Many polymers need to be joined allowing complex shapes to be processed, and welding is one process that may be employed. Transmission laser welding of thermoplastic polymers is a well established process, and is often used. To ensure that the weld occurs between the layers of plastic it is necessary for one of the plastics to be transmissive to laser light and the other to absorb the laser energy, and convert it to instant heat sufficiently high enough to melt the polymers on each internal face. The traditional method has been to load the lower component with carbon black but nowadays new laser absorbing compounds have been developed that allow transparent or coloured parts to be transmission laser welded. Transmission laser welding is capable of welding thick parts because the heat affected zone is confined to the joint region so no marking of the outer surfaces occurs. This paper describes the transmission laser welding process and its traditional limitations, and details laser absorbing compounds that have been developed allowing clear and coloured materials to be welded.

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