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Old 18-07-2017, 21:08   #1
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Vacuum bag - first time - dry spot

This weekend, I finished putting together my vacuum bag system (vacuum press from joewoodworker.com / veneersupplies.com), and tested with a couple of small panels (6" x 6" x 4 layers), one wet layup and one resin infusion. Those went well ( also use butyl tape on those)

My first attempt at vacuum bagging several layers of fiberglass had a small dry spot. This job was resin infusion.

What did I do wrong? I couldn't see the dry spot until after all was finished, I took everything apart and flipped the panel upside down. It was not visible during layup.

Do I need to put some kind of flow medium between the layup table (UHMW board) and the layers of fiberglass?

Maybe I had too much vacuum and the dry area got pinched off by the pressure?

I ended up with a dry spot near the suction port. Here is a rough diagram of the issue (attached picture)

I slowly let the resin flow thorough to the vacuum bag connector and it was starting to suck up into the tube (and into the resin can)

Dimension of laid-up fiberglass panel is about 6" x 18" and the 12 layers ended up being about 3/8" thick.

I layered 1708, mat, 10 oz cloth, mat, 1708, mat, 10 oz cloth, etc... for about 12 or 15 layers

I also had a bunch of tiny air leaks. I expected the leaks because i didn't have any butyl tape and used clear packing tape. That was a mistake.

Even with the leaks, the vacuum pump kept up, vacuum ranged from 21" to 25" (that is the swing of my vacuum valve the cycles the pump.

When flowing resin into dry material, what vacuum value should be used? 10", 15", 25"?

Thanks for the tips.
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Old 18-07-2017, 23:15   #2
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Re: Vacuum bag - first time - dry spot

The leaks may have been enough to prevent the pump from pulling resin throughout the whole layup, as part of it's pumping power was being used up to handle them.
You might try posting over on www.boatdesign.net/forums as there's a fair bit of knowledge on construction over there. But let us know what you find out
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Old 19-07-2017, 05:55   #3
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Re: Vacuum bag - first time - dry spot

Thanks uncivilized, I'll ask over there.
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Old 19-07-2017, 05:57   #4
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Re: Vacuum bag - first time - dry spot

De nada
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Old 19-07-2017, 10:41   #5
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Re: Vacuum bag - first time - dry spot

Looks like the resin took the path of least resistance. These issues have been worked out many time over and the advice already given will get you the answer you seek.
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Old 21-07-2017, 05:23   #6
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Re: Vacuum bag - first time - dry spot

Thanks,

Looks like I've been setting up inefficient flow pattern (in one end, out the other.

I will run inlet around the part, with suction in center and see how that works out.

I did three tests using the same failed method and can reproduce the failure, so it is definitely the in-one-end-out-the-other layout that is not working.

The forum suggested by uncivilized has my thread in it. It also have other apparently useful info and helpful folks.

https://www.boatdesign.net/threads/f...ry-spot.58268/
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