I'm looking for some general advice on making an intake plenum to suit a low boost turbo engine (7 PSI) out of carbon fiber using vacuum infusion, utilising my 3D printer to create the mould. This would be the first carbon fibre part that I've made, based on a design drawn up in Autodesk Fusion 360 shown below.

So far I've spent a few days doing research online, reading other posts in this forum relating to intake manifolds and 3D printing moulds and watched numerous videos on producing carbon fibre parts. However I still have a few open points I'd like to get peoples opinion on. 

• Based on the plenum shape I'm assuming I would create a positive mould.
• Should I use tooling gel or something like 3M Super 77 sparingly to ensure the CF attaches to the shape.  
  
• Does this sound right to create a 3D printed mould; use PLA filament, smooth down the edges, spray a few coats of plastic filler primer and seal with S120 mould sealer.
• For the 4 bolt end flange I'm guessing this can't really be formed using CF. Instead could I go about inserting an aluminium flange that could form part of the mould and be covered in CF. Would this hold up under boost? 
• Layer up wall thickness to be around 2mm, from what I've read this would be strong enough.

-Are you sure, that you are able to get a positive mould out of the part? If so, a positive mould would be a good option. But resin infusion might be tricky either way. Possible, but no beginner part.
-tooling gel definitely not, spray tack might be very helpful
-i don't know if the mould sealer works on top of primer. You should use 2 component primer, as the one component stuff causes many problems while demoulding.
-why that. I see no reason this can't be made out of crp. ideally in one piece with the rest, by attaching a removable plate at the end of your positive mould. This might make infusion even harder.
-for low positive pressure, this should be enough.

A positive mould would be something you would remove after the resin has cured. This is just the part printed as you want it to be in the final shape, am I right? There is no way you could remove the printed model out of the carbon piece, without destroying either. Plus, your measurements would be wrong in the end, because both flanges will be of by the thickness of the print.
A positive mould should look like the body that fills your 3d model. Without the flanges. To get your carbon part like your drawing, this body has to be placed on a flat board, that "forms" the big flange, and a small metal plate could be attached to the intake end of the body, to create a plane to form the small flange. After the resin is cured, you could remove the board and the metal plate, and if your geometry is right, you should be able to draw the 3d printed positive mould out of your crp part.
As I read your post again, I guess you are thinking about just covering the 3d printed part with carbon. I don't think this is a good idea, because of the temperatures inside the motor bay. especially with a turbo. This could cause countless problems.
In this case, primer and mould sealer would be unnecessary products.
In my opinion, you might think about a far easier and not structural part to start your composite experience. The way I see it, there is much to learn for you, before you could master such a slightly advanced piece.

An old post but, just in case the OP is still here.

I would reconsider making the plenum out of carbon. A friend, who builds race cars, stopped me from doing just this.

On a N/A engine there shouldn't be any issues. However, a turbo engine is a different matter. The pulsing that occurs in the plenum is such that there is a risk of the carbon failing and being sucked into the engine, which happened to my friend.

The small saving in weight isn't worth risking an engine.

I'm sorry but that is far fetched... I build lots of carbon intake plenums as well as various other engine components for lots of different race cars, from little weekend 200hp NA Civic's, to 700hp Japanese drift cars, to 1600hp twin turbo V8 drag cars... None of them have ever had a carbon plenum fail due to air pressure or pulsing or anything of the sort. I've had one set of plenums break on me, and that was due to the engine shifting 3mm under load and crushing the plenums against the strut brace - even then, the owner simply put some race tape over the crack to seal it up before continuing the race for the rest of the weekend. 

If you have a carbon plenum fail from anything other than an impact, then it wasn't built strong enough or there's something else funny going on there. Keep in mind that carbon is stronger than steel and ali - have you ever seen a steel or ali intake plenum collapse simply due to engine load?

I would like to kind of disagree with both of you.

-Except for interlaminar stress, carbon has excellent duration fatigue strength. And as the plenum should be under pressure from the turbo, I don't see any reason, why interlaminar stresses should be an big issue here. So I don't see any reason why pulsing forces should be a problem here.

-to say: carbon is stronger than steel, have you ever seen an aluminium or steel plenum collapse?, is in many ways a misleading point.
Carbon is in almost all cases not stronger than steel. The fibre itself under tension, yes. But most real life multidirectional laminates with 50-60% fibre content under complex loads are not. The higher the resin contend (to a certain degree) and the more the load differs from an simple unidirectional tension (so you have to include different fibre orientations) the better steel gets in comparison. In most cases, you could get strengths that exceed those of steel, but usually not with "lay up 4 layers of 240g". This needs knowlege and understanding of the exact forces and fibre orientation, as well as some calculations. Far more crucial here is the fact, that both steel and aluminium fail through deformation, while carbon is far more likely to break into pieces, which could enter the engine.
And the new plenum has a diffent shape, different wall thickness, etc.. that's like comparing apples and oranges.
Especially the "joint box profiles" with many plain surfaces and corners, shown earlier here, could be a real problem when applying internal pressure. And those are very likely bad from an aerodynamical standpoint as well.

It happened, whether you believe it or not :-)

However, it was a while ago so, hopefully, designs, techniques and materials have solved the issues. BUT, I think making one of these without the necessary skill and experience is asking for trouble. I certainly wouldn't risk it as an amateur.

I've realised my earlier post probably came across as quite confrontational - probably a combination of having stayed up to watch the Germany game last night, as well as only being half way through my first coffee when I posted! So I apologise, I didn't mean to sound so fired up.

I also don't doubt that it happened, I just don't think that it happened due to the material being unsuitable - there has to have been a fault in the design. 

Oekmont, I don't disagree with you either, I'm aware of the difference in properties between steel/ali/carbon. But a plenum isn't really a part under complex load, it doesn't take any real engineering knowledge to build one that is sufficiently strong to not just fall apart. Maybe if you are trying to save every last gram, but that is rarely the case.

I do believe you that it happend. But it was likely a construction problem. Wrong shape or insufficient wall thickness. Maybe insufficient overlapping of the layers of a two part mold.

As long as we are not talking about aerodynamics, intake plenums are really not that much of a challenge for experienced laminators. The second half of the sentence is quite important though.