Hi,
I'm not so new to using carbon for my projects but I'm a total newbie in using prepregs. I've built a few (around 80) bamboo bikes where the tubes are joined with carbon fibre.
I am planning to advance my technology skills a bit by building a full carbon bicycle using prepregs. (I've built one for my son with standard woven mats and tubes made on an aluminium mandrel.)
However, I ran into unexpected problems with my tube-building technique:
Since I wanted to make everything perfect I bought UD prepreg from Haufler Composites in Germany (300 grm/m2) and built an oven large enough to hold 3 bicycle frames. I also bought some shrink tape (
http://www.carbonmods.co.uk/products/composites-shrink-tape-roll.aspx). I then took an aluminium mandrel with 35 mm diameter, waxed it and rolled strips of UD fibre at +-45° and 0° - in total 5 layers on the mandrel taking care that the strips would sit neatly next to each other (not gap) and that there is little air caught between the layers. Then I wrapped it with the shrink tape spiralling over the mandrel and put it into the oven at 110° celsius.
The result was worse than expected - the tube was not uniform at all on the outside. On some locations, there was a gap in the shrink tape. There, the fibre "escaped" the pressure of the tape and "ooozed" out of the gap. On other locations the tape overlapped and leading to locally higher pressure creating non-uniform wall thickness. I don't see how I could improve the process prevent that other than using less compression force, which would probably lead to delamination inside the tube. A heat shrink tube would not have any gaps or overlaps, but it would still compress the fibre. I guess the fibre would "meander" under the compression force as the outside diameter would shrink a bit.
On a sidenote - I could not get out the mandrel either - but actually I might just have taken a non-waxed one off the shelve.
Any suggestions on what I shoud change?