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.