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I have a 240W table top exposure unit from ryonet like the one below that i use to expose my screens.

I am using QX-1 emulsion and my screens arent coming out as crisp as I want them to. The website say that it Exposes Dual Cure Diazo Photopolymer Emulsions in 5 minutes, but im sure thats not the case for QX-1. I've seen the post about the step test and im not understanding how that works either.

If anyone has a similar set up...what exposure times are you using?
 

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qx-1 is a dual cure emulsion. It should take longer to burn than a pure photopolymer emulsion especially with a weak light source. Research step wedge test or just trial and error your exposure times. Every set-up requires a different optimal exposure time.
 

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Exposure Time / Step-Wedge Test:

Like anything else in life screen printing is a pragmatic series of events, you must look at it in each stage and determine the problem. After looking at the entire series of events you’ll find that art, screen, printing and troubleshooting will fall effortlessly in place.
Concerning the problem of exposure and washout let’s break it down into events;

1. You created the art, output the film and had a screen coated.

2. You exposed the screen, attempted to wash out with no results.

Since you’re early in the pre-press stages the answers to your problem are obvious and easy to repair, don’t worry that you didn’t recognize them as even the most seasoned printers overlook minor details.

The troubleshooting begins at you’re art;

Was the positive opaque enough? (Could you see light through the film?)

If light is creeping through the black images of your film it’s NOT dark enough and will expose as the rest of the screen would, only slower.


How long has your screen been sitting and has it been exposed to any light before burning?

Screens that have been exposed OR subjected to excessive heat during drying will expose. I’ve had screens ruined and exposed by keeping my screen box temperature too high. Screens that have been sitting over a couple of weeks and having the door opened and closed will pre-expose as well.

Is your exposure time long enough?

Over time light sources weaken, especially HUV (Black light) units and to compensate you increase your exposure time but make sure your positive is OPAQUE!


If your unsure of an exposure time perform a “Step-Wedge” test, this involves dividing your screen into 4 sections and marking each with a specific exposure starting with the manufacturers then increasing in increments of 2 minutes per section.



Mark the screen and at the top of each section write 4 minutes, 6, 8 and 10. Produce a piece of art with type, shapes and even some halftones and place it in the 4 minute section, cover the remaining 3 sections with the black sheet; expose for 4 minutes.



After the initial exposure move your art and sheet to the next section, covering the 4 minute section with a light safe sheet as well, you only want the 6 minute section exposed to your light source. Expose this section for 6 minutes and follow this procedure until ALL 4 sections have been exposed at the times indicated.



Wash out the screen as you normally would, completely wetting both sides the let it sit for a few minutes allowing the emulsion to soften (your washout room should be yellow lit as well) then spray softly again on both sides until you see your image washing out.

IMPORTANT; Always do your final washout from the SHIRT side of the screen as this is the side exposed. To determine the best exposure look for; edge definition and degree of unexposed emulsion on the squeegee side while washing out. A properly exposed screen will have less slime on the inside as the exposure light has burned 75 to 90% of the emulsion. Be aware on finer detail you may want to cut back time to maintain the details.

After you have gone through all these parameters, found your time, washed, dried and blocked out your screen and ready to tape don’t forget to post-harden your screen for strength and longevity. This is nothing more than exposing the screen on the unit for 10 to 15 minutes or setting it in the sun for 5 minutes, why? During your initial exposure your only trying to burn the image on a screen and maintain detail, post-hardening will ensure a strong, long lasting screen.
 

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Can you recommended a good exposure unit I need to purchase one.

Mayleen
If you're buying, the best are metal halide systems with vacuum blankets from companies such as AmerGraph, Richardson, and NuArc (M&R).

The next best would be the multi-bulb systems that use unfiltered fluorescent blacklights with vacuum blankets. Vastex makes a couple, M&R might make one, Workhorse makes one. Most commercial press manufacturers make one.

After that, if you're handy with tools, you can make your own multi-bulb systems as mentioned above for about $200, using pond liner from Home Depot as your vacuum blanket material. I've built two this way and they worked great.

You can also use halogen work lights as a light source, but they have very little UV light and take forever to expose your emulsion.

If you're making your own, you can also use the old foam rubber pad over a piece of black felt on the inside of your screen frame and weight it down with a stack of books to get your film and emulsion fairly tight against the glass. It works, it's cheap, but if you want to do halftones or any really fine lines, it's the worst method in terms of holding image definition in your stencil.

I've never seen anyone complain about investing in a really good exposure unit, even with all the workarounds.
 

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I just switched to QX-1 and have this exact exposure unit. It's a lot faster than the other emulsion I was using. I did a step wedge test and the QX-1 exposes in 2 minutes for me. Your mileage may vary.

It took me a while to find the right exposure time because my first step wedge test started at 3 minutes, so I was over exposing the entire test. I did a second test at 30 second intervals and finally found that 2 minutes worked best for me.

I know the thread starter already figured out his exposure time a long time ago but thought anyone else searching the forum who has this setup would like a ballpark exposure time for use in doing their own testing.
 

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Hi All!

Before I joined I read this post many times because I was having the same problem. I have to say that the step wedge test was totally worth it! I really thought that I could just "find" my correct exposure time by trial and error and it proved to be a total waste of time. Just wanted to say thanks... now that I am member. No more underexposure slime on my screens!
~j
 
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