Radio-Frequency Glue-Up Failures with Hardwood

A door manufacturer gets advice on troubleshooting adhesion failures in an RF glueing process (that occurs only with certain wood species). March 26, 2010

We are an interior door manufacturer of primarily poplar doors, but have recently seen an influx of whole house jobs using red oak, maple, ash, etc. Our door construction consists of a two-piece laminated door stile from 4/4 lumber. Given the volume, our process of laminating these stiles using I-beams and clamps is no longer efficient enough, so we have been trying to use our radio frequency gluer for laminating door stiles. We have no problems gluing poplar stiles, but have been very unsuccessful gluing up any other species, with upwards of a 50% failure rate. We can glue up 4/4 billets for panels and 8/4 billets for parts, but not stiles.

The machine company has been working with us to try to figure out our problem, as has the adhesive company, to no avail. As far as we can tell, we are following all the recommended specs for laminating door stiles with our RF. Surface prep is the same with any species, cook times and pressure are adjusted accordingly, etc. We do water drop tests with each batch, check moisture content, note building humidity and temp, and anything else we can think of.

Short of sending test pieces to whoever, which we are ready to do, is there something we are missing when it comes to gluing what amounts to 21/4 or 23/4 lumber versus 4/4 or 8/4? It's getting really expensive for us to glue up twice as much as we should have to, expecting a 50% failure rate.

Forum Responses
(Adhesive Forum)
From contributor K:
I will try to help you with this problem but need additional information. What type of adhesive are you using? Is it a pre-catalyzed cross linking PVA? How is the RF field being applied to the load? Is it what we call a parallel application, where the glue lines are parallel to the RF field? Or is it a perpendicular application where the glue lines are perpendicular to the field? Also, you give no indication of the characteristics of the failures you are experiencing. Is the glue line wet after the RF cycle? Is the failure in the glue line, or are you getting some wood failure? Is the adhesive transferring from the spread surface to the unspread surface? Are you seeing any arcing? The denser hardwoods would probably take more time to heat up if you have to heat up the entire load, as in a perpendicular application. They will also be significantly more difficult to wet out. If the adhesive is drying out too fast during assembly, or is much too high in viscosity, insufficient wetting could result on the denser hardwoods.

From the original questioner:
Thank you. We are using Advantage 310. I am also looking into changing adhesives that would meet our needs closer. The RF field is applied perpendicular to the load. From what I know, while we are reaching the upper end of our machine's capabilities, we should still be capable of face gluing two pieces of 21/4 blanks. The failures are separation between the two stile parts that is not evident when the stiles come out of the machine. We have cut the bad stiles and split them back in half to try to figure out what is going on. It appears the material is cupping after glue up, and the edges are separating. The wood fibers are bonding in the middle, but not at the edges. We have gone as far as wetting both pieces, but with no success. The glue is tacky afterwards, but not what I would consider wet. We have not measured MC before and after, but that is our next step. We are not seeing any arcing. Franklin and L&L are working with us to resolve this issue, as it is completely hit and miss. About a month ago, we glued up 6 1/4 hard maple with 100% success. We measure the MC of all of our lumber before it comes off a truck.

From contributor K:

You indicated that the bonds seem to be tight in the center of the construction, which makes sense, because the RF will cure from the center outward. However you said that the edges are still tacky and unbonded, which indicates that the glue is not fully cured. This can be the result of too short a cycle, too high moisture content of the wood, or too little RF energy getting applied from the generator. High MC could result in a demand for more total RF energy, so with the same cycle you would get undercure of the adhesive, especially toward the outside of the glue lines. This theory would also explain the cupping of the wood. Have you tried significant increases in the cycle? Even if you can get the adhesive cured, high MC can manifest itself in twisting and instability of the final stile.

From Professor Gene Wengert, forum technical advisor:
RF machines are very unforgiving, especially with respect to non-uniform pressure and non-uniform MC. They also do not always have a uniform RF field.

Oak is quite unforgiving. It is dense and it moves a lot with small moisture changes. In my experience, you will have trouble with such thick oak material in an RF gluer. However, 50% is certainly excessive, so something is out of control.

The warping afterwards can be because of the moisture in the adhesive. If the wood were wet, we would see cupping in several directions and the individual pieces would warp overnight in the plant before gluing if they were wrong. I think you are saying that cupping is always the same, so it is water in the adhesive. However, the cure time in the press is supposed to develop enough strength in the joint to resist this effect.

Although you indicate that you want to send the failed pieces to someone, I believe you need someone to visit your operation and analyze all the factors. Oftentimes, the gluing people or the press people do not look at the entire operation and all the potential factors, but rather focus in on their areas of expertise. In my experience, you have two or more factors that are causing the problem, with each one by itself not being too serious.

Here is what I suggest you do in the meantime. (Do not change two variables at once.) Machine some of the pieces within 15 minutes of gluing, giving you a fresh, true surface. See if they behave the same as your normal production that is RF glued at the same time. Next, sort some of your incoming material into two groups, very dry and not so dry. Then see if the problem occurs with both groups. Next, reduce the glue spread with oak by 30% or so. Run some pieces in the RF gluer without any glue. Find out if the heat alone is causing moisture in the wood to move and warp pieces. Next, clamp up some of the same pieces that you are RF gluing, but do not RF glue the clamped pieces. If the clamps work, I would become real suspicious of the pressure in the RF machine, perhaps because the individual strips are not uniform in size. (This is my best guess - of all the items - as to why you are having problems.)

Note: Avoid adding moisture to the plant's environment. Moisture will keep the joints from opening perhaps, and will hide the fact that you have a weak joint (until it gets into the field).

I do believe that a proper consultant can solve your problem quickly, but if you run the tests above before he arrives, that might make his visit unnecessary. (I may have talked myself out of a job!) Feel free to post any results you obtain.
Finally, you indicate that you measure the MC of the wood. How to you measure (meter brand and style) and what are the values?

From contributor J:
We use Multi-bond 2000 and have a 99 percent success rate on our glue joints, and we glue everything from poplar to oak to cherry. Room temp has a big play in the matter, as well as setup time. Is your RF set up to let you know when the glue is cured? Does your machine pull enough ks to dry the glue? Your harder woods need a rougher finish to allow the glue to penetrate deep enough in the wood to get a good bond.