Composites/Bosywork

[mechart87]

Hey Everyone,

I will be working on the bodywork for the San Jose State car. Being that our school hasn't had a formula car in quite some time, we are really working things from the floor up. Does anyone have any recommendations for material (books, videos, etc.) about using composites for semi-structural purposes?

Ideally we plan to use our mock up chassis to help create a foam plug, which will then be used to create a reverse mold to for the body. Since our bodywork will not be structural for the most part, I was thinking of running 3-4 layers of 4-6oz glass.

If you have any feedback, suggestions, or criticism, it would be very much appreciated.

Thanks,
Art

[mechart87]

Hey Everyone,

I will be working on the bodywork for the San Jose State car. Being that our school hasn't had a formula car in quite some time, we are really working things from the floor up. Does anyone have any recommendations for material (books, videos, etc.) about using composites for semi-structural purposes?

Ideally we plan to use our mock up chassis to help create a foam plug, which will then be used to create a reverse mold to for the body. Since our bodywork will not be structural for the most part, I was thinking of running 3-4 layers of 4-6oz glass.

If you have any feedback, suggestions, or criticism, it would be very much appreciated.

Thanks,
Art

[alumasteel]

in case you haven't checked these out... (http://www.youtube.com/watch?v=q_fKlDzemPY)

This video series is pretty good, covers most everything. There are several other videos on youtube that I would check out, as well.

As far as books go, the only one I've read is Fiberglass & Other Composite Materials by Forbes Aird. It is a good reference if you're new to composites since it covers a lot of the basics.

[Wes Burk]

For 06-07 I made our bodywork with no previous fiberglass experience. Just went out and had some glass fabric donated, we had plenty of resin from previous years, and it turned out very well so you should be ok.

Your mold will be much harder to make than the actual panels. An easy way to go that i would suggest is to have your rough body lines follow the frame, then just fill in the openings in your mock frame with poster board and lay felt/fabric over this. For our nose cone we took blocks of foam, glued them together with polyeurethane glue and mounted it to the front, then sanded it down to the desired shape (then fiberglassed over the foam shape). Be prepared for A LOT of bondo and sanding. These pieces would be your male molds/plugs. Be sure that your 'mock up' frame is accurate or wait until your actual frame is complete.

Only because I know how long it takes to go through 2-3 gallons of bondo for our panels (75+ hours), this year we are having our molds cut by a gantry CNC machine in town. I have also seen some nice bodywork made of things such as parachute fabric, which just requires bending and making a small structure underneith. Would save a lot of frustration.

[RiNaZ]

Most teams have access to a CNC machine of some type that can carve out the plug/master from your 3D model. But if you dont have access to it, you can start with your idea of using your mock up chassis to make your bodywork.

Depending on your bodywork design, if you have a wide or long surface area, 3-4 layers of 4-6oz glass probably wouldnt be strong enough to hold its weight. Im assuming that when you say 4-6oz, you mean a woven glass, similar to a surfboard glass? Reason i said this is becoz a 4-6oz chopped mat is too heavy for bodywork.

Also, you might have to double up on spots where you'll put a lot of stress on. If your car needs rework or things need to be changed at the competition, you'll be putting the bodywork off and on all the time. Just by doing this, will put a lot of cracks on your body. So, i would suggest 6oz (woven) + 1.5oz (chopped) + 6oz (woven). Or maybe 1.5+1.5oz or 1.5oz+3oz (chopped) ... (notice that my suggestion starts to get thicker and thicker).

Also, i personally think that bondo is used to fix dings and not to sculpt something. So if you're using 2-3 gallons of bondo or more, you're better off buying some type of primer. I would suggest that you buy a primer from DURATEC. I dont know how much it is per gallon, but you can google the name and call them up for price.

I strongly advised that you use some kind of primer that you can spray with, especially, if you're going to have your foam CNC. A foam that is milled out from a CNC is an almost perfect replica of the 3d model you have on your computer. So if you're slapping gallons worth of bondo on there, you might as well carve the foam using your hands.

In case you have no idea why im talking about primer and bondo, here's a sketch on how i think you should build a plug/master.

http://i301.photobucket.com/albums/nn63/NaZaeM/mold.jpg

I'll give you a brief explanation on the steps. First thing you do when you CNC or carve your foam, you should fill out those gaps with some type of resin that wont react with your foam. If you're using insulation foam that you buy from home depot or the like, you have to use epoxy to cover the whole foam, becoz any type of ester resin will eat out the foam.

Knowing that you will carve out your foam by hand, im predicting that you will have big gaps in the foam. So mix your epoxy with some type of filler (microballoons, cabosil, aerosil or whatever it is that you have, use a cat litter if you want to) and fill in those gaps. And then sand it down to make it smooth with the surface.

And when your foam is smooth out, apply epoxy on the entire surface, including the bottom. The plug/master bottom should already attached to a base at this time, but if it hasnt, apply epoxy on the entire foam including the bottom or base. Let it cured.

And after it cured, apply another layer. And then after it cured, apply another layer. I usually put 3 layers, but you can probably do it more since tolerances are probably not an issue for you since you're carving the foam by hand. I suggest that you brush this epoxy on there instead of spraying or wiping it rag. But if you get any runs from the epoxy, do wipe it out with rag (or you can probably sand it later).

