Vacuum pump question

I have modified a fish tank air pump to produce a vacuum rather than blow air bubbles. It only produces 2.5" of Hg. Is that enough vacuum for sheeting white beaded foam? I keep reading about pumps that produce 20-30", but the foam will fail at high vacuum.

The pump is small quiet, and cheap. My goal was to make a vacuum bag system that didn't have a noisy, large expensive compressure for foam wing construction. I had the pumps sitting in my shop.

The mod is real simple, I'll post in in a few days if there is interest. The pumps are only $10, the vacuum guage cost 2x that.

>I have modified a fish tank air pump to produce a vacuum
>rather than blow air bubbles. It only produces 2.5" of Hg.
>Is that enough vacuum for sheeting white beaded foam? I keep
>reading about pumps that produce 20-30", but the foam will
>fail at high vacuum.
>
> The pump is small quiet, and cheap. My goal was to make
>a vacuum bag system that didn't have a noisy, large
>expensive compressure for foam wing construction. I had the
>pumps sitting in my shop.
>
>The mod is real simple, I'll post in in a few days if there
>is interest. The pumps are only $10, the vacuum guage cost
>2x that.
>
>Dave

You can build a vacuum pump from a compressor unit from an old refrigator, freezer or air conditioner. Refrigator units are small, quite and work great. One copper line is vacuum and the other copper line is pressure. These units seem to be able to suck a vacuum equal to a real vacuum pump. The high pressure side on my unit pumps up to 280 psi. I have a real vacuum pump and a refrigator vacuum pump both will suck a vacuum of 28.5 lbs. If you suck a vacuum on a glass of water the water will boil at room temperature. These units are designed to run in oil. Make sure the unit is upright when you use it. It will blow out a lot of oil but after a while the oil will stop blowing out. If you run the unit a short time and not 24 hours a day like it is designed to do it will work fine. I sometimes suck up some oil in the vacuum line and let it blow out the other line just to make sure it stays lubrcated inside.

You probably have a system that will meet your needs and let you learn how to effectively vacuum bag parts. A real gross simplification of the clamping aspect of vacuum bagging is 1/2 PSI per inch of vacuum. So your 2.5" of vacuum represents about 1.25 PSI clamping force. That doesn't sound like much until you start comparing total force applied to overall wing.

For instance, a 200 sq inch wing would have the equivalent of 250 pounds of pressure applied to clamp the wing skin to the foam. How many books or bricks would you need to stack on the wing to reach that clamping weight?

But wait, there's more!

The clamping action from a vacuum bag is applied all the way around the sealed enclosure. So the total clamping pressure is twice the single surface number. Or in the example about, about 500 pounds.

I have a system that will go to 18" of vacuum. I have found that much over 12" can crush some white bead board foam structures. 15" can crush some some blue/pink foam structures. Depending on the peel ply I use, 18" can pull too much epoxy out of a wood lay up.

The higher levels of vacuum are really only needed if you are making tooling (molds). I have a friend that has a system that can hit close to 24", which is pretty good for Denver (roughly 5000' above sea level).

When you are ready for more capability, you can build some real interesting systems. You can also buy some pretty good systems for the price of a new engine or two.

You don't want to exceed 8" Hg for 1 lb foam. Run a test prior to skinning your wing. Core the foam after vacuuming skins.
Derek Moran