How best to set the emulsion?

[BartJY]

Somewhere I read something about re-exposing the screen to sunlight to harden the emulsion after you've washed out the uncured emulsion? Is that correct? If so, for how long?

Thanks
Bart

[inkman996]

That is only necessary if the emulsion is undercured or if the screen is for an ink system that is tough on the emulsion like water base or discharge. For plastisol under cured is not really a big deal unless you use hot solvents like a screen opener, that can potentially harden emulsion into the mesh, but really its not a normal worry. If you are not getting a good enough cure after developing then putting it in the sun squeegee side of the screen will post harden it really really quick. The amount of UV from the sun is magnitudes higher than our light sources, even if cloudy there is way more than enough UV to post harden.

I  suggest after you develop a screen run your finger along the emulsion on the squeegee side while still wet from the water and check if any emulsion came off on your finger, that indicates really undercured. You can also do it with a white rag to check.

[RICK STEFANICK]

That is only necessary if the emulsion is undercured or if the screen is for an ink system that is tough on the emulsion like water base or discharge. For plastisol under cured is not really a big deal unless you use hot solvents like a screen opener, that can potentially harden emulsion into the mesh, but really its not a normal worry. If you are not getting a good enough cure after developing then putting it in the sun squeegee side of the screen will post harden it really really quick. The amount of UV from the sun is magnitudes higher than our light sources, even if cloudy there is way more than enough UV to post harden.

I  suggest after you develop a screen run your finger along the emulsion on the squeegee side while still wet from the water and check if any emulsion came off on your finger, that indicates really undercured. You can also do it with a white rag to check.

Exactly. Get your exposure times right on and don't worry about post exposing. Emulsion chemistry has changed over the years and  with some emulsions  post exposing helps zero. Another suggestion would be to use a hardner if necessary.

[Frog]

with some emulsions  post exposing helps zero.

Post exposure only works with photopolymer emulsions, or with the photopolymer component of dual cure.

As touched upon, besides the added durability on long waterbase runs, post exposure was/is often touted as a band-aid for underexposed screens, something some do on purpose in the belief that it helps hold better detail (fact is that with a proper exposure unit, correct exposure should be correct exposure, whether the image is detailed or blocky). Also, since many of us dry our screens outside anyway, our photopolymer screens get this extra dose of UV without trying.

[ABuffington]

As mentioned, only pure photo-polymer emulsion continues to cross link in the sun.  The issue with many exposure systems out today is complete cross linking is not possible.  Metal halide lights cross link better but there is a huge difference in a 1k vs an 8k in terms of exposure quality strength.  Getting maximum resolution and full exposure are two competing outcomes.  Unfortunately under exposure to get fine halftones has become the norm which creates havoc with water base and discharge inks.  Plastisol inks are far less aggressive.  So what I see in my demos are screen rooms using this "plastisol under exposure technique" for LED lights and the result is water base, discharge and HSA ink screens breakdown quickly.  Durability is directly related to how long you expose on the first shot and the quality and amplitude of the light used.  Post exposure as mentioned is a band aid.

Maximum initial exposure quality is time, wattage and wave length related (since diazo and pure photopolymers like different wavelengths.) With thin coating techniques and pushing LED exposure up as far as possible LED can yield a screen capable of water base or discharge, but take an 8k metal halide multi spectral lamp and maximize exposure time and the screen can be bullet proof with a greater degree of cross linking.  However in today's churn and burn screen rooms CTS/LED systems have significant ROI that can't be ignored.

This is one reason why we developed a pure photopolymer for LED that is water resistant and can print plastisol as well.  T9 can be post exposed for added strength, while our SP1400 would just be dried well in the sun but still has good durability with LED systems and complete exposure.  Always post expose the squeegee side of the screen, this side is not completely cross linked with LED exposure and will be affected by water base ink systems.  For S Mesh Screens avoid baking screens for long periods since the frames can expand and go past max tension for the mesh.