View Full Version : Vacuum Pump for Rubber degassing question

01-06-2010, 09:06 AM
Hi there,

Not certain if this is the right spot for this topic, so feel free to move at your convenience. I am trying to make a silicone rubber mold of a machined part so that I can copy multiple times. With silicone rubber, it is imminet to degas (remove air bubbles) in order to maintain plug detail.

Question is that everybody specifies the pump being able to achieve at least 29 inches of mercury but I have not found any pump that rates the vacuum capabilities in inches of mercury, but on microns.

Per example, I am looking at a Robinair 15500 which is a 5 CFM, 1/3 HP vacuum pump capable of achieving 40 microns of vacuum. Will this take me to the 29 inches of mercury? Already tried the 3 CFM Harbor freight pump and it did not work.

Thanks for your input!


01-06-2010, 10:06 AM
I had not run across microns as a measure of vacuum.

A micron is a millionth of a meter, or a thousandth of a millimeter. One inch is 25.400 mm so 1" is 25400 microns; atmospheric pressure is about 30" of mercury (Hg) so atmospheric pressure is about 762,000 microns.

For vacuum degassing you need about 29"Hg of vacuum which is equal to about 25400 microns so a pump that can reach 50 microns, theoretically is more than enough.

Do you have a good vacuum gauge and have you tested your Harbor Freight pump just with the gauge, blanked off so you know there are no leaks; rig up an adapter to fit the gauge directly to the pump inlet.

I have successfully degassed RTV silicone at around 26" Hg which should be within reach of your pump provided there are no leaks.

Incidentally I think microns for vacuum is a totally hokey measurement; it sounds more technical than inches of mercury but doesn't conform to any recognised system of measurement.

01-06-2010, 10:14 AM
According to this calculator: http://www.brandtech.com/vac_unit.asp 40 microns (.04mm) works out to 29.92 inches mercury

01-06-2010, 10:28 AM
Incidentally I think microns for vacuum is a totally hokey measurement; it sounds more technical than inches of mercury but doesn't conform to any recognised system of measurement.

While not a standard unit, it is commonly used in the HVAC industry.


From http://www.heatcraftrpd.com/res/pdf/support/Evacuation.pdf :

"Vacuum levels between 29 in Hg and 30 in.Hg (absolute vacuum) are measured in Microns. One thousand microns is equal to one millimeter of mercury and are measured with an electronic vacuum gauge."


01-06-2010, 10:34 AM
While not a standard unit, it is commonly used in the HVAC industry....bob

That is what I discovered and I still think it is hokey.:)

01-06-2010, 12:12 PM
Excellent information! Thanks to all!

That being said, what pump can you then recommend? I took the Harbor Freight back to the store as I thought it would not work. I will definitely not buy this pump again, not because it may not work, but because their deal is off and I am not willing to pay $170 for a pump I know will die at some point in time in the near future. HF always dies prematurely, so only if something is dirt cheap will I consider it.

I have been trying to search the CL and ebay, but dread buying a pump just to find that it is not enough. What has worked for you before?

How many CFM?
How many HP?

With these data points I am willing to jump...

Thanks again for your great input!


01-06-2010, 02:02 PM
Have you seen this from Freeman Supply? http://www.freemanvideos.com/moldmaking/vacuum-degassing.html

This is the script for the video. They use this for degassing: http://www.freemansupply.com/GasVacIIIndustrial.htm

The pump they use is 6 CFM 20 microns. A 5 CFM pump would be a bit slower while the difference between a 20 micron and a 40 micron rating is insignificant for this purpose. In fact the plumbing is probably as important as the pump itself. One of the eBay sellers has this to say about tubing size:

Evacuation Rates - the "CFM Wars": It's true! The capacity of a standard 1/4"ID hose is limited to 3/4 (0.75) CFM. And that's ONLY if you remove the valve core depressors from your hose ends AND Schrader valve cores from the system's high and low side access valves.

I haven't verified his claim and he goes on to try and sell you tubing so take it with a grain of salt but it does highlight the fact that how you connect things together is important.


01-17-2010, 10:31 PM
To further the confusion my Busch R5 vacuum pump is 15Torr or 29.3Hg,1Hp at 600V at 3400RPM,20CFM.This baby cost 2,000 20 years ago.After 29.3Hg pumps are very,super,mucho denero.To degass one or 2 gallons of rtv your vacuum vessel or tank should be at least 5gallons.A 10 gallon paint pressure pot would suffice as you can drop a 5 gallon plastic pail in the pot to avoid cleaning of the tank.
The blurb on the hoses is true.Larger lines will act as increased storage.For lower capacity pumps a storage tank may help similar to air storage as similar to an air compressor.But an ordinary air tank will collapse at 29Hg.Propane tanks can take the negative pressure,but if you put the mix in the vacuum tank and suddenly open to the storage,the mix will suddenly expand to 2 to3 times the volume as foam.This is why you need a 5 gallon pain to de gass 1 or 2 gallons
To confuse more,29.5Hg is equal to 14.5PSI.To get absolute vacuum of 14.7 you must be at sea level or on the shuttle.I don't think I would want to have a beer on the shuttle.