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Laser makes invisible welds in plastics

Connect, no.100, May/June 1999, p.8

TWI's Advanced Materials and Processes department has recently developed a technique for laser welding plastics, creating a joint almost invisible to the human eye.

Until now a black absorber, most commonly carbon black, or some other visible absorber acted as the medium that heats in the laser beam to create the weld. However, this new approach enables two similar clear plastics to be joined with no visible weld line.

Laser welded sample (top) using carbon black absorber beside a sample using the new technique with low visibility absorber
Laser welded sample (top) using carbon black absorber beside a sample using the new technique with low visibility absorber

Unlike many other methods, laser welding is carried out without vibration and with the minimum of heat input and is therefore unlikely to damage or distort a component. This method, transmission laser welding, can be used for welds in film, sheet and moulded plastics. Diode or Nd:YAG lasers are normally used in either a 'single-shot' or continuous seam welding process. Both methods are very rapid and produce little or no flash.

The novel adaptation to this technique works by using an almost colourless medium which absorbs the infrared laser light very efficiently, converting the absorbed light energy to heat without significantly absorbing visible light. The medium is applied to one of the components to be welded, either to the surface by painting or printing, or into the bulk of the plastic. It can also be applied in a thin film inserted at the joint. The infrared absorbing media are proprietary, complex molecules, not typically available off-the-shelf.

The use of an infrared absorbing medium offers the opportunity of producing a low visibility weld between two materials that need to be clear or have a pre-defined colour. Potential applications include finished products where appearance is important. Packaging, fabrics, visual displays and optical products, and automotive components are being considered.

LP Frieder III of GENTEX Corp., a leading supplier of this type of absorber, says 'These dyes are used in various applications which require absorbing IR light while maintaining visible transmission, a typical example being laser protective eyewear. Using them as a means of generating heat for welding is entirely new. This is a very exciting development in the use of these products and gives a new dimension to the light management capabilities of Filtron�dyes.'

The work has been carried out under TWI's Core Research Programme and TWI has applied for a patent for the use of these absorbers in laser welding of plastics.

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