Adjustable, easy to remove form for heat shrinking
Just recently for the first time I tried heat shrinking some small soda bottles. I did not realize just how tight they get when they shrink over a form. I tried to think of a way to make a form that could be reduced in size after the heating to make it easy to remove. Then I realized that such a tool already exists.
It’s called an automotive tail pipe expander! It has a good range of minutely adjustable diameters and all you have to do to remove the shrunken bottle is turn its handle to reduce its size. You won’t need an expensive professional grade tool for this, a bargain priced one should do. I picked up the largest size one at a Harbor Freight Tools store for about $10 U.S. on sale with a coupon, smaller ones are much less.
Two metal cone-shaped pieces are on a threaded metal shaft. Eight metal bars are held around the two cones by heavy rubber o-rings, making it look somewhat like a wooden barrel. When the shaft is turned the cones move together, forcing the metal bars away from the shaft and expanding the circumference of the tool.
To use it as a heat shrinking form, wrap it with a piece of cereal box cardboard, unprinted side out, to smooth out the small gaps between the metal bars. Cut the cardboard to the exact circumference that you want with no overlapping seam. Start with a size obviously too big, heat shrink, remove and test the fit, and then reduce the size very gradually and repeat until you get a good fit.
Since the treaded shaft extends out the end, a piece of bottle neck size tubing (PVC launch tube) can be adapted over it to pass through the neck of the bottle you are heat shrinking to make sure the neck stays aligned and doesn’t distort.
The large size expander is adjustable from 2 1/8 to 3 1/4 inches in diameter. This should handle bottles up to 1 liter. The small size expander is for 1 1/8 to 1 3/4 inches and probably would work with the smallest bottles being fit to T-12 Fluorescent Tube Covers. For 2 liter bottles you will need to make your own 3/8 inch thick custom cover for the tool to expand it out to 4 inches. Perhaps a woodworker could make his own larger version of the tool using this design.
Other possible uses for this tool:
Clamp the handle in your vise and use it to hold pieces of PVC pipe or bottle sections while you work on them.
Use it as an expanding inside clamp while gluing struts to ring fins.
Possibly use it to expand heated pieces of PVC pipe. I haven’t tried this yet.
Possibly use it as an expanding chuck on a lathe. I have not tried this yet, it probably isn’t balanced enough and it would be very DANGEROUS at anything more than extremely slow speeds.
I would not suggest doing any rough work such as hammering on parts held by this tool. When heat shrinking, don’t over heat this tool because it is held together by large rubber o-rings that melt.