Welding: It's light work

Tony Hoult, IPG Photonics discusses welding clear to clear polymers with laser light

It is widely accepted that absorption of electromagnetic radiation of most widely used natural non-filled, un-pigmented polymers can be neglected in the wavelength range of the widely used near infra-red lasers, In the longer wavelength range from 1.2-2.5µm, most polymers start to absorb and this is known to produce molecular vibration not electronic vibration. This absorption is very strongly dependent on the molecular structure of the material.

Light wavelengths longer than 2.5µm will be strongly absorbed as they very effectively induce molecular oscillations in optically clear polymers. For some years, the longer established older laser technology in the form of CO₂ gas lasers have been used for welding thin clear polymers, but the strong absorption of this wavelength only takes place at or very close to the surface. If any depth to the weld is required, weld times are excessively long and weld speeds are very slow as only thermal conduction can create this melting – and the low thermal conductivity of polymers is well established. So for controlled melting and welding of polymers an intermediate absorption to a depth within the material (volumetric absorption) is required. It is also well established that for realistic industrial welding speeds to be achieved between polymers, average powers > 50 watts are often required.

Currently these is only one industrialised laser source that can provide this level of average power with a wavelength in this intermediate range and this is the thulium fibre laser which is now commercially available at up to 200 watts average power.

Semi crystalline v amorphous polymers

Thermoplastic melt-processable polymers are either amorphous or semi-crystalline. These behave differently when heated with any thermal source and in this sense laser heating is no different. What is different about laser sources – and this is seen in many other industries such as the semi-conductor and microelectronics industry – is the fact that they are highly controllable; a laser beam can be switched on and off in nanoseconds and can be focused down consistently to produce features in the range of a few microns, try doing that with a heat gun! This spatial and temporal controllability of laser sources makes the behaviour of polymers very easy to observe and control during laser heating and welding. The practical effect of this is that with semi-crystalline polymers slower heating rates must be applied to avoid degradation of the polymer. This moderate volume absorption produces highly controlled melting in all clear thermally weldable polymers over a range of material thickness, typically from 0.25mm to 3mm thick. Fortunately, it is very common for material thicknesses in this range to be employed in precision moulded polymer components that may require a precision welding process such as laser welding.

Laser welding of polymers using the TTIR welding technique

This laser technique that is now established in a number of industries requires that one component of the joint transmits the laser beam and another component absorbs the laser beam. This means that either the absorbing component of the joint needs to be black or dark coloured or special absorbing inks such as Clearweld need to be used. This has clearly limited the industrial application of this technique, particularly in the medical device industry where lighter colours are the accepted norm and additional additives are not accepted lightly and the market, although growing, is limited.

Thulium fibre laser systems

Although thulium fibre lasers are already employed in industry for producing consumer items such as twin walled drinks containers, the market that is most likely to prefer the precision and controllability of laser welding over competitive techniques such as ultrasonic welding is the medical device industry. Introducing a new technology into this industry clearly takes time and systems are now starting to find their way into the R&D laboratories of some larger companies. Similarly, manufacturers of ultrasonic equipment are also seeing this as a technique that complements and widens their existing product range.

Additionally, as the process has become better understood, laser welding of micro-fluidic devices is increasing in importance. The galvanometer scanning technology and related software that is widely used for laser marking requires some subtle modification to allow the usually complex weld paths required to be welded successfully and good progress is also been made in this area.

This is certainly a technology to watch.