all my bangles start life like this: an armature of scrap polymer clay.
the easiest way to make one is to use an extruder gun with a half circle disc.
i then wrap the extruded clay around a thick cardboard tube but just use whatever you can find thats the correct diameter and oven proof (old straight glasses and bean cans etc) then bake
polymer clay central has an easy zipper cane tutorial: http://www.polymerclaycentral.com/zipper_cane.html
though i altered this by following the tutorial up to step 5 then squashing and lengthening the log until it was 2in wide, 6+ inches long and about 4mm thick.
i then put a sheet of white polymer clay through the thickest setting on my pasta machine (mines only got 7 settings so use your judgement here
) place the flattening log on top of the sheet of white (make sure the sheet of white is wider and longer than the log!) and run through the pasta machine again. i cut strips from this sheet and stacked them as in step 9.
yellow and blue zipper canes
cut slices from the canes and cover the out side of the armature. trim away any excess from the inside of the bangle and bake
i then roughly sand the bangles using wet/dry sandpaper correcting the shape as needed. you can see the difference between the sanded bangle on the left and the unsanded one on the right.
i then add a strip of my chosen colour to the inside of the bangle and trim the excess (a small amount of liquid clay smeared on the inside should help it to adhere easily) bake again.
then sand. i work up through the grades (from 400 to 600 grit paper depending how much reshaping i feel the object needs) until i reach 1000grit paper then buff.
bangles made with the yellow zipper cane:
blue zipper cane:
as always comments or advice are more than welcome