emulsion troubleshooting

[Prince Art]

Had a strange problem occur that I need to troubleshoot: Burned a set of 4 screens. During washout, the emulsion on 2 of them washed out as if it was severely underexposed - not just the image area, but most of the emulsion fell off the screen, using a spray nozzle, not a pressure washer. The other 2 screens were fine, even when I put a little extra pressure on them. The good ones held detail, too - no ragged edges on image area to indicate underexposure.

All were burned for established times per the mesh counts used.

The obvious answer is that the LEDs on my DIY exposure unit didn't turn on properly. I'm aware of some weak connections in the wiring. But I've never seen anything close to this before, and I've been using it without issue for years. (Only thing that changed is that these were the first burns after a week of vacation. But I don't know how that would cause a problem.) This is where my troubleshooting will start.

But I want to ask: Is there anything else that could cause this besides a problem with the exposure unit? Just want to make sure I'm not missing something less obvious.

Additional info:
230 mesh - emulsion fell off during rinse easily
200 mesh - emulsion fell off in most areas, but clung a little more & seemed more exposed than the 230.
156 & 110 - no problems, behaved normally

All standard mesh. Using Saati Textil PV emulsion, coated 1/1 on everything.

The only other variable I know of is that the higher mesh counts may have been coated longer ago than the low meshes. But I'd expect older emulsion be harder to wash out, not easier.

[Colin]

Mesh Color?

New emulsion bucket?  When?

New Screens?

Were they stored in different areas?  Humidity in those areas?

When was the last time you dialed in your higher mesh counts?

Film change?

What IS your exposure times between your mesh counts?

[Prince Art]

Mesh Color? yellow on 200, 230, white on 110, 156

New emulsion bucket?  When? Emulsion was probably about 10 mos old. It is possible the 200 mesh had been coated with an older batch & sitting for awhile, as I rarely use that count. But 230 should have been as recent as anything else.

New Screens? No.

Were they stored in different areas?  Humidity in those areas? Same screen rack & room, indoors, under air.

When was the last time you dialed in your higher mesh counts? Not recently. But burned good halftones on 230 mesh just before vacation.

Film change? One "bad" screen used film from a new batch, but the other used older film that was the same as the 2 good screens. (Reburn of a 3col order.)

What IS your exposure times between your mesh counts?
230= 1:25
200= 1:30
156= 1:42
110= 1:54

[Colin]

I think you are underexposing.  It is also possible that those screens that washed out, may have been coated a little thicker than previous screen, making the washout worse.

White mesh exposes MUCH faster than yellow.  Wow, been a long time, I forgot what the conversion is between same mesh count different colors.... like 30%-40% longer for yellow than white I think...

I would try giving your 200-230 mesh another 30 seconds and see what happens.

[Prince Art]

Thanks for the input, Colin.

Here's the update: Reburns today went fine, using the same times as above. Burned on the same mesh counts, same emulsion, etc. Got crisp detail, no hint of emulsion falling out in non-image areas. Everything burned just like I'm accustomed to. And the only thing I did differently was to make sure all light strips in the screen area were definitely working. (I've been aware of a few bad connections on the farthest edge of the unit.)

Ultimately, I'm not sure what caused the problem. My best guess is that somehow those first 2 screens didn't get a full-strength burn - presumably due to an electrical or wiring issue I've never experienced before and haven't reproduced today. So... who knows for sure. At least I've got good screens now!

...And as for the theory of a thicker coat of emulsion on the bad screens, that wouldn't have been it. I have a pretty consistent technique, and coat thinner than most people do: 1 outside, 1 inside, scrape outside, scrape inside. Atypical, I know. But thin enough to not likely be the issue.

I also think that the faster exposure time of white mesh could be why my low mesh & higher mesh times are as close as they are - if I were using white on high mesh, I'd expect LOWER times for those.

Nonetheless, I appreciate the help brainstorming!