| Author | Message | ||
     Torquehd |
For you guys who have some experience making your own carbon parts. i've made a few little projects out of carbon and glass, like a radiator shroud for my 1125. i'm ready to step up to something bigger now. a couple questions. 1. how thick should the average body panel be? the 5.7 gram 3k 2x2 twill that i've been using is .009" thick. that would require almost 14 layers to get up to even 1/8" thick. 2. are plain-weave, biaxial, triaxial as "conformable" as 2x2 twill? the 2x2 twill i have is super conformable and easy to work into the nooks and crannies. if i switch to a thicker (12k, .022" thick)biaxial or plain weave, will it still be workable into all the little nooks and crannies? I intend to also step up to vacuum bagging, i believe that should help with this. | ||
| Slaughter |
The only answer is "it depends." Designing for stiffness? Designing for strength? Bearing? Max Temperatures? Going to paint when done? Vacuum bagging can help reduce weight but it's a pain in the ass when you only get to save a little weight in the excess resin you've removed. Tools must be designed around the vacuum bagging or you have to envelope bag the entire tool. Vacuum/oven curing works really well if you can get prepreg materials instead of bucket and brush (wet layup). If you go this route, you'll be disappointed in the visual quality until you get some practice... plus the commercial grade prepregs are about $60 a pound. If you can find them in less-than-full-roll, you're going to be paying closer to $100 a pound. You CAN get surplus but it's hard to find somebody who'll sell less than a full roll - and that will still set you back at more than $25 a pound. Racing bodywork can be made very light -about 5 plies of the twill but you MUST support/mount. MUST build up around mounting points because bearing strength sucks. Carbon looks cool but doesn't tolerate abuse as well as glass. I can get carbon fiber at cost at work but build fiberglass race parts for the bikes I race. | ||
| Torquehd |
Yeah, i know, everything is dependent upon circumstance... i just wondered if there was a rule of thumb or "norm" for non structural body components. I'm still curious about the thicker material - how it conforms in the mold. i'm not worried about the parts i'll be bagging, just the un-vacuum-bagged ones. thanks for the input. | ||
| Aesquire |
The triaxial material is stiffer, doesn't conform as well, but has more strength in multiple directions, the others conform better. home cheater "vacuum bagging" can be done with a female mold and a bladder/innertube to apply pressure to the part. Not as elegant, but workable and only a few percent difference in resin squeeze out. I've even seen folk wrap a tubular part in heavy mylar coated in mold release and wrap bicycle inner tubes around it for pressure. In Aircraft applications, most of the work involved is the sanding after, to get a smooth surface. The really hardcore builders on Lancairs & the like spend nearly as much time in surface prep as they do in building the structure in the first place. Safety tip, always, always use appropriate gloves, respirator, and face shield. A lot of folk have had to abandon a $50,000 project because they developed an allergy to the resins. Being paranoid about skin contact, and breathing fumes helps a lot at keeping you both safe, and able to keep using the stuff. | ||
| Sifo |
I'm curious if you just use the same epoxy resin that you would use for glass? How thick something should be has many variables and depends greatly on the design. Stiffening ribs can be used, or you can sandwich balsa wood or foam and get some incredible results. Ribs can be built up by sandwiching foam or balsa. Bottom line is you need to have some understanding of the forces involved in the part you are planning to build. A typical body panel doesn't need much strength over most of it's surface, especially if it has a compound curve built into it. It will probably need areas of support for mounts etc. Also how well do you expect it to survive abuse is a big factor. One nice thing, you can always add more layers, ribs, or what have you. I only experimented with CF once when building a model airplane wing. I never got the chance to play with destructive testing on that wing though. It added a surprising amount of strength to the area that was laminated though. One thing I did learn working with glass... When you are laying it up you tend to get far too much resin in the cloth. Be liberal to blot out excess resin with paper towels. I'm not sure that it's possible to blot out too much, but it's real easy to get dry spots if you are stingy with the resin when wetting the cloth. This method give much better consistency. Excess resin of course does little more than add weight. | ||
| Slaughter |
There are tons of techniques - make a stippling brush by cutting the bristles down to half-inch. You can apply the resin AND work out the excess. Latex squeegees or any squeegee that you can grab ahold of that will flex to follow the contour will help you remove excess resin. Good to get the feel for fiberglass before experimenting with carbon since you're now unable to see the proper wet-out or air trapped in and below the fabric. If you use fiberglass, you can use polyester resin (commonly available at boat shops and hardware stores). Polyester doesn't work well with carbon/graphite. Long story there. It's fun stuff to do. What are you making your molds/tooling out of? Composite? Plaster? Need recommendations? | ||
| Sifo |
Polyester doesn't work well with carbon/graphite. So I assume there's no special epoxy resin for CF work then. I knew polyester wasn't the way to go. It's got numerous good qualities, but isn't the best for high strength to weight ratio products. The technique of blotting with paper towels will get more resin out than a squeegee. It's not a replacement for the squeegee, but just another messy step. As I understand it you won't ever get to the cloth/resin ratio of prepreg CF, but I think that stuff pretty much requires vacuum bagging or some similar process to keep thing together while curing. It' been quite a few years since I've read much about using prepreg though, so some things may change. I have had the chance to test sailboards that were pretty identical with the exception of on having CF strands in the lay up, in only one direction. There was no difference in the weight of these boards, but the stiffness of the CF reinforced board was undeniable. The finished product is up to the creative ingenuity and the amount of engineering that you put into it. Most CF bike parts are nowhere near what they could be IMO. The problem is that development and labor costs go up dramatically and a very simple CF fender is already incredibly light. Is it worth four times the price to drop another 40% in weight? For most people the clear answer is no. When you see how it's used in big projects like aircraft wings though, they do a bit more engineering and labor than on your bike fender. | ||
