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Design with Adhesives in Mind

Review your challenge with the adhesive manufacturer.  Due to the uniqueness of each application, there are no adhesive design standards or safety margin limits to aid designers working with adhesives.  Very often there are multiple factors which can affect a bond such as chemical exposure, stress or load, heat or time.  There are no calculations to prove that the adhesive will tolerate the conditions.  Simple life estimation plots are generally inaccurate for adhesive purposes due to the large number of affecting factors that build upon each other.

To effectively design with adhesives, a designer considers the individual requirements and the effects those requirements have upon each other.  For example, an adhesive may have good peel strength on the substrates used, resist from -30°C (-20°F) to 90°C (200°F) and resist a 10% chemical solution.  But will it do all of these things at the same time?  And how fast will the assembly transition from -30°C to 90°C be?  Resisting a temperature continually is vastly different from the thermal shock of going from -30°C to 90°C instantly!  And thermal shock effects vary depending on the coefficients of thermal expansion of the substrates.  Some factors create a negative effect and others may not.  As such, do not expect the adhesive manufacturer to sanction your application as foolproof and infallible but do expect some guidance on which factors and conditions should be tested and considered.

Additionally, keep the following adhesive design points in mind:

Types of Stresses
Adhesives work best in compression, shear and tensile.  Many resist impact and vibration very well.  The most challenging stress for most adhesives is peel or cleavage.  To prove this to yourself, put a post-it note on a piece of paper and try to pull it off the paper in shear stress, then try to peel it off.  Keep that demo in mind when designing…avoid peel stress!

Surface Area
Consider also that strength is directly related to surface area.  If you can design a tongue and groove joint, not only do you provide a good surface to dispense the adhesive into but you also multiple your bond area thus increasing strength and resistance to environmental factors.  Similarly, a rough surface has more “area” than a smooth surface.

Gap
Unlike solvents, adhesives take up space.  Gap -filling adhesives provide savings by reducing the need for accurate machining tolerances.  Check the optimum gap -filling capability of the adhesive to ensure a good bond.

For further help and advice with adhesive design, please contact Permabond.