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Bambooride
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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?
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f1rob
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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
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Group: Forum Members
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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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carbonfibreworks
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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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brainfart
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Excellent advice, rob. I like that slide hammer idea.
Mine would have been overlapping the shrink tape more, too.
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Fasta
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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.
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Bambooride
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Group: Forum Members
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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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Drew Diller
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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.JPG2) 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
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Group: Forum Members
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Hi Drew, sorry for the late reply - I had actually written the response a while ago but didn't hit the "post reply" button. 1. Nice bike you've got. It's quite a funny coincidence that a friend of mine showed me a picture of your bike as he's planning to build a fat-tyre carbon/bamboo bike. My website is Bambooride.com. I run weeken-bike-building-workshops where you build your bike in a weekend. Some of the results can be seen here: https://onedrive.live.com/redir?resid=BBF286F67D81C572%21103 Since about half a year I switched from using rovings for the joints to woven fabric which is a bit harder to use, but gives a nicer appearance. 2. Thanks for the advice but I think I'll stick to aluminium as I didn not have any problems removing the mandrel in the last trial with the new release agent. --------------------------------------------------------------------------------- To everybody else (and Drew- of course) Still - I face a problem with making tubes - I tried 50% overlap and the result is much better than the first try, but still not satisfactory. I took good care to overlap the heat shrink tape by exactly 50%. But it seems that it shrink laterally as well, leading to a sprial wound "unevenness" (I don't know how to express that - but I think you'll see it in the picture. Is there a way to make a tube with perfect appearance without the need for sanding/painting? (Is using a mold the only way?) Is peel ply/vacuum bagging more promising? |
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cornelp
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Hi Alex, One way I found easy for tubes is to use a full heat shrink tube. This way you will get an uniform thickness. The only issues are: you need to find a tube that has no adhesive at all (the usual "battery" shrink tube is ok) and it is not that easy to take off. Usually you should heat it to about 150degC with a heat gun and you can tear it out. I tried waxing the inside of the tube before shrinking, but I do not like the embedded wax in the final product... After removing the tube, I sand a bit and apply https://www.easycomposites.co.uk/xcr-epoxy-coating-resinor a thick epoxy of your choice, but the above is very glossy and crystal clear without any yellowish shades. Cornel
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