John Tuttle discusses bending copper tubing in 140905 MMD.
I'll pass along a trick the old plumbing trade used when bending
old-time lead water lines.  The pipe would be packed with sand
before being bent, and the sand would prevent the tubing from
developing a kink.  A properly-sized spring around the outside
diameter would also work.

Hope this can help you.

Ed Gloeggler
Long Beach, New York