Exposure Woes

[jjkage84]

All, friends, professionals, PLEASE help me!

Ok, I've burned hundreds (approximately) of screens in the worst possible conditions in my garage using a 500 watt, 15 dollar shop light, with better than expected results.  I knew, using Ryonet's dual cure pink emulsion, to burn my screen for 7:45 seconds, good exposure to screen contact, it would come right out easy peasy.

Now, here's where the trouble begins...

I have recently moved into a big shop, and was recently, temporarily, gifted a Lawson EXPO-LITE.  When I got the exposure unit, it had Sylvania Daylight deluxe F40/DX bulbs in it.  Some worked, some didn't, but enough worked that I could burn a 20x24 screen on it with no missed spots.  The neoprene vacuum cover has holes and is no use at this time.

When I would attempt to burn a screen on the exposure unit, using that little 1-20 exposure graph, I could not get any sort of consistent results.  Burn it for this long, it would over expose, burn it shorter duration it would be under exposed.  Try to fine tune it, the top portion would be exposed, the lower over, or under, or well, you get the idea. 

I attempted to go back to my trusty shop light to no avail.  The emulsion just wouldn't hold the lines.  Bought new emulsion, same results. 

So I ended up buying 12 new GE 10526 40 Watt T12 blacklight fluorescent bulbs for the exposure unit.  First screen I burned looked great at 3 minutes.  Was like "BOOM! I figured it out!"  Next screen I burned, I did it at three minutes, and it was way, way over exposed! 

So I've bounced around from and have had a couple decent results at 45 seconds on a 180 screen with Ryonet's dual cure pink emulsion.  I lift the lid, put my screen down, put a black piece of foam board in the back and a large heavy platen inside of the screen on top of the foam board which is sitting against the screen.  I can't imagine the screen isn't getting bad contact on the glass.

I just cannot get consistent results.  No clue.  I have literally burned and reburned screens dozens of times now and it's driving me bonkers because you would think, "Okay, its under exposing, I'll up the time."  I up the time, it's over exposed, then it's back down in small increments which is how I got to the 45 seconds.  Problem with the 45 seconds is that I'm still getting inconsistent results at the 45 seconds.  I can burn a screen, and on the exact same transparency, it'll burn perfect on the top but bad on the bottom.  I've turned my screen inside the exposure unit, I've put black vinyl over the foam board thinking maybe light was seeping in behind my screen... I've done about everything I can think of.

All I know is that not being able to burn screens has basically been a slice to my Achilles tendon and I cannot function.  Any help would be GREATLY APPRECIATED!

Thank you all so much!
Joe

[Croft]

I had a similar problem with Ulano QTX a long time ago ( the garage days) it was fast exposing so even walking to the light table would expose it, No darkroom here . I switched to RLX for a while but now am using KIWO Polycol Crossover that is giving me great results

[larryk]

Must be something you changed from your previous location.... like maybe how you are handling the screens from the time they are coated to when they are exposed. Too much time in the wrong light can give bad as well as inconsistant results.

[Frog]

So, you mentioned success at three minutes and then another failure. Are these screens you are having mixed results with the exact same mesh coated the exact same way? (and as also brought up) are they all handled the exact same way from coating, to drying, to exposure, to development?

My buddy Step-Wedge Man would encourage you to do a manual step wedge test and try to actually see an exposure going from under to over with hopefully a sweet spot somewhere in-between. Here's how http://murakamiscreen.com/technical-support/step-test-instructions/

[larryk]

The step wedge process will be a great help to you once you are consistantly getting the same results with the same exposure times. Good tip Frog.

[Frog]

  I lift the lid, put my screen down, put a black piece of foam board in the back and a large heavy platen inside of the screen on top of the foam board which is sitting against the screen.  I can't imagine the screen isn't getting bad contact on the glass.

This description does not really tell me that you're indeed getting good contact. How heavy is the "larger heavy platen"? Maybe some pics?
You may want to get out your patch kit and try to make use of the strongest feature of that exposure unit, its vacuum top!

[CBCB]

How is your darkroom setup? If screens are not drying efficiently after coating it will give wonky results all around. I find this pops up and is overlooked especially when rushing screens through, like in a troubleshooting scenario.

If the emulsion is sticky to the touch, you'll have a problem. The rest of the advice in here is spot on. Different mesh holds different amounts of emulsion, so different exposure times. Use your vacuum lid. Do a step test. Profit.


