Hello,
I want to make a bicycle frame by prepreg vacuum bagging process . I am going to use foam as a mold.
I am looking for a foam to withstand the temperature that the prepreg needs to cure.
what type of foam do you suggest?

Are you planning to wrap the foam, and so get the finished frame (positive), or do you want to make moulds out of foam (negative moulding). This determines wich kind of foam you want to use. For negative moulding, you would want to use epoxy tooling board, for positive foam wrapping, you might need low temp prepreg (ooa) and a foam like airex or rohacell.
It is not trivial to get a nice and straight frame with positive wrapping methods, as the vacuum bagging easily warps the flexible foam model during the cure process. A process called "Tube to tube" is easier to manage in terms of the quality of the final product, compared to get everything (or just the front triangle) in one go.

Also be aware that most Polyurethane foams and model boards cause cure inhibition with many epoxy based pre-pregs when cured at elevated temperature.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thank you for the replies,

could you please give some more information about the tube to tube process?
is it like making all the parts seperately and connecting them togheter after curing?
but how about making a mould for tube to tube process? what material do you suggest for mould making?
( i don't have the equipment and machines to do negative moulding)

thanks

Tube to tube is a process for producing bicycle frames, in wich you first make each tube, and then join them on a jig.
The easiest way to get tubes is to get an aluminium tube where the outer diameter equals the inner diameter of the later carbon tube.
Then get some cash register tape an wind it with constant thickness around the tube. Then "seal" the cash register tape with a layer of film tape. After that apply release agent, and wrap your prepreg around the tube. The aluminium tube should be longer than your desired carbon tube. Vacuum bag everything, follow your cure cycle, and just pull of the aluminium tube. Because of the cash register tape, this should be very easy. Finally, with the help of a stick or thinner tube, remove the tape from the inside of your tube.

I too think tube to tube is probably the best method for you. I would recommend having a look at the instagram page of werking cycles (https://www.instagram.com/werkingcycle/?hl=en) and others to see how those kind of frames are made.

Furthermore I made quite a bit of tubing in the past without the cash register roll technique. If you can cure the tubing at 80° to 120°C on an aluminum or POM mandrel and you are working with carbon fiber, the tube comes off the core really easily due to the smaller thermal expansion of the carbon compared to aluminum or plastic. Obviously you need to use a mold release agent on the mandrels.
The laminate can be compacted with shrink tape as well, which might lead to less porosity in the tubing compared to vacuum.

Cheers, Reto

Hi,
I actually changed my mind since making this parts with prepreg is gonna cost me a lot . i want to switch to hand layup process.
is it possible to use Styrofoam as a mold in this case? i mean to shape the Styrofoam, and place the carbon layers and then peel ply . i want to use a resin which would be cured in room temperature. do you think Styrofoam is suitable for this process? if not what kind of foam do you recommend ?
if yes, my next question would be about how to make the Styrofoam stiff enough before placing the layers? is there any kind of wax to do this? just to prevent the Styrofoam from being damaged.

thank you for the helps

You could absolutely use Styrofoam, but not as a structural foam, because it is far to soft.
When using Styrofoam, I would recommend tube to tube even more, because a whole frame made of this material will be extremely flexible, resulting in a warped carbon frame.
I would recommend getting a straight aluminium tube from a hardware store. The higher the diameter, the better. Then make a template of your tube cross section, with a whole in it for the aluminium tube. Cut of as many Styrofoam pieces with your template as you need for the tube length, and stack them over the aluminium tube. This way you get a relatively straight plug, with your desired cross section. After that wrap the whole plug tightly in cling film to reduce resin soak up from the foam.
Wet out a carbon cloth of the right size on a piece of bagging film, and carefully place the plug on top and roll wrap it with the carbon (not the bagging film). After that, repeat the roll wrapping with (dry) peel ply. Finally, you should compress the laminate slightly. The easiest way should be to wrap everything with insulation tape. But because of the soft nature of Styrofoam, you should be very carefully with only slight tension in the tape while wrapping, to avoid deforming the foam core.
After a day you can unwrap the tape and the peel ply, pull out the aluminium tube and push out the foam core and you got a low cost carbon tube.

I have used acetal rod as a mandrel for making small pieces of tube and it works fantastic. There is expansion of the acetal of course but this is predictable and the difference means that the acetal core/mandrel just falls out. No release agents needed either.
For a wet laid mould you should consider the tooling board materials and seal it well like easy composites do with their coating material. Never used it but it certainly looks good.

Hi,
 I  have been searching for Styrofoams which would be suitable for tube to tube process, but when I search I find many different types of Styrofoams and I don't know which one is suitable,
and the ones I find in the DIY stores are all used for insulation and as I understand they are not suitable for this process.

Could you please mention some exact names and material properties of the foam to search for?

regards,
Hadi