The adhesives ripple effect

When building a new adhesive process or amending an existing one, it pays not to make assumptions. Any change to a process, from the substrate or adhesive to the processing method or packaging, can trigger a ripple effect that could mean production equipment is no longer suitable or compatible, or the finished article no longer passes the final design specification. Often drop-in replacements are not as simple as first expected and can lead to a series of knock-on effects. 

When an adhesive bond starts to underperform, or fails to meet QA procedures, the most common first reaction is to blame the adhesive. Is there a batch problem or another material fault? In our experience, changes in process are actually the more likely culprit. Process variables and their control play a vital role in the overall success and performance of an adhesive, and even aspects perceived as seemingly minor can have a much greater impact than initially thought.

For example, recently we supported a customer who had inherited a process from their parent company. The process involved filling a void with an optically clear silicone elastomer to prevent moisture ingress within the cavity. However, upon cure the optically clear elastomer appeared to be crackled and didn’t meet the optical requirements. Although there was no visible source of contamination within their process, and the handling, mixing and dispensing process looked within specification, we investigated a few different production elements, changing one at a time and then retesting.

In a pre-potting cleaning stage, the customer used a cloth, which was professionally cleaned and dried after each use. Nevertheless, changing this for a single use isopropyl alcohol (IPA) wipe solved the problem, as on the next batch it became apparent that the cloth must have been the source for enough contamination to interfere with the silicone cure. All subsequent batches have cured with the desired optical clarity with no recurrence of the crackled finish. 

Another example comes from a customer who was bonding aluminium substrates that had been cleaned with acetone. Optimal adhesion will normally require a substrate that is clean and free from contamination. Acetone is a highly effective solvent that can dissolve impurities and contamination on a substrate. These organic contaminants become suspended in the acetone and have the potential to remain on the substrate if not removed before the acetone evaporates, leaving a film of contamination behind.

In this instance, the very fast evaporation rate of the acetone allowed some dissolved contamination to remain on the aluminium, despite its high solvency power. This compromised the bond strength. We suggested a change to isopropyl alcohol (IPA) as a cleaning agent, with its slower evaporation rate, which significantly improved the structural strength of the assembly with no other process changes.

Preventing the ripple effect

Best practice suggests that when developing a new product, consider adhesive selection and its associated processes as early as you can, to avoid having to make changes much later on (when it can be much more expensive). At the prototyping stage, it’s important to use the actual substrates to qualify the adhesive process. A change of substrates can require a new adhesive, curing process, and/or an alternative bondline geometry. We recently supported a customer bonding plastic LEDs to a plastic housing using a UV curing adhesive. The company later developed a second version of the product, which involved bonding the plastic LEDs to anodised aluminium instead. The new design did not pass the required QA testing, and a new adhesive had to be tested and validated late in the day.

Sometimes design engineers will prototype their new product using an adhesive available in the lab but have to change it later as production orientated factors come into play. Designers should be careful about selecting an adhesive on data sheet information alone, as what works on paper does not always work in practice.

Marrying design and production

One common reason we see for changing an adhesive at a late stage in a project is the production engineering requirements. Design engineers can help their production engineering colleagues, as well as avoid the need for costly respecifying, by considering both the functional and process requirements when specifying an adhesive.

We worked with one automotive customer who was using a two-part epoxy cured in an oven to bond parts. Months later, as the production cell was being finalised, the company swapped to a UV material, because the epoxy’s cure time was too long. However, this design bonded with the new adhesive didn’t pass their functional testing, and this meant that a revalidation was required, adding time and cost to the project.

Small changes, big improvements

Thankfully, the flipside is also true – small improvements to a process can make a big difference. Improving productivity is not always about large increases in output or overhauling an entire line, but often about slight increases in yield, reductions in waste, decreases in material usage, or improvements to quality.

With a good understanding of your process, you are well equipped to improve it. First, establish your specific goals, be it improved quality, repeatability or removing bottlenecks. Then, you can identify areas of improvement and devise a step-by-step plan to achieve these objectives. One example is by identifying and removing redundant steps in a process, such as by purchasing a material in a more optimal package size to remove a decanting stage.

Incremental improvement can also include purchasing a preliminary technology that takes you part of the way, later adding more equipment with little to no redundancy. We worked with LED illuminator manufacturer GJD Manufacturing in this way – the customer initially purchased a Fisnar dispensing robot with a pneumatic dispensing valve to improve efficiency, and later retrofitted a preeflow eco-PEN 600 volumetric dispensing pump to achieve higher repeatability and accuracy. These incremental improvements are more manageable for production teams, as well as easier to justify from a return-on-investment perspective.

Instead of starting from scratch with a trial-and-error approach, working with a partner with extensive experience in adhesives applications can help contextualise what’s happening in your process. You can therefore make changes in a planned, controlled way, equipped with understanding of the possible implications.