Light cures vehicle assembly problems

Dr John Amod of Dymax describes some new advances in aerobic adhesive technology

New advances in aerobic acrylic adhesives technology continue to broaden the areas where the `instant curing’ advantages of light can be used in vehicle lighting assembly. With a long history of successful use on both glass and plastic automotive head lamps, UV aerobic acrylic adhesives are a proven method of assembly for a variety of vehicle lighting applications.

Beginning in the early 1980s, glass headlamp lenses were bonded to housings, vent holes were sealed, and leads tacked in place to assure correct alignment with UV technology. The first systems using aerobic acrylic adhesives were developed in partnership with the Inland Fisher Guide Division of General Motors. Shortly thereafter, polycarbonate plastic lenses were being bonded to filled nylon housings by the Wagner Division of McGraw Edison. Other successful uses and applications followed.

In the late 1980s changes in lighting assemblies and assembly requirements led to the adoption of new materials of construction. Weight reduction, aerodynamic drag and a more stringent non-yellowing requirement, led to the use of new engineering plastics, many of which blocked UV light. The industry had to rely on mechanical fastening methods or use slower curing (4-48hr) multicomponent urethane, silicone and epoxy adhesive systems, replacing the instant UV curing adhesives and sealants.

Recent developments in photocuring technology at Dymax, have yielded faster curing adhesives that are able to cure through UV blocked, clear plastics. The new products, called Ultra Fast and Ultra Light-Weld, are making possible bonding processes that provide the faster assembly speed and over all efficiency gains possible with light curing technology.

ADHESIVE JOINING METHODS

Besides UV aerobic acrylics, other acrylated urethanes, modified acrylics, silicones, UV silicones, urethanes, epoxies, hot melt adhesives and two part acrylic adhesives have been used in head lamp assembly. Sonic welding has long been considered a reliable method of joining plastics, but with inconsistently mating parts, dissimilar plastics, and more complex part and joint geometries, its utility is limited for producing consistently bonded and sealed joints.

WHY LIGHT CURING?

Ultraviolet light curing technology has been an attractive bonding and sealing alternative for vehicle lighting assembly for years because of the dramatic process cost savings the technology affords. UV adhesives are strong, stable and do not cure until exposed to light of the proper wavelength. Then, upon exposure, cures in a few seconds are typical, facilitating high speed automated assembly and the use of robotics.

Curing stations are compact compared with long conveyorised heating ovens and drying racks. Capital equipment and labour costs are lower as well. In addition most manufacturers are able to accomplish 100% in-line quality testing to assure 100% defect-free product without having to wait for parts to cure or cool (off line) before quality testing (see Table I)

The UV aerobic acrylic acrylate adhesives commonly used are also available in a range of thixotropic, easily dispensed viscosities that can eliminate stringing, uneven dispensing, and clogged dispensers. Clean up is easily accomplished and the lack of pot life considerations means that there is virtually no waste. The urethane acrylate adhesives keep regulatory costs to a minimum due to their 100% solids composition and lack of hazardous isocyanates. In summary, UV adhesives used for vehicle lighting have reduced costs because they make assembly easier, reduce scrap and waste, facilitate inventory management for just-in-time delivery, use very little floor space, require less labour, reduce regulatory compliance cost and eliminate the need for clips and clamps

NEW DEVELOPMENTS

UV adhesives were initially limited to headlamps or lenses that used UV transparent polycarbonate. Recent developments in photoinitiator technology have allowed visible as well as UV light curing, permitting cure through UV-blocked plastics. Additionally, certain new grades of UV/visible curing adhesives have more aggressive adhesion to a wider range of plastics, as well as higher performance characteristics.

Higher bond strength, clarity, moisture resistance, thermal cycle and shock and vibration endurance combine with the new UV/visible curing capability to provide a wider range of application opportunities to the lighting manufacturer.

SAVING SPACE

Because of the fast cure of the UV/visible adhesives compared with a two component silicone, an automotive parts supplier was able to reduce the space required for its manufacturing operation significantly. This supplier was charged with providing the tail light for a 1999 model luxury vehicle. The application involved bonding a red polycarbonate lens to a clear acrylic diffuser. Though both plastics blocked all but 2% of the ultraviolet light, the visible cure capability of the resin allowed polymerisation through the clear polycarbonate. In addition to opening up additional floor space to allow for expansion, the change to the photocuring resin eliminated waste and scrap that could result from producing defective parts, and lowered the total labour content of the process.

Because of the experience this manufacturer had with photocuring technology, it began to investigate how it could be used in PET and acrylic headlamp bonding operations. This led to evaluation of a light curing material for a foamed-in-place gasket on a nylon reflector. A hot melt adhesive was considered, but had poor adhesion to the nylon. The Dymax foamed in-place gasket bonds well to the reflector. The lens is then screwed onto the reflector and the gasket provides the seal.

SIDELIGHTING

Grote Electronics, a Division of Grote Manufacturing of Canada, had experienced significant warranty cost as a result of the failure of side lighting assemblies for heavy duty trucks and passenger vehicles. To correct this situation, an extensive investigation of sealants, coatings and potting materials was instituted to protect the sensitive LED printed circuit board assemblies. Epoxies, known for their durability and resistance to moisture and difficult conditions were a natural choice. However, the hard, rigid nature of these products caused their delamination and cracking under the extreme temperature cycling and vibration conditions to which the assemblies were exposed. Moisture and salt spray quickly found their way through the resulting leak path and attacked the delicate electronics.

UV sealants and coatings appeared an attractive alternative, but this assembly was more of a potting than coating application, and some of the resin would be under the electronic components and would not be able to be exposed to UV light for cure.

It was found that a Dymax resin combined light curing chemistry with a room temperature shadow cure capability. Upon evaluation of the `shadow curing’ coating and potting material, it looked as if the processing benefits of light curing technology might be available for this application. The quality and durability issue, however, were of primary concern. But after extensive evaluations and road testing on vehicles enduring extremes of temperature, and all other adverse environmental conditions including constant vibration and impact, salt spray, snow, sleet and hail, performance of the lighting system was not compromised. The electronics remained fully protected and warranty cost dropped substantially.

CONCLUSION

The years of use of light curing adhesives in vehicle lighting construction has led to wider acceptance of the technology into a broad variety of applications. Advances in adhesive technology have pushed this trend further. This has led to more opportunities for process cost savings as well as improved lamp performance for vehicle lamp manufacturers.

Recent developments in photocuring technology have yielded faster curing adhesives that are able to cure through UV blocked, clear plastics. The new products make possible bonding processes that provide faster assembly speed and over all efficiency gains

Information: Dymax Corporation Tel: +1 860 482 1010