Plastics welding and adhesive bonding - innovative applications

Based on an article originally published in Welding and Metal Fabrication, 2000, Vol. 68, No. 10,
November/December, pp 13-15 by DMG World Media
http://www.dmgworldmedia.com/

Ewen joined TWI in 1996 and now works in the Advanced Materials and Processes Department as a Principal Project Leader in Adhesives Technology. He graduated with a BSc (Hons) in Chemistry at Edinburgh University followed by a doctorate at University of East Anglia. Since then he has worked in a range of areas including polymer science, biomedical applications, spectroscopy, opto-electronics, adhesives and composite materials. E-mail: ewen.kellar@twi.co.uk

Fig. 1. Generic AdhFAST ^TM components with separate spacing element

Fig. 2. Injection of adhesive into a joint cavity using AdhFAST ^TM fasteners

Employing AdhFAST ^TM enables the following benefits to be attained:

• Little or no external jigging
• Simplified dry assembly with accurate location and checks of tolerance
• Protection of pre-treated surfaces prior to bonding from excessive atmospheric exposure and operator contamination
• Minimal operator exposure to uncured adhesive
• Simplified adhesive application process
• Ability to fill complex joint geometries
• Accurate bond-line control and metering of adhesive within the joint
• Potential to control fillet profile accurately
• Ability to consider semi- or full automation.

The key word is system - using 'glue' as a primary structural joining method is often doubted but if the adhesive is employed in a highly controlled process which achieves elevated levels of quality assurance then total confidence can be developed. A good example of where this has already been achieved is with gluelam technology where small sections of wood are bonded together to create large structural beams used in buildings. Gluelam is presented as a high quality system and not as bits of wood glued together. The AdhFAST ^TM system has the potential to offer this same degree of confidence throughout manufacturing industry.

Many industry sectors can benefit from this technology. Complex joints can be filled in one go enabling more efficient better-designed structures to be made. The aerospace industry consumes hundreds of thousands of rivets per aircraft. Aluminium is still the primary structural material but with increasing use of composites and other materials within the fuselage, structural adhesive bonding will become a necessity presenting opportunities for the AdhFAST ^TM system.

Bridge stiffening and strengthening is often accomplished by bonding, bolting or welding steel plate to the underneath of the structure. The processes although well established, are messy and time consuming where adhesives are used; not so effective in the case of bolting and labour intensive where welding is required. Using an AdhFAST ^TM fastening system could simplify the process considerably and remove the need for welding.

The marine and shipbuilding industries are also likely candidates for AdhFAST ^TM , either in the fabrication of bulkheads (composite structures) or for the attachment of secondary structures. The specialist automotive sector is increasingly looking at 'new' materials and adhesives to join them, for better, lighter, faster cars. High levels of quality control are required to maximise consumer confidence.

The fasteners could also be used in repair situations e.g. damaged car bodywork, defects in tunnel walls, bulkhead damage in ships, cracks in pipes etc.

TWI has filed a patent on AdhFAST ^TM and is in detailed discussions with a global fastener manufacturer. Both parties are assessing key applications to establish this enabling technology within the manufacturing world.

Clearweld®

In conventional transmission laser welding technique, a transmissive plastic material is used 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. An example can be seen in Figure 3.

Fig. 3. 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

An extension of the transmission laser welding process which allows completely clear or similarly coloured components to be welded by using an absorbing medium, clear in the visible range of the spectrum, but tailored to absorb heavily the specific wavelength of the laser beam being used, has been developed. ^[1] This technique has been termed Clearweld®.

The nature of the medium means the laser wavelength is absorbed with high efficiency, thus requiring relatively small amounts of the consumable 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 an impregnated film to the joint region between two transparent sheets of 3mm thick PMMA can be seen in Figure 4. When the incident laser light is absorbed, the molecules present in the medium dissipate the absorbed energy principally as heat.

Fig. 4. Transmission laser overlap weld in clear PMMA made with infrared absorbing impregnated film at the interface

Although the example in Figure 4 is shown with two visibly clear sheets of PMMA, an absorbing medium applied in this way can be used to join several other materials, coloured or otherwise, or indeed, textiles and flexible materials.

For welding to take place, the infrared absorbing consumables must be absent from the upper plastic material and must be localised at least at the surface of the lower plastic. The following application methods can be used:

• The absorbent welding consumables can be incorporated into a thin film which can be placed at the interface of the plastic pieces to be welded
• The absorbing medium can be introduced into the bulk of the polymer.
• An impregnated film may be added as a mould insert to coat a moulded article.
• The surface of the solid or fabric material may be coated in the absorbing media. This may be by dip coating, infusion, painting, spraying, printing, dry burnishing, paste application, etc.
• The material to be welded can be coextruded with polymer containing the absorbing medium.
• A plastic piece can be overmoulded to provide a narrow strip to a selected area.

The welding process using infrared absorbing consumables can also be applied to films and fabrics or coated fabrics. Figure 5 shows continuous and hermetic overlap welds made in the waterproof fabric Goretex ^TM using this technique, with a Nd:YAG laser beam of approximately 100W in power. The welding speed was 500mm/min.

Fig. 5. Continuous overlap welds made using infrared absorbing consumables in the fabric Goretex ^TM

Figure 6 shows a polyester/viscose shirt welded using a diode laser. This illustrates the potential for further automation of garment manufacturing in areas such as waterproof clothing, protective clothing, and other textile products.

To summarise, polymer materials can now be laser welded using near infrared absorbing media as a mechanism to produce heat and localised melting. The welds produced are cosmetically appealing 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.

The Clearweld® process was invented, and is the subject of a patent application, by TWI. It is being commercialised by Gentex Corporation. The process uses lasers that are commercially available, and infrared absorbing welding consumables that are currently under development, but the intention is to create a product line of consumables compatible with commercially available delivery systems (e.g. inkjet inks, inks for liquid dispensing systems, thin films etc.).

Additional information can be found on the Clearweld® website - www.clearweld.com.