And when you have the whole foam covered in epoxy and cured, i would spray some type of primer (DURATEC for example) on the surface. And when the primer cures, start sanding.

If no one in the team has any experience in sanding, one of them would probably sand thru the foam. If this happens, apply epoxy again on that area. Apply a couple of coats if you're spreading it thin in that area (do it in layers as in, apply cure, apply cure, apply cure).

When you're done sanding, spray another layer of primer. Let it cure, and then start sanding again. At this point, you will see the high and lows of your surface. This time, apply bondo to those highs and lows (provided that those spots are not exposed foam).

Spray another coat of primer when you're done sanding. And fill in the highs and lows again with bondo. After you're done with this, and quite happy with the surface, spray a surface primer or a tooling gelcoat (you have to get some gelcoat anyways for your mold) as your final layer. And then wet sand it smooth. If you're painting it, i wouldnt sand the surface more than 320 or 400 grit.

The difference between a surface primer and a tooling gelcoat is, with a surface primer, you can sand it much easier than a gelcoat. It will be harder to sand than a regular primer, but that's why you only use it as the last layer, where you'll be sanding the surface the least.

With a tooling gelcoat, you have to add wax additive for your last layer to make it easier to sand.

When you're done with all this, spray your gelcoat on your plug/master, after you wax/release/PVA the surface. Remember not to add wax additive with this gelcoat.

[MalcolmG]

I feel like I should add my 2c because I think a lot of teams could benefit from a small piece of advice about non-structural composites.
I have a bit of a love affair with core. I especially like urethane foam but honeycombs are better for anything that's going to be put through an autoclave. Anyway I have found (not exactly a scientific breakthrough) that you can make a very light part nice & stiff by adding thin strips of core, with a thickness of about 3-5mm and a width of maybe 20-40mm, around the perimeter and across any flat surfaces of the part. This, combined with geometric stiffness of certain shapes – such as curves and returns/flanges on edges would help immensely with making bodywork much more lightweight than the technique I often see on SAE cars of simply putting more layers of reinforcement until the part is stiff enough.

[pucksaver]

RiNaZ,

I am not the bodyworks guy on my team but your explanation of the fiberglass process is interesting. I was confused a little bit about the plug and mold part. I'm sure this is a simple questions but do you lay the fiberglass on the plug directly and use that as the body? Or do you create a mold using the plug and then lay the fiberglass on that mold?

Also, what kind of epoxy and foam do you use? Is the epoxy the same used while laying up the fiberglass?

-Clayton

[alumasteel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I was confused a little bit about the plug and mold part. I'm sure this is a simple questions but do you lay the fiberglass on the plug directly and use that as the body? Or do you create a mold using the plug and then lay the fiberglass on that mold? </div></BLOCKQUOTE>

I think his intention was to use the plug to create a mold, which in turn would be used to make the final part. This would be the most desirable way to do bodywork. The advantage of doing it this way is it is much easier to get a smooth finish without as much additional sanding on the final part. The downside is the extra time spent, as well as the extra material required.

Neither the pros or cons necessarily outweigh each other; it depends on the individual situation. If you're a relatively new team with an untested car and the competition is weeks away, you do not have enough time to justify using the plug-mold-part approach.

Our team found ourselves in that boat the past couple years. What we did is lay foam insulation blocks on the chassis of the car and shape them into the rough shape of the bodywork using wood rasps. Then rather than applying bondo or some other filler to smooth the surface for the glass, we covered the foam with clear packing tape and applied the fiberglass over that. The tape doesn't stick at all, and if you do a couple coats of matte covered with a layer of cloth, and then a thin coat of resin after the cloth layer has set up, you should have a decent surface with which to work and it will be strong enough for the application (albeit a little heavy, but if you're in a time crunch you aren't worried about ~3lbs). It may not be the prettiest bodywork you've ever seen, but the turn around time can be 72 hours or less from starting of foam molding to the paint drying. Besides, a broken-in and functional car with lumpy bodywork is better than a car that is going to breakdown consistently but look good doing it.

[RiNaZ]

Clayton, you get your body (or part) out of your mold. Here's just a quick sketch if you really dont know how it works.

http://i301.photobucket.com/albums/nn63/NaZaeM/mold2.jpg

You can also lay the glass on your plug/master, and directly use that as your body if you want to, but finish surface will be in the inside of the part, instead of the outside.

Or you can go with alumasteel suggestion, which is to use a clear packing tape (you still have to wax your tape though).

Good bodywork is as important as getting the car running. Dont settle for less.

[Rob Klyver]

Think core materials - loads of wet lay add weight and it will do everything but make you happy - added weight and brittleness.
Also remember how often the body will be man handled, we see damages to more things with off/on head scratching...Do you still have a car from the last effort?

[Mustang Mac]

I must agree with MalcolmG. If I remember this correctlt, as it has been a few years, out body was 2 layers of 6oz glass for the most part, a third layer in some areas and we used foam strips in strategic locations to provide stiffness. Just look at the body design, it usage, and it's not to hard to figure out what you need and where to place it. Also, using the vacuum bag technique saves some weight as well due to less resin usage. We had a nice body, quite strong and extreamly light. Was almost 50% lighter the our previous body.