Coating, Exposing Screens

[Dochertyscott]

Hi All,

After reading a couple of forums and tech newsletters last night i tried today coating my a few screens in a different way than usual...
I usually coat starting substrate side 1:1:1:1(the last being a more horizontal scrape type method to gather the excess emusion and push through to the substrate side)
This is how i was taught at previous workshops and how i have coated screens now in my own workshop for years
Im using KIWO SWF

Today i tried a new methods after reading up on the EMO Last night....
So i tried a 1:1 this gave me a much thicker build on the substrate side...
I tried a 1:2 Even more build... obviously
And then my usual 1:1:1:1

I exposed the 1:1 screen firstly at my usuall time of 240secs
The lines look good... high releif. but in some spots the emulsion has fallen off the mesh...
I exposed another 1:1 at 300 secs and the same.
I then exposed my 1:1:1:1 at 240 Secs and great as usual but not that nice high build therefore not as crisp lines especially for fleece.
now i just exposed my 1:2 for 420 Secs and the same the emusion is just falling off the screen.

I do want a good build to get those nice sharp lines especially for fleece...

Any ideas? my initial thoughts was that the screen hadnt been degreased... my worker assured me it was.
Then i thought underexposure... (because there was more emulsion it needed more time...) near on doubled the exposure with no luck.

I need to coat more screens and want to try the way recommended by most but dont really want to waste too much time

Should i dry them squeegee side up to form more of a bond? (I've always been told squeegee down.)

Id appreciate any feedback.

[Evo]

a) buy an exposure calculator film. Ulano and Chromaline make excellent ones. It will really help you narrow down your exposure times to be optimum.

b) if your exposure unit does not have an integrator, then re-calibrate with the exposure calculator every couple months, and at every lamp change

c) To build thick stencils, I always coat 1:1 starting at the print side, moving slowly with firm pressure. Dry, then add one or more coats to the print side.

d) For each coating method and mesh combo, run the calculator. Yes this may mean dozens of exposures. Do it.

[Dochertyscott]

So do you think this is from under exposure?

[Dochertyscott]

a) buy an exposure calculator film. Ulano and Chromaline make excellent ones. It will really help you narrow down your exposure times to be optimum.

b) if your exposure unit does not have an integrator, then re-calibrate with the exposure calculator every couple months, and at every lamp change

c) To build thick stencils, I always coat 1:1 starting at the print side, moving slowly with firm pressure. Dry, then add one or more coats to the print side.

d) For each coating method and mesh combo, run the calculator. Yes this may mean dozens of exposures. Do it.

I will get a calculator and get the exposures sorted asap but at the moment was hoping someone could tell me from experience why the emulsion is falling off the screen in this way...

Cheers

[abchung]

What type of exposure unit are you using?

[Dochertyscott]

1100w metal halide home made jobbie.

[Dochertyscott]

After reading the Ulano calc sheet I think it is still underexposed and not 'cross linking' the emulsion. Does this sound right?

[chubsetc]

Just a shot at it but not fully rinsing out a degreased screen can cause issues like that.

[Denis Kolar]

1100w metal halide and you still need over 400 seconds?

That is a bit fishy. You sure your emulsion is OK?
My Nuarc 40-1K which is Mercury (Slower) exposed ChromaBlue in less that 90 seconds.

[Gilligan]

1100w metal halide and you still need over 400 seconds?

That is a bit fishy. You sure your emulsion is OK?
My Nuarc 40-1K which is Mercury (Slower) exposed ChromaBlue in less that 90 seconds.

Agreed, when I had a home made MH I was running hmm... 3 minutes? I'd have to ask my guy if he remembers, I've never looked back after getting my 3140 (best purchase I ever made!!!).

Also, we recently had some "under exposed" issues and it was due to the emulsion getting "rank".  Smelt like doo doo in the bucket but my guy couldn't see throwing out 1/4 of a bucket of emulsion and was still getting "good" stencils out of it, but our exposure times were getting higher and higher and we were still technically "under" (we run an exposure calc strip on EVERY screen).  I finally told him to stop using it and he cut back exposer, WAY back and we over exposed a couple of steps... so definitely our problem.

