Small pressure chamber made from rated NPT components... Safety questions. - Page 3
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    Quote Originally Posted by Laverda View Post
    Just so you have more to be worried about, the semi truck next to you on the freeway has over 100PSI inside its rubber tires!
    I'm a weirdo. I have my windows down when I drive, all the time, unless it's pouring rain, even on the interstate.

    I had a semi tire burst right next to my head/ear at 75mph.

    It was loud.

    Then there's this classic video of what happens when you purposely stab a large truck tire:
    Extortionist Stabbing a Shop Owner's Tire Has it Blow Up In His Face - YouTube

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    My son has become involved in casting small parts, too; and asked me about methods to avoid voids.

    The more i read, the more sensible and convenient that vacuum sounds compared to pressure.


    If when using vacuum to degas epoxy, you experience boiling, It is the volatilizes leaving the mix. NOT GOOD!

    The vacuum is too hard.

    Reduce the level of your vacuum. 1/2 an atmosphere is sufficient. 200 microns for a short period of time will pull all entrained air from any two part.
    In other apps, i once used a large glass peanut butter jar (probably no longer made?) as a perfectly safe and easy view vacuum chamber with a Cenco megavac. (Fairly hard vacuum) Glass bell jars & vacuum grease were the old lab standard. Ball & Kerr make canning jars in various sizes for safe storage under vacuum.

    smt

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    Worked one summer at a cannery as a USDA inspector. A good can of food had 7-9 PSI vacuum after it had cooled and been stored for a while. So a glass canning jar should be safe at that level. I think it had to be at least 5PSI or load was rejected. It may have been 3 PSI?
    Max vacuum is 14.7 PSI.
    Bill D

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    A refrigerant filter housing would be ideal for your application. They are 4" dia. and varry in length from from about 12 to 30 ". The closed end is domed and the open end is flanged with a grooved gasket. They are rated over 400#. You would have to braze shut the inlet and outlet. Look for trade name Sporlan.

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    Doing this the hard way. Don’t use a threaded cap. Use a Victaulic clamp and grooved pipe. Rated for 300 psi or 600 psi and never leak. Used in fire sprinkler systems. We built RO systems with 2” stainless piping running at 1200 psi. For your purpose get a quick acting lever clamp, no tools to open it. Fire sprinkler guys can groove the pipe for you too.

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    Suppose if push came to shove a length of pipe and a couple of bolt on flanges may be a way, plenty of scrap short flanged bits in scrap piles due to fabrication “adjustments” they all seem to be in the 12-18” range ( not guilty your honour) mistakes do happen, you’d have no trouble with high pressure then,
    Mark

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    Quote Originally Posted by stephen thomas View Post
    My son has become involved in casting small parts, too; and asked me about methods to avoid voids.

    The more i read, the more sensible and convenient that vacuum sounds compared to pressure.




    In other apps, i once used a large glass peanut butter jar (probably no longer made?) as a perfectly safe and easy view vacuum chamber with a Cenco megavac. (Fairly hard vacuum) Glass bell jars & vacuum grease were the old lab standard. Ball & Kerr make canning jars in various sizes for safe storage under vacuum.

    smt
    ok, now THAT'S a really, really bad idea, a peanut butter jar for a "fairly hard vacuum"..."perfectly safe", LOL!. No, just NO. come on, Stephen, you know better.

    as to all the chatter about plumbing and "my pipes are fine", this ia a cyclicly loaded application that is literally "in your face". 0 to pressure and back, totally different than a relatively narrow variation of pressure in pipe in the wall, or running shop air or such. the chances that it blows while you are within arms reach is at least 3 orders of magnitude higher than a random burst in a piping system, I'd hazard a guess.

    having said that, I totally agree hydrotest is way overkill IF you take the simple, relatively cheap, readily accessible precaution of getting SCH 80 made in usa pipe and steel fittings. and why not just go seamless while you are at it. I'd be way, way more confident with that than any setup that used cast parts, hydrotest or not.

    anyone who has even a rudimentary grasp of fatigue failure or brittle fracture would understand, and it's so easy. why wouldn't you?

