A clear future for plastic bonding

Transparent and lightly coloured plastics can be welded together by laser, with invisible joins, if a near infrared absorbing ink like material is used at the interface. The process was invented some ten years ago at TWI and was subsequently licensed to a US defence firm. However, it is now being used by numerous other companies that make a variety of components. Inventor Ian Jones says: "The infrared absorber is either coated on the plastic used for one side of the joint or built into one of the plastics where it provides a means for localising the heat. The infrared absorbing layer only has to be 100nm thick to absorb 94% of the incident thermal energy." However, the process and joining ink has since been sublicensed to various other companies, including Leeds based Barkston Plastics which is the sole licensed distributor in the UK. Barkston's marketing manager, Jo Dell, says: "Traditional joining methods work well for some clients, but not all. For example, adhesive bonding can degrade, requires long curing times and creates aesthetic problems. We also found that design improvements could be made by moving away from hot plate welding and heat treatments." Laser welding allows precise control of manufacturing parameters, provides stronger hermetic seals compared to other joining methods, reduces the risk of mechanical and thermal stress, and avoids vibration. The process allows parts to be preassembled, enables high weld speeds and the making of three dimensional contour joints. It is suitable for mass production, small batch work and product development. The technique also has the advantage of avoiding possible vibration damage caused by ultrasonic welding and the build up of welding beads: a problem associated with hot gas welding. And, Dell claims, the clear weld produces superior weld strengths and speed. "We also found that design improvements could be made by moving away from hot plate welding and heat treatments," says Dell. "Using laser welding, we are able to produce spot and seam welds in the micron range, even in hard to reach places. Sensitive components previously impossible to weld with other processes can be joined, enabling new product designs that were impossible or not economic beforehand. One example that we have manufactured is a new, improved oil reservoir as well as sensors, filters, tube connectors, micro-fluidic devices, oil tanks, infant incubators, isolators, bottles and containers, industrial batteries and micro fluidic 'lab chips'." "Sensitive components previously impossible to weld with other processes can be now joined," she says. "We have used the process to join together dissimilar materials, both clear and coloured, including almost all thermoplastic materials." Other applications that Ian Jones mentions include electronic packaging, where components have to be hermetically sealed, and textiles that need to be stitched together by pulsing a laser on and off. As well as waterproof clothing, the process allows the manufacture of automotive air bags from component parts, instead of their having to be made from more expensive, single pieces of woven fabric. It is also now, apparently being used to make Italian designer furniture and ink containers for computer printers. Jones says: "We are still doing research work, mainly for specific applications, and developing new welding procedures using the process."