This year our team scored some pre-preg carbon fiber material that was destined for the dumpster. We’re operating on a shoe-string budget, and we’re looking at ways to make sandwich-style components. We’re thinking lightweight panels for either protection, or ramps, if needed.
This stuff already has the epoxy mixed in, (pre-preg is short for pre-impregnated with resin) and the epoxy must be cured at 250 deg F. So our core material must be able to handle that. You basic sheet insulation from home supply stores melts below this temperature, though I’ve found some stuff from Dow that can take 250, but some quick toaster-oven tests show that it might deform.
Does anyone know of a cheap, high temp core material, that could take 300 deg F or so?
Yes, I should have mentioned that I was looking for the core material; the carbon-fiber goes on the outside, with a lightweight core on the inside. I’m looking for the core material. Aircraft use this stuff, and we could go out and buy this stuff for $100 or so a sheet; I’m looking for bargain prices, things like home insulation material, or other stuff that’s cheap.
Steel would not be a good core, unless it was honeycomb material, so it would be light. But I think that stuff is pretty expensive.
If ChrisH comes across this thread, you’d probably get the best information from him. I don’t really know much about working with composites, but have you considered wood? I imagine you could make some nice and light sheets with CF and balsa (or birch or hard maple if you want some more strength). The low price requirement kinda puts a bit of a limit on what you could use (polycarb and garolite, for example, get rather pricey in large amounts). Industrial cardboard can also be quite strong… I imagine a healthy diet of CF would make it even stronger.
I don’t like being a party pooper, but based on the rules last year, I think you need to do a little research on the pedigree of your carbon fiber before you consider using it on your robot.
Specifically, anything you use on your robot has to be (reasonably) commonly available to all teams to purchase. If your brand of pre-preg isn’t commonly sold to the public somewhere that has a reasonable amount of stock on hand, then it’s not legal for use on the robot.
Moreover, you have to account for the cost of your carbon fiber in your cost accounting, whether or not you paid full price, 90% off, or saved it from the dumpster. The upshot of last years rules is that you’re either allowed to use the fair market value of the entire amount you bought, or a quasi-prorated amount that’s probably almost always less. Quasi-prorating means you’re allowed to use the price of the smallest amount you could actually buy that would cover the amount you actually used. No finding a unit price and multiplying by the amount you used. So if carbon fiber comes in rolls of 100 sq ft for $500 and 20 sq ft for $150, and you buy a 100 sq ft roll but only use 10 sq ft, then you can use the $150 price of the 20 sq ft roll in your robot BoM, but it’s the full $150 that you account for.
This is an important rule because you only get $3500 total to spend on your robot. And much more importantly in this case, no individual component can cost over $400. In the above example, if your carbon fiber only comes in $500 rolls, then it’s not legal to use.
Again, this is all according to last year’s rules and things might drastically change, but I think it’d be a good idea if you found out the specifics of the carbon fiber you have and whether it or something extremely equivalent is available to other teams at a legal price.
As for a filler material, I’d consider stacks of insulating foam, machinable foam, and possibly even a light wood like balsa or a red cedar.
I would experiment with Balsa wood, since it can handle the curing temperatures, is stable and fairly light. Aircraft use it, as well as honeycomb aluminum - got a small airport nearby (of course you do!) - call the flight school and ask them what airplanes use as a core for prepreg, the’yy usually be happy to spend 15 minuts on the phone.
There are some high-temperature foams, but the weight savings (perhaps a few ounces) versus the costs (you don’t want to know) might not be a good deal.
Kevin: Thanks for heads up on the pricing, while the exact material is not technically available to the public, equivalent material is, and is priced fairly reasonably (for composite material) and is sold by the square foot. So even applying the rule you spoke of, which I’m also not sure is correct, since you can buy it by the square foot, then I think I’m OK on the pricing. Plus, I remember seeing at the NJ regional a team that used a fair amount of CF fabric, with a wet-layup arrangement, and they obviously got through inspections.
The general evolution of the rules at FIRST has been that more and more exotic materials have been allowed over the years. When I started with this in 2000, if it wasn’t in the kit of parts, it had to be either on the approved materials list, or have come from Small Parts Inc. The approved materials list was pretty small also, pretty much wood, fiberglass, and aluminum or steel from the home stores was the limit.
I have also looked at Balsa, but that looks kind of expensive also, what would be perfect would be some kind of home insulation material that can take the temperatures needed for curing. I’m actually testing some stuff right now, we’ll see how it turns out.
<R51> The total cost of all non-Kit Of Parts items must not exceed $3,500.00 USD. No individual item shall have a value of over $400.00. The total cost of components purchased in bulk may exceed $400.00 USD as long as the cost of an individual component does not exceed $400.00. The following items are EXCLUDED from the total cost calculation:
The cost of any non-functional decorations
The cost of individual fasteners, adhesives, or lubricants, unless any one component exceeds $1.00
The costs of SPARE PARTS. A SPARE PART used as a direct replacement for a failed or defective ROBOT part (either Kit part or non-Kit part) that has already been included in the cost accounting is covered by the accounting for the original part
All costs for the construction of the OPERATOR CONSOLE
The cost of items purchased in bulk or large quantities may be prorated on the basis of the smallest commonly available unit that satisfies the need for the item.
