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Old 16th July 2004, 07:36 AM   #1
amo is offline amo  United States
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Default Plywood not glueing well

I have been experimenting with plywood, and my results are not as good as I have hoped. I tried to make a 1.5 inch thick piece from 2 x 0.75 inch pieces, and I was using regular grade birch ply. I smothered both sides with high quality wood glue (yellow stuff), put the two pieces on the floor, and weighed them down with bricks. I made sure the glue was set enough that the two pieces would not move. In the morning I discovered two things:

1. The top board, under the weight of the brick moved a good 3 inches, ruining the experiment.

2. Although the bond is dead strong, I can see a .3 to .5 mm void between the two bords.

Questions: Did I not use enough glue? Do I make sure the glue gashes out of the sides, so that there is no void? Should I roughen the sides with sand paper before glueing so that the boards do not slide around (I guess this is why it is easier to veneer MDF then ply). Do I abandon the bricks, and spend a bunch of money on clamps??? These things are EXPENSIVE! You can buy some big transformers for that money...
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Old 16th July 2004, 08:24 AM   #2
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Default Screw and Glue

amo,

Never heard of the wood shifting quite that much without a jolt, or left to dry at an angle.

Usually, the opposed surfaces should both me covered liberally with glue, then clamped together. Use a cheap brush to spread the glue evenly. You can use a damp cloth to wipe down each piece before glueing up, then again after screwing/clamping/weighting down to clean off the ooze before it sets. The glue actually activates from moisture in the wood, so a damp wipe will only insure good set-up while cleaning any particles from the surface. And, yes, there should be ooze uniformly along the edges, even drips. Don't wait for the tack of the glue to set before joining. Stick em together while wet, scoot em around some for uniform coverage, and make tight.

Just screw it together! You can use screws on the side to be hidden, spaced about 4"- 6" apart. This will straighten panels with a slight bow or warp so that the bond is strong, the panels tight to one other. Allow at least 8 hrs. to dry. Actually, the smaller the gap between the work pieces, the better, so long as not glue-starved. This is the preferred method. You can always take the screws out after drying if they bother you. Wood filler will take care of the holes. I still clamp, even after screwing if the panels are bowed at all.

Polyurethane glue can shift panel alignment because it expands as it sets. Takes lots of clamps and/or screws and 8 hrs. drying time at room temp. Faster drying if hot.

With ordinary yellow wood glue you can use heavy duty packaging tape to fasten around the perimeter of the panels in strips toward center to keep the surfaces from shifting. Just block the bottom piece up a couple of inches to allow wrapping the strip under it. Then remove the spacers and add your weight.
Check for flatness if large panels.

Tim
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Old 16th July 2004, 09:37 AM   #3
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more pressure

http://www.sign-lang.uni-hamburg.de/...R/L2/L2430.jpg
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Old 16th July 2004, 09:59 AM   #4
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Sounds like you had air trapped between the boards. You only need just enough glue to thinly cover the surfaces.

When you lay your boards together you should make the top one squirm around a few inches in every direction to spread the glue around and squeeze air out. This creates a kind of vacuum action and it will become progressively harder to do.

Weight evenly distributed around the board would then be my preferred holding method. Oversize both boards then trim them square after drying.
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Old 16th July 2004, 09:22 PM   #5
amo is offline amo  United States
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Thank you guys for your replies! To screw the boards in place makes a lot of sense, as does trimming the boards to size after the glueing. I really do not know how much energy will be transfered to the outside of the box by the screws, but I have a feeling it is not much! Attention to other details may prove more productive.

Till- If I had a press like that, I would make my own composites! The possibilites would be endless
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Old 16th July 2004, 09:34 PM   #6
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you can alway take the srews out after a day or so, if you worry about them.
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Old 17th July 2004, 02:13 AM   #7
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Till was exactly right, you need a lot more pressure, plus you need a lot less glue. Once you get it lined up properly, use a few finishing nails to hold it in place. The glue bottle tells you 100 lbs per sq in, which is a lot of bricks to glue 2 side panels together. You just ended up with 2 panels with glue in between when what you needed is the 2 panels joined together with the glue bonding them. Screws will help but the hole on the panel where the screw enters needs to be a larger diameter than the screw so it pulls the panels together. Screwing through both will usually just hold them the same distance apart. Excess glue needs a way to flow out of the joint, so start in the middle and work your way to the sides so the excess glue is squeezed out. Use a very thin layer of glue to begin with. It's not the same as gluing a joint together where the excess glue is easily squeezed out allowing the wood to join together.
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Old 17th July 2004, 02:27 AM   #8
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As stated above too much glue.
When glueing two pieces of wood like that with yellow glue you should not cover them both with glue, you are setting up yourself for some warping. Use a criss cross pattern like a "tic tack toe" or squiggly lines or whatever design you like with about 2 inches between the lines of glue on each sheet of wood.
Instead of yellow glue why not just use contact cement? Put on both surfaces to be stuck together, wait for it to get very tacky or even past the tacky stage and be dry to the touch, put them together and voila! (just make sure you line them up before bringing them together). Put some pressure on it so it all sticks together and you have no air spaces in between. You can work the newly joined wood immediately after.

Andrew
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