I tried printing flexible PLA as a mold for PU. I used silicon spray as release agent. => The mold got destroyed after only one use.
My grey soft PLA behaved like chewing gum :S very little shape memory.

For small quick and cheap stuff I now use ogoo (acetic silicone + corn starch). It sticks on PLA while wet and goes off easily when dry - just perfect.
I used wax based release agent to quickly make around a dozen PU copies from a single mold from a specific PLA part (PLA positive -> ogoo negative -> PU positive)
An other release agent might be better (less viscous)