Stick to the rules
July 1st 2007

Bob Goss, of Henkel, maker of Loctite brand products, maintains that involving the adhesives supplier at an early stage helps design engineers

Ask some design engineers why they seem reluctant to specify adhesives and you'll likely get one of two main reasons. First, they believe the performance of adhesives is not always predictable in advance. And secondly, a previous bad experience has led them to suppose 'glue' simply doesn't work.

The truth is, however, the performance can be ascertained and, providing the right adhesive is used, there is no reason why the joint or seal cannot be as effective – if not better – than other methods. So, let's dispel both myths.

Predicting performance Those adhesive manufacturers that have invested in appropriate laboratory facilities can replicate, with great accuracy, the actual process that will be utilised in the production environment. That enables designers not only to test out their designs before costly production runs are started, but also to seek expert advice about a number of issues involving the use of adhesives. So, just what kind of advice can you expect from an adhesives supplier? Alongside the selection of the right kind of adhesive technology (more of that later), perhaps one of the most important areas is that of joint design.

Bonded assemblies must uphold a multiplicity of forces. To achieve maximum strength (and therefore, reliability), adhesive joints should be designed such that cleavage and peel stresses are, wherever possible, avoided. In addition, the substrate material in question, its thickness and the type of adhesive should be evaluated to establish the optimum area of overlap.

On a simple lap shear joint the highest stress points are at the leading and trailing edge of the joint. The length of the overlap increases strength - but by diminishing amounts. On the other hand, expanding the width of the overlap creates a proportional increase in strength.

Where a butt joint has been used as a previous assembly design, a simple bevelling of the two mating surfaces can produce the area required for an effective bonded joint.

Selecting the right product As stated earlier, one of the traditional objections to bonding is that adhesives do not 'work'. Often this conclusion is reached because the wrong type of adhesive has been used in the past. There is a plethora of adhesives available in the market place, and just because a product works well on one type of material, does not necessarily mean it will be effective on another substrate.

Within the scope of this article it is not possible to provide in depth details of each adhesive technology, but an overview will offer some useful information regarding the types of product that are available.

Cyanoacrylates or instant adhesives cure in a matter of seconds at room temperature through reaction with moisture on the component surface. As well as the regular liquid form, instant adhesives are also available as gels.

These products are generally ideally suited for small parts plastics and rubber bonding.

Anaerobics are single component acrylic adhesives that harden when air is excluded from the joint - in other words, when the mating surfaces, one of which is usually metal, are brought into contact.

These are the "engineering adhesives" – so called because they will lock threads, bond cylindrical metal parts and seal flanges.

Epoxies mostly consist of two part components - a resin and a hardener. Only when these are mixed in the correct proportions will the adhesive begin to cure. These are normally used where high strength structural bonding is required.

Light Curing products are adhesives that react to the presence of light – either Ultra Violet or natural.

Typical applications include bonding glass-to-glass, potting electronic and electrical components and the surface conformal coating of printed circuit boards.

Polyurethanes are highly flexible adhesives used for direct glazing windscreens, most metals, woods, various plastics and rubbers. They are also suitable for bonding polyurethane foam, polyester film and composite body panels.

Modified Silanes are mainly onecomponent primerless products that are used for sealing the joints and seams on containers, railway carriages, shower screens, air conditioning and ventilation equipment.

Silicones are one or two part products, generally used for sealing and gasketing, vulcanise at room temperature by reacting with ambient moisture.

That's just a summary – and any adhesives manufacturer will be happy to provide more detailed information. Clearly, using the wrong adhesive will result in an ineffective bond – and could, potentially, have serious consequences.

Applying the solution Having selected the correct adhesive and joint design, the method of assembly is the next consideration. This involves both where and how to apply the adhesive on the component. If only a few components need bonding it is likely the adhesive will be simply applied direct from the bottle or pack in which it is supplied. However, if a production process is involved, a range of adhesive applicators are available that can be coupled up to automated assembly lines and robotics.

Better - by design Many designs have been enhanced through the use of adhesives. Smaller and lighter components, increased versatility of use, and sealing against the elements are just a few of the benefits. Where adhesives have replaced components such as clips, pins and gaskets, the inventory of the engineering stores has been considerably reduced.

The perfect time to consider adhesives is at concept stage. A great deal of time and heartache can be avoided by building bonding technologies into those early designs. And that's just what an increasing number of design engineers are doing.

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