Once More: Prepreg-Tubes

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Once More: Prepreg-Tubes

Author	Message
Bambooride	Bambooride posted 10 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 (687 views, 253.00 KB) image002.jpg (693 views, 158.00 KB) image003.jpg (635 views, 192.00 KB) image004.jpg (632 views, 173.00 KB) image005.jpg (654 views, 195.00 KB)
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Dravis	Dravis posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 592, Visits: 1.9K	How about doing it with a mecanical system for adding outside pressure? Get someone with a machine shop to make an outside mold from aluminium... bolt together two long bars along the edge, set them up in a lathe and bore a suitable hole through (outside diameter of tube) Then lay up on the mandrel, and clamp the outside around it.. You will need to figure out the correct inside and outside diameters, before boring the outside "mold" If you polish it up, you will get perfect surface finish as well ... This is what I have done for short tubes (up to around 30 cm, probably a lot easier than long ones ) I have not used prepreg this way though, just high temp resin and CF/Kevlar fabric .. did a hand layup on the inside mandrel then clamped the outside around, and vac-bagged the whole lot. You could also simply make a very strong outside mold from CF reinforced Epoxy on a slightly thicker mandrel ( add 2 x desired thickness to the outside diameter of your inside mandrel) Use a tube for the inside mandrel and heat it from the inside, using hot oil... ... or air... or simply leave an aluminium mandrel sticking out each end, and put it in an oven.. The aluminium will conduct the heat inside through the mandrel.. Aluminium expands quite a bit when warmed up, so it will put some pressure on the prepreg. Just a few ideas,,, "Sapere Aude"... Dare to KNOW! The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind... 103% of all people do not understand statistics... Do not adjust our mind, theres a fault in reality :-) Edited 10 Years Ago by Dravis
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Matthieu Libeert	Matthieu Libeert posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 955, Visits: 3.4K	Great imput from Dravis! you will need to try it with vacuumbagging as soon as possible... As long as you try it with heat shrink tape you wont get the same results as with a good vacuum... Keep in mind that you are not using an out of autoclave prepreg... the prepreg you are using is probably made for an autoclave, this might cause some problems as well. You also say that your oven is not really accurate? this might cause some problems as well with the prepreg... Sometimes its better to master the process with wet lay up and then go to prepreg once you master everything and get perfect tubes with wet lay up and vacuumbagged... I have some tutorials coming online soon on my youtube-channel about making tubed shaped parts https://www.facebook.com/video.php?v=452455014897809&set=vb.223877301088916&type=2&theater on that link you can see a small video... Like Dravis says, there are a lot of ways to make tubed shaped objects Matthieu Libeert Founder MAT2 Composites X Sports website: www.mat2composites.com
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ChrisR	ChrisR posted 10 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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Fasta	Fasta posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 468, Visits: 3.5K	These pictures all look like the shrink tape may not have enough overlapping? A 3 thickness/layer tape needs to overlap 2/3 on itself as you go. Is this how you are doing it? Fastacraft Facebook page
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fgayford	fgayford posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 433, Visits: 1.2K	I have watched a youtube clip on making a carbon fiber mast for a multi million dollar sail boat that showed the process clearly. They mandrel wound the unidirectional prepreg onto the mandrel and after every few layers they debulk using a plastic tape. They removed it and added a few more layers and debulked again, until they had the full layers needed. Then they put on the shrink tape and vacuum bagged the whole thing and rolled it into their oven. Look it up on youtube and watch it yourself I think this will help you. Fred
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Bambooride	Bambooride posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 11, Visits: 37	Reading the responses and thinking about it a bit more I came to the conclusion that the most likely cause is the loose winding. so for the next trial I plan to debulk as they do in the mast production. I will keep you updated on the results. Thanks to everybody for the valuable input.
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