DIY vac chambers

[trav]

From perusing other posts, I see that most people recommend the 1/4" stockpot, and the 2.5 CFM Harbor Freight Vac Pump for doing things on the cheap. I have a couple of questions regarding that though...

Of the posts I sifted through, the above pump was used with a smaller container, I would guess ~ 1 gallon chamber. From what info I have been able to piece together, it sounds like this pump can pull the necessary air to degas just fine. But, will it take significantly more time to pull that air from a 40 quart container, than it would from the much smaller one? This maybe a stupid question, but I gotta learn from someone!

Second question is regarding the lid. I understand Lexan seems to be preferred to regular acrylic plates. So lets say I go with 1" Lexan for the lid. My concern is the seal. Is it necessary to route the trench out to form the seal, or can you just use a neoprene type ring, that would fit between the lid and the pot?

 

[clonesix]

I question that pump's capacity to pull a strong enough vacuum. Everything else sounds right. I use an industrial stock pot, a sheet of neoprene rubber with a hole cut in it for the gasket, and 1" plex + assorted fittings.

You need 29" of mercury to sufficiently degass silicone, or other liquid.

 

[exoray]

I question that pump's capacity to pull a strong enough vacuum.

29" inches should not be a problem with any AC service pump like that, the cheaper ones just take a little longer to get there, they get to about 25" and then they slow down drastically, they hit 27" and it even gets slower but at that point you are degassing and if the pump is still running it will continue to go down...

But, will it take significantly more time to pull that air from a 40 quart container

Yeah, that pump is a 2.5 CFM, that means it pumps 2.5 cubic feet of air in one minute under no load...

40 quarts is only 1.34 cubic feet but you have to factor in the load that is ever increasing, just a guess but I would say you should get to a vacuum of 25" or so in about a minute or so with that pump and a 40 quart tank... Then you run into the stall where it takes significantly more time to get the last smidgen of air out down to 29" probably a several minutes depending on the pump...

To give you an idea I just tested my 2½ gallon (10 quart) paint tank with my 3 CFM pump... Mind you my AC service pump is probably an antique by now, I picked it up at a garage sale for chump change, it's old but still works like a champ but probably not as well as a new one... Also the seal on my paint tank isn't the best, like the pump the tank is old and the original rubber seal was shot, so I cut a new cork seal, hardly a good seal material as it leaks, but it still functions well enough that I have never bothered to change it...

I hit 25" in 25 seconds, took another 20 seconds to hit 27" and it can take several minutes to get down to 29"

When you are degassing silicone where you generally have 15-30 minutes of work time, it's more then adequate for most applications...

 

[Rebelscum]

I might add this bit about the top. 1" of lexan is over-kill. 1" of acrylic is easily fine for a very large container and MUCH less expensive. I used 1.5" acrylic on my 40 quart container just to be safe and it doesn't even begin to move during vacuuming. Acrylic has the benefit of being much easier to machine as well. I routed a 1/4" x 3/4" wide groove in mine for the seal. Just filled it up with the same mold max 30 silicone I use for my molds. Works perfect.

I have a pretty big two stage vacuum and it does take quite a bit longer to pull this one down over my little 1 gallon one. Due to this I only use the large one when I need a lot of silicone for a mold so it doesn't see much use.

 

[clayslinger]

Yep,
All my chambers are 1/4" thick walled acrylic and have 1/2" acrylic tops.
Used them for almost 20 years now without a hitch for molding and casting.

To me, it's the pressure pots you have to be super careful with.
A buddy had the top of one of those things hit him in the chest when the lock bolts snapped during filling.

Chuck...

 

[trav]

Thanks for all the replies. This is all great info for me, since I know next to nothing about the process at all. I am most worried about getting the acrylic (not lexan, thanks for that moenysaving tip) tapped, and a decent seal on it.

In regards to pressure pots, those scare the hell out of me, I would probably kill myself, and take off a side of my house.

