Once More: Prepreg-Tubes

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Once More: Prepreg-Tubes

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Bambooride	Bambooride posted 11 Years Ago HOT Topic Details
Group: Forum Members Posts: 11, Visits: 37	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! Attachments image001.jpg (727 views, 253.00 KB) image002.jpg (727 views, 158.00 KB) image003.jpg (677 views, 192.00 KB) image004.jpg (671 views, 173.00 KB) image005.jpg (695 views, 195.00 KB)
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ChrisR	ChrisR posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 363, Visits: 3K	Out of interest, how tight are you winding the prepreg onto the tube? From my albeit limited working with prepreg, you need to be extremely firm with it during layup, just wrapping it around a tube and adding heatshrink could lead to similar issues. For a smoother surface finish you could use a final layer of heatshrink tube rather than tape (or number of layers) then vac bag. Other options include as mentioned, an external mould but it gets a bit more complicated with layups and compression from expansion etc. Another option (if you can find it!) is wet lay (or DIY prepreg) braided sleeve with tube overlay and heat. I couldn't find a data sheet on the haufler website but does it require debaulking? And what is the recommended cure cycle?
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Once More: Prepreg-Tubes

Bambooride - 11 Years Ago

How about doing it with a mecanical system for adding outside pressure? Get someone with a machine...

Dravis - 11 Years Ago

Great imput from Dravis! you will need to try it with vacuumbagging as soon as possible... As long...

matthieutje65 - 11 Years Ago

Out of interest, how tight are you winding the prepreg onto the tube? From my albeit limited working...

ChrisR - 11 Years Ago

These pictures all look like the shrink tape may not have enough overlapping? A 3 thickness/layer...

Fasta - 11 Years Ago

I have watched a youtube clip on making a carbon fiber mast for a multi million dollar sail boat...

fgayford - 11 Years Ago

Reading the responses and thinking about it a bit more I came to the conclusion that the most likely...

Bambooride - 11 Years Ago

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