The vacuum pumps that home builders use show an amazing variety and range of capability. Everything from aquarium pumps to vacuum cleaners. A professional outfit at retail prices can easily cost $1000 US. That’s a steep price to pay for an amateur builder. So somewhere between a $1000 pump and your home vacuum cleaner there must be a rig to get the job done without bankrupting the builder.
The two important things are that a pump be reliable (if it quits during the cure you are screwed) and it has to have enough capacity to tolerate minor leaks in the bag or mold. Theoretically, if a bag is totally sealed, a very small pump is all you would need. Real life is very different. Even if you do a complete dress rehearsal with the bag and pump before committing to wetting out the reinforcements, and the bag proves to be tight, there are still things that happen during the actual lay up. For instance, a drop of wet resin where the bag seals to the mold will prevent the sealant tape from sticking and will cause a leak. There may be a minor leak in the pleats and folds of the bag that is nearly impossible to find. For these reasons, some significant pump capacity is needed – the more the better.
I’m sure there are a lot of opinions on this but to vacuum bag something the size of a Tornado hull side; I would recommend a pump of a least 4 cubic feet of air per minute. This equates to around 1/3 horse power which is a little bit more than the average HYVAC air conditioning service pump. There are oil filled pumps that can develop more vacuum than dry rotary vane pumps but they are messy and require a lot of maintenance. I have acquired several pumps over the years, a couple of Gast 1/3 horsepower rotary vane units, a one horse power (14 cubic feet per minute) Gast vane pump, and a two horse power oil filled vane pump (16 cubic feet per minute). Although most pumps can produce 27 to 29 inches of vacuum, 20 inches will produce good quality parts. If you can’t get above 15, the quality of the part will definitely start to suffer.
Finally, this brings us to the issue where to buy a pump. I swear by Ebay. If you search under ‘vacuum pump’ there will be a hundred or more hits. The 1/3 horse power Gast units are common in computer rooms and machine shops. A unit in excellent condition can be had for around $100. They retail for around $400. My 14 cubic foot pump cost $180 on Ebay and was brand new surplus. Retails for about $800. So, the deals are out there if you look around a little.
