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Step 16Checking For and Dealing With Leaks
DrCrash Shared some good ideas on testing the system for leaks, as well as splicing a clever vacuum reservoir into the system. At some point I'll make one to illustrate, but for now, just text:
You can use the vacuum pump itself as a crude vacuum gauge, to tell whether your bag is leaking. Just stop up the end of the hose, and pull on the pump, and feel the resistance to "calibrate" your judgment of the vacuum level. Assuming the pump is working right, the resistance will top out at about 5/6 of a full throw.
If you feel noticeably less resistance after pumping the bag down, you have a leak or significant outgassing. If it's loosing air, carefully look around your bag assembly for any possible wrinkles in the plastic which would let air by. I've sometimes found that by carefully stretching the seal along the length of the tape where a wrinkle has occurred can help get the tape to adhere to the wrinkled plastic.
You can also enhance the setup with a vacuum reservoir, so that the vacuum level drops more slowly when you have a very tiny leakage, or outgassing from your resin.
The vacuum reservoir can just be a glass gallon jug, if you put it in something like a sturdy trash can in case it implodes. (Not likely just due to vacuum, but if you accidentally drop it or bang something against it, it could implode with a bang, and send glass flying on the rebound.)
That will maintain the vacuum level in the face of moderate outgassing and very tiny leaks. If you come back every now and then and pump until you feel the usual resistance, you can "top off" the vacuum. (If the resistance has dropped much, you need to be doing it more often.)
The usual way to add a vacuum reservoir is to just use a tee between the pump and the bag, and connect the reservoir "off to the side". I got a rubber stopper and a double-ended hose barb (hose butt splice) for my (apple juice) jug, drilled a hole in the stopper, and shoved the barb in. (I think the rubber stopper was about $3 and the nylon 1/4" ID hose butt splice was about a dollar, both at Lowe's.)
Another way, which doesn't require the extra plumbing, is to make the bag oversized and put the vacuum reservoir right IN the bag. For example, you could use a quart mason jar with a few holes punched in the lid, and some breather material taped over the holes to keep them from getting blocked. (Wrap the mason jar in duct tape to lessen the risk if it gets broken somehow. It shouldn't break due to the vacuum, but you never know.)
The idea there is that the volume of air outside the layup but inside the bag is normally very small. Putting the vacuum reservoir on on or in the bag greatly increases the internal volume, so that any small amount of gas weakens the vacuum within that volume much less.
Any kind of reservoir will soak up minor outgassing, and maintain your vacuum level longer.
It will also make it easier to judge the vacuum level using the pump resistance. If the internal volume of your setup is very small, you'll raise the vacuum level a lot with one stroke of the pump, and might not notice the low resistance at the beginning of the stroke. With a quart of volume, and especially a gallon, you'll notice the weakened but increasing resistance over a few strokes.
One of my concerns about simple vacuum bagging setups like the Roarockit kit is how long they actually maintain the vacuum. I'm wondering if the roarockit kit loses most of its vacuum almost immediately, because any little bit of outgassing pollutes tiny volume within the bag. For vacuum pressing wood laminates, one of the sources of gas is just the air in the wood. Wood is very porous and dry wood contains a lot of air. (It's so porous that that air may mostly flow out immediately, while you're pumping, but I'm not sure more doesn't seep out over the next few seconds or minutes.)
For resins like most polyester fiberglassing resins, there's a fair bit of solven that evaporates out fairly quickly---in the case of polyester, it's styrene monomer. Liquids generally expand tremendously when they boil or evaporate, so that may be significant, too.
Without a reservoir or a continuously operating pump, I'm wondering if you only get one good squeeze at the beginning, and then your vacuum level rapidly drops due to outgassing. (That would not be obvious from just looking at the bag---a tiny amount of gas would be enough to lose most of your vacuum, but the bag would still look tight.)
One good squeeze at the beginning may be be good enough for many purposes, to squeeze out excess resin, but holding reasonably high vacuum until the resin is set would be better.
You can use the vacuum pump itself as a crude vacuum gauge, to tell whether your bag is leaking. Just stop up the end of the hose, and pull on the pump, and feel the resistance to "calibrate" your judgment of the vacuum level. Assuming the pump is working right, the resistance will top out at about 5/6 of a full throw.
