Darryl Keil Last Activity 2020-05-06 10:41 AM
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Aaron Paris

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Joined: 2014-11-28

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Subject : Serpentine Laminations
Posted : 2020-01-19 7:49 PM
Post #37868

I have a large project that is made up of several vanities,desks and credenza's. All of which are of different shapes made up of convex and concave radius's. (Thanks a lot Interior Designer's!) For each cabinet I've had to make forms to press the doors and drawer fronts to the desired shape. I couldn't find any suitable bendable plywood that would bend tightly and that didn't have such a stiff core, so I created my own. I took 1/4" plywood and sanded down the outer face veneer which left just the poplar core. At the plant this core is softened or tenderized somehow which makes it very flexible once the outer veneer is gone. So my laminations are a 2 ply maple( grain running vertically on the face,horizontal on the back)5 layers of the poplar(grain running vertically) then the 2 ply maple again for the backside( grain running same as face)creating a 3/4" finished thickness. On some of the cabinets I had to use an aluminum phenolic instead of the maple. All laminations were glued with pva Titebond Extend,pressed for 48 hours and left to cure for another two days before trimming to final size.
Still with me?? Sorry, I'm getting to the point now.
The shapes that have the maple want to twist after I cut them apart into drawer fronts. The one's with the aluminum phenolic stay perfect as when they came out of the vacuum bag! I thought I might have an issue with the laminated panels drying out,so after the 4 days out of the bag I put a good seal coat of conversion varnish on all sides to help seal things up. That didn't help,there was still twists after I cut it into the separate drawer fronts. These fronts are 6" and 12 " high and have at least an 1/8" twist!
Not sure what's going on?? Any thoughts are welcome!
Thanks Aaron.




 
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Don Stephan

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Joined: 2003-07-18
Location: Cincinnati, Ohio

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Subject : RE: Serpentine Laminations
Posted : 2020-01-20 10:08 AM
Post #37869 - In reply to #37868

Here in Cincinnati is a branch of DSI (Distributors Service Inc?). Several years ago when I had to make a bent lamination I purchased several sheets of I think 1/8" poplar bending plywood, which I used without sanding off any of the sheet and it worked very well. It was not "wiggle wood" which I have found much less desirable.


 
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Enrico Konig

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Joined: 2006-01-06
Location: Vancouver, BC

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Subject : RE: Serpentine Laminations
Posted : 2020-01-20 11:29 AM
Post #37870 - In reply to #37868

Sounds like a great commission but a bit of a nightmare at this point. My instinct would be to use a catalyzed glue like the unibond, because it dries rigid. It actually seems as hard as glass when fully cured. I have done a fair number of such projects, using rubber ply for the core, and Italian bending poplar for the substrate immediately beneath the veneer. The thin rubberply (5mm or 3/16" bends pretty tight. I have also used an inexpensive thin meranti 3 ply that is less than an 1/8" thick as well. This means more glue lines which means greater strength and less springback.

I have found I sometimes get a slight bit of twist when cutting a panel into curved parts doing it this way, but usually not. So sometimes need to fair 1/32 or less to flush faces up again before applying front and back veneers. But I always use an urea resin for these things. I also like to leave it sit after pressing for several days or longer with good air flow all round just to let it completely finish drying (but many of my laminations are 2.5" - 3.5" thick, and the curve can tighten a teeny bit as everything cures and dries).



 
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Darryl Keil

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Joined: 2003-05-22
Location: Maine

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Subject : RE: Serpentine Laminations
Posted : 2020-01-20 4:50 PM
Post #37871 - In reply to #37868

I would suggest 1/8” Italian bending poplar as well. This material is designed for bent lamination work like what you are doing. Taking the face of a ply that is not designed for bending may be the issue.


 
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craig tufankjian

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Joined: 2004-02-01
Location: syracuse ny 13208

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Subject : RE: Serpentine Laminations
Posted : 2020-01-20 5:58 PM
Post #37872 - In reply to #37871

Ditto on the 16 th inch bending ply. It’s actually A heavy 16th. However this has its limitations. Depending on how tight the radi is on these fronts you may still experience some spring back.

