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Selecting a Vacuum Pump

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Selecting a Vacuum Pump

Postby mangozac » Thu Mar 08, 2012 9:32 am

Well lately my clunky old vacuum pump that was gifted to me has been starting to play up a bit and isn't reaching as good a vacuum any more, so I think it's time for it to be retired with a new, higher flow unit.

I'm looking at pumps on eBay, each with a final vacuum of 5 or 10 Pascals (0.005 or 0.01 kPa), which correspond to better than 99.99% vacuum according to the table here. Where the prices differ is when it comes to the flow rate (CFM).

Now I'm using a 10L pressure pot so in order to work out the evacuation time I first convert this to cubic feet using google: 10L = 0.353ft^3. I can then use these figures with the calculator provided on engineering toolbox to calculate the evacuation time for various CFM pumps. Note that the absolute initial pressure will be atmospheric pressure = 101.4kPa, with a final pressure of 10Pa (0.01kPa).

1CFM = 3.26 minutes
2CFM = 1.63 minutes
3CFM = 1.09 minutes
4CFM = 0.81 minutes (48.6 seconds)
5CFM = 0.65 minutes (39 seconds)
6CFM = 0.54 minutes (32.4 seconds)
7CFM = 0.47 minutes (28.2 seconds)
8CFM = 0.41 minutes (24.6 seconds)

As you can see, the fill time does not correspond linearly to the flow rate/CFM, and once you get above 6CFM the decrease in fill time starts to drop quite a bit. Remember also that these times are best-case and do not account for leaks in the plumbing or lid seal (which effectively reduce the flow rate further at higher vacuum).

Which one is most suitable? Well it comes down to your budget and your resin's pot life. Anything under 2CFM is out of the question as it simply takes too long. 3CFM is acceptable if you're using the same pot for your pressure and vacuum. Me? I'm going to go with the 8CFM pump, which can be had for just over AU$200. For the amount of casting I do it's worth the investment.

Disclaimer:I know in the past I've made comments about suitable lower CFM rates but I had been ignorant to the fact that the evacuation rate is logarithmic, making evacuations times longer as the pressure gets closer to complete vacuum. So please take this post as guidance for pump selection.
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Re: Selecting a Vacuum Pump

Postby blind pig » Thu Mar 08, 2012 10:42 am

I have a 9cu/ft min vacuum pump that I bought of ebay. It will have silicone degassing and water boiling at room temperature, in under 20 seconds.

When searching for vacuum pumps on ebay, have a look for "vacuume pumps", as I did, and scored my pump for under $30, brand new in box. 8-)
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Re: Selecting a Vacuum Pump

Postby mangozac » Thu Mar 08, 2012 1:45 pm

9CFM degassing in 20 seconds sounds pretty agreeable with the maths above. Of course those times are to the absolute maximum volume - the point at which the resin is becoming sufficiently degassed would be a bit sooner.

blind pig wrote:When searching for vacuum pumps on ebay, have a look for "vacuume pumps", as I did, and scored my pump for under $30, brand new in box. 8-)

I tried that, but eBay noticed the error and automatically corrected it! :roll:
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Re: Selecting a Vacuum Pump

Postby Anvils Hammer » Thu Mar 08, 2012 8:57 pm

I got my pump for about £300 with all the bits, I could have had a cheaper one probably if Id shopped around.


Its a really nice unit, 5.3CFM. which will empty my pot fast enouhg, but not as fast as Id like it to.

I really want to get a 10CFM pump with a 1" bore outlet, which will really pull the vacuum chamber down fast! at the moment It takes maby a minute.. which is fine but not ideal if I was pouring larger ammounts of resin or using a faster resin for any reason..

Just gotta justify the price tag on a larger "proffessional" pump, which even second hand, is the best part of £1000!

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Re: Selecting a Vacuum Pump

Postby mangozac » Sat Mar 10, 2012 8:00 am

Well the auction has ended and my new 8CFM pump is on it's way! I'm expecting to get even better results from my casting now, although may have to go around my pot and improve some of the seals ;)
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Re: Selecting a Vacuum Pump

Postby mangozac » Thu Mar 15, 2012 8:39 pm

My new vacuum pump arrived today so I wasted no time in testing it out! I need to pick up a correct fitting for it but in the mean time bodged something together to do the job ;)

I did a couple of "dry" runs first to see how fast it reached vacuum and just how deep a vacuum it would go. Well the speed was about double that of my old pump, reaching -25 in/Hg in about 20 seconds. Left running it only made it to about -27.5 in/Hg, likely because of the poor seal of my dodgy temporary connection. I have no doubt I will be able to run it to at least -28 in/Hg during normal operation.

So after the successful dry runs I prepped some moulds and did a proper run. After the resin was mixed I dropped the moulds in and popped the lid in place. I then turned on the vacuum pump and while the pot is evacuating I do up the lid clamps. Normally I have to leave the pump running for a bit after I finish doing up the clamps in order to reach a decent enough vacuum but in this case the good vacuum was reached long before I'd finished ratcheting them up! The pump was switched off, 50PSI of pressure applied and it was left for an hour for the resin to cure.

Usually many of my parts have tended to collect tiny sub-surface bubbles near the pour spouts if the part does not have a steep enough angle. Since they generally just back from the surface they're normally not a problem, but sometimes were and were at the least a bit annoying. I'm pleased to report that the new vacuum pump seems to have done away with almost every single one of these bubbles!

So all in all the new pump was money well spent!

Would anybody like my old vacuum pump for free?
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Re: Selecting a Vacuum Pump

Postby blind pig » Thu Mar 15, 2012 9:34 pm

Why do you bother to do up the clamps when vacuuming? The vacuum provides the clamping force if your seals are good enough.....
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Re: Selecting a Vacuum Pump

Postby mangozac » Thu Mar 15, 2012 9:47 pm

I do up the clamps for when the vacuumimg is finished and I'm ready to apply pressure - remember I use the same pot for both ;)
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Re: Selecting a Vacuum Pump

Postby blind pig » Thu Mar 15, 2012 11:12 pm

mangozac wrote:I do up the clamps for when the vacuumimg is finished and I'm ready to apply pressure - remember I use the same pot for both ;)


Sorry, forgot that little detail.

Supercheap keep having sales, get another one and live the dream! :lol:
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Re: Selecting a Vacuum Pump

Postby mangozac » Fri Mar 16, 2012 6:33 am

Tempting, but unnecessary ;)

How much, out of interest? The catalogue isn't available online :(
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Re: Selecting a Vacuum Pump

Postby blind pig » Fri Mar 16, 2012 9:31 am

$90
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Re: Selecting a Vacuum Pump

Postby Fleafa » Fri Mar 16, 2012 11:18 am

Why does no one sell them that cheap in my country? WAAAAAAAAAAAAAAAH!!!
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Re: Selecting a Vacuum Pump

Postby Eltnot » Wed Mar 21, 2012 7:12 pm

They're on special right now $98 for a 10L pot. I just bought one this afternoon.
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Re: Selecting a Vacuum Pump

Postby blind pig » Wed Mar 21, 2012 9:10 pm

Eltnot wrote:They're on special right now $98 for a 10L pot. I just bought one this afternoon.



I wonder how much the'd cost to post to Germany? 8-)
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Re: Selecting a Vacuum Pump

Postby Eltnot » Wed Mar 21, 2012 9:40 pm

blind pig wrote:
Eltnot wrote:They're on special right now $98 for a 10L pot. I just bought one this afternoon.



I wonder how much the'd cost to post to Germany? 8-)

Didn't notice yours and Fleafa posts before I hit reply.
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