AV8TOR

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I run a Product Development Model Lab so molds are no big deal. I usually CNC machine my master plugs and have made several plane and helicopters. This winter for fun I would like to make a mold for a carbon fiber prop. Last year at Toledo I bought one at a pretty good price but lost the contact info. I am thinking of using it as a master and cast a mold off of it. What I am not sure about is in joining the two prop halves together. I am planning on vacuum bagging the carbon cloth in the mold, trimming, then put the mold together (with mold alignment pins) at join the two halves together. I just am not sure if a prop needs more gluing surface.
Has anyone built any carbon props?
What weight cloth did you use?
Did you use a wood center?
Do you have any assembly pictures?
Thanks

ptxman

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Ive only dabbled & it was pre-digital camera so undocumented & mostly with CF tow so solid like a Bolly vrs CF cloth hollow blades like a Meszlik ?sp. My advice if you have cnc capability is to skip the male plug machining & go straight to milling the female molds if you can. Here are some links that will give you a feel for the mold shape & considerations for layup. Using a prop as a plug involves a lot of messing around with center pins, filler to get the draft angles, care that the thin tips dont get distorted during the process, mold pins & alignment etc. You could mitigate a lot of that with cnc female molds & thats what the big boys do anyway. If you do go the prop plug, id use a casting urethane vrs epoxy layup. Re joining the hollow blade layup halves, Im guesing but from what I saw of a busted one, it looked like CF tow was wound around the hub & tow and/or possibly CF chop slurry along the adjoining l/e & t/e (it was a neat jobwhatever it was). The surface was very well finished which leads me to believe they were clear coated in the mold or after mold release & cleanup. Take some pics of yours!

http://www.delago.com/ariane/EProps.htm
- shows a good cnc female mold cavity milled from tooling urethane Im guessing

http://digilander.libero.it/mfer/molds/PMpart1.htm
- (3 parts) shows the classic plug with built-up draft angle & mold box etc

http://winshipmodels.tripod.com/h_t_mold-prop.htm#top
- ditto to above

http://www.rentooling.com/CustSucces...?lngProfileID=
- full size effort, same basic process

AV8TOR

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Ptxman,
Thanks for the reply. The reason I do not want to CNC the mold is design. To CNC it a surface or solid computer model is necessary. This would be a risky choice because you would not know if your design was a good one. Making a mold from a “good” flying design ensures that your effort will work. Basically I want to reproduce a blade not experiment with a new design. One comment on what you said about CNC machining a cavity mold. This sounds easier with a big step eliminated but having done this before it does not leave you with the best results. Especially if you want a glossy quality finish. It is very difficult to sand and polish a cavity, and when you do it is almost impossible not to round over the edges some at the parting line. This leaves thick flashing that after cleaning does not look like a quality piece. Starting with a plug you can easily sand the master and polish the master giving you a high quality mold finish. In the case of using a new carbon fiber blade the finish step is eliminated because it is already glossy. Yes the mold will be cast from a tooling urethane casting resin.
The mold making as I said is not the problem for me as I do this for a living. I would like to make the lightweight cloth hollow blades. I do not have any broken blades to cut cross sections with to see the joining technique. Mostly right now that is my questions dilemma.
I will take pictures to post. This will be a winter project so it will be a while. I am in the information-gathering phase right now. Thanks for the links.

ptxman

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You seem to have a good handle on the cnc mold aspect. Ive seen aluminum wing & stab molds polished to a mirror finish, but they obviously present a larger working area & are less finicky. If you want to do a knock off of an existing blade & play with the layup technique, then pretty much follow the hand-made mold steps shown on the links. What Im trying to point out is that reproducing efficient props that originated from precision molds requires extra care & attention with hand methods. Have a close look at the tip of an APC prop, its almost transparent because it is so thin. Adding a couple thou here & there or distorting the blade angle slightly by packing in the draft filler material is pretty easy to do while trying to be careful. Anyway, I had fun with my little test project (made a 3 blade mold from a plug of 3 spliced single blades). You will probably have fun too. Try to get hold of a broken Mezslik ?sp? to disect, it will probably teach you a few things.

