resin degassing and casting tank

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Hi,I've been lurking this site for awhile and thought I'd pose a question to everyone here about a project I'm working on....will this work or am I wasting my time?! Goal: 32"x48" low pressure (about 50 psi and 25" Hg vacuum) tank for thin castings of resinSo my pressure casting/vacuum chamber for thin panels is underway butI wanted some advice if you have any. I have not found any informationon rectangular pressure vessels as it seems to go against physics.Every welding forum (and this one) discourages it highly but I thought if it'soverbuilt it it would still work and thats also the consensus of most ofmy friends. If panels are done in large factories they build a flatarea inside a large cylindrical tank (industrial autoclaves). I'veheard/read that the corners are subject to the greatest pressures so Iwas going to cut down a piece of 3" pipe to round/radius the insidecorners. The second hurdle is "pillowing" - where flat areas would tryto become round I thought if I used enough channel / stiffeners I couldprevent this. My build now is 1/4" thick plate that I've mig welded(> 75% penetration) from both sides thenground flat for the lid and bottom. This will be coated with epoxy andlined with a thin silicone membrane to make it easier to clean andsolve any tiny air leaks. The channel that makes up the sides iscompletely mig welded all the way around and corners were welded bothsides, top and bottom and ground flat. The sides of the tank are 3/16"thick - 4" high channel. The channels run across the width of bottomand top flat areas as stiffeners.(I've got 6 -3" chanel for the bottomand 10 pieces of 2" chanel for the lid (everything was made fromremnants)  The window would have 3/4" thick acrylic through bolted andBUNNA seal gasket. This gasket material would also run the edge of the lidand would be compressed with 3" C-clamps every 6" around theperimeter. All the info I find online discourages this rectangular design as anytype of pressure vessel. Physics dictate that pressure tanks need tobe round as all pressure is distributed equally across the material.Since I'm only pressurizing to about 50 psi to crush "champagnebubbles" and vacuum to about 25" Hg....Do you think this will work oris it going to collapse or explode?? I still have quite a bit of workleft on it and don't want to be making mistakes early on. What do youthink? Thanks for any suggestions Attached Images
Reply:i suspect that will bulge out longbefore you reach 50 psi...
Reply:unless it leaks before it reaches high enough pressure to bulge...
Reply:First off, it is not that the "corners are subject to the greatest pressure".It is that the corners of the 'tank' are subject to the greatest STRESS caused by the pressure.Next, do you realize that a vessel 32 inch x 48 inch subjected to a pressure of 'just' 50 psi will have a force lifting that 'plate' of 76,800 pounds?There is a reason why pressure vessels are round/cylindrical.  They work 'better' that way.  The best laid schemes ... Gang oft agley ...
Reply:Or consider  24,576 pounds of crushing force at full vacuum  (-14psi).  ever boil water in a metal can, remove from the heat and put the top on???  Crinkle Crinkle.  maybe not as flat as a pancake, but pretty crushed in any event!!edit oh 25" hg = -12 psi = 18432 poundsLast edited by fredf; 12-01-2010 at 06:14 PM.-- fredLincoln 180C MIG
Reply:I have no idea what the intended use is of this, but I have built my own vacuum chamber for degassing resin for mold making & I can tell you that -25 is not going to be enough vacuum to fully degas the resin, it will start to cure before fully degassing... you want to hit like 29"hg vacuum. even with a vacuum chamber that goes down below 29"hg it can be a little challenging fully degassing before the resin starts to thermally react. I have just enough time to degass & inject before it starts to react & even there sometimes I'm cutting it a little closethe maximum attainable vacuum possible is something like 29.4"hg. this creates a whole lot of pressure on the vessel, enough to crush things that you would normally consider "strong"this is also dependent on the resin used... the purest forms of resin will be very thick viscosity & much harder to degas than some thinned down resins too... but the thicker resins will be higher performing resins, again, depending on the applicationI built my vacuum chamber from a very thick aluminum stock pot & used a 1 1/4" thick plexi lid... full vacuum is not a force to be taken lightly, you are dealing with an extreme amount of force here... your square box will crush itself imploding from the amount of forces exerted on it.. it would probably have to be several inches thick to have any hope of surviving...find yourself a very thick stock pot & build one from that. also don't bother playing around with only 25"hg, your going to need to hit over 28" at least to get resultsLast edited by turbocad6; 12-01-2010 at 07:36 PM.miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:Thanks for responses!Lets say weight isnt a concern since the lid will be raised mechanically. I have 3/8" plate I could add to the bottom. Lots of scrap steel also. Would it be better to frame a step for the window & thicker channel? Does anyone have a reference for flexion/strength of steel in plate or channel. How is the force determined with known pressure? I would never have imagined 76k lbs! Thanks again.
