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Design guide

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Hot plate welding examined A look at an alternative and simple way of joining together irregular and fragile thermoplastics parts The PRW monthly Design Guides provide practical guidance for designers, toolmakers and moulders. Every month a different aspect of design technology is tackled and together these guides are becoming an indispensable reference point for those designing successful products. Having started with relatively basic design guidance, more sophisticated and detailed issues are now being addressed as the series progresses. In this month’s Design Guide we look at another way of joining thermoplastics parts, this time by hot plate welding. The process uses a heated flat plate to melt the joint surfaces. This means that the joint must be planar and cannot have locating features, so the parts must be held and aligned in jigs. The parts are brought into contact with the hot plate until the joint surfaces melt, then the hot plate is withdrawn and the melted surfaces are pressed together to create a weld that is held under pressure until it is sufficiently cooled to remain stable.

Typical joint designs

Material Hot plate temperature (˚C ) High density polyethylene 190-220 Polypropylene 190-240 PVC unplasticised 230-250 PVC plasticised 130-200 Acetal 210-230 Guideline process conditions for hot plate welding

Heating time (seconds) 30-80 30-120 40-60 20-60 10-40

Source: New Horizons in Plastics, ed J Murphy, WEKA Publishing, 1991

The hot plate itself is usually electrically heated and is provided with a non-stick coating to prevent the melted plastics from sticking to the plate with subsequent “stringing” or degradation. The key process parameters are hot plate temperature, the duration, pressure and displacement during the heating and welding stages, and cooling time. The process is most often used for welding polyolefins, PVC and acetals. The table gives guideline times and temperatures for these materials, but the precise conditions will depend on grade, particularly on

part thickness, and should always be established by experiment. Materials with a tendency to oxidise rapidly when heated in air are not suitable for this process; polyamides in particular should not be hot plate welded. Joint designs are usually quite simple. As a guide, the joint width should be at least two and a half times the part wall thickness. During the welding process, flash is produced when beads of molten material are squeezed out on either side of the joint. To improve the appearance of the assembly, these can be concealed by providing

The welding sequence

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Designer’s notes ● Hot plate welding works best with like materials and planar joints ● You can get good results with polyolefins, PVC and acetals ● Do not use hot plate welding for polyamides

recessed flash wells. The recess can take any form that is convenient for mouldmaking and will only be needed on the outer edge in the case of closed assemblies. Assembly tolerances can be improved by providing preset stops to control the extent of squeeze-down during heating and welding. Hot plate welding is a simple and robust process, and can be used to join asymmetrical parts and parts that are too fragile for ultrasonic or vibration welding. However, there are limitations. The hot plate requires that both surfaces melt at the same temperature. In practice this usually means the process is used to join like materials only. Perhaps the greatest disadvantage compared to ultrasonic or vibration welding is the long cycle time required for heating and cooling. Some of these disadvantages can be overcome by process variants such as non-contact hot plates that heat by radiation, double heating plates for joining dissimilar materials, and contoured hot plates for non-planar components, but the basic mono-plate process is by far the most common. Clive Maier, Econology

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Material Hot plate temperature (˚C ) Heating time (seconds) High density polyethylene 190-220 30-80 Polypropylene 190-240 30-120 PVC unplast...

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