BernieG

I wonder what exactly the process is to mold carbon reinforced plastic parts. On his site, BV describe that pretty well http://www.bvmjets.com/Accessories/carbon.htm , but I wonder if there would not be a more amateurish process allowing an individual to do something similar for smaller quantities, a kind of casting.

Does anyone have some insight about that ?

Thanks,
Bernard

ptxman

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Here is a project I tried that kind of fits your description. Its a copy of a spoke/lightened aluminum spinner backplate. The mold was poured from a semi rigid form of urethane. It happens to be a Smooth-on product, but others are available. Urethane are good for stuff like this because: it captures details well, it is 50/50 mix, it has reasonable viscosity, it demolds in a matter of minutes, it has some flexibility to demold the cf part when reverse curves are inevitable, it is relatively inexpensive. The only neg factors are when you require super accuracy or are involved in larger parts & distortionbecome an issue. The #1 part was so-so, the subsequent ones were basically perfect once the tow length formula was worked out. It was a finicky little part to make with the web intersections & varying cross section thicknesses but it was fun.

ptxman

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the modl & cf part

ptxman

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closeup of CF back side, first attempt, warts 'n all

BernieG

Yes, Yes, that is the kind of realisation I had in mind. Could you please give some references for the products used, both for the mold and the parts ?

Thanks a bunch,

Bernard

Rcaerosport

Any idea on the weight savings? Looks great!

TOYMAKER

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What size back plate is that spinner from? Have you tested it at speed yet? A 3 1/4 backplate can be lightend almost by 1oz with the geo cuts like in yours. So if you go c.f... maybe just a few grams? How does the spinner cone fit your c.f part?

I like it if it works.

rpmtech

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Injection molding uses carbon reinforced resin that is like silly putty in consistency, it is forced into a Steel mold under many tons of pressure.

I have thought of making molds out of silicone RTV or urethane, and mixing some epoxy with graphite powder and injecting it into the mold with a syringe, could work...

ptxman

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The plug was obviously the finished machined aluminum part. The mold material is Synair Por-a-mold 555 urethane. I chose this 55 durometer material as a compromise, its stiff enough to hold dimensions pretty well & still has some flexibility to demold complex shapes. (It feels about like an eraser for reference). Synair is nice stuff for this type of application - equal mix ratio, rapid set up time, resilient to demolding. Just have to watch you don’t introduce bubbles during mixing, or vacuum them out or something. I used their recommended releasing agent. The layup was done with carbon tow & laminating grade epoxy resin. I don’t know the final weight off hand but recall it ended up about 70% of the aluminum part. That works out pretty close to what you calculate based on material densities. The carbon part ‘feel’ is incredibly strong & it fits the spinner cone perfectly.

Here are the links to the products

http://www.synair.com/art/grid.html

http://www.synair.com/products/releases/sl531.html

Here is a link showing some nice work that is similar.

http://winshiprc.tripod.com/index.htm

ptxman

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Silicone from what Ive seen will work the same as urethane. It usually doesnt have the same 1:1 mix ratio so is a bit more sensitive that way, & maybe not as many durometer selections as urethane but new formulations arrive all the time.

I dont think Id recommend carbon powder over tow based on my own informal coupon break tests unless its a very tiny, low stress piece (in which case you may as well make it out of something else). Tow is much stronger. Carbon chop & powder have their place in filling in smaller voids & fillets here & there that the tow has trouble conforming to. I used a bit on my subsequent trial & it worked well to fix the voids you see on this #1 version. You are striving to cram as much wetted fibers in the cavity. Any excess epoxy reduces strength but it cant have dry spots either.

TOYMAKER

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Why not just make it out of a solid backplate instead of the lightened one. Molding it would be much easier. I'll bet it would still be as light or lighter then a lightened aluminum backplate.

ptxman

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You could, it would, it might be..... but then it wouldnt be a spoked carbon fiber back plate would it?

TOYMAKER

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Now... lets make a carbon cone to go with the backplate.

seriously... I would like to try making a complete spinner in 3.25 in FAI size. We already have our pattern plane built with c.f fuse, c.f. landing gear and a c.f. tune exhaust pipe. Why not a
c.f. spinner.... lets not forget we can get a c.f prop too... Wow I LIKE C.F dont I.

ptxman

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Sounds great, show us your pictures of the process & results! Note, a CF spinner topic has been started on this forum, see some comments & considerations there.