I would go the route of the skateboard approach. By that I mean you can buy several skateboard kits which consist of maple veneer (16th). And lay up the drawer that way. Rule of thumb with this procedure is the more plies the less spring back. These kits have 7 pieces and they have both face veneer and crossband. So the reversing of the grain has already been done for you.

I would also use urea resin glue. Never pva glue.

You can find these skate board kits at certainly wood. On the pull down menu you go to special packages . The get the 3/4 final thickness you’ll need two kits. 20.00 per kit.

Because of the process that veneer under goes you can get a much tighter radius.


 
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Aaron Paris

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Subject : RE: Serpentine Laminations
Posted : 2020-01-21 6:05 AM
Post #37873 - In reply to #37872

Thanks everyone. I'm checking with my supplier's about the poplar bending material. I still would like to know why the panels are twisting when I cut them apart?? Is it because air is now getting to spots that may not be as cured as the outer portion of the panel?? And I suppose too,as mentioned previously my glue choice may not be correct causing an uneven cure rate.


 
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Don Stephan

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Location: Cincinnati, Ohio

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Subject : RE: Serpentine Laminations
Posted : 2020-01-21 7:26 PM
Post #37874 - In reply to #37868

As you no doubt know, something in the process created stress in the laminated panel that was not strong enough to twist the entire panel but strong enough to twist the pieces you cut apart. PVA contains water, and there were a number of glue lines in the assembly. And cured PVA is still somewhat flexible, meaning it may not have the strength to resist stress in the lamination.

Moisture causes wood movement, more along growth rings than across them. The poplar core of your plywood may have been rotary peeled, along the growth rings. The added water would not be able to move out of the 3/4" lamination in only a few days, and the sprayed coating will only slow down the moisture movement out of the lamination.

I'm not familiar with aluminum phenolic, but it might be stiffer than two maple and thus able to counter the stress in the lamination. Or perhaps because wood is not perfectly uniform, and glue layers seldom perfectly identical, there just happened to be less stress in that lamination.

If 1/8" Italian bending ply isn't flexible enough, there is no shame in telling the designer the concept is not workable and show what is possible with the 1/8" Italian bending ply.




 
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Aaron Paris

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Joined: 2014-11-28

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Subject : RE: Serpentine Laminations
Posted : 2020-01-22 6:20 AM
Post #37875 - In reply to #37874

Thanks Don. That's a very detailed and sound explanation, I appreciate it.



 
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Aaron Paris

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Subject : RE: Serpentine Laminations
Posted : 2020-01-27 3:37 PM
Post #37885 - In reply to #37875

Would applying a heat blanket over my shapes that are in the vacuum bag help them cure faster the same way we do with the Unibond 800?? Or would this cause even more warping,with heat on one side??


 
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Don Stephan

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Location: Cincinnati, Ohio

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Subject : RE: Serpentine Laminations
Posted : 2020-01-27 6:38 PM
Post #37886 - In reply to #37885

Not going to use a rigid glue? You are much braver than I, especially with the earlier experience cutting out the pieces.


 
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Don Stephan

Posts: 819

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Location: Cincinnati, Ohio

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Subject : RE: Serpentine Laminations
Posted : 2020-01-27 6:39 PM
Post #37887 - In reply to #37885

Not going to use a rigid glue? You are much braver than I, especially with the earlier experience cutting out the pieces.


 
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Darryl Keil

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Joined: 2003-05-22
Location: Maine

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Subject : RE: Serpentine Laminations
Posted : 2020-01-27 6:53 PM
Post #37888 - In reply to #37885

Urea resin glues like Unibond 800 are very responsive to the addition of heat due to the chemical reaction that is taking place. A PVA glue like Titebond is not effected so much by heat unless the heat gets quite high. A common heating blanket will usually not warp a panel as the one sided heat created is too low to cause warpage. In general, one sided heat needs to be above 110 degrees to begin to warp a panel. Most heating blankets only achieve, in the 90’s, which will speed up curing considerably with a urea resin glue but have little effect on a PVA glue at these temps.


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