I needed some black plastic tubes with several different IDs, which usually calls for some tedious machining. Then I realized I could just shrink some heat-shrink tubing around mandrels.
Drill rod worked fine, as did a socket wrench. The only catch was avoiding the engraved lettering, which tends to lock the tubing firmly in place.
In a pinch, I suppose you could turn a rod to the right OD and make a mandrel. That would likely be faster than machining a tube from solid stock, at least for me.
Chuck the mandrel in the lathe, lean a box cutter against the tubing, turn it by hand, and cut to the right length with nice flat ends. Pry it off by sliding a fine needle between the tubing and the mandrel.
You knew that already, right?
The Smell of Molten Projects in the Morning 
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2 responses to “Shrinking Heat-shrink Tubing to a Specific Diameter”
The only catch is that if the heatshrink tubing gets warm, it will continue to shrink until it reaches the size determined by its shrink ratio.
I’d like to see a graph of shrink rate vs. temperature for common heatshrink types. Does it start shrinking suddenly at a specific threshold, or will it very slowly shrink, millimeter by millimeter, on a hot day?
Irradiated polyolefin, right? Fascinating stuff.
if the heatshrink tubing gets warm, it will continue to shrink
And on days like we’ve had recently, maybe I should be worried!
Actually, though, I think there’s a threshold in there somewhere: below a certain temperature, the tubing doesn’t shrink at all. That’s certainly true for shrinking around a solid metal rod, because any patch in contact with the metal stops shrinking until the metal acquires some serious heat.
A bit of rummaging turned up a random 3M datasheet that specifies a minimum shrink temperature of 100 C, so it seems there really is a threshold.
Irradiated polyolefin, right?
I’ve always wondered whether anyone would buy the stuff if the retail package had “Radiation-treated” in big letters… [grin]
Sort of like bandages and medical supplies: they’re all sterilized by ionizing radiation and nobody seems to know or care.
But try irradiating food and all hell breaks loose.