Making tubes out of prepreg

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Making tubes out of prepreg

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Drew Diller	Drew Diller posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 35, Visits: 199	Hi there Bambooride, two things: 1) Can I see some of your bikes? I make bamboo bikes as well: http://dillerdesign.com/random/bike/fat_bamboo_v2/DSC_0185.JPG 2) I suggest a turned polypropylene mandrel. Polypro has a very high thermal expansion rate, does not bond to epoxy. Basically you put carbon around your mandrel as you otherwise would, shrink tape it as you otherwise would (use Kapton tape to deal with the heat). After cure at 200F or higher, remove the whole arrangement and either let it sit or put it in a freezer. You should be able to remove the carbon tube with minimal effort compared to with a metal mandrel. One thing to keep in mind here is that the expanded size of the mandrel in the oven will change the tube's final inner dimensions a little bit compared to the diameter of the mandrel at room temperature. You can get "polypropylene rod stock" at Interstate Plastics.
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Bambooride	Bambooride posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 11, Visits: 37	Thanks to everybody for the useful advice. I ordered new release agent and will do a second trial as soon as I've got it. Alex
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Fasta	Fasta posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 468, Visits: 3.5K	Looks like your problem is no overlapping as said above. Most tubes I have seen done with tape are three layers (overlapping in thirds?) or if the laminate is just three layers or less then just overlapping 1/2 is ok. Fastacraft Facebook page
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brainfart	brainfart posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 198, Visits: 1.4K	Excellent advice, rob. I like that slide hammer idea. Mine would have been overlapping the shrink tape more, too.
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carbonfibreworks	carbonfibreworks posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 292, Visits: 4.5K	Hello Alex I have made a few tubes with shrink tape and a alloy mandrel, make sure you polish the mandrel. I use Easy Lease which works fine in the oven environment, the alloy will expand slightly with the heat and when it cools down it will reduce down to its original size but the carbon will will not shrink so you mandrel should slide out of you hew carbon tube with no effort. Chris Carbon fibre works LTD PS I make sure I have a good overlap on my shrink tape.
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Bambooride	Bambooride posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 11, Visits: 37	Hi f1rob thanks a lot. I check and the wax was indeed not rated for the temperature. I'll try the overlap on the shrink tape and also your method for removing the aluminum tube. Alex
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f1rob	f1rob posted 11 Years Ago HOT Post Details
Group: Forum Members Posts: 237, Visits: 4.8K	Was your wax rated for 110 ? we only ever use wax on LTM tooling 60 deg cure 110/125 cures boil most waxes and they don't work To demould we tap one end of the bar (m8) and bolt a plate on 20 mm O/D bigger than the bar so we can lay up a shoulder on one end of the pipe When its cured remove the plate and then we use a slide hammer screwed into the now empty thread in the bar and the shoulder gives you a way of holding the pipe Helps to drop it in the freezer for a few hrs as well When you shrink tape you shouldn't move along more than 25% of the width of the tape each rotation (even less is better and the less it is the better the finnish)yours is nearly 100%
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Bambooride	Bambooride posted 11 Years Ago HOT Topic Details
Group: Forum Members Posts: 11, Visits: 37	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? Alex
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