Our customers regularly ask us for guidance on creating strong glue joint. Here’s a quick guide to help you achieve the perfect joint.
Good glue joints start with the lumber itself. You want your wood to be well seasoned, free from internal stress and dried to a moisture content that corresponds to an EMC (Equilibrium Moisture Content) representative of the destination market. In the US, the equilibrium moisture content of wood averages six to eight percent over the course of the year for 80 to 90 percent of the nation, while the EMC in Europe is closer to 10-12% , whereas wood will stabilize closer to 12-14% in Asia. Being mindful of the EMNC of the destination market is key. Wood glued at too high a moisture content for the EMC of the destination market will dry out after gluing, causing shrinkage of the wood producing stress. The stress that this shrinkage produces causes weakest part of the joint, whether at the glue line or in the wood, to fail.
Because wood tends to equalize to the environment, you want to store the dried lumber in a controlled environment to maintain the proper moisture content and seal or use it promptly. Glue the joint within 24 hours – at most – from the time your machine the lumber and prepare the joint. Quick glue-up will prevent movement of the wood through changes in moisture as well as prevent migration of oils from the wood to the surface which impede adhesive penetration. As a rule of thumb, the higher the moisture content of the wood the longer the cure rate of the adhesive.
Also, before you prepare the edge joint, we recommend that you prepare at least the bottom surface of the wood to ensure the ability to make a 90-degree cut on the joint. If you are using radio frequency to glue the edge joint, you want to prepare both the top and bottom surfaces of the wood.
When gluing, spread a sufficient amount of adhesive on the wood. Typically, a one-sided glue application will get the job done. Look for a small continuous bead of glue squeeze-out along the entire length of the panel and at the ends.
When it comes to clamping, pay close attention to clamp spacing and pressure to ensure that all gluing surfaces are in close contact. Recommended clamp times vary with species, humidity, temperature (of the wood, glue and air), wood moisture content and the type of glue you use; however, most joints will set within 30 minutes and two hours. And be patient: Give the joints plenty of time to cure – preferably, through the night – before working with the pieces.
You can get more information on joint gluing in our new series of videos on joint preparation, found
here. Or, feel free to post any comments or questions you have below, or email them to marketing@franklininternational.com.