Need Help!- Vacuum pump for degassing resin & silicone

[jontidy]

Hi, I'm looking to buy a vacuum pump to connect to a chamber for degassing polyester resin and RTV silicone. I've seen this one for sale in an air conditioner sales shop but not sure if it will do the job. I understand I need around 29 inch of mercury vacuum to degass successfully but this model is in PA.

The specs are
1 stage vacuum pump
3.5 CFM,
ultimate vacuum - 10PA
1/3 HP

Anybody know if this is good enough?
thanks

[johnohara]

By comparison, this HF vacuum pump is 3CFM and pulls down to 25 microns:
Air Conditioning Vacuum Pump - Two Stage, 3 CFM

Some other (hopefully) helpful links:
Vacuum Gauge Tech
Vacuum Pressure - Converting Units
Pressure Units Online Converter

~John

[jontidy]

Thanks. Some interesting information there. The harbour freight pump looks like it would do the job from the reviews, But as I'm living in Phuket / Thailand, this is the only pump available to me. I tried some of the online conversion tools but apparently 29 inches of mercury is equal to 100000 PA (and this pump only creates 10 PA!!). Surely that cant mean this vacuum pump needs an ultimate vacuum 10,000 times more to create 29 inHG ??

[RomanLini]

It's probably 10kPa. Single stage pumps will work ok for vacuum bagging and some light degassing, but generally you need a dual-stage pump to get the higher vacuums needed for good degassing, especially on very thick liquids like silicone.

No single stage pump of that type will give you 29 inches of mercury!

[johnohara]

jontidy~

Here's are some links to Freeman Supply that should help.

Vacuum Degassing & Pressurizing
FMSC - Gas Vac II Industrial Vacuum Degassing Unit

Freeman Supply's web site is full of great information. They are very generous with their product knowledge and expertise. A must visit.

~john

[Geof]

Normal atmospheric pressure is 101kPa, i.e. 101,000Pa (as mentioned).

If that pump can indeed pump down to 10Pa it is pumping down to less than 1 ten thousandth of an atmosphere; 10/101,000

That is a good vacuum and is much better than 29"Hg.

[jontidy]

Originally Posted by Geof Normal atmospheric pressure is 101kPa, i.e. 101,000Pa (as mentioned). If that pump can indeed pump down to 10Pa it is pumping down to less than 1 ten thousandth of an atmosphere; 10/101,000 That is a good vacuum and is much better than 29"Hg.

So even though this is only a single stage pump, at 10PA it would be able to degass silicone right?

[Geof]

Yes the vacuum it develops will be more than enough. I notice from the nameplate that it mentions oil capacity so it must be an oil immersed vane pump. These are the type used in laboratory applications.

If you do get a pump like this and use it for degassing silicone you MUST make sure you have a reservoir tank between it and the silicone and also a filter between it and the tank. These will slightly reduce the efficiency of the system but if you omit them and fill the pump with silicon froth the efficiency will be reduced to nothing.

I am guessing you have not done any silicone degassing yet. I have and fortunately I was not using an expensive pump the first time, I was using an air aspirator, a venturi valve that created vacuum from a high pressure water flow. When I applied the vacuum nothing much happened at first. Then when the vacuum got down to somewhere about 25" Hg the silicon frothed up and 'boiled' out of its container and filled all the tubing with silicone. In this way I discovered that you needed a container with a volume about three times larger than the silicone being degassed and you need something of larger volume between this and the source of the vacuum.

Once the silicone has frothed and then settled down you know that it is successfully degassed.

[jontidy]

Thanks for the info Geoff. I think I did see on the box it comes with that this is a rotary vane pump. Its for sale in a Air conditioner repair shop so I guess its for removing the moisture from Aircon units. They also sell a smaller pump (just 2.5 cfm and 5 PA ultimate vacuum) for 1900 thai baht (about $60), but I guess this wouldn't be good enough because its so cheap. The bigger one is about $100.

When you say a reservoir tank and filter, Do you mean an air tight container with the pipe from the vacuum chamber going into it, then out of it and into a normal air filter (the types usually connected to air powered machinery to remove the moisture). Then the pipe running into the vacuum pump?

Sorry for all the questions, just want to get the right equipment as cheaply as I can as I've already wasted enough cash on ruined silicone moulds full of bubbles.

[Geof]

For the filter enquire at the place you get the pump. Just using a regular compressed air filter may not work because the filter housing could collapse when subject to vacuum.

The reservoir tank can be anything that does not collapse due to vacuum. You could construct one out of heavy plastic pipe using varnished plywood as the end caps. All that is needed is a volume that would allow any silicone bubbles sucked down the line to settle out before reaching the pump with the filter as a secondary backup.

The plumbing sequence you describe is correct: Degassing chamber to reservoir, other end of reservoir to filter, filter to pump. Put a ball valve between the degassing chamber so you can pump the reservoir down while you are mixing the silicone and you will save time.

[RomanLini]

Originally Posted by Geof Normal atmospheric pressure is 101kPa, i.e. 101,000Pa (as mentioned). If that pump can indeed pump down to 10Pa it is pumping down to less than 1 ten thousandth of an atmosphere; 10/101,000 That is a good vacuum and is much better than 29"Hg.

That's impressive! My single stage pump won't pull anything close to that. Normally it's recommended to use a 2-stage pump for good degassing.

I hope it's not a case of "enthusiastic specs label" that happens on some Asian equipment sold on ebay?

[rowbare]

jontidy, watch the vaccum degassing video that John linked to and you will see what Geof means, the silicone will increase in volume and then collapse as the air is removed.

A lot of people use a paint pressure pot for degassing (something like this: Paint Pressure Tank - 2-1/2 Gallon). You would put your container inside the pot. In a pot that size, you would probably do 1 litre at a time in a 4 litre pail.

bob