trimming Kevlar and edge finish

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#1

Hi,

This is my first post and have to say brilliant forum !

I want to ask advice on edge finishing using Kevlar. I layed up a piece wet and then bagged it. On trimming the edges the fibers were very dry, which made it very difficult to get a good line when cutting, I brushed resin on the edges after to get rid of the frayed edge and seal the edge. Is this frayed dry edge a normal thing when working with Kevlar or is the breather / bleeder I’m using absorbing too much resin leaving the piece too dry. Also I’m using polyester resin but read epoxy is much more suited to Kevlar layups, is this a possible factor ?

I’ve been looking at a few bonnets made from carbon fibre and their is a nice resin finished edge on them. Is this been got from exactly laying up the fabric to the edge in the mold and then using infusion ??

Thanks for any help and advice !

Joe

#2

Welcome!

Epoxy resin is better for Kevlar.

How are you cutting the cured Kevlar? Try a power bandsaw inverted (installed upside down).

#3

Or use a Dremel tool w/ a cut off disc.

#4

you will always get a bit of fraying…kevlar IS meant to be cut resistant!

#5

Thanks for the advice ! I used a thin cutting disk on a 4" grinder, definitely not the way to go from the finish and fraying I got. Will try out what you suggest and report back !

Anyone have any comments on what I was asking about the edge finish on the carbon fiber bonnets I was looking at ?

#6

The neat edge on the bonnets comes from the way your molds are made.Usually called a rolled edge instead of just having a sharp cut edge .Takes a bit of time when you are making your flanges to get it right.

#7

depending on the part you can just end the kevlar just shy of the edge of your part

#8

The best I seem to do is to sand the edge with progressively finer grits of sand paper. I usually start with 80 grit and work down all the way to 600. I agree with the post above. Where it’s possible I do the edge with carbon. I,ve been watching this post hoping to hear a solution(miracle). Also as posted previously kevlar doesn’t absorb resin, it’s only encapsolated by it. Thus the dry nature or fraying of the material.

#9

I’ve tried cutting Kevlar/carbon composites (3 layers kevlar, 1 carbon) a number of ways.

I’ve used a dremel with diamond cutoff wheel, and that produces a lot of smoke, and really just melts it’s way through the epoxy/Kevlar more than anything…but does leave a respectable edge finish.

I can’t remember if I’ve tried an abrasive cutoff wheel instead of the diamond one…but that might be worth trying.

The best results I’ve had were doing my cut work while the part is still curing - when it can easily hold it’s own shape, but is still semi-pliable.

I use Stanley MaxSteel long-cut aviation shears - for $20 or less, they cut Kevlar like butter when it’s dry, and with practice, you can cut Kevlar composites without the edges delaminating. The things are strong enough that cutting totally cured Kevlar parts is possible, too.

Hope this helps!

#10

Waterjet leaves the cleanest cuts on Kevlar.
The problem with frayed edges is not usually caused by low resin content. What happens is that when a kevlar fiber is flexed by the cutting tool, the resins “pops” off. This leaves an unsupported fiber, of a material that’s abrasion resistant already. The material essentially ends up tearing instead of being sliced cleanly. Whereas glass and carbon snap across the fiber, aramid (Kevlar is just a brand name from DuPont) snaps along the fiber basically turning it into a bunch of smaller fibers.
Epoxy or at least vinylester helps prevent this, as polyester has poor adhesive qualities and forms only a weak bond to the Kevlar.

If possible, it’s very important to support the Kevlar laminate on both sides while cutting. For flat panels, a piece of plexiglass on each side will hold the material steady during trimming, and result in much cleaner edges. Also, wet sanding with 400 grit will clean up the edges to a smooth finish, and is what DuPont recommends.

#11

This is my first post so I’m glad to not start with a question.

I had the same problem whe I made this:

http://www.flickr.com/photos/polytube-cycles/3313782572/

The dropouts were made of 8 mm plate with aramid as a top and bottom layer. I used a scrollsaw and about 20 blades to cut the shape out of the plate. I found the easiest way to trim the frayed ends off was with an exacto-knife. They get dull fast so have many replacement blades on hand.

Cheers

#12

Hey man! Ever spend time at any…“frame forums”? :wink:

Good to see you over here, small place, this inter-web is!

(I think my username over there is CarbonKev)

Cheers!

#13

HI,

it is a small wolrd.:slight_smile:

Cheers

#14

i was compositepro on frameforum until like everyone else who has ever had an opinion other than kissing neils arse quit with it

maybe i shoud have said why but there you go

#15

Hi guys I did a vacuum bag sheet and cut the edges with a dremel cutting wheel. I got all of these frayed edges. Tried sanding but this is not working. Adding a new epoxy end sanding it can work?

#16

Has anyone ever tried to “flame polish” kevlar egdes using a hydrogen torch like used to clean up acrilic cut edges? Just a quick pass of the flame that wouldn’t even heat the part. I think the kevlar fray would melt/burn pretty easily. Might end up with little balls of melted kevlar… anyway, just a thought. Just to test, could use propane but may discolor the part a bit.

I’ve always thought this would work… some one try it an report back… no kevlar in my shop!

#17

grinding Kevlar is like sanding sheep

#18

It doesn’t work,I tried to burn the edges with a lighter once, the best way is to sand the edges until you remove the fraying.

#19

Dremel with a mini cut off wheel, lots of sand paper of various grits (wet sanding paper works fine for me. And Sherwin Williams HPC15 clear coat.

I trim, dab the 15 min cure clear coat over the Kevlar, then wet sand from 220 - 1000. It gives me very clean edges with out the head ache. The clear coat works real well because its a soft material that wont crack and fall off the Kevlar fibers.

I wouldn’t call it a miracle cure… But its pretty close :). Hopefully one day they’ll make a cutting disk specially for aramid fabrics. I’d pay up to $100 for a mini cut off disk for the dremel tool.

#20

wet sanding paper works fine for me. And Sherwin Williams HPC15 clear coat.

Great forum guys. Glad I found it. This sounds good to me, superglue well cured (overnight) on the edge and then wetsand has worked for me also. Depending on how clean the original cut is you may have to do it twice.