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Making L-Brackets
by Alex Tung
created 1/21/1999
submitted 3/8/2002 10:49:56 PM

These are notes on how to wet-layup carbon L-brackets which were used to hold the Stanford car together in 99 and 01.

How to Make Wet Lay-up Carbon Fiber L-Brackets

Objective: Make Carbon Fiber L-Brackets to bond into the folds of the carbon fiber panel chassis to make it really freakin' strong.

Implementation: The basic idea behind the L-brackets is to lay up three plies of carbon in three different directions around a steel tube with square cross-section and rounded corners. This makes four 12" brackets in one shot, which, once cured, can be cut from the steel tube. The following list of instructions will guide you through the process.

1) Put on Gloves and Prep the Table. Carbon splinters are not the best thing for you, and they will find their way into you if you don't wear gloves. Wrapping the table in vacuum bagging or some other protective plastic is probably a good idea.

2) Cut the Carbon. We've decided on a 90/45/135 degree orientation with respect to the length of the steel tube. To make this you need to cut one rectangle of carbon and two parallelograms to make the angled orientations. For a 12" wide roll of carbon fiber, the cuts should be made as below (hint: use a diagonally folded sheet of paper to mark the 45 degree lines.)

3) Prep the tube. To keep the carbon from sticking to the tube when all is said and done, you have to spread mold release on the tube. Mold Release is a water-soluble thick green liquid, usually found in a small TAP Plastics bottle. Before applying the mold release, clean the tube with water, being sure to get as much old mold release and other junk off as possible. Then dry the tube, and prop it up on both sides so that the middle of the tube doesn't touch the table. Pour a small amount of mold release into a bowl, and use a paintbrush to spread it onto the tube. One layer is plenty, and because the mold release is self-soluble, it won't spread thickly on the tube. Let the mold release set for about 15 minutes.

4) Mix the epoxy. In the yellow chemicals cabinet there are two huge five-gallon black barrels of epoxy resin, as well as a few containers of medium epoxy hardener, all generously donated by TAP Plastics (a $675 value, don't spill it). Take one of the big jugs, and using a plastic spoon or other implement, put some into a small medicine cup or container, being sure not to leave epoxy around the lip of the opening. Cap that thing and put 1 part hardener into the cup for 4 parts of the epoxy. Do not put hardener in the large epoxy container. Stir up the mixture well. The epoxy will set in approximately 25 minutes, so be careful not to mix in the hardener until you are ready to use the epoxy.

5) Apply the epoxy. Using a plastic spoon or your fingers, spread the epoxy on one side of the sheet of carbon, making sure to use just enough to allow the epoxy to be impregnated. Flip the sheet over and check to see that the epoxy went through to the other side. If it didn't, apply more. Be sure to impregnate the entire sheet, especially the edges, and always smear with the grain of the carbon.

6) Wrap the carbon around the tube. For the first layer the lay-up is pretty simple. Start with the carbon grains going perpendicular to the tube, and lay it so that the starting edge is in the middle of one face of the tube (Usually it's easier if you start the carbon on the face of the tube facing away from you.) Press the impregnated carbon around the tube, making sure that the carbon is tight against the steel, especially around the corners. Take your time with this part, because if you don't the brackets will come out like shite.

7) Repeat 5 and 6 for the second and third layers. Wrapping the carbon will be a little trickier this time, since the second and third layers are cut in an odd shape to wrap around the tube. The parallelogram shape, if wrapped correctly, will come around and meet itself. Begin by placing the grain of the carbon at 45 degrees to the length of the tube. Place the carbon so that the corner begins at the middle of the side facing away from you. The transverse edge of the parallelogram (the edge going against the grain) should line up with the right edge of the first layer as you wrap the carbon around. Be sure to keep the carbon tight as before, and add more epoxy as needed, since the sides will tend to flare out. Be sure to smooth the carbon in the direction of the grain, and don't worry if it doesn't line up exactly. Tightness is the key factor here.

8) Peel Ply. Cut out about an 18" X 16" piece of peel ply to wrap around the tube. secure it with vacuum bag tape. The peel ply is there so that the outer layer of breather won't stick to the carbon and will be easier to remove.

9) Breather. Cut out a similarly sized piece of breather and wrap that around the tube. This allows the vacuum to not suck itself to the bagging or the tube, and it also soaks up any excess epoxy.

10) Vacuum Bag it. Cut out a big sheet of vacuum bagging, enough to fold over the tube so that you have one large rectangle folded over the tube. Take some vacuum bag tape and put down a rectangle without a side where the fold is around the tube, being sure to leave enough room to be able to fold the top of the bag down. Make sure as you lay the tape down that you make an airtight seal, and check the underside for bubbles. Take off the tape backing and carefully fold down the top portion of the bag, making sure that the tape seals all holes. Start on one end and work your way around the tape, focusing more on getting a good seal than on making the bag look pretty. You'll probably end up with a hole in the bag where the tape doesn't match up; don't worry, you'll need to use that hole later.

11) Hose it. Wrap a little bit of breather around the end of the vacuum hose, get plenty of vacuum bag tape and wrap it around the inner part of the hole in the bagging. Stick the hose in so that it lies close to the tube, and mash the tape around the tube, again being sure to get all the air holes out.

12) Test Vacuum. Plug in the vacuum pump and make sure that they're aren't any leaks. Good vacuum should be around 27 in-Hg. Mash in more tape or smooth out the tape where the leaks are. Be sure to smooth out the vacuumed bag around the corners so that there are no wrinkles. This tends to create wrinkles in the L-bracket corners, which makes them much weaker than they could be.

13) Heat it up. Place the whole apparatus into the "oven" (in this case an insulated wooden box), cover it, and aim a heat gun into the box through a hole in the cover. Let the brackets cure for about an hour.

14) Take them out. After an hour, turn off the vacuum and take the apparatus out of the box. Cut open the bag and cut off the peel ply/breather layer. Using a utility knife, cut down the middle of each tube face and pry the brackets off.

15) Build a solar car. Now that you have L-brackets, you just need to bond them onto the chassis, build a suspension and electrical system, modify an array, and build a shell bottom and you've got your very own solar car!! It's that easy!!

1292 words | tungsten

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