MYP Design Forming and Bending Notes

Forming Reshapes Material Instead of Cutting It Away

1. Forming bends, stretches or presses a material into a new shape, and the amount of material at the end is the same as at the start.
2. That makes it cheaper than cutting for most curved shapes, because there is no offcut and no swarf on the floor.
3. Thermoplastics such as acrylic and HIPS go soft when heated and hold the new shape once cool, and they can be heated and reshaped again later.
4. Ductile metals bend cold instead, with layers of atoms sliding past each other and staying where they land.
5. A formed shape is also stiffer than a flat one of the same thickness, which is why a 15 mm folded lip along the front of a steel shelf stops it sagging.

Line Bending: Heat One Strip of Acrylic and Fold It

1. A line bender, also called a strip heater, runs a hot element under a narrow slot so heat goes into one line of the sheet rather than all of it.
2. A 3 mm acrylic sheet needs roughly two to four minutes over the element, turned over once, and it is ready when the heated strip goes floppy like warm rubber.
3. Lift it out, fold it away from the heated line in one smooth movement, then hold it against a former for about a minute while it cools.
4. Peel the protective film off the bend area first, since the adhesive bakes into the surface and leaves a cloudy stripe that will not polish out.
5. Bubbles just under the surface mean the acrylic was overheated, and that trapped gas is permanent.
6. A 90 degree fold uses up a few millimetres of flat length, so cut the blank a little long and trim the ends after bending rather than before.
   1. A line bender only makes straight folds, so a curved edge needs a different process entirely.

Vacuum Forming: Atmospheric Pressure Does the Pressing

1. A sheet of thermoplastic, usually 1 to 3 mm HIPS, is clamped in a frame and heated until it sags slightly.
2. The mould rises on a platen underneath, the pump pulls the air out, and atmospheric pressure of about 100 kPa pushes the soft sheet down onto every surface of the mould.
   1. The vacuum does not pull the sheet down, the air above pushes it, which is why the process stalls if the seal around the frame leaks.
3. Every vertical face of the mould needs a draft angle of around 5 degrees, or the cooled part grips the mould and tears as you pull it off.
4. Round internal corners to at least 3 mm, because a sharp corner stretches the sheet thinnest exactly where the part needs to be strongest.
5. Drill 1 mm vent holes through the deepest hollows of the mould so trapped air cannot hold the sheet off the surface.
6. The process suits shallow open shapes such as chocolate box trays, seed trays and the shell of a display stand, and it cannot make a closed hollow.

Laminating: Thin Strips Bend Where a Thick One Snaps

1. Laminating builds a curve by gluing several thin strips together while they are clamped around a former.
2. A 2 mm strip of ash wraps round a 150 mm radius without complaint, while a single 12 mm board of the same ash cracks long before it gets there.
3. Six 2 mm strips glued together rebuild the 12 mm thickness, and the cured glue lines stop the strips sliding over each other, so the curve is locked in.
4. PVA is fine for indoor work, while urea formaldehyde or polyurethane glue is used where the piece will meet damp.
5. Leave the clamps on for the full cure, often 8 to 24 hours, because unclamping early lets the strips creep back towards flat before the glue has any strength.
6. Skateboard decks, bentwood chair backs and curved plywood table legs are all laminations for this reason.

Sheet Metal Folding: Bending Metal Past Its Yield Point

1. Sheet metal is folded cold, and the bend is permanent because the metal has been pushed past its yield point into plastic deformation.
2. Folding bars clamp the sheet along the bend line in a vice, and you push the free edge over with a hand or a soft faced mallet.
3. A bench mounted box and pan folder has removable fingers, so you can fold all four sides of a tray without an earlier fold blocking the next one.
4. The outside of a bend stretches and the inside squashes, so the inside radius should be at least the sheet thickness or the outer surface cracks open.
5. Metal at the bend takes up extra length, so work out the flat blank as the sum of the sides plus a small bend allowance rather than measuring the finished box.
6. Plan the order before you start, since folding the short ends of a tray first can leave the long sides with nowhere to go.

Formers Turn One Good Bend Into Twenty Identical Ones

1. A former holds the accuracy so the person does not have to, which is what lets a year 9 student match a shape they could not judge by eye.
2. Cut formers from 18 mm MDF or plywood on the bandsaw, then sand the curved face smooth, because every bandsaw ripple prints straight onto the part.
3. Wax the face or cover it with packing tape so squeeze out glue does not bond the workpiece to the former permanently.
4. A two part former, male and female, presses from both sides at once and keeps the pressure even along the whole length of the curve.
5. Formers only repay the time spent making them when you need repeats, and the same logic drives jigs and templates, which have their own article in this unit.

Springback: Every Bend Tries to Come Back

1. Springback is the amount a formed part opens up again the moment you release the pressure.
2. It happens because only some of the deformation was plastic, and the elastic part of the bend recovers as soon as nothing is holding it.
3. Fold 1 mm aluminium to a dead 90 degrees in a folder and it may sit at 92 or 93 degrees once you let go.
4. The fix is overbending: aim two or three degrees past the target so the part springs back onto it.
5. Springier, harder materials recover more, so stainless steel opens up further than annealed aluminium of the same thickness.
6. Laminations and heated acrylic spring back too if released early, which is why the part stays on the former until it is cold or the glue is fully cured.