Vacuum Pump

[Kipper]

Originally Posted by ger21 Check Surplus Center. I got a Gast pump there for $90

I got 2 Piaff vacuum generators for £3.95

[ALpmc]

I can honestly tell you you that I tried my very best to take every short-cut to avoid paying the high price of a proper vacuum pump. I tried everything and nothing worked! After I added the overall cost of my short-cut attempts together, I could have bought a nice pump.

29 to 29.9 inches in Hg is what you need to successfully degass silicone and most casting resins. Anything less and you may as well skip it.

The size of the vacuum pump is dictated by the size of the vacuum chamber and the time that is needed to achieve an adaquate vacuum for degassing. SO.........No matter what! There is no escaping MATH in order to determine the best possible vacuum pump you need for your task!

In order to calculate the total volume of any round chamber you must multiply radius x radius x height x 3.14. Same principle as estimating the volume of a round mold box.

Note that a radius is half the diameter. A 10 inch circle has a 5 inch radius and that’s why we multipy it twice.

For example, if I had a chamber measuring 8 inches around (ID) by 10.5 inches tall, this is what the math would look like:

4 x 4 x 10.5 x 3.14 = 527.52 cubic inches of air volume inside my chamber.

We must convert inches into feet now. There are 1728 cubic inches in a cubic foot. In order to convert cubic inches into cubic feet you must divide the cubic inches by 1728.

So......... 527.52 divided by 1728 = 0.30 of a cubic foot.

Now that we know our volume, lets build a foundation for calculating an adaquate vacuum pump size.
We are going to use these rules:

PS = Pump Size or CFM
V = Volume
T = Vacuum time
PDF = Pump down factor. 1 Torr is 7. 10 Torr is 5.

This is our first question concerning pump size!

How long will it take for a 5 CFM vacuum pump to pull 1 Torr (1000 microns or 29.88 in-Hg) in my .30cf chamber?

Here is our formula: T = V x PDF divided by PS

What is our V? It is .30cf
What is our PDF? For 1 Torr it is 7
What is our PS? It is 5 (CFM)
.30 x 7 = 2.1 divided by 5 = .42 of a minute. That’s less than a minute to pull an adaquate vacuum for degassing in a chamber of this size.

What is the minimum size vacuum pump required for achieving a 1 Torr (1000 microns or 29.88 in-Hg) vacuum level within 1 minute in my .30cf vacuum chamber?

Here is the formula: PS = V x PDF divided by T

What is our V? It is .30cf
What is our PDF? For 1 Torr it is 7
What is our T? 1 (min)
.30 x 7 = 2.1 divided by 1 = 2.1 CFM pump would pull 1 Torr of vacuum in one minute. This would be a minimum but adaquate pump for degassing in a chamber of this size.

Here are some basic guidelines when cosidering a vacuum pump for vacuum degassing or casting.

I personally believe that you must pull the vacuum as fast as you can! I like to let my resins or silicone set until it almost completely stops bubbling. That can take up to 5 minutes or more in the chamber. Every minute in the chamber is lost working time for me! This is one time when faster is better!

After many hours of searching, studying, and pricing I finally decided on a pump that was used by home air conditioning servicemen to pull vacuums on refrigerant lines in order to eliminate water condensation. It was a RobinAir VacuMaster 5 CFM SPX 15500.

I was going to buy a brand new one but, as luck would have it, I found a used one at a local pawn shop for $150.00 US. This pump works great and will pull a vacuum in my homemade PVC 8” x 10.5” vacuum chamber in about 1 minute.

Good luck!

[Avago]

If you want a cheap 'n' cheerfull vacpump,try one off a fridge/freezer.These will pull a big vacuum if needed,are very quiete and are readily available.

I got my fridge pump from the local dump/re-cycle centre for only £2.

A lot of us composite modellers use them for vacuum bagging and they work very well.

Tim

Also if they don't pull the vacuum fast enough for you ,just add a reservoir with a valve on it.

[ranchak]

Thanks for the good info guys. I figured my chamber is about .91cf. Using 3.75cfm vacuum pull (the pump I got is 5.5cfm, but allowing for loss) it will take 1.7 min to degass. I plan on using 6" PVC pipe, using 2 lengths at 5' each. This will be a large reservoir, so I can pull vacuum fast on the silicone. I also am going to use this for vacuum clamping on my mill, that's why I'm going with the large pipe. I can live with this if it works. I went with the V2 Premium pump. I'll let you guys know how it works out.

Has anybody casted with the water clear urethanes?

[ranchak]

ALpmc, how much silicon have you de-gassed at one time with your pump?

[ALpmc]

ranchak - Sorry for the long wait. I think around 40 oz's of some slow setting silicone in one pour is the most I have degassed at one time. I can't get a larger container in my chamber that will allow for the rise of anything over 40. Most of what I pour is on a smaller scale. In addition to degassing, I also pressure cast all of my molds, both silicone and resin.

[ranchak]

I've spent most of the week working on a vacuum table. The venturi pump wouldn't pull past 25"Hg, so I ordered a 6CFM Robinair pump. It's pulling 27"Hg. I'm going to rework my vacuum tanks, I'm sure I have small leaks. I used teflon tape, but I'm not convinced it's the best for vacuum applications. Permatex makes a PST sealant for vacuum, but it isn't cheap. I should be able to draw close to 29"Hg then. I need to get a seal for the lid of my chamber before I can de-gas and do any mold making. I'm also going to get some of Dow Corning's High Vacuum grease. Right now I'm using Vaseline on my vac table. Ron

[ookpic]

This is a great option depending on how serious of a degassing chamber you want you..

I was able to make a degassing vacuum by poking a hole in the top of a mason jar and simply sucking out the air with a brake bleeder i was able to pull a max of 27 in of mercury just enough to degass my HS III silicone

it cant boil water but it seemed sufficient for removing bubbles from the silicone. I was able to set it up in literally one minute and for only $22

[SteveS]

Originally Posted by ger21 I use a Joewoodworker venturi pump for veneering. Imo it should work just fine for what you want to do, provided you have a decent size reservoir, and your degassing chamber isn't a lot larger than the reservoir.

Gerry,

I just got some veneer given to me and want to experiment with it.

I notice going to joes site now that he is dissing the harbor freight venturi for the new one he is selling. I believe in an earlier post you mentioned using the HF one.

Did you use the harbor freight one?
If you changed to the new one, what was your reason?

Steve.