If you feel noticeably less resistance after pumping the bag down, you have a leak or significant outgassing. If it's loosing air, carefully look around your bag assembly for any possible wrinkles in the plastic which would let air by. I've sometimes found that by carefully stretching the seal along the length of the tape where a wrinkle has occurred can help get the tape to adhere to the wrinkled plastic.
You can also enhance the setup with a vacuum reservoir, so that the vacuum level drops more slowly when you have a very tiny leakage, or outgassing from your resin.
The vacuum reservoir can just be a glass gallon jug, if you put it in something like a sturdy trash can in case it implodes. (Not likely just due to vacuum, but if you accidentally drop it or bang something against it, it could implode with a bang, and send glass flying on the rebound.)
That will maintain the vacuum level in the face of moderate outgassing and very tiny leaks. If you come back every now and then and pump until you feel the usual resistance, you can "top off" the vacuum. (If the resistance has dropped much, you need to be doing it more often.)
The usual way to add a vacuum reservoir is to just use a tee between the pump and the bag, and connect the reservoir "off to the side". I got a rubber stopper and a double-ended hose barb (hose butt splice) for my (apple juice) jug, drilled a hole in the stopper, and shoved the barb in. (I think the rubber stopper was about $3 and the nylon 1/4" ID hose butt splice was about a dollar, both at Lowe's.)
Another way, which doesn't require the extra plumbing, is to make the bag oversized and put the vacuum reservoir right IN the bag. For example, you could use a quart mason jar with a few holes punched in the lid, and some breather material taped over the holes to keep them from getting blocked. (Wrap the mason jar in duct tape to lessen the risk if it gets broken somehow. It shouldn't break due to the vacuum, but you never know.)
The idea there is that the volume of air outside the layup but inside the bag is normally very small. Putting the vacuum reservoir on on or in the bag greatly increases the internal volume, so that any small amount of gas weakens the vacuum within that volume much less.
Any kind of reservoir will soak up minor outgassing, and maintain your vacuum level longer.
It will also make it easier to judge the vacuum level using the pump resistance. If the internal volume of your setup is very small, you'll raise the vacuum level a lot with one stroke of the pump, and might not notice the low resistance at the beginning of the stroke. With a quart of volume, and especially a gallon, you'll notice the weakened but increasing resistance over a few strokes.
One of my concerns about simple vacuum bagging setups like the Roarockit kit is how long they actually maintain the vacuum. I'm wondering if the roarockit kit loses most of its vacuum almost immediately, because any little bit of outgassing pollutes tiny volume within the bag. For vacuum pressing wood laminates, one of the sources of gas is just the air in the wood. Wood is very porous and dry wood contains a lot of air. (It's so porous that that air may mostly flow out immediately, while you're pumping, but I'm not sure more doesn't seep out over the next few seconds or minutes.)
For resins like most polyester fiberglassing resins, there's a fair bit of solven that evaporates out fairly quickly---in the case of polyester, it's styrene monomer. Liquids generally expand tremendously when they boil or evaporate, so that may be significant, too.
Without a reservoir or a continuously operating pump, I'm wondering if you only get one good squeeze at the beginning, and then your vacuum level rapidly drops due to outgassing. (That would not be obvious from just looking at the bag---a tiny amount of gas would be enough to lose most of your vacuum, but the bag would still look tight.)
One good squeeze at the beginning may be be good enough for many purposes, to squeeze out excess resin, but holding reasonably high vacuum until the resin is set would be better.
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Oct 7, 2007. 11:53 AMRagged Edge
says:
Shortshift thanks for mentioning our kit. We have never really had a problem with offgasing causing loss of vacuum in our Roarockit Thin Air Press system. You may see a small loss of vacuum right after the initial pressing. I check my pressing for the first five minutes. Ten seconds of pumping with the manual pump always beings it back up to full vacuum. The small loss of vacuum is cause from the trapped air being pulled from the skateboard veneers and foam mold. Even when use epoxy in the Thin Air Press there seems to be no problem with holding vacuum. We have actually left sealed bags sit for months while travelling to find them still holding vacuum when we return. Ted from Roarockit
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