If you have a super thin layer of emulsion then your times could be dramatically different.  Also what is the distance from bulb to film?  Inverse square law comes into play on your exposure time.  It's possible that your time is "correct" for your distance.

[alan802]

Looks underexposed to me.  Just coat using the glisten method, it's been talked about quite a bit here (even in the last few days) and it's truly the best way to coat a screen for textile printing.  It's the method that all the emulsion companies recommend and I know at our shop when using it the EOM is right where it's supposed to be when I measure.  As long as you're not using a watery emulsion then you can build a sufficient EOM coating wet on wet without face coating.  ALWAYS dry your screens with the shirt side down, squeegee side up.  And it looks like your bulb is toast but it's been years since I tried Kiwo SWF but I seem to remember it taking a long time to fully expose.  On our 10K metal halide dual cure emulsions may take 20-25 light units and for us with a decent bulb is just under 200 seconds to 250 seconds.  How old is your bulb? 

If I were you, which I'm not, I'd use an SBQ pure photopolymer emulsion with a 45% solids content or higher then coat all your screens using the glisten method, get an exposure calculator like others have said and get to testing.  Oh, and check how old that bulb is.

I did some exposure testing yesterday since I hadn't done it yet with the HVP emulsion and it was one of the few times we've used an exposure calculator and it turned out we were dead on with our exposure times already.  It's nice to know our not so scientific way of determining exposure time is right on with the calculator. 

[Dochertyscott]

1100w metal halide and you still need over 400 seconds?

That is a bit fishy. You sure your emulsion is OK?
My Nuarc 40-1K which is Mercury (Slower) exposed ChromaBlue in less that 90 seconds.

Agreed, when I had a home made MH I was running hmm... 3 minutes? I'd have to ask my guy if he remembers, I've never looked back after getting my 3140 (best purchase I ever made!!!).

Also, we recently had some "under exposed" issues and it was due to the emulsion getting "rank".  Smelt like doo doo in the bucket but my guy couldn't see throwing out 1/4 of a bucket of emulsion and was still getting "good" stencils out of it, but our exposure times were getting higher and higher and we were still technically "under" (we run an exposure calc strip on EVERY screen).  I finally told him to stop using it and he cut back exposer, WAY back and we over exposed a couple of steps... so definitely our problem.

If you have a super thin layer of emulsion then your times could be dramatically different.  Also what is the distance from bulb to film?  Inverse square law comes into play on your exposure time.  It's possible that your time is "correct" for your distance.

yeah i have been looking at getting a 3140. my new 6/6 Sidewinder has turned up today... (getting setup on Thursday) so i'm thinking that the next investment has to be a good exposure unit.
This home made exposure unit has served me well over the last 3 years... same bulb... perhaps thats the problem?
It seemed fine untill coating with this new method of 1:1  leaving a 'glisten' as they call it and a much higher build than usual.
The method i have always used of 1:1:1:1 last storke being an excess clearer and pushing emulsion to the substrate side. which has always exposed in 180 secs nicely.
I tried another one just now 1:1 on a 59s with exposure of 360 secs and the same problem.
I'm going to stick with my same old method tried and tested and when i get my new exposure unit i will try the glisten method again and recalculate.

[mk162]

i used to coat that way, it seemed to work fine.  I might play around with it again.  I know it's not "proper" but if your prints are good, your halftones look good and you are happy, sometimes it's not worth changing.

[Gilligan]

We do the scrape thing when we screw up and want to try and "start over".

[screenprintguy]

if the m/h unit is home made, what kind of light is it? is it an m/h lamp for security lighting, or shop lighting, or is it an actual m/h bulb for photo exposure. There really is a difference. Just like led lights that have zero uv, and led lights that when configured right, seem to have serious amounts of uv output. We bought a UV meter a few years back and we saw an extreme difference between the uv output of a 1000 watt photosharp, and the 1,000 watt m/h shop lights that were in our ceiling fixtures, and I can tell you that those shop lights would have had a hard time exposing a screen. I could be wrong, you may have the proper lamp set up in your home made unit, but for others, there is a difference between actual lamps.