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Dual cure emulsion 30 days but I had one for over 60. Pure photopolymer they say 30 days I just exposed 3 that were 66 days. I keep my screen box full all the time.
 

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i know emulsion only lasts 3 months or so..
What type emulsion are you using that only lasts 3 months ? We use Ulano LX660 and I have a gallon that was mixed in November 2011 that still works great. I've had this stuff go over 6 months before without any problems. As far as exposure times with an screen that's been coated a little too long, seems all it needs is extra time to wash out... the expose time seems to be the same. I've pushed some beyond the extreme also... just took way longer to wash out.
 

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Is there a need to increase the exposure time?
I don't, if any thing it may shorten the time by a few sec, I also have used dual cure over 6 months old. I think it has a lot to do with storage and how it's handled.

The little extra wash out time is usally cause as it gets older in the 6-8 month it needs less exposure time. Just my experience yours may vary.

I switched to pure photopolymer and 1-2 year shelf life
 
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I'm not quite sure why so many people are having problems with emulsion that's a few months old--unless it's stored in a very warm, or perhaps coated screens in a light-unsafe environment.

I just exposed a 125 I coated at least three months ago--as Jaime mentioned, it just took longer to wash out.

If you're running thousands of shirts, or shooting some sort of insane halftone work, it may make a difference if it's sat around for a few months, but I don't think most people who are being told to throw out their emulsion are doing either of those things... I would, however, be quite interested if anyone has info on the science behind how age changes a diazo sensitized emulsion...
 

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Problems with diazo. I think a lot of people use tap water to mix. I am no chemist but what I have read minerals, chlorine, and all the other stuff in tap water is very bad for diazo. Heat and humidity also desensitize the diazo. Even bottled water contain contaminates and distilled water is preferred.

It goes bad with age due to the water from what I understand. In powder form last a very long time soon as water is added the clock starts ticking not as fast as others claim though. I actually under mixed with water most the time then after adding to pot put a small amout of water in the diazo bottle shake and add to get all the diazo in the mixture I could. And never used all the water required maybe why I was able to obtain a very long shelf life.
 

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I've let them sit a few months with no issues. At my work, we burn only a few screens/month and I'm still using a quart of TZ I mixed back in early November. Refrigerating it was the key there.

I will agree that for finer detail, the time you allow them to sit will affect you, at least that's been my experience (tight lines closing, etc).

I don't think the time is as much a factor as the environment you're keeping them in. It's my understanding that the breakdown happens mostly due to the water, you're evaporating the water, so a dry screen should be good to go for a while as long as it's dark, dry, and a good cool consistent temperature. That's how I keep my dark room. It's on the other side of the shop, no heat in there so the winter was kind to my stored screens, just keep a space heater in there and run it up before I need to do some work.
 
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