It has been ten years since I built my 360
and am now into APBA stock outboard racing. The following post on
Hydroracer.net is very informative and should be useful to you guys pertaining
to tips and techniques working with glass, carbon and Kevlar layups. Enjoy.
“Working with
Kevlar
I'm not quite sure where all the mystery of working
with Kevlar has come from. It simply isn't nearly as bad as everyone seems to make
it out to be. Here are some hints and observations that I have learned by using
it for quite a while now.
1. Scissors: You don't need to buy the expensive special Kevlar shears that are
out there. I've used the same pairs of cheapo scissors to cut glass, carbon and
kevlar. I used to build inboard safety cells. Between those and regular
outboard stuff I've probably gone through close to 100yd of Kevlar and it will
still cut just fine. A neat trick that really helps is to give them a light
sharpening on a grinding wheel. Run the blades perpendicular to the wheel and
leave the slight burr that is created. They should feel "gritty" when
you work them now. This will help hold the fibers and keep them from
"squirting" forward away from the cut. The other thing I do is to
pull them back as I'm doing each cut. It is like using 4" of blade to cut
2" of material. Sort of hard to explain, but I think you will figure it
out.
2. Surface prep: I have seen many times where people talk about pouring epoxy
over the bare wood and then laying on whatever fiber they are using. Next time
you epoxy coat something pay attention to how some spots wind up glossy and
others look dry when it is cured. This same thing will happen when there is
fiber over it. The dry looking spots are now resin starved layups where the
part will have far less strength. It is better to coat the part, let it cure
and then clean it, and scuff sand it. This primer coat ensures that your fiber
will maintain the resin content you want and not be too lean in some spots and
too rich in others.
3. Bagging: Kevlar doesn't need to be bagged. While I say this, also bear in
mind that it is lighter than you resin and will actually float. If you keep
putting on resin, it will just keep floating up. I simply put on the pre-cut
kevlar on my prepped panel and pour resin down the middle of the part. Gently
work the resin through the fiber with a body filler squeege. The more of it you
get wet out, the more aggresively you can work the resin through. You don't
want to squeege fast or you will induce a lot of tiny air bubbles to the resin.
Just work with gradually harder pressure and you will see the color change
slightly. You will also be able to see any spots that need more resin by the
way they have a flat look rather than glossy. Let is set for 10 minutes or so
and then add resin where necessary and squeege one more time for uniformity.
Remember to only put enough resin to wet it out completely and then quit. Any
more is just extra weight and wasted money.
4. Peel ply: Dave hit this one in another post. My test at the fabric store is
to try to blow through the cloth. If you get one that is very hard to blow
through, it will peel off easily. I just get 100% polyester cloth with a very
fine weave. There is alway something on sale in the bargain corner. Usually the
only problem is getting the cashier to believe that the small stain or whatever
put it in the sale rack won't affect my use of it. Don't even try to get into
that discussion at the store! Once my layup is wet out evenly, I put a piece of
the peel ply over it and squeege again. I use a good bit of pressure here. If
you have the right amount of resin, you will see the pattern on the peel ply
change from just the dots at each high
point of contact with the weave to one solid color.
any excess resin will work up through the tiny holes in the peel ply and come
off when it is removed. The peel ply can even be used to hold things in corners
much like bagging. What happens is that between the surface tension and the difficulty
with which air passes through it, it will hold the fibers in some fairly
complex shapes with no vaccuum. A hint when peeling it is to pull it off 45
degrees to the weave direction. This way, just like gaining strength by laying
up gusset strips on the bias, you are pulling on tiny strings in two directions
rather than one. It should pull up without too much effort if you work slowly
and work from side to side.
5. Green trimming: I alway time the lay up the Kevlar so that I will be able to
come back later and trim the parts when it is say 80% cured. I don't mean to
full cure, but rather when it is no longer tacky, but still pliable. At this
point the peel ply will come off very easily. Don't try this too soon, or it
can actually lift the fibers you are keeping down. After you have the peel ply
removed and you are done admiring your very flat surface, trim the edges with a
new utility knife blade, You will quickly find the ideal angle to hold it by
experimentation. I always have all the cuts made in the wood first so that this
becomes a simple tracing step of following the lines.
6. Dry trimming: If you need to make any cuts after the stuff is cured, make
the cuts however you normally would. Use carbide tools or you may as well throw
them away. It is very hard on cutting edges. You will see the edges fuzz up
like crazy. Don't sand them or they will only get worse. A lot of this can be
cut off with a brand new utility knife blade. It won't get it all though. You
will need to wet out the fuzz, and trim it in the green state or let it cure
and sand and repeat a time or two. With patience it will all come out with no
ragged edges.
7. Flat straight cuts: If you have an edge that you want straight across a
panel that extends beyond the fiber, here is a neat trick. Put down a piece of
2" wide tape that butts up to the desired edge. Lay the fiber so that it
extends on to the tape. When it is in the trimming state of part 5 above,
simply come back 2" from the exposed edge of the tape, and cut it. Peel up
the bottom layer of tape and all the wierd little stuff that happens at the raw
edge of cloth is gone and you are left with a perfect crisp edge that looks
great.
8. Not quite a vaccuum bag: A neat trick for holding any fiber into complex
shapes is to use a garbage bag with water in it. Put down the peel ply and then
use a garbage bag with some water in it. As long as there is slack in the bag
and it isn't bridging over any shape detail you will get modest pressure in all
directions to hold the cloth to the more complex shapes. You will be amazed at
just how little water it can take to accomplish this. To make this method one
better use warm water and accelerate the cure!
That is about all that I can think of right now. I probably have photos
somewhere that show some of this if anyone needs more to go on.
Steve Roskowski “
Submitted by Bob Smiley 94RJ 360