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[jjkage84]

I use the exact same "bug bulb" that I used in my other safe area in the garage and was able to get consistent results in the garage.

Am yet to rush a screen after its been coated. An ample 12-18 hours dry time on usual. Screens are dry to the touch prior to exposing.

Thought maybe my "bug light" being left on, although it was always on in the garage while screens were drying, may be playing a part and I was going up to bat already half exposed so I quit leaving that on while the screens were drying, same results.

I also use a fan blowing on the screens, which now sit in complete darkness while drying.

As far as not getting a good contact, I can't hardly imagine its not, but I guess I could be wrong. The black foam board is the exact inside dimensions to fit in the back side of the screen. So it goes, glass, transparency, coated screen, black vinyl over black foam board, approximately 20lb platen which sits inside of the screen and on top of the foam board, neoprene, then a portable air tank (only thing I have laying around that fits on top of the platen.

Screens range from 110-180. All coated 2 on the face 2 on the inside.

I'll try to do a step wedge, but what type of variation in time should I use? 30 seconds I know was too short as the lines did not hold and the back side was a little slippery, 45 seconds seems to work perfectly in most areas of the screen but not others. 55 seconds was over exposed. I've bounced between 45 seconds and 52 seconds with variable results.

I considered my transparencies may be bad, or may not be sufficient for the black lights vs the shop light since they worked fine on the shop light, but that wouldn't make sense since I get a good burn on a portion of the screen and a bad portion using the same transparency. Maybe the ink isn't as thick on the transparency in different locations. Although it'd not detectable by my eyes at least.

Maybe there's brighter spots inside the exposure unit?

New neoprene is the next thing to buy, just waiting on my next paycheck from my real job.

The kicker of all this is, none of it makes sense as now I can't even burn a screen with my 500 watt shop light and get good results.

As far as how the screens are prepared, they're prepared in a dark room using my "bug light", coated, then stacked on 2x2's face down as if they were in a shelf with a fan blowing on them, then dried in complete darkness in a various swing of temperatures, but that's no different than how it was in my garage. In the range of 40-65 degrees right now. Maybe even the amount of time, they're not fully dried even though they're dry to the touch? But again, I worked in worse conditions in my garage as far as moisture content, temperature, etc...

[Frog]

 Ideally, when held up to a light source, films won't seem as you're looking through dark sunglasses. You shouldn't be able to discern anything through the dark areas (like moving fingers) It's also not impossible that your films are patchy, more likely than uneven light with all of your new tubes.
How are you making your films? Printer, ink, specific media?

Exposure times go up exponentially with coarser meshes. Assuming both are white, a 110 may easily take half again as long as a 180.

With a starting time of 45 seconds, I'd do a half dozen steps ranging from 25 to 90 seconds. When you find a potential sweet spot, you can narrow it down.
Remember, a test should be done for each emulsion,  each different mesh count, mesh color, and coating technique.

[jjkage84]

Ideally, when held up to a light source, films won't seem as you're looking through dark sunglasses. You should be able to discern anything through the dark areas (like moving fingers) It's also not impossible that your films are patchy, more likely than uneven light with all of your new tubes.
How are you making your films? Printer, ink, specific media?

Exposure times go up exponentially with coarser meshes. Assuming both are white, a 110 may easily take half again as long as a 180.

With a starting time of 45 seconds, I'd do a half dozen steps ranging from 25 to 90 seconds. When you find a potential sweet spot, you can narrow it down.
Remember, a test should be done for each emulsion,  each different mesh count, mesh color, and coating technique.

I use an epson 1280 for printing my transparencies.  I've have had great results in the past with it, and it has new ink.

Great example is a business shirt I'm printing, or attempting to rather, the top portion of the logo has a large text that burns out perfectly fine, the lower portion has a finer text and drawing on it, but it's thicker than what I've done before which is driving me nuts as I've done smaller lpi (? I think that's what it's called) and didn't have these type of issues, but the bottom side of the image, the finer stuff, over exposes and is nearly impossible to wash out without ruining it.

I new that there was a variation on exposure times depending on mesh count but couldn't remember.  That's awesome info and I'll give it a whirl. 

Do screens really burn in like 25 seconds?  That's crazy to me coming from my shop light and a nearly 8 minute exposure time.

[Frog]

Fine detail open areas will fill in with overexposure, while big areas are more forgiving. Both work with correct exposure and good contact. You can hold more detail with vacuum than with weights.
And yes, with a fast-exposing emulsion and lights more powerful in the correct uv range, times can go down to a few seconds. Not always a good thing as it leave very little wiggle room.
Back when you were using your funky work light, a photopolymer emulsion would have really helped. That diazo slows it down.

