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New Advances in Plastics Joining for High Speed Production (September 2001)

   

Gareth McGrath, I A Jones, P A Hilton, E J C Kellar and A Taylor
Polymer Technology Group, TWI Ltd

R Sallavanti
Gentex

Paper presented at 2001 Automotive Transportation Technology Conference, Barcelona, 30 Sept - 3 Oct 2001

The vast majority of automobile components contain joints, typically:

3,000 spot welds
2 metres of welds (arc or laser)
20 friction and MIAB Welds
150 Welded plastics components
500 mechanical fasteners
10,000 electrical connections
8Kg adhesives/sealants

Thus, 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.

An ability to use high productivity and integrity joining at competitive rates is fundamental to the evolution in the automobile sector. There is no real choice regarding the adoption of new technology: it is a pre-condition for survival.

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

AdhFAST TM

AdhFAST TM is a hybrid joining system, where adhesives and fasteners are combined to maximize 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 applied to the surfaces, the joint closed and a fastening system used 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, the storage of dry assembled parts ready for bonding is possible, with 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
  • 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 side 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 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 integrated into the fastener body (not shown) or 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.

Fig.1. Generic AdhFAST TM components with separate spacing element
Fig.1. Generic AdhFAST TM components with separate spacing element
Fig.2. Injection of adhesive into a joint cavity using AdhFAST TM fasteners
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 geometry's
  • 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 Gulled® technology where small sections of wood are bonded together to create large structural beams used in buildings. Gulled 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 the 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 essential applications to establish this enabling technology within the manufacturing world.

Clearweld ®

A process has been developed for laser welding plastics with an infrared absorbing medium, 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.

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
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 clear or similarly coloured components to be welded by using a medium, clear in the visible range of the spectrum and tailored to absorb the specific wavelength of the laser has been developed. This process 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 medium at the interface between the two components to be welded. Development work on the process has been carried out using PMMA; and example of an overlap weld can be seen in Figure 4. When the incident laser light is absorbed, the medium molecules dissipate the absorbed energy principally as heat to the medium molecules and their local environment.

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

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

Thus, in order to have the weld occur, the medium must be absent from the front plastic entity and must be localised at least at the surface of the lower plastic. Methods of applying the medium are as follows:

  • The medium can be incorporated into a thin film which can be placed at the interface of the plastic pieces to be welded
  • The medium can be introduced into the bulk of the polymer
  • A medium laden 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 medium from solution. This may be by dip coating, medium infusion, painting, spraying, printing, dry burnishing, paste application, etc.
  • The material to be welded can be co-extruded with polymer containing the medium
  • A plastic piece can be over-moulded to provide a narrow strip to a selected area

The welding occurs, as the heat generated in the medium is sufficient to melt of the order 0.1mm of the polymer fabric. The heat generation at the interface is controlled by the absorption coefficient of the medium layer and the processing parameters. The main parameters, for a given width of weld, are laser power, energy distribution in the focus, and the welding speed.

The welding process using infrared absorbing mediums 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 and a Nd:YAG laser beam of approximately 100W in power. The welding speed was 500mm/min. Table 1 presents results for the mechanical testing of the materials.

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

Table 1. Results of mechanical testing on a range of woven fabrics.

Material ColourThickness,
mm
Peel Strength,
N/mm
Lap/Shear Strength,
N/mm
Parent Strength,
N/mm
Brown
Orange
0.19
0.23
0.70
2.16
2.08
5.22
8.47
13.95
Bronze
Yellow
0.16
0.41
2.07
4.40
2.76
6.79
9.38
16.12

Figure 6 shows a polyester/viscose shirt welded using a diode laser. The potential therefore arises for further automation of garment making procedures for waterproof clothing, protective clothing, and other textile products.

Fig.6, A polyester/viscose shirt welded using a diode laser
Fig.6, A polyester/viscose shirt welded using a diode laser

To summarise, polymer materials can now be laser welded using near infrared absorbing medium as a mechanism to produce heat and localised melting. The welds produced are cosmetically appealing and the upper and lower surfaces of the material 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 process of laser welding using an infrared absorbing medium has been given the trademark Clearweld ®. In addition, TWI has initiated a patent protection on this process. Gentex Corporation, who manufactures suitable mediums for the process, is working with TWI to develop consumables and procedures for the process.

Vitresyn TM

The objective of this innovation is to deposit protective coatings onto soft substrates such as aluminium and plastics. The prime driving force behind the development of abrasion resistant coatings for plastics is the development of alternative glazing technologies. Glass is a well established glazing material its main advantages of transparency and abrasion resistance ensures it is the only commercially viable material. However, there are disadvantages to glass, it is denser than transparent plastics, cannot be readily and economically formed into complex shapes and does not significantly contribute to passenger safety.

