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quinn
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+xThere's a few theoretical differences, things like crimp in the weave etc. Theoretically, a lightweight woven cloth will be stronger than a heavyweight woven cloth because as you say, there is less crimp in the weave and hence less of a tendency for the fibres to want to straighten out under load. In the real world, how much difference this actually makes is going to depend on the application. For most applications, the economic benefit of heavier weight materials being cheaper and taking less time to layup is more of an influence than the theoretical difference in strength. The other factor of lighter fabrics is you have more opportunity for fibre orientation for a given thickness, and they are easier to manipulate around geometry. For flat sheet, this obviously isn't normally too much of a concern, and so heavier weight materials are an advantage. You'll also find that especially with carbon, one layer of 800g biaxial infuses easier than 2 layers of 400g biaxial. All in all, I think the heavier fabrics are very suited to your application - you wont get much benefit out of using lighter weight fabrics. Good to know. Definitely potential for saving some money there. Next time I lay up some plate I'll order a couple different biaxials and test a few samples. Shouldnt be too hard to get some data. I can cut a few strips of different lay ups, anchor one end and measure deflection when applying different amounts of pressure.
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Hanaldo
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There's a few theoretical differences, things like crimp in the weave etc. Theoretically, a lightweight woven cloth will be stronger than a heavyweight woven cloth because as you say, there is less crimp in the weave and hence less of a tendency for the fibres to want to straighten out under load. In the real world, how much difference this actually makes is going to depend on the application. For most applications, the economic benefit of heavier weight materials being cheaper and taking less time to layup is more of an influence than the theoretical difference in strength.
The other factor of lighter fabrics is you have more opportunity for fibre orientation for a given thickness, and they are easier to manipulate around geometry. For flat sheet, this obviously isn't normally too much of a concern, and so heavier weight materials are an advantage. You'll also find that especially with carbon, one layer of 800g biaxial infuses easier than 2 layers of 400g biaxial.
All in all, I think the heavier fabrics are very suited to your application - you wont get much benefit out of using lighter weight fabrics.
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Steve Broad
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My limited experience tells me that 2mm thick panel made from any style of twill carbon will be bloody strong :-) The complete front clip of my car is mostly 2 layers of 220gm with 4 layers around the edges and a couple of stiffening ribs and weighs less than 7kg which includes the hinge mechanism and catches. With a twill there is no real need to orientate the weave although you could, if you wished, twist the 600gm layers 45 degrees.
I think 200gm is about 0.25mm thick so you would need 8 layers. Using a couple of 600gm layers sandwiched between 200gm cloth would do the job nicely and be a lot cheaper :-) Whether there are any issues with infusing thick carbon I will leave to the experts on here as I am a prepreg convert :-)
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quinn
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Another question about carbon panels, when doing just a solid carbon infused panel, no core, let's say 2mm thick for example, what would be the best choice for layers on the lay up? I understand how to orient weave depending on what direction strength is needed in, but what about heavier weave vs lighter weave vs biaxial? The couple of panels I've made were just 8 layers of twill 200g weave. Looks like it would be far more economical to use some biaxial for the inner layers since it seems cheaper for the thickness, also would it give better stiffness since there's no kinks in the fiber like weave? Furthermore, if it is a good idea to use biaxial for inner layers, how does different thickness affect strength? I could use 200g twill weave for outer skins, then 4 layers of 400g biaxial, or 2 layers of 800g biaxial. The 800g is more economical, but how does the 2 options affect the lay up?
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Hanaldo
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Pretty easy to make a solid foam core infusion ready, but it does make them heavy. I've played around with this a far bit. To get good wet-out, you need to punch holes at 2" centers, and lightly grooving the tool side will help avoid pinholes between the holes (this only needs to be as much as a light score with a flathead screw driver). As with all solid infusion cores with thin skins, you will get some print through.
It does come out somewhat lighter than Soric, but there's negligible difference between this and 3D PET core (at least in thicknesses up to 5mm), which makes it somewhat pointless given the effort. I cant remember the exact numbers, I posted a thread on here about it a couple years ago. From memory 3D PET core consumes about 300g/m2 per mm thickness, drilled and scored foam core is 295g or so. Heavier than a 2-shot process anyway.
