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Old Oct 13, 2002, 08:09 PM
Stuck on a rock
epilot's Avatar
Germany, NDS, WOB
Joined Nov 2000
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Making carbon props

Hi Gang

I have been making props over the weekend. Attached is a pic of how it is done. A suitable prop is fixed on a piece of wood. Since I have no release agent I simply rub the prop with a candle. Apply some epoxy and 2 layers of light carbon cloth. Cover with cling film which is stretched over the prop with sticky tape. Gives a nice surface. I cook mine in the oven for a couple of hours at 70-80 degrees C. Viola! Cheap prop.

Finished prop on the geared KP-00 is made on a Graupner speed 280 folder with the folder blades glued together. It is my favourite for small models since it has good thrust compared to the amps it draws.

Michael
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Last edited by epilot; Oct 13, 2002 at 08:11 PM.
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Old Oct 13, 2002, 08:50 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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Michael,
Great. I've been thinking of doing something like this, but haven't gotten around to it. Why do you cook it? Won't it cure on its own? I've wondered about doing something like this with the food vacum bagger technique written up elsewhere on the Ezone. And, now you've given me an idea. Why not use some heat shrink tubing for battery packs. Just slip the wood with the prop and CF covered with clear plastic in the shrink tube and then hit it with a heat gun. Then, put it in the oven. What do you think? Will it work?

Can you give us an idea what one of these finished weighs? Also, what are you doing to mount them?

I've been thinking for a while that now that we are using light LiPoly cells, suddenly the props we thought were light aren't quite so light after all. I've been meaning to find the thread where Aeronutz Mark (I think) did something kind of like this. Any additional details would be appreciated. Now I've just got to try this.

Gordon
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Old Oct 13, 2002, 09:07 PM
FLB
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Montreal-Toulouse
Joined Jul 2002
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Hi Michael,

Very nice job. Like Gordon I would be interested to have more details, if possible.

Here is another link for home made carbon props (in german). I think I found it on this forum but don't no anymore in which thread:

http://134.169.96.2/~modellflug/klau.../prop/prop.htm

What is "cling film"?

FLB
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Old Oct 13, 2002, 09:15 PM
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Hi Gordon

The epoxy I use wont harden properly unless cooked a bit. I really must try another brand.

I have attached a couple of shots of the prop hub. It's a small piece of ply and some teflon tube. Tight fit on a 1mm axle.

I thinks vaccum bagging or heat shrink is overkill but would certainly work. I am happy with the results I get from using cling film.

The prop on the KP-00 is a 4.7X2.3 (Graupner folder) and weighs 0.6 grams. Would be lighter with only one layer of carbon but a bit too weak IMHO.
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Old Oct 13, 2002, 09:18 PM
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Second shot
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Old Oct 13, 2002, 09:20 PM
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Quote:
Originally posted by FLB


What is "cling film"?

FLB
Known as Reynolds wrap or Saran wrap in North America I belive.
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Old Oct 13, 2002, 09:48 PM
FLB
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epilot,

Thanks for your answer. I was not sure.

FLB
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Old Oct 13, 2002, 10:36 PM
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SuperCat's Avatar
Wichita, KS
Joined Oct 2002
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Carbon Props

It looks like you guys are doing a fantastic job. I run the composite lab at K-State and would like to answer some of your questions.

Why do you cook it? Heat generally speeds up the curring time. For instance, if you have a 24 hour pot life (cure time), you could cook the part and accelerate the cure to an hour or so. It all depends on the polymer (resin) that you are using.
You can use any type of resin (epoxy) on carbon, the type of epoxy used only determines the overall strength of your specimen. When you guys buy your cloth, ask the retailer which resin system is supported. These props don't see near the forces exerted on them as they do in industry, therefore you should not have any problems with using any time of epoxy. There are basically three types of resins that are normally used: polyester, vinyl ester, and epoxy. Stick with epocy, it does not creep over time as the others do.

For the cloth, you can buy dry fiber or what they call "prepreg" material where the resin is already embedded in the material. There are specific curing cycles that should be adhered to when using prepreg material.

