FASTENINGS & ADHESIVES FEATURE W Right: Even if the


selection of adhesive and substrates results in a good adhesive bond, the subsequent environment needs to be considered. The adhesive must be resistant to any


temperature extremes, solvents, other


chemicals, weather, stress and vibration that befalls it during the


lifetime of the assembly


hen preparing to manufacture a new product, it is essential


to think ahead to ensure you have specified the materials, processes and resources needed when production starts – and this includes the adhesives that should be used. In an ideal world, all surfaces would be


bonded by one perfect adhesive. However, a single adhesive that cures instantly on demand, can operate at any temperature, resists all solvents, sticks to everything, and is completely and instantly reworkable, is not possible. Compromise is almost inevitable, but how can design engineers ensure they get as close as possible to the perfect adhesive?


KNOW THE SUBSTRATES Successful bonding can be a matter of choosing substrates and adhesives wisely. Normally, an adherent force can only be established when the molecules in the substrate are closer than five ångströms (5 x 10-7


mm) to


the molecules in the adhesive. For this to occur, wetting of the adhesive to the substrate must be achieved. If the adhesive behaves like water on a greasy plate and forms little balls rather than a homogenous film, wetting, and therefore adhesion, will not occur. For wetting to be achieved, the


adhesive must have a lower surface energy than the substrate. For example, stainless steel has a much higher surface energy than most adhesives, so is usually readily bondable. On the other hand, polytetrafluoroethylene (PTFE) is a substrate with an extremely low surface energy, which makes it almost impossible to bond. Choosing bondable surfaces is an


A CLEAR WAY TO BOND CARBON FIBRE COMPOSITE PARTS


In response to demands from companies manufacturing and assembling products made of composite materials, that needed a product that could bond carbon fibre parts quickly and neatly, Techsil has introduced EP25880 Clear. According to the company, this clear epoxy adhesive cures in five minutes and provides a strong, tough and durable bond on carbon fibre composite parts. Techsil EP25880 Clear cures to a completely water clear


product with a shore hardness of 80D, a shear strength on steel over 20MPa and tensile strength on steel of over 30MPa. Being a multi-purpose adhesive, it is also suitable for bonding substrates including wood, ceramics and metals. Commenting on its strength, Techsil’s application


engineer, Steve Green, said: “In tests, two carbon fibre parts were ‘butt joined’ and a bench weight test of 56lb was applied for 30 minutes – and didn’t trouble the joint at all. The part was then cleaned down and the joint line was found to be aesthetically acceptable.”


Techsil www.techsil.co.uk


Sticking to the facts With multiple factors to consider, how


should design engineers go about specifying the most appropriate adhesive for their


application? Peter Swanson, managing director of Intertronics, shares some tips


important part of the design process. If UV light can pass through the


substrates, light curing adhesives are often a great choice. These offer very fast cures and multiple process advantages, including no mixing requirement. Because the processing cost may be a significant portion of the overall part cost, choosing process-friendly substrates may bring commercial benefits.


THINK ABOUT THE REQUIREMENTS Even if the selection of adhesive and substrates results in a good adhesive bond, the subsequent environment needs to be considered. The adhesive must be resistant to any temperature extremes, solvents, other chemicals, weather, stress and vibration that befalls it during the lifetime of the assembly. If the coefficient of thermal expansion


(CTE) is different for each substrate, the adhesive may need to be tough or flexible to withstand the different amounts of thermal expansion. For example, the CTE is 17ppm/˚C for stainless steel and 6ppm/˚C for glass. Depending on the size of the assembly and the temperature, there may be significant stress imposed on the bondline, which the adhesive will need to cope with. Although the priority is to choose


an adhesive that will effectively bond together the required substrates, design engineers must also think about the requirements of the production environment. For example, an adhesive that takes a long time to cure may not be suitable for a high


/ DESIGNSOLUTIONS


production volume industry. If an adhesive is to be applied to a


vertical surface, the more viscous the better. Whereas if it is for filling in gaps, a runnier adhesive is more suitable. Some adhesives are viscous under normal conditions but become runny when shaken, agitated or otherwise stressed – known as thixotropy. Highly thixotropic adhesives can make dispensing easier, but a combination of viscosity and thixotropy should inform adhesive choice. Adhesives are commonly specified


during product development, often based on functional requirements. When production is upscaled, the chosen adhesive can be less than optimal – application and curing may be difficult, unreliable, challenging to automate, or take too much time. The intended production quantities for the product should therefore influence adhesive choice.


PRIORITISE PROPERTIES It’s rarely possible to find an adhesive that ticks all boxes. To make it easier to prioritise the properties needed, do not request more strength or heat resistance than is really needed, because this may exclude adhesives that are equally optimal, at a lower cost and with simpler production capabilities. Ideally, the substrate fails before the adhesive, so a low strength substrate does not need a high strength adhesive.


Intertronics www.intertronics.co.uk


DESIGN SOLUTIONS | NOVEMBER 2018 11


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