Vacuum Bagging explanation

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Forum topic by tangible posted 02-09-2014 01:15 AM 1106 views 0 times favorited 7 replies Add to Favorites Watch
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02-09-2014 01:15 AM

Topic tags/keywords: vacuum bag help question veneering clamp

So we have been having a pretty spirited debate this week in our shop about how a vacuum bag actually works. By spirited I mean people getting dang close to throwing down at work. I was wondering if anyone has an online resource I can use to show that its not actually the bag pulling down but the atmosphere outside the bag that does all the work. I have looked a bit but cant find anything that shows it clearly.

The misunderstanding over how the vacuum bag works has led some guys into thinking that there should be a gap around the perimeter of whatever is in the bag so that the bag stretches and therefore clamps down harder. It has led to quite a bit of rework on projects which has eaten up all the profit we would have made.

Any help would be really appreciated!


7 replies so far

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#1 posted 02-09-2014 01:19 AM

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#2 posted 02-09-2014 01:20 AM

Are you not using cauls?

It’s atmospheric pressure that does the clamping as I understand it.

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#3 posted 02-09-2014 05:08 AM

Read up on hammer veneering or have a look at this video.
When you veneer in this method with hot hide glue you are actually performing the same function as you are in vacuum bagging except …... without the bag.
As the hot glue is forced out from under the veneer it cools and gels. This seals the edges and prevents air from getting back under it. With no air under the veneer, the atmospheric pressure becomes the press. So it’s not the bag, it’s the atmosphere.
You can see the same effect when a shower curtain sticks to a wet wall. Air prevented from getting under …. atmosphere presses it. The bag is just another way of excluding the air.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

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Fred Hargis

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#4 posted 02-09-2014 12:30 PM

Take a look here. It is atmosphere pressure. Nor sure what to say about the “gap around the edges” you mentioned. Ypu do need a path for the air to evacuate from the bag, Joe Woodworker sells a mesh that is often used for this.

-- Our village hasn't lost it's idiot, he was elected to congress.

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Lee Barker

2170 posts in 2849 days

#5 posted 02-09-2014 04:59 PM

My vac bag setup uses a 3” square grid of kerfs cut 1/8” deep on a platen. The tube from the vacuum pump is hooked into one of those.

The bag, though quite thick, conforms amazingly to the object, which should have no sharp areas.

By removing the atmosphere, indeed, the air outside is the weight, or clamp.

Nature abhors a vacuum nearly as much as she abhors arguments at work.



-- " his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"

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#6 posted 02-09-2014 06:48 PM

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#7 posted 02-10-2014 04:51 AM

Thanks for the responses! I think I have enough here to take to work tomorrow.


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