Example: A team purchases a 4’ x 4’ sheet of aluminum, but only uses a piece 10” x 10” on their ROBOT. The team identifies a source that sells aluminum sheet in 1’ x1’ pieces.
The team may cost their part on the basis of a 1’ x 1’ piece, even though they cut the piece from a larger bulk purchase. They do not have to account for the entire 4’ x 4’ bulk purchase item.
So first, if each individual component costs under $400 for something bought in bulk, then things are fine. Second, if the hypothetical team couldn’t find aluminum smaller than 4’ x 4’, then they’d have to use the price of the entire 4’ x 4’ piece instead of the price of the 1’ x 1’ sheet.
Admittedly, I’m unsure how exactly you account for something like a 6’ length of 1" titanium tube that’d usually cost you $600 when it’s available in 1’ lengths for $145. If all your parts are under 1’ long, is your cost $600 or $870?
Still, I think it’s pretty clear that if 1000 sq ft is the minimum size of your carbon fiber that Joe-Firster could ever find and it costs $500, then it’s not a legal part. The whole rule is aimed at preventing teams from doing something like making a carbon fiber robot from exotic and nigh unobtainable weaves and resins donated by their sponsor. So to Ted, if your pre-preg is going to be significantly stronger, lighter, etc. than pre-preg Joe-Firster could buy, then I’d be uncomfortable using it under last year’s rules. I’m not saying all CF was illegal under those rules, just any exotic extra strong fabrics or resins that, say, Team 57 couldn’t also acquire. FIRST is definitely allowing many more exotic materials than 2000, but they’re trying to keep the playing field level.
They also have a variety of foam core materials. We used the small cell styrofoam, 1/2" thick, inside the panels and embeded hardwood mounting points for our kickstand and hinges. We also added some heavy fiberglass cross spars as well. The ramps could probably hold hundreds of pounds yet weighed just a few pounds.
My professional curiousity wants to know what resin and fiber you are using and who manufactured it. This information would tell me just how “custom” the material is. As you can see from the above posts this is an important consideration in figuring out whether or not a material is “legal”.
For core materials, anything that will handle 250F is not going to be terribly cheap. Rohacell will certainly work in the proper grade. It will go over 350F if you keep it from picking up moisture and give it the proper “heat treat”. You can also form it if you handle it right. There is another product called Divinycell F from DIAB Products that will also work. DIAB also sells an end-grain balsa product called ProBalsa that wll probably work.
Rohacell and DIAB poroducts are readily available, but you have to know where to look for them. When I go to SAMPE (Society for the Advancement of Materials and Process Engineering) shows DIAB is always there handing out samples. If I had to pick one company to approach for “sponsorship” of materials I’d start with them. They seem to be happy just to have thier materials out there. It seems like they give out tons of 6"x6" samples.
All these materials come in various thicknesses and densities. There is a direct relationship between density and strength. For sandwich cores the most critical property is typically shear strength, followed by compression. If there is a significant amount of tension on a face sheet you are doing something wrong. Check the manufacturer’s data sheet to find a material that works for your application.
Here’s a rough fabrication sequence:
trim core to manufacturing size. Composite panels are generally built oversize and then trimmed back. this is so all the fiber goes all the way out to the edge. The Stress guys don’t like it when a ply stops short of where it is supposed to.
Apply the proper release agent to a clean metal or composite mold or tool. This can be a flat plate for a flat part. Do not apply release to the outer two inches or so. Follow any cure instructions from the release agent manufacturer.
Lay up the plies of the outer skin. Depending on how sticky the prepreg is and how much contour your mold has you might need to “debulk” periodically during layup. Since this is expired material it is likely to be pretty dry or “boardy” so count on at least a couple of debulks. For a debulk you construct a vacuum bag over the part. First cover the part with a release film such as teflon film. Then put a breather material such as fiber glass cloth over the release film and out to the edge of the released area of the tool. Apply vacuum bag sealant to the edge of the tool. Bag sealant comes in rolls and is typicaly 1/2" wide x 1/8" thick. There are high temp and room temp bag sealants. For room tempertature debulks you can get away with the cheap room temperature stuff. Place plastic bagging film over the part and tool and stick it down to the bag sealant. Make sure you have vacuum probes that go through the bag. Hook the probes up to a vacuum source and let the bag “pull down” onto the part. The bag will compress the part and help the plies stick together.
Repeat the lay-up/debulk sequence untill all of outer skinn plies are laid up. Do a final debulk and then position your core. Debulk the core into place and repeat the lay-up/debulk cycles for the inner skin.
Now rip off your trusty debulk bag and throw it away. Rebuild it with high temp capable bag film and sealant tape. Make sure the bag does not leak.
