I have a lot of empty toilet paper tubes, that I'm using for a project (see my previous question). However, sometimes, before I have a chance to staple them, the tubes get crushed. (I keep them in my room and I'm not the most neat person...)
They end up like this:
They then end up rather... square when I unbend them:
Is there any way to uncrush them so that they're circular again?
Unfortunately you won't be able to completely "uncrush" the tube again.
Remember that we are dealing with cardboard here, which is essentially a mass of compacted fibers. When you create a crease (or two as in your photo), you are effectively destroying the fibers' structure and tearing some of them. Straightening the crease with mean you still have torn fibers at the former crease, no matter which method you choose.
If you want to reshape nevertheless, pressure on the two creased ends alone will create a parabolic shape and likely that new crease in the middle (or at a weak point somewhere) and the "square" result. At minimum, you need to "hold back" the center with your second hand, molding the original shape. Combined with a cylindrical balloon (blow up once first and deflate again for easier use) from Willeke's answer, you should be fine.
But there's a big caveat:
If you need tubes with maximum stability for some "load bearing" in your project, the reshaped ones might look good, but will bend again at the creases almost immediately. You might want to discard them and resort to intact ones.
Going by a tool we use work, I would try with blowing up a thin narrow balloon inside them.
The kind of balloon they use for balloon figures would work.
The tool we use at work has a two part beak and uses a cylinder to power the beaks apart. But there we do not need to unbend just cardboard tubes but rolls of material with a card board tube center.
I do not see a way to immitate that tool, but then, I also do not see the need.
