Warping Ply

I'm working on a stitched ply boat and would like to increase curvature. Bending wood for furniture, I’ve used water and I’ve heard that a solution of ammonia and water will allow even more. Is either of these ok for ply? I wondering if it’ll crack after drying… or if the ammonia might weaken the wood. Thanks for all your help.

 

Ammonia works very well. It softens the "glue" that hold the fibers of the wood together. An ammonia saturated stringer can be bent in an amazing curve without breaking. When the ammonia dries out, it is as if the wood grew in that shape.

I don't know that it would work on plywood, the grain in each layer is at 90 degrees to the next. Inducing bend also creates a sheer load at the glue joint between the laminations.

Try a test part. Cut two sections. Take section one as your control, bend it until it fails and note the deflection. Soak your other section for 24hrs in ammonia and see it it fails at the same deflection.

 

I read an article, forgive me for I remember not whence it came, that talked of cat hulls made of ply. It might have been in wooden boat. But any way, these hull had an extrodinary amount of complex curvature introduced in the method of assembly.

In a nut shell, the two hull halves are assembled with epoxy and glass tape along the hull centerline. The halves are still flat. Once the epoxy is cured, the halves are "opened" up and bulkheads are introduced to give it the desired shape.

The down side is that flat panel shape is a trial and error process using poster board and tape. This is all extraneous information since it looks like you've aready started cutting the hull panels. I thought it was as interesting and insightful article though.

Good luck with you project.

 

a long time ago my father used anhydrous ammonia (NH3) to bend ash. He put the ash strips in a 55 gallon drum with a few gallons of NH3 in the bottom. Covered it quickly and left it for 24 hours. When the wood came out it was quite flexible and tainted in color. He put the strips in between two curved pieces of wood and clamped it.

I think there was a bit of spring back.

this took place 30 years ago so this is all I remember about it.

NH3 is available from farm supply as a fertilizer. It smells bad and you have to wear protection when handling it. Nasty stuff

 

Nero's post reminded me... I believe furniture builders sometimes use amonia to bend wood. Also to give a distinctive color to the wood. Might be some more info at a furniture building website...

 

The problem is not knowing how all this will effect subsequent coatings (epoxy/glass, and paint). Its could really be a disaster if have adhesion problems later do to the wood being soaked in something. I would prefer a safer solution, like cutting the plywood into strips or making shallow kerf cuts to allow it to bend easier. Good luck.

 

DO NOT kerf ANY PARTS of boat parts. No one knows what strength is left. Plus, I can see every kerf cut in the bent surfaces.

 

Your right, I was assuming the wood was a core and kerfs would be filled. You can kerf if this is the case. Without know what sort of boat is being built - I shouldnt say anything.

 

Funny you mention that. I was reading System Three’s “The Epoxy Book” and the chemistry section mentioned that the hardener was “not unlike” ammonia. I am now a little concerned about the curing and mechanical properties… as I have already used the solution to help warp the ply. That part worked like a champ... I got far more curvature than if I left them dry.

Anyway, I had emailed System Three about the situation and will defer to the hard-core resin chemists. I’ll let you know when and what they respond.

 

It's the ammonia like construction of the molecules in the hardener that gives off the smell in epoxy. The resins used are very similar to the high molecular weight, long chain polymers found in the structure of sugar. In fact, it is this same stuff (sugar) and it's construction, that provides wood is unique strength. "Cellulose" - literally means cells made of sugar (the suffix -ose- means sugar) and is the reasonably aligned tubules that make up about 50% of the weight of wood. The wood cells are bound by "lignin", which is a rather tough resin like material, making up about 30% of weight in wood. From a chemical and molecular stand point, wood is a uni-directional composite. This basic structure has been the base for many man made materials, as has many other naturally found structures.

Plywood will only follow a certain radius with any level of reliability. These minimum radius are noted in a number of texts (none of which I have handy at the moment) and are dependant on the thickness, number of plys and species of wood. Trying to "talk" a piece of ply into bending beyond these limits usually sets up internal stresses that can cause the ply to buckle, crack, delaminate, crush, deform and other wise have less then desirable things happen. The damage may not occur at first, but may take weeks, months or years to show up, typically after you've got a really nice paint or varnish job on it (at least with my luck anyway)

 

Thanks again PAR.
I hope all the others appreciate the wisdom in your posts.

 

PAR,

After looking at wood at the lumberyard the other day, I think I'd take issue with you statement about wood being a unidirectional material......

Yoke.

 

I certainly appreciate the wisdom…

Reading the chemistry section of System Three’s: “The Epoxy Book” and not being a chemist I was concerned that the some ammonia might be left in the wood might react with the epoxy and cause it to not cure properly. Although not as informative as your message, here is System Three’s response: We do not recommend bonding to wet or damp wood. If the wood is dry and the water/ammonia has evaporated there will be no problems. This is not a problem... my workshop is way dry this time of year!

If you happen across those references, could you pass the titles/authors along? I’d like to find out if what I have is going to be a problem. I currently have 5 mm ply with a bending radius of 432 mm.

I found one reference on-line. However, it appears that the reference was more furniture based instead of marine based. It said a 7 mm thickness requires a minimum bending radius of 600 mm. Since bending is a function of (EI) and I is a function of (thick^3)… I’m guessing my 5mm laminate could stand 600(5/7)^3 = 220 mm. If I go more conservative and say its just linear, I get 600(5/7)=429 which says I’m still good. Any thoughts?

Thank you for the information.