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Compressed layup versus vacuum bagged layup

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Bill Strahan

Member
Joined
Oct 8, 2012
Messages
11
Location
Plano, Tx
I recently needed some firmed up foam for some modifications I was doing to my wingtips. Once the shaping was done, I would be ripping the foam out of the tip and glassing the inside, so I had no need for structural quality foam, I just needed it to stay stiff enough to let me shape it.

I have also been playing with peel ply, so used this as an experiment. I cut a piece of 1" pink insulation foam about 6" wide by 30" long to laminate a single layer of fiberglass cloth on each side. Here's how I did it: I wetted out the foam, waited a few minutes for it to soak up the epoxy, then wetted it again. Then I laid a sheet of glass cloth and wetted that out thoroughly. It was fully saturated, no light spots. Peel ply went over that, then I sandwiched the whole thing between several layers of paper towels, a sheet of plastic, and a smooth piece of wood. Put weights on top of that and left it to cure.

I was expecting the excess epoxy to be forced through the peel ply and get absorbed by the paper towel. I only had 50 pounds on top of it, so 50 pounds/180 square inches...not even a third pound per square inch. What I found surprised me. Parts of the paper towels were soaked in hardened epoxy, and the layup was starved of resin. I should have taken some photos, it shocked me. It was just really really dry compared to my normal peel ply/squeege layup. And it was weaker as well. The very light areas would peel off the foam very easily compared to the wetter areas.

None of it mattered for what I was doing. It provided the support I needed, I shaped my tips, ripped the foam out and chunked it.

But it has me curious. I've never done any vacuum bagging, but if I had done something similar with a vacuum bag I would think the results would be even more severe. What part of vacuum bagging am I not understanding, such that it does not create resin starved layups like I did in this experiment?
 
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