Hi everybody,
I had a thread open about my troubles in getting nice carbon tubes out of prepreg. I've made 5-6 trial tubes now and I still have troubles getting good quality tubes so I come back for help again.
My goal is to find a process for creating carbon tubes for bicycle building. Straight tubes are OK, I do not need fancy curvatures in my bikes. I would like to use prepregs as they are clean to use and give a good weight/stiffness ratio. I thought it would be easiest to use an aluminium mandrel and wrap the prepreg around it. So I got some mandrels and an oven and did some trials.
The problem is, that I cannot get a tube where the carbon is pressed well against the mandrel. In all my trials the carbon lifted from the mandrel. This resulted in an uneven surface (but no delamination - I cut crossections of the tubes to see if there was delamination between the layers)
All tubes you can see in the pictures have 4 layers +45° / -45° / +45° / -45° of 300 gram prepreg from Haufler composites.
I used curing temperatures of 120-150°C for all trials. The oven has bad regualtion with a hystersis of about 20°C.
I tried two "winding" processes and three compression processes.
I did spiral winding of long strips of prepreg but found it very difficult to align the prepreg nicely, so I switched to cutting rectangular sheets in the length of the tube and the other dimension being slightly larger than the circumference of the tube to get an overlapping 45° surface (I know this is sacrificing quality for ease of building, but that's OK for me)
I tried:
- spiral wound strips, 2 layers of heat shrink TAPE.
Result: shown in previous thread - really bad - very "wavy surface"
- overlapping rectangular sheets with +-45° fibre orientation (i.e. you get a "seam" along the tube). 4 layers of heat shrink TAPE.
Result: picture 1 and picture 2. Better, but still not good enough - very uneven surface which would need an additional coating and lots of sanding.
- overlapping rectangular sheets with +-45° fibre orientation (i.e. you get a "seam" along the tube). 4 layers of heat shrink TUBE with and without peel pyl.
Result: picture 3 and picture 4. Uneven surface and carbon not pressed against the mandrel in some locations.
- overlapping rectangular sheets with +-45° fibre orientation (i.e. you get a "seam" along the tube). vacuum bagging - but the bag got a hole 15 minutes after putting it into the oven.
Result: picture 5. Even surface and carbon not pressed against the mandrel in some locations.
I do not understand the physics involved: what makes the carbon want to lift off the mandrel? Is it the lateral compression?
Tomorrow I plan to re-run the last trial with vacuum bagging. The next step would be to build an autoclave for tubes - that shouldn't be too difficult, but I'm pretty sure there is a more simple solution!
Any comments are weclome!