I put the sheets with the curled negatives in alternating layers, back to back, face to face ()()()()() instead of all facing the same way, (((((((( in a soft binder and placed the whole thing under a box to keep a constant pressure to encourage them to flatten.
It's Kodak safety film 5035 and still as flexible now as when I bought it 30 years ago.
Anybody know how fast this type of film will the curl after it is developed and stored unrestrained? Has anyone tried to flatten it and how long to get a reasonable flattening?