Plastics Welding/Adhesive Bonding: Innovative Applications

TWI, Granta Park, Great Abington, Cambridge CB1 6AL

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.

 

 

Ian joined TWI in 1989 after a degree in Materials Science at Jesus College, Cambridge. Since then he has worked on a wide range of laser processing applications and development studies, including development of high power laser welding of a wide range of metals and non-metals. In particular, these have involved improving quality in laser welding of aluminium for light weight vehicles, guidelines for laser welding structural steel for ship construction and a fundamental study into transmission laser welding of plastics.

For virtually all commercial products, especially those produced from more than one component, joints are a fact of life. Joining technology therefore is crucial within the manufacturing world. Although many traditional joining methods can be, and are applied successfully, the opportunity to innovate still exists. Indeed the innovative approach can have significant economic benefits associated with reduced manufacturing costs, increased product quality and a greater freedom in material selection to design and manufacture new products.

Recently TWI has been active in the innovation of two distinctly different joining technologies - adhesive bonding and plastics welding. The resultant processes are termed AdhFAST ^TM and Clearweld® respectively. Both offer opportunities in the areas of increased manufacturing opportunity/flexibility, reduced time to market and improved component functionality.

AdhFAST ^TM

AdhFAST ^TM is in essence a hybrid joining system where adhesives and fasteners are combined to maximise the benefits of each technique i.e. rapidity and ease of use of the fastener coupled with the sealing ability and high fatigue resistance of the adhesive. Conventional hybrid joints are formed in a continuous linear process where an adhesive is first applied to the surfaces, the joint is then closed and a fastening system (e.g. resistance weld, rivet, nut & bolt etc.) is then employed to secure and hold the structure together while the adhesive cures. Such processes are used extensively within the automotive sector.

By adopting an innovative approach, the process can be broken down into two stages i.e. a 'dry' assembly stage followed by an adhesive injection stage. In reality this means that these operations could be done in different geographical locations or at different times depending upon production and manpower resources. With the correct selection of surface pre-treatment, where a bonding window of days or weeks is possible, one could envisage the storage of dry assembled parts ready for bonding with the possibility of disassembly and re-use should an order be changed or amended.

What makes this process different is the addition of new functionality to the fastener employed. The traditional fastener has two primary functions:

• Location - holding the components in the correct position
  and
• Retention - fastening the pieces together

The AdhFAST ^TM fastener adds:

• Spacing - controls the spacing between the materials to be joined, thereby enabling the adhesive to be easily injected and defining the final thickness of the adhesive in the joint, this second factor aids the calculation of the mechanical properties of the bonded joint
• Injection - accomplished either through a central hole or down features on the sides of the fastener.

The fastener design is completely flexible and can take many forms, from nuts and bolts through blind riveting systems to the wood screw. Generic designs are shown in Figure 1. The fastener element can take a range of forms incorporating either a central hole with exit ports or a modified head combined with vertical grooves or flats along the length of the fastener. The spacing element can either be integral to the fastener body (not shown) or can take the form of a shaped washer, which contains features to allow flow of adhesive into the joint cavity. The adhesive can therefore be pumped through the fastener, filling the joint from the inside out as shown in Figure 2. Escape of adhesive from the joint edges is controlled by a number of methods including gasketing or adhesive tape depending upon application.

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.

This technology has the potential to be adapted to virtually any industry sector whether it be a high value, high precision titanium rivet for aerospace, a low cost, low precision steel screw and spacer for DIY or a modified structural nut and stud arrangement for the building trade.

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®

A technique has been developed for laser welding plastics with infrared absorbing welding consumables, creating a joint almost invisible to the human eye. Typically carbon black would be used as the absorbing medium for the laser light, however, 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 a Nd:YAG laser (1064nm wavelength), or using the relatively new high power diode lasers (typically 808nm and 940nm wavelengths).

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.

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.

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.

Welding occurs as heat is generated in the absorbent media. Typically, the weld depth is of the order ≈ 0.1mm. The heat generation at the interface is controlled by the absorption coefficient of the medium and the processing parameters. The main parameters, are laser power, beam spot size, and the welding speed.

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.

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.).

N°	Author	Title
1	Jones I A, Hilton PA, Sallavanti R and Griffiths:	'Use of Infrared Dyes for Transmission Laser Welding of Plastics'. ICALEO99

Further information

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

A related paper on Clearweld - 'Use of infrared dyes for transmission laser welding of plastics' was also presented at the SPE ANTEC 2000 conference, held in Orlando, Florida, USA in May 2000.