I've finally gotten a mold made for my bmw e30 racecar trunk/boot and I am starting to plan out the layers of materials so I can infuse the part.

Are there any general recommendations on how many layers and core material to use? This will be basically just a skin part (I won't be making the inner structure like was done in that mini cooper bonnet video from easy composites) that will be pinned to the car. Obviously the car is only for track use so lightweight is the goal, without being to flimsy.

Thanks for your help!

For a race car, I would be doing a single layer of ~200g carbon either side of a 2mm Rohacell foam core. You will get a bit of print-through, but on a race car I prefer that over extra weight for the sake of cosmetics.

Also a good idea to identify where you are going to place your pins beforehand, then cut the foam core out in those areas and replace it with solid carbon (so for a 2mm core you would need ~8-9 layers of a 200g material). The pins can place quite high compressive loads on the panel which the foam core won't like, so better to have monolithic laminate in those areas. Not as important on a bootlid, but good practice.

For the Rohacell foam, would Mycell foam be a suitable alternative? The supplier I usually order from only has the Mycell, they also have Soric core (but I remember you saying before that it is super heavy for it's application).

Also, do the more rigid foams like Rohacell and Mycell provide a significantly larger stiffness than the Soric core? I have made sandwich panels with the Soric and just a single layer on either side and I think it would be stiff enough for the trunk piece but would feel better knowing that the foam cores are a lot stiffer than the Soric.

Thanks again!
Patrick

It's not so much about the stiffness, it's about the weight. Rigidity is largely a function of thickness, so if you had 2 laminates with the exact same layup and thickness but different types of core, then you likely wouldn't notice much difference in rigidity (as far as real world rigidity goes anyway, there will be some differences if you measured them scientifically). Certainly for something like a bootlid, which isn't really a loaded panel, it doesn't really matter what core you use from a structural perspective.

But the weight differences can be huge, relatively speaking. And a lot of it comes down to how much resin uptake the core has. PVC core has quite a high resin uptake. Divinycell isn't bad. Rohacell is the best. I dont personally know Mycell, it is probably a competitor to Divinycell which is the most common. 

Realistically, it isn't going to matter much. When it comes down to it, you're probably talking less than a few hundred grams difference. But Rohacell is the best, so I tend to just recommend it.

I Agree with Hanaldo, Rohacell is more expensive but the low resin uptake and good strength and quality means you can have a really light core.  Also being slightly stronger it means often you can use a lower density core compared to say PVC which is another weight saving. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

As always, great information, and wonderful help.

Im trying to secure some Rohacell foam core now, and looking at prices it may be less expensive for me to buy it from Easy Composites and have it shipped state side than to buy it from any supplier in the states!

Thanks again everyone, hopefully Ill have the core soon and can start the infusion!

I was able to get the rohacell foam from easy composites, so now that I have it was it the best method of using it for infusion?

Should I drill holes in it using a square pattern, score it, just leave it as is and infuse?

Thanks!

Yeh just punch fairly small holes (I would probably stick to around 4mm) roughly 2-3 inches apart. You probably don't need to score, but if you want to be safe then just score very lightly between the holes with a ruler or screwdriver or something. Just enough to make a light groove, you certainly don't need heavy channels. Just keep in mind the more holes you put in and the deeper you make the grooves, the more resin uptake you will have and the heavier your finished part will be, and the more print through you will have.

Personally, I would consider doing it as a 2 shot process. Infuse the outer skin first, then remove the consumables without demoulding the outer skin, and wet lay vac bag the core and outer skin in place. This is actually a lighter method and will leave you with a better surface finish, but is obviously more work. Food for thought.

Still going with this project :-)

Im having a hard time getting the Rohacell to conform to the near 90 curve in the trunk lid. I'm using a heat gun and that seems to be fine for slight bends but this angle seems like too much maybe? I'm thinking about splitting the core material into 2 pieces - 1 for the larger flat part of the lid, and a 2nd that spans the lower portion with the emblem (as seen in the poorly edited photo below). This seems less than ideal in my mind but I feel like I am otherwise potentially cracking the core. Any issue with doing it in 2 pieces, or advice on getting it to conform more to the curve?

Thanks as always!

No issue doing it in two pieces, save that it will be slightly heavier due to the more surface area being open to resin and the inevitable slight amount of bridging. You will also want to use more reinforcement in the gap between the cores to make up the loss of rigidity there, which is also a touch heavier. All more theoretical issues than functional ones, again you're realistically speaking grams.