Author Topic: Emulsion with Diazo??  (Read 1887 times)

Offline chubsetc

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Emulsion with Diazo??
« on: March 15, 2018, 07:21:13 PM »
If you add Diazo to an emulsion which gives it a certain pot life, say 4 weeks.  What is the usable life span of a screen that is coated and dried with that emulsion awaiting exposure. 

I’ve been testing CCI TX emulsion and it is pretty rock solid to WB and discharge when diazo is added.  The issue is that when I went to the TDS it says the pot life after diazo addition is 7-10 Days.  This is OK in our system as long as the coated and dried screen waiting for exposure is still good long after the 7-10 Days. 

Thanks!



Offline mimosatexas

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Re: Emulsion with Diazo??
« Reply #1 on: March 15, 2018, 08:35:34 PM »
I use SP1400 with diazo and honestly I can tell the difference when exposing halftones from a screen that is 2 hours old and one that is 12 hours old. the halftone range is less the older the screen gets.  For spot color stuff I have used screens that are 2 or 3 weeks old without issue, but past 4 or 5 you will likely run into issues in my experience.

Offline ScreenFoo

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Re: Emulsion with Diazo??
« Reply #2 on: March 16, 2018, 10:14:49 AM »
Sounds interesting.
Sure the two hour old one isn't holding smaller dots underexposing a hair because it's still got moisture?

FWIW, using a purpose made dual cure (Usually either poly plus, or LX6xx series) I have exposed and developed six month old screens stored well, have had issues with three week old screens that weren't.  Not sure how different it is for PP's you can add diazo to or not. 




Offline ABuffington

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Re: Emulsion with Diazo??
« Reply #3 on: March 16, 2018, 11:15:47 AM »
Light contamination of the stored screens can also affect screen sensitivity.  To check your screen storage lighting just place a coated screen on the floor before you go home, put a few coins on the screen and leave the lights on overnight.  Wash it out in the morning.  If the coin's circles fall out first then you have some light contamination that will add up over time and pre flash your screens making details hard to wash out.  Also wash out of exposed screens should be in a light safe area.  To test this take a 5 gallon bucket of water into the light safe exposing area and develop the image with wet sponges.  Sometimes this helps fine details that can get flashed during wash out.  (I have had many companies complain that the emulsion is bad when the development is done outside!  Even in shade, reflected light and blue sky have enough UV to expose emulsion.)

The other issue with diazo is heat.  Above 80 degrees, diazo starts to cook and dark harden.  So stored screens are really affected by high summer temps.  Anything in the 90-100 degree range is going to cook the coated screen in about a week.  You will notice saw toothing and excessive pressure needed to wash out.  SBQ and pure photopolymers are unaffected by heat, and can be stored in temps up to 104.  As long as you are coating and shooting diazo screens within a couple of days in high heat conditions you are fine.

The one other key area I am seeing is overcoating.  Just because one emulsion can be coated 3:2, 2:2 doesn't apply to all emulsions.  High solids content, high viscosity emulsions like SP1400, T9, HVP need only 1:1 dull or 1:2 sharp, or 1:1 sharp on really fine meshes.  Too much EOM affects details.  Great for spot opacity, but typically I look for 6-7% EOM for halftones, 10-15% EOM for spot color.

Al
« Last Edit: March 19, 2018, 12:16:54 PM by ABuffington »
Alan Buffington
Murakami Screen USA  - Technical Support and Sales
www.murakamiscreen.com

Offline chubsetc

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Re: Emulsion with Diazo??
« Reply #4 on: March 18, 2018, 12:45:50 PM »
So am I to infer that the “pot life”  in the bucket in the emulsions “wet state” is where the degradation occurs and once dried on a screen and properly stored the screen has equal success with exposure and on press durability 1 or 2 months after coating as it would 1 day or 1 week after coating?

Offline ABuffington

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Re: Emulsion with Diazo??
« Reply #5 on: March 19, 2018, 12:23:51 PM »
Pot life is one term that refers to the life in the bucket once diazo is added.  Diazo emulsions in the bucket, without diazo being adde, is good for a year.  Once you add the diazo you have 4-6 weeks of "pot life".  A coated screen is less.  2-3 weeks in optimum storage conditions.  Really hot/humid screen rooms would be less: 1 week of optimum sensitivity for the finest of halftones.

Pure photopolymers: 1 year of pot life, 4-6 weeks on coated screens.  Once you add diazo to an SBQ pure photopolymer it has a pot life of 4-6 weeks, 2-3 weeks on  a coated screens.

Al
Alan Buffington
Murakami Screen USA  - Technical Support and Sales
www.murakamiscreen.com

Offline chubsetc

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Re: Emulsion with Diazo??
« Reply #6 on: March 19, 2018, 05:06:46 PM »
Perfect, thanks Al!!