Glue Line Failure with Hickory

Trouble shooting glue line failures when gluing hickory - 1999

Q. I would like some tips on edge gluing hickory. We us a Taylor clamp carrier with approximately 200 psi of pressure on each clamp. The glue we use at this time is Franklin Titebond 50. We are having problems with glue line failure.

A. There a number of possible factors that could be contributing to your glue line failures. Some things to consider when troubleshooting include temperature, joint preparation, pressure consistency, and moisture content.

During the winter months temperature becomes a major concern when working with PVAs (Polyvinyl Acetates). Most standard PVAs have a minimum use temperature of around 40 degrees F. Some can be as high as 50 degrees F and many cross-linking PVAs have minimum use temperatures closer to 60 degrees F. If the wood you are using is stored outside, it may take a while before it rises to the right temperature. Often, the wood on the inside of a stack will take weeks to reach room temperature after being stored outside. It is important that all components of the gluing process are above the minimum use temperature; this includes the wood, glue, ambient temperature and gluing equipment. If this isn't the case, the result is often an effect called "chalking". A chalked glue line has a white chalky appearance and has no strength. At this time of year you should check this first.

Joint preparation and pressure application are equally important factors. By examining your failed glue lines you can tell much about how well a joint is prepared and how well pressure has been applied. Areas of glue thickness in the glue line indicate poor joint preparation. If there are areas where there is no glue it could be a problem of "wetting". To avoid burnishing be sure your blades are sharp and true whether cutting or planing. Also, it's a good idea to use freshly machined stock (within the last 24 hours). Failure at one end of a board or on one side of a joint indicates inconsistent pressure. This is often a result of the clamps being too tight at one end and not tight enough at the other.

Finally, open joints at the ends of panels can be the result of moisture content problems. Especially in the winter months, if the moisture content of your wood exceeds 8-10% you might have problems after the panel is glued up and stored in a heated room where the moisture content of the air is very low. The panels tend to loose the excess moisture out the ends, which results in shrinkage and failed glue joints.

If, after investigating these points, you are still experiencing problems you should contact your adhesive supplier for more technical assistance or send me samples of the failed parts for evaluation. Samples can be sent to Custom-Pak Adhesives, 11047 Lambs Lane, Newark, Ohio 43055. Be sure to include your company name and a short description of your gluing process and equipment.

Jeff Pitcher is Marketing Director for Custom-Pak Adhesives in Newark, Ohio.