Just come across this site, thought others may be interetested in this. Carbon prepreg board on a hand shaped PU core that I made a few years ago. Been used a lot and still in fine condition. Totally ding proof, gets a bit hot in the sun though!
Just finished a long board with a CNC machined Styrofoam core and carbon prepreg. Very nice but no photos yet
Please expand on the entire process of using prepreg. Where you get it, how you apply it, what you have to be careful of, how you cure it, how you finish it, advantages/disadvantages. I think a lot of people besides myself would be interested.
Sweet looking color/ pattern scheme. Hell of a fin, it won’t slide out on ya though.
…carbon fibre boards are something I don’t see many photos of [I mainly have seen kitesurfers and tow-in boards here made of the stuff …and fins , of course !]
I’m with KeepOnKeepnOn … what IS pre-preg ? A rundown / “talkthrough” of it WOULD be good …cheers !
Ok, heres a quick summary. Prepregs are the industry standard for making anything expensive, race cars, aeroplanes, americas cup yachts etc. By taking the fabric and impregnating it in a controlled manner on a machine it is possible to have much more accurate control over the resin weight and fibre volume fraction. Furthermore, it is possible to use much more sophisticated epoxy resin systems that will have much better mechanical, thermal and chemical properties than a wet lay-up type epoxy. Generally wet lay-up epoxies have been diluted with reactive diluetents to bring the viscosity down to a suitable level for laminating. Prepregs can be laminated much more accurately than dry fabrics and wet resin and are much easier to handle, hence there use for critical structures such as formula 1 cars and fighter jets. However, there are some significant disadvanntages.
Temperature. All epoxy resin used in prepregs need to be cured at elevated temperature. Generally this is either 120 deg C or 180 deg C. I work for The Advanced Composites Group (www.advanced-composites.com) who specialise in low temperature cure prepregs. We have resin systems that will cure as low at 50 deg C, which makes it possible to use them with the typical low density polymer foam cores used for surfboard manufacture. However, these low temperature cure materials have very specialist curing agents that mean in some cases they will react with the typical gases emmited from blown foam at temperature leading to undercure. So with a polyurethane foam core (such as Clarke foam) isocyanates left over from the chemical used to manufacture the foam can inhibit the cure of the prepreg. Similarly with Styroam residual styrene can cause problems. There are ways round this though.
Consolidation. These epoxy resins are basically like chewing gum at room temperature. At elevated temperatures they drop in viscosity and then cure. To ensure good laminate consolidation and bonding to the foam core they have to be vacuum bagged, at good vacuum levels (mimimmum 28" Hg). So vacuum bagging is a must.
Outlife. These resins are mixed prior to prepregging so are basically curing all the time. This is prevented (or slowed down significantly) by storing in a freezer at -18 Deg C. Typically a prepreg that will cure at a minimmum of 65 deg C, will be usable for 30 days at room temperature (18-20 deg C) and will have a freezer life of a year at -18 deg C. A 50 deg C curing prepreg will be usable for 4 days at room temp and will have a freezer life of 6 months at -18 deg C. So storage in a freezer, and in most cases, shipping in refrigerated transport is necessary.
Cost. All the above add up. I cant give accurate prices but expect to pay anywhere between £30-£50/m2 for a 200 gram woven carbon low temp cure prepreg depending upon the amount purchased.
The advantages are its very easy, quick and clean to use. Just remove from the freezer, allow to warm up, cut the pieces to shape, remove the protective polythene film and stick on the foam core, then vacuum bag and stick in an oven/heated box. A typical cure would be 16 hours at 65 deg C. It only really makes sense for carbon
boards, glass prepregs are made, but wont give the clear translucent finish that you get with a wet lay-up resin so the board would have to be painted/hot coated or similar to look nice.
My two carbon boards are very nice, but very, very stiff, I am only a beginner so dont think this a a problem, but others have said that some people would find them too stiff. Also, a non painted, black carbon board will get very hot in the sun which could lead to blistering of the foam, and certainly makes the wax melt, I generally keep them in a bag and have had no problems in the British sun!
Thanks for taking the time to explain TomC, that is very interesting info. With a rough guess on percentages, if you used the same carbon cloth, how much stronger and how much lighter do you think the prepreg process makes things as compared to a normal lay up then vac bagged?
