emulsion slime on one side of screen after exposed/rinsed

[Mark @ Hurricane Printing]

I burned two screens last night. I'm using the emulsion you mix together. I dried the screens in a drying cabinet in 2 days...meaning when i coated them i put them in the cabinet (which is small..can only hold six screens) with a small heater in it and also has a vent for the heat to escape....I let it run for about 30 minutes and then unplugged the heater then called it a night (I had no intention of burning the screens that night)...the next night when i came home from work i plugged the heater in again and let it run for about another 30 minutes to eliminate any moisture that may have built up within the cabinet....after i let them dry I burned one screen...rinsed..and used news paper to absorb some of the water...I know not to wipe the screen...i lay a full piece over each side and let it absorb...some slight patting going on but thats it..i peel it off and flip the screen around and do the opposite side with the same paper and same method.

Well last night i did this like i always have and when i peeled the newspaper off of the squeege side of screen the emulsion was slimey and put a slight film of emulsion on the paper. The other side, which I dry off first always, did not do this...its was only the squeegee side.....on BOTH screens i burned.

Why is this? Under exposed? Didn't get the moisture out of the cabinet?

[Prosperi-Tees]

Under exposed for sure. Did you change your coating technique from previous screens? Did you change emulsion?

[Denis Kolar]

Underexposed

[Mark @ Hurricane Printing]

Under exposed for sure. Did you change your coating technique from previous screens? Did you change emulsion?

previously i had the ready mix emulsion....but this isnt the first time i had the kind u mix. I just dont recall this ever happening before with ready mix or the kind u mix.

[Prosperi-Tees]

There are so many variables in this situation. Most will say you need to do a exposure calc test which I have never done. But if you changed emulsion brands you need to find out what times you need to burn.   The difference between pure photopolymer and diazo (the kind you mix) emulsion burn times are huge. I can burn a pure photopolymer in 30-45 seconds but the diazo emulsion I use takes between 4-6 minutes. Again alot of variables here.

[Mark @ Hurricane Printing]

There are so many variables in this situation. Most will say you need to do a exposure calc test which I have never done. But if you changed emulsion brands you need to find out what times you need to burn.   The difference between pure photopolymer and diazo (the kind you mix) emulsion burn times are huge. I can burn a pure photopolymer in 30-45 seconds but the diazo emulsion I use takes between 4-6 minutes. Again alot of variables here.

aahhh...well i had my exposure times set by my mentor and never deviated from that....i used the same burn times for ready mix and diazo....cant say i ever had a problem...or maybe i just never noticed it but that would be impossible...because last night its was VERY noticeable...i was like: "wow.....wth???..why is it so slimey and only on this one side???"

[3Deep]

Mark, simple answer is to increase your exposure time, dual cure you can burn a little longer if your using yellow mesh longer burn time white mesh shorter.

Darryl

[kirkage]

could it be the humidity? i keep my box at 45% humidity max. also, this orange emulsion by ulano is hard as a rock. (tricky reclaim though, but never any slime) Its probably under exposure but humidity is a factor in naptown during the summer.

[Mark @ Hurricane Printing]

could it be the humidity? i keep my box at 45% humidity max. also, this orange emulsion by ulano is hard as a rock. (tricky reclaim though, but never any slime) Its probably under exposure but humidity is a factor in naptown during the summer.

i had a free sample of that...it was pretty decent..thats what I had before this diazo i am using, which i have used this brand of diazo before.

And no humidity on these screens ..i let the heater (that blows heated air) run for a good 30 minutes and didnt let them set in the box before i burned the screens...I made sure of that.

[Frog]

There are so many variables in this situation. Most will say you need to do a exposure calc test which I have never done. But if you changed emulsion brands you need to find out what times you need to burn.   The difference between pure photopolymer and diazo (the kind you mix) emulsion burn times are huge. I can burn a pure photopolymer in 30-45 seconds but the diazo emulsion I use takes between 4-6 minutes. Again alot of variables here.

aahhh...well i had my exposure times set by my mentor and never deviated from that....i used the same burn times for ready mix and diazo....cant say i ever had a problem...or maybe i just never noticed it but that would be impossible...because last night its was VERY noticeable...i was like: "wow.....wth???..why is it so slimey and only on this one side???"

Why Gerry would specifically avoid using an exposure calculator is anyone's guess, but I (and a certain superhero friend) could never advise that. At the very least, try a manual step wedge test, the instructions which I have included. Previously posted at Screenprinters.net by Ken Brown And, what the hell, if this isn't a place to bring back Step Wedge Man, I don't know what is!

