joints of low strength.

The hot plate temperature was found to be a less critical parameter than either heating pressure or time. Although material displacement increased substantially with increasing pressure, the joint strength remained around 30 N/mm.sup.2 for platen temperatures between 215[deg.]C and 260[deg.]C.

Two process parameters were involved in the weld consolidation stage--consolidation pressure and time. The pressure affected the material displacement; with the major portion occurring within 2 sec of application. The consolidation time showed no discernible trends on either material displacement or tensile strength beyond 5 sec.



Welds of the same strength as the parent material were readily achieved. However, the amount of material displaced was large. An essential requirement for the control of material displacement was the use of displacement limit stops. The position of the stops was important, since improved joint strength was achieved for stops set at 1.0 mm, the maximum position on the equipment used. The weld consolidation period could be limited to <5 sec as an aid to process efficiency, as both pressure and time had little effect on tensile strength.

It is apparent that sufficient heat soaked material, behind the melt fronts, must be available to provide >2.5-mm material displacement in the consolidation stage. At a hot plate temperature of 205[deg.]C, a minimum of 20-sec heating time was required to provide the required heat soaked material. However, using a hot plate temperature of up to 255[deg.]C, a reduced heating time of <15 sec would fulfill the >2.5-mm displacement requirement and create joints with high tensile strengths.

Polyphenylene Oxide

Heat soak time was found to be the critical factor in providing weld strengths approaching that of the parent compound. As contact is made with the stops in 4 to 8 sec, the remaining heating time allows increased material softening without further displacement. AT 268[deg.]C, 30 to 40-sec heating time was necessary to acquire sufficient heat soaked material. Increasing the hot plate temperature would allow a reduction in heating time. However, an alternative to the PTFE sleeve would be required to prevent components from sticking to the hot plate.

Although the other heating parameters affected the displacement, their influence on tensile strength was minimal. However, in the weld consolidation period, the effect of pressure was substantial. A decrease in tensile strength of more than 10 N/mm.sup.2 occurred over the pressure range 0.33-1.17 N/mm.sup.2. This was presumably because a greater proportion of the material was squeezed from the joint, thereby reducing the amount of material available at the required temperature. For optimum properties, a low pressure and short time, 0.35 N/mm.sup.2 and 5 sec, would be advantageous in the consolidation stage.


The hot plate process has demonstrated its suitability for the three thermoplastics investigated, proving to be reliable and providing high strength joints. HIPS was the most tolerant of procedural variations, giving high fracture strengths over a wide procedural band, followed by PP, with PPO having the least tolerance.

The importance of heating time and position of displacement limit stops has been shown. The most important parameter was heating time, particularly for PP and PPO. It should be of sufficient duration that an adequate amount of heat soaked material can form. This was aided by the use of stops to limit displacement in the heating phase. The amount of material required to be displaced varied for each polymer. PPO produced the least displacement, followed closely by PP, with HIPS giving over 1.0-mm greater weld flash.

Within the ranges studied, control of consolidation phase parameters was not critical. Short times, around 5 sec, at low consolidation pressures were satisfactory--a finding that would be beneficial in reducing weld cycle times.
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Author:Watson, M.N.; Murch, M.G.
Publication:Plastics Engineering
Date:Jun 1, 1989
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