Over the past 20 years, the automotive design community has successfully pioneered the use of a broad range of structural materials.Simultaneously, the processes and products required to join and assemble these often dissimilar materials have become critical to achieving optimum levels of part performance.The increasing use of engineering thermoplastics in bumper systems is a prime example of this phenomenon -- a typical thermoplastic bumper design features an outer fascia and an inner reinforcing structure (beam), which are combined to form the bumper to a complete assembly. The challenge of effectively joining these two separate elements in bumper systems to achieve optimum impact performance is the subject of this discussion.Initially, plastic bumper systems were joined by vibration welding. This method provided a solid bond, but performance was often inconsistent, and the joint was too rigid to withstand even minor impacts. Engineers soon realized they needed a more ductile joint -- one which could absorb shock and remain intact. Mechanical joints were also evaluated, and while they survived impact tests, exterior bolt heads were considered to be aesthetically objectionable, and cracks tended to develop around bolt locations because of increased stresses concentrated in those areas.Adhesives provided an optimal solution to both problems. These materials can be formulated to bond tenaciously to their substrates, yet be flexible enough to absorb shocks. Since an adhesive is applied entirely inside the assembly, the fascia surface remains smooth and uninterrupted, reducing finishing costs. Further, adhesives provide a uniform stress distribution over a large bonded area, rather than concentrating stresses at fastening points like mechanical fasteners.With few exceptions, adhesives can bond effectively to most combinations of substrate materials, including metals and plastics. Usually, the weight of an adhesively bonded assembly is less than that of one fastened by other methods. And, because an adhesive bead can be continuous, it provides the additional function of sealing out the environment, which prolongs the service life of a part. Finally, adhesives are formulated and manufactured in forms that are practical for dispensing by pumps, so automating the application of adhesives on line is a relatively simple and economical process.