vacuum bag

Can the weight of water be used to pull a vacuum?

If I place a small mold inside a vacuum bag at waist height and connect it to an elevated water reservoir which is in turn connected to a tube, will releasing water into the tube evacuate the bag? Any physics to support your answer(s)?

Seems to me that this isn't going to work. In order to have a complete vacuum in the mold bag (top bag) with no air bubbles then you would have to have a completely closed system. By this I mean that there would have to be no air in the reservoir (bottom bag). If there is air in the bottom bag or if you were looking to do this over a bucket or something you would then have air moving up into the top bag to replace the void of the water. In a closed system I don't think you are going to find gravity, your only force, creating enough of a "vacuum" to do what you are looking for. Please someone correct me if I'm wrong, I have no experience in Vacuum Bagging, just throwing out what I think is possible.

No reason that this wouldn't work except for practicality. You'll have to displace more water than you have air to remove for it to work. It's basic fluid dynamics.

The difficulty as I see it would be the volume of air to be removed relative to the amount of water you are able to "drop". i.e. once you've pulled all the vacuum that you can with the water, it may not be enough to remove the air from the carbon/resin and produce a vacuum of sufficient quantity if at all.

In order to pull 1 atmosphere of vacuum with water you need 33 feet. Not very practical. Not sure I understand your description of the system, but as mentioned it must be closed.

There is that too, lol

It isn't that I doubt you, but how do you know that? Would a 33' coil work?

I'm considering a very small volume and considering using Luer lock, standard I.V. equipment. In fact, I was thinking of an i.v. bag on a pole.

Pete, just buy a cheap vaccum pump at Harbor Freight.
http://www.harborfreight.com/catalogsearch/result?q=vaccum+pump

I'm not buying any more gadgets to get used once, then gather dust for decades.

Besides, I'm obsessed with the question...

Well, on the surface, absolute vacuum will only generate atmospheric pressure on the item in the bag...which equates to about 14.5 psi. You can try to pull "more" vacuum on it but once you have removed all of the air, there is nothing else to pull. So, the vacuum bag can only match the differential pressure between the outside and the inside of the bag...i.e. 14.5psi atmospheric and 0psi inside the bag = 14.5psi total...and you can't generate less than 0 inside the bag because there's nothing left to draw out.

That said, you CAN increase the external pressure on the bag to increase the differential and the pressure on the item inside the bag. This is the principle of an autoclave. You first draw a vacuum on the bag, insert the bag into a pressure vessel, pump up the pressure inside the vessel and put a lot more pressure on the part. Hall Spars builds masts this way (and the Autoclaves normally employ heat as well).

You ~could~ also take your vacuum bag and lower it into water to create more differential in the pressure in the same way the autoclave works.

Pete,
You could always go this route.
www.spacebag.com

That is a basic SCUBA concept, every 33' you go down the pressure increases by 1atm, so no a coil will not work to create that additional atm you would need a column of water. To elaborate on Jake's idea, the idea of a one-way valve on the bag and attaching the bag to an anchor in 33' of water comes to mind (try this on something replaceable first) with the valve pointing up so you don't get an air bubble.

Is there a standard? X amount of resin per square meter of cloth, for instance?

IF .5 atmospheres is all that is required for most applications, I suspect vacuum bagging operations are designed to handle relatively large amounts of air in commercial operations.

Ahh, but now I see that you just want to generate the vacuum with a column of water.

The coil won't work - you need 33.5 vertical feet in a column of water to match 14.5 psi. It's the only way to get the gravity to add up.

Umm, I'm probably using the wrong approach. I need to know how much resin my piece needs for maximum strength, then I can just roll it out by hand.

Straight out of Felder and Rousseau, "Elementary Principle of Chemical Process"

1atm = 33.9 ft H20 at 4 deg C.

Jake, I disagree. If the pressure (vacuum) gauge reads lower (less) than 0 psia then you are exerting more than 1 atm of pressure on the item in the bag. The benefit of the autoclave is that you can reach the high activation temperatures required for prepeg AND put it under pressure.

"Straight out of Felder and Rousseau, "Elementary Principle of Chemical Process"

thx

I think we're saying the same thing - you can't have a bag out in the open space (at atmospheric pressure), at full vacuum, and have more than one atmosphere on it. You can't achieve more vacuum than complete vacuum. You CAN increase the pressure outside the bag in order to increase the pressure inside the bag by using an autoclave to get better than atmospheric pressure.

OK smarty pants......why do beer cans explode after you shake them? What is it about Mentos that makes coke bottles blow up? Those of us on the wrong side of the bell curve need real world knowledge!

If you have a column of water descending 33 feet creating a vacuum space above it, then the water at the surface of that column will, in essence, boil. The vacuum space will have a considerable amount of water vapor in that volume. Will the presence of that "dampness" have an adverse effect on the curing process?

Ahhhh...great point! Because the water will boil/evaporate, you will never achieve a consistent maximum vacuum that way - the water vapor will continue to fill the vacuum space and reduce it somewhat.

