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Infra-red alternative scores highly in plastics joining task

Connect, no. 156, November/December 2008

Schematic of equipment set-up for IR heating trials
Schematic of equipment set-up for IR heating trials

Hot plate techniques, for heating plastics components prior to welding, certainly have their place in plastics joining. But when an Industrial Member found that it created a variation in the melt depth of its components to be join edit asked TWI for assistance.

The Member Company manufactures cartridge filters where a 56mm diameter polyester end cap is heated using a hot plate to produce an accurate and consistent melt depth of sub 2mm. The filter material is then embedded into the molten plastic hopefully producing a sound joint. Regrettably the hot plate technique can produce a variation in melt depth across the end cap and cause discoloration of the polymer.

Finding another heating source was not straightforward. TWI was asked to investigate the use of infrared lamps as a potential means of melting the end caps evenly and consistently.

The objectives of the work included establishing both the melt depth, and variation in melt depth, across the end cap for different infrared heating conditions, and secondly determining suitable heating conditions to create the required sub 2mm of melt depth.

Work began using a Bielomatik hot plate welding machine retro fitted with an array of five medium wavelength IR lamps. Each lamp has a max power of 1.8kW and was variably controlled. Using a custom built rotary aluminium jig the endcaps were clamped parallel to the IR heaters and made capable of being rotated.

The consistency of infrared heating of the polyester end cap was to be measured using TWI's Agema thermal imaging camera.

Heating trials confidential to the client were undertaken varying the parameters; eg exposure time, standoff distance, heating power.

The melt depth was measured using the client's own melt depth testing equipment.

The results clearly showed that the required end cap melt depth was consistently achievable using infrared heating. The conclusions drawn included; IR is a very consistent, even and repeatable method of heating the end caps. It is also a potentially effective method of joining the end cap to the filter membrane. Finally TWI also advised that thermal imaging proved to be an extremely effective method of measuring the consistency of infrared heating.

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