De-gassing RTV Silicon in apressure pot??

redfinger · Post by **redfinger** » Thu Oct 09, 2008 11:16 pm

I know that you can degass the RTV in a vacuum chamber but what about a pressure pot? Would a cup full of RTV under enough pressure loose it's air bubbles? Just interested if anyone has ever tried it.

Ashton

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Thu Oct 09, 2008 11:19 pm

No it would compress the air bubbles.
The bubbles would then reappear when the pressure was released.

If you poured your mold, and then pressurized while it cured, any
air bubbles would also reappear once the pressure was released, and
wreak havoc on your mold and parts.

You can degass RTV in a pressure pot, but it requires setting it up
as a vacuum chamber, and investing in a vacuum pump.

redfinger · Post by **redfinger** » Thu Oct 09, 2008 11:29 pm

Umi_Ryuzuki wrote:No it would compress the air bubbles.
The bubbles would then reappear when the pressure was released.

If you poured your mold, and then pressurized while it cured, any
air bubbles would also reappear once the pressure was released, and
wreak havoc on your mold and parts.

You can degass RTV in a pressure pot, but it requires setting it up
as a vacuum chamber, and investing in a vacuum pump.

your so smart Umi

just thought I would ask.....any good pump suggestions?

Ashton

modelrealms · Post by **modelrealms** » Thu Oct 09, 2008 11:31 pm

Vacuum chamber
http://www.artmolds.com/product_details ... ccessories

Vacuum pump/compressor
http://www.artmolds.com/product_details ... ccessories

Pressure casting pot
http://www.artmolds.com/product_details ... ccessories

This is the lowest prices I have found on the pot.

Joseph Osborn · Post by **Joseph Osborn** » Fri Oct 10, 2008 2:02 am

Yes, you can pressurize the RTV during cure and eliminate air bubbles. The only real caveat to pressure curing is that the parts either can't be hollow or they must very strong hollow shapes-- else the pressure can distort the shape. Pressure curing produces absolutely beautiful, solid molds in minimal time-- just mix, pour, and put in the pot. I also use vacuum degassing with a pump & chamber when I'm pouring a bunch of molds and don't want to tie up the pressure pots. Just tonight, I made several Smooth-Sil 910 platinum molds and cured them in pots at 35psi. This rubber mixes fairly thick and takes longer than usual to de-air in a vac chamber, but the bubbles don't stand a chance against 35psi. Maybe it's the type of parts I make, but I never have any problems with surface defects from molds I've cured under pressure, and I've cast a lot of parts for my own line and others. My rule of thumb is cure at 35; cast at 35. Your mileage may vary

Stu Pidasso · Post by **Stu Pidasso** » Fri Oct 10, 2008 5:56 am

warrensgc wrote: Pressure casting pot
http://www.artmolds.com/product_details ... ccessories

This is the lowest prices I have found on the pot.

TA- DAAA!!!!

DLMatthys · Post by **DLMatthys** » Fri Oct 10, 2008 7:11 am

redfinger wrote:... but what about a pressure pot?

Uff Duh! What about that stuff you smoked befor High School science classes? Make physics work for you. Here is some ingrediants:

Bell Jar $50
.5"Lexan Sheet Plastic scrap $5
3cfm AC Vac-Pump $300

Castings... flawless

Mark Yungblut · Post by **Mark Yungblut** » Fri Oct 10, 2008 7:21 am

We always used a system like Don is tlking. I prefer the bell jar as i can more closely monitor the degasing process and prevent spill over with ease. De gasing is really the only way to go. Additionally, if we were molding smaller highly detailed parts we would put the mold in the pot after pouring the degasses rubber. Made for near to flawless molds...

Just my 2 cents,

Mark

Stu Pidasso · Post by **Stu Pidasso** » Fri Oct 10, 2008 7:30 am

The real trick is to find a scrap piece of PLexi or lexan. The holy grail around here, there's absolutely NO waste scrap, everything is used.

And I thought you had to get at least a 5cfm to attain the 29.92" of mercury to degas?

schweinhund227 · Post by **schweinhund227** » Fri Oct 10, 2008 7:43 pm

I got this Venturi style Vacuum pump "No moving parts"... Runs in line from your Air Compressor... * 90+ PSI not your Iwata compressor...

I had to use an 1/4" Adapter on the Nozzle since it's a Air conditioner vacuum pump... but for the price I paid ! It does the job...

http://www.redhillsupply.com/images/MTN8405.jpg

16$ on Ebay through a reputable store front ! and -28"hg
Now... I am sniffing around for that Elusive 1/2" Plexi...

Or I can still do it Blind in my Pressure pot.. using a Neg. Pres Gauge.

take care !

