Monday, January 11, 2010

Using a Bending Mold

I had a request recently to show bamboo being bent on a bending mold (what they call a himage-jigu, meaning "fire bending jig", here in Japan; 火曲げ治具)so here I'll go into a little more detail about how the bending mold is used to bend bamboo, and keep it that way. Once again, a picture of the mold we use at our school above. Shaped like a wedding cake, there are numerous tiers with different circumferences to bend the bamboo at tighter and tighter curves. Underneath this hollow wedding cake is a propane burner, which heats the jig from within. You can control the level of heat with the burner, but the jig is also made to release extraneous heat through an adjustable vent at the top.

Bending bamboo on the mold is both conceptually and technically a fairly simple procedure. At school there are specially made wooden wedges that fit in between the round central part and the arms that stick up. Wedges of varying thickness are on stock, but because most of the bamboo we bend at school is between 2-3 mm thick we almost always use the same wedges. You start by placing the end of the strip you are bending in between the central tier and the arm, and hammer in a wooden wedge between the strip and the arm, pressing the strip against the central tier and locking it in place. You bend the strip around the tier as far as the next arm, and hammer in another wedge (after the first wedge it's best to point the remaining wedges in the direction you are bending the strip, to keep the strip from floating away from the central tier, but my picture shows the opposite). You continue this at each arm until you get to the end of your strip. When bending strips to make rims, the strip will always double up on itself, in which case you have to bend the strip around in a slight spiral; this doesn't have much affect on the final product. Also, its best to bend a strip that's longer than the final strip's length, because bending the very ends of the strip is most difficult (in the picture I used the handle of the hammer to press the end of the strip against the mold). You measure the length of the strip you need before bending, and then cut off the extra ends after bending.
A couple other pointers about bending molds, or bending bamboo with heat in general:
  • Always use bamboo that's dry. Moisture keeps bamboo from setting after it's heated. Semi-dry bamboo will release moisture from the ends when bending it, which is usually a sign that it's being heated enough. Even days with high humidity can have an effect on how well bamboo retains its shape after molding.

  • It's easy to burn bamboo on a bending mold. Once the jig is warmed up or has been in use long enough, it only takes a minute or two to heat the bamboo enough to shape it.

  • As a basic rule, bend bamboo with the vascular bundles on the outside. When bending strips with the skin/vascular bundles on the inside, bend the strip more slowly, allowing the heat to soften the strip as you bend it, otherwise it could break (especially for thicker strips).

  • After the strip is molded, take it off the mold by hammering out the wedges. Bamboo that is still hot won't retain it's shape, so it's important either to hold the bamboo with gloved hands, or use another non-heated mold to keep it bent until it's cooled. At school we place the strips in tin containers roughly the same circumference as the final rim (see picture).

  • It's best to bend a strip more tightly (at a smaller diameter) than the curve it will be used for later. It's easier to unbend a strip without breaking than bend it more tightly after it was heated.

The bending mold we use at school is a rather complex piece of equipment that had cost the equivalent of several hundred dollars to have specially made. It's useful on an industrial scale, but remember that you don't need as fancy a device when making your own crafts at home. I've been successful bending 2.5 mm thick strips at home buy using a metal cake tin I bought at a 100-yen (1 dollar) store, screw-on clamps, and my kitchen stove. It takes a little longer but works just as well. Also, keep in mind that Japanese craftsmen long ago and some even today didn't/don't even use a round bending mold like this to make rims; they just rubbed the bamboo across their knees to soften and bend the bamboo enough to make a circle. Cutting the strip extra long before bending makes this method much easier.

I think that's about it. The process has become so natural to me by now that I can't think of anything else that might be worth mentioning. If you have any questions please leave a comment.


  1. Thanks Bunches Stephen! It was wonderful to see how this bending is done on the other side of the world. The minute I saw the wedges I understood why there was such a big gap between the wall of the mold and the upright pegs.
    It is true that you don't need fancy equiptment to bend bamboo. I have been clipping strips to tin cans of different sizes with handmade wodden clips, then using a heat gun to bend them. Also, if I'm bending a lot of strips I can put the cans in the oven with the bamboo on them. You should submit this to Betty Shor at the ABS mag for publication. Email me about it.

  2. There are so many different way to bend them.
    After reading Charissa note..she has another way to do it.
    It matter what available your need and limit.
    Keiji and I DO Enjoyed reading your article...
    Wonder how much that equipment cost? How big is it?
    Is it heavy?
    Keiji and Stefani Oshima Oshima Bamboo School

  3. Hmmm... I've never picked it up but I'm sure it's quite heavy. It would probably take a team of two people to pick it up without breaking your back. That's why the stand is on wheels. The set of tiers (not including the stand) are maybe 2.5 feet high altogether. I'm not sure how much it cost, but it was specially ordered/made. I remember hearing an estimate between several hundred dollars to over a thousand. The school has a couple sets, each one a different size for different circumferences.