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Tube-like positive mold in high density PU

Clouddancer

Well Known Member
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.

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Carving the positive PU mold

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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?

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That?s what the part will be used for?
 
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.
 
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.
 
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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.
 
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.
 
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
 
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
 
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
 
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!:rolleyes:

I thought about slitting the part. But as I see some other possible improvements I will redo the part again.
 
Additional methods

I use blue or pink common insulation foam to rough shape the part. I finish the more difficult transitions (compound curves etc.) and inevitable dings and low spots with plain old artist modeling clay. It sticks well to the rough foam and forms easily.

A quick coat of packing tape, and ready to go. If you use small enough pieces of packing tape in the tight areas, it goes on nice and smooth. I also agree clear can be difficult to see. Brown is a better choice. (I tried glassing directly over the clay since it is oil based. Epoxy soaked into the clay surface and made a mess)

If I want a more robust mold, coat the foam with a brush coat of epoxy, then use Bondo to finish. No shrink, sets extremely fast and sands easily. Packing tape or PVA for a release agent.
 
Andreas, did you miss the sheetrock mud idea? Might have a different name over there. Drywall compound, gypsum board finishing compound, spackling compound, etc.

Easy to fill holes, gaps, etc, and sands easily to a smooth finish. Then wax the mold and glass it. Residue will wash out with water.
 
rv7charlie, you are completly right. I forgot that method. I was always wondering what "drywall mud" is? It was mentioned several times in other threads. Now, I finally understood and got the translation right (in german: "Gipsspachtel"). The good thing: l even have it already at home. I will definitiv add it to my list and give it a try.
 
Making the part out of paraffin wax is called "Lost wax methood". Very common method of manufacturing precision parts and jewelry. Carve whatever you want in warm wax, fiberglass it, plaster it to make a mold, depending what you want to make, melt out the wax and away you go.

RV6A CGLRV
 
If it doesn't have corners to go around a latex condom works really well as a release barrier. It can be stretched and nothing sticks to it. (let the gutter comments begin :rolleyes:)
I used to do that when I was making ducts for RC jets.
 
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Andreas, I may be an obvious comment, but if you want to melt the plug away with acetone the filler must be soluble in acetone! Perhaps use plenty of mould release and/or packing tape and then slit the part in half. If the central part is parallel would it be better/easier to use 2 wraps of glass cloth rather than tape?
Pete
 
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