How to avoid bond strength variations due to operator influence? Even in highly automated processes, operators can still influence the success and effectiveness of the operation. However clear and well-written an instruction manual may be, there will always be room for variation wherever humans are involved. It is important to find ways for various operators to be on the same page. The following looks at two examples where subtle variations in technique by the operators caused variations in the outcome. And ways that the problem was overcome.
From one shift to another, a customer was experiencing bond strength variations with a product that utilized surface-activated acrylic adhesives. The bond strength was well within the quality control limits the manufacturer had set. But the variation was still large enough to cause concern.
The process called for a wipe of activator on one piece and the application of a serpentine bead of adhesive with a time pressure dispenser. As the amount of adhesive was automated, it was thought to be a fail-safe method, and attention turned instead to the activator wipe. The instructions simply said to wipe the part with the activator, leaving a thin film, though ‘thin’ was not quantified.
The examination of parts destroyed in previously conducted quality-control testing determined that the amount of activator was not the factor influencing the variance in bond strength.
Upon further investigation, they found that the adhesive dispensing pattern on the parts with the higher bond strength was significantly different and that the ones with the lower bond strength had sharper curves, which allowed for air entrapment.
Solution — Adding an image of what appropriate and inappropriate serpentine adhesive patterns should look like made it easier for operators to know what they were looking for in terms of spray pattern, and greatly reduced variation.
A customer was setting up an incoming quality control procedure to verify the set time of an adhesive. Two quality control technicians ran the tests side by side, though one was getting much faster results than the other.
The process called for dispensing a bead of part B over a bead of part A on steel lap shears. Then assemble the lap shears and test for fixture time after a set amount of time.
To troubleshoot the difference, they ran the tests together watching the other’s process. They learned that upon lap shear assembly, one technician was moving the laps back and forth. While the other technician applied the top lap shear as you would a coverslip on a slide. The movement created mixing of the adhesive, which caused a faster fixture time.
Solution — Including a detailed diagram along with the written instructions in the operator’s manual. This helped to clarify procedures and reduce the operator differential.
It seems that the best kind of training is simple and clear instructions and a practical demonstration of the task. Detailed diagrams are also always a fantastic resource for workers. They can refer to them if they are unsure about best practices.
For further help and advice, please contact Permabond.