Troubleshooting Bowing in Engineered Flooring Layups

Veneer balancing issues, as well as glue characteristics, could explain why laminated engineered wood flooring panels are curling badly. December 29, 2008

We have been pressing engineered flooring up using a 9 ply Baltic birch core with a 3/16" sawn white oak face and a 3/16" sawn poplar back and have gotten some very bowed panels. We have taken the 9 ply and run the face grain 90 degrees to the face veneers, making 10' long panels about 8" wide. We are using EPI glue in a vac press. We have done this before using an oak face and oak backer and the panels seem to be very flat. We thought we should be able to use a poplar backer instead, but now we are getting very bowed panels. They are flat in the width but have a big bow from end to end. Any ideas? Moisture is around 7-8% on both woods. Is the vac press not enough pressure? The panels are flat in the press, but bow up when taken out.

Forum Responses
(Veneer Forum)
From contributor C:
Rule #1 in Veneer Club: Same material, same thickness on both sides. Sometimes you can cheat, but certainly not on thin material. Sounds like you knew better but took the chance. W. oak is the toughest. You could get away with a hickory backer, maybe even red oak, but why bother?

From contributor F:
Balanced panel, balanced panel... balanced panel!

From John Van Brussel, forum technical advisor:
Is EPI glue a PVA?

From contributor F:
Emulsion Polymer Isocyanate

From the original questioner:
EPI glue we are using is Franklin EP925, and I do believe it is a form of PVA, but has an acid catalyst, not like any other glue we have used. It looks like white PVA glue and after adding catalyst it foams and becomes a thick whipped consistency. Does not spread too bad but has a very short open time from what we found. The only reason we are trying it is the fact it is for engineered flooring, very water resistant and supposed to have very little water in it and no formaldehyde. Also short press time of 45 minutes to 1 hour and works in 45 degree on up temps, but we have found that in a cold press or vac, it has about a 2-3 hour set at 60-70 degrees and short open times.

I think we may have concluded the problem. We pressed 1 side of the veneer first, then pressed the back on another press. Seems to stay very stable when pressed together at once.

Also, anyone engineering flooring with thick sawn veneers? What glues are you using? We have used urea glue, but with colder temps the set time is long. Also the EPI is supposed to have some elasticity to the bond. Seems to be very rigid to us, and urea is supposed to be brittle compared to EPI for engineered flooring? Any remarks? Any solution for speeding up set time with urea in a cold press?

From John Van Brussel, forum technical advisor:
I looked up EPI and the reason you are getting bowed panels is because EPI is thermoplastic. It does not dry rigid and the wood can pull it where it wants basically. I would also check the UF content of the EPI because it has been explained to me that wood itself contains urea formaldehyde. Urea glue would not allow the spring back you are experiencing with your current glue and also has a higher water resistance.

From the original questioner:
I agree. We have used urea in the past with good results, and were lured to the dark side by tales of fast set time and lower use temp point. Kind of a real pain to use EPI and clean up. You live you learn. Any suggestions on how to get reasonable set times with urea in a cold press? We have used electric blankets with our vac press, but for production we use a cold press. Would love a nice hot press, but we do a lot of 10-15' long pieces and do not have the budget yet for one. We keep the press room at about 65-70 degrees.

From contributor F:
You need to talk with Jeff Pitcher about the cure time of urea. As the temp goes up, the cure time drops like a meteorite. For engineered flooring the moisture resistance of urea would have benefits on the sales end as well.

From contributor E:
The flooring industry is moving toward using a Poly Urethane Reactive Hot Melt adhesive for engineered flooring. The advantages of the PUR are as follows:
1) It is a solid adhesive with no water to transfer.
2) It is interior/exterior and will not let water break the bond.
3) It is applied through a roll coater and the panel composition is pressed through a multi-section pinch roll.
4) Fast process.

If you're a small producer, though, you may find the cost of the adhesive equipment a bit high.