CLOGGED SCREENS

[dirkdiggler]

sometimes you cant see it till ink hits it, hence the spit later!

[tonypep]

Give me a PM and we can discuss if you like!

[Frog]

Ya know, we've all skirted the issue of underexposure being the underlying reason for scumming.

[screenxpress]

Ya know, we've all skirted the issue of underexposure being the underlying reason for scumming.

Considering the post was started by Mike, I gave him credit to have discounted that.  Perhaps mistakenly, lol........

[tonypep]

It is often difficult to see "scumming" on a block out table. Too much direct light usually. And yes, underexposure and/or back flow from a single sink contributes to this. This is why I often see many  printers holding screens up to different angles on the production floor. It is subtle as it is frustrating. Yes, so as a solvent based screen cleaner will get the ink (plastisol) out, it is possible to re-introduce spit or water. Many of these chemicals can lock up the stencil however,. Most will re-burn
So..... back to the beginning. Mesh counts/EOM/Vs /emulsion/vs/drying time/vs relative temperature due to climate variances. Cross contamination is also a consideration. Just getting started.
best tp

[bimmridder]

Simple cure. Proper exposure

[GaryG]

Ulano Orange is about as soft as an emulsion as one can get. Rubbing saliva, water or chemical, I have seen cured emulsion loosen and smear into open screen and "re-scum" to drying. I was once baffled by fine halftones closing up after using screen wash. The rep said screen wash (in this thread case, water) actually loosened up emulsion and swept it back into mesh open area, clogging it. Yes even with properly cross linked set emulsion depending upon how hard you scrub.

Pure Photopolymer emulsions are notorious for this being softest of the lot.

In my mind kind of a (Autotype) 2000, 7000, 9000 rule:
Autotype brand emulsion going up in hardness. Higher number, studier the cross linking, better detail, less chance of being dragged back into open mesh.

[mooseman]

I am pretty sure under exposure is not involved here. We are pretty simple in our operation, by that I mean we use a commercial exposure unit 4 tube unit with an upgrade to 40 watt tubes and change the tube every couple of years.
We have run the step wedge test and used an exposure meter. From those results we got 5 minutes give or take then added an additional 25% time just to compensate for the weakening of the bulbs over time and the various EOM we get with different mesh counts.

At the risk of giving many of you a cardiac event when it comes to time and efficiency we expose at 6.5 minutes normally and when using half tones add another 20% to that. On a busy day we might burn 10 screens, not a big operation here.
If under exposure is the culprit I am going to take up stamp collecting.
I have not paid too much attention to this specific element BUT it seems like I have this situation happen mostly on my 156 mesh screens and within that mostly on the older screens we have.
At the end of the day it is a PITA situation but the fix is relatively simple with the product we use. Even using a white cotton cloth there is barely any transfer of the emulsion to the cloth after the application.
mooseman

[Prince Art]

I notice the thread started as a statement, not a question. Whatever the cause of the problem, you found a solution that works. If it allows you to get a quality product out the door, in some ways "what is the real problem" and "why does it work" and "how else could this be addressed" may not matter much. I can see where, yes, it would matter in a bigger operation, where a tight control over variables is highly beneficial. But small shops have more freedom to play fast & loose & do whatever works. My only question with the product you're using would be whether it negatively affects mesh, emulsion, or ink.

But... since scum has proved such a popular topic, here's two cents more:
-When talking about exposure, it's easy to consider the image area only. But emulsion can frequently be thicker at the edges of the screen, where the coater starts, stops, and leaves side trails. During washout, attention may be given only to the image itself - not the whole screen. Result: Not all underexposed emulsion is washed off in outlying areas. If any emulsion-laced water runs back into your image area, and then dries, it can create a blockage.
-This is why surface drying each screen as soon as it comes out of the sink is a good practice: if you remove enough water that it can't run & streak, and can't bridge in the open mesh, the potential for a problem is greatly minimized. Blasting with compressed air is one of the best approaches I know of, but anything the gets the water out is good.

[Sbrem]

It is often difficult to see "scumming" on a block out table. Too much direct light usually. And yes, underexposure and/or back flow from a single sink contributes to this. This is why I often see many  printers holding screens up to different angles on the production floor. It is subtle as it is frustrating. Yes, so as a solvent based screen cleaner will get the ink (plastisol) out, it is possible to re-introduce spit or water. Many of these chemicals can lock up the stencil however,. Most will re-burn
So..... back to the beginning. Mesh counts/EOM/Vs /emulsion/vs/drying time/vs relative temperature due to climate variances. Cross contamination is also a consideration. Just getting started.
best tp

We also started holding screens up to bounce light off them to see scumming, wayyyy back, and we eventually switched to a wet vac with a custom head which solved that. Prior to that, we blotted screens.

Steve

[3Deep]

To help stop scumming I always use those blue shop towels once I wash out a screen and dry the image area on both sides and then towel dry the edges of the screen where most water will hold up and slowly run down the screen back into the image area.  Even though under exposure has been said scumming can still happen on very good exposed screens which is cause by the residue from the unexposed emulsion being washed out.  One thing I can say I know everyone will agree with is it is a pain in the butt once you've set everything up and test print to fine a bad spot in print and now you got fix it

[tonypep]

Tricky fix. Even though auto coaters are great and many manual pre-press people as well however;  Many overlook the top and bottom. The image may be perfect but that is not the end all. That is where much of this can occur. Microscopic water mixed with chemical is a common the case.