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Ra V's unique machined body core
by chris churchill
created 10/21/2005
submitted 10/21/2005 08:11:24 PM

Ra V has the distinction of being the only solar car to construct a complex body shape without producing a mold of any sort.

Our problem: construct a solar car body using foam core. Reasons for a foam core over a honeycomb composite are many. The driver is much more comfortable, as the solar car tends to be 10 degrees cooler than the outside air, rather than 15 degrees hotter in a CF body. Also, closed cell foam does not require as much epoxy to get good adhesion to fabric, so some weight savings is possible if the correct amount of epoxy is used in a wet layup.

Why we didn't want the traditional mold/plug process: Anyone who has gone through the plug/mold/layup process knows what a long process it can be. The machining tolerances to make a good plug make for a very expensive job (or difficult to get donation) and the challenge of making a smooth mold is best left to professionals or at least a seasoned mold veteran. The process consumes several times as much material as is actually required to make the body. Finally, we thought it would be impossible to squeeze flat sheets of foam into the complex curves that we wanted on our car's corners and integrated canopy.

Solution: why not just machine the foam core itself? What started off as sort of a joke developed into a process that gave us the freedom of a molded shape and the insulating benefit of foam. For reasons I will explain, we didn't exactly get the weight benefit we were hoping for...

When we approached sponsors about this idea, we were initially laughed at, but eventually we found a company willing to humor our crazy idea. We started by gluing together 3" thick blocks of foam in a shape large enough to contain one half of the clamshell body. The type of foam used was the usual green or blue foam that most foam-core solar car bodies use (see Rolla's Solar Miner cars, Ra IV, Ra VI).

Next, we milled out the inner surface of the body on a large 3-axis mill. This milling process is well known to most teams. After the inside surface was complete, we did a wet layup of single ply kevlar on the foam block. The kevlar was laid up under a light vacuum, enough to get good adhesion but not enough to compress the foam. The layup needed to be done in between machining steps so that the foam would be stiff enough to withstand the pressure of machining the outside. After the layup cured, we glued temporary plywood ribs to the inner body, giving it stiffness. These ribs would stay on until the full layup was complete.

After the plywood ribs were glued in, the entire block was flipped over and the upper surface was machined away. At this point, the remaining core was shipped back to the Principia workspace, where we completed the layup on the upper surface. All that remained was to cut the excess core off the sides, do a final wet layup on the edges, and glue in some foam ribs for extra stability.

The result? Acceptable, but by no means ideal. There were plenty of problems that kept this body from being all that it could have been. I will try to explain a few of the pitfalls inherent to the process.

- Though they look flat, 3" sheets of foam are by no means perfect. When they are glued together, significant gaps still remain. They were partly filled with glue, so we figured that it would be enough, but after the layup we found that the interface between blocks was a serious weak spot. It took a month and probably a gallon of epoxy to fill the empty space, usually with a syringe, a painstaking process that could be avoided by choosing a stiffer glue and making sure we used a lot more of it. Gorilla Glue is what we used, by the way.

- Cutting the excess foam from the edge of the body was a difficult affair, since the machining sponsor had neglected to cut the score line we had indicated in the computer model. Bottom line: we cut too far and ended up with a minimum 1/2" gap across the whole body.

- Edging: We did the final edge layup after we had removed the plywood supports. Big mistake. The body sagged a little bit, and we ended up laying up fabric on the edge of the car that changed its shape enough that it would not fit well. We managed to bend the body back by forcing it into place during the rib layup, but the resulting shape was a little curvy and not exactly what we wanted. If I did it again, I would be very careful about how I laid up the edges, so that the body stayed in its desired shape with the help of the internal ribs as long as possible. I may even go as far as to make a supporting structure for the upper surface, so that when the internal ribs were removed, the body would stay true while the foam internal ribs were laid up.

- wet layup problems: Maybe it was the humidity, maybe someone didn't mix the epoxy enough, maybe our mixing machine was a little off, and maybe it was the residual stresses resulting from our series of layups on different body shapes. The bottom line was that the epoxy never quite cured, resulting in a slightly floppy body that fit a lot better when it was hot out than when it was cold.

- air pockets: this is probably related to the glue problem and the bad wet layup. When the body would heat up, the kevlar would randomly delaminate and swell, creating a bubble. Needless to say, this was very bad on our array. It only happened in a few places, but where it, it looked pretty bad. We would fix it by popping the bubble like a zit and then filling it in with epoxy.

Well that's about all I can tell you about the process. If we did it again, I'm sure we could cut 1/3 of the weight off and make the shape perfect. The biggest downside of the process, of course, is that you can only make one. If you ever decide to go this route, I recommend you talk to Robyn Goacher. She was the main architect of the process and can be contacted through the yahoo solar raycing group.

1062 words | cbc1920

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	Ra V body core Photo by chris churchill submitted 10/21/2005 Ra V's unique machined body core, before layup

	Ra V body core Photo by Chris Churchill submitted 10/21/2005 Ra V's unique machined body core, before layup

	Ra V body finished Photo by chris churchill submitted 1/26/2003 Ra V's unique machined body core after its layup of single ply kevlar and the final paint job.

	Ra V body finished Photo by Chris Churchill submitted 1/26/2003 Ra V's unique machined body core after its layup of single ply kevlar and the final paint job.


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