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FEATURE
FASTENINGS & ADHESIVES CONSIDERING ADHESIVE PRO
Although the primary consideration when specify an adhesive is its functionality, thinking about the processability of an adhesive at the design stage will save time, money and effort.
Kevin Brownsill, head of technical: learning and development at Intertronics, comments
the material will work in production? An adhesive’s processability covers everything
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from how it is removed from its packaging, to mixing, dispensing and curing it. Understanding the processability of a material for your application involves weighing up production requirements, such as speed and scale, against the adhesive’s properties. If a material hits all of the technical criteria, but can’t be repeatably and accurately applied into a final part at a speed the production team are happy with, it’s probably not a good fit. Similarly, if the process can be perfected by hand during the R&D stage, but not easily replicated by automation, it is unlikely to work in large scale production.
UNDERSTANDING THE PROJECT Design engineers typically start the specification process by assessing the technical characteristics of a material as explained on its datasheet. But, data sheets are not exhaustive – knowledge, experience and tests are needed to establish if the data can be extrapolated for your unique application. A good adhesives supplier will understand the full complexity of the project, and make recommendations for materials and dispensing technology for you to evaluate based on all of the relevant factors. In many cases, there will be multiple materials
that offer the required functionality. We recommend trialling more than one material – you may find that two adhesives give similar technical performance, but one offers significant process or implementation ROI benefits.
DISPENSING TECHNOLOGIES Adhesives are supplied in everything from small syringes or vials containing a few
millilitres to large 220 litre drums and beyond. If the material you select is only available in 10kg batches, but you require a 1nl shot, you may wish to consider an alternative. Following this, you can then think about how to
apply it. This involves trialling possible dispensing methodologies, which can be anything from simple time pressure dispensing up to fully automated volumetric dispensing, depending on the repeatability and accuracy requirements. A material’s viscosity often impacts its
suitability for an application, both technically and in terms of the process. Adhesives range from a water-like viscosity (e.g. a cyanoacrylate adhesive), to very thick pastes (e.g. a thermally conductive silicone). Each extreme can be difficult to process for
different reasons. While a low viscosity material may cause problems with dripping, a high viscosity material may be a challenge to remove from the packaging or apply to the part. For example, if you need a material to flow into certain areas and its high viscosity means this can only be achieved with coaxing by hand, the process is unlikely to be scalable into high volume production. Your adhesive supplier can talk you through which materials will be easy to dispense for your application based on their physical properties.
PROCESSING STEPS As part of an adhesive project, there are numerous processing steps that may be required, such as thawing, decanting, warming and mixing.
28 DESIGN SOLUTIONS NOVEMBER 2023
Typically, the fewer steps involved in the process the better. If a material requires a great deal of processing steps, an alternative that has similar technical properties but fewer processing requirements may be available. Single part adhesives are easier to dispense and handle than two-part ones, which must be mixed; while UV curing adhesives enable a fast cure, no-mix process, that improves efficiency and overall economy. Two-part materials – such as epoxies,
polyurethanes, silicones and methacrylates – begin to gel and start to change in viscosity immediately upon mixing. An understanding of how a material changes over time can dictate how it is used. With two-part materials, for example, a metering and mixing system can be employed at the point of dispense. Another consideration is working life,
the amount of time a mixed material remains low enough in viscosity so that it can still be readily applied with the appropriate accuracy and tolerance required in your application, and whether the working life suits your process. In some cases, you may need to modify dispensing technology to accommodate. For example, if you are dispensing with a two-part epoxy that cures in five to ten minutes, you may wish to use a dispensing system with an automatic purge, to keep fresh material in the nozzle. Another consideration is cure, which can range
from seconds to hours, or even days in the case of slow-curing epoxies and silicones. A long cure regime means parts may need to be set
ften, functionality is the first aspect that is thought about when specifying an adhesive, but what about its processability – or how
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