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screen printing => Screen Making => Topic started by: Frog on April 28, 2014, 11:01:32 AM
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The way I understand it, only photopolymer emulsions benefit from this second exposure, made after development. Conventional diazo emulsions need not apply, sorry guys.
My question, assuming this is indeed the case, does adding diazo to a photopolymer emulsion for added toughness now negate this (ability to benefit from post exposure)?
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Nice Pun! This may be a philosophical question :o
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Adding diazo increases exposure times by at least 25%, maybe that is where the added toughness derives from...or not? We need a chemist for this one. :-\
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Adding diazo increases exposure times by at least 25%, maybe that is where the added toughness derives from...or not? We need a chemist for this one. :-\
I went back and made my question clearer.
I want to know if merely adding Diazo to a Photopolymer emulsion now makes post exposure non-effective.
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Maybe the addition of diazo does not negate the benefits of post exposure...
• SBQ emulsion remains light sensitive after development. Post expose squeegee side to sun or lamp on long WB or discharge print runs.
http://murakamiscreen.com/creating-durable-screens-for-waterbase-discharge-and-plastisol-ink-systems-2/ (http://murakamiscreen.com/creating-durable-screens-for-waterbase-discharge-and-plastisol-ink-systems-2/)
Since Murikami recommends the addition of diazo to HVP for dc/wb printing, the above statement in their technical literature, would lead me to believe that post exposure is effective.
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Personal experience here says that post-exposure does indeed help dual cure emulsions.
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Adding diazo increases exposure times by at least 25%, maybe that is where the added toughness derives from...or not? We need a chemist for this one. :-\
Not to derail too badly, but I swear that Kiwo used to have this sort of instruction in their one coat tech sheet. Now all it says is that adding a diazo will increase water resistance. As far as I can tell, my previous exposure times still work, no overexposing. Speaking of, even in this SBQ's tech sheet, Kiwo sort of poo-poos the idea of post exposure.
Post-exposing the screen after developing and drying is not very
effective. To improve the resistance 10-15% the post-exposure time
needs to be four times the original exposure time. Exposing the
screen fully with the primary exposure offers better resistance than
under exposing initially, then post-exposing to improve resistance.
Post exposure is most often used for long printing runs when water
based and/or abrasive inks are used.
Though I suppose if it only takes 15 seconds to expose, a one minute post expose isn't a big deal.
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Kiwo does not require a post exposure. Really non of them do if you expose it correctly the first time. We do not post expose any of our discharge screens. We have even stopped leaving them out in the sun after hardener has been applied.
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IMHO if you expose screens that are always slimy inside because of a weak light, or really thick stencils, it's worth hitting the inside with the sun or light again, but sounds like everyone is on the right page in exposing properly the first time...
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A rep from Saati sill be here tomorrow. I'll get his input and report back if you'd like.
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I think there's a lot more in common between "pure photopolymer" and "dual cure", etc. these days. I've heard that they are all dual cures, some get diazo, some don't. Then you have ones like Nova and HVP, etc. that are "pure photopolymer" which often get diazo added....not to mention whatever the hell these "hybrid" presensistized ones are.
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Ulano Rep will be at my shop tomorrow, tons of questions for her.
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Quoted from a 1999 article.
Yeah that time known as way back when the internet wasn't a place to go for information and you had to actually do the testing work yourself on expensive consumables to find out what did, and didn't work.
"Post-exposure is often widely believed to compensate for an underexposed stencil. Some emulsion manufacturers claim it offers no benefit, while others say it can marginally improve durability. All agree that it does nothing to reverse the dot gain or loss brought about by underexposure. They also agree that benefits would only be seen on slightly underexposed screens, and the first line of attack should be to expose screens correctly. And that, they say, frequently means changing one or more of the variables in the exposure process."
From an article on Screen Web about correct exposure. (http://www.screenweb.com/content/calculating-a-good-dose-exposure#.U18nd_ldWSo)
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Maybe the addition of diazo does not negate the benefits of post exposure...
