All you really need to
make your own carbon fiber prepreg is the right prepreg plastic
embossed sheet. You
can use just flat plastic but it gets an airlock of sorts and is almost
impossible to pull off the carbon fiber.
I make an 8' by 4' table at a time by just spreading the correct amount
out on the fiber and then putting the top sheet of plastic on it, and
then cut the parts right there and then and put the lot in the freezer.
Over time and experience I have formulated the right amount of epoxy to
use for first layers and then less for the next layers, you always want
a richer layer on the tool surface. I tried blending different
hardeners to get the cure temp and time I needed but have had the best
results with the high temp hardeners, this gives you a wider B stage
time so with the low pressures that a vacuum gives (as opposed to an
autoclave cure) you need enough time to bleed out gasses and general
resin flow. The hardener I use needs about 7 hours at 180F, or about 2
hours at 250F. I tried using B staged prepreg with vacuum like you have
to with autoclaved parts but the low pressure doesen't work well with
that, so all my vacuum prepreg is A stage stuff. Finished parts I
generally get a 40% resin content, and autoclave parts under
20%. With a vacuum I can get very close to a pinhole free
surface, and with B staged fiber in an autoclave the parts are pinhole
free. You generally need to look at the back side of any layup to check
it's quality, this is where you see the usual overuse of resin, giving
a typical fluid lock as too much resin is impossible to bleed out, with
a fluid lock like that the fiber isn't really interlocking over each
other and you give up a lot of part stiffness, and add a lot more
weight that the vehicle has to carry around for life.
Prepreg always has either plastic on both sides, or plastic
and waxed paper, leave off one of those and your cutting table or
patterns are somewhat gooey...
I don't know what the viscosity of this resin is, but I heat
it slightly to get it thin enough, bit thick as it is cold.
I just use a spreader to spread the resin out, a
Bondo spreader to be exact. I do it in quarters of the table and
measure the poured amount to get it even.
Believe me, don't use
flat plastic to make your own carbon fiber prepreg, especially if you
very rich batch.
All types of fiber, be it 3K 2x2 carbon, 7544 fiberglass (my favourite
for moulds) or 6K uni carbon needs a different amount of resin. Carbon
first layers I use 600 grams of base resin, before adding hardener, 450
for layers after that. This is lean enough that the plastic will almost
fall off itself, and for the autoclave stuff it does fall off, and you
would swear that there is NO resin in that carbon.
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