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  #1  
Old 06-23-2009, 11:09 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,345
Default Tip: Small Fiberglass Parts

Sometimes small fiberglass parts have complex shapes. The shape may not allow neat wrapping in gloss packing tape in order to facilitate mold release. You'll need to make a plug with a firm, slick finish of its own. So how do you do it quickly?

Start with a dense, small cell foam. You can order foam from the aircraft supply of course. I just hit the local big box home supply and buy a sheet of dense pink or blue wall insulation board, usually available 2" thick. Not expensive, and a whole lot more foam for your money. Cut a few sections bigger than your intended part and laminate them together with dry micro. That gives you a solid foam block of any desired size.

How to shape the plug? I use an ordinary crosscut hand saw for the big cuts, then a hacksaw blade for the finer cuts. From there use 80 grit paper to rough it out, then 180 to detail the shape. If there's a critical outside dimension, make the plug about a 1/16" small.

When you have the shape nailed, mix some epoxy. Pour a little off in another cup and mix in some micro. Use a little plastic squeegee to wipe some micro mix into any surface flaw. Now, without waiting, paint the foam surface with neat epoxy. The idea is to form a thin epoxy shell and seal the foam.

Shaped and sealed plugs, unsanded:



When cured, lightly sand the epoxy shell until smooth. Little defects don't matter much. They will be duplicated on the inside of the finished part, but a little sanding will remove them later. Do NOT sand through the epoxy skin.

Now wax the surface, two coats, fully dry between coats, no buffing. I've used the same old can of carnuba wax for ages. When dry, spray or brush over the wax with some PVA mold release.

If the part needs a mounting flange, cover a sheet of aluminum or wood with packing tape. Screw the foam to the sheet with a few coarse deck screws or similar.

Do your layups. Three or four plies of 8.9 oz 8-harness is typical.



When cured, dig the foam out. The epoxy shell-wax-PVA combination will peel cleanly and leave a finished surface. A little touchup sanding will remove any raised defect. If you accidentally allowed a few air bubbles between the plug and the layup, fill the indentations with micro and sand the surface flat later.

A finished part after 10 minutes of trimming and sanding, and a plug ready for layup:



BTW, if this doesn't seem to fit your definition of "quick", consider how the work is scheduled. Cut and laminate a few foam blocks, ten minutes. Shape the plug and paint with epoxy, an hour. Sand, wax, PVA, and layup, maybe two hours, and some of that is waiting for the wax and PVA to dry. Remove the foam and trim, another half hour. Between these tasks you work on something else.

Have fun!
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RV-8 SS
Barrett IO-390

Last edited by DanH : 07-17-2018 at 07:09 AM.
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  #2  
Old 06-23-2009, 12:10 PM
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erich weaver erich weaver is offline
 
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Location: santa barbara, CA
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Default

Very cool - thanks for the tips.

You describe the basic foam plug shaping methods - various saws and sandpaper - but your plugs look like they were shaped by Michaelangelo and I think mine would come out more like a Manny Moe and Jack piece. I assume you used something more than freehand shaping in at least some critical areas - for e.g., where you need a circular cross section shape? Perhaps some sandpaper glued to the inside of some tubing would help in areas like this?

erich
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  #3  
Old 06-23-2009, 12:29 PM
jarvis jarvis is offline
 
Join Date: Jul 2005
Location: lexington, KY
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Default Dan...

You sir, are an artist!!! Thank you so very much for the detailed write-ups and pix. Great aids for a glass-challenged individual like myself.
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  #4  
Old 06-23-2009, 02:53 PM
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Bubblehead Bubblehead is offline
 
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Location: Keller, TX
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Default

These look an awfull lot like the urethane parts you did about a month ago! Are these different parts, or did something change your mind about using urethane?

This fall I want to tackle the induction system. Instead of the "FAB" unit my -8 uses scat tubing but when I put the lower cowling on it has to compress the scat just a little to get into place. I rather not have the cowling under load like that so I was planning to replace the scat with a urethane duct.
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  #5  
Old 06-23-2009, 02:56 PM
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DanH DanH is offline
 
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I'm the least artistic guy you know.

Really, to do the shapes you just make plane cuts (draw them right on the block) and then round off the corners. If you screw it up, big deal. Throw away $1 worth of foam and do better on the next one.

Quote:
<<I assume you used something more than freehand shaping in at least some critical areas - for e.g., where you need a circular cross section shape? Perhaps some sandpaper glued to the inside of some tubing would help in areas like this?
I just traced a circle of the correct diameter on the appropriate plane cut and sanded to the line, but do whatever makes you comfortable. More than one way to skin a cat.

The real focus is how to easily fabricate a one-shot throwaway plug which releases easily from the finished part and leaves a near-finished surface.
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Last edited by DanH : 07-17-2018 at 07:09 AM.
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  #6  
Old 06-23-2009, 03:08 PM
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DanH DanH is offline
 
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John,
The urethane/glass ducts are much larger. These are just some little cooler ducts which will couple with SCEET.

I'm experimenting with the urethane duct idea. This weekend I laid up a new set using a harder Shore 70 industrial urethane and a single ply of 9 oz 14x16 coarse glass. Popped them off the plugs this AM. They're quite a lot more flexible than the 4 ply ducts. More in the duct thread later.

BTW, I'm a little hesitant to recommend the urethane ducts for an application where they might see a lot of exposure to fuel. Samples submerged in fuel did ok short term (a week), but I dunno about years. Apparently nobody does; the manufacturer was no help ("Sorry, we've not tested for that").
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  #7  
Old 06-23-2009, 03:56 PM
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RV7Guy RV7Guy is offline
 
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Location: Chandler, AZ
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Default Great Stuff

Great stuff Dan. Guys, glass work is quite easy. It is also very rewarding. On this project another simple way to get rid of the foam is to dissolve it with Acetone. Quick and easy then peal away the tape from earlier.

To add to Dan's great "How to," you can start very simple with rudder cable fairing covers. I won't hijack Dan's deal but will so a write up on those in a separate topic.
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  #8  
Old 06-24-2009, 08:11 AM
Gary 40274 Gary 40274 is offline
 
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Location: Conyers GA
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Default Additional thoughts

Not all foams and epoxys are compatable. Test what you have on a sample. I know Vinyl esters and styrofoam are not compatable. Safetypoxy and styrofoam are, West and styrofoam are, I can't remember about polyester and styrofoam. Test yours first.

If it is incompatable, you are still in luck, Coat your plug with sheetrock mud. Sand like Dan says and coat again with wax, it will take a couple more coats. and PVA. Otherwise go on like He says.

Gary Specketer
Dragonfly, Glasair III. and asisting in bunch of others
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  #9  
Old 06-24-2009, 10:23 AM
shorbird shorbird is offline
 
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Default

What is the size of the plenum inlet 4" or 3"? It also looks as if it may be for the larger six cylinder coolers, is that correct?

Thanks
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  #10  
Old 06-26-2009, 08:06 PM
MJarreau MJarreau is offline
 
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Location: LA (Lower Alabama)
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Default Amazing

Dan,

Your work is amazing. Actually, it's inspirational. I saw your shop about 10 months ago. Guys, all of his work is this impressive.

Thanks for the ideas!
Michael
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