OK...here is a part that I am making. Here is the mould. I did it with the EC gelcoat and paste for high temperature, anticipating that I could then use it for hand layup and prepreg. I am making my first part by hand layup (because I need to check all dimensions are good at the end). But man it was a mess with all the shapes...do you guys think that I would really be better with prepreg or would it be just as hard ?
Thanks !

Pre-preg would be easier to achieve a good result. In terms of lay up, you can actually use wet-lay in much the same way as pre-preg by creating what is called a 'wet-preg'. Basically just putting your reinforcement between two layers of plastic, pouring your resin over it, and then using a squeegee to wet out the reinforcement and squeeze out excess resin. Then if you leave the plastic in place, you can cut the reinforcement as accurately as you could a pre-preg, allowing you to use templates to kit out for your mould geometry. If you leave the resin to cure for a little bit, you can choose how tacky you want your material to be for laying into the mould. You can use it wet, or you can let it cure to a b-stage, where it would handle exactly the same as a pre-preg. You can even stick it in the fridge or freezer to slow the cure or keep it for another day. 

In terms of the result you will get using that method, it is still a wet-lay and wont be comparable to a proper pre-preg. But it may make the process much easier for you, allowing you to achieve a better result than you might have with traditional wet-lay methods. Certainly if you have the facilities and equipment to use pre-preg, that is a far superior process and definitely a path worth going down.

Hi Hanaldo,

Thank you for your feedback ! I can have access to a fairly good oven, I may give a shot to the prepreg to see how it goes.

But watching the EC video on out-of-autoclave prepreg, they make their part cutting multiple small pieces with some overlap between them. That makes the positioning of the prepreg easier, that I can understand, but then the thickness of the final part is not homogenous as it will be thicker everytime two parts overlap. I need a quite homogenous thickness of 2 mm. I have never worked with prepreg. Is it

Thanks!

No, that won't work. And if you might get it down the part will look horrible, because the fibres will be disoriented quite a lot. The main problem is the prismatic shape in the middle. Fibres which go across this shape will need to be much longer than fibres which run flat along the shape. This will leave a heavily diagonally streched area on both sides of the shape. This isn't a prepreg problem, this will always be the case.

Thanks ! Now if I don't mind something that is not good aesthetically speaking, is it doable ? If not, do you think that I can use a single piece for the entire part except for that prismatic shape in the middle, cutting my prepreg just at the edge of it, and then use separate parts for that prismatic form ?

As I would consider it possible done with infusion, it should be possible with prepreg with the help of a heat gun. I would definitely not do it. I would cut out the middle bump for the first layer, and do it with a second piece with a 1cm overlap. The next layer than leaves out the 1cm overlap section. The third one again with overlap and so on. This looks like a small car interior part. 2mm would be way overengineered for that.

Yeh you just need to plan your overlaps correctly in order to not get a step in the laminate thickness.

Doing this part in one piece without any joints in the material may be possible, but very very difficult. Expect several unsuccessful attempts while you learn how the material behaves, you will certainly end up with bridging all over the place initially. 

I agree with everybody who says don't try it in one piece!  Hanaldo's idea of using a wet-preg method with plastic on both sides is what I'd do - especially if you don't have / haven't used pre-pregs.  It is a great way to really controlled wet-layup.  Infusion would be good too.

Here's a video (scroll down a bit in link) of the wet-preg method - not super helpful for a flat panel - but it's just to show the process: 
https://explorecomposites.com/materials-library/laminate-sample-15-vacuum-bagged-wet-layup-carbon-with-foam-core/

You'll need to pattern everything pretty well before you start the layup and with 1600-2000g of material (2mm) you have plenty of room for slip joints / laps at the joints with other plies cut back and butted - if it's really critical to have it all 2mm.  You may need to do a little grinding but it should come out well if you are careful about slip joints in the material (and bagging consumables) and leave plenty of bag in the inside corners.

Dear all,

Thanks a lot for all your advices ! I did give a shot to OOA prepreg , and the results are stunning. First, it was so much nicer to manipulate...no epoxy and pieces of carbone fiber all over the place, less toxic, easier to put in place (I did spend a lot of time on how to cut the different parts and it was great, with a very consistent thickness). Then at the end of the day, in one afternoon, the part is ready, so faster than with the epoxy...and the results are awesome. OK, I can improve my technique a bit (a few microholes there and there...but to be honest, I forgot to vaccuum-bag for a while after the first layer as suggested on the EC  website...and I got some left-over prepreg from a friend, which is pre-preg for autoclave, not oven...so I applied the same curing time than suggested for the EC OOA prepreg and it worked just fine, but the prepreg I used is not optimised for that...so I am sure with the proper prepreg, it will be even better).
Anyway,  I am sold ! I will keep using wet layup for large and simple parts, but for smaller and complicated one, I will do it with prepreg for sure.
THanks  for all the tips !!!

Yeh it's super tough to stomach going back to wet-lay after having worked with proper pre-preg. Hell, even infusion is just a bit unpleasant with all the extra consumables etc. 

Wait till you've tried tooling pre-pregs! Regular tooling is just agony after that.