TSB
screen printing => Screen Making => Topic started by: jvanick on February 26, 2016, 01:20:30 PM
-
all the discussion over on the LED thread got me to thinking about the science behind a complete crosslink of emulsion on a screen.
how do you test for a full cross link?
It seems like the 21-step stouffer strip has it's issues as the 'solid 7' is arbriatary.
you can end up with great detail on say the Kiwo 10-step test, but end up with a soft screen.
you can do your own 'detail' testing, and still end up with a soft screen.
Other than putting screens on press and seeing how they perform, is there some kind of test one could do to know for sure?
As was mentioned in the LED thread, SBQ's are preferred on LED units, and that the longer times of Diazo emulsions can cause under-cutting and other not-so-optimal effects, so let's just focus on SBQ emulsions for now...
is there some kind of hardness test that could be run???? Some kind of chemical??? etc.
-
Great question, however I would think a swab test although ideal would only tell you if the swabbed surface was converted. Still have to deal with the full film thickness to insure total conversion.
-
Great question, however I would think a swab test although ideal would only tell you if the swabbed surface was converted. Still have to deal with the full film thickness to insure total conversion.
if there was such a thing, after exposure, the swab test could be done on the 'inside' of the screen.. if it passed, you'd know if the light source penetrated ALL the way through and completely cross linked the emulsion.
due to the directionality of the light, the inside being cured would mean that the outside and middle would have to be as welll.
-
Great question, however I would think a swab test although ideal would only tell you if the swabbed surface was converted. Still have to deal with the full film thickness to insure total conversion.
if there was such a thing, after exposure, the swab test could be done on the 'inside' of the screen.. if it passed, you'd know if the light source penetrated ALL the way through and completely cross linked the emulsion.
due to the directionality of the light, the inside being cured would mean that the outside and middle would have to be as welll.
Quite correct, sorry for thinking unidirectionaly...
-
I've been thinking about this lately as well. There's gotta be a Prussian blue type test.
-
We recommend the swab test on the inside of a screen being developed. White fabric wiped on the wet area of the screen away from the image should not show emulsion color. We use high powered microscopes, even electron microscopes to see what is going on. Looking at halftones is one way to see how the exposure went, you can visibly see undercutting of overexposure or weak exposure, but the real exposure info on cross linking is molecular in nature and not visible. There are chemists who can explain how many milli joules are needed to excite a molecule of sensitizer to make a handshake, but this is all overkill IMO for the average printer. Keeping a log of every screen with all screen parameters, EOM, Tension, exposure time, etc etc is a cheap method to back track to an issue. The Stouffer scale seems to be the best tool, but as you mention Jason, it doesn't completely tell you if the exposure was complete. If it breaks down, the exposure isn't correct, but the lamp condition, light quality , drying procedure, EOM, and other variables all play a part. Documentation can reveal issues. If 2 different personnel coat and one achieves an EOM at 12% and the other is at 2% and the 2% design has tons of pinholes after 2k then a log would pinpoint the EOM as the difference and one cause of the pinholes. If EOM is the same, then set up, inks, drying, could be the issue.
Al
-
We recommend the swab test on the inside of a screen being developed. White fabric wiped on the wet area of the screen away from the image should not show emulsion color. We use high powered microscopes, even electron microscopes to see what is going on. Looking at halftones is one way to see how the exposure went, you can visibly see undercutting of overexposure or weak exposure, but the real exposure info on cross linking is molecular in nature and not visible. There are chemists who can explain how many milli joules are needed to excite a molecule of sensitizer to make a handshake, but this is all overkill IMO for the average printer. Keeping a log of every screen with all screen parameters, EOM, Tension, exposure time, etc etc is a cheap method to back track to an issue. The Stouffer scale seems to be the best tool, but as you mention Jason, it doesn't completely tell you if the exposure was complete. If it breaks down, the exposure isn't correct, but the lamp condition, light quality , drying procedure, EOM, and other variables all play a part. Documentation can reveal issues. If 2 different personnel coat and one achieves an EOM at 12% and the other is at 2% and the 2% design has tons of pinholes after 2k then a log would pinpoint the EOM as the difference and one cause of the pinholes. If EOM is the same, then set up, inks, drying, could be the issue.
Al
Did the swap test this morning on our 150-S mesh screens , coated with Murakami Aquasol TS sharp edge 2/1. At an exposure time of 30 seconds on our LED exposure unit we still managed to get underexposed screens. It`s really baffling to me how the exposure times with that unit can be so long and still not expose a screen proper.
