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  1. #16
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    May 2012
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    I'm not an expert and no doubts others will chime in, but in my limited experience acrylic will always develop cracks under vacuum or when using resin, especially more so when it has holes drilled for connectors or gauges. I started with a 12mm thick one, which promptly died, I then went with a 20mm thick one, and found it developed stress fractures really quickly, but I have been using it for some time now and it seems to hold up.

    The really stupid thing is you always see these tops with holes drilled for fittings exactly in the centre, the worst spot possible to hasten it's demise!

    I plan to make another top, and move all the fittings to the top side of the tank to make it last longer. I've also looked into the thick glass tops you can get from the States, but they're pretty expensive. I'll have to save my pennies for one though, I've heard their the bees knee's.

    With the bubbles, so long as your pulling them, i'd leave the vacuum running. When it's no longer producing bubbles, all the air it can pull has been pulled. Some like Casey Martin have said they leave their pump running for weeks sometimes depending on the wood.

    The system should be capable of using any vacuum pump, so I wouldn't worry about the design, usually the acrylic lid will actually be the weakest point. Pulling a vacuum is a finite process, depending on a number of factors, including relative sea level, it's not like an air compressor that you can keep increasing the pressure well beyond a safe point.

    I've had different results with different pumps, my fave one for low noise and long running is an oilless US made diaphragm pump, a little more, and doesn't seem to pull quite as much as the cheap Chinese oil pumps, but more than adequate for the task.

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  3. #17
    Join Date
    Jun 2016
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    Thanks for your advice, makes me feel more comfortable with running the system.

  4. #18
    Join Date
    Apr 2015
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    Warragul Vic
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    My experience is only in labs not for stabilising wood with CJ. Neil has a lot of practical experience .. but that wont stop me from giving my opinion )

    Acrylic crazing may be a result of chemical attack from resin or solvents (some plastics are NOT chemically resistant and will craze first and can crack disintegrate catastrophically.

    Vacuums and pressure are dangerous to deal with especially with some equipment and lack of knowledge or care. Implosions can be as bad as explosions. The large the lower the quality and bigger size of the gear the more dangerous.

    There are a few ways of making a vacuum. The purpose is to remove all the air and mosisture from the cells and tissue of wood and replace them with a resin. Pumps are used for the removal of air & volatile. Air pressure pushes the resin in from around the wood. These are pumps I’m familiar and of course there are others.

    Venturi pumps (water or air pressure) offer moderate vacuum at low cost but have risks in use, but they are small and inexpensive. Some will work from just household / town water pressure but water use and sucking back can be a problem. I believe others can run on air pressure. Some have ball valves to stop suck-back but a trap offers extra security from a change in water pressure. They good for reducing air pressure (1 atm about 760 mm down to 15-25 mm or 20 mbar ie removing 97% of the air from 5-10 L vessels. Never turn them off at water supply before admitting air into the vac line or vessel and only after isolating the vac chamber first.

    Diaphram pumps are also good for moderate to low vacuum. They are medium-low cost, reliable & don't use oil just power for pump which creates vacuum by a reciprocating movement of rubber or Teflon diaphragms. Pump design is oil-free environment which is easy to maintain and are corrosion resistant (with proper diaphram selection) and easy to maintain. They will remove 97-99% of air.

    Piston and displacement high vac pumps use vacuum oil and produce the best vacuum BUT require care, oil changing etc. We always used dry ice traps (to condense water, volatiles / fumes while can cause internal corrosion and damage) and particle filters (sometimes) to protect them and their oil from corrosion and contamination. They are the most expensive to buy and run. There are overkill in my opinion when the above pumps will do. They can be useful for ultra drying of wood though.

    Neil may disagree with some of this but these are my views. Euge

  5. #19
    Join Date
    Apr 2005
    Location
    Nerang Queensland
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    66
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    Quote Originally Posted by Dave Reed View Post

    • Am I running the pump for too long at a time? (if I turn it off the bubbles slowly stop after about 20 minutes and then restart quite strongly if I turn the pump back on even though the gauge doesn't show any difference in level of vacuum. But I'm afraid of leaving the pump running to maintain this higher rate of bubbles evacuating in case the lid fails altogether)




    • Can a cheap vacuum pump produce more vacuum than the system is designed for?




    • Is it just a crappy system?
    What you bought was a degassing system, not a stabilising system. It is made to degas resins under low vacuum levels.

    The acrylic lids are only designed for low levels of vacuum and will fail, as you have found out, under larger vacuum levels required for stabilisation. Even thick acrylics (depends on pot diameter, but I've seen 50mm fail), although will last longer, will ultimately fail. This is because of the flexure (why you have cracks on inside as the lid bends down/in) but also because continued contact with most stabilising resins will slowly dissolve the acrylic. Polycarbonate may be stronger but is more flexible and also turns white from the stabilising resin.

    Best for those sort of pots are strengthened glass with the hose/connections in the side of the pot. I have a 400mm pot with 20mm strengthens glass off Ebay that is still going. All my other chambers are from Curtis in America (Cactus Juice Stabilizing Resin, Chambers, and Accessories) and can be ordered on line, although the Cactus Juice that he makes is available here in Aus (Timberbits).

    Stabilising is not a cheap process and trying to do it on the cheap will lead to failure, both in achieving a stabilised piece of material (timber/stone etc), but also as you have found structural failure that can be extremely dangerous. To make up a decent setup with a number of coloured stabilising resins etc your not going to get much change from $1K. There is the vacuum chamber(s), vacuum pump, and resin and dyes, oven, refrigeration, storage containers, the list goes on and it all adds up. It also takes time, often with Aus hardwoods, a lot of it.

    Good luck
    Neil
    ____________________________________________
    Every day presents an opportunity to learn something new

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