TWI Knowledge Summary
Clearweld ®
by Ian Jones
A technique has been developed for laser welding plastics with a range of colours and degrees of clarity with a laser specific infrared absorber, creating a joint which does not change the appearance of the material. Typically carbon black would be used as the absorbing medium for the laser light, which obviously affects the appearance of any product that might be manufactured in this way. This new approach enables two similar clear (or coloured) plastics to be joined with a minimal mark weld line. Welding may be carried out using Nd:YAG lasers (1064nm wavelength), high power diode lasers (typically 808nm and 940nm), and fibre lasers (1000-1100nm).
Since first being reported in 1985, transmission laser welding has been carried out with an infrared transmissive plastic material for the upper section and a carbon black loaded plastic for the lower layer. The carbon black absorbs
and heats in the laser beam to generate a weld at the interface between the two pieces. The process is limited by the fact that one side of the component has to be black. The other side of the joint must also transmit a proportion of
the laser energy (more than 10% is usually enough). An example can seen in
Figure 1.
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Fig. 1. Laser transmission weld in 4mm thick polypropylene using a 100W Nd:YAG laser at a speed of 1.6m/min. The weld is at the interface between the light and dark materials |
A more general extension of the transmission laser welding process has now been developed, which allows completely clear or similarly coloured components to be welded by using an absorber material, which is clear in the visible range of the spectrum, but tailored to absorb heavily the specific wavelength of the laser beam being used. [1] This technique has been termed Clearweld ®.
The nature of the absorber means the laser wavelength is absorbed with high efficiency, thus requiring relatively small amounts of the absorber material at the interface between the two components to be welded. Development work on the process has been carried out using polymethylmethacrylate (PMMA) test specimens, and an example of an overlap weld made by applying absorber by ink jet printing to the joint surface between two transparent sheets of PMMA can be seen in Figure 2. When the incident laser light is applied, the absorber molecules dissipate the absorbed energy principally as heat, which melts the joint only at the thin region either side of the absorber coating. In this way the outer surfaces of the component are unaffected.
Although the example in
Figure 2 is shown with two visibly clear sheets of PMMA, an absorber applied in this way can be used to join a wide range of plastics, in moulded or sheet form, coloured or otherwise, or indeed, textiles and flexible film
materials.
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Fig. 2. Laser overlap weld in clear PMMA made with Clearweld absorber at the interface |
Development work to select the most suitable absorbers and dispensing systems has also been carried out at TWI and Gentex Corporation. The vast majority of near-infrared absorbers can be discounted on the grounds that they have a pronounced visual colour. Others are unstable or have low absorption at the laser wavelength. Examples of three absorber types which can satisfy all of the requirements are organic molecules such as cyanine, aminium/diimonium and squarylium. Materials such as these are used in the absorber systems for Clearweld.
The effectiveness of the absorber system depends upon its compatibility with specific process parameters. The systems are custom-formulated taking into account the substrate materials, part design and process requirements. Critical to the success of the Clearweld process is the accurate delivery of the absorbing material to the joint interface in terms of the amount and location of the deposit. The absorbers now available are certified for use with specific delivery methods such as ink jetting and liquid dispensing. A delivery method using a dry film absorber at the weld interface is also feasible, though commercial products are not yet available in this area.
Thus, in order to have the weld occur, the absorber material must be absent from the upper plastic material and must be localised at least at the surface of the lower plastic. The welding occurs as the heat generated in the
absorber. Typically the weld depth is of the order
0.1mm of the polymer material. The heat generation at the interface is controlled by the absorption coefficient of the absorber layer and the processing
parameters. The main welding parameters are laser power, beam spot size, and the welding speed. Intimate contact and clamping is required for the welding to be effective.
Application areas include:
- Medical devices
- Packaging
- Automotive components
- Consumer products
- Electronic packages
- Textiles
The application to textiles is very interesting. The Clearweld process also offers a new method of welding textiles that does not melt the outer surfaces of the material.
Figure 3 shows continuous and hermetic overlap welds made in waterproof fabric laminate. The potential therefore arises for further automation of garment manufacturing for waterproof clothing, personal protective clothing, and
other textile products.
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Fig. 3. Continuous overlap welds in waterproof material made using Clearweld absorbing system. A Nd:YAG laser beam of approximately 100W in power was used with a welding speed of 500mm/min |
Summary
Polymer materials can now be laser welded using a near infrared absorber system as a mechanism to produce heat and localised melting. The welds produced are of high strength, cosmetically appealing with minimal visible colour and the upper and lower surfaces of the material are unaffected by the process.
The welding process is efficiently achieved using the very compact diode laser sources now commercially available, and lends itself easily to high levels of automation. Potential applications of this technology exist in a wide variety of industry sectors with plastic joining requirements.
TWI has patented the Clearweld process of plastic laser welding. Gentex Corporation has worldwide exclusive license to commercialise Clearweld and supply consumable products for welding applications.
More information on how to proceed with applications testing may be obtained from Ian Jones at TWI ( ian.jones@twi.co.uk), Gareth McGrath at Gentex ( gmcgrath@gentexcorp.com) or from the web site ( www.clearweld.com).
- Jones IA, Taylor N S, Sallavanti R, Griffiths J: 'Use of Infrared Dyes for Transmission Laser Welding of Plastics'. proc. SPE ANTEC 2000 conference, May 7-11 2000, Orlando, Florida. Vol. 1, pp1166-1170.