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chriscnf
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Hi guys...I'm messing around with hand layup of twill carbon in a complex three part mould. The three parts are laid up seperately and then joined with an inflation bladder. There is an advantage for me in letting the parts cure to B stage (not sure wha range of cure that covers) and then joining with fresh uncured epoxy. The question is what state can you let epoxy cure to before it needs sanding for good adhesion and what is the chemical process that stops expoxing adhereing well once cured?
Cheers...Chris
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Warren (Staff)
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Once you go past the B stage and it begins to loose tack, you start to see a reduction in cross linking as the epoxy is getting closer to being cured. Realistically there will be some cross linking in a room cured epoxy even for day or two after reaching demould hardness. However it won't be much hence the need to key the surface to aid with the mechanical adhesion to the resin. You also find with a smooth surface, wet laying the next layer on without keying can lead it to fisheying off causing even more problems long before you get to potential delamination issues.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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chriscnf
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Group: Forum Members
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+xOnce you go past the B stage and it begins to loose tack, you start to see a reduction in cross linking as the epoxy is getting closer to being cured. Realistically there will be some cross linking in a room cured epoxy even for day or two after reaching demould hardness. However it won't be much hence the need to key the surface to aid with the mechanical adhesion to the resin. You also find with a smooth surface, wet laying the next layer on without keying can lead it to fisheying off causing even more problems long before you get to potential delamination issues. Thanks Warren...so I'm assuming B stage is the classic tacky but not coming off on your finger that's displayed in some of your videos.
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Warren (Staff)
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Yes that is correct.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Matt.F
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+x Hello both of you, please could I jump in to ask about this theme of joining already cured carbon fibre parts together rather than attempting a single massive lay up. Details on my project are below but my project is load bearing so I'd like to know if significant strength is lost in carbon fibre if you lay up and 100% cure a section and then key it and begin layup on the next section? In my project I think this technique will work best as it will allow me to compress & cure each layer with confidence i've done it well and it will give me time to focus on laying the trickier part of my design (two attachment points with steel inserts). PROJECT: I am rebuilding a carbon fibre bracket with two attachment points that is bonded around a carbon fibre spinnaker pole which is used to crane a 500kg tender off the front deck of a sailboat. There are big factors outside of the bracket itself which caused this current bracket to start cracking (which I'll address) but I have decided to strip it all away and start again, upsizing the width and depth of the band and mixing unidirectional & plain weave. I haven't done any testing yet but my plan was to do wet lay up by hand using heat shrink tape to compress & cure perhaps two or three steps to rebuild this bracket. 1. Wet lay up a 5mm deep band around the pole using 50mm CF tape both unidirectional & plain. Heat shrink it and leave to cure. 2. Key the surface from step 1. Wet lay up around the two steel inserts by building cut sections of 50mm CF tape both unidirectional & plain. Heat shrink it and leave to cure or go straight into step 3. 3. Key the surface from step 2. Hand lay up another 5mm deep band around steps 1 & 2 using 50mm CF tape both unidirectional & plain. Heat shrink it and leave to cure. Any advice would be massively appreciated, cheers! Matt  
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Warren (Staff)
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Although the bond strength wont be quite as high, in that kind of design, it won't be an issue as you are wrapping around the tube so anything below is mechanically locked into place and there is no real mechanism to shear off the layers.
A lot of UD fibre in the lay up will certainly help with the hoop strength on those brackets with a bit of plain for all round strength.
Weather you stop and let it cure after step 2 is probably more an issue of practicality,
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Matt.F
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+xAlthough the bond strength wont be quite as high, in that kind of design, it won't be an issue as you are wrapping around the tube so anything below is mechanically locked into place and there is no real mechanism to shear off the layers. A lot of UD fibre in the lay up will certainly help with the hoop strength on those brackets with a bit of plain for all round strength. Weather you stop and let it cure after step 2 is probably more an issue of practicality, Thanks Warren, that is the kind of answer I was hoping for! I'll do a small test piece to check out the 'tack time' you've mentioned with Chris above. Again many thanks for taking the time to reply. Matt
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Matt.F
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+xAlthough the bond strength wont be quite as high, in that kind of design, it won't be an issue as you are wrapping around the tube so anything below is mechanically locked into place and there is no real mechanism to shear off the layers. A lot of UD fibre in the lay up will certainly help with the hoop strength on those brackets with a bit of plain for all round strength. Weather you stop and let it cure after step 2 is probably more an issue of practicality, Hi Warren, please could I send another question your way? I'm pretty happy with the foundation band (step 1 from my previous post) I've now cured on my pole bracket project. Step 2 I think I've massively underestimated building the material required to bed in the two steel inserts. Bedding the inserts with hand lay up would take masses of cured sheets so I'm wondering is there a fairing compound / filler I could use in this instance? I'm hoping to do a test project with a quick fix style epoxy putty (Quicksteel or similar) over the weekend to see if it bonds well and won't crush, that would be very easy to work with? Then I can move to step 3 banding the entire project with unidirectional & plain to give the required strength. Thanks in advance for any wisdom! Matt 
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Chris Rogers
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Best option would be something like G10 fiberglass sheet used to make ramps/wedges that fair the tubes into the tube. Since you are not vacuum bagging, you'll have to keep the whole thing convex to get pressure from your shrink tape. You could also use epoxy and high-density filler / milled fibers to cast a ramp in place using a stick and some packing tape to get a nice surface. It would be ok to tack the tubes in place with some 5-minute epoxy or something before casting in any filler - just so things don't move. Not sure of the load situation but if its just tension - pulling the bushing off the tube - and no twisting on the tube it should be ok. Maybe cut a few of your woven plies so the fiber is running +/-45 to the uni hoop. It wouldn't be a bad idea to lap some of these outer plies of carbon onto the tube covering the inside band. The band may not have to be a consistent width like the old one - and more bonding area is good.
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Matt.F
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+xBest option would be something like G10 fiberglass sheet used to make ramps/wedges that fair the tubes into the tube. Since you are not vacuum bagging, you'll have to keep the whole thing convex to get pressure from your shrink tape. You could also use epoxy and high-density filler / milled fibers to cast a ramp in place using a stick and some packing tape to get a nice surface. It would be ok to tack the tubes in place with some 5-minute epoxy or something before casting in any filler - just so things don't move. Not sure of the load situation but if its just tension - pulling the bushing off the tube - and no twisting on the tube it should be ok. Maybe cut a few of your woven plies so the fiber is running +/-45 to the uni hoop. It wouldn't be a bad idea to lap some of these outer plies of carbon onto the tube covering the inside band. The band may not have to be a consistent width like the old one - and more bonding area is good. Excellent ideas thanks Chris, I'm at home using epoxy filler for fairing so if I pick up some high density that may be my preference, good tip about creating bonding area from the outer layers all the way to the tube, I hadn't thought about that and the finish will be painted so I'm not aiming for amazing carbon looks, better to aim for strong bonds! The loading is very much tension only, the tube becomes a crane arm, this bracket is the attachment point to hold the crane arm up at 90 degrees and attach the 3:1 lifting tackle to the load on the lower side. Thanks for thinking this through!
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