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10-23-2017, 08:01 AM
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Join Date: Aug 2009
Location: Switzerland
Posts: 178
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Tube-like positive mold in high density PU
I?m trying to fabricate a tube-like glass fiber duct between a NACA inlet and an fresh air vent. I thought the most straight forward method would be to use a positive mold of high density PU, wrap it with glass fiber and after curing, dissolve the PU with acetone.

Carving the positive PU mold

Vacuum bagging the mold with the glass lay-up
The question that rose was how to smoothen the surface of the PU foam and to fill the pores and cavities? A first attempt was done with RTV silicon. After dissolving the PU foam with acetone, this silicone layer showed to be very hard to remove from the glass surface, i.e. only with mechanical scraping, which is not possible within the curved tube. Any ideas what to use that can be removed without scraping?

That?s what the part will be used for?
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Andreas Uebersax
RV-4 SN2416 in slow progress
Switzerland
VAF donation 2019/12/13
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10-23-2017, 08:06 AM
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Join Date: Jul 2006
Location: Stone Mountain, Georgia
Posts: 483
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Packaging tape?
Would it work the next time to cover the foam with clear packaging tape before applying the glass? That should give you a easy release once the glass is cured.
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Steve Ashby
Stone Mountain, GA
N184RW (reserved)
RV-8A
YIO-360 180 hp Lycoming
Hartzell Blended Airfoil Prop
Engine Hung
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10-23-2017, 08:46 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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It's a bit of a pain in the backside, but sheet rock mud can be used to fill open cell foam, and then sanded smooth. Then use conventional waxing methods to prevent the 'glass from sticking.
I used vinyl tape (electrical tape) around one of my diffuser molds. Problem with tape is that every wrap leaves a ridge, which is a *royal* pain in the backside to get rid of.
Oh, I should have added: I've also used clear packing tape. As strange as it may sound, when using clear tape it was surprisingly hard to see where I'd covered the mold, and where I'd missed spots. If I used packing tape again, I'd try the brown version; much easier to see.
Last edited by rv7charlie : 10-23-2017 at 11:48 AM.
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10-23-2017, 08:49 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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BTW, for something like that, have you seen the 'glass/RTV (instead of epoxy) method? Dan Horton has documented the method somewhere on this forum.
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10-23-2017, 10:57 AM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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Nice looking part!! Lots of things that can be used, but something that can be waxed or sprayed/brushed with PVA is best. Plain water based paint after filling the pinholes. That could be done with a squeegee a couple of times, light sanding or use some neat epoxy resin. Then wax and/or PVA. The PVA can be washed off afterward with warm water sprayed through the part.
Light coatings of plaster of paris works too, to smooth/fill any pinholes. the pink insulation board sands pretty smooth with fine sandpaper.
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Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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10-23-2017, 01:14 PM
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Join Date: Jul 2017
Location: Napa Valley
Posts: 28
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Tube clean out
You could slit the part into two along the long axis, scrape the insides smooth, then glass the halves back together. Maybe faster than making an entire new one.
Just a thought.....I try to have at least one a day.
jordan
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10-23-2017, 05:59 PM
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Join Date: Oct 2006
Location: torrance, ca
Posts: 650
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How about using today's technology and grow one? You can download SolidWorks from the EAA website for free, make an STL file from that and then have it 3-d printed via SLS in Nylon-12. This is the process that Airbus uses for the ventilation ducts that feed the eyeball vents above each passenger's head.
Heinrich
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10-23-2017, 06:21 PM
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Join Date: Jun 2006
Location: Castaic, CA
Posts: 549
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How about something like parafin wax that could be melted out with a hair dryer?
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Mike Sumner
Castaic, Ca
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10-24-2017, 03:00 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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Mike, that's a good idea. The wax ought to be reusable, and I'd expect that the epoxy would release from it well.
Definitely worth a test run any way.
Dave
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10-26-2017, 01:19 AM
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Join Date: Aug 2009
Location: Switzerland
Posts: 178
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Thank you very much for all the ideas!
I think, I will do some test pieces with packing tape (just to see if I can get the curvatures) and give the parafin wax and the PVA a try.
My friend suggested to use dissolved sugar in water, that will dry. As weird as it sounds, we did a test piece because we were curious. It worked out quite acceptable and it is water soluble!
I thought about slitting the part. But as I see some other possible improvements I will redo the part again.
__________________
Andreas Uebersax
RV-4 SN2416 in slow progress
Switzerland
VAF donation 2019/12/13
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