David gets the prize on this one, I didn't even think about that, but he is absolutly right.

1) They don't. Until they are opened. Fluid dynamics and answer #2
2) Nucleation sites

Unless the interface area is protected by a non-volatile solid/liquid

Pete,

Must you complicate everything?
The whole point is to apply "pressure" so you consolodate the laminate and remove excess resin. Vacuum is a conveinient way to get about 14lb/in. You can get 80-90 out of cheep home compressor if you can manage to build a vessel to hold it. A pressure chamber is a lot more complicated than tacky tape and plastic film, but it is possible to achieve the same net effect.
Rollers and sqeegees work to some extent with open molding, but if there any loft to your fiber pack it is extremely difficult to get a good fiber to resin ratio.

Yes, but you just end up with a really crappy 1 stroke piston pump

Look at a Manometer. Fill a tube taller than 33 ft with water. Upend the tube of water into a container of water. The column of water will drop until it is supported by the air pressure (~33 ft). The pressure in the void in the top of the tube will be the vapor pressure of water (~0.5 psi). If you use an IV bag and a flexible tube, start with the tube full of water, the open end in the container of water and then just hoist the bag up a tree.

Lets assume you are going to use this thing to pull a vacuum. Assume 1 ft of that tube has the same volume as what you are trying to pull a vacuum on. If you make your tube 43 ft high the best vacuum you can pull will be 14.7/11 +0.5(Vapor pressure of water). There are loses and other factors but I don't remember what they are.

I actually saw someone try this once as a college project. They were able to pull a couple of psi of vacuum. They then showed a hand powered lab vacuum pump beat the hell out it.

A better system is a vacuum oil changer. I have a metal one leftover from my big boat days but it works like this

http://www.liquivac.com/buyonline

I can get about a negative 7-8 psi per the gauge, if I work really hard and when I finish with, I can change the oil in my lawnmower

the nice thing about the vacuum, though, is that it tends to expand, burst, and evacuate bubbles in the resin of the layup. Applying just external pressure would compress those bubbles and they would remain in place.

True, but the nice thing about large differentials in pressure (this includes the difference between atmospheric pressure and the applied vacuum, or atmospheric and the applied pressure) is that as long as the material is flowing, the air will rush to the flow front. In production comression molding uses this to get void free laminates. Puddle the resin in the center and as you apply pressure the resin flows out and takes the air with it. Air voids (short circuits)under vacuum will not "move" either without flow.

So which is the superior method for home building dagger boards, rudders and other relatively "flat" panels, vacuum or pressure?

For pressure I'm assuming you could put the part between two flat plates and pile weight on it. No? Yes?

Inquiring minds...

And I got this in the "mail" this morning. Anyone know anything about it? http://www.jamestowndistributors.com/userportal/main.do?&campaign=email060512

you could park a car on it...but, lets say you had a mild level of vacuum at 20 inches of mercury (-9.8psi) on a daggerboard that measures 60 inches by 10 inches, you will need to park 5,880 lbs on it and somehow get that pressure evenly distributed on the layup.

Ah, enlightenment...

Dude, just go with explosive bonding and be done with it. You could have an aluminum/nobium boards in no time...

...there's one in every crowd.

http://www.youtube.com/watch?v=QDNX...ad=5295696183&kw=explosive%20bonding

My Blade could handle that, no pro'lem

Pete, if you don't want noise and do not require lots of air evacuation, try a water venturi vacumn pump, you probably remember them in the school lab, cheap simple, use a surprising little amount of water and can pull a surprising amount of vacumn. The thing they cannot do is evacuate huge amounts of air quickly like a mechanical pump. We used to use them for calibrating aircraft instruments and 50000ft was pretty achievable in a round glass goldfish bowl with a 10mm ali plate lid. A 10K dollar rig for about $ 100.00

http://www.youtube.com/watch?v=VodfQcrXpxc&feature=related

I've no experience with that but I've quite a bit of experience with this: http://www.youtube.com/watch?v=SDprRTHw4I0

Use a stethoscope to find small leaks.

To do it right, I guess you really need the proper equipment.

You could always rent a compressor if you didnt want to buy one.

www.unitedrentals.com

Plus this kit, (which you need anyway) and you have a tried and true system for making composite parts.

http://www.jamestowndistributors.co...amilyName=WEST+System+Vacuum+Bagging+Kit

Pete what are you trying to build?

Pete,
You can make a vacuum pump from on an compressor from a cars air conditioner and an electric motor that delivers around 500 revs per minute. A couple of pulleys and a belt can get that for you.
Trust me you will use it a lot more than once.
I had a friend who wanted to laminate some timber to make a gun stock. I'm not into guns at all but offered him some advice. I told him to work out the area of the wood and the pressure he could get under vacuum (around 14 psi) and just get bricks and stack them on top of his wood. That way he would get good pressure without a vacuum. On the way home I did some mental calculations and worked out the stack of bricks would need to be nearly 28feet high (8.5m) just to get the pressure he could get with a simple vacuum.

