How Important Is Bond Line Thickness?
Mind the gap!
Occasionally, we are asked why the glue line thickness is so important.
Glue line thickness? Let me explain……when gluing two parts together, a film of glue coats both parts, thus bonding them together. The thickness of this glue film, is referred to as the glue line thickness.
How important is glue line thickness? Take this example, all two part structural glues need to attain certain cure schedule for to achieve optimum performance. Within this cure schedule, we have a working time and a fixture time. The working time should never be exceeded and the fixture time must never be reduced. The lesser known but equally as important factor is the glue line thickness. Within the Scigrip range of structural glues, both minimum and maximum suggested glue line thickness. These range from 0.5 mm to a maximum of 2.0 mm for Scigrip SG5000 to 0.5 mm to 9.5 mm for Scigrip SG300. Unfortunately, maintaining the glue line thickness is often overlooked and very seldom monitored, occasionally leading to manufacturing issues.
What happens if glue line thickness is under of over what is suggested? If under, the glue will cure to a rubbery or semi-hard consistency. If the bond line thickness is over that suggested, you may not get ‘thru cure’, in other words, the glue may not cure right through. In both instances, the bond strength will be defective.
How can you ensure that the correct glue line thickness is achieved? Both in our laboratories and during customer line trials, our technicians always use a shim. This can be in the form or a thin plastic or metal spacer, or indeed a tab of one of our structural tapes. Using the tab of structural tape gives you an added benefit, it helps keep the parts together during that all important fixture time.
There are several hard rules which accompany every type of industrial glue. Keeping the glue line gap, is one of the key factors which bonds your materials, permanently!