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  12. #48
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    The most pressure on the outside of a chamber you can get with a perfect, "hard" vacuum inside that chamber is atmospheric pressure at your location. This is generally about 14.7 PSI. While there may not be any official rating on a peanut butter jar, I am completely confident that it will withstand two, three, even as much as five times that much pressure, either from the outside or from the inside. 14.7 PSI is just not that much pressure.

    I think a pair of shop safety goggles would be more than enough of a safety back-up while using such a vacuum chamber at close distances. But please do not think that I am recommending a peanut butter jar, be it glass or plastic.

    However, moving on to the basis of the discussion here, namely removing air bubbles in an epoxy (plastic) casting. The stated goal of the OP is to "...crush any entrapped bubbles down to nothing." It would seem that the Combined Gas Equation (PV=kT) would not allow that. At best the use of higher pressures would reduce the size of any bubbles, but never eliminate them completely. Assuming that the temperature (T) remains constant, then pressure (P) and volume (V) of the air bubbles would be inversely proportional. If you double the pressure the volume is reduced to half (and the diameter of the bubble would only be reduced by the third root of 0.5 or 0.79 times. Not very impressive.

    But 120 PSI is greater than two times 14.7: it is about eight times greater. So, then the volume is reduced by a factor of eight (1/8 or 0.125X). But again, the diameter of the bubble is only reduced by the third root of that amount or about 0.5. Yes, that is correct. The reduction in the size, the diameter of the bubbles is only 50%. They will still be half the size as they were at normal atmospheric pressure.

    This third root thing makes it necessary to increase the pressure by the cube of the inverse of the desired size reduction factor. For a reduction to 1/10 of the original diameter, you must increase the pressure by 1000 times.

    Can you say "diminishing returns"?

    It seems to me that the vacuum route, which makes the bubbles larger, which in turn makes them float to the surface faster a much better option. Again that same Combined Gas Equation shows the volume (and diameter) of the bubbles going toward infinity as the pressure is reduced towards zero (a hard vacuum). Of course, long before the vacuum is perfect the bubble will have expanded to a size where some part of it does reach the surface even if it does not move upward at all. At that point the gas (air) in the bubble will be removed by the vacuum pump and the liquid epoxy will simply flow to fill the void. Therefore no bubbles in the casting.

    Vacuum (14.7 PSI in the negative direction) easily outperforms any reasonable amount of pressure that can be applied in the positive direction (120 PSI, 1000 PSI, or whatever).

    Note: Yes, there is a temperature term in that Combined Gas Equation and the temperature will NOT remain constant as a pressure or vacuum is applied. In the case of added pressure the temperature will increase and therefore the reduction in the size of the bubbles will be even less than I estimated above. So it works against you in that case. When a vacuum is applied the temperature will decrease and the expansion of the size of the bubbles will be somewhat less, but it will still tend towards infinity. At some point the temperature may be reduced enough to condense the gas into a liquid, but the bubble will reach the surface long before that point so the process still works to eliminate the bubbles.

    My point is: FORGET USING PRESSURE!

    DO USE A VACUUM! Vacuum is both safer and it works better to remove the bubbles.



    Quote Originally Posted by stephen thomas View Post
    My son has become involved in casting small parts, too; and asked me about methods to avoid voids.

    The more i read, the more sensible and convenient that vacuum sounds compared to pressure.




    In other apps, i once used a large glass peanut butter jar (probably no longer made?) as a perfectly safe and easy view vacuum chamber with a Cenco megavac. (Fairly hard vacuum) Glass bell jars & vacuum grease were the old lab standard. Ball & Kerr make canning jars in various sizes for safe storage under vacuum.

    smt

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    Quote Originally Posted by cyanidekid View Post
    ok, now THAT'S a really, really bad idea, a peanut butter jar for a "fairly hard vacuum"..."perfectly safe", LOL!. No, just NO. come on, Stephen, you know better.

    as to all the chatter about plumbing and "my pipes are fine", this ia a cyclicly loaded application that is literally "in your face". 0 to pressure and back, totally different than a relatively narrow variation of pressure in pipe in the wall, or running shop air or such. the chances that it blows while you are within arms reach is at least 3 orders of magnitude higher than a random burst in a piping system, I'd hazard a guess.

    having said that, I totally agree hydrotest is way overkill IF you take the simple, relatively cheap, readily accessible precaution of getting SCH 80 made in usa pipe and steel fittings. and why not just go seamless while you are at it. I'd be way, way more confident with that than any setup that used cast parts, hydrotest or not.

    anyone who has even a rudimentary grasp of fatigue failure or brittle fracture would understand, and it's so easy. why wouldn't you?
    14.7 psi is a really really bad idea?