There is one important link I forgot, supercool. You can see some more molds for model props & for sure read up on his airfoil section analysis work

http://www.supercoolprops.eftel.com/...article_15.htm

probligo

One of my flying mates is into C/L speed. He has established a small cottage industry making props for FAI speed models.

He gave us a run-down on how he does it at club.

He actually lays out the prop in the mould from CF tow, NOT cloth. The tow is laid in a specific sequence with the intention of spreading the stresses. One the cf is down he literally floods the mould with matrix, puts the mould top in and bolts it down.

The props he makes are (something like) 4.3x4 single bladers and are designed to turn in excess of 38krpm.

SCALECRAFT

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Hi
All this talk on composite blades motivated me to make one.

This is the "plug" for my 80" 109G6.

It took around an hour to make using power tools to get to the rough shape.

Now to hand finish the "airfoil" so it works as a scale flying 3 blade. It will be a manual variable pitch assembly.

The layup will be in a pressure mold. It will be layed up with a mix of composites that will insure stiffness and strenth.

Will post a pic of the flying one once done.

AV8TOR

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Great motivation Scalecraft keep us informed

Mluvara

My Feedback: (10)

I don't know of anyone that bags their props, although some probably do. Most props that I have seen are laid up with carbon tow, chopped carbon, and some ~5oz cloth. It takes some practice to lay up the props because the whole unit is clamped together wet.

I made one mold for a 10" size prop. The props are made out of carbon. I would start by brushing on a thin layer of resin and letting it set as the surface. Once it tacked off, I would then place a layer of carbon cloth on a 45 deg bias where the blades are. It is mostly for looks, but does help with lessening the possiblity of the prop twisting. Then start running carbon tow tip to tip. The carbon is wet out with on a piece of glass and a squegee used to work the resin in when it is on the glass. The carbon is then laid in wet. Obviously, as you move inward on the prop the tow needs to get shorter. This is done until the proper heigh of carbon is built up. Chopped carbon is put near the hub during the layup process, and once it is built up on each prop halve, the mold is closed and clamped together. It took me awhile to figure out what the right amount of material was. You either have stuff oozing out everywhere if you use too much, or a cavity in the prop with not enough material.

Michael

probligo

There is at least one -

A club member was recently seen showing off his latest acqisition - a 20" carbon fibre prop, hollow, and with two pinhole "breathing holes" at the base of the blade.

Very well made, very well balanced, very expensive.

georgemay

Hi Scalecraft,
How about CNC the blade?
I'm doing it everyday


Make 3D solid in Rhino and I'll carve it for you.

George

SCALECRAFT

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Hi George

I would guess that, as side from the fact that I'm married, have a house, kids and roomer has it I'm cheap. I always try the most economical method before taking my piggy bank out.

After all " r/c is a waste of money" from what I hear around here.

But, it helps me escape and cope. But if mine dosn't work and its not too expensive, its an option. can anything be CNCed??

Steve

georgemay

Steve,
I do not know why everyone thinks that CNC service is expensive.
Actual machining may cost you around $20/hr and that blade can be carved in less than an hour. Of course preparation might be killer here. 3D image is required to prepare tool path, but if you can draw it then I do not see any major cost.

And yes, anything can be CNC'ed

Click here to see some examples.