Vacuum bagging is normally used to reduce porosity and voids within the matrix. It also makes it easier to get a near complete part without much rework. A tip when sanding or grinding on your props. If you are getting individual fibers that fray when ground, you need to alter your curing process or wet the material down more. In general it takes less resin than you might expect and the resin will pool in low areas, which is why a lot of people like to vacuum bag their parts.

Anyway, I could go on and on and I am sure you would rather practice building props than reading posts. If you have any questions about any composite related issues, I could probably steer you in the right direction, including online retailers.

Again fantastic jobs with the props and hope you continue to mess around with carbon, it is marvelous stuff

Here are a few articles on carbon parts.

http://www.cstsales.com/articles_for_model_builders.htm


http://www.netcomposites.com/education.asp

http://www.raypubs.com/ctyp/ypoverview.html
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Old Oct 14, 2002, 06:02 AM
Sticky Shepherd
Graham Stabler's Avatar
Oxford/England
Joined May 2001
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Nice one Michael, excellent work as ever.

On a general composites note, another alternative to vacume bagging is heatshrink tubing, if you get the right stuff it works a treat. I made some lovely carbon fibre tube using some braid Michael gave me and a piece of acrylic rod. Just realised Gordon suggested it below and so yes it does work. I think shiny heatshrink is the best.

I bought a load of layflat tubing (as used to make small bags) I was going to get a heat sealer as well and then vacume bag the prop and then seal it. That way I could make a few and stick them in the oven together. I genrally bake my props on top of the radiator over night it does the trick but I will often take the carbon off the mould in the morning and then put it back on to let the "green" epoxy cure to super hardness.

Yes it is overkill although vacbagging/heat shrinking do squeese out a lot of excess epoxy and that leads to really light props.

Graham
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Old Oct 14, 2002, 06:08 AM
Sticky Shepherd
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Oxford/England
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oh, there is a thread on homebrew cf props I did but I was using a full two part mould, my experiences proved that making a plug and using vac/shrink/film was probably better.


Oh yes another silly idea I had, wouldn't be ovenable but I'll mention it. Take plug, add carbon, place in a tube (drain pipe?), insert balloon, blow up balloon. Anti-vacuum pressure moulding, it might work

Graham
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Old Oct 14, 2002, 08:34 AM
Was that my 15 minutes?
Andy Birkett's Avatar
Leicester. UK
Joined Jan 2002
117 Posts
And whilst your using vaccum bags don't forget pressure cookers, for the whole boil in a bag prop experience

Heat and squeeze all in one

Andy

Ps Graham's props work really well until I break them
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Old Oct 14, 2002, 10:55 AM
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Seems like our Aeronutz friends have been sniffing a lot of solder fumes lately

I thought about making proper moulds but being lazy by nature I decided to take the least complicated approach and it seems to work just fine. I'm happy with an 0.6 gram prop that I know will take a beating. The single layer props tend to break on impact.

HOWEVER.... Since I have no life anyway I will have a go at producing a candle wax mould and see how it works out.

Michael
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Old Oct 14, 2002, 11:09 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,432 Posts
Michael,
I like your technique of using a prop for the mold. Personally I would avoid making a mold unless I needed a prop that didn't already exist somewhere that I could use as a mold. I'll probably try one in the next few weeks on a GWS prop or something. I really like the idea of using an existing prop. Do you have a pin sticking up from the wood you mount the prop on to hold the prop in position?

Graham, when you say shiny heat shrink, what is the advantage is "shiny"? I have clear heat srhink from Airdyn, and also while heavier duty stuff from SR batteries. I wonder which will do better?

Gordon
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Old Oct 14, 2002, 11:44 AM
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Hi Gordon

No, I don't have a pin in the mold. I drill the hole in the prop after it is cured.

You sometimes have to cut/sand some of the hub off on the prop for the mold. I like to use folder blades and just glue them back to back. The small Graupner CAM folders seem to be an excellent choice and are a lot more efficient than the old grey folders.

Michael
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Old Oct 14, 2002, 12:23 PM
Sticky Shepherd
Graham Stabler's Avatar
Oxford/England
Joined May 2001
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Gordon, the heatshrink I used was quite thin and the epoxy didn't stick to it. Most heatshrink used for battery packs is probably the same, just check on a sample. I have other heakshrink from eletronic suppliers that is quite rough in texture, degrades the surface finish and is more likely to stick.

Graham
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