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carbon fiber install question

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Old 06-25-2013, 05:47 AM
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raron455
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Default carbon fiber install question



I have a few questions for you guys who are familiar with carbon fiber,, (YES I HAVE SEARCHED and am still confused)



I am in the process of repairing a 3w 40% airframe and am using carbon fiber but Im not too sure about what to use, I am familiar with fiberglass and how to lay it, but I am going to use carbon fiber on this one, and got a few questions.



1. The carbon sheeting is easy enough to figure out, its just stiffer than the glass I have used, but I am not sure about which resin to use, for the glass I have been using this zpoxywww3.towerhobbies.com/cgi-bin/wti0001p but I am not sure which to use on the carbon fiber, I have searched and the one thing that people agree on is to use an epoxy resin, but I am not sure if what I am using is an epoxy or what?? Which resin do I use?



2. the previous owner had already laid down some carbon fiber sheet in the landing gear area, I need to put a little more sheeting going around the top inside of the cowl because it was cracked during his dead-stick landing. I am going to make 2 formers that run from one support to another, and they sit on top of the carbon fiber sheeting that he has installed, the formers I am going to use are cut out of a carbon fiber honey comb panel, When I put these formers in, what epoxy do I use to get a strong bond, I have read about some kind of GREEN? epoxy, just not too sure on this one guys,, dont want to get it wrong



thanks for any help,,



Ronnie Dean

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Old 06-25-2013, 08:13 AM
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Default RE: carbon fiber install question

Hi.

Use epoxy resin. One of the many other types of resins is, polyester. In the RC world most resins are either epoxy or polyester.

Polyester being the cheap inferior of the two. Polyester smells like Bondo auto body filler. Your fuse may be polyester, if it smells like Bondo when sanded or cut, it's polyester.

No big deal, just sand and clean area to bond with epoxy.

Today the choices are vast for epoxy resins. I use West Systems 105 epoxy and a hardner for the time I need to work. Take climate into consideration. Hot = faster cure.

Very easy to mix ratios with plastic economy pumps. And easy to work with. Always cures. Cure time determined by hardner one chooses.

Note the per ounce cost when buying zpoxy type resins. Very high cost per ounce.

The green epoxy resin I have not heard of. There are however green colored polyester resins, along with blue, red ect. They are tinted. Epoxy is normally an amber gold/clear color, and has no smell to speak of. Still use respirator and use in well ventilated area. Green also can mean a stage of cure for a layup. The green stage (gummy) is when the resin is at a point of cure that allows easy razor trimming, yet holding the shape of the "part".

On your bulkheads, use an epoxy paste. Mix in cabosil, (fluffed silica, USE RESPIRATOR!!!) maybe micro balloons. Many additive to thicken epoxy these days. Make it like peanut butter or pudding. Don't use to fast a cure on this type of bond. The epoxy will cure to fast and fail to penetrate the sanded surface.

There are many good composite guys here that can also help. This is just what works for us.

steve
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Old 06-25-2013, 09:15 AM
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Default RE: carbon fiber install question

Hi,

Everything Steve said is on the money.

A couple additions if I may. You may purchase West Systems epoxy at various places, such as CST Sales, ACP, or Aircrfat Spruce and Specialty. The only "negative" is the fact that the smallest quantity is a quart. But this stuff doesn't really go bad so it's usable for many years. The hardener is also stable as long as it is tightly sealed in storage.

If you do go to AS&S website, look up something they call Poly Epoxy. It is extremely strong stuff and is much less expensive than the premium epoxies. A point about mixing epoxy....mix it well with a stick and then with a brush like a solder paste stiff bristle brush. Pay close attention to the ratios of resin to hardener. Weigh out if you can

US Composites and CJ Composites also sell pretty good epoxies as well as carbon, kevlarand glass cloth, at very reasonable prices. You want to use the slower hardener as Steve mentioned. For a 40% landing gear mount repair, stay with carbon. It's the strongest by far

It sounds like you have a source for the carbon cloth and/or carbon sheet. For a 40% IMAC plane I would use a couple layers of 3K cloth. Wetting the cloth is not a big deal but is very important to be thorough for full strength. Place the material inside a large ziploc or similar bag and roll the epoxy into the weave, rolling on the outside of the bag. Makes much less of a mess and makes certain the cloth is saturated. Don't squeeze too much of the epoxy out of the cloth but you don't want it dripping either. Leave it rest for 10 minutes to make the cloth easy to fashion into whatever shape you need. 3K cloth will not go into deep, sharp, concave corners easily by itself. You should first build a rounded corner (like a fillet) with some of the paste Steve described and then follow up with the saturated cloth.

Cabosil is a good thickener. To add some strength I often will chop either glass or carbon scraps with scissors and admix them into the epoxy/cabosil mixture. The cut length is fairly short less than 1/4". It should take the look of wet cat hair. After placing the mix on the formers or reinforcing doubler corners, I would addsome strips of of 2 ounce cloth over the top to hold everything in place while it cures. Good luck with it
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Old 06-25-2013, 04:50 PM
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Default RE: carbon fiber install question

Ronnie, your 3W Extra is an epoxy/glass layup so yes you must use epoxy laminating resin. As a side note CF and polyester resins are usually a bad idea. Are you using cloth or tow? Be mindful of how much weight you add during the repair as you end up with a CG shift. Those 3W airplanes have huge cutouts in the rudder and elevator for a reason. Make sure all areas to be bonded are sanded well and clean. Use only enough resin to saturate the cloth, it will actually seem a little dry. I would dab a gloss coat of resin on the inside of the fuse where you plan to attach the CF if you are using cloth. Once in place with the wings and stabs on the airplane to make sure you are square you can place a hair dryer set to low inside the fuse. This will speed up the cure and increase the TG ( strength ) of the resin. About 10 years ago I repaired a Fiberclassics 202 that had the core delam from the back of the canopy forward using a single layer of 6K ( 5.7 oz ) using the described method.
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