Post Exposure - what's the scoop?

[Frog]

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)?

[kirkage]

Nice Pun! This may be a philosophical question 

[Orion]

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.   

[Frog]

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.

[Orion]

 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/

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.

[ebscreen]

Personal experience here says that post-exposure does indeed help dual cure emulsions.

[StuJohnston]

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.

[JBLUE]

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.

[ScreenFoo]

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...

[bimmridder]

A rep from Saati sill be here tomorrow. I'll get his input and report back if you'd like.

[ZooCity]

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.

[dirkdiggler]

Ulano Rep will be at my shop tomorrow, tons of questions for her.

[jsheridan]

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.

[Underbase37]

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

[tonypep]

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