Make a chair with bent sheets
The technique of bending plastic sheets offers great applications in furniture design. In this How-to, we will explain how to build a chair from bent PS sheets and a steel frame.
It’s split up in two parts:
Step 1-7: Making the jigs and moulds
Step 8-27: Building the chair —
Step 1 - Get ready
To make the moulds and metal parts, you will need access to a wood and metal workshop.
You can find the exact amounts and dimensions of materials in the attached parts list.
As the chair is made with a plastic sheet, you’ll need to get plastic sheets or a sheet press to make the sheet (link to sheet press video) and an oven to heat it for bending.
How to bend plastic sheets 👉 https://tinyurl.com/bendsheets
Step 2 - Print your plans
Step 3 - Cut your parts
Take the part list and the drawings and prepare all the wooden parts (number 1-14) needed for jigs and moulds. Make sure to put the corresponding part number on each piece, so you don’t mix them up later. Also mark the center line on parts 4 and 6-8.
Step 4 - Laminate the drawings
Apply spray glue on the back of one sheet, align it according to the instructions on the drawings and apply it evenly by pushing with your hands from the middle outwards.
Step 5 - Build the jigs
Leg bending: Transfer the measurements from the plan to part 4, cut the required wooden pieces and screw them in place.
Bending angle: Cut according to drawings (or adapt to your bending device)
Cross-brace: Drill the 18mm tube holes, then saw along the cutting line. Make sure to stay on the hatched side! Assemble by screwing parts 5 in place.
Sheet shape: Pre-drill holes and cut parts according to the drawing, then sand them smooth.
Put some extra effort into the precision during this phase, as all the flaws made here are hereditary.
Step 6 - Build the mould
Step 7 - Mould done!
Step 8 - Cut your plastic
Begin with cutting the 14mm PS sheets for your seat and backrest to the dimensions in the part list. For a first rough shape, transfer the shape of the wooden jig to the plastic sheet with a thick marker and cut along that line with the jigsaw (more info about this process in the How to cut plastic: jigsaw 👉 https://tinyurl.com/cutjigsaw
❗️In this process, you’ll probably create a lot of plastic dust. Please be conscious and try to save it from going into the environment.
Step 9 - Smoothen the edges
Before removing the jig from the plastic, don’t forget to transfer the vertical centerline from the jig to the plastic parts as indicated in the drawings.
Step 10 - Bend the plastic sheets
Heat up your oven to 190°C and heat your sheets (parts A+B) for about 7min. Turn it half way through to ensure a more even heat distribution. We recommend using Teflon fabric or baking paper to prevent the plastic from sticking to the oven.
When soft enough for bending, put the sheet into its mould and align it in the center and along the fence. After pre-bending by hand, put the upper part of the mould on top and make it snap into place. After a couple of minutes it should be cooled down and ready to be taken out of the mould.
Step 11 - Bend the metal legs
Once you are done make sure the leg is still planar and correct any deformation by clamping and bending the piece straight.
Step 12 - Weld the legs
Take the legs out of the jig to finish the weld.
☝️ Melting metal can get really warm, so reduce the welding time in the jig to the minimum and have common fire safety measures handy.
Step 13 - Trim the legs
Step 14 - Make the cross-braces
Then bend them to the shape of the jig by clamping or hammering. This step requires a bit of experience, but it doesn’t need to be perfectly precise, as you will have the opportunity to make corrections once the seat is screwed together.
Now drill and countersink the previously marked holes, approximately perpendicular to the curvature. Take your tube/round bar for the cross-brace (part G) and notch it as shown in step 11.
Put everything aside for welding during the assembly.
Step 15 - Backrest connector: Drill the blind holes
The outcome should be a blind hole, going as deep as possible without breaking the inside of the backrest. From the moment when the drill is orthogonal and the entire height of the drill bit is cutting, dive at least 7mm into the plastic to have enough space for the locking pins.
Step 16 - Backrest connector: Drill the locking pin holes
Avoid clamping the piece too tightly as it might crack, hence support the backrest on both sides with two pieces of identical thickness to prevent it from tipping to one side. Precision is crucial in this step!
Chose a drill around 0.5-1mm smaller than the diameter of the locking pins. Now align the x-axis to the center of the blind holes (step 13) and the y-axis to the middle of the material thickness. Drill until passing the blind holes and further in as much as possible. This is crucial to make the connecting plugs shake proof in both directions.
Step 17 - Backrest connector: Make the plugs
Then take it off and move to the drill press. Choose a diameter slightly bigger than the locking pin and countersink the top side. Drill a M5 center hole (Ø4.2mm), 6mm from the other end of the plug and tap it.
Step 18 - Backrest connector: Assemble the plugs
After drilling, mark the immersion depth of the drill bit on your locking pins to see how far you can hammer them in without cracking the material. Finally, stick the plugs into the blind holes, hammer the locking pins in and cut the excess off. Use a file to smoothen and align the ends to the plastic edge.
Step 19 - Backrest connector: Let the frame immerse
Put your connecting plugs in place and use them to guide your hole saw while you’re cutting a smooth slot. Remember that PS melts quickly, so give the tool some cooling time and sharpen the teeth regularly.
Step 20 - Drill holes for bolts
Step 21 - Assemble the frame
After welding, grind all the beats and make them look beautiful.
Remember: Grinder and paint make you the welder you ain't! :)
Step 22 - Stamp your product
You can use a simply heated metal wire or specially engraved stamps from the bazar. For the stamps, heat them up and clamp them to your plastic part.
Step 23 - Put everything together
Put the backrest back in place. Then, enlarge the two pre-drilled holes in the seat and attach the frame with screws. While doing so, make sure that the seat’s alignment is straight.
For the other four screws, clamp the cross-brace to the seat, pre-drill holes and screw it together. It’s recommended to use euro screws (the ones you know from attaching hinges to a cupboard), as they give a solid halt in the plastic.
Step 24 - Alignment
Step 25 - Finish the frame
Remove the plastic parts for painting and put them back on once the paint is dry
Step 26 - Add pipe plugs
As it might be tricky to find the perfect fitting for the angled legs, we attached an STL-file for fitiing plugs. So if you have access to a 3D printer you can easily print them yourselves (preferably with recycled filament). The letter on the inside should be aligned as shown in picture 2.
Step 27 - Make it live long!
This chair is designed to be disassembled easily. So in case a part breaks or you want to change something, just unscrew the seat from the frame, unbolt the backrest and take it off. And lastly, hit the metal plug with a hammer to break the backrest apart (here we recommend wearing safety glasses to protect your eyes).
Once you have all materials separate again, they can be reused or recycled for new products. Make sure to bring them to your local Precious Plastic workspace or recycle in another responsible way :)
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