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Hello Dan
Post cure is used to increase the cross link density of a material. In other
words, molecules of resin and cross linker (this is also somewhat arbitrary
since which is the resin and which is the cross linker can be argued, however
the amine is typically called the cross linker). Also note that the cross
linker is often incorrectly called "catalyst". There are catalysts for
epoxies but this is not the same!
At any rate, these molecules of part "A" and "B: have to "find" each other in
order to harden. In a perfect stoichiometeric ratio, one molecule of resin
is matched to one molecule of cross linker. However not all molecules are
equally reactive. At room temperature, there is perhaps about 96-97% cross
link desity, i.e. there are still about 3-4% of the molecules that have not
found there counterparts. These are just some less reactive molecules that
need a little heat to finally cross link. Note, too that most epoxies shrink
about 3% when they cure (vs. about 9% for polyester/isopthalic/vinylesters)
based on volume. The reason you will see more print through with a part that
you finish with a nice gloss is the shrinkage from the final cross linking.
This happens because at some point when the part gets hot enough to post
cure, this is the last cure phase adds some more shrinkage to the part.
Hence the print through.
Anyway, post curing will add some extra strength and isn't a bad thing to do
in the least. It is also a time/temperature thing. You can get it near the
HDT (heat distortion temperature) of the resin and cure it up pretty quickly
or have it below there for a bit longer. If I rememebr correctly, the Jeffco
has an HDT of 190 degrees F (with the glass transition temperature about
15-20 degrees above that). So say you had an oven and took it to 170 degrees
for 4 hours should do it. Or 150 degrees for 6 hours. It will be something
like that. You shouldn't go above the HDT although I have it on good
authority that it doesn't really hurt the resin much as long as there is no
stress on the part at the time. However the HDT is exactly that. It's the
temperature at which the resin becomes a bit plastic and can move around
under stress. Here again, it's a bit arbitrary since actually, it's a curve
so you can see the beginning of the softening a bit before the HDT (whereas
the glass transition temperature is actually a definate fixed number).
Last thing is make sure the PARTS see the temperature required, not just the
air in the oven! A great example is some guys I know in Australia were
building a 78' all carbon uni prepreg boat, (Steinlager IV designed and built
for the around the world sailboat race). They built an ENORMOUS oven for the
job. When they built the hull, they did a beautiful job! Anyway after
cooking for X number of hours they popped it off and started work on it. So
a few days into it, they noticed that when they ground the carbon to mount
bulkheads and such, they didn't get dust, they had little balls of resin and
carbon rolling up. The layers inside were uncured! It turns out that while
the oven temperatures were adequate, they hadn't placed thermocouples on the
hull and against the plug so they didn't realise that the inside never got
hot enough to cure.
A hard lesson but that was one of the first prepreg built large boats ever
and the learning curve was steep and painful!
Anyway, the best thing is to check with Jeffco. They can give you the
time/temperature curves.
Good luck!
Dan Newland
Super ES #61
Alameda, California
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