CP Harrison
Posts: 11
Joined: 2015-04-08
Location: Afton, VA 22920
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CP
It would be very helpful if you posted these followup comments in the original discussion so they will be found when anyone wants to read all the discussion about pearwood.
Regarding the glueups that failed, you didn't mention how you confirmed that sufficient time had elapsed for the glue to cure, or the starting temperature of the platen. I usually leave the thermostat up to 70 deg overnight, and turn on my heating blanket when I come in to warm the bag and platen further while mixing the adhesive and applying. Additionally, I pour a bit excess adhesive in a zip lock bag and place on top of the bag, as an indicator when the adhesive has reached a rubbery state in curing. If possible, I will wait an additional hour or two however for insurance before opening the bag.
***
Thanks Don,
Allow me to summarize my procedures with the pear wood.
Original Post:
I'm trying to veneer pear wood to a number of panels of which the largest is 25x25 inch MDF substrate with Titebond III.
I'm cold pressing at 19.5. However, every press comes out wrinkled.
The supplier warned me that pear wood is hydroscopic—very prone to picking up moisture. I try and get the materials into the press ASAP, but still no success.
Subsequent Posts replying to questions:
I'm leaving in the press 2-3 hours.
The wrinkles are just in a couple of places.
I used less Titebond III glue the last time and it came out better.
I'm now using a couple of large pieces of ¾" plywood underneath the piece to be veneered as "sponges" to soak up some the ambient moisture, then the ¾" MDF, then the glued veneer, then Kraft paper over, and lastly a (same-sized as veneer) piece of MDF over everything.
(I add to this: This last piece of MDF has its upper corners routed to a 3/8" quarter round to prevent tearing the upper section of the press bag during gluing or I use a larger piece of cardboard over the MDF.)
I try and get everything into the bag and start pressing ASAP. I'm cold pressing at 18.5-19 psi/hg for 2-3 hours minimum.
Pressure is at 19.5 millibars and it is a 4x8 plastic bag with a flat 4" box grid table of scored MDF.
I'm putting glue only on the MDF substate.
I'm just wondering if Titebond III is the problem. If I use too little I'm afraid I won't get good adhesion. My supplier mentioned that I only have to put on the thinnest layer of glue, just enough to easily see through it down to the substate.
I love challenges, but my deadline is fast approaching for this client. I've softened curly and burl veneers and had no problems, but this pear wood sucks up so much moisture so rapidly that I'm wondering if I shouldn't offer the client another choice.
I'm applying the glue with a mini 4" foam roller.
I've tried to find urea resin glue. All I can find is DAP Plastic Resin which still mixes with water. Is there another type?
(Aaron Paris)
Unibond 800 from Vacupress.
***
I purchased the Unibond 800 as Aaron suggested. It does work, there are no wrinkles in the pear wood. That problem solved.
However, the first couple of times I used the Unibond my ambient temperature was hovering around 70*F and the veneers did not adhere. The instructions say no work should be done with the product under 70* and I thought I was within specs., but I guess not and decided to make sure.
I did several things. One, I trained heat lamps to warm up the substrate about 15 mins. to at least 80* using a thermometer;
2nd, I placed an electric blanket over the press (an old Sears and set it on high which brought the bag to about 80*);
3rd, I placed the mixed glue in a 90* bath of hot water after mixing and let it sit a couple of mins.
I used a black foam roller and placed a thin coating on the surface of the substrate, waited a couple of minutes with the heat lamps on and then installed a second coat making sure the edges were well coated.
I know from the instructions that I had 15 mins. maximum to install in press.
The time the glue was in the water bath, applied to the substrate, and the veneer installed in the press was about ten minutes.
I pressed for 5 hrs. and the edges were tight. I pressed a second time for 3 hrs. and the edges were iffy. So now I plan to press for minimum for 5 hrs.
I want to thank you all for your help and advice. My client is anxious about the time but I explained the process and stated that I couldn't cut corners and still offer a guarantee. Many thanks again to Aaron, Don and Darryl.
***
(Darryl Kell)
Your efforts to warm things up is definitely to your advantage.
A couple of points. Dont warm up the glue itself, you run the risk of it catalizing prematurely. Keeping it cold extends your open work time considerably. As soon as the mixed glue is on the wood it will come up to that temperature almost immediately as it is spread out so thinly.
Second, make sure you do not test the glue line until 24hrs later. When you take it out of the press after 3-5 hrs it is only in the firm rubber stage and needs overnight, and sometimes longer to reach a good cure where you can properly test the bond strength.
***
(Don Stephan)
Typically I will set the furnace thermostat to 70 deg the night before pressing, and first thing in the morning I'll turn on the elec blanket over the bag/platen/bottom caul to warm further. Even with everything set out the night before it will be 30-45 minutes before I get the pressing in the bag and vacuum drawn. So I leave the materials all room temp so curing doesn't start too soon.
Over the years I've kept track of how much adhesive per square foot is needed for different substrates, and always aim to have some left over. Long long and far far away, Darryl suggested pouring some of the excess into a tightly sealed zip lock sandwich bag (which I place in a 2nd tightly sealed bag) and placing above the vacuum bag under the heating pad. When the adhesive in this bag becomes rubbery the adhesive should be sufficiently cured that the vacuum can be turned off - but I'll usually wait an extra couple hours for a margin of safety.
***
All,
Thank you again for all your input. I'm just starting out using Unibond 800 and you folks are the experts. I intend to use Unibond 800 for all my problem pressings.
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