Stu Pidasso · Post by **Stu Pidasso** » Fri Oct 10, 2008 10:05 pm

So you're telling me that this will work to de-gass RTV rubber???

I can get 1/2" plexi ( 16" x 16", to go on top of this) for $30 at a local plastic store. HHHMMM....

DLMatthys · Post by **DLMatthys** » Fri Oct 10, 2008 11:23 pm

$16.99? Ya got to be kidding. I will have to run down to my local HF store and check that out!

OK then...with such a savings, go blow your wad of cash for a sheet of new cut to size Lexan!
Go nuts!

Gouf · Post by **Gouf** » Sat Oct 11, 2008 12:39 am

I've actually had success with just a pressure pot.

Mind you, these guys are telling you the 'right way' to do it.

But it doesn't take much frankenzoiding to get good result with just a pressure pot. I put the mold in the pot, then crank it up to 40psi. Hold for 5 minutes , then decompress. All the expanding air moves to the surface. you can repeat as needed. And I get good clear castings from these molds.

Works for me. But I do limited hobby casting, not production casting.

modelrealms · Post by **modelrealms** » Mon Nov 10, 2008 10:16 am

Stu Pidasso wrote:
warrensgc wrote: Pressure casting pot
http://www.artmolds.com/product_details ... ccessories

This is the lowest prices I have found on the pot.
TA- DAAA!!!!

79.99 online but if you go to the site and look for a HF store near you they are 54.99 in the store! picked up 4 this morning!

Chas · Post by **Chas** » Mon Nov 10, 2008 7:36 pm

I used to work in a prototype shop. We used urethane resin that was mixed and degassed in the upper portion of a scratch built two level vacuum chamber, the lower section of which was 3.5'X3.5'X8', and was where the moulds were placed. once the resin was fully mixed it was poured into the mould, which was still under vacuum. When the mould was filled the vacuum pump was shut off and air let back into the chamber. after this the moulds were removed and placed in a 300 degree (F) oven to cure.

DX-SFX · Post by **DX-SFX** » Mon Nov 10, 2008 8:51 pm

How did you manage to manipulate the mixed resin to pour it while it was still in the chamber?

Chas · Post by **Chas** » Mon Nov 10, 2008 9:37 pm

Well we called it a robot. Essentially the upper chamber contained two bowls of approx. 2.5L volume, both on pivot rods. One was above the other and over to the right, the other held a mixing paddle attached to a motor. Below these was a funnel with a replaceable liner in it and attached to the funnel was a disposable plastic tube than ran into the mould. Once there was sufficient vacuum in the chamber the simple push of a button was all it took to turn the tilt rod on the upper bowl and empty its contents into the lower bowl. Pushing another button engaged the motor that turned the paddle in the lower bucket thus mixing the resin and catalyst together. When they were sufficiently mixed - you guessed it- another button was pushed to tip the lower bucket dumping the resin into the funnel where it would, via the attached tube, enter the mould. It sounds a lot more complex than it was. The complex part happened when we tried to program the machine to dump the first bucket, mix for the appropriate time, and then dump the second bucket on its own. We also used closed two part silicone moulds that were wrapped in duct-tape in the cambers. We knew the moulds were filled when the resin escaped up through the vents in the mould.

GKvfx · Post by **GKvfx** » Mon Nov 10, 2008 11:26 pm

This sounds like the beginning of RoboCop 2 where bad things happen to the ED-209 replacements........

Gene

DX-SFX · Post by **DX-SFX** » Tue Nov 11, 2008 10:37 am

Chas wrote:Well we called it a robot. Essentially the upper chamber contained two bowls of approx. 2.5L volume, both on pivot rods. One was above the other and over to the right, the other held a mixing paddle attached to a motor. Below these ...

SNIP

..that were wrapped in duct-tape in the cambers. We knew the moulds were filled when the resin escaped up through the vents in the mould.

Purpose made unit then. I'm only interested because I'm working with Epoxy resins for laminating and the white gel coat I use incorporates bubbles very easily and they steadfastly refuse to pop even with degassing and full vacuum. I've made a mixing platform from a geared motor mounted above the bowl. The power supply to the motor is external and passes through the vacuum chamber wall via some insulated brass machine screws and wing nuts so I can start and stop the motor at any time. The power supply is variable voltage so I can control the speed of the motor too. Basically, I mix the epoxy thoroughly by hand in the bowl outside the chamber to make sure it's all mixed properly then put it in the vac chamber. As the bubbles grow, I turn on the motor which further stirs the mixture although now the paddles burst the bubbles so when I subsequently return the pressure to normal, I have a homogenous bubble free mix. The epoxy takes many hours to go off and you have to laminate the next layer while the layer below is still sticky so pressure pots are a non starter in this particular case.