If the side-windows of a family car were replaced with plastic glazing, a weight saving of at least 10 kilograms could be made. Whilst improving the fuel efficiency of the car, this would give greater design flexibility and also contribute significantly to improving the vehicle's safety. Since plastic windows would not shatter during a rollover, the passenger compartment is more likely to stay intact. Furthermore, the high toughness of the plastic would reduce the number of complete or partial passenger ejection during accidents, of which there are almost 8000 occurrences annually in the USA.

Whilst there are a number of significant advantages to plastic glazing, the fundamental problem is that of abrasion damage. Plastics such as polycarbonate and acrylic which are candidate replacement materials for glazing applications are also soft and readily abraded. To be considered for automotive glazing, these materials have to show glass-like abrasion resistance, FMVSS 205 sets this standard as an increase in haze of less than 2% after 1000 abrasion cycles. This is the target that a number of organisations, including TWI, are aiming towards.

TWI identified sol-gel as a potentially useful technology in this quest since it allowed glass and ceramics to be fabricated in room temperature solutions which could then be deposited as coatings on plastic substrates. A selection of coating formulations have been developed, together with the definition of the important criteria that a coating system would have to match if it was to attain technical and commercial acceptance.

The technical hurdles that needed to be overcome before a viable coating system was achieved were also identified. These included the need to increase the achievable coating thickness from a maximum of ~1.5µm to ~10µm. This was essential, otherwise the ease of deformation of plastic substrates would always cause brittle failure in the coating. This is commonly referred to as the 'ice on mud' scenario, where a hard and brittle coating is easily compromised by the soft substrate until it has a minimum degree of integrity.

Another challenge was that of existing competitive commercial solutions, with silicone hardcoats identified as the benchmark. These are routinely used on many products and have particular specific processing windows and operating performance specifications. The solution had to at least match the performance of silicone hardcoats, and preferably have less arduous processing conditions.

The route selected was to devise a method of combining the inorganic network produced using the sol-gel method with an organic network. A similar approach has been tried by other workers in the field, but gave transparency problems when the inorganic content exceeded 25%. The innovative process that has been developed minimised this problem and achieved many of the target performance values.

Whilst the development of the new technology was continued, a patent application on the fabrication process was submitted and a brand name coined. The name Vitresyn TM reflects the origin of the materials produced (the coatings are vitreous and synthetic).

Vitresyn TM coatings compare very favourably with the best silicone hardcoats. Whilst greater protection against abrasion is afforded to the substrate (as measured using industry standard Taber testing), Vitresyn TM coatings do not need a primer, and can be rapidly cured at room temperature using UV light.

Since the raw materials for the Vitresyn TM process are not particularly expensive, it is likely that the commercial coating systems will be competitive with silicones. The abrasion results are shown in Table 2

Future technical developments aim to improve the properties of the existing coatings and develop new coatings for different markets, such as protecting painted metal components. The commercial considerations are related to how the technology will be made available for the large markets that exist for solution based hardcoats.

Table 2. Comparison of properties of commercial silicone hardcoats and Vitresyn TM. The increase in haze was measured after 300 Taber cycles. The coatings were heated to 130°C on Polycarbonate and 80°C on acrylic substrates for one hour.

Coating TypeCoating IdentitySubstratePrimer RequiredIncrease in Haze,
%
Commercial
Silicone
hardcoats
SHC 1200 Polycarbonate Yes 4.1
AS 4000 Polycarbonate Yes 4.0
SHC 1200 Acrylic Yes 7.5
Vitresyn TM C80A Polycarbonate No 2.4
C95A Polycarbonate No 1.8
C95A Acrylic No 2.8

Conclusion

There is no shortage of problems to tackle in the world. We may already have the means to tackle them: what we need are the innovative solutions and the will to implement them. If we do not have either of these, the outlook could be bleak for individuals, organisations, countries and the global economy.

Finally, the innovation process needs good management that balances the need for good definition, resource planning, team building and commitment with the need for variety, flexibility and rapid response. It needs innovators and adaptors; it leaves scope for individual creativity; it balances pressure of time and resources to achieve agreed goals with the opportunity to develop new ideas.

Acknowledgments

The authors would like to thank their colleagues for assistance and support during the development of these innovations.

References

  1. Silvers H J Jr and Wachtell S: 'Perforating, welding and cutting plastic films with a continuous CO 2 laser'. PA State University, Eng. Proc, pp.88-97, August 1970.
  2. Jones I A, Hilton P A, Sallavanti R and Griffiths: 'Use of Infrared Mediums for Transmission Laser Welding of Plastics'. ICALE099

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