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Need to add more to the controversy, first by admitting I MADE A
MISTAKE by drawing from memory, rather than checking notes. (Second mistake
this year. First time I thought I was mistaken....)
Recommended grit per the seminar I attended is 150 not the 220-320
that I recalled and wrote earlier. I have transcribed key points from my
lecture notes below. Keep in mind that the speaker works on repairs (not
fabrication) of composite panels on air carriers. Also keep in mind that our
kits are designed with massive safety margins on the bond lines and BID tape
reinforcements, much more than "professional" stuff, to compensate for
builder variability. The Boeing stuff is made to much tighter margins and
with less safety factor to gain the weight and structural properties of
composites. We do not have the quality control, environmental control,
autoclaves, shirt shelf life on our chemicals, etc., in our garages. So keep
in mind that the speaker is talking about repairing parts that are probably
designed much closer to their limits than our homebuilt stuff. That said,
here are the notes:
1) Start with an alkaline wash (soap) for hydrocarbon removal (Note that
Poly-Fiber makes some for use prior to painting composites).
2) Solvent wash with acetone. (MC is better for reasons cited earlier, not
the least of which is propensity for acetone vapor to burn easily and ruin
you whole day.) Commercial grades are not good enough. Use reagent grade
solvents.
3) Scuff with 150 grit aluminum carbide sand paper, and use only sharp sand
paper. Bond within 4 hours as the sanding creates a high energy surface
that is best for bonding. Recommended Merit Adhesives as a good supplier of
the sandpaper. (I personally like the 3M stuff, green and yellow.)
4) When fixing damaged area, remove damage plus 1/2 inch.
5) Taper beyond the cut out area 1/2 inch per ply of composite.
6) Sand an additional 1 inch past tapered area to permit overlap of full
thickness parent material.
7) Vacuum dust off using a clean brush. Do not use compressed air.
8) Solvent wash again.
9) Do a water bead test. Spray clean water using a hand spritzer and fine
spray. The water should not bead, but should wet surface. If water beads
within 10-15 seconds, redo washing and cleaning sequence until water test
produces desired result which shows that all free hydrocarbons are removed.
10) Warm air, 120F for 30 minutes to drive off all water and solvents.
11) Kevlar has its own special requirements. It will wick water, having
hollow fibers, so needs to have ends of fibers sealed. No raw unsanded edges
when finished or using water break test. (I guess you don't use it on Kevlar,
then ...)
12) Keep in mind that resins suck water from the air. Keep containers
closed. Moisture contamination of resin can cause a 50% drop in strength.
Work in an environment that is less than 50% relative humidity. (Easier said
than done for some of us.) However, epoxy is much less susceptible to
moisture than polyester or vinylesters.
Now some editorial comments.
1) Keep in mind that we work with larger safety margins, so we need not
comply with all these guidelines. But by working towards them, the quality
of our work will be higher.
2) On the surface area issue, all is not as it appears to the eyeball.
Remember that charcoal (cooked carbon, all hydrogen and oxygen driven out
leaving a carbon matrix) has a huge surface area, and is used to suck up
large quantities of hydrocarbon vapor including chemicals with quite low
boiling points.
So my thought is as follows. Too coarse on the sand paper and you damage the
underlying fiber (bad). The experts suggest 150 grit (at least this expert
did) as sufficiently coarse, but fine enough to create the desired increase
in surface area and roughness that can be wet by the epoxy. However, it
would seem that solvents can be captured in the fine scratches, and remain
even after bulk liquid solvent has evaporated. So it seems to me that if you
want to solvent wash after sanding (in addition to solvent wash before
sanding, to prevent contaminating the sandpaper with surface junk) you should
follow with a gentle heading with your heat gun to drive out the solvent
stuck to the surface. I do this for all core areas since I found that MC gets
into the core, and then the vapor pushes out after the lay-up as the material
warms (exotherm) and blows little bubbles after I worked diligently to get
rid of them during the lay up.
Final comment: don't panic if you have not followed these rigorous
procedures. Lots of Lancair's are flying using factory recommended
procedures followed by builders with widely varying patience and skill
levels. As far as I know, no structural failures have occurred in normal
flight conditions. This would seem to be silent testimony to the large
safety factors used in bonding practices in our aircraft and the satisfactory
nature of the procedures in the manuals. However, it doesn't mean you
shouldn't try to do better.
So go forth, be fruitful, and build airplanes!!
Fred
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LML homepage: http://www.olsusa.com/Users/Mkaye/maillist.html
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