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Hot plate welding examinedA look at an alternative and simple way of joining together irregular and fragile thermoplastics parts

The PRW monthly Design Guides provide prac-tical guidance for designers, toolmakers and moulders. Every month a different aspect of de-sign technology is tackled and together these guides are becoming an indispensable reference point for those designing successful products. Having started with relatively basic design guid-ance, 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The parts are brought into contact with the hot plate until the joint surfaces melt, then the hot plate is withdrawn and the melted surfaces are pressed together to cre-ate a weld that is held under pressure until it is sufficiently cooled to remain stable.

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

Typical joint designs The welding sequence

Material Hot plate temperature (˚C ) Heating time (seconds)High density polyethylene 190-220 30-80Polypropylene 190-240 30-120PVC unplasticised 230-250 40-60PVC plasticised 130-200 20-60Acetal 210-230 10-40Guideline process conditions for hot plate weldingSource: 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 weld-ing polyolefins, PVC and acetals. The table gives guideline times and temperatures for these materials, but the precise condi-tions will depend on grade, particularly on

part thickness, and should always be estab-lished by experiment.

Materials with a tendency to oxidise rap-idly 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 assem-bly, these can be concealed by providing

recessed flash wells.The recess can take any form that is con-

venient 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 asymmet-rical 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 great-est disadvantage compared to ultrasonic or vibration welding is the long cycle time re-quired for heating and cooling.

Some of these disadvantages can be over-come by process variants such as non-con-tact hot plates that heat by radiation, double heating plates for joining dissimilar materi-als, and contoured hot plates for non-planar components, but the basic mono-plate proc-ess is by far the most common.Clive Maier, Econology