Gouf · Post by **Gouf** » Wed Nov 12, 2008 10:00 am

DX-SFX wrote:How did you manage to manipulate the mixed resin to pour it while it was still in the chamber?

I pour the resin into the mold before putting it into the pot.

DX-SFX · Post by **DX-SFX** » Wed Nov 12, 2008 12:54 pm

Thanks. The question was actually aimed at Chas but run by me again how a pressure pot makes bubbles move to the surface?

Hold for 5 minutes , then decompress. All the expanding air moves to the surface.

Hasn't the resin set or at least thickened by then?

Gouf · Post by **Gouf** » Wed Nov 12, 2008 2:31 pm

DX-SFX wrote:Thanks. The question was actually aimed at Chas but run by me again how a pressure pot makes bubbles move to the surface?

Hold for 5 minutes , then decompress. All the expanding air moves to the surface.
Hasn't the resin set or at least thickened by then?

When you release the pressure form a pressure pot all at once, it's the scale equal of the RTV undergoing explosive decompression. I.E. all the gas tries to escape at once and you usually get a large air bubble that pops on the surface of the mold. So, no more air bubbles. For RTV, I use Dow Corning HS-3 (repackaged by allumilite in smaller quantities.)

As for resin, different resins have different pot lifes. I use a resin with a longer pot life, like Smooth-on Smooth cast 322, and have 15 minutes of time before the resin even begins to harden. Air escapes easily in this liquid state.

Sparky · Post by **Sparky** » Wed Nov 12, 2008 5:36 pm

Compressed air bubbles are very buoyant, this is way scuba divers must where several pounds of lead weight, to make them neutrally buoyant. So compressed air bubble will float to the surface of a mold even faster when compressed.

http://www.kc6sye.com/techmages_10_10_05_b.html
http://www.kc6sye.com/advancedcasting_c.html

One thing to note about the ventri pumps, they require a lot of time to pull a 2.5 gallon pot down to vacuum. There ar every efficient ones out there, but surprise surprise they cost as much as a regular vacuum pump (~200 dollars). Also you must ensure that all tubing and connectors are good and tight. I even found that large Inside diameter tubing helped my vacuum pump get a deeper vacuum in the pot. This was even more evident with the ventri pump.

Basically the CFM rating tells you how fast the device will empty the air out of a pot of given size, Cubic Feet per Minute. when degassing large amounts of RTV or multiple molds, and vacuum casting, the CFM of your vacuum system will impact the ability of the system to do what you need. In the case of vacuum casting a low CFM pump may not even work.

Yes, a paint pot works great for vacuum pots. One thing to note, is that you can also use very thick walled PVC pipe. We recently got access to scarp Pipe of this nature as the local town ship refited the water mains. That's some thick PVC pipe. There are some how two's on this setup. I'll see if I can find the link...

PS I moved this to construction. Seems like we should have a sticky how to on this. . .

DX-SFX · Post by **DX-SFX** » Wed Nov 12, 2008 9:18 pm

Sparky wrote:Compressed air bubbles are very buoyant, this is way scuba divers must where several pounds of lead weight, to make them neutrally buoyant. So compressed air bubble will float to the surface of a mold even faster when compressed.

(Emphasis added)

Sorry but I don't believe that's true based on simple physics. Bouyancy is based on the weight of liquid displaced. A large bubble will always rise quicker than a small one. The lead weights on a diver are to make a diver weigh the same as the water he displaces making him neutrally bouyant. A compressed air bubble displaces less liquid than an expanded one. If what you say were true, sticking a silicone mould in a pressure pot and keeping it pressurised would make bubbles rise more quickly which clearly they don't.

What the pressure pot is doing is compressing the bubbles to a smaller size while the resin sets which of course prevents them from growing again when you decompress the pot. You've actually got very small but high pressure bubbles incorporated into the casting.

Quick compression and decompression may encourage the bubbles to let go of what ever surface they're clinging to but that's more about liquid flow and breaking surface tension. Uncompressed air has a greater volume and so the forces compelling it to rise are greater hence a degassing chamber to make tiny bubbles bigger which in turn rise more quickly out of the liquid.

Sparky · Post by **Sparky** » Thu Nov 13, 2008 12:00 pm

That's right, the tank on a diver actually has negative buoyancy due to the compressed air's weight.

I think what makes the compressed bubbles rise easier might be more simple, the smaller volume of the bubble means a smaller point is trying to push through a viscous material.

I don't think its the cyclic natural of compressing and decompressing, as folks that use this method say they leave the mold in the pressure pot till the RTV has fully cured. Any small bubbles on the surface of the master will compress small enough so that they don't leave a defect in the newly formed mold, and when the pressure is released they just push on the mold and master until it is popped out, or they leak out along the master and mold to an open edge.

