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Old Apr 13, 2015, 07:45 AM
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Disposable F/G Molds?

Has anyone tried one-time use molds, perhaps like this process:

1. CNC mill a cavity in a block of rigid foam;

2. Coat the inside with something sandable... maybe plaster, sanded smooth on the "inside" before laying 'glass, or a layer of gelcoat to sand after pulling the hull;

3. Lay the 'glass inside this;

4. Carve and/or dissolve away the foam;

5. Finish sand the hull.



I'm thinking the sanding could be easier here than what's needed if you lay mat glass over a male foam plug...
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Old Apr 13, 2015, 07:57 AM
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I have known guy's that have made a quick, lightweight, fiberglass mold, just good enough, to make 1-3 parts. But that is still using, the conventional methods.
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Old Apr 13, 2015, 08:25 AM
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I could never sand inside a mold very well. Gorilla hands...

But I have made a wooden plug, vacuumed plastic over it, and then laid up inside the plastic.

Andy
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Old Apr 13, 2015, 08:29 AM
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Can't help you very much here Pat, as I learned to do fiberglass the old-fashioned way from Glenn Staubitz.

That involved making a reusable mold over a cleanly finished master.

You never know if you'll use it for more than just a few copies anyway---.
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Old Apr 13, 2015, 08:32 AM
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working in reverse is never easy, you don't really know for sure if things are correct until things are removed from the mold.. some of the surfboard glass out now is crazy supple.. I've got some stuff that will just drop into a complex mold.. with careful work it would work just as great on foam plug, not much post glassing sanding and filling needed..
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Old Apr 13, 2015, 08:38 AM
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Sanding inside: Yep, sounds debatable.

I'd lean towards gelcoating the CNC'd cavity, and sanding after pulling the part- gelcoat sands so easily!

Another variation: If you have rough glass on a plug (or even a glassed-over wood hull), can you apply external coats of gelcoat, and sand that instead of sanding glass?
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Old Apr 13, 2015, 08:51 AM
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top-coating with gel-coat is done all the time in 1/1 scale boat repairs, plus you can always wet-sand (no dust no fuss) most of the motorcycle glasswork I do is never dry sanded.. faster and cleaner..
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Old Apr 13, 2015, 08:57 AM
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Cool.
The advantage of working inside a cavity is that the part's outer skin will be close to correct size.

Of course, it will only be as true as what my ball end mill produces... good for most surfaces, not so good for crisp crease lines and small details... even a 1/4" ball will leave a 1/8" radius on everything.
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Old Apr 13, 2015, 10:22 AM
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Preval spray kits (fillable/disposable spray bottle) can be used to spray gel coat with good results so you don't have to worry about using the stuff in a good but hard to clean out spray gun..
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Old Apr 13, 2015, 10:40 AM
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The exact opposite is done, quite often.
Foam is CNC'd to the shape of the hull, then fiberglassed, and then the foam carved or melted out.
I have also seen the hulls CNC'd in two inch sections from bow to stern.
The sections then assembled, and the assembled hull fiberglassed for a one off hull.
If hull accuracy is that much of an issue, the CNC can be programmed to carve at a
negative offset to undersize the foam blank.

The blank could then still be used to create a quick negative mold as Charlie points out.

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Old Apr 13, 2015, 10:52 AM
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But but... the SANDING! I'm still thinking that the gelcoat layer applied first in a cavity mold would be easier to finish...
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Old Apr 13, 2015, 11:17 AM
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Since you have both access and the ability to use a CNC, I would mill it into Corian and lay up directly into the resulting mold. It is possible to polish the Corian so that you get an excellent surface and the finish of your part will only be as good as that of the mold. The glider builders often create a finished painted surface on the part by painting in the mold before doing the lay up. This is a whole subject in itself.

You can often purchase Corian sink cut outs at counter top fabricators very inexpensively or you can make your own DIY Corian for molds that would be larger than the sink cut outs. See http://www.rcgroups.com/forums/showthread.php?t=2165394

If you can mill a plug rather than a mold, then using UltraCal 30 (a cement plaster - much harder than Plaster of Paris and it does not shrink as it cures) either in commercial form from a ceramics shop or a DIY substitute http://www.rcgroups.com/forums/showthread.php?t=2254757 makes very quick and cheap molds. These are good for up to 10 pulls if treated kindly.

I have poured UltraCal over a foam plug that was surfaced with wall board joint compound and then sanded and sealed, but unless your mold was very simple - as in few curves - it will be difficult to keep the original lines without sanding hollows.
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Old Apr 13, 2015, 11:26 AM
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The biggest issue I see with the CNC process that Pat is proposing is that you limit the head movement milling out the inside to get the out hull shape. most of the cnc hulls I have seen need the multi spinning head to rotate around the outer (or male) plug to get the surface smooth and correct. I think if it was a good process, you would see more manufacturing if fiberglass that way. I think also the cost would stop most from wanting to even experiment with that unless they can sell multiples to make up the cost of doing one in such a way. Just saying.
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Old Apr 13, 2015, 11:44 AM
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Pat,

EASY, is just giving the master to your fiberglass guy, and telling him to call you when he has the first hull pulled!!
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Old Apr 13, 2015, 12:51 PM
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The mold could be CNC'd in two halves.
The keys could be cut directly into the halves and be ready to assemble off the table.
That would solve the narrow aspects of machining space.
And you could go right to sealing and finish.

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