-
Would it work if you slightly under exposed inorder to get more detail and then re exposed the screen to strengthen it?
-
Would it work if you slightly under exposed inorder to get more detail and then re exposed the screen to strengthen it?
Not sure if this was directed at me, but just to make it more clear, the screens are underexposed at 30 seconds.
-
Would it work if you slightly under exposed inorder to get more detail and then re exposed the screen to strengthen it?
you get maximum emulsion strength if the emulsion is cured correctly on initial exposure, that being said, if you were doing short runs with plastisol, you may be able to get away with under exposing a SBQ only (diazo's don't work the same way, and after the initial exposure, post-exposing doesn't do as much).
-
We recommend the swab test on the inside of a screen being developed. White fabric wiped on the wet area of the screen away from the image should not show emulsion color. We use high powered microscopes, even electron microscopes to see what is going on. Looking at halftones is one way to see how the exposure went, you can visibly see undercutting of overexposure or weak exposure, but the real exposure info on cross linking is molecular in nature and not visible. There are chemists who can explain how many milli joules are needed to excite a molecule of sensitizer to make a handshake, but this is all overkill IMO for the average printer. Keeping a log of every screen with all screen parameters, EOM, Tension, exposure time, etc etc is a cheap method to back track to an issue. The Stouffer scale seems to be the best tool, but as you mention Jason, it doesn't completely tell you if the exposure was complete. If it breaks down, the exposure isn't correct, but the lamp condition, light quality , drying procedure, EOM, and other variables all play a part. Documentation can reveal issues. If 2 different personnel coat and one achieves an EOM at 12% and the other is at 2% and the 2% design has tons of pinholes after 2k then a log would pinpoint the EOM as the difference and one cause of the pinholes. If EOM is the same, then set up, inks, drying, could be the issue.
Al
Did the swap test this morning on our 150-S mesh screens , coated with Murakami Aquasol TS sharp edge 2/1. At an exposure time of 30 seconds on our LED exposure unit we still managed to get underexposed screens. It`s really baffling to me how the exposure times with that unit can be so long and still not expose a screen proper.
on 150S that is going to be too much emulsion. Get an EOM meter to find out what's going on. You are probably at 50%. We were at 15s for 20% EOM with Aquasol HV (if you have an early unit this would be the same, they redesigned the light layout shortly after release and the times would go up if you have a newer unit than what we tested.)
pierre
-
To brush up on the subject of exposure and type of control tools, you can go back to this article from 1999...
http://www.screenweb.com/content/calculating-a-good-dose-exposure (http://www.screenweb.com/content/calculating-a-good-dose-exposure)
This is back in the day when trade mags used to feature some really technical stuff.
-
To brush up on the subject of exposure and type of control tools, you can go back to this article from 1999...
[url]http://www.screenweb.com/content/calculating-a-good-dose-exposure[/url] ([url]http://www.screenweb.com/content/calculating-a-good-dose-exposure[/url])
This is back in the day when trade mags used to feature some really technical stuff.
awesome article... now I know what toy Danny was talking about...
it's a digital radiometer (light meter) with a uv-transmissive filter...
Extech LT300 Precision Digital Light Meter
-
Honestly I`m not getting any useable result using either the Chromaline or Saati 21 Step Guide. White T-mesh coated 2/2 sharp edge with Murakami Aquasol TS . 60 seconds on our LED. Chromaline calculator says multiply time by 0.33 Saati says half the time and swap test on the squeegee side of the screen gives me plenty of blue on the cloth at 60 seconds. So any shorter and the stencil will be even more underexposed.
-
To brush up on the subject of exposure and type of control tools, you can go back to this article from 1999...
[url]http://www.screenweb.com/content/calculating-a-good-dose-exposure[/url] ([url]http://www.screenweb.com/content/calculating-a-good-dose-exposure[/url])
This is back in the day when trade mags used to feature some really technical stuff.
awesome article... now I know what toy Danny was talking about...
it's a digital radiometer (light meter) with a uv-transmissive filter...
Extech LT300 Precision Digital Light Meter
I talked to somebody about this, I think it was Ross, and this method does not work with SBQ emulsions. From what I remember, the SBQ skims over right away as soon as it is hit and the opacity does not change with time. Diazo turns milky and gets denser as it cross links.
Pierre