Reply:@ Turbocad6: Yes. I agree.  I assumed i couldnt structurally design this for as great a vacuum as needed and thought id have to compromise by degassing before injection/pour...mostly this project is for pressure casting thin panels.
Reply:also, I forgot to mention that the 29+"hg of vacuum I'm talking about is at sea level, not sure where you are but if your at lets say a 5k' altitude then yeah, 25"hg is about all you'll even be able to achieve. vacuum level attainable is all Dependant on atmospheric pressure...not any old vacuum pump will do for degassing either, you really want something that can hit one micron, which is over 28"hg... anything less will not degas well or at all...miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:it doesn't matter how minor the project is, either it will degas or it won't. if you need to extract all the air bubbles from the resin then you will really have to hit the one micron range or it just won't work period. it'll bubble & bubble until it hardens, never fully degassing...I built my chamber just to cast some clear reflectors in my headlights, that's it, simple small project, but any less than 28"hg it just would not work at all... if your only going to hit 25" then don't even bother, it won't work & your wasting your timemiller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:@Turbocad6: U may have solved one of the problems i was overlooking...not completely degassing. Im gonna do some more vacuum tests & maybe pressure casting for ths project can be avoided.Last edited by wooly; 12-01-2010 at 10:00 PM.Reason: re
Reply:a venturi system/dual stage pump sounds like the kind of setup that will go well below the 25" you originally mentioned. why are you so determined to make the chamber square? also I can honestly say that I don't know much about pressure casting at all, I assumed pressure casting was just using pressure to fill the mold but from what you've said about it I gather thats not it. are you planning on vacuuming & then casting in the same vessel? my thoughts are you will have a better chance of building a rectangular chamber to hold 50 psi than to hold 29"hg of vacuum... it's much easier to contain 50 psi than to resist collapsing at full vacuum. might be worth considering a separate chamber for vacuuming & another for your pressure casting?miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:Originally Posted by woolyThanks for responses!Lets say weight isn't a concern since the lid will be raised mechanically. I have 3/8" plate I could add to the bottom. Lots of scrap steel also. Would it be better to frame a step for the window & thicker channel? Does anyone have a reference for flexion/strength of steel in plate or channel. How is the force determined with known pressure? I would never have imagined 76k lbs! Thanks again.
Reply:Originally Posted by turbocad6are you planning on vacuuming & then casting in the same vessel? my thoughts are you will have a better chance of building a rectangular chamber to hold 50 psi than to hold 29"hg of vacuum... it's much easier to contain 50 psi than to resist collapsing at full vacuum. might be worth considering a separate chamber for vacuuming & another for your pressure casting?
Reply:Wooly, You may want to consider vacuum infusion for your vacuum resin impregnation, it is completely safe, not too expensive to set up and works well. This process generally doesn't produce air bubbles in the first place that you have to remove. You have a variety of different polymers or epoxies to select from, with also different types of curing times, viscosities and curing methods, heat, catalyst and UV. You can even make a vacuum holding tank like I did out of an old 300lt water heater. I have had mine down to 29.5mm Hg (2 stage 4 cubic ft/minute pump), you can even string a couple of tanks together with taps connected to a main line to add capacity.Miller Auto Invision 456 + S-62 wire feederC6240B1 Gap bed lathe16 ft3 air compressor16 speed pedestal drillHafco BS-912 Bandsaw
Reply:no argument from me that a failure on the vacuum end is much much safer, but that don't change the fact that it's easier to contain pressure than vacuum. take an automotive gas tank as an example... you could pressurize it to 50 psi & see no deformation at all (as long as it don't blow ), yet apply 29"hg to it & watch it crumple up   I do agree though that playing around with pressurizing a square container to 50 psi is not a very safe thing to do in the first place, as many others have also mentionedLast edited by turbocad6; 12-01-2010 at 11:02 PM.miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:Aqua Chem single effect evaporators are about that size and run 3/16ths CuNi side plating at 25" Hg.  I haven't seen one implode yet but I have seen many cracks after 30 years of use and salt water exposure.
