Laser welding is the process of choice in many industry sectors for joining plastics due to its ability for precision of the joint location and in the amount of heat applied. It is perfectly suited to the manufacture of complex products such as microfluidic devices, where channels and structure resolution below 100 μm are used regularly.
As industry seeks to produce even smaller scale complex plastic components, there is a call for new research to allow welds in plastics of 10 μm and below. Until recently, the smallest laser welds possible measured between 10 and 20 μm in width. However, this is at an infrared laser’s resolution limit, and smaller welds are not possible using control of the laser focus spot size alone.
It was work carried out by TWI and the University of Cambridge’s Cavendish Laboratory into precise patterning of laser absorber dye on a plastic surface to define weld position, that sparked off the latest advance. Having proved that the process using laser absorber dye resist material could allow joints to be made between plastics, the project team studied the use of electron beam lithography to pattern the absorber and enable welds with a width smaller than 10 μm, mimicking methods used to build microelectronic circuits. The challenge was to generate micro-channels and infrared absorber tracks at their edges simultaneously, and to seal the channels.
With polymethyl methacrylate (PMMA) thermoplastic as a base material, the team carried out trials, following the principle of transmission laser welding using a thin coating of infrared absorbent material at the joint interface. The coating was patterned using electron beam lithography to the required resolution in a reproducible manner, so it could be retained after welding. Joint strength was ratified using larger-scale samples.
The trials culminated in successful demonstration of a series of laser welded joints with widths of 1 μm and channels of 5 μm. Smaller welds with 0.5 μm were also demonstrated leading to the conclusion that it is possible to make plastics components with a high density of structure and resolution below 1 μm, and that welding can be applied without excessively heating regions outside the weld lines.
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