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Hi all--I am using a Yudu machine for the first time. I got some Ulano QTX instead of the cap film and got a great thin coat but could not get the image burned. I saw a faint faint outline of the image but when washing out nothing. Then it dried and I saw nothing. I am thinking that there was too much ambient light in my basement when I took the screen out of the drier unit and the dry emulsion exposed perhaps. I am going to reclaim, recoat, and try again, but I am hoping that perhaps someone has a recommendation on exposure time with this emulsion and the low-light Yudu (which I really only wanted as an exposure unit!). And would it be helpful to double up the transparency to get a darker outline of the image? thanks for any tips!
 

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I used an inkjet printer (Epson Stylus) on inkjet transparencies. When I learned screenprinting at the art center, we used oiled paper or vellum, but they had a nice, big exposure unit and dark room. With that typical time was about 7-9 minutes. I don't remember the exact emulsion used then, except that I'm pretty sure they mixed it there (wasn't presensitized) and it was light brown in color. thanks!
 

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How long did you expose the QTX? If you are going from a capillary film to a very thin coat of QTX your exposure time should drop significantly.
 

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that's why I wondered if I messed up the screen with ambient light, because I exposed it for 8 minutes and nothing. I wondered if the Yudu lights are small and it needed more, but after 4 more minutes that is when I faintly saw the image and yet nothing washed out and upon drying saw nothing. With the cap film, I tried burning and saw the typical color change where the image had burned (from green film to yellow) but when I went to wash out, big chunks of the emulsion came out too. And since I hated applying it, didn't do it well, and it's expensive , and I feel comfortable scoop coating a screen I bought the QTX.
 

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I have never used a Yudo, so I dont know much about its bulbs, but my guess is it would be similar to standard fluorescent exposure unit. I would say cut it back to 3-4 minutes total and see what you get. On a standard fluorescent unit you can get burn times in the 2-3 minute range when using a photopolymer.
 

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Process of elimination to diagnose development troubles

Color Change
The private label Yudu direct film you used changed color as the 'yellowish' diazo sensitizer reacted to UV energy. Ulano QTX is sensitized with SBQ and doesn't change color.


Storage or Positive Failure
Your emulsion is performing well, but if your image area didn't dissolve with water, it means your positive failed you like a raincoat made of cheese cloth, or you've had storage failure, and your storage area is not safe.

Your positives worked before, so I tend to think your storage area isn't safe for the much faster SBQ emulsion.


Experiments
Your new job is to discover where/how your stencil is getting exposed. After coating, as soon as the stencil begins to dry, tape a dime or some aluminum foil in an out-of-the-way area of your coating. Since you covered a patch of stencil with a guaranteed UV stopper it should wash out like a dream, even if you store it for months.

Storage Test
Check the safety of you storage room by coating a small area of a screen, then putting a coin on the screen each hour/day/whatever - for a day/week/month, then wash out the screen with water. This will tell you how long you can store screens in that area without exposing the stencil.



Establish a washout time

To experience a base time for washout, make a very small coating (3 inch by 6 inch?), on a screen and let it dry to the touch - then take it to your washout area and time how long it takes to dissolve.

Don't expose it, just wash it out. That's your standard time for washout for that mesh & coating. Watch the clock as you develop the screen. Average screens don't take longer than 1-3 minutes at most. Unexposed, there is nothing to to stop it from dissolving - just like a properly covered image area.

Measuring Stencil Hardness
Without a step wedge test positive, you can't tell if you actually cross-linked all the sensitizer in your stencil -especially an SBQ stencil that doesn't change color.

The best exposure test for stencil hardness is a US$10 Stouffer T-2115 21 Step Transmission Gray Scale.
Transmission Step Wedges

A transmission gray scale is a narrow 5" film positive with darker and darker filters next to each other in steps. This is a standard photographic darkroom test positive that's been used since the 1930's.

When you tape one on the stencil as you expose, you'll simulate 21 different exposures to the stencil and you get visual feedback of how well your stencil is cured.

When you develop the stencil, areas that didn’t get enough exposure will dissolve with water and rinse down the drain. Aim for a minimum of a Solid Step 7 that adheres to the mesh and survives development. More exposure will make your stencil more durable and less will make the stencil less durable, but light scatter could start to choke fine lines or halftones.

Properly used it should last a lifetime, so I want you to put one on every screen you expose for the rest of your life. It's visual feedback you exposed the stencil properly.

Stouffer Instructions To Measure Exposure
Exposure FAQ Screen Making Products how to measure exposure Exposure FAQ Screen Making Products how to measure exposure


For me, discuss with the person that suggested you buy SBQ sensitized QTX instead of whatever you used before - how they could sell you a different emulsion without asking if you had a way of measuring exposure of a stencil that doesn't change color.

If you just bought online without talking, never mind.
 

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Discussion Starter #9
thank you *so* much for the detailed info. It helps me a lot to understand the reasons behind everything, such as the color change with diazo and not with SBQ. I did manage to get a stencil made today. I reclaimed and recoated in very low light, dried overnight and exposed the screen for 4 minutes and the image washed out fairly easily. It's not the crispest at the edges but I attribute this to the positive not printing super clean off the inkjet printer. I will print with it once to see if it clean enough.

The guy at the store didn't seem to know anything about the Yudu machine, barely seeming to know if its existence. This was at a many-decades old Chicago silkscreen shop. I just said that I did not want to mix in sensitizer and he recommended this.

Seems I've figured out the exposure time now though. thanks so much to all for the help!
 

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I will still ask the Screen Police to arrest him for selling emulsion without checking to see if you had a way to measure exposure.
 
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