Hi,

I make carbon fiber fins for freediving
with OOA prepreg twill 200 gsm
I use a mold and a counter mold in press
and a heating system under the mold

The issue I am having:
Sometimes my part comes out curved lengthwise instead of being perfectly flat.

If anyone has an idea of to solve this issue it will help me a lot.

Thanks

My guess would be either an asymmetrical, unbalanced (hard with cloth) laminate schedule or an unevenly cured or resin-bled part. 

How many plies?  If you are using twill it should be that each ply is symmetrical and so if just 0/90 orientation, the whole layup would be necessarily balanced (just cross-ply) and symmetrical.  Are any of the plies at +/-45 or some other orientation?

Does resin bleed out during cure?  Could that be a source of variability?  Is the mold temperature ramp rate controlled by a PID or is it just a switch?

If you cured it faster on one side than the other - which would be hard if it is just a few plies in a heavy metal mold in a press - then it could have cured-in stresses.  Do you have a way to check the temperature across the top and bottom mold/platen halves?  It sounds like this is a sometimes problem - which is puzzling! 

Good luck!  Please follow up with anything you learn and let us know!.