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Background

I have always assumed (or heard) that adhesives require a minimum thickness of adhesive to function properly. One example of this is not clamping two pieces of wood together so tightly that all of the glue squeezes out.

This assumption seems to be backed up by the following articles on minimum adhesive bond line:

  • From MasterBond:

    Bond line thickness is one of the important factors to be considered in designing a bond joint. Most commonly used bond line thicknesses in a bond joint range from about 0.002 to 0.007 inches.

  • From Epotek:

    Minimum bond line thickness often receives special attention in part specifications but, is often overlooked and seldom controlled or monitored during process and manufacturing engineering. The risks of inadequate bond line could be low strength...

Why this might not be the case

I ask about this because recently a machine shop that was fabricating a part for my design asked about adding adhesive to the connection of two components where no adhesive was specified.

The part is a press (interference) fit between two steel components.

The machine shop was offering to use a product that specifically states that it is used in interference fit situations. The idea of a minimum bond line seems to directly contradict the use of an adhesive in an interference fit. This is a white paper from the supplier that also talks about the use:

Retaining compounds are used ... for interference fitted joints: • added to existing interference fit designs to increase joint strength and reliability; ...

So which is it?

  • Do adhesives have a minimum thickness (bond line)?
  • Do adhesives in an interference fit have a use?
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  • $\begingroup$ I have a feeling that this is very material-dependent. Not at all familiar with interference fit adhesives, though. $\endgroup$
    – grfrazee
    Feb 12, 2016 at 22:36

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Many adhesives work best when you have the minimum thickness of adhesive you can get while still effectively wetting the joint.

This is particularly the case for unfilled resin based adhesives which tend to be somewhat brittle. In this case a thinner bond thickness results in a more flexible joint with less risk of cracking. However different adhesives do vary widely in their cap filling performance. In this case the minimum will often be specified to ensure that the joint is properly wetted and the maximum is the point at which mechanical strength starts to suffer.

Adhesives are applicable to certain types of interference fit, particularly in fitting bearings. Often this will be used as an alternative to a very tight pressed or shrunk fit. Similarly thread locking compounds for threaded fasteners fall into the same general category and are generally used to provide a seal and resist vibration rather than carry the principal load.

Surface finish and porosity will also be important as this will cause local variations in the gap between abutting surfaces.

There is also the consideration that some applications rely on capillary action to completely fill a joint, in which case gap thickness is critical.

The strength of adhesive joints is usually also strongly dependant on the direction of load and here it is also consider the mechanical strength of the adhesive itself compared to the adhesion strength to the substrate. For example adhesive joints to wod will often fail by tearing off the wood surface before the adhesive itself fails in which case a thicker bond line would tend to be preferable to ensure good wetting and penetration. In metal to metal joints it is more commons for the adhesive itself to fail so a thinner line may help to minimise strains (especially in the 'peel' mode).

Having said all of that it is hard to generalise too much as 'adhesive' covers a huge range of materials from low viscosity cyanoacrylates to reinforced resins to rubber based materials.

Often adhesives intended for larger gasps will have a higher viscosity, lower hardness and will often be a liquid filled with a powder of fiber reinforcement (JB weld is a good example). By extension consider materials like epoxy putty or fibreglass (GRP) where the material itself can form adhesive bonds to other surfaces but there isn't really a meaningful bond line thickness as such.

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Polymers Physical Properties states "Joints made with smaller bond lines are epoxy-starved and have lower strengths."

I am not a chemist, but there is some physical amount of space required for the polymerization process. It may be that in confined spaces the polymer chains are more parallel to the bond line instead of randomized (might be a good question for Chemistry SE). In my internet research many companies state that they use a filler with a constant particle size to control the minimum bond thickness (glass micro beads in your masterbond link).

Interference fit adhesives such as thread locking compounds dont have the bond strengths that other "minimum bond line thickness" epoxies and adhesives do and they are not exposed to tension, only shear. Perhaps they too would be stronger at a specific thickness, but in this case bond strength is less important than increasing the friction between the two tightly mechanically fit components. Natures thread lock, rust, does a pretty good job of it.

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  • $\begingroup$ I like thinking about rust as "nature's thread lock" $\endgroup$
    – hazzey
    Feb 18, 2016 at 12:11
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It will depend on the type of glue you will use

Super glue (cyanoacrylate adhesives) requires very thin bond line and their final strength weakness as the bond line thickness increases.

2K Adhesive is quite the opposite as they are a gap filling material and if the bond line is smaller than a certain value (depending on the one you use) the final strength decreases and off-course it has a maximum bond line thickness

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