| Slaughter |
This is what we're building at work. The wing ALONE will be slightly over 300,000 pounds of carbon fiber: http://www.stratolaunch.com I'm kinda the head materials geek for Scaled Composites. Fun stuff. | ||
| Sifo |
Scaled Composites! They never got back to me about the resume I sent them many years ago! | ||
| Slaughter |
Sheeit... send in a resume NOW to their online site http://scaled.com/careers/ They're still looking but being really picky. They staffed up most of their open slots in technical and engineering but they ARE continuing to look because there's ALWAYS turnover. | ||
| Uncle |
WOW. Both those Stratolaunch fuselages are only connected by the wing. That's pretty impressive. | ||
| Aesquire |
Sweet. As a recoverable first stage that's bold, and quite workable. The last such I saw that looked as good used 8 SR-71 engines on a broad flying wing platform first stage for the Shuttle. ( reusable, flyback, like the Stratolauncher above. ) The vid above shows 4 stages, with the second, ( first rocket stage ) probably recoverable. Stage 3 looks like a burn up, and so does 4. ( which is the orbital change bus ) http://www.projectrho.com/public_html/rocket/multi stage.php The Grail is the SSTO ( single stage to orbit ) which is just at the edge of our material technology. http://www.projectrho.com/public_html/rocket/surfa ceorbit.php I had hopes for the DC-X technology demonstrator getting a follow up, but political considerations trashed that. The "Grasshopper" flights from Space-X are the private efforts in that direction, and look promising. I wonder if SC needs old school machinists? It has a very good rep, and many of the employees are making homebuilts in the hangers at the spaceport. Very nice stuff. Record breaking airplanes have come out of there for years. That Stratolauncher is the next best thing to the Shield Helicarrier. ( being built in the secret underground base nearby, I'm sure ) | ||
| Rohorn |
I've had good results stiffening up areas that needed it with Coremat XM, sort of like honeycomb for wet layups. It conforms to simple curves very well and compound curves somewhat, but only if it is oriented the right way. I've only used the 2mm core - 2 ply of 8.9 8HS glass on either side makes a very stiff and light seat base, among other things. I also used it for fabricating a gas tank (4 ply on one side, 2 on the other), which crash tested badly when the faces delaminated from the core and I ended up wearing a freshly filled gas tank's contents with the bike on top of me. An educational moment... | ||
| Torquehd |
The wing ALONE will be slightly over 300,000 pounds of carbon fiber unreal! that's got to coast a dollar or two! Hey thanks for the help guys. I love badweb, everyone here brings something to the table. | ||
| Reepicheep |
Cool concept! And even mocked up with GE engines, though the proportions looked wrong for the GE-90. | ||
| Sifo |
Sheeit... send in a resume NOW to their online site http://scaled.com/careers/ They're still looking but being really picky. They staffed up most of their open slots in technical and engineering but they ARE continuing to look because there's ALWAYS turnover. It was a very long shot back in the '80s. The skills/knowledge that I though might be of value back then have not been focused on for a long time now. Meanwhile the tech world moves forward. I recognized Rutan back then as a person who can think outside the box and make work what others say can't work. Truly an amazing innovator. I wonder what would get built if Burt and Erik ever collaborated? It strains the imagination. | ||
| Torquehd |
Rohorn, I'm interested in making a composite coolant reservoir. What kind of resin did you use for your gas tank? I've heard that vinyl esters are more chemical-resistant but i don't know where to find information as to whether or not they'd be compatible with engine coolant. | ||
| Rohorn |
West Systems epoxy, 105 resin/206 hardener. I have no idea how coolant friendly that might be - or if the 207 hardener would work better for that. | ||
| Etennuly |
Ahhhhh.....so it IS rocket science!  | ||
| Slaughter |
Generally Vinyl Esters will outlast stainless in corrosive environments and will do better than epoxies. West (Gougeon) has great tech support. http://gougeon.com/ If you can't HEAT it, you'll not be able to use it at temperature or in oddball environments (fuel, water, hydraulic fluids) for at least a week. ProSet tech support is really good. Heck, tell them you talked to Slaughter from Scaled - they know me there. If you want specific vinylester recommendations, it gets more involved in formulating/promoting/catalyzing. | ||
| Torquehd |
I emailed proset and west, we'll see what they say. I'm also considering rotocasting, but it looks like the resins for rotocasting are about the same as for laminating. Unless there's something I'm missing. I wonder if I could heat some kind of PET polymer or something like that to it's melting point, and rotocast it... more research to follow. | ||
| Slaughter |
Rotomolding requires more tooling pain than it's worth. Too much work unless you're looking at dozens to hundreds. | ||
| Aesquire |
Finally found the data I was looking for in back issues of Kitplanes magazine. Bolt holes should be at least 3 bolt diameters from edges of composite structures, Bolt torque should be just above finger tight, and no more than half the torque you'd use on metal structure. Crushing the composite will weaken the joint..... a Lot. Also the orientation of fibers should be across the shortest distance from bolt hole to edge. In other words, if the fibers are running from the hole to the edge, they will fail long before fibers at right angles or 45 deg. And...... A repeat of my earlier warning. Avoid skin contact & fume inhalation of your chosen resins. Even the far more user friendly vinyl esters can cause allergic reactions. Being careful really helps prevent this. I've done quite a bit of work with ABS plastic sheet heated in a home oven and hand molded around forms. We use ABS for "sport armor" and form breast plates, shoulder cops, etc. directly on the person, protecting them with a damp heavy towel, using heavy leather gloves to protect the hands. A cold water bath "freezes" the shape nicely, You need to do some practice pieces to get a feel for spring back. |