As a side note when vacuuming my F85SR floats I worked out I would need 53 tons of weight evenly distributed to apply the same amount of pressure as a simple vacuum pump. I am now working with 3 different vacuum pumps depending on the job I'm working on with a Milking machine in the wings to turn into a vacuum pump should I get the time. Even with the simple process of glueing two bits of ply together. Do it under a vacuum and you won't be able to see what is your glue joint and what is the production ply glue joint.

Think about it a little and see how it falls.

Regards,
Phill

There are a lot of economical vacuum pumps available now for around $100 US that are good for building composites (but maybe not for HVAC work).

Links? Ebay?

try:
http://www.ebay.com/sch/i.html?_trksid=p5197.m570.l1313&_nkw=gast+vacuum+pump&_sacat=0

Gast is a common industrial brand in North America. There are others.

I bought a oil less rotary vane pump with tanks, manifold, gauge, and pressure controller for ~$100 10 years ago. I see several similar units in the link.

Suggestion, use a filter. Many of the small units have a short life without them. I use a jar with paper towel media backed up with an inline water filter.

How about venturie vacum pumps?

Decent, but they really consume a lot of compressed air. They are good when you need to remove a lot of air quickly or have a very leaky setup. I have a couple of them and used them in conjunction with my mechanical vane pump when I reinforced a failing block basement wall with uni directional carbon.

I use this one:
http://www.veneersupplies.com/products/Vacuum-Filter-5-Micron-High-Flow.html

This pump is plenty for doing composite work:
http://www.harborfreight.com/25-cfm-vacuum-pump-98076.html

I have one of those walls myself. A crack is forming and it is beginning to buckle ever so slightly inwards.

How did you prepare the surface and apply the carbon?

http://www.teamseacats.com/2008/06/19/carbon-and-concrete/

I have used a 25 gallon Craftsman shop vac on numerous occasions, running it for up to 6 hrs straight without it burning up...but cant' guarantee yours won't....not a very controlled way but it gets the job done...unlike a dedicated vacuum pump it will pull all the air out of a big bag in seconds...We use to use one to evacuate all
the air out of large projects before turning on the dedicated vacuum pump.

I like to note that when I used the shop vac itself for vacuum bagging I cleaned it completely out and ran it without the filter in place...it would suck so hard that the HD base would suck the sidewalks of the plastic tub. Not the best way but if you want to do it on the cheap and your resin jell time is less than 1-1/2 hrs it will do.

Regards,
Robert

This is my current Vacuum pump setup the pump is also from Harbor Freight. If you watch for sales and get an Insiders Club Card the price is not too bad.
Thinking about changing the pump system some to not work the pump as much - but more the method as I pulled too much epoxy last time when I had a perfect seal with my bag and smaller parts.
I started working on it before I left to go work on new cruise ship build a couple of months ago. I’ve been back to the US for week and I’m still weeding through the “honey do” list. I’m also interviewing for a new job my current job keeps me too busy and can’t seem to find time for my boat and that is starting to bother me. Also is picture of the “Bong” I built to capture epoxy and have some extra vacuum volume. I have the conversion of the vacuum pump and building the bong on my website http://www.themanshed.com/tms-20-trimaran/8-18-09-layup-and-vacuum-bagging.html I hope that helps.

Okay Phil, I'm sold. But, I'll buy something off the shelf, trying to engineer my own rig would be a disaster.

You may be getting a PM or two when I get lost. Or, hell I may as chronicle my little saga right here. This crowd can always use a good laugh.

Right now my hands are full so there'll be a little delay.

Mike, you guys amaze me! I wouldn't come up with that in a million years...

Great stuff Jake, thanks! I might have to try the same thign to save myself some serious digging and construction work.

YEP -- Great stuff !!

Only change i'd make is to use Carbon-UNI instead
of the woven carbon tape that's pictured. UNI places ALL the strength
in one direction , unlike the tape that strengthens
on both the + - 90deg. Ya really don't Need the carbon
in the cross-fibers, and the fibers are actually about
30% stronger in compression and tension loads,when NOT woven.

Bille

One of my vacuum pumps for small jobs, was made
from the compressor off an old refrigerator.
It's a 110v compressor. An AC compressor would work
but i'm Scared of 220v.

It sucks the air out of a Beer-keg, and has a one
way valve in line to keep the vacuum in the keg.

Add a valve and an adjustable Hg switch and Ya got something that
can pull almost a full atmosphere and only run for
5 minutes every hour if Ya got a good tight seal on
the bag.

It's held up well now for four years. In the first following winter (2008), my buddy swore the wall was moving and was comparing it against an open stud wall he built in front of the block wall. I didn't believe that was the case because the carbon and epoxy looked like the day they cured but I couldn't convince him that his studs were moving. To prove it, I epoxied a glass slide (laboratory type that is used with a microscope) to the wall on both sides of the crack between two of the carbon spans in the most offending area. It's still intact and hasn't broken loose or cracked proving that the block wall has not moved at all. His stud wall, however, appears to bow/move as the humidity changes in his leaky basement (at least the crown in the studs are in the correct direction! ;-).