ElMostro, in our application, we typically run the pump for a minute to a minute and a half. This is the approx time required to pull the vacuum down to 29" of Hg and degas the material (allow it to rise and then collapse on its own). We typically don't degas very long. Once you've removed the air from the material, we remove it from the vacuum chamber and pour perfectly bubble free material into the mold or mold box. However if we did wish to leave it under 29" of Hg for an extended period of time, you can turn the pump off and it does hold the vacuum in there. Once in awhile you will have a chamber that will slowly let air back in and it will eventually lose vacuum but if you have a good seal around your chamber it should hold your vacuum in there no problem with the pump off.
Mikey, good questions. To be honest I had to make a phone call to get a little help with it. In Kalamazoo, MI we are approximately 700'-800' above sea level. With any 5 or 6 cfm pump we have used here, we have always been able to reach 29" of Hg and even 29.5" per the gauge. I have heard of some people claim that due to their altitude they could not reach 29" of Hg simply due to there not being enough air to pull that much vacuum. So after reading your questions, I picked up the phone and called a long time customer and excellent mold maker in Colorado (elevation of where he is at is 5,280' above sea level). After I asked him the question of how much vacuum he is able to pull with his vacuum pump (same pump as Robinair but has a Procraft label on it), he said hold on because he was mixing a cup of HS II right then (moldmaking rubber - thick pancake batter consistancy material that traps a ton of air). I heard him turn on his vacuum pump which sounded exactly like the Robinair and he reached 24" of Hg. He said he normally leaves it in there for about 2 minutes in which time the material rises and collapses and is fully deaired. That was amazing to me because if I were to stop my vacuum chamber at 24" of Hg and hold it there, my HS II would never degas in 45 minutes (maybe even in the full amount of time the material sets - 1.5 hours) let alone 2 minutes.
So you are correct when stating that many people might not be able to pull 29" of Hg. However, the vacuum those pumps pull at those elevations seems to be equivalent due to the amount of air each of the pumps are able to pull down (less air at 5,280' than 750'). So in otherwords, my customers 24" of Hg at 5,280' might be equivalent to my 29" of Hg at 750' above sea level. I will investigate this more and see if I can verify these assumptions from my conversation with my customer with someone who knows the physics behind it.
Mike
Alumilite
800 447-9344
