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ranchak
04-15-2009, 02:14 AM
I'm looking for a vacuum pump for de-gassing silicon and urethanes, any recommendations? I don't want nor can I afford a complete setup. I have a 2 gal paint pot, so I really only need the pump. Can a Robinair pump that is used in HVAC pull enough vacuum to de-gas?

Geof
04-15-2009, 09:42 AM
Yes if it is working correctly. You will need to make sure there are no leaks in your system becasue these pumps have a limited pumping capacity.

I take it you have not done any vacuum degassing like this before? It can get really messy. At the point where the vacuum is strong enough that the mixture starts degassing it can foam up to several times its volume spilling out of the container and getting into vacuum lines. Start off with a very small sample in your bigger container and if you can rig a second container ahead of the pump as a trap it is a good idea. Vacuum pumps don't work very well when they are full of silicone.

Years ago I did quite a bit of silicone degassing with a system using an open top tank something like a paint tank and found the best approach was to put a thick piece of acrylic on the top so I could watch what was happening and open a vent valve if the foaming was getting out of hand. Once the degassing is complete the mix settles back down and the casting results with properly degassed material are far better than you can get without degassing; well worth the hassle.

rowbare
04-15-2009, 10:19 AM
To get an idea of what Geof is talking about check out the videos at: http://www.freemansupply.com/video.htm

In several of them they show the vacuum degassing process and you can get a sense of how the mixture expands before collapsing.

ranchak
04-15-2009, 12:52 PM
I've seen the video already, that's what got all of this started. I've been looking for a pump and there are too many choices, so I figured I see what you guys are using and if it fits in the budget then I'll get one. So what are you guys using?

ranchak
04-15-2009, 12:58 PM
This pump caught my eye:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=220390939520


I really don't wan't to spend this much, but I know you get what you pay for.

ger21
04-15-2009, 01:38 PM
Check Surplus Center. I got a Gast pump there for $90

ckirchen
04-15-2009, 02:04 PM
http://www.veneersupplies.com/product_info.php?cPath=60_36&products_id=1099

I bought one of these for vacuum clamping, but it should work well for degassing. They have others that are bigger; click "Vacuum Press Parts" and scroll about halfway down.

Chris Kirchen

ranchak
04-19-2009, 01:53 AM
ckerchin, thanks for the link. I'm going to give one of their venturi pumps a try.

ckirchen
04-19-2009, 11:05 AM
I'm going to give one of their venturi pumps a try.
Make sure your air compressor has the available CFM to handle it.

I was going to get a venturi, until I realized that my compressor would have to provide 5 cfm to make about 3 cfm. That's why I got the pump I posted instead.

Might not be a factor for you though. Just thought I'd put it out there.

Chris Kirchen

ranchak
04-20-2009, 02:31 AM
Chris, I have 10cfm @90psi, it won't be perfect, but will definately work. The V2 "requires" 7.8CFM @80 so I will be close when you figure in the flow restriction with fittings. I'm going to use large vacuum reservoir tanks to aid in pulling down the vacuum. I also plan on using this system to hold some parts that I have to mill.

ckirchen
04-20-2009, 10:49 AM
Chris, I have 10cfm @90psi, it won't be perfect, but will definately work. The V2 "requires" 7.8CFM @80 so I will be close when you figure in the flow restriction with fittings. I'm going to use large vacuum reservoir tanks to aid in pulling down the vacuum. I also plan on using this system to hold some parts that I have to mill.

That won't leave much CFM left over for anything else. It also means that you compressor will be running at almost 100%.

Why not use an electric pump with those same tanks? My Rietschle Thomas from Veneer Supplies needs new seals, but it has run (in always on mode) eight hours a day for the last two years. I can't imagine how a compressor would have faired after the same duty, a bunch of oil changes at the minimum; plus the compressor is much noisier.

Chris Kirchen

ger21
04-20-2009, 11:12 AM
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.

Kipper
04-20-2009, 04:39 PM
Check Surplus Center. I got a Gast pump there for $90 I got 2 Piaff vacuum generators for £3.95 :D :p

ALpmc
04-20-2009, 08:46 PM
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
04-21-2009, 08:41 PM
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
04-22-2009, 12:48 AM
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
04-26-2009, 02:54 PM
ALpmc, how much silicon have you de-gassed at one time with your pump?

ALpmc
05-03-2009, 12:19 PM
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
05-04-2009, 01:45 AM
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
08-10-2009, 04:47 PM
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
10-26-2009, 01:13 PM
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.