My cousin actually works for the water co in his hometown, so I could probably get a good sized chunk of PVC pipe, but am worried about an even cut on the top and bottom for a seal.... Seems a pot would work just as well.

 

[exoray]

but am worried about an even cut on the top and bottom for a seal....

For a vacuum pot from pipe, you only need to get the ends close, then lay a thick bead of 100% pure silicone caulk around the end and mount the bottom, you can lay a bead of caulk around the outside for extra measure if you want... For the top or if you want a removable top and bottom Vaseline the top or bottom lid up, just a thin film of Vaseline, and then squish the lid(s) into the caulk bead, apply light pressure and let the silicone cure overnight, once cured peal the lids off and you will have a built in silicone sealing washer that is perfectly flat...

 

[trav]

Exo, so I just let the caulk dry around the top of the pipe? There is no actual gasket on the acrylic, it just stay around the top of the pipe?

 

[exoray]

Exo, so I just let the caulk dry around the top of the pipe? There is no actual gasket on the acrylic, it just stay around the top of the pipe?

Yep, the caulk will create a flat gasket on the top of the pipe, just drop the lid on and vacuum, the vacuum will pull the lid tight against the pip/caulk...

 

[OzDean]

I used a pressure tank with the top cut off. I then placed 20mm acrylic on top used the homemade gasket method. If the vacuum pump is a good one, it will suck the lid onto the pot.

 

[racecardriver03]

I wouldn't do a 2.5 cfm pump with 40 quarts. I have a 5 cfm pump and it takes close to 4 minutes to get to 29". It's not a cheapo harbour freight brand either. It's a pretty good one.

 

[exoray]

I wouldn't do a 2.5 cfm pump with 40 quarts. I have a 5 cfm pump and it takes close to 4 minutes to get to 29". It's not a cheapo harbour freight brand either. It's a pretty good one.

At that point it's not the CFM rating that is causing the slow down it's the nature of the beast when you get to the point where there is already a strong vacuum and you are trying to remove just that much more... I'll bet under the same circumstances the time difference between your 5 CFM and a 2.5 CFM to 29" would be literally a difference of a few seconds in the end... I'm not saying there won't be a difference but in the end you are playing with seconds not minutes in this case...

40 quarts is only 1.3 cubic feet, a 2.5 cfm will pull all the air out in about 32 seconds assuming it didn't fight back, but it's fighting it's own tug of war as it creates a vacuum... A 5 CFM will do it in 16 seconds, a savings of 16 seconds best case no fight back, but that isn't the end of it as you have said it actually takes 4 minutes to pull since it's fighting back and when you stretch the difference out over that 4 minute period I believe you will find and end total time savings between the two pumps to be maybe 10 seconds, likely less...

If you want to get faster you have to use a high CFM multi-stage pump, one that uses a high CFM to pull say the first 10" and then kicks into a secondary pump to pull the remaining vacuum... But again unless you are spending a ton of money on a super high CFM the time savings isn't going to be that great...

 

[zorg]

I'm building my own rig at the moment, i have a qusetion about some of the times quoted here.


if i pull a vacuum and lets say it takes 3 mins, then i burp the silicone, it shouldn't be another 3 mins to get back to where i was ?

these times seem very long to me.

when will the silicone start to properly degass 25 or higher?

 

[exoray]

if i pull a vacuum and lets say it takes 3 mins, then i burp the silicone, it shouldn't be another 3 mins to get back to where i was ?

Little confused about the question...

The silicone is 'burping' the entire time you are pulling the vacuum, it doesn't all the sudden happen at 29", by the time you get to 29" there shouldn't be enough air left to worry about (for most practical uses) and most of the 'burping' should be nearly over at that point, but you should give it enough time to stabilize at full pull and let any micro bubbles escape, before removing the vacuum...

