Laminating two pieces of ply together

[Peter Quinn]

First, keep in mind that while titebond's recommendations for pressure are a helpful suggestion for some clamp ups, on a 2X4 sheet of plywood, 150PSI clamping pressure would be almost 180,000PSI of pressure...thats a lot of bags of salt! Logic and experience tell me somehow the suggestions for clamping joints in hard and soft wood don't apply to these types of laminations. I worked in a place that had a 50 ton hot press, thats only 100,000 pounds, does anybody doubt that a 50 ton press makes plywood stick together? I've also seen a guy at work put a handful of cast iron weight plates on an 18X24 inch lamination...and seen it fail miserably. The glue has water, the plywood moves when you wet one side, the glue won't stick if the plywood doesn't touch.

The idea of the cauls/clamps/platten approach is that you use a flat platten above/below the work, curved cauls on one side of the glue up, flat cauls on the other, the curves in the cauls apply the pressure in the middle of the assembly, the platens spread the pressure over a larger area evenly. It works surprisingly well, though it gets more difficult to achieve as the assemblies get larger. My favorite method is a vacuum bag. We have been making lots of 1" shelves lately the require 1" plywood due to span. Much of the 1" plywood available has only one show veneer, the back side can be a 3 or 4 and suitable only for paint grade. We've also done a bunch of 1 1/2" shelves in rift oak by laminating 3/4" sheets. I'm pleased to let you know we have had no failures to date in the vacuum bag, and it only applies 13psi at best! I've used it to do some pretty sharp curves in bent lamentations, works great. My point is not that you need to buy a vacuum press (you should of course because they are very useful and great fun, but not required), you do not need a 50 ton hydraulic press, and you do not need to buy 40,000 bags of salt. Just make sure your method applies enough pressure to keep the two surface close together with no gaps. A few cauls and platens is usually the cheapest readily available approach.

For rolling the glue I like the cheap black foam rollers, for big stuff I use 9" roller and put the glue in a paint roller pan, I also have 4" and 6" foam rollers for smaller stuff. I just cut the 9" ones to fit my roller. I also have some skinny white foam rollers that don't hold as much glue but work well for small assemblies. Regular nap paint rollers work fine too but ten to shed a bit of fuzz at first which I find annoying, though it doesn't seem to cause any problems. Plywood glues quite easily, try applying a thin layer of glue to two pieces and rubbing them together until they feel stuck then just leaving them to dry. no clamps, you will still have a pretty hard time if even possible to separate them.

[Pat Barry]

How does this apply pressure to the center of the pieces?

In concert with your sandbags - ie: still use the sand bags for the middle area

[Joe Jensen]

Unless this is for structural use all that clamping pressure is not necessary. Just use regular wood glue or titebond III if outdoors. If you really want to clamp it well use a vacuum press.

[Art Mann]

I agree with you Peter in that the recommendation I referred to is for edge gluing. However, I wanted to emphasize that 150 pounds over the whole surface area is essentially zero from a clamping perspective. As I stated before, I have done a lot of this kind of glue ups - enough to experience what happens when you don't supply enough pressure. The assembly can look perfectly flat and strong after you are done but if you rip it in half, you can see where the 2 surfaces didn't touch enough to bond at all. They will separate with no prying. Appearances can be very deceiving.

By the way, the video of making plywood from the TV show "How It's Made" shows the plies being glued up in a press with something like 180,000 pounds of force like you calculated. As you said, that is a lot of bags of salt. That is why I recommend something better - like screws.

[Peter Quinn]

By the way, the video of making plywood from the TV show "How It's Made" shows the plies being glued up in a press with something like 180,000 pounds of force like you calculated. As you said, that is a lot of bags of salt. That is why I recommend something better - like screws.

They also use a pretty hot platen to set the cure immediately. If you figure a sheet of plywood has 32'/sq, thats 4608 square inches, multiply times 150#/sq inch...thats 691,200 pounds of force required to press a sheet of plywood, or a 350 ton press. Not sure they make anything that large for wood. Thats almost 9000 bags of salt...going to need tall ceilings to glue up a full sheet that way!


