Hi all, New to the forum
I'm currently designing a carbon fibre bicycle frame. I plan on making a
monoque mould out of fibreglass, filling this with an expanding foam to
make a blank frame, then wrapping the foam in carbon. The final step is
to dissolve out the foam once it's all cured. My question is, which foam
would be best to use?
Now, polystyrene is the obvious one as it's dissolved by acetone, but I
can't seem to get hold of any PS that hasn't been fully expanded to use
in my own mold. Plus, I'm not sure how easy it would be to get steam
throughout the mould. So I'm hoping for something I can get from a DIY
store. It seems the yellow builders foam is impervious to everything but
lava so that isn't suitable (it will probably be used for the prototype
SO, can anyone point me in the direction of an easily get-hold-able
expanding foam that can be dissolved out with a liquid? Oh, and the
liquid can't be damaging to the epoxy.
Thanks for your thoughts!
> On Apr 29, 3:36*pm, OneLooseCrank
Hmm, not in bicycles. I'm happy to leave it in for the prototype, but I
wouldn't want to for future builds. Riders are an anal bunch who will
spend many many hundreds of pounds to shave tens of grams...
On Monday, April 29, 2013 7:52:19 PM UTC+1, OneLooseCrank wrote:
1) Could you persuade them the foam is part of the strength for the tubes -
allowing the tubes to be lighter as a result? In that case you'd be looking for
a foam that has a high young's modulus.
2) how about making the blank frame out of wax and melting it out afterwards.
Instead of dissolving, which would require large amounts of rather nasty,
flammable and toxic liquids, no matter which dissolvable [plastic] material you
chose, why not design the form to be removable? Not at once of course but piece
You could slice it up to pieces smaller than the larger opening in the finished
body. Then slide the one next to the opening out, move the adjacent one to the
space that was freed, then slide that one out as well. And continue removing
pieces like that as pieces of a puzzle. It will most likely require some 3D
modeling of the optimal way to slice it and I would imagine a clever way to hold
pieces together before they are wrapped with carbon fiber will have to be
devised. Perhaps put the pieces in order, then shrink-wrap before starting with
carbon fiber? Then tear shrink wrap at the opening and proceed to remove the
I understand this is not a complete, ready to go description of the process. You
will have to adjust it to the actual shape and perhaps some parts may and up not
being removable this way. But there may also be a chance that you might want
some of the internal crevices filled with foam rather than empty for structural
reasons. Anyway, many options exist, I just wanted to say that "dissolving" as
such may not be the only approach to look at.
> replying to OneLooseCrank , DA wrote:-
It's a thoughful suggestion... I will be making most of the tubes
individually, then wrapping them into the final frame. I could scrape
the insides out at the very least. Some of the tubes are very straight,
and whilst there aren't any mechanical locks, it would be tough to get
the building blocks out due to friction. The CF will be vacuumed
compressed upon the mold.
It might be worth you looking on some of the boatbuilding groups too -
there are carbon masts, which are 20ft long tapered carbon tubes, and
some of the boats have carbon racks which are assemblies of tubes not
unlike bike frames.
I'd suggest first looking for the International Moth class. You may find
links on the Yachts and Yachting magazine forums - it's not something I
follow. You could always try to find my brother Jim on-line, he's far
more into these hi-tech things than me. He has boats made of carbon and
epoxy, whereas mine is made of wood!
If foam isn't a goer, what about Wood's metal (bike sized parts would
probably require an unreasonably large amount) or wax, then you can melt
it out rather than dissolve it ... or is the resin setting too exothermic?
Get yourself some chrome moly tube and build a frame that is durable and fit for
purpose, whilst possessing predictable modes of failure that are usually non
catastrophic, and so both you and the bike will thus be around for a period
longer than a few weeks or months :)
Alternatively build the first few plies in two halves of a female mould, then
remove, trim and bond the two halves, then layup a few more plies on the
exterior wrapping over the existing structure. You can still vacuum bag all the
layups, there is no need to dissolve any foam and you can accurately locate and
bond any metal inserts with good control over the thickness of the bond line.
Then ride it, get crippled and post details of your experiences here.
I don't think there is anything that will easily do exactly what you
have asked for. The PU foams crosslink and will not dissolve in anything
safe to use - they also expand violently enough to destroy moulds.
Expanded polystyrene might but then the solvent used to remove it might
well weaken your newly constructed epoxy frame.
The closest foam I can think of is urea-formaldehyde as used to insulate
houses. It might be strong enough for this duty. eg.
I don't know if you can buy it in small amounts though! I am afraid you
would have to physically drill it out, but it is quite soft.
ISTR there is a water soluble PVA resin used for 3D printer supports
which can be removed by dilute caustic. If you could obtain that and
foam it then it would be ideal. There are also the biodegradeable foamed
starch blobs box fillers used by packaging companies. Some of those
disintegrate on contact with water but might not be tough enough.
A very brief google search suggests that carbon fibre products are not
usually made this way commercially. Could that be because of the
difficulty of getting rid of the foam?
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