J

JimWoo50

Well Known Member
I decided to sandwich a layer of kevlar into my windscreen to canopy gap filler so it would be stronger. It certainly is strong and was very difficult to cut. The rotary cutter wouldn't work so I borrowed a pair of my wifes sewing scissors and was able to slowly cut it. Its now on the canopy but I'm finding it is very hard to sand and otherwise work now that it is hard with epoxy. I would like to sand off the rough edges and cover it with another layer of e glass. Any tips on how to work with this material would be appreciated. Thanks Jim.
 
This is one of those cases where I wish you had asked about kevlar before using it. You have already found out why I don't recommend it for very many uses. It is so strong that once it is in a layup none of our tools will do anything but fray the edges. Unless you are using something like a water jet cutter. About your only choice is to completely encapsulate it in the glass epoxy layup so when you sand you are only sanding glass and epoxy. Another option is to dig it out and redo it with glass. I know that isn't what you were wanting to hear. If someone else has a better suggestion I would love to know what it is.

The sad thing about kevlar is you very seldom get to use it where it actually contributes to the strength. It stretches more than glass or carbon so if it is used with either of these the glass or carbon will fail before the kevlar can start contributing to the strength.

Sorry,

Steve Eberhart
RV-7A bolting on the tail
 
Any stainless steel scissor can be quickly and easily converted to cut Kevlar. Just run the blade across a disk sander at a 90 degree angle, one or two passes on each blade is all it takes to modify new scissors or to "sharpen" them after they get "dull". (if I could post a picture here It would be eaiser)
 
Yikes. Any attempt to sand the stuff will probably make it look like it went on a trip to Hair Club for Men. Can you say "fuzz"?

I don't know how much room you have but my suggestion would be:

1) Use a SHARP blade to cut through the Kevlar and trim it back as much as needed (or as much as you can). #11 X-acto blades or a single edge razor. Buy a whole box....you'll need them. Just keep scoring over the same spot and you'll eventually get through. And keep the blade SHARP.

2) When you're done, maybe some micro around the edges to give you something to sand smooth.

3) Put a layer of glass over it so you have a reasonable material to work with for your finish work

Maybe someone else has better advice but this is all I've ever gotten to work for me. My least favorite cloth to work with by far. I normally take great pains to cut it to EXACT size, or slightly undersize. What am I saying? I normally take great pains to use glass or carbon fibre instead :D

Good Luck! Let us know how it turns out and how you eventually were able to work it.
 
And One more Thing....

You do realize that you probably owe your wife a new pair of sewing scissors as well, right? :rolleyes:

Well, you did ask for advice.... :D
 
Peel

The nice thing about laminating Kevlar into a layup is that when you want to remove it, it easily peels off the layup because the bond between the epoxy and the kevlar is very poor.

If you want to remove the kevlar, start at an edge and begin the separation with a knife, then you can wedge or peel off the laminate above the kevlar along with the kevlar.
 
It is sheeeet material. I personally removed all of the kevlar out of all the Fairing panels / fairings for a major airlines A-320 fleet and replaced it with fiberglass.. It will suck water into you project and blow your panel out.. Just you wait!! Do your self a favor and remove it now!
 
As I remember from my composite days, Kevlar is very good for impact resistance (as in bullet proof vests), but tends to be brittle and does not stick to epoxy well. For strength/ stiffness, graphite cloth is preferred.
 
Peeled right off.

Started under a corner, grabbed it with a pliers and off it came. Took about 5 minutes. I wouldn't have guessed it was bonded soo poorly to the glass underneath. Thanks to all for the advice. Now I'm just going to put a couple more layers of e glass and that will be strong enough.
 
Would Carbon Fiber be a good addition for strength? Or are there any gotchas with that as well?
 
Composites

Hi Jim,

As has been pointed out, you're kind of screwed. Definitely should have asked first. But, since we can't unring that bell.....

There are special scissors for kevlar if you need to cut the stuff in the future.

Also has been pointed out you may have compromised the integrity of the bond by using Kevlar as it doesn't bond well to dissimilar materials. If there is a way to remove it, I would. This may involve using a sanding drum on a high speed air tool. If you aren't interested in removing the Kevlar, the next option would be to add a couple of layers of glass cloth over the Kevlar. The Kevlar needs to be completely covered, it is not sandable.

Regarding carbon, it will not give you any significant advantage for use on the wind screen. Carbon is great where you want strength without flexing. Carbon in thin laminations is very brittle and splinters easily. It only becomes strong and durable with thickness. You can sandwich it between glass layers to give strength and save some weight.

I remember reading that someone was talking about making Carbon wheel pants. They would be very light but durability would be nearly non existent. Same with empennage tips. Not to mention, expensive.
 
Well, now I'm curious

I have a big roll of Kevlar that I was planning to use during construction (and a $50 set of scissors), but I will follow the warnings of this thread and dispense with that idea.

I am curious about the poor bonding comments. I had assumed that an epoxy matrix was the best way to create a Kevlar layup. If the stuff peels off that easily, the interlaminar shear strengh of the solid composite must be low. Anybody know how that is dealt with on the "real" kevlar parts? Autoclave?
 
Yea, we threw it away! we had a giant autoclave and we never used it.
. I can say the only thing we did with it was remove production kevlar and replace it with fiberglass. .. sell it on Ebay
 
Bagging

rzbill said:
I have a big roll of Kevlar that I was planning to use during construction (and a $50 set of scissors), but I will follow the warnings of this thread and dispense with that idea.
Click to expand...


Most of the work with Kevlar is pre preg or vacuum bagging. I have a couple of fuselages for high end RC sailplanes made in the Check Republic that are carbon/kevlar weave. These are finished in the mold and vacuum bagged and therefored don't require work because the kevlar is never exposed.

You can sell the Kevlar on Ebay and probably do very well. It is tough to get right now, as is Carbon because of the Military needs.
 
Kelvar

Kevlar is great for ballistics and impacts because of the amount of energy required to break the fiber. But nothing bonds to it very well. On commercial aircraft, Kevlar is mainly used on the containment shroud around the fan blades. They are designed to absorb an engine fan blade being shed and to keep it from penetrating the cabin.

It does not help to autoclave cure the stuff. It still does not bond to the matrix.
 
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