I have a number of the 32L technic axles, but most of them have some bend to them. Is there a straight-forward way to straighten these? Trying to bend them by hand has not seemed to change anything, and I'm afraid I'm going to snap them if I try harder.
I have had some success straightening bent train level crossing gate barriers by first immersing the pieces in a bowl of hot water*, for around 5 minutes to soften the plastic and then immediately trapping them between two hardback books and applying weights until they have cooled.
So by using Joubarc's method to hold the axles straight combined with a pre-soak you may get better results.
*LEGO is plastic and plastic can melt, you only want to soften the plastic not melt it further out of shape. See this question for more.
I'd suggest storing them straightened up for a while, if possible in 32 adjoining 1x2 bricks with cross hole, and add a few layers of bricks to hold them in place. At least it's a pure LEGO solution, but I'm not sure how efficient.
I got one severely curved 32L axle which I straightened with hot water in a plastic tub.
Leaving the axle in water about 70-80°C for a few minutes managed to do most of the straightening job.
Try not going much above 80°C as otherwise you reach the temperature at which ABS will start to become rubbery (used in injection moulding). At about 95- 105°C your part’s shape will be ruined because of ‘melting’.
To further straighten the axle, I repeatedly put the axle in hot water for 2-3minutes, then slightly “over-bent” the axle (to overcompensate the remaining curvature), and then keeping the axle held this way under running cold water for half a minute. This way the axle becomes rigid again, in the desired shape.
On subsequent iterations I applied the same method very locally to cancel out remaining local curvature.
Eventually the axle can be straightened enough for it to do its intended job again. It took me no more than 20 minutes.