Help! I’ve got air in the bond line!

The troubleshooting process begins with a thorough knowledge of the processes leading up to, and including, the application of the adhesive.

There are many possible causes for failure of bonded components. Before suspecting air in the bond line as the source of failure, double check the adhesive selection, the application process and the working environment. Make any necessary corrections.

A careful examination of the bond areas may provide clues to the cause and possible solution. When using adhesives or substrates that are translucent, trapped air pockets can be easily found by visual inspection of parts.

The difficulty of determining if air in the bond line is the source of the bond (or seal) failure can be difficult when the substrates are opaque.

Using your knowledge of the entire process, determine where the air is coming from. The four primary causes of air in a bond line are:

* Insufficient Adhesive
* Dispensing
* Entrapment
* Separation


Insufficient Adhesive

* Shrinkage— Most adhesives shrink to some degree. Generally the adhesive will shrink in a direction approaching the cure mechanism. For example: the adhesive closest to the UV light source will cure first and adhesive furthest from the light will be pulled toward the cure. Examine the failed parts for evidence of shrinkage, and adjust the amount of adhesive or cure process accordingly.
* Flow—The adhesive may slump or flow out of the desired location before cure. If the product is cured with heat, the heat can reduce the viscosity causing flow prior to the onset of curing. Examine the failed parts for evidence of inconsistent flow. Implement process controls to manage variables of positioning parts and applying heat to cure. Or switch to a heat cure epoxy that is designed not to flow.
* Static displacement – Some manufacturing lines can produce sufficient static to cause the adhesive to defy gravity and leap away from the intended area. This is rare but can be seen on large plastic components. Obtain recommendations for reducing or eliminating static from engineering.

Storage, Handling & Dispensing
Avoid introducing air into the adhesive system using this checklist:

* Packaging, shipping – store product in upright position and avoid shaking.
* Loading the dispenser – depending on the product viscosity, pour adhesive slowly into vessel, or use a pressure pot that is sized to accept the adhesive package.
* If using an air pressure/ time dispenser, pressure should be minimized to avoid bubbling air through the adhesive.
* If using a suck back valve, minimize the return so the dispense process begins with adhesive, (not air).

Entrapment

* If the dispense pattern overlaps, an air pocket may form under the second layer of adhesive. Modify the dispense pattern to avoid overlapping layers of adhesive.
* For large surfaces areas, dispense in a serpentine manner. Avoid dispensing circles of adhesive as it will trap air in the center.
* Consider adding an air release path into the joint design, such as a tapered tongue in a tongue and groove joint.
* Assembly technique for flat surface to surface bonding is key to success. Two flat components should be mated in the way a book is closed, rather than stacking one component on top of the other part.

Separation

If, upon close examination the adhesive appears striated, it is likely that separation prior to cure has occurred. This can happen when two pieces are assembled under pressure. When the pressure is removed before achieving full cure, the adhesive which was squeezed thin may cling to each piece but no longer fill the space between the two.

In summary, it can be seen that there are a number of possible sources of air in a bond line. The solution may be as simple as changing a dispense pattern to avoid entrapment, or modifying the assembly process. The manufacturer of dispensing equipment, or the adhesive manufacturer is qualified to assist in troubleshooting.