• SBQ emulsion remains light sensitive after development. Post expose squeegee side to sun or lamp on long WB or discharge print runs.
From what the saati rep said that was in my shop a few weeks ago, this is how he explained it too from what I remember & understand.
Murphy37
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To clarify, when I refer to post exposure it is for DC over 200 pcs; using emulsion as a blockout and the post exposing. And yes harden and dry overnight. Extra steps for sure but payoff is POM
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Yes everyone, Pure Photopolymers remain sensitive to post exposure.
Even when it is part of a dual cure, or adding diazo, the SBQ sensitizers can still be exposed further.
For diazo emulsions, once the diazo is exposed to water it loses most of its' exposure capability. Post exposing in the
sun only helps dry out the emulsion film, which is a good idea with discharge.
Some good reasons to post expose:
Pure Photopolymer is easier to reclaim. If you underexpose SBQ emulsion and use Screen Opener it will flash any unexposed sensitizer and make reclaiming difficult.
Dry screens are better for water base and discharge. It is possible to overdry in really cold dry conditions. 30-35% is an optimum humidity to dry in.
By aiming the squeegee side toward the sun or lamp you expose the side that had weak light during the exposure process. This is the side that needs strength for Discharge.
Post exposure however is a band aid on the exposure process. Well dried screens, good light source, proper time are at least 85-95% of the strength of the emulsion during print. Hardeners and Post exposure help, but should not be a substitute for proper intial exposure.
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Agreed
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Yes everyone, Pure Photopolymers remain sensitive to post exposure.
Even when it is part of a dual cure, or adding diazo, the SBQ sensitizers can still be exposed further.
For diazo emulsions, once the diazo is exposed to water it loses most of its' exposure capability. Post exposing in the
sun only helps dry out the emulsion film, which is a good idea with discharge.
Some good reasons to post expose:
Pure Photopolymer is easier to reclaim. If you underexpose SBQ emulsion and use Screen Opener it will flash any unexposed sensitizer and make reclaiming difficult.
Dry screens are better for water base and discharge. It is possible to overdry in really cold dry conditions. 30-35% is an optimum humidity to dry in.
By aiming the squeegee side toward the sun or lamp you expose the side that had weak light during the exposure process. This is the side that needs strength for Discharge.
Post exposure however is a band aid on the exposure process. Well dried screens, good light source, proper time are at least 85-95% of the strength of the emulsion during print. Hardeners and Post exposure help, but should not be a substitute for proper intial exposure.
Bingo, that's what I was wanting to see!
Not that others didn't express similar points, but hearing it from a representative of the the manufacturer of the first pure photopolymer emulsion that I ever saw and worked with, can be accepted (at least by me) as gospel.
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Should also clarify the same as Tony, we put screens out in the sun squeegee side up. The effect has probably as much to do with post exposure as it does thoroughly drying the screen. Hell, its free insurance.
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spent 3 hrs with the Ulano rep today, and the verdict was, EXPOSE CORRECTLY THE FIRST TIME, post exposure is NOT necessary. So they say...
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I can tell you from years printing discharge it helps to post expose, not all SBQ pure photopolymer sensitizer molecules within the emulsion are completely cross linked. Consider that the print side got full wattage of light, while the squeegee side,(the side that breaks down) received far less light due to the weakening of the light ray as it goes through the emulsion. This takes into account in all my travels to hundreds of shops that I hardly ever find a shop exposing emulsion enough. There is quite a bit of extra cross linking that occurs in pure photopolymers during post exposure. The trouble is most shops do not have high wattage systems anymore. Even a metal halide at 8k exposes better than a 5k. So post exposure is insurance with benefits. Screens reclaim easier, discharge runs with my emulsion can hit non stop production, and post exposure costs nothing! Low wattage systems, Fluo tubes, 1k, LED (as good as they are), benefit from post exposure. For diazo emulsions the sun is a fast drying oven, once water hits diazo during development sensitivity drops. So expose your dual cures and diazos completely since post exposure of these emulsions dries them well in the sun and provides some additional strength for dual cure emulsions.
Al
Murakami Screen