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Aug 30, 2019, 08:27 PM
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Ward Hagaman's Avatar
Quote:
Originally Posted by plane_tech
Just out of curiosity, those that do skin the foam before, how much do you take off?
0.125"? 0.25"?
Both sides?
Just the skin...whatever it takes to bury the wire, like 1 mm.

Both sides for sure.
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Aug 30, 2019, 09:39 PM
Balsa dust maker
plane_tech's Avatar
Quote:
Originally Posted by Ward Hagaman
Just the skin...whatever it takes to bury the wire, like 1 mm.

Both sides for sure.
Thanks!
Aug 30, 2019, 11:38 PM
Entropy is happening!
Jim.Thompson's Avatar
I am using some XPS foam that was given to me as it was to used as insulation otherwise.
It also is of only fair quality with imperfections.
I am content to fill and sand the flaws and small holes with powder filler and Q cells mixed together.
After final sanding, I will brush with warmed up or thinned laminating epoxy resin to ensure adhesion with the later bagged skins.
Sep 04, 2019, 07:01 AM
Barney Fife, Vigilante
tom43004's Avatar
Skinning can depend on the manufacturer and even the batch of the foam. I generally skin about 2-3mm to be safe. The problem I've found with some manufacturers of foam is that the density of the foam is variable, starting with lowest density in the center of the panel and increasing toward each skin. The change in density isn't linear and after talking to a chemist at Owens Corning (one of the US manufacturers) I learned that temperature and humidity when the foam is made had a large role. So if you're looking for the lightest possible cores, cut from the middle of thicker sheets. Our sheets here in the US are commonly available in 2" thickness which is about 51mm. My templates (and now CNC programming) are set to 45mm block height.

Side note... Rohacell has a similar density variability in that the bottom of the block is more dense and the top is less.... more due to gravity in that case.
Sep 05, 2019, 06:17 AM
supreme being of leisure
ZAGNUT's Avatar
the pack of 20mm thick sheets i just bought has no "skin", each sheet has been cut from what must have been a big block. this is a 40ish psi pink XPS made in turkey under licence from owens corning. last time i bought the same brand was ~10 years ago and it did have "skins" back then.

oh, and the cuts are very nice and clean so no trash in the foam.
Sep 05, 2019, 10:07 AM
Detail Freak
target's Avatar
20mm is pretty thin, you must be making only DLG's or other narrow chord wings?
Sep 05, 2019, 01:01 PM
supreme being of leisure
ZAGNUT's Avatar
it's getting cut into 6x20mm strips for something non-RC
Sep 05, 2019, 02:15 PM
Entropy is happening!
Jim.Thompson's Avatar
I just layed up and bagged a wing yesterday that will go in the hot box today before removing the mylars.
It was cut from some very poor quality XPS; many small defects and some contamination particles which result in voids, dragging etc.
I was given a large quantity of it, so I'm giving it a go using my usual treatment consisting of filling with powder filler/Q cells mix and sanding (x3).
Then, I brushed on laminating resin diluted with alcohol about 10%. Allow to tack off, then bag.
This will add around 50 grams on an 1800mm span wing, but it is only a sloper, so no problem about that. If it was a thermal glider, this would be a consideration.
I will report back on the results tomorrow some time, after post curing.
Sep 06, 2019, 07:35 AM
Barney Fife, Vigilante
tom43004's Avatar
You'll be fine Jim. It's amazing what the bagging process can hide.
Sep 06, 2019, 02:48 PM
Entropy is happening!
Jim.Thompson's Avatar
Yes, I fully expect so Tom.
Wings in the hot box as I write @ 45 deg. C.
Mylars still on them.
I will start cooling down the box this afternoon and strip them tonight.


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