Exposure/Washout problems

Here you can discuss issues related to exposing screens for screen printing. This includes determining the exposure, light sources and washing out the stencil in the washout booth. Emulsion issues may also be addressed here.

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reactiongfx
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Exposure/Washout problems

Postby reactiongfx » Sat Oct 29, 2016 4:07 pm

Sorry if this is long-winded, but I'm hoping someone will have the patience and knowledge to point me in the right direction.

I run a small sign/graphics/screen printing business part-time (soon to be full-time), and I work at a small sign shop during the day that used to do lots of graphic screen printing. We have since stopped screen printing entirely (just digital and vinyl graphics now), but we do still make screens for some of our customers who do their own printing. We no longer have a screen printer on staff and I've taken on the duties of making the few screens we still do. I also have the luxury of using their equipment to make screens for my own garment printing business.

Lately I've been having a lot of issues with stencils not washing out very well. It seems by the time the stencil is fully washed out, the surrounding exposed emulsion is soft enough to wash away, blow through easily, or it just runs down and contaminates my washed out stencil area.

Setup is as follows: 6'x8' glass vacuum frame (we used to do A LOT of large-format printing), Violux 5002S exposure unit (guessing it's a metal halide, bulb was replaced 12/2011) with Douthitt Magic 83 light integrator. Front of light reflector is 4' from the glass, set on HIGH for 600 (it's not seconds, it's just some arbitrary unit of measure on this thing, apparently...I'm guessing it's about 4 minutes). Kiwo Poly Plus Z diazo emulsion. For work I do 280/380 mesh screens, but for my garment printing I only do 130-180 mesh. Two coats emulsion on stencil side with rounded edge of scoop coater, one on well side. Wash out with 1700psi pressure washer from about 6" away. I use the same process and settings for everything, as the guys before me did all their screens this way for years and years. I've made dozens and dozens of screens for work and myself over the past year or so, both detailed and simple, with great results. I've even done a 200 mesh screen with halftones and it turned out great. The problem has only really started in the last few months. Now sometimes it takes me 2-3 tries to get a decent screen, but I have to be very careful to use as little water as possible so as not to soften/blow through the exposed areas.

First thought was the emulsion was bad. Admittedly, it was maybe 6 months old, so we got a new can. Still having issues. I thought it might be my films not being opaque enough, even though they appear pretty darn opaque--I use an Epson Artisan 1430 with all-black bulk ink system on waterproof film (no RIP, I just bump all CMYK values up to 100% in Illustrator or Photoshop)--but I've also had the same issues using a plotter-cut rubylith positive, so I'm ruling out my films.

Maybe the bulb needs replacing? We're not a super high-volume shop, so it hasn't been used a ton over the 6 years since we last replaced it. We maybe do a few screens a week. It sure seems pretty darn bright to me. Maybe the second can of emulsion is bad? We've had some stuff in the past that was on the shelf for a long time at our supplier and it gave us issues. Am I applying the emulsion too thick? Should I opt for a thinner emulsion layer using the sharp edge of the coater in hopes that the stencil will wash out faster, resulting in less water used and thus less softening of the exposed areas? I may have only recently changed to the rounded edge in hopes that a thicker emulsion layer would withstand the washout better (I've tried so many different things over the past few months it's hard to keep track), but perhaps my logic is backwards and I actually want a thinner layer of emulsion so the stencil washes out quicker. I'm at a loss.

One thought I did have today, I have a tendency to use hot water to wash out screens. Maybe this is bad? I guess just our of habit I only turn on the hot tap to wash out coaters, etc, and I realized I do the same for screens. Should I try cold water? Warm? I usually just quickly wet the entire screen from about 2' away to start the emulsion developing, wait about 1 minute, then come in with the pressure washer at about 6" to wash out the stencil. Should I just use the garden hose sprayer instead of the pressure washer? I've used the pressure washer many, many times in the past successfully, and it seems the consensus on various boards is that a properly exposed screen should be able to withstand a 1700psi pressure washer at 6".

