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  #1  
Old 10-23-2017, 08:01 AM
Clouddancer Clouddancer is offline
 
Join Date: Aug 2009
Location: Switzerland
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Default 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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  #2  
Old 10-23-2017, 08:06 AM
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Steve Ashby Steve Ashby is offline
 
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Default 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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  #3  
Old 10-23-2017, 08:46 AM
rv7charlie rv7charlie is offline
 
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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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Old 10-23-2017, 08:49 AM
rv7charlie rv7charlie is offline
 
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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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  #5  
Old 10-23-2017, 10:57 AM
BillL BillL is offline
 
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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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  #6  
Old 10-23-2017, 01:14 PM
Pixair Pixair is offline
 
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Default 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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  #7  
Old 10-23-2017, 05:59 PM
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hgerhardt hgerhardt is offline
 
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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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Old 10-23-2017, 06:21 PM
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G-force G-force is offline
 
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How about something like parafin wax that could be melted out with a hair dryer?
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Old 10-24-2017, 03:00 PM
David Paule David Paule is offline
 
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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  
Old 10-26-2017, 01:19 AM
Clouddancer Clouddancer is offline
 
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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.
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