Whether you use vacuuming or compression, you need to relieve any voids in the master, a pin hole or a pipe. If you are making a 2 part, you can make this hole on the bottom side of the first pour, with relief out the bottom of the mold box. Or relief out a mold vent point, if you use a tube material to form the vent. I've use brass tube in the past for very large caverns.
http://www.kc6sye.com/images/images_11_ ... endcap.jpg

DX-SFX · Post by **DX-SFX** » Thu Nov 13, 2008 6:21 pm

I think what makes the compressed bubbles rise easier might be more simple, the smaller volume of the bubble means a smaller point is trying to push through a viscous material.

Sorry but again, the physicist in me tells me I don't think that's right. If that were the case, the smaller bubbles would reach the top of the silicone rubber before the large ones and they clearly don't. The same would be true in water. The whole method of degassing silicone rubber in a vacuum chamber is to reduce the surrounding pressure making tiny bubbles which would otherwise stay in the mix near the bottom (or at least not rise enough before the rubber sets) expand and rise to the surface of the rubber almost instantly. That's not just bubbles sticking to the subject but bubbles in the mix.

Q.E.D. Big bubbles - quick risers. Small bubbles - slow risers.

Stu Pidasso · Post by **Stu Pidasso** » Thu Nov 13, 2008 6:38 pm

All I know is that it works. I don't care how.

modelrealms · Post by **modelrealms** » Thu Nov 13, 2008 9:02 pm

Stu Pidasso wrote:All I know is that it works. I don't care how.

same here

DX-SFX · Post by **DX-SFX** » Fri Nov 14, 2008 9:10 pm

All I know is that it works. I don't care how.

Nice to see the future of the human race hasn't lost it's thirst for knowledge. Sigh!

Antenociti · Post by **Antenociti** » Sat Dec 13, 2008 7:43 am

Sparky wrote:Compressed air bubbles are very buoyant, this is way scuba divers must where several pounds of lead weight, to make them neutrally buoyant. So compressed air bubble will float to the surface of a mold even faster when compressed.
.

(i'm qualified as a BSAC international level insturctor - so i vaguely remember a bit about a diving)

dont compare/analogise it to bubbles in water from scuba diving - totally different environments - scuba gear produces compressed air at the pressure of the depth of the diver in the water - so at 10 meters it gives air at 2x atmospheric pressure aka 2bar (+1bar for every 10 meters of water depth ~14.7psi).

As the air rises through the water the pressure drops and the bubble expands dramatically (especially in the last 10 meters as pressure halves) with bigger bubbles rising faster than little ones as they have greater bouyancy and a greater expansion rate.

As it expands it also increases its bouyancy and travels faster toward the surface (up to around 60 feet per minute), in the (very) old days they used to teach you not to ascend any faster than your smallest bubble - but as they reach 60 feet per minute that's enough to radicaly increase the chance of decompression problems, so the max safe rate is now called at 30 feet per minute - the old guide of "watching your bubbles" is totally defunct.

This does not occur in a pressure pot as the pressure remains constant (to all intents and purposes) throughout the mould, resin and air in the pot.

I think what makes the compressed bubbles rise easier might be more simple, the smaller volume of the bubble means a smaller point is trying to push through a viscous material.

(I'm into dodgy science territory now...)

as the pressure does not change, the rate of ascent will not increase as the bouyancy of the bubbles will not change (the bubble will not expand and increase its bouyancy) - after that i guess you are in the realms of viscosity and flow-dynamics....

but if the boyancy of a bubble is not great enough to break through the viscosity of the resin then it will not rise at all and compressing bubbles reduces their bouyancy... so little bubbles wont go anywhere they'll just get smaller.

Taking the pressure off will allow the bubbles to expand back to their original size but if the bubble didnt have enough buoyancy to rise through the fully liquid resin it certainly wont have enough to travel up through the resin as it thickens.

As such I can't see any reason why releasing the pressure (and then re-applying it) would be of benefit... you would get the same effect (indeed a BETTER effect as you wont copress the bubbles therby lowering their boyancy) by just leaving the pressure off for those 5 minutes....

and that is exactly what we do when making our moulds - pour in the fluid silicone and let any bubbles anturally rise out of it over about 20-30 minutes - then whack it in the pressure pot. If you just whack it straight into the pressure pot there is a tendency for large bubbles to form within the silicone along a surface (not neccesarily "the" surface).

I dont know why that should be, but we have had it happen a number of times.

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De-gassing RTV Silicon in apressure pot??

De-gassing RTV Silicon in apressure pot??

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