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Old 11th May 2009, 03:53 PM   #1
pjanda1 is offline pjanda1  United States
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Default Cheap and easy vacuum impregnation

A couple of weeks ago, I was bemoaning a buzzy power transformer. Poinz was kind enough to share his experiences with vacuum impregnating power transformers and it got me to thinkin'. I messed around with a couple of ideas, but I also had several conversations with my neighbor, a materials scientist who has done quite a bit of vacuum impregnation both in industrial and diy settings. He suggested that I use ziploc vacuum food storage bags, as he figured I could pull about as much vacuum with them as with any other DIY method. He also figured that it would be highly advantageous to first pull the vacuum, and then introduce the varnish. So, here is the rig I setup:
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Old 11th May 2009, 04:04 PM   #2
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Everything is sealed up with flexible butyl caulk that my engineer friend refers to as "dum dum". You can get it at the hardware store, though I don't remember the exact name as I bought it years ago to use for speaker gasketing. The bag is 1qt. The most difficult thing is getting a good seal to the tube. In addition to the caulk, I used a homemade plastic washer.

The pitcher was chosen for it's stiffness, sealed lid, and lid hole size (more about that later). I drilled a hole in the bottom and sealed the PVC tube in with more butyl caulk.

The tube (IIRC) is 3/8" id. I needle nose vicegrips for a valve.

I used a regular natural varnish. I didn't think that the UV protectors in Spar would do anything good. And urethane varnishes aren't as good for heat and vibration. I heated it on the grill (see the silicon remnants of a failed vacuum experiment on the pan's rim). At around 150 degrees F, it was smoking and I decided to stop there. I don't think it was actually smoke, but rather some plasticizers evaporating/boiling off. Still, I didn't want to mess a potential fire, so I took it off the heat.
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Old 11th May 2009, 04:10 PM   #3
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When I had everything warm (I warmed the transformer in a toaster oven), I bagged the transformer, clamped my valve, and pulled the vacuum. (Then took the pic below). I dumped the varnish into the jug. To get it started down the tube, I loosened the vice grips a bit and let the vacuum pull some down. I clamped the vice grips again just before the varnish passed. Then I held the pitcher up and managed to get all the air out of the tube. I then evacuated the bag again.
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Old 11th May 2009, 04:11 PM   #4
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Then I opened up the valve and let the varnish flow:
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Old 11th May 2009, 04:22 PM   #5
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(I actually took that last pic after the next step.) No matter what I did (I practiced with a junk transformer and water to perfect my technique), I still had a few air bubbles. So, once I introduced the varnish, I toss the lid on the jug. I had one of the bag valves sealed into the lid (over the hole) with caulk. So, once I had varnish in the bag, I vac'd a bit on the jug. That pulled (most) of the bubbles out of the bag. Then I clamped the 'valve' and took the above pic. I admired it for a couple of minutes.

I then picked the bag up higher than the jug, and unclamped the valve. I inverted the jug over the pan and dumped most of the varnish. Gravity sucks most of the varnish out of the bag. Eventually a buggle crawls up the tube and you loose vacuum.

It is still curing, so I can't tell you whether or not it worked. I wanted to post this prior to potentially discovering it was for naught. I think the method is pretty solid, especially for a total investment of $10 or so. If it doesn't work, I'll probably give it another shot, as I've got nothing to lose. The transformer had quite a buzz, so I think it is possible that nothing will work.

You could also use the above method with a real vacuum pump. You don't need to use ziploc bags either. I haven't done it, but evidently you can take sheets of plastic and build whatever size bags you want with the butyl caulk. Of course, you'd need another hose and valve, one for the vac pump, one for the varnish.

happy sucking,

pj
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Old 11th May 2009, 06:28 PM   #6
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cool. What are you using to create a vacuum (i actually have an old Russion dental vacuum pump, might be useful for yjos kind of thing)

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Old 11th May 2009, 06:43 PM   #7
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It's a Ziploc hand pump. I picked it up for a couple of bucks at my grocery store. Reynolds has a battery powered system, but I can't imagine it can pull as hard. I'd imagine some folks have "Foodsaver" systems, and that would work well (electric vacuum sealing machine). They even have some marinating containers that have a hose and a valve that one could adapt to other pumps (dental or otherwise).

None of those things pulls as high a vacuum as dedicated pumps. My friends view is that you don't need an incredible amount of vacuum, but that it is highly advantageous to pull the vacuum before introducing the varnish. He/his factory is getting a big uber-expensive screw vacuum pump, so perhaps if I fail at this, we can subject it to some real serious suckage.

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Old 11th May 2009, 06:47 PM   #8
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Your post on this subject has also goosed an idea that i've had before... turning (rough) tweeter horns out of cedar, drying them to an extreme extent, and then vacuum imprednating them (a low viscosity epoxy has been suggested as perhaps the best bet)

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Old 11th May 2009, 07:00 PM   #9
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That's much closer to what my friend has used diy vacuum impregnation for: various canoe and sailboat parts. Depending on the size of horn you are thinking of, the bag has some huge advantages. You can squeeze most of the air out before sealing it, so you don't have to spend nearly as much time vac'ing as you would with a rigid container. As I was googling to figure this out, I also ran across folks who use vacuum to laminate big diy airplane parts. I followed one guy's example and glued a hose barb over the air intake of a little 12V air compressor. It did pull some vacuum, but I was skeptical. The originator of the idea claimed he could pull 25 inches of mercury in seven gallon bags for over an hour. My friend figured I'd be better off with the ziploc pump.

I just impregnated the transformer last night. I really want to put it together tonight. But I know I should wait a day or two.

pj
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Old 11th May 2009, 07:12 PM   #10
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As I think about it, perhaps what you really want for wood horns is vacuum and then pressure. Those machines are expensive! If you just wanted to get some pressure, but not a crazy amount, perhaps you could pull it off in an large old pressure cooker. I occasionally stumble upon some that look to be around 12 Qts. Figure out how much pressure they take, rig up some valves, and you'd be set.

pj
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