The ability to bend plywood opens up a number of interesting design possibilities for amateur craftspeople as well as professionals in a number of trades, such as boat building. Because it's made from a series of cross-laminated sheets, though, plywood is more difficult to bend than raw wood. You can use steam to soften plywood for bending, but it isn't the best way to do it, because it requires a steamer large enough to accommodate a plywood sheet, which can be difficult to construct and expensive to buy. An easier option is to saturate all the layers of the plywood, using warm wet towels, prior to bending.
Not All Plywood is Suitable for Bending
The best plywood for bending has a close-grained surface that won't splinter and separate when you bend it. For example, construction-grade fir plywood is not the best choice for bending, because the softwood fibers tend to separate under pressure, especially around knots. On the other hand, plywood with a close-grained hardwood face is less likely to splinter. Birch, mahogany and poplar are great choices.
The thickness of the plywood and the direction in which you bend it both determine the minimum bend radius you can achieve. The bend radius is a measure of the size of the circle you would draw if you complete the semi-circle formed by the curvature of the wood -- a smaller bend radius denotes sharper curvature. You can get a bend radius of as little as two feet by bending 1/4-inch plywood, whereas the minimum bend radius for 3/4-inch plywood is 12 feet. These radii increase if you bend the sheet along the grain of the wood instead of across it, because splinters are more likely to form when bending along the grain.
You Need a Mold
Creating a mold to which to clamp the plywood is an important part of the bending process. This needs to be made from a rigid material that won't lose its shape when you clamp the plywood to it. The mold doesn't have to be elaborate -- a metal bar may do the job -- but keep in mind that it will determine the accuracy of the bend.
Step 1 Mark the Crease Line
The crease line is exactly in the middle of the bend and is where the maximum stress will be applied. Depending on the curvature, stress points will extend some distance from this line.
Step 2 Soak the Plywood
Apply towels or rags saturated with warm water to the crease line and the parts of the plywood that will be under stress. If possible, apply the rags on both sides of the sheet. It could take several days for the plywood to become soft enough to bend. During this time, be sure to re-soak the towels with warm water to keep them -- and the plywood -- wet.
Step 3 Start Bending
Use clamps to bend the plywood around your mold. Use bar clamps for plywood thicker than 1/2 inch and use strap clamps for thinner plywood. Strap clamps won't form depressions in the wood, but they aren't strong enough to bend thick plywood.
Step 4 Apply Pressure Gradually
Slowly tighten the clamps to force the plywood into the shape dictated by the mold. You may be able to do this over a matter of hours for thinner plywood, but for thick plywood, expect the process to take days. Be sure to keep the crease line and stress points soaked with warm water while the plywood is conforming to its new shape.
Step 5 Let It Dry
Remove the towels when the plywood has achieved the shape you desire. Let the wood air dry for several days. You can use fans to expedite this process, but it's best if the wood dries slowly. This way, it will be less likely to splinter and crack.
Step 6 Sand and Fill
Remove the clamps when the plywood has dried and sand the bend with 120-grit sandpaper. If any of the wood has separated from the surface, you may need to cut it off with a knife and fill the resulting depression with wood filler.