If anyone can advise me on what my issue might be, or what I could try, I would surely appreciate it. It gets pretty frustrating when everything looks great and them BLAM! You notice you washed away an edge of your stencil, or blew through the surrounding exposed emulsion, or the screen looks good, and you hold it up to the light and there's an emulsion "scum" that has run into the stencil area that I'm now afraid to try and wash out. I've reclaimed so many screens!!

I REALLY appreciate any help!


reactiongfx
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Re: Exposure/Washout problems

Postby reactiongfx » Sat Oct 29, 2016 8:06 pm

After doing a bunch of reading today, I'm convinced at least one of my problems is that I'm putting down way too much emulsion. I'm going to try a screen tomorrow with one coat each side using the sharp edge of the coater.

Dip.Schlitz
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Re: Exposure/Washout problems

Postby Dip.Schlitz » Sat Oct 29, 2016 10:25 pm

Sounds like it could be underexposure. You could bump up your exposure time in 30 second increments. Make sure your emulsion is mixed well, diazo adds another variable. Maybe 1 and 1 on the coating with the rounded edge.

reactiongfx
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Re: Exposure/Washout problems

Postby reactiongfx » Sun Oct 30, 2016 8:33 am

Good news...I had great success with a screen today, now we'll just see if I can keep it going. I used much less emulsion--one coat stencil side, one coat squeegee side, then a final coat on the stencil side, using the SHARP edge of the coater. I knew I was on the right track because the screen dried much faster. Shot it at our normal settings we've been using (600 light units--now I know!), washed out with hose sprayer (NOT pressure washer) using COLD water. Worked like a charm! No blow-out of exposed emulsion, no emulsion scum running down into my clean stencil area, nice sharp edges, and not a single pinhole.

I love it when I'm able to solve my own problems!! BUT-- I'd still love to hear any input from some of you more experienced folks out there. Thanks!

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Re: Exposure/Washout problems

Postby Catspit Productions » Wed Nov 02, 2016 10:50 am

Yes, the more emulsion you coat the screen with, the more time it will take to dry and the longer the exposure time will be. Also try using cold water on a garden hose with a spray nozzle. A pressure washer is way too powerful for washing out the stencil and it can be an issue. Also I agree, this may be an under exposure due to the extra emulsion on the screen. A 1/1 coat is all that is necessary using the thin side of the dual edge scoop coater.





Also metal halide bulbs can be specific to emulsion. This info comes from our friends at CPS:

"Mercury Lamp
These are the common lamps that are used in UV driers. Their spectral output is in the region of 365nm and this is to match the absorbance of screen printing UV inks and varnishes.

Iron Iodide Lamp
Iron Iodide is a broad emitter and enhances the spectral output of the lamp in the 380nm region. This can be used with good effect on photopolymer and daylight film exposure systems.

Gallium Iodide Lamp
Gallium Iodide has the effect of introducing spectral lines at 403nm and 417nm of the electromagnetic spectrum. This is particularly useful with Diazo type exposure processes.

For a lamp to be suitable to cure a particular emulsion it must emit light in the region that the photosensitive component of the emulsion absorbs. So a Gallium Iodide doped lamp would not be suitable to cure a photopolymer emulsion since the photopolymer does not absorb above 400nm but the lamp mainly emits at 405 and 417nm. A Gallium Iodide lamp would be perfect for a dual cure emulsion because the sensitizers absorb at 405 and 417nm and the longer wavelengths penetrate better giving better through cure than a pure mercury lamp."

Black lamp industrial fluorescent and LED exposure units tend to work well with both photopolymer and dual cure emulsions.

It sounds like you already solved the issue: "Shot it at our normal settings we've been using (600 light units--now I know!), washed out with hose sprayer (NOT pressure washer) using COLD water. Worked like a charm! No blow-out of exposed emulsion, no emulsion scum running down into my clean stencil area, nice sharp edges, and not a single pinhole."

Just remember, thinner stencils are easier to expose and get detail with. Using more emulsion makes exposing screens very difficult and increases exposure times.

I hope this added info helps out some.
Jonathan Monaco
Catspit Productions, LLC
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http://catspitscreenprintsupply.com/
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