Reply:years ago i did a project at dc's blue plains sheet plant..corp of engineers experimental project..included an ozonation chamber that some genius designed as a box..had to be 20' x 10' x 10'..top was a big plate with tons of channel stiffeners, had to be three hundred bolts around that top, maybe 3/8 plate, heavy gasket..it wqas supposed to hold very minimal pressure , less than 0ne psi, as ozone was bubbled into the tank..never ever worked no matter how mch we fottsed with it..leaked ozone at the top..youyr taxpayer dollars at work...
Reply:Thanks for all the informative replies. I'm shelving this one for now in consideration of how much more work is involved w still a high likelyhood of various failures. I can build a strong, safer cylindrical vacuume chamber just large enough to degass imediately after mixing the components & maybe try other composite manufacturers product. I thought I could degass and compress all in one step to help w curing window requirements. I'm considering earlier tests weren't degassed @ strong enough vacuume (29.5/1 micron as Turbocad6 suggested).
Reply:Originally Posted by turbocad6a venturi system/dual stage pump sounds like the kind of setup that will go well below the 25" you originally mentioned. why are you so determined to make the chamber square? also I can honestly say that I don't know much about pressure casting at all, I assumed pressure casting was just using pressure to fill the mold but from what you've said about it I gather thats not it. are you planning on vacuuming & then casting in the same vessel? my thoughts are you will have a better chance of building a rectangular chamber to hold 50 psi than to hold 29"hg of vacuum... it's much easier to contain 50 psi than to resist collapsing at full vacuum. might be worth considering a separate chamber for vacuuming & another for your pressure casting?
Reply:Not sure what product you are using. I can remember coming across some products like this when I was doing urethane rubber casting to make stamps. I don't remember which company it was anymore because of the length of time. You might look at Smoothon or Polytek for the resin. Both were very helpful in getting thru some of the wrinkles when we were working on stamps..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:in my opinion smooth-on is great for urethanes, silicones and even plastics, but for resin I've found a pretty good source of high quality stuff. check out polymerproducts out of canada. he does sell on ebay & also has a website. I spoke to the guy there that runs the show (forgot his name now) & I can't begin to tell you how very helpful he was... the guy actually spent well over 1/2 hour on the phone with me discussing so many aspects of resins with me to help with my project. turns out that the resins I've used before were all thinner viscosities but I was having a problem of my thin clear reflectors yellowing in the headlight capsules after a while from the heat of the hid's & the uv exposure from the sun... the guy was extremely helpful & his product really did the trick for me . he even helped me select a pump for my vacuum chamber, it's rare to find someone so helpful for such a small sale but the guy seems to have a genuine love for what he's doing & I got the feeling that he would have stayed on the phone with me all day had I been so inclined... he also has a lot of videos up showing his products usage & stuff... I'd recommend looking into it at least...from my experience once the resin is degassed then you can get water clear results as long as you don't reintroduce air bubbles in your pour. the resin I used was so thick that I could not fill my mold with just pouring so I developed a way to inject it into my mold & I was getting perfectly water clear lenses that would stand up well to heat & uv rays too... if you want some info on the way I did my injection lmk & I'll get you a few shots of what I did...miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:http://r3mfg.com/index.html
Reply:Just a thought. If for some reason you do try the rect tank then you will want to pressure test it with water first, and a lot higher than design pressure. First test should be vacuum though.