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Some ‘untraditional’ uses for adhesives

01 April 2010

Riveting and spot welding are commonly used to join structural components; for example, the fixing of sheet materials to supporting frames. But this time-honoured method does have its drawbacks. Colin Chapman provides some interesting insights into where adhesives have replaced these ‘traditional’ joining methods

Designers of items such as vehicle bodies, office partitions, metal furniture and doors frequently consider the only viable ways of joining components to be riveting or spot welding. After all, these ‘traditional’ methods do provide sufficient strength to hold those pieces together.

Nevertheless, it also has to be admitted that these two methods do present some shortcomings if the surface of the finished item is to remain unblemished. For example, spot welding can cause distortion of parts that, consequently, need dressing prior to painting. Welding may not always be possible, as dissimilar materials could be involved. Riveting, too, can present some serious disadvantages. Perhaps the most obvious are the facts that the sight of rivets can create an unpleasant appearance to completed objects and holes have to be drilled to accommodate the rivets.

Reducing weight
Fortunately, adhesives can provide some satisfactory answers in, what is sometimes seen as, an ‘untraditional’ market for this technology. Having said that, there are instances in which a mixture of joining techniques can be utilised very productively.

For example, the aerospace industry has successfully employed a mixture of adhesive bonding and self-piercing rivets for joining aluminium to other substrates. This combination has now been transferred to the automotive industry to assist those manufacturers who are producing aluminium vehicles. Compared with seam and spot welding on a steel body, there is a considerable weight reduction, coupled with gains in strength and torsional stiffness.
Having witnessed the benefits of adhesives, a growing number of designers have begun to specify structural bonders for a broad diversity of joining applications. To go further, in situations where a combination of appearance and strength is required, adhesives have become a vital design component.

In another example, designers decided that traditional methods of joining would spoil the look of an aluminium bonnet. Trials revealed that the use of a suitable two-part acrylic metal bonding adhesive would not only secure the joints, but also provide a cosmetically acceptable finish. In this case, an air gun fitted with a bi-mixer nozzle was employed to guarantee the correct ratio of adhesive was applied. Within minutes of application, handling strength was achieved – with the cure being completed in a low-bake oven.

Ramping up production
Increasingly, buses and coaches are being supplied with ramps that will allow easy access for wheelchairs and prams. One company that needed to produce a folding ramp for this purpose created a design that required a cassette-type unit that was compact, light, modular and flush fitting. In addition, the ramp needed to be dropped straight into the vehicle chassis and connected to its operating mechanism during bus construction.

To achieve the necessary lightweight finish, aluminium was used for the base and ramp floor, while steel was employed for the frame. An aluminium-based interfacing box section, along with wedge shaped stringers, was incorporated to provide stiffening for the floor.

When consideration was given to the assembly method, rivets were first considered. However, the number involved was considerable – so much so that the cost was prohibitive. Not only that, the appearance of the ramp would have been less than acceptable. But, more crucially, when the ramp was in operation it would repeatedly need to unfold on to an uneven surface. This action would result in a high degree of twisting and flexing. Further, as the ramp was retracted, it would need to revert to the original profile to fit into its housing.

Tests revealed that rivets couldn’t meet these requirements, whereas adhesives were deemed viable. By using an air-operated cartridge gun to apply a structural adhesive to the various components during assembly, the stringers and the floor were securely joined together. The adhesive in question ensured a more flexible finished product and did not deface the surfaces. Beyond that, the lightweight ramp proved to be extremely tough – withstanding loads of 254kg (560lbs) during testing. The manufacturer was so impressed by these results that it changed to using the adhesive for bonding panels on bus doors.

Eliminating vibration
Another application involved the attachment of fan-mounting rails to the galvanised cases of air conditioning units. Initially, the specification called for mechanical fasteners, but testing showed the units resonated when the fans were operated. Naturally, this was unacceptable and so a switch to an adhesive was made.

Comprehensive testing revealed that even when six times operational weight was applied to the rails, the adhesive met every criterion. Indeed, not only did the resonance disappear, but the adhesive also proved to be less expensive and the workforce found application to be quicker and easier.

One further example involves bonding sheet steel to the aluminium die cast frame of metal furniture. Again, rivets had previously been specified for the task. Although this joining method provided the strength required, the result was unsightly. By switching to a structural bonder a far more pleasing appearance was created, without compromise to the furniture’s strength specifications.

So, adhesives offer an aesthetically enhanced finish, they do not distort component materials and, significantly, they can be used to bond dissimilar substrates. Moreover, there are products to suit each application, and your adhesives supplier will assist in determining the most appropriate. Indeed, it is wiser to review each application prior to embarking on formal assembly so that the right adhesive technology is chosen for the job. And to complement the adhesives, most suppliers will offer a selection of dispensing equipment that will make application more precise, and easier with minimum wastage.

Adhesives are a viable alternative in many markets that use mechanical fixing methods. In fact, it might be said that these ‘untraditional’ areas of use are rapidly becoming ‘traditional’ for today’s modern family of adhesives.

Colin Chapman is with Henkel Loctite

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