Post Exposure - what's the scoop?

[ABuffington]

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.

[tonypep]

Agreed

[Frog]

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.

[ebscreen]

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.

[dirkdiggler]

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...

[ABuffington]

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