AV8TOR

My Feedback: (2)

George I have a 5-axis Thermwood CNC router and 3 Fadal machining centers. I am with you on the anything can be CNC’ed. The desire is for weight reduction and I guess the cool factor for carbon props. Do you have any CAD surface/solid models of blade designs that you would be willing to share? I wish I had a digitizer. You can email me if you do. [email protected]

georgemay

My machine is not even close to thermwood but I can use it as digitizer and it is accurate too.
Most of the work I did is for clients and I may not have the file I can share now. They might be mad. Most of them are full scale props anyway.
But I can come up with sample I can post here.
George

SCALECRAFT

My Feedback: (13)

Ok,

My question is, would your method work If I were to want a "plug" of , lets say, a BF109G6 cowl made from a material (economical) I can detail, can one do this from the flying scale plan? Or do you need detailed dimensions. Of course the more I provide the lesser the cost I presume.

Please advise.

Steve

georgemay

Steve,
Plans, photographs, anything at all that you have would help to reconstruct the cowl or a whole fuse. Few crossections drawings wouldn't hurt too. The more info you have the more accurate it will be. It has nothing to do with time involved. But once it is done you can carve thousands of those plugs fast.
George

SCALECRAFT

My Feedback: (13)

George

Can you email me a phone number. I'm interested.

[email protected]

Steve

probligo

This thread "bothered" me when it first came up, and I have been having a look back through my small library.

The definitive word on cf prop moulding is in Aeromodeller October 1995.

The article covers everything from mould preparation to layup to finishing and balancing is very complete.

Resin/Hardener recommended is acknowledged as out of date (Ciba-Geigy LY1927GB/HY1947GB) but alternatives should be available.

Obviously copyright will prevent putting the whole article out on the net.

KrisG

Some links here show excellent pictures of the process of moldmaking for props, but keep referring to the actual prop molding as "part two" or "a different story" . Are there any illustrated online articles on the prop molding too?

My composites retailer recommended some polyurethane resin for casting the mold, because it has a very low shrink rate. Does anybody have experiences with this, or does anybody know any obvious downsides to it?

Thanks to all for a great thread.

Kris

ptxman

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Ive played around with hard casting urethanes like what you ar referring to & they are pretty good in certain applications. Their shrink rates are comparably low even to epoxy. They generally cure quite rapidly so overnight de-molding is possible. They tend to exotherm more (higher temp) during cure & prefer their own recommended releasing agents. They are good for relativly small block pours. They dont lend themselves quite as well to reinforcements like the addition of milled fibers etc but I have done it. The finished part generally does not have high temp capability if post-curing a part is an issue & can be brittle & 'chippy' on sharp edges. As long as care is taken to de-airate the mix, they yield as good a finish as other resin systems & comprable hardness. Urethanes generally have a shorter shelf life than epoxy & are adversly affected by moisture. If you are talking about large molds for a fuse etc, I would not recommend casting urethanes.

probligo

In the Aeromodeller article I referred to, one point that is stressed repeatedly is the difficulty of getting the right matrix/cf mix.

The technique that is recommended is to wet a small area of glass with resin, place a small amount of cf tow on top and then (my words) "squeegee the resin through the tow". Then squeegee as much of the resin out of the tow as you possibly can.

The idea is apparently that there should be very little to no bleed from the mould as it is tightened down. Conversely the ratio of fibre to matrix should be very high.

This being the case, I would suggest most strongly that a moulding resin or urethane is not suitable for the purpose.

ptxman

My Feedback: (2)

We are talking about 2 different things here. The post inquired about urethane as a potential mold making material, not for making the composite component (ie the prop) itself. Urethane is a very good choice for an application like this unless post curing is involved in which case specific urethanes or additives must be considered.

The article kind of sounds like like the classic method of wetting out CF tow (strand bundles) into both mold half cavities & then clamping them together. Although props like those are very strong, they have been around for at least 18 years that I know of, probably longer in the UC speed circles. A current commercial example of these are Bolly's. But there at least 2 manufacturers that make exceptional hollow blade props from CF cloth through to TOC sizes (Mezlick ?sp) for example, which are lighter that solid uni-fiber props & sufficiently rigid. The hubs are generally solid though.

probligo

Where is that danged crow.....OW!!! Bit me mumblemumble...