I completely agree, 150# on a 12" square is just over 1psi...that doesn't feel sufficient, or at least its not much security. We did some bent laminations last year or two that wouldn't fit in the press, they brought in an entire pallet of play sand, I think it was around 3000# over a 7'X5' area, which looked like a lot but doesn't add up to much weight per inch. It did hold the parts closely together and so far they have stayed that way, I was surprised it worked frankly. I'm not really sure where the happy spot is between almost no pressure and the ideal pressure which is very hard to recreate in the shop, but I know the vacuum press is very effective. I get none of those voids that I've seen too many times using traditional pressing techniques. Strangest thing Ive seen is one guy parked the forklift on a glue up over night, thats around 12,000 pounds....it worked! Not sure ultimately how many PSI a forklift delivers when you figure its weight is divided over four wheels and only one or two of them are on the glue up.

[John T Barker]

glue roller.jpg
Woodcraft supply. I'd go with regular wood glue, flat surface and put everything in the shop on top of it. Do it at night, come back in the morning. Are there tremendous forces acting to pull it apart? The bond will do.

[Allan Speers]

You could also use a large veneering bag.

Also, I wouldn't be overly concerned about 150 lbs/ in3. That's the recommendation for a small joint, which will later take a lot of stress. This "project" will have a huge surface area, and there won't be much force, per inch, trying to pull it apart. You could probably use chewed bubble gum and be OK.

The main thing is to give it a LOT of time to dry, as the glue in the center won't be exposed to much air. If it were me, I'd give it a week or more.

[Dale Coons]

You sound concerned about appearance and how well the lamination will be stuck together--if that's the case, common sense suggests you just go buy the 1 inch plywood. No hassle, no worries, and it's ready to go. Also, you may find some of the suggestions here will rival or exceed the cost differential between the half and one inch ply, and of course, there's the 'hassle factor' of gluing the two pieces up.

OTOH, many have laminated two pieces of plywood or mdf just by spreading glue over the surface and clamping the edges with some weight in the middle. Works fine, have never had a problem.

[John T Barker]

You could probably use chewed bubble gum and be OK.

What brand?

[Todd Burch]

What brand?

I owned a church full of pew seat bottoms for a period of time (until I sold them and doubled my money), and there was chewing gum stuck to the bottom of those that required a hammer and chisel to remove.

[Steve Peterson]

Titebond suggests clamping force of 100 PSI for softwoods. I don't think its possible to put enough bags of salt down to get that much pressure.

This was debated on one of the forums about a year ago. Titebond recommends 100 PSI for absolute maximum bonding strength. I recall that the strength drops off to somewhere around 50% strength with no clamping pressure. Considering the huge surface area, I think it will be just fine with just enough weight to keep it flat.
Steve

[Mike Nolan]

I agree that you don't need more pressure than enough to flatten the plywood. You don't need much glue strength. For the same reason you don't need perfect glue coverage.
I would be mostly worried about having a very flat surface to put the plywood on for glueing.
Avoid having too much glue. It won't squeeze out very well.
if you can clamp the edges, so much the better. The edges are the only part that can separate.

[Pat Barry]

This was debated on one of the forums about a year ago. Titebond recommends 100 PSI for absolute maximum bonding strength. I recall that the strength drops off to somewhere around 50% strength with no clamping pressure. Considering the huge surface area, I think it will be just fine with just enough weight to keep it flat.
Steve

Oh, actually I agree with you. I was just pointing out what the manufacturer recommendations were. I actually think all this is much ado about nothing and maybe all he needs is to get the edges to come together.

[roger wiegand]

A vacuum bag will provide 14.7 PSI, a long way from 100-150, but yet seemingly perfectly adequate for many purposes. Unless you have a very odd application you won't have the kind of shear forces that would demand maximum strength. In the absence of a vacuum press I'd jut put it on a flat surface and pile every heavy object I could find in top of it. If I could I'd make it oversize and use screws/staples around the edge and then cut them off prior to use.

[Alan Lightstone]

Just finished laminating two 3/4" Baltic Birch plywood sheets with a center 1/2" Baltic Birch Plywood sheet for a swivel top tool cart (along with two larger 3/4" Baltic Birch plywood sheets for the sides). Used my vacuum bag, and it looks perfectly straight and quite well glued.

Used Unibond 1 for an adhesive, but for this, I'm sure lots of choices would have been fine.

Even though, as Roger said above, the 22PSI I used is a long way from 100-150, it sure seems like enough.