As I stated earlier

"I hit 25" in 25 seconds, took another 20 seconds to hit 27" and it can take several minutes to get down to 29""

In 25 seconds @ 25" there is PLENTY of burping going on already, the rest is pretty much just gravy at that point... There is no magical vacuum number that all the sudden starts de-gassing it's happening the entire time you are just speeding it up...

when will the silicone start to properly degass 25 or higher?

It will honestly de-gas just fine on it's own without vacuum if it's given enough time before it kicks... The vacuum just accelerates the process by removing atmospheric pressure and allowing any trapped air to drastically increase in volume and rise to the surface much quicker... This effect is realized as soon as any vacuum is applied, you don't really need 29" to effectively de-gas silicone for most uses...

 

[clonesix]

I'm building my own rig at the moment, i have a qusetion about some of the times quoted here.


if i pull a vacuum and lets say it takes 3 mins, then i burp the silicone, it shouldn't be another 3 mins to get back to where i was ?

these times seem very long to me.

when will the silicone start to properly degass 25 or higher?

Not sure I get your question. But you will need around 29" of vacuum to get the silicone to boil. You will need to let it boil for 10 -20 minutes, or until the last of the air has bubbled out.

A pump may take 3 minutes to get to 25", but another 5-10 minutes to get from 25" to 29"

 

[zorg]

there are several vids showing the silicone rising to the top of containers during a pull,

then the person lets some air back in to burp it,

the silicone drops back down then rises back up fairly quickly (i assume this is at the start of the burping when there is a larger volume of trapped air)

i was trying to ask/confirm is when the person lets some air back in, i take it the vacuum does not go all the way back to the beginning

i have not bought my vessel yet, i was gonna use a stock pot
possibly this one or the size down (26cm)
30CM HEAVY DUTY STOCK POT SAUCEPAN WITH GLASS LID 19L on eBay (end time 04-Nov-10 16:38:17 GMT)

i have a 20mm lid and one of those HF venturi pumps, i have read conflicting reports on these and there ability to pull a good strong vacuum.

using the pot in the link above would i be wasting my time with it or should i go with a smaller pot with the pump i have?

-z

 

[exoray]

there are several vids showing the silicone rising to the top of containers during a pull,

then the person lets some air back in to burp it,

They are simply trying to avoid a boil over, the same as if you remove a pot full of noodles from heat when it starts foaming up... It's only necessary when you don't have the available room to let it boil on it's own... If your container can't hold 4 or 5 times the volume of the silicone in it, it will likely boil over...

i have a 20mm lid and one of those HF venturi pumps, i have read conflicting reports on these and there ability to pull a good strong vacuum.

With a decent air compressor supplying a decent volume of air they can pull 28-29" more then good enough for this application... I used one for many years, with my big v-twin high CFM air compressor I would get that 28-29" without issue, but when I was using my cheap pancake compressor I generally only realized 25-27" that was unless I started to shut off valves on the tank, let the compressor fully charge, and do small burst pulls, then I could get the 28-29" even with that compressor...

using the pot in the link above would i be wasting my time with it or should i go with a smaller pot with the pump i have?

Without actually seen the pot in question it's hard to say, if it's thin walled you might very well collapse it... Based on the price and pictures I would be very cautious though, it doesn't really appear to be that heavy duty...

I highly recommend paint pressure pots, they are reasonably priced handle the vacuum well and can double as a pressure casting pot...

 

[Eaglewood]

just got a Robinair 6 CFM vacuum pump --Will this one work well?

 

[exoray]

just got a Robinair 6 CFM vacuum pump --Will this one work well?

It should work very well...

 

[racecardriver03]

I would just stay away from the cheapie 2.5 cfm pumps. If you are paying less than $100 for a new one, it's probably very low quality. Robinaire is a good brand. I just bought some vinyl hose and some barbed fittings and realized you can buy pvc airline for like $9.99 and not have to spend money on barbed fittings. Doh! I do like the vinyl better because PVC hose is really stiff stuff. But if your a on a budget grab the campbell hausfield hoses from Walmart.