Cure per the resin manufacturer’s instructions. You can cheat a little bit here. Sometimes you can cure at a lower temperature for a longer time or a higher temp for a shorter time and get the same results. But you need to check with the manufacturer before doing this. You will need at least an oven big enough to hold your part and the mold it is made on. You might need an autoclave, which applies pressure as well as heat, to properly cure the material. I do not recommend using a home oven for curing parts. Too much nasty stuff comes off to think about mixing it with food preparation.
Let the part cool and remove the bag. Throw the bag materials away, they are not generally re-usable. If this was the aircraft biz the part would then go to Non-destructive Evaluation (ultrasonic and x-ray inspection) to see if it was any good. Depening on the location, voids or other problems as small as 0.25" dia can cause a part to be scrapped. But whatever you get will probably work just fine for FIRST.
Trim and drill the part to final dimensions. Trim with a high speed rotary grinder like a radiac. Carbide spade drills should be used for holes. These are drill bits designed especially for composites. You can use conventional bits, but then you are talking drills/hole rather than holes/drill.
As you can see this is an expensive process in terms of labor. The ancillary materials like the teflon release film aren’t cheap either and the mold can be more expensive than all of the above. While things are better than when I started 26 years ago, there is still a sgnificant mount of “art” to composite fabrication. One giant hole in the cost rules is they don’t address the cost of ancillary materials required for fabrication. Of couse if they did you could probably forget custom fabricated composites. This stuff just isn’t cheap.
For work I just finished a graphite/epoxy test panel with a sandwich core about a week before Christmas. We used 1.5" thick titanium core. Ti core would certainly work for this application and it was the easiest core to work with that I have ever used. But the cost of the core block could fund at least one team for an entire season.:ahh: So it is probably out of your price range.
I couldn’t help noticing the poster’s location and closeness to a certain aerospace company that begins with a B. If those mid temp prepegs came from there then they are probably of a grade and quality that are not available to all teams in small quantities. I doubt that you could justify the cost and availability with last years rules. Remember the prepegs are shipped refrigerated next day delivery from the manufacturer. The shipping costs almost guarantee the cost is over the rule limit. Another thing about aerospace prepegs that you should be aware of is that there may be additives in the prepeg that are very toxic in the uncured state. These can be added to modify the surface tension of the resin or increase the fire rating of the material. You need to get info on what you have. Lay up today and loose your kidneys or liver next month. Now I’m not saying first teams should not use composites. Just stay away from the super stuff. Our team has used normal dry cloth and room temperature cure epoxy on our robot for years. Every team should put some fiber in their robot diet. As for core material, You can get AL and arimid honey comb material. Not cheap. If you want a flat board for your electronics or for the bottom of the robot, Try some Baltic birch plywood. This year our team will probably use a piece of 6 MM Baltic birch with a layer of S2 fiberglass laminated on both sides. It will stiffen our frame and give us something to mount electronics and other components to. Also don’t forget the fiberglass pultrusions. They come in many structural shapes and are available from McMaster Carr.
I’m almost positive that you don’t include shipping costs in your accounting. It’s far too variable from team to team for it to make sense. Including shipping would probably cut the robot budgets of Hawaii and Alaska teams by a significant margin. Similarly, I don’t think you include sales taxes or VAT.
Without going into detail, the material is somewhat custom, but not the chemical makeup of the epoxy or the fiber size/density/layup. I believe the only thing “custom” about this stuff is the type of roll it comes on, for the machine it goes into. MSDS data has also been checked, this is your basic epoxy; avoid skin contact when uncured; vapors are not a big concern (use ventilation) and wash hands with soap and water if you do get it on your skin.
So the equivalent material is available on the open market, just in a different width form, which for the finished product, is moot. For the cost accounting, we’ll cross that bridge when we get to it.
Even if it ends up not being allowed, the students are learning how advanced materials are made, and that’s worth the effort!
BTW: While I’m not an expert in composites manufacturing, I did do the 2 week course at Abaris training, in Reno, NV, for aircraft composite manufacturing and repair, so I guess I know enough to be dangerous! We covered wet lay-up, prepreg, vacuum bagging, and repair of honeycomb and foam core structures.
My original question was looking for cheap core material; I think I found it in Polyisocyanurate Foam, which is used for aicraft core material, but luckily is also a nice building insulation material sold at Lowes, in 8’X4’ sheets, at about $13 per sheet in 5/8" thickness. It seemed to handle the toaster oven test at 300 deg F. Dow brand Super Tuff-R. Just peel off the foil insulation first.
Team Fusion has used composite material in 2005 and 2007 for the electrical board, the ramps last year, and armor in 2005 (we had a bulletproof robot). We get our composite material from a local company that one of our student’s dad work for. We have had good experience with carbon fiber and fiberglass with a balsa wood core. As to make it, I’m not really sure. I know you need resin, heat, and a vacuum bag to suck the air out.