    Oh well.....

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    Quote Originally Posted by EPAIII View Post
    The most pressure on the outside of a chamber you can get with a perfect, "hard" vacuum inside that chamber is atmospheric pressure at your location. This is generally about 14.7 PSI. While there may not be any official rating on a peanut butter jar, I am completely confident that it will withstand two, three, even as much as five times that much pressure, either from the outside or from the inside. 14.7 PSI is just not that much pressure.

    I think a pair of shop safety goggles would be more than enough of a safety back-up while using such a vacuum chamber at close distances. But please do not think that I am recommending a peanut butter jar, be it glass or plastic.

    However, moving on to the basis of the discussion here, namely removing air bubbles in an epoxy (plastic) casting. The stated goal of the OP is to "...crush any entrapped bubbles down to nothing." It would seem that the Combined Gas Equation (PV=kT) would not allow that. At best the use of higher pressures would reduce the size of any bubbles, but never eliminate them completely. Assuming that the temperature (T) remains constant, then pressure (P) and volume (V) of the air bubbles would be inversely proportional. If you double the pressure the volume is reduced to half (and the diameter of the bubble would only be reduced by the third root of 0.5 or 0.79 times. Not very impressive.

    But 120 PSI is greater than two times 14.7: it is about eight times greater. So, then the volume is reduced by a factor of eight (1/8 or 0.125X). But again, the diameter of the bubble is only reduced by the third root of that amount or about 0.5. Yes, that is correct. The reduction in the size, the diameter of the bubbles is only 50%. They will still be half the size as they were at normal atmospheric pressure.

    This third root thing makes it necessary to increase the pressure by the cube of the inverse of the desired size reduction factor. For a reduction to 1/10 of the original diameter, you must increase the pressure by 1000 times.

    Can you say "diminishing returns"?

    It seems to me that the vacuum route, which makes the bubbles larger, which in turn makes them float to the surface faster a much better option. Again that same Combined Gas Equation shows the volume (and diameter) of the bubbles going toward infinity as the pressure is reduced towards zero (a hard vacuum). Of course, long before the vacuum is perfect the bubble will have expanded to a size where some part of it does reach the surface even if it does not move upward at all. At that point the gas (air) in the bubble will be removed by the vacuum pump and the liquid epoxy will simply flow to fill the void. Therefore no bubbles in the casting.

    Vacuum (14.7 PSI in the negative direction) easily outperforms any reasonable amount of pressure that can be applied in the positive direction (120 PSI, 1000 PSI, or whatever).

    Note: Yes, there is a temperature term in that Combined Gas Equation and the temperature will NOT remain constant as a pressure or vacuum is applied. In the case of added pressure the temperature will increase and therefore the reduction in the size of the bubbles will be even less than I estimated above. So it works against you in that case. When a vacuum is applied the temperature will decrease and the expansion of the size of the bubbles will be somewhat less, but it will still tend towards infinity. At some point the temperature may be reduced enough to condense the gas into a liquid, but the bubble will reach the surface long before that point so the process still works to eliminate the bubbles.

    My point is: FORGET USING PRESSURE!

    DO USE A VACUUM! Vacuum is both safer and it works better to remove the bubbles.
    I agree on every point presented here.

    BUT

    Under hard vacuum, the volitiles in two part epoxy will generate their own bubbles.

    600mm of Hg is more than enough

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    Thanks, EPA & Cal!

    There's nothing wrong with the peanut butter jar as a vacuum chamber. I used it for other vacuum apps, as well.