Corecork is still the lightest infusion core I have used at about 190g/m2/mm resin uptake. But has less compressive strength than other cores.
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oekmont
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I fear .3 square grooves won't do you any good. You could however drill a pattern in the foam, and infuse through the core, with flow media on top of the stack. If this is lighter than the glueing has to be tested.
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Steve Broad
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+x+x+x+x+x+xWith regard to bolting, I use these. Length of tube equals thickness of composite so crushing is not possible. Machined out of aluminium round bar and glued into the composite with a suitable adhesive. This one is actually threaded (M6 which will give you an idea of size) as it sits in the car floor with the bolt securing an undertray. The washer spreads the load over the carbon undertray.  These are bigger and secure the pedal box to the floor. They aren't threaded so are just top hat washers but were countersunk prior to gluing in. Just an idea.  Those look pretty handy, although I think I would gain more weight back than I lost going with light core. About 30 bolt holes in a 200mm x 500mm frame side. Although it would be nice to come up with a solution that doesn't involve reinforcing the bolt locations during lay up. Maybe when cuttimg out side frames, drill an m3 hole to 4mm and epoxy in short sections of 4mm OD 3mm ID tube in each hole or something like that. I'll probably just experiment with bolting through the sandwich and see how it does. Might be fine as is, especially the soric that pretty much ends up with a rigid honeycomb pattern inside. A foam core with no honeycomb would probably be more likely to have issues in bolt locations. Another possible solution could be a tiny key hole cutter with a 1mm shank and opens up to 3mm diameter by 1.5 high. After cnc drills all holes, it could come in with the key hole cutter and open up just the core to 5mm diameter at each 3mm hole. Fill those back in with resin and then drill to 3mm. Each bolt hole would end up with a 1mm wall column of resin between outer skins for reinforcement. Actually I could even run the keyhole cutter around the outside edges as well and have a 1mm thick wall of resin bridging the skins at the edge. At that point a lighter foam core could probably be used. Starts to become a lot of work though at that point though Aluminium is approx 2.7 times as heavy as epoxy resin. A quick calculation using aluminium top hats 6mm dia and 2mm deep with a 10mm dia flat top, 30 of these will weigh 7.5gms :-) Are you sure that's right? Each one is a quarter of a gram? That doest seem right. An aluminum m3x8 screw weighs that much. How thick is the 10mm flat top? And are you figuring for a 3mm hole or 4mm hole? I think so, it seemed light to me also so I checked and checked again :-) 6mm diameter tube with a 3mm dia hole and 2mm long (thickness of composite) = 0.1145gms 10mm diameter 'washer' with 3mm diameter hole and 0.75mm thick = 0.1447gms 0.2592gms x 30 = 7.776gms :-) There is, of course, the extra weight of the bonding adhesive :-) A lot easier that faffing about with resin and a lot stronger, as long as you have access to a lathe. This is the way I have done it on my car and I am so anal about weight that I even centre drill titanium bolts! Interesting, yeah that is pretty light, and its actually even lighter because you need to figure the 2mm thick, 6mm OD 3mm ID section of lay up being removed that the aluminum replaces. Gosh, yes, forgot about that :-)
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quinn
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+xCarbon+rohacell has clearly the best stiffness to weight ratio. Because of the thin core, bonding cured skins to the core might take away a big part of the savings. But if you aren't messing around with the glue, you should come out lighter than a lrc solution. So do they make rohacell or something similar with grooves in it to allow resin flow if I was to try infusing it in one shot? Or maybe If not, I could mill these grooves myself? I could do like .3mm square grooves every few mm on both sides. Do you think that might make it an infusable sandwich?
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oekmont
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Carbon+rohacell has clearly the best stiffness to weight ratio. Because of the thin core, bonding cured skins to the core might take away a big part of the savings. But if you aren't messing around with the glue, you should come out lighter than a lrc solution.
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quinn
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+xIf you're open to doing a 2 or 3 shot process, then honeycomb cores are by far the lightest (they barely add any weight to the skins, aside from the adhesive). So basically you just infuse 2 skins, spread a thin layer of resin on the back of them, and then clamp them to the honeycomb?
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