Visible Bond Lines

Question:

We are a smaller manufacturing company that makes high-end loudspeakers. The enclosures are made mostly from phenolic based paper composites and high density wood composites. Our problem is seeing the glue seam in our high-gloss urethane painted finish. Generally, the seam is very slight and can only be seen at certain angles. However, sometimes when the paint is wet sanded and polished, the seam can be felt on the surface. As the surface cools, the seam shrinks back. Our current theories on the cause of this visible seam are as follows:

• Insufficient cure of glue before paint is applied (causing a high amount of outgassing).
• Too great of a difference in the thermal coefficient of expansion between the glue and the enclosure material.
• Insufficient glue in the seam, creating an air pocket under the paint.
• Using an adhesive that is not solvent resistant and is thus attacked by the solvents in the paint, softening the glue, adding more solvents to the seam area, ultimately causing the paint to swell under the bond line due to the solvents trying to evaporate.

So far we have found the best results when we use a high strength, solvent resistant, low outgassing, epoxy adhesive. We give the glue five days to cure and then we are gel coating all of the enclosures to provide a barrier between the adhesive and the paint. Are you familiar with this problem and do you have any suggestions that may improve or simplify the steps necessary to eliminate these bond lines from appearing in the painted product? Thank you for your help. K.K.

Answer:

Yes, I am familiar with the problem. The problem you described can not only happen in wood products but in metal and composite products as well. Any time an adhesive joint or fillers are used in the seam of butt joints, the “line” could be visible. Unfortunately, it is exacerbated by all the reasons mentioned in your question. Most often the problem is solved by a lot of cosmetic treatment of the affected area. This involves a lot of hand labor by skilled workers.

In this case, you described the seam as very slight. This could be a blessing because any 100% solids liquid should work well. You also said your best solution involves the use of an epoxy adhesive. However, the 5-day curing time is unacceptable for your production schedule. You can speed up the cure of the epoxy adhesive by heating. This curing of the adhesive must be done before painting. Using the “10 deg” rule, the cure time is cut in half for each increase of 10 deg centigrade (that’s Celsius for those living in Dunbar, PA) in cure temperature. Since I don’t know the upper temperature limit of your product, I won’t suggest what temperature to use. You will have to do some experimenting to establish the right temperature.