My first curved tube!

It has been literally 364 days since I came up with my method of making curved tubes. Today I finally made one. It isn’t perfect, but it definitely shows that the concept works.

The basic concept of this method is that you have a mold of how the outside should be shaped and you have a tube that inflates the carbon fiber to fill this mold. This process is used in commercial carbon fiber part making, but it usually requires you to make a custom latex “bladder”. With my method, the tube is simply a thin silicone tube with a stopper on one end and a connector on the other end to connect it to the air compressor. This makes it much easier and cheaper to make this type of CF tube.

The mold is made from polypropylene plastic and is milled out on my CNC milling machine. Each half of the mold takes about 45 minutes to machine which yields a very good surface finish. Eventually they will probably be made from aluminum, but for now I am using plastic because it is much cheaper, much easier to machine, and these molds will probably be replaced after one use so they don’t need to last. The two halves of the mold are lined up with 1/4″ aluminum pegs. When I switch to aluminum molds, these pegs will switch to precision dowel pins.

Pretty much the only problems I encountered with this molding method were in making the molds. I’ve spent the last few months saving up for, buying, retrofitting, breaking, fixing, and learning to use my CNC mill and I still don’t have everything figured out, so these molds aren’t correct. They were quite a bit smaller than they were supposed to be so that meant that the carbon fiber didn’t quite fit in the mold and bled over the sides.

Here’s the process I went through to make this tube:

1. Design the molds using Autodesk Inventor.
2. Create the toolpaths for milling it using CamBam.
3. Mill it using the CNC mill and clean up edges with knife.
4. Make pegs using the lathe.
5. Prime the mold using mold release wax.
6. Put the quick disconnect air connector on the silicone tube.
7. Mix epoxy thoroughly in correct ratio.
8. Put one layer of carbon fiber tube fabric over the silicone tube and wet it out with epoxy. Make sure the carbon fiber is stretched out over the tube and doesn’t protrude over the end of the tube.
9. Repeat this for a few more layers. This tube was made with three layers total. Four layers is ideal and should be possible with the correctly sized mold.
10. Put the plug and clamp onto the end of the tube.
11. Put all of this into the mold and put the other half of the mold on top. Make sure no carbon fiber is sticking out of the mold channel.
12. Clamp the mold in as many places as possible very tightly.
13. Connect the silicone tube to the air compressor which should be set at 20-30 psi. I don’t know what the ideal pressure range is, but this worked well enough for the first time.
14. Let the epoxy cure and remove the mold halves. They can be pried apart with a flat-head screwdriver. I let it cure for four hours before removing it from the mold. The epoxy was not fully cured yet, so next time I will let it cure a full 24 hours before opening the mold and I’ll see which way is better.
15. Remove the tube from the mold and pull out the silicone tube.

As you can see, this is a very complex and labor intensive process, but it will probably become very quick and easy once I do it a few times. Steps 4-15 can be repeated many times for the same mold. Steps 1-3 take the most time and effort, particularly step 2. Getting the mold right determines the quality of the tube, so it is very important.

Here’s a picture of the whole thing in the process of curing: