Out of curiosity, I just browsed a bunch of online data sheets and
tech papers from various mfr's of epoxy resins. The first obvious
thing is that the postcuring effect is a function of the formulation,
with greatly varying effects, among formulations, of postcure and
recommended time/temp. So really, you have to refer specifics to the
resin mfr.
I can't see any advantage re filler either way. I found statements
that insignificant shrinkage occurs in postcuring at temps the foam
can stand, and the effect of postcure on filler is moot since it's not
structural and contains little resin. Also, they say (except at
high-temp initial cure) that no chemical is released, except for some
possible moisture, except you're not supposed to cure in high-humidity
conditions in the first place. SP Systems, for example, states that
max strength is NOT achieved without a 4-hour, 80C postcure, but foam
may not stand that. They only otherwise document the effect of
overnight cure + 16 hours at 50C, but hint that just 28 days at room
temp is similar, with glass transition temps either way well above
what you need for white paint on a sunny day.
What I'm getting at is that the strength issue is likely moot on the
flying surfaces, namely the maybe 10% improvement in postcure strength
isn't an airworthiness issue. Of greater concern is the softening in
the sun's heat. But another paper describes exactly what happens at
the molecular level, and only when the transition temp is exceeded
does the molecular structure magically change (thermo-setting,
cross-linked polymer--epoxy, verses common plastics which soften
linearly with temp). My attorney (a 6-ft. rabbit named Harvey) is
here kicking me in the shins, but with proper mixing of the resin and
safe margins per the data sheet, postcure is neither critical nor done
incorrectly, below a temp harmful to the foam. Course, I still did
the postcuring, right after layup to play safe, especially since I
have Mk 1 wings.
Specific to straightening trailing edges in postcure, it will neither
work per above citations, nor IME did it when I tried it at 50C. If
you do it the following morning before initial cure is complete,
yes...and I found that useful in straightening small parts, like gear
leg fairings, for the complete cure.
Regards,
Fred F., A063
> Hello all,
> This is an add-on to the recent questions about applying filler to
> the control surfaces. I would like to hear opinions on whether
> the filler should be applied before or after oven curing?
>
> I have also heard that it is relatively easy to straighten trailing
> edges with heat and clamped straight edges, and that it's possible
> to put in, or take out, some minor twist in the ailerons under
> heat. Is this best done during the oven curing or with a heat gun
> localized on one area (I'm nervous about melting the blue foam under
> the bid).
> Thanks for any input.
> Dan Geldermann
> A-139
>
|