Radio-Frequency Gluing, Humidity, and Wood Moisture

Humidity in the shop is a factor for RF gluing, but wood moisture content is the critical issue. January 12, 2012

We are considering an RF line gluer for our shop in south Louisiana. We will be gluing alder, oak, mahogany, Spanish cedar. The shop is not climate controlled. Has anyone had experience using one in a high humidity area?

Forum Responses
(Cabinetmaking Forum)
From contributor B:
Why do you think high humidity would matter? You are using heat to set the joint, which should happen as fast as anywhere else. After you set it, it still has to fully dry, at which point the humidity does matter.

From contributor M:
I am concerned that the humidity content of wood, when heated in an RF machine, may affect the RF glue.

From contributor B:
RF glue is just a Titebond II type glue. I hope someone with more knowledge than me pipes up, but I don't think it will be significant. Did you call your glue manufacturer? I buy drums from National Casein. When I call I can get with the guys in the lab to discuss the glue and its properties.

From contributor C:
We used a L & L RF gluer at my former employer to glue all the species you listed and more at thicknesses from 3/4 up to 5 1/2. The humidity in your shop will only matter as it affects the moisture content of the stock being glued. We were in Central Ohio, with shop humidity ranging from less than 10% in the winter to 80%+ in the summer.

Edge prep and machine setup were the two most important factors for us. We used many different glues with the same results. We settled on Titebond 2. One thing we did learn is the various adhesives manufacturers helped us immensely with machine setup and troubleshooting.

From Professor Gene Wengert, Sawin and Drying Forum technical advisor:
The moisture of the lumber is much more important than the RH of the air. You need to make sure that the lumber is properly dried as wet pieces in an RF gluer will tend to arc and ruin the section being glued. This arcing is worse when there are wet and dry pieces in the same panel.

You might need to consider a dry lumber storage room that is able to dry the wood to the correct MC. I would expect that you will see a lot of wood that is a bit too wet, especially Spanish cedar and mahogany.

Of the four species mentioned, oak is the hardest to glue... That is, everything must be perfect or better. The other three are fairly forgiving.

You need to keep the glue cool during the summertime, maybe 75 F in storage. You need to keep the cut strips or staves in a controlled RH storage condition if they are not used right away.

In most of the USA, the outside RH is 65%, which is 12% EMC. So your plant will likely be 9 or 10% EMC. (Our body might say that it is hot and humid, but usually the humidity is much lower than we think.) Be aware that if you ship such high MC wood (10% MC) to the northern interior climates (6% EMC), the wood will dry out in the wintertime and can cause cracking. Again, the correct MC of the wood is critical.

Usually an RF gluer is best when you have a consistent sized product for a long time. A clamp carrier is better when you have a few pieces of this size and a few of another size, etc.

Finally, the RH will affect the glued panel. You need to keep the panel in a fairly dry condition to avoid moisture regain and also allow the moisture in the glue to evaporate from the panel. Depending on what you are making, it might be necessary to dehumidify the shop, especially for the 16 hours it is shut down every day and also weekends.