Need tips/ideas for molding complex high strength Carbon fiber part 2

[EvOne]

Hi all,
I need to produce a part that looks almost exactly like an aircooled disc brake rotor. IE: a shaft mounted flat disc with fins supporting a disc shaped ring whose inner hole does not touch the center shaft. Approx 12" in diameter, approx .750 thick. This part will be highly loaded axially at the outer edge and I need minimal axial deflection, maximum stiffness. Mental picture describing the stresses on this would be to take an aircooled disc brake rotor, lay it in a hydraulic press supporting it above the table only on the outer diameter with the ram pushing down on the hub with approx 1,000 lbs of force. Considering making the part out of CF as light weight is a plus. Lower thermal conduction is desirable. Also seems it should be easier and cheaper to fabricate a prototype this way. Have heard of an "Aquagel" cavity filling material that sounds promising. I have very little experience with CF for this type of part, need advice on material and resin selection, resin impregnating methodology or whether pre-preg cloth is advisable. What are the best layup practices for maximum strength? I am currently thinking along the lines of laying down the full disc, positioning precast formers (aquagel?) then temporarily tacking on the "fins" or "ribs" then laying the "ring" on top of those. I am mostly worried about bonding the "fins/ribs" to both disc and ring. Ribs will be in compression and seems that the part will need to derive most of its stiffness from them. Could use any tips or advice you care to offer.

 

[DROSSI]

the only thing i can say is if your going to epxoy everything together make sure its balanced if your rotating it at a high rpm!!

have you thought about loss core process with a light alloy??

 

[MikeHalloran]

My limited experience with composites says that if you can't make the finished part in one cure cycle, you won't be able to trust its structural integrity.

Look around. How many _complex_ CF parts can you find? There's a reason.

If you had the production numbers to support a big tooling program, your part might be a candidate for injection molding, but it doesn't sound like a good candidate for frp, whether the f is carbon, or glass, or cotton.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[EvOne]

I don't think balance will be too much of a problem, planned operating rpm will be around 300 rpm with a VNE of 450-500 rpm. On the lost core idea with a low temp alloy, sounds like an interesting idea but seems harder to work with than the stuff I have found in Tucson from ACR. Trade name "Aquacore", would highly recommend having a look at it if thinking of doing any lost core fabbing. Looks ideal for CF lost core items, fairly cheap @ 125.00 for 5 gallons, disolves in cold tap water and is spec`d for resins curing under 380 Deg F. Apparently they are also willing to make high temp versions etc. Since I am in Phx I have been planning to take a drive down to Tucson and discuss the app directly with ACR. I definately am planning to do this part in one cure cycle, mostly worried about resin flow issues between the mating parts. Thought about using prepreg cloth cut to shape and placed into the aluminum mold cavity, then placing the Aquacore "spacers" with strips of prepreg between them to form the fins then placing the final ring disc on top with an aluminum plate clamped on the top to close the mold and put slight pressure on the whole assy. My main concerns are cutting the CF prepreg cloth into a fairly accurate disc, cutting the rib strips and getting the resin between the discs and the rib faces so they are properly joined. This is mainly a prototype piece so I guess I am envisioning assembling the prepreg CF pieces into the mold and adding the Aquacore "spacers" and the filler resin by hand catsup bottle type method. I guess for a production piece the top cap on the mold could be drilled for resin injection but I am not sure how you do the resin injecting then get the residue out of the feed holes before you cure the whole shebang. I could still use a source for the CF prepreg cloth and resin, especially from a supplier/manufacturer that could offer suggestions on product application and suitablilty. Thanks to all that have replied.

David Chapman
Arizona Electropulsion / Fine-Junque

http://www.stores.ebay.com/theworldoffinejunque

 

[MikeHalloran]

The whole point of using prepreg is that all the resin you need is already in the cloth and partially cured so it's semi- sticky, and sticks to itself pretty well, so you can build it up. Adding neat resin just makes a mess and weakens the resulting product.

There is another process that may be of interest. Resin Transfer Molding/ Infusion molding has taken the boatbuilding industry by storm. In that, you preplace the reinforcement, possibly retained in a very lean prepreg, bag the assembly, and pump resin into and through it, then cure the resin.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[EvOne]

Hi Mike,
Yes the Resin Transfer/Infusion looks like something that might be a good idea. I was planning on an aluminum cavity mold anyway. I am not sure how to prevent the excess resin from clogging the "sprue" holes, will have to ponder that. Do some research on the topic. Any suggestions for good suppliers for small quantities of prepreg CF and resins? Seems that there aren't a lot of suppliers when I Googled on "prepreg CF" the other night. Thanks again, David Chapman.

David Chapman
Arizona Electropulsion / Fine-Junque

http://www.stores.ebay.com/theworldoffinejunque

 

[MikeHalloran]

In a closed cavity mold, the resin does indeed clog the sprue and vent holes. That's why closed cavity molds are made in multiple pieces, that's why molds are expensive, and that's why closed cavity molds are rarely use in frp manufacture.

I suggest that you read up on frp manufacture, and on injection molding. Take special note of the subject of 'debulking', which is essential to frp, and extra difficult in injection molding.

Before you invest a fortune in a roll of CF prepreg, I suggest that you develop your tooling and process with glass fiber prepreg. Contact me privately for the name of a supplier of both.

I wish you well, but I suspect that you face some nasty and expensive surprises on the way to making a good part.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[doogie48084]

evone
Mike is right. run some prototype parts using soft tooling (FRP tools).
I run vacuum infused parts all day long and I dont use prepreg. I use dry carbon fiber material, (specifically 5.7 oz 2 x 2 twill). I bag my parts and vacuum degass the bag and check for leaks, then I allow the resin (vinyl ester) to get sucked through the tool and it wets out the carbon. when it reaches the end, I shut off my vacuum and allow it to gel, then I remove the bag and let it finish curing. This makes for gorgeous clear carbon cosmetic parts.

http://photos.yafro.com/pics3/i/20050331/14/d/3/9/d39d3106f864b02899bf82f02382c6a30_full.jpg

this is an example of what you can do without expensive tooling and without using an autoclave for prepreg.