Laminated plywood bending (bowing)

[grainmaster]

Dear forum members.

I'm trying to laminated thin strips of hardwood plywood, which will replace something similar to window sill in a yacht.
I have a conundrum, they are bowing and bending.

18 mm marine plywood was cut to 15 mm x 300 mm blocks, then glued together in way that I got 300 x 900 X 15 mm panel, kind of an end grain plywood.
Initially I glued them to a thin 8 mm plywood during lamination, after taking out of a press, 24 hours later, it looked nice and straight, I sanded and oiled it.
The next day It was bowed
I tried on a thicker and moisture resistant chipboard as a base, same result (a little less)

Any ideas whats causing it and how to prevent bowing?
BTW how can I post images ?

Thank you
Garik

 

[Mike.S]

BTW how can I post images ?

Garik

New members (welcome, BTW) are prevented from posting images stored externally to prevent spammers. This restriction is lifted after about 3 posts. However, if you upload the pic to the forums server you can post images immediately. Guidance here.

 

[grainmaster]

Thank you.

Here are images ...

After sanding and oiling (Danish Oil)

The next day ...

 

[custard]

What glue did you use Garik?

The bending problem might well come from gluing the ply strips to another board (8mm ply or chipboard), although I would have instinctively expected the bend to be in the other direction as the secondary board is preventing the moisture in the glue from evaporating (assuming you're using PVA or UF and not epoxy).

Cool effect by the way (the oiled ply "end grain", not the bend!).

 

[Mike.S]

Until more experienced/knowledgeable members come along, my guess is the oil is only being applied to the top surface causing that to expand - hence the bow. But that might be utter tosh....

 

[grainmaster]

What glue did you use Garik?

I'm using PVA - EN 204 (D3), when assembled on its own the laminate is not bowing, deformation occurs only when bonded to the base material.
Next I'll use 30 min Polyurethane adhesive (D4) which actually should extract moisture from wood when curing.

BTW when strips are laminated lengthwise (15 strips 15 x 950 mm) no deformation occurs, and the panel is strong on it's own.

 

[Mar_mite]

Until more experienced/knowledgeable members come along, my guess is the oil is only being applied to the top surface causing that to expand - hence the bow. But that might be utter tosh....

+1

 

[custard]

How long can you leave it before bonding to the base material? PVA is adding a huge amount of moisture and even though it's reached full working strength in 24 hours it takes much, much longer for all the added water to dissapate. When laminating for chair components with UF adhesives I try and leave the components cramped in the formers for 7-10 days if I can.

You say the laminate isn't bowing on it's own, only when glued to the base material, that just adds to the probability that the problem is with the sub base not the laminate (and not the oil which is a relatively trivial part of the equation in terms of adding moisture or constituting a moisture barrier).

Please keep us posted, the "plywood end grain" effect is very interesting, like a Danish Modern version of quarter sawn Wenge! I'd like to learn more about that.

 

[Droogs]

Hi Grainmaster, welcome
I would suggest that you breakdrown the glueing process a little. Instead of glueing the laminate strips to the baseboard, I would place some waxed paper between the board and your strips. Glue the stips up, place some more waxed paper on top and then another piece of board and clamp up (using cauls). Leave to dry for at least 48hrs and then remove. Give it a day to setlle out of the clamps and then if needed glue to the 8mm baseboard.

regards
droogs

 

[Mr T]

I would say that the configuration you are using is inherently unbalanced so the bending problem will be ongoing depending on humidity. Do you need to stick the laminates to the ply for strength reasons? Perhaps some other method of strengthening could be used.

I agree with Custard, an interesting effect.

Chris

 

[Zeddedhed]

I've done something similar to this before for a client who wanted exactly this effect for staircase risers (they weren't structural but simply glued onto the main risers.
I found that using the wax paper as you've been advised worked well, a piece either side with a clamping block (caul?) either side to keep the whole lot flat and then a careful sanding once out of clamps.

The whole thing was then surprisingly strong and was easily glued onto the riser. (with a few headless pins to help)

 

[Sgian Dubh]

As Chris said, you've created an inherently unbalanced structure. It's similar to attaching veneer to just one side of a piece of wood. The end result of that construction will almost always try to become concave on the veneered side. Similarly, if you wet a piece of paper it curls towards the wet side. You have the essentially the same effect occurring, but as you've noticed the thicker board you used in your second attempt didn't bend as much.

If you can glue your 'edge appearance' plywood to a base board firmly attached to a rigid sub-structure you will probably fair better, although there is the possibility your edge on block of plywood might lift away from the base board at some point in the future, probably at the outer extremities first. Slainte.

 

[grainmaster]

Thank you for responses and advise,
I'm not a professional woodworker, I'm a designer and software engineer so I might use some methods or try to do things that shouldn't be done ...
Today I experimented with Polyurethane adhesive.

This is the source material ... 18 mm marine plywood cut to 15 x 270 mm, very hard to machine, lots of splinters, and it's so sharp on edges that it will cut your skin, if not careful.

This is the jig / press

Makeshift press

Out of the press

Still got a slight bend, arching upwards ...
I'll leave this to dry for 2-3 days, and then try to bond it to a base material, I will use 12 mm birch plywood as it is supposed to be the most stable one.

 

[grainmaster]

How long can you leave it before bonding to the base material?

I left it in the press for 24 hours, somewhere I read that you shouldn't leave it in the press for long time to prevent creep, not sure when it will creep, in the press or when the compression is released?
The new samples I will leave for several days ...

 

[grainmaster]

Solved, but at a high cost