Expanding foam + easily removable

[OneLooseCrank]

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 though).

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!

[harry]

It's normal to leave the foam in.

[Mr Pounder]

Take it to uk.rec.cycling They love cyclists in that group.

[DA]

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 by 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 pieces?

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.

Cheers!

[Andy Burns]

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?

[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...

[OneLooseCrank]

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.

[The Natural Philosopher]

why are you using expanding foam in the first place?

[The Other Mike]

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.

[harry]

If you're vacuum forming it you won't need foam? It would be crushed.

[Martin Brown]

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.

[GB]

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?

[The Natural Philosopher]

exactly. If you are using a female mould the easiest thing is to sculpt the frame out of expanded polystyrene and then carbon wrap that. Acetone or cellulose thinners will make short work of it.

[Andy Champ]

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!

Andy

[RobertL]

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.

Robert