I had a few weeks to build a chair and decided to experiment with bending plywood. Instead of laminating veneers or using a product like bendy ply, I tried laminating thin furniture grade plywood to make thicker plywood. My results were quite successful and the chair was very sturdy and relatively cheap to make. The molds were simple to build using hand tools; they can also be CNC machined for faster, more precise results. The seat pan and seat back are made from the same mold, the top and back legs are made from the same mold, and the bottom leg has its own mold. So, three molds make all of the parts for this chair.
Step 1: Tools and Materials
- One gallon woodglue - I prefer to use Titebond III for its longer open time
- 1/8 inch 3 ply baltic birch plywood sheets
- Wood finish - I used Minwax polyacrylic
- 3/4 inch MDF sheets
- Bandsaw with a fence
- Lots of clamps!
Step 2: Design and Dimension
To keep the design simple, I designed the chair to be built from three sets of parts. I sketched up a few different concepts and really liked the merging triangle. The side view sketch was very helpful for determining the seat height, leg angles, and seat back angle. This was going to be more of lounge chair so the angles were relatively steep so the person sitting would slide down into the chair. I used Solidworks to refine my chair design and layout the mold shapes.
Before I began building the molds, I tested various bending techniques and wood types. I settled on using 1/8 inch 3 ply baltic birch plywood. It was relatively cheap and bent quite nicely.
Minimum bend radius:
When you bend a material, stress starts to build up in the part. The inside curve of the bend is under compression, while the outside curve is under tension. If the bend is too tight, the material will fail from the tensile stress. Plywood is a unique material since it is a sandwich of thin layers of wood. Wood also has a grain direction. Each layer alternates the direction of the grain direction that that the plywood sheet is nice and rigid. This is a slight problem when trying to bend plywood.
I chose to use the thinnest plywood I could find. With the baltic birch I was able to achieve roughly a 4 inch bend diameter. So, my minimum bend radius would be about 2 inches before the material would start to break. Since wood is a fibrous material, bending slowly helps stretch the fibers gradually which helps reduce breakage. See the sketchbook photo for more explanation.
Step 3: Build the Molds
Use the 3/4 inch MDF sheet to make the molds. I traced the profile of the bend onto the MDF and then cut out the pieces using a jigsaw. Since the molds are 2D extrusions you can replicate the first piece you cut using a flush trim router bit if you have one. Otherwise, keep using the jigsaw to cut out identical pieces until the MDF mold stacks together at the right depth. The leg molds are 4 inches deep and the seat pan/back mold is 17.35 inches deep. You can make them deeper if you want because the plywood parts will be trimmed later.
Step 4: Mold the Legs
Use a table saw or bandsaw to cut the 1/8 inch baltic birch into 4 inch strips. Each leg needs 4 strips of baltic birch plywood laminated together. Place the plywood strips into the mold one at a time and liberally cover the top side with wood glue before placing another piece on top. Work quickly as the glue will start to set after about 10-15 minutes.
Once all of the plywood pieces are glued together in the mold, clamp the entire mold together tightly. More clamps are always better. Let the parts set and rest for at least 8 hours before de-molding them. Set them aside for 24 hours for a full cure.
Step 5: Mold the Seat Pan and Back
Molding the front was the most difficult task because of the size of the part. I ran into problems of the glue setting before I had clamped everything down. I would recommend using the slowest setting glue you can find for this part.
Step 6: Trimming the Parts and Test Fit
Once all of the parts have been molded, trim the uneven edges off using a bandsaw with a fence. The fence will help you cut nice and straight. I traced the front leg profiles onto the large seat pan part and cut out a section with a jigsaw.
Once all of the parts have been trimmed and cut out, it is time to test fit everything together. I used small clamps to hold the chair together to I could look at it from a distance to make some last minute design changes.
Step 7: Add Hardware and Finishes
I used socket head hex screws to secure the parts together. Each screw and nut was recessed and covered with a wood plug that I sanded down flush to the baltic birch. I wanted to hide the hardware as much as possible.
The Minwax polyacrylic finish went on nicely and gave the chair a nice gloss.
Step 8: Photography
Building the chair was great project. Bending plywood to make simple furniture is not as intimidating once you get into it. The laminated baltic birch plywood turned out to be incredibly strong.