View Full Version : gravity forming / free blowing cold rubber to make molds

12-22-2006, 03:51 AM
I got to thinking about gravity forming and free blowing, and came up with a similar technique for easy mold-making.

With gravity forming, you just sag hot plastic through a cutout, to get a nice smooth catenary-type curve. With free blowing, you blow or suck the plastic to get a more bubble-like curve. (With gravity forming, the curves will never exceed 180 degrees, and will be steeper on the sides. With free blowing you can get more that 180 degrees, and the shapes tend to be more spherical.)

The great thing about gravity forming and free blowing is that you don't have to make a mold. The awful thing is that you don't get a mold---the process is difficult to control precisely, and the results vary.

So I thought, why not gravity-form some plastic, and then cast plaster in the result, to get a nice smooth plaster buck? You could do it several times, and cast a buck in the best one.

Then I realized that if you're going to do that, you don' t need to use hot plastic at all, and you don't need exquisite timing in applying vacuum, etc.

You can use cold rubber, fill it with plaster, and let the weight of the wet plaster shape the rubber. This gives you a lot more time to adjust the shape, by

1. adjusting the tension of the rubber around the cutout

2. lowering the bulging rubber onto a flat surface (to make a flat-topped dome shape, if that's what you want),

3. tilting the cutout to shift plaster toward one end or other, to change the proportions of the domed shape along its length,


I tried this on a very small scale with a piece of rubber cut from a latex glove, over a teardrop-shaped cutout. I didn't get a very deep dome, because it wouldn't hold much plaster, but I found the results encouraging.

For something larger, with a big enough sheet of rubber, I think this should work pretty well.

By adjusting choosing a different thickness of rubber, you should be able to make this work for a reasonable range of sizes. (Maybe not things several feet across, but probably a foot or so.)

By building a dam around the cutout, you can also use more plaster to increase the weight, essentially doing pressure forming. (I tried this, a bit, on the spur of the moment when I saw how shallow my little dome was; it seemed to help, but then my little dam broke. Oops.)

Or by combining the weight of plaster with suction from below, putting the cutout over a box and pulling the air out of the box, you should be able to get the kinds of more-bubbly shapes you get with free blowing.

(On the other hand, if you mostly use vacuum to get the bulge, you won't be able to vary the shape as much by tilting the cutout, because gravity won't be as important.)

I'll be looking for some nice sheet rubber to try this on a larger scale, and an acrylic aquarium to use as a vacuum box, so I can see what's going on...

Any thoughts on this?