[RoboLove]

Hello

I recently haven't had many problems but this week I exposed a screen and washed it out all fine ready to start printing however when I helf it up to the light there were some areas which had been washed out which looked almost misty or like a glue had been put over the mesh, I tried printing through it to see what would happen and the misty areas passed hardly any ink through.

I cleaned down, reclaimed and degreased and recoated the screen, left it over night and then re-exposed and washed out but with the same problem. Any ideas what is causing this?

Thanks

Paul

 

[Alan Buffington]

Probably underexposed, the emulsion on the inside was partially dissolved and a very thin transparent film has formed over your screen. You can dry this screen, wash with two wet rags and get this out of the screen.
I would bump your exposure up 20% more time to see if this fixes the issue, or run a step wedge test.

Alan Buffington
Murakami Screen USA

 

[Greatzky]

are you using a diazo emulsion? like dual cure emulsion where you have to mix in the diazo?
if that's the case then that is the diazo dripping back into the image and blocking it out.

I use photopolymer emulsion now, but what I used to do was lightly pat down the screen with papertowel then use an air compressor to blow all the water off the image area. This will also blow any of the diazo out of the image area as well...

LMK if that helps you!

 

[ScreenFoo]

I've seen shops that blotted underexposed developed screens as SOP, and then would take wet cotton swabs to any areas like this--usually you can salvage a screen if it's not too bad, but be nice to the stencil edges, it's easy to damage when it's underexposed.

As Alan suggests, it's a lot easier to properly expose the stencil in the first place...

 

[RoboLove]

Hello

So I bought some new emulsion coated up 4 screens and exposed them all, all of them came out absolutely fine except for the design I had the issue with last time.

My stencils are all made exactly the same and are plenty opaque. Have never had this problem before except on this one design and I have no idea how to fix it :/

 

[Alan Buffington]

The goal in exposure is to use the maximum amount of UV light almost to over exposure. Imaging a design occurs all the way from under exposed emulsion to completely exposed emulsion. A completely exposed screen has no slime on the inside of the screen and should never need blotting, or touch up of the open area. Run a step test to really know how long you can expose the screen, not how short you expose the screen. Typically on fluo tubes you can easily go to 15-19 minutes for complete exposure, 1K - 9-12 minutes, 3K 2-3 minutes, and 90 seconds or less on a good 5k system. Here is a link to run a step test.
[media]http://www.murakamiscreen.com/documents/StepTestInstructions.pdf[/media] Again try to reach over exposure, if after 20 minute exposure you still have the image blocked, you need a stronger light with more UV output.

 

[RoboLove]

Hello

I don't think I am explaining the problem quite right.

Basically I expose the screen as normal which normally works absolutely fine. I wash it and then leave it to dry (usually over night) At a glance the screen looks fine but then when you look up close areas of the design look like they are still wet with water although it is 100% dry.

If you try and pass ink through the design it comes through perfectly in areas and then patchy in the other areas.

I have tried new emulsion, new screens, different exposure times, different inks yet always get the same problem.

 

[superD70]

you explained it fine, it is un-exposed emulsion running back into the open areas, I have this issue sometimes on ultra fine lines or halftones.
I use a blower and my compressor to blow the stencil clear of all moisture before leaving it to dry.

 

[Alan Buffington]

Still an underexposure issue IMO. More time will eliminate this.

 

[ScreenFoo]

In total agreement--believe me Paul, I was forced to do crappy screens like this for some time--if anything I have more experience than I could ever want with this problem.

If you try the same thing, and take one screen right after you develop it, lay it on clean newsprint, and blot it completely dry with paper towels, I bet it won't happen. Not to say you should keep doing it that way.

 

[RoboLove]

Ok cool, will give that a try. Thanks