Heated Press Clamping Time
Pros and experts puzzle over a delamination problem with panels veneered on a heated press. October 4, 2005
A customer uses our PVAc adhesive to laminate wood on a heated press. They usually run at about 10 minute close time at 80°C, however, they need to increase the output, and are aiming at a 7 minute close time at 90-100°C. They now get occasional end delamination. Are there any suggestions as to the cause and the resolution of this problem? I suspect that the thermoplastic nature of the adhesive could be a problem coming out of a hot press, but would this be a problem if the wood was under no dimensional stresses? Would a thermoset adhesive be best for such an application?
From contributor J:
Press time is a factor of veneer thickness and glue type. How thick is the veneer? Is it a two ply? Is it flat panel pressing or curved?
1. How fast are the panels cooling?
2. Are they being stickered and fanned when brought out of the press?
3. What is the mil thickness spread of the adhesive that is being put down on core? How does this compare to manufacturer specs? What are adhesive manufacturer specs on time and temp at X thickness plus X time for additional thickness?
Please provide a bit more info on the panel and we can try to narrow down the most likely culprit.
From Professor Gene Wengert:
I suspect that you will find that the end delamination is caused by the ends heating up, moisture flowing to the outside, and the end then being drier than the rest of the panel. The heat and low moisture at the end leads to a poor glue joint. This problem gets worse as higher temperatures are used. Why don't they cold laminate, using a stack of many panels at a time? This makes more sense to me.
From Jeff Pitcher, forum technical advisor:
Both contributor J and Dr. Wengert make some good points. I might add that the use of a urea resin at a higher temperature may solve the problem and get them to the time they're looking for. The UF resin will be nailed coming out of the press and won't allow the movement that a still soft PVA glue line could. This said, if the moisture content is correct, there shouldn't be enough stress at the ends to cause movement.
From the original questioner:
Thanks for all the useful input. A UF resin may be the solution, however we are confined to using a PVAc based adhesive. I suspect that Gene is correct about moisture loss from the ends causing the problem. I have asked our customer to see if they can change the settings to have higher temperature at the centre, and lower temperature at the ends. This said, when they press for 10 minutes, they have no problem, but reducing time to 7 minutes results in delamination, so perhaps the moisture loss theory is not correct. Unfortunately, the machine supplier is unable to advise as the machine is being run beyond the factory settings! Cannot use cold pressing, as the machine is already a production bottleneck, so they need the heat. On this note, whilst the press is set at 90-100°C, the surface of the wood laminate is approximately 60°C coming out of the press, and only about 45°C in the joint. The laminates are air cooled before further processing. Laminates are 3/4 inch thick, and approximately 3" wide, and are butt laminated. Possibly the only solution is a PVAc adhesive with sufficiently high solids of say 55-60%.