TSB
screen printing => Screen Making => Topic started by: Dottonedan on February 01, 2016, 07:17:15 AM
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Mesh coating. Type the first thing that comes to mind.
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Dried vertically instead of horizontally?
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Mesh coating. Type the first thing that comes to mind.
Inadequate emulsion level in the coater, tried to coat too many screens prior to refiling