[Printficient]

New neoprene is the next thing to buy, just waiting on my next paycheck from my real job.

[/quote]
Pond liner at Home Depot.  You are most probably putting too much emulsion on.  If you run an auto I would do 1 and 1 with sharp edge of coater.  If manual then 1 pass shirt side with round edge.

[blue moon]

the problem here is that the list of possible causes is almost endless! If I was you, I'd go back to what you were doing and was working and set up a reference point. get that working consistently and then start switching things around ONE AT A TIME.
Few things to think about. If your coating is not even, you might need more light on one side of the screen then the other. You should contact Lawson and get the correct bulbs for your unit. Buying blacklight bulbs off the shelf does not guarantee that you are exposing with the right wavelength. It would be easy to get a bicycle tire repair kit and fix the vacuum on the unit. This will ensure good contact between the film and screen.
when you say underexposed or overexposed, what do you really mean?
How dark are your films?

pierre

[jjkage84]

Update on my exposure woes,

Well, they're still there for the most part.

As stated previously in my prior post, my film negatives are Dark.  I honestly cannot imagine them being any more dense as they allow absolutely No light to pass through when held up to a light and close inspection.

The previous way I did it, the 15 dollar shop light approximately 18" away from my screen using Ryonet's dual cure pink emulsion, does not work now.  I'm not sure if the bulb has lost it's umf or what, but it currently does not produce consistent results.  Another point of confusion.

With regards to the leaky vacuum rubber/neoprene it is beyond repair, hence the ordering of new 1/8" neoprene to fix it.  Had it been one pinhole or rip, yes, I would have utilized a bicycle repair kit, but it is ripped numerous places along the edge, with approximately 3-6" rips where it is dry rotted.

With regards to the replacement bulbs.  According to Lawson's manual for the Exposure unit, it suggest I use 40 Watt "rapid start" day light bulbs 48" w/ 20,000 hours OR rapid start black-light bulbs to decrease exposure time which is what I purchased.

I then went to my local supplier and bought some blue emulsion designed for plastisol.  I cannot think of the brand of the top of my head, but I did not have to add diazo to it, and my supplier informed me it would take approximately 2 minutes to expose.

I explained to him what type of emulsion I had been using, and he explained to me that a dual cure emulsion should take approximately 10-11 minutes to expose on usual.  I explained to him I was over exposing screens on anything over a minute. 

*To answer the question for me to elaborate on what I mean when I say under exposed/over exposed  Under exposed is when, A. the squeegee side of the screen will be slimey, and B. the stencil will blow out and not hold an edge.  Now, how to tell if the screen was over exposed, A. The image will be filled in for a lack of better words.  As in, when you go to rinse out, you can rinse some out, but not all the image will wash out.

So one of the first screens I burned with the new emulsion turned out literally perfect.  Nice crisp lines, held detail, and burned at a 7 on the graph.  I exposed that screen for a minute.

The next five screens I exposed, attempting to do the second color of the job, one side was perfect, and the other edge was not.  It was either over exposed or under exposed.  I upped the time by a few seconds, decreased the time by a few seconds, but no matter where I would place the screen, thinking there might be a hotter spot of the bulbs, etc, I would get th same result.  ON five different screens which made me think, surly it wasn't a contact problem.

Long and short of it is, I do believe it is a contact problem.  I guess.  Still don't understand how one screen can burn perfectly, and the next five burn horribly.  I use a platen approximately 1" less width and 8" less top to bottom of a 20x24" screen so I have approximately 1/2" on either side left to right, and approximately 4" blank space on either side top to bottom.  I obviously position the platen over the transparency section of the screen, then place a weight over that.  But I guess, as hard as it is for me to imagine, I am still not getting good contact on the glass.

My new vacuum cover should be in tomorrow, I will b coating screens tonight, and trying again tomorrow.  Oh, on that, as far as screens being coated evenly, they are. 

There it is.  I honestly don't know what else to do but to try to fix anything that could possibly be an issue.

[Frog]

With all of that explanation, what still seems to be missing is a damn step test as was suggested! One screen, five or six exposures to compare!

That said, yes, poor contact can certainly screw up edge definition and add to detailed open areas filling in as well. An array of tubes, even more so than a single point light source really needs good contact
Also, did you get real unfiltered black light tubes like this rather than the party type?