+xYes I was thinking of mould and counter mould made of rigid material.
I already did some helmets with mould and film on the other side and it was okay, not perfect because of pinholes and some bridging.
What do you recommend? silicone or something else to get as good surface as it could be on both sides?
And additional question, what would be best way to make serial production of helmets made of carbon, with which process?
bridging and porosity are not going to be solved with a counter mould. this probably just comes down to skills and techniques of the laminator and will come with time...
What process for serial production depends a lot of how many parts you intend to make, the shape, the resin system used, ...
The perfect process can be Infusion, autoclaved prepreg, RTM, BIM, compression moulding, ... I'm not 100% sure but i would guess my downhill bike helmet is made by hand laminating in a negative mould and then compacted with a pneumatic bladder. This means high up front investment in the tooling, but short-ish cycle times.