Comparing a carbon skin to a glass skin, a typical woven carbon/epoxy laminate has an elastic modulus (stiffness) of around 40 GPa compared to a glass/epoxy laminate of around 15-18Gpa. There are more expensive grades of carbon that can be anything up to five times as stiff as a glass laminate. The density of a carbon laminate is around 1500Kg/m3 compared to a glass one of about 1900Kg/m3
So assuming the foam core used was the same for both, you could make a board of the same stiffness but half the skin weight, or the same weight with thicker skins. Although its probably not as simple as that, a super light carbon board with say only half the skin thickness of a glass one would be very prone to dinging and maybe the skins buckling of the core. A carbon board made to a similar weight to a glass one would have a thicker skin and be around twice as stiff and strong, also the thicker skins will be much more ding proof. The one I have been using for around four years now is still pristine.
It does open up other possibilities, you could make a thinner board (core thickness) with a a similar strength to a standard thickness board by using carbon skins rather than glass ones. Or by using carbon skins instead of glass you could go to a lower foam density and not worry about the damage as much.
By using prepreg carbon rather than just standard vac bagged wet epoxy and dry carbon cloth there would only be a minimal weight saving, maybe 10 to 20 percent. There would be some weight saving in the finishing as there would be no need to hot coat. The beauty is that it is much easier, cleaner and stress free to apply. Especially if you have free access to the stuff at work!
As has been done with glass boards there would be a lot of scope for development and optimising.
Someone posted a while back (I think it was Kenz) about making his own pre-pregs. If I recall correctly he used regular fibreglass, spread it on a sheet of plastic, laminated with epoxy, rolled it up and froze it. I’ve been meaning to try it. Does this work? Does the freezing discolor the resin? I’ll probalby give it a go on something small scale.
Some years ago, I’ve a friend who had a femur broken by a carbon fiber board. Has anybody experienced something like that? I know that in F1 racing cars carbon fiber represents advancement and security. What in relation to surfing?
Ive made two, the CNC machined long board was HD300/F flooring grade Styrofoam from Dow, density is 45Kg/m3 which is the highest density one they do. I just had a sheet available. You could go much lower they do a 28 and 30Kg/m3 which would make more sense. The long board with two plies of 200 gram carbon prepreg and a fair amount of epoxy paint on it is still much lighter than the hand shaped/glassed standard type construction that I copied it from.
The previous one (with the yellow pin stripes) was a blown white PU blank which I then hand shaped. Am guessing it was around 25-35 Kg/m3. It also had two plies of 200 gram carbon all over and a small amount of paint and acrylic lacquer. It is much, much lighter than the wet lay glass/foam one I copied and much stronger/ding proof Although I have never weighed them, I would say it was 2/3rds the weight.
If you were really after a a pukka prepreg type construction then the best foam would be the low density grade Rohacell which is a PMI (poly methacrylimide) foam used widely in advanced composite construction. Density down to 30Kg/m3 can be brought (http://www.rohacell.com). Its very nice stuff to work with, and much more structural than EPS, PU or PVC, wont outgas and wont inhibit low temp prepreg cures. Expensive though. You could probably get away with one 200 gram ply for the skin and have a super light, rock solid board. Would be radar transparent as well which is always good for those stealthy surf trips!
Yes, the ding proof qualities of these boards mean that they will certainly do you more damage if landing on your head from a height than a glass board. Dont know about breaking legs though, I guess if any board hits you that hard something has to give. The F1 guys have some huge crashes, the stuff absorbs a lot of energy when fracturing/splintering/delaminating so is very good for crash structures.
If you have to use 28" HG - there’ll be a limit to how low you can go in core density without crushing it. Be interesting to hear how your styrofoam core holds up.
What sort of price difference is there between Prepreg skins and drycloth/epoxy. Roughly twice? I like the stiffness in a deck, but wouldn’t do both sides with it. How well would a regular fibreglass lap stick to it?
And as for that frozen ocean you got in hawaii, I thought us Canadians had it bad… until I found out that guys surf in Norway. You guys have to be ‘Berserk’. (Yes Haavard, I too would do it despite the thick neoprene armour that is no doubt needed. That could also be from my norse heritage though too.)