I decided to start a separate page on the process of making a fiberglass nose cone. Although my title is not very accurate, you don't cast a fiberglass nose cone, it's laid up. But the mold is cast, so I'm close I guess. This being my first attempt at such an endeavor, please keep the laughter to a minimum!
The first thing I did was to form a nose cone out of wood using my wood lathe. You can see the details of the wood nose cone at the fiberglass rocket body page. I ordered a quart of a two part silicone mold casting compound from Micro-Mark, part number 82083. The molding rubber is mixed 1:1, so you don't need a scale to get the proper proportions, just a measuring cup will do. The silicone sets up at room temperature in 4 hours.
This odd looking thing is an outside form for my nose cone
mold. Such a large nose cone would require a huge mold box to hold the silicone casting resin. So I cut up some
old plastic bottles and formed them around the nose cone. There is about a 1/2" space around the nose in the
form. After it was assembled, I placed the nose in position and measured how much water it took to fill the form,
about 2.5 cups were required. The casting resin I ordered is only a quart. So I'll only have one shot at this,
as I'll only have 1.5 cups left after my first casting.
The nose cone was inserted into the form, I had planned on hanging it from a string into the form, to keep the nose cone away from the sides of the form. But I wasn't thinking, as I filled the mold the nose cone floated in the mold compound, so I had to hold it into the form with a dowel pressing it from above. You can't see it in this picture, but there is a board running above the top of the work bench, that's what's supporting the dowel.
After about 4 hours I removed the form from its stand.
After peeling off the plastic form, this is what I was left with.
Peeling the mold off the nose cone wasn't very difficult. The silicone mold compound really doesn't stick to much anything but itself.
Here's the mold next to the nose cone.
The mold was a little flimsy, so I added a couple of loose wraps of fiberglass cloth and polyester resin to firm up the mold. It was more than I bargained for, now I couldn't get the nose cone out of the mold again. I had to cut some slits in the fiberglass support to get the nose cone out. Even then, I put two small tears in the mold.
In the picture above I have already added the first coat of epoxy/fiberglass to the inside of the mold. I cut triangular strips of cloth for the pointed tip, and rectangles for the base of the nose. The inside was coated with a layer of epoxy using a foam brush, then each little strip of cloth was inserted and worked into position. This part was easier than I expected. The inside glassing went very well, as the cloth held in place readily.
After the second inside glassing was complete I decided to remove the nose cone before it was too stiff to get out. There is about 3 layers of cloth covering all surfaces because of the overlap in the layers. It will still need two more applications of glass I think. That should bring the the layers of cloth up to about 6. Once I get the base trimmed we'll see if it will fit the body tube.
Here is the nose cone after its final inside layer of glass. The base of the nose was a little rough, as it was hard to get the fiberglass cloth into that area. So I used some body filler (the pinkish stuff) to smooth out the base. The surface of the fiberglass nose cone really was very smooth, but it did have a few small voids (bubbles) here and there. So I applied some spot filler (red stuff) to any voids left by air bubbles.
Overall I think the nose cone turned out pretty well. It was a lot of work, and a lot of time making the mold. But the actual laying up of the nose cone went pretty fast. I still need to attach the recovery line to it somehow, I suppose I'll just glass in a "D" ring of some sort.
The silicone mold compound worked ok, but it really is designed for smaller parts. A heavier, stiffer mold compound made as a two part mold would work better. Just for fun, I used a little excess mold compound to make a mold of a one ounce silver coin I had. It worked remakably well on the coin. Reproducing every detail, even minute scraches were reproduced. Without the fiberglass shell I made to support the silicone mold, the nose cone would have been badly disformed. I had to slit the fiberglass shell down one side to get the fiberglass nose cone out. That's not a big deal, as I can tape the fiberglass shell back together again and reuse it.
Without a "D" ring installed, the nose cone weighs in at very light 111 grams. To see the nose cone finished and put to action, see the fiberglass rocket page.
Here is the second nose cone out of the mold. For this one, I used polyester resin instead of epoxy. It worked fairly well, but the epoxy isn't as sticky and really does work better. Epoxy is stonger too, not that I think the extra strength will be needed for the nose cone though. It didn't take long to make this one, I used larger pieces of fiberglass cloth and overlapped them. So I laid up this nose cone in just two applications. I did manage to enlarge one of the small tears in the mold while demolding the finished nose cone. I think I'll have to try to repair the mold before making any more nose cones.