Sorry this is so long winded - Here are some basic step wedge instructions.

STEP WEDGE INSTRUCTIONS
Start with a properly coated, and dried screen.
If you would like basic coating instructions let me know.

Print out a full size film or vellum (enough to cover most of the screen). Use the common fonts, line sizes,halftones that you often print. This becomes your sample film for the step wedge.
Tape the film to the "print side" of the screen (not the glass)
Use an estimated exposure time based on the emulsioin manufactures recommendations a guide and then multiply them by 2.
For example, 110 White Mesh with estimated exposure time of 3 minutes would double to 6 minutes.
Expose the entire screen for 1/6th of that time (1 minute).
Split the screen into 6 equal sections, cover the screen with something thin and opaque like a manila folder, or if you have it, use Rubylith.
Cover 5 of the 6 sections of the screen leaving 1 section exposed to the light and expose for 1 min.
Slide it over to expose 2 sections of the screen and expose for 1 min, etc .
When you are finished this will give you a 1, 2, 3, 4, & 5 minute exposure test on 1 screen.- you can also vary the times from 1 min to 30 or 15 seconds when you get it dialed in closer.
When washing out the screen, pay careful attention to how it is washing out:
If it is under exposed the inside (squeegee side) of the screen will be slimy
If over exposed the fine detail and or halftones will fill in and you may start to see some saw tooth or jagged
edges. If you have weak density on your film or vellum the light may burn through some areas so underexposing the screen may be your only choice.
Somewhere within this exposure test you should be able to find a good exposure time for that mesh count.
You can use that exposure time for that mesh count to estimate times on your other mesh counts.
Keep in mind that the courser mesh counts will have longer exposure times because the mesh is thicker and it holds more emulsion (more emulsion = longer exposure).


Ken Brown

[Prosperi-Tees]

I agree it should be something that I/we all should do, I didn't mean to suggest it was a waste of time. I guess I have been just lazy. Although I am unsure of what I would accomplish with doing one as I do not have trouble with my screens at all. That being said I don't do much of any high end type art where every dot is crucial to the print. We are 1-3 color mostly bold type work with a sprinkle of 2-4 color low end halftone work. I have always used diazo emulsions and only dabbled in pure photopolymers and my exposure times for all brands of diazo emulsion I have tried have been in the 4-6 minute mark.

[Frog]

I just wanted to emphasize its value and not make it seem unimportant.

[Sbrem]

Follow Frog's advice. Anytime you change something in your recipe, you have to test, because things are no longer the same. Your mentor's instructions were fine, but you changed the emulsion, so his exposure times no longer apply. You can bet we make tests for every emulsion we use, and log them into a book. And yes, it was underexposed. The exposure light has to get all the way through the coat, just the same as the heat in your dryer has to get all the way to the bottom of the ink, so it stays on the shirt. Hope this helps.

Steve

[Evo]

Might I add:

Run an exposure test for every emulsion AND mesh combo you use. Example, if you use two different emulsions and say six different mesh counts, that's at least twelve exposures to calculate.

Make sure the coating method is consistent from screen to screen, as varying emulsion thickness can have a drastic effect on exposure times.

Note that it takes a certain amount of total UV energy penetrating the emulsion from the print side to the squeegee side for the emulsion molecules to cross link. This is what is referred to as "units" with exposure integrators on most exposure equipment. You want a total sum of energy to hit the screen per exposure, and a good integrator will measure this and add time to the exposure as the lamp degrades.

If you don't have an integrator, then you need to re-do your exposure calculations every time you change the lamp, AND every 3-6 months or so or whenever you notice screens are not getting proper exposure (over OR under).

A properly exposed screen will hold detail, but not "pinch" it.

Diazo emulsions often have a wider exposure latitude than photopolymer and/or dual cure emulsions, but it's still important to narrow down the optimum exposure for long term durability of the stencil and to avoid reclaiming difficulties.

I am a firm believer in Ulano's exposure calculator kit, or Chromaline's. (the two-piece kits with the included film of neutral density filters) It makes quick work of the process.

[Evo]

Oh and get the heater out of your screen box! A cheap dehumidifier placed in the screen room is a better choice. You want to pull the moisture out of the air and let the screens dry gently. Adding heat can affect the stencil exposure and quality.