    Under hard vacuum, the volitiles in two part epoxy will generate their own bubbles.
    This was the limiting factor in the referenced experiments. Vacuum never made enough difference in penetration to be worth the effort and wasted epoxy for the materials i was trying to impregnate.

    It is also the self limiting factor for molding resins discussed by the OP - watch, and if the substance boils, back off.
    Ideally use a vac gage in the system somewhere so once the vacuum induced BP is established (for a given ambient temp) a practical lower target can be used for succeeding efforts. (process control )

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    Quote Originally Posted by stephen thomas View Post
    Thanks, EPA & Cal!


    It is also the self limiting factor for molding resins discussed by the OP - watch, and if the substance boils, back off.
    Ideally use a vac gage in the system somewhere so once the vacuum induced BP is established (for a given ambient temp) a practical lower target can be used for succeeding efforts. (process control )
    The suction side of a "dry" Bell and Gossett diaphragm, or rocking piston air compressor is all that is required.

    Heck, the air pump out of an Audi Allroad would do the trick!

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    Quote Originally Posted by CalG View Post
    The suction side of a "dry" Bell and Gossett diaphragm, or rocking piston air compressor is all that is required.

    Heck, the air pump out of an Audi Allroad would do the trick!
    Or a "curbside find" de-humidifier compressor....

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    Ditch the threaded water pipe idea.

    Buy a heavy duty pipe spool of the desired length and diameter with bolt flanges. Also buy 2 blank plates and gaskets plus a few spare gaskets. Then drill and tap an end plate for the desired fittings. If you machine "half moon" thread plates for the removable end the bolts can quickly be run in and tightened (by a cordless impact driver) without the need for a wrench holding the nuts.

    PS: they sell segment nuts for flanged fittings but they are usually only 2 hole and properly fitted half moon units would be much quicker to place.

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    Quote Originally Posted by CalG View Post
    I agree on every point presented here.

    BUT

    Under hard vacuum, the volitiles in two part epoxy will generate their own bubbles.

    600mm of Hg is more than enough
    Very little volatiles in most casting epoxies. Takes me about 15 seconds under vacuum to degass epoxy samples. The air comes out that fast.
    Biggest danger is using too small a container so the material foams over the edge and goops up the bell jar. 1) use a larger container than you think
    is needed, 2) put some kim-wipes in the bottom just in case.

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    Just a general statement for those who are saying "vacuum is less than 15 psi, what are you afraid of?"

    Vacuum is a different animal than pressure entirely. One might not think so because it is simply just pressure, but the shapes of things are all wrong. In an enclosed internal pressure vessel, the tensile forces are pulling at every molecule in the vessel, this causes it to naturally stretch into a uniform round shape. Under vacuum, the compressive forces are pushing against each other in less-predictable ways, which can cause any number of distortions.

    Put simply:

    Every differential unit of strain from internal pressure makes your vessel better at holding it.

    Every differential unit of strain from external pressure makes your vessel worse at holding it.

    Glass is pretty stiff so the effects are minimal, but as others mentioned any changes in thickness or material flaws are going to amplify those effects at the weakest point in the vessel, that's why it's more hazardous than it seems at first glance.

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    Vacuum is the standard when using epoxies for metallographic analysis of porous materials (coatings, etc). In this application you need as much infiltration as possible and bubbles will ruin the specimen. The vessels are clear, so you can reduce the vacuum if you get too much bubbling too fast. They are also set-up to pour under vacuum:

    https://users.4dlabs.ca/uploads/docu...-Embedding.pdf

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    Quote Originally Posted by Hightemp View Post
    ... The vessels are clear, so you can reduce the vacuum if you get too much bubbling too fast. ...
    Looks good.

    Looks good.

    Looks good.

    (blooorp!!)

    AAAAAAHhh!!!!

    (stycast all over the inside the vacuum jar)

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    Quote Originally Posted by digger doug View Post
    Or a "curbside find" de-humidifier compressor....

    That's an oil sealed mechanical pump. It sucks too hard for degassing epoxy! ;-)

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    Quote Originally Posted by CalG View Post
    That's an oil sealed mechanical pump. It sucks too hard for degassing epoxy! ;-)
    I'm sure you can regulate it down....


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