Vacuum pumps question revisited!

I'm considering using a vacuum pump to deair my RTV and to possibly use while casting. What sort of pump do you use?

I already have the little red one from Harbor Freight but I don't know if it'll pull enough air fast enough.

 

I've seen people deair silicone with a 1 gallon empty paint can and one of those inexpensive pistol grip plastic vac pumps. Look for the one that has a built in gauge.

With RTV I don't think time is of the essence as much as ability to pull atmospheres.

If you are talking about the red pump from harborfreight that you hook up to a compressor I think it pulls between 28 and 29.

When you say "and to possibly use while casting" if you mean setting resin in a RTV mold then I would think you would want to use pressure not vaccum.

 

With the HF venturi, I can get about 27/28 inches on my big compressor in the garage, with a smaller compressor that I have in the house I only get about 23/24 inches... So real world results will vary depending on your compressor and the volume of air it supplies...

Without spending a fortune you won't get much more speed on the pull then the HF one does, but you might get a little better vacuum... Try a Google search (or ebay) for Gast brand vacuum pumps... You can also go on the cheap and rip one out of an old fridge or freezer but results will vary and you will need to probably put an oil blocker inline...

 

Doug, didn't you get my Hi-vac?

 

One shop I worked out of bought a vac pump from a auto parts supply store. Its used to vacuum out radiators or something. Its been awhile so I can't remember exactly what its used for. I imagine if you called and asked around you could get more info. Supposed to cost a few hundred less.

 

he he. I'm not going to say it. :lol :lol

 

No baby, that bulge in my pants when I left your house was all me.

 

The auto parts store vacuum pump is used to clear brake lines etc. of air. Works great. I have one if anyone wants it, and I have the Red Harbor Freight pump. Both of these I planned to use to test my vacuum forming system, but now don't need them.

 

I KNEW there was more going on in his workshop than vacforming when i was talking to Rylo on the Phone the other week.....

Marc

 

Thats what I'm talkin' about. *, I'm gettin' old and memory is getting rusty.

 

Thanks for the info, fellas. I'll have to fire up the Harbor Freight one I already have and see how well it works. I'll shoot for the auto parts version if I don't like the first one.

 

Okay, I bought a 30 gallon compressor and attached my Harbor Freight vacuum pump to my new vacuum chamber.

The Harbor Freight pump requires a constant 90 PSI to maintain the 28-ish inches of mercury. The tank holds 155 PSI and is drained rather quickly. I'd say the air supply dips under 90 PSI in 1-2 minutes and then the vacuum isn't as effective.

Should I look at a different vacuum pump? I'm not opposed to the idea but I don't want to have to buy a larger compressor. I've only had this one for 2 weeks.

 

Put an on/off valve on each side of the HF pump, when the compressor tank drains, close the valve between the pump and tank, and then the valve between pump and air compressor... Let the compressor build back up then open the pump to compressor valve immediatly followed by the pump to tank valve and it should then continue to pull down... Once you get down, and assuming you don't have a leak, closing the valve should hold the vacuum just fine...

 

I was wondering if closing a valve would hold the vacuum. I figured it would but then again, I wasn't sure if the bubbles would continue to rise with a closed valve.

Then again, I don't think everyone that vacuums RTV has a 60 gallon compressor at their disposal either.

 

Mine tank holds vacuum fine overnight, with a closed valve... Get the 1/4 turn ones, they make life a lot easier... And if you are around you can always fire up the compressor and bring it back down if it starts to rise after a few hours...

On the flipside it leaks a little while under pressure so I need to leave the compressor hooked up when pressure casting...

 

I use the motor from a Fridge. It pulls a high vacuum but takes a while, but then the motor is oil cooled.

 

<div class='quotetop'>(terryr @ Feb 25 2006, 07:23 AM) [snapback]1193133[/snapback]</div>

This is the same thing I use for my Vacuum Chamber and it pulls 26 to 27 inches of mercury.

From what I understand 28 inches of mercury is as deep as you can get on earth and it changes depending on altitude.
People in Denver will get a better vacuum then someone in Death Valley.

Scot

 

if the mold is small enough to fit in your pressure pot, there is no need to vac the rubber. Just cure the mold under pressure and it will have better results than vac.

 

That's been my impression as well. If you have a good pressure pot, the vacuum chamber normally is not even neccesary.

 

My vacuum pump goes to 29-30 inches of mercury. The people at Silicones Inc. told me you had to have 29 inches to properly deair RTV.




<div class='quotetop'>(OhioAstromech @ Aug 7 2006, 01:06 PM) [snapback]1295587[/snapback]</div>

This is the same thing I use for my Vacuum Chamber and it pulls 26 to 27 inches of mercury.

From what I understand 28 inches of mercury is as deep as you can get on earth and it changes depending on altitude.
People in Denver will get a better vacuum then someone in Death Valley.

Scot
[/b][/quote]

 

<div class='quotetop'>(darthscifi @ Aug 7 2006, 06:49 PM) [snapback]1295644[/snapback]</div>

Two totally separate issues. A vacuum chamber removes bubbles from silicone by making them grow large enough that they rise to the surface and pop. A Pressure pot helps cure resin during casting by not allowing bubble to form in the first place.

Just as a side note: silicone molds that were not de-gassed will distort in a pressure pot.

One more question for those who are using pumps at the moment: Do you use the filters (babyfood jars) with the pump? If so, where can you find them?

 

<div class='quotetop'>(clonesix @ Aug 7 2006, 04:58 PM) [snapback]1295750[/snapback]</div>

Two totally separate issues. A vacuum chamber removes bubbles from silicone by making them grow large enough that they rise to the surface and pop. A Pressure pot helps cure resin during casting by not allowing bubble to form in the first place.

Just as a side note: silicone molds that were not de-gassed will distort in a pressure pot.
[/b][/quote]
Both rubber and resin react the same way. If they are both done in the pressure pot, bubbles can not form. I cast dragon skin in the pressure pot all the time and it is transparent rubber so you can see there are no bubbles

 

Re: Vacuum pumps question revisited!

My impression is that most mold rubber compounds are either fairly runny, and designed for use without degassing equipment, or fairly viscous and designed for use with a vacuum source that can pull 28 inches of mercury.

My understanding is that you don't need that strong a vacuum to degas the runny ones---24 inches or maybe less should be enough...? Maybe you won't get the durability of the viscous mold rubbers, but your easy-to-use runny compounds will degas okay. (The bubbles will swell enough to float out of the less-viscous liquid, even if they'd stay stuck in the thick stuff.)

Does anybody know if that's correct?

If so, there are a lot of cheap vacuum sources you could use to degas the runny ones, like a converted bike pump (under $20) or electric tire inflator, or a kitchen-type vacuum sealer from a thrift shop.

I've also heard that some people get good results degassing with with piston vacuum pumps that only pull 25 inches of mercury, even for compounds designed for degassing. I'm not sure how good those results actually are, though.

 

Re: Vacuum pumps question revisited!

24 or 25 inches is a * good pull and will do most degassing just fine, 28 or greater is unpractical for most people and not really needed in most cases...

If you use a slow setting rubber or resin, and keep it under vac for an extended period of time you will be fine... Just don't try and degass 5 minute resin with a hand pump

And if you have an air compressor to do the pressure casting, then get the cheapy Harbor Freight Venturi pump it works GREAT for the price, and is easy enough to rip apart and pipe up...

Get the cheaper of the two (they flip flop pricing on these) the guts that you need to remove and use from the units are identical...

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92475

or

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=3952

I put a female air quick connect on the tank and already have one on the compressor hose as that is the norm... Then I put 2 male quick connects on the venturi pump (one on each end) and presto you are good to go... For the pressure casting that is using the same tank, I picked up a pressure regulator and put 2 male fitting on that as well... So now you have a pressure and vacuum adapter for your tank that is interchangeable... Very simple to switch from vac to pressure...

This will be switched out soon as I picked up 3 electric vacuum pumps this year at garage sales... Total for all 3 = $25 The old "I'll give you $5 for this boat anchor..." implying it's junk, line works...

 

Re: Vacuum pumps question revisited!

Very interesting. My converted bike pump or electric tire inflator can do that, and my Food Saver can pull 23.5 inches. I got the inflator and the food saver for about $2 or $3 each at the Goodwill Outlet Store. (a.k.a. Blue Hanger store.) I've actually collected a half-dozen devices like that at the Blue Hanger, for a few bucks each.

Here's an Instructable about converting a bike pump to a vacuum pump:

http://www.instructables.com/id/EA58LR1F35J1I9N/

and another one about converting a 12V tire inflator-type compressor:

http://www.instructables.com/id/E791HNXF23Z39P6/

Do you mean it works for the thick stuff designed for degassing, not just the runny stuff that isn't, if you give it long enough? Very cool.

I don't like stuff that sets up fast anyway. I'm slow.

I suppose I could adapt my two-stage plumbing for my vacuum former to work with a vacuum chamber instead, though:

1. use a slow cheap pump to draw down a tank over several minutes, then

2. mix the resin, put it in the vacuum, chamber,

3. open the valve to the tank.

4. close a shutoff valve to the tank, leaving the pump connected to the chamber and

5 use the pump to pull the vacuum chamber down all the way to its max

That way, the tank sucks most of the air out of the vacuum chamber (very quickly) and the pump does the rest of the job. You want to shut the tank off so that the vacuum pump pulls more air from the vacuum chamber, rather than pulling the air back out of the tank.

Here's a thread about my 2-stage plumbing for vacuum forming, using a FoodSaver as a vacuum pump:

http://www.tk560.com/phpBB2/viewtopic.php?t=424

That setup is overkill for degassing, though, and I have a more elegant version now.

I have a little compressor, but it's too wimpy to run those. (They need something like 4 CFM at 90 PSI.)

My understanding is that there are much more efficient 2-stage venturi gadgets you can use with small compressors, but that they're crazy expensive. (For no good reason, because they're not complicated machinery.)

If I knew more about the theory, I might try to come up with a DIY 2-stage venturi.

 

Re: Vacuum pumps question revisited!

What do some of you use for chambers on your homemade rigs?