carbon gurus, female mold construction?
- Thread starter dhkid
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i seem to recall from his blog he uses rivets to join the two halves together. possibly bonded as well.
as long as there was sufficient overlap bonding the two halves together would be fine. That is how F1 panels are jouned to the chassis. Think of a intense or a V10. they are effectivley 2 halves 'bonded' through welding. Personally if i was him id bond those two halves then add another few layers on top for extra safety.
as long as there was sufficient overlap bonding the two halves together would be fine. That is how F1 panels are jouned to the chassis. Think of a intense or a V10. they are effectivley 2 halves 'bonded' through welding. Personally if i was him id bond those two halves then add another few layers on top for extra safety.
what's the blog's adress?
Thats correct, I used two female mold halves, with a double lap joint in between. The gussets go inside the halves to support the inside walls of the head tube and downtube. Obviously strength of the joints is a huge concern, and why much time is spent on eliminating joints, ie, closed molding bladder techniques, or designing suitably strong joints, (such as the racecar industry does). A double lap joint is one of the strongest ways to assembly parts, because the adhesives used in the joint are strongest in shear, and the "double" lap joint reduces peeling, where adhesives are the weakest. (http://www.roymech.co.uk/Useful_Tables/Adhesives/Adhesives.html) So with a suitably large and well placed joint, the seam can be as strong or stronger than the rest of the frame. Kind of like a suitably designed welded aluminum monocoque.
As for placement, I put it on the edges to move it as far from the nuetral axis as possible, where shear is the highest due to bending. Something, surprisingly not all racecar manufacturers do. (Racecar Engineering had some interesting articles about this) The joint will primarily see shear due to torsion then. its also the most apparent method of manufacture too (to split it down the middle). -derek
As for placement, I put it on the edges to move it as far from the nuetral axis as possible, where shear is the highest due to bending. Something, surprisingly not all racecar manufacturers do. (Racecar Engineering had some interesting articles about this) The joint will primarily see shear due to torsion then. its also the most apparent method of manufacture too (to split it down the middle). -derek
so after the two halves are bonded together did you add another few layers to cover and reinforce the visible edge of the 'female' half?
if so what cure method was used for these outer layers? and was there a visible difference in the surface finish quality vs the female molded halves
if so what cure method was used for these outer layers? and was there a visible difference in the surface finish quality vs the female molded halves
The female mold is really only needed to establish shape. In theory you could do 2 layers in the mold, then joint the 2 halves, and lay all subsequent layers. Vacuum bagging is a common technique for clamping when a mold is not used.
yeah, there are several layers on teh outside of the seam. These were cured just like the layers in the mold, with peel ply, bleeder cloth, release ply, and breather cloth inside a vacuum bag. The surface finish is not as nice because of the fabric edges, the peel ply, and the vacuum bag and so requires some finishing. THis can entail filling, fairing, and painting; or sanding, clearcoating, and polishing. Im workin on a few techniques which will hopefully allow for a vacuum bag compression with a nice, or at least better, surface finish.
true, although It can be hard to maintain a consistent surface finish without the use of a female mold, the material can be a real pain in the ass... fraying with the slightest touch, or pulling the weave out of shape accidentally.
Id say aside from molding a 'seam' out of carbon fibre, prepreg would possibly be the best solution here, it wont fray, its 'sticky' so wont move about too much, it will keape the weave in shape and it can be vacuum bagged as you say.
Thats true, and the main reason I went with the female mold. I looked at prepregs too, but moved away from them because of several reasons. At the time they were difficult to get in small quantities due to the carbon shortage, they must be stored in a freezer, and cured at an elevated temperature (although this is not terribly difficlut). For the seam, I am thinking of working with a poor mans prepreg, in which you wet out flat pieces of carbon and allow them to get tacky in a vaccuum bag. You can take em out after a suitable time, and have a tacky weave that you can cut without fraying. THe big question mark with this technique, which will require some experimentation, is how long to let them "pre-cure" before laying up so as to ensure a quality layup. Initial tests resuslts look positive.
I am also considering this technique or something similar and some sort of male mold for the next frame, with a mating female mold to get a consistant inside surface for bonding, and a decent surface finish.
physical work ont he CVT has paused since Interbike as I catch up on other obligations I had to put off to get ready for the show. In the meantime though we have been developing some new ideas to improve the CVT and on a 10 speed gearbox as well.
yeah the having to have them shipped frozen put me off..the home made prepregs is a very good idea! parts i have cured before at just above room temperature have that 'tacky' feel after about 1 hour.
if you could create female molds for a sort of lip that would bond onto the top and sides of the frame (then repeat with the back and underside of the frame) you would have that uniform finish throughout.. the frame you have already made could be used to make such a mold out of fibreglass for example. it could also be made to look pretty good too if you get creative with the shape of the lip.
I think I can visualize what you are saying. If I got it right, you have a good point. ONe question I have though, is if you mold an additional piece to reinforce the seam, how do you control both the outer surface for finish, and in addition, the surface that will be bonded to the frame itself. In other words, how do you control the thickness of hte layup to ensure it has both a good finish and a dimensionally accurates surface for bonding. Do you just be careful and unsure a layup goes down smooth and vacuums down with no wrinkles? Its my impression that this is the problem all carbon manufacturers have. Many monocoque frame builders go back and machine all the inner mating surfaces for bonding. Theres definitely some room for creative manufacturing yet with carbon, especially in terms of reducing labor and cost.
it could be tricky, so i suppose it really depends on the tolerance of the mold, and as you say youd have to be careful to avoid wrinkles.
with the inner surface of the lip, it would be desirable to have it rough, to get a better bond. likewise a rough surface on the frame areas that the lip would cover...so some machine work on the inner surface of the lip to remove bubbles etc would be advantageous Its likely that your frame is strong enough even without a lip so taking off excess carbon wouldnt be such a bad thing. as long as the visible outer surface was good there should not be a problem.
another way to do it would be to sand the area on the main frame that the lip would bond to. Then use your homemade prepreg technique and take the lip out of the vacuum bag when 1/2 cured.
place it opon the main frame (perhaps with a light coat of epoxy) thus getting a good fit. stick it in the vacuum bag again and complete the cure. Im sure the edge that you are trying to hide on the main frame could be sanded, perhaps with use of a curved sanding block so that no imperfections show through to the lip.
with this way you could just cut the edges, whereas if it was left to cure they would have to be sanded clean.
i hear what you are saying about creative manufacturing! theres a vast amount of ways to get the job done and as you have done yourself, you can even through your own solutions into the mix!