How much vacuum is required to cast?

[BigguyZ]

OK, I'm looking at eventually getting into casting my own blanks. I know that the HF pressure pot and vacuum pump are a popular combination, but my wimpy PC pancake compressor just won't do the trick when it comes to the CFM required to pull a vacuum. So, I'm looking at some options.

As far as I see it, there are two solutions: Buy a new compressor that's big and will be enough to pull a vacuum. Or see if you can buy a vacuum pump for a veneer press and use it for both applications.

I really like the latter option best, as I think it'd be the ideal solution and allow me to do veneering for flatwork, vacuum chucking for bowls, and the casting. However, I noticed that the stand alone kits I've seen for vacuum systems available online only pull 25" of Hg. I know I've seen systems around here pull around 30" of Hg. But is 25" enough? Or is my best option to buy a compressor and use the HF vacuum pump?

Thanks,
Travis

 

[Firefyter-emt]

IMHO, if you can buy the compressor, go for it. You hardly ever hear someone say that they need a smaller one because the big one is just too much for them. (Not enough room for it, yes... but just "too big", never)

 

[MesquiteMan]

Vacuum? That is for cleaning up your shop, not casting!

Seriously, I cast a ton of blanks and do not use vacuum at all. I have tried it both ways and have found no benefit to the added step of vacuum. Then again, I am using Alumilite resin instead or PR so maybe that is the difference.

 

[toolcrazy]

Get a ultasonic cleaner, for de-gassing and use pressure. For casting that is all you need. I don't have a cleaner, but I use the HF pressure pot and don't have any problems with bubbles.

Pressure is for stablizing blanks and that is a totally diffrent animal.

 

[stevers]

I also use either nothing or just pressure for 95 percent of my castings. You shouldn't use vacuum on any casting with a secondary media, like worthless wood, cactus or for casting label blanks. The vacuum just pulls out more air bubbles than it removes. Pressure compresses the air bubbles to a size so small that they are basically invisible to the naked eye.
And you are correct about trying to pull that kind of vacuum with a pancake comp. We just had a fella try it and it wouldn't work. When he borrowed a full size comp, it worked fine.

 

[BigguyZ]

OK, so for casting a vacuum isn't needed. What about for stabilizing wood? I saw the tutuorial about using poly to stabilize wood, and that's something I'd also be interested it. So would 25" of Hg be sufficient there?

Thanks!

 

[skiprat]

Originally posted by toolcrazy

Get a ultasonic cleaner, for de-gassing and use pressure. For casting that is all you need. I don't have a cleaner, but I use the HF pressure pot and don't have any problems with bubbles.

Pressure is for stablizing blanks and that is a totally diffrent animal.

Is this a typo?
I thought that the generally accept methods are;
Vacuum for stabilizing (or de-gassing)
Pressure for casting

I assumed that the stabilized blank would be vac'ed to get the fluid in, then air dried. Like if you were using varnish. Of course, when you release the vac, you get an extra 1bar pushing the fluid in further.

Casting to me means holding it under pressure till the 'resin' has set hard. If you try to 'pressure stabilize' then the still wet fluid will be forced out when the pressure is released. Or a lot of it anyway.

 

[alamocdc]

I have a wimpy pancake compressor and the HF venturi vacuum pump and they work fine together. You only need to pull a vacuum for about 5 seconds to get what you need. I installed a ball valve and simply close it when the vacuum gauge bottoms out. Then I disconnect the compressor from the VP (quick disconnect).

 

[armchi]

Originally posted by alamocdc

I have a wimpy pancake compressor and the HF venturi vacuum pump and they work fine together. You only need to pull a vacuum for about 5 seconds to get what you need. I installed a ball valve and simply close it when the vacuum gauge bottoms out. Then I disconnect the compressor from the VP (quick disconnect).

 

[armchi]

Opps.. sorry about my last post. Pressed the wrong button.

Anyways, I just built my pressure pot and I found some minor leaks which I haven't fixed. I'm not worried since I don't think I need to hold pressure or vacuum for too long. I also have a small compressor and when hooked up to the venturi vacuum pump, it will pull at most 15-20" HG. I managed to get it up to 30" HG and here's how I did it.

- Start with the compressor's tank empty.
- turn on the compressor so the pump itself only draws the vacuum.
- Once the vacuum stabilizes (15-20" on mine), close the valve on the pot.
- disconnect the hose (from either the pot end or compressor end), but keep the pump going so it fills the tank up (mine is set to 90 psi but it may go as high as 120 which I've not tried)
- Once it reaches its max pressure, hook up the hose back up and quickly open up the valve on the pop. The higher pressure will draw up move vacuum (around 30" hg before it stabilizes).
- Close the valve and unhook the hose.

Hope that helps.

Originally posted by armchi

Originally posted by alamocdc

I have a wimpy pancake compressor and the HF venturi vacuum pump and they work fine together. You only need to pull a vacuum for about 5 seconds to get what you need. I installed a ball valve and simply close it when the vacuum gauge bottoms out. Then I disconnect the compressor from the VP (quick disconnect).

 

[toolcrazy]

I only use pressure. My tank leaks slightly, so I start at 40psi and it looses about 5 psi overnight. I can find the @#%$% leak, no biggie.

 

[gketell]

Steve,

If you want to find the leak mix up a table spoon of dishwashing soap in a squirt bottle of water. Apply pressure to your tank and then soak everything including around the gasket. Mine leaked THROUGH the cast-brass fittings. Not where they joined; THROUGH the cast material itself.

GK

 

[toolcrazy]

Originally posted by gketell

Steve,

If you want to find the leak mix up a table spoon of dishwashing soap in a squirt bottle of water. Apply pressure to your tank and then soak everything including around the gasket. Mine leaked THROUGH the cast-brass fittings. Not where they joined; THROUGH the cast material itself.

GK

I have soapy spray bottle, just haven't tried to track down the leak. I think it may be the pressure release valve. thanks for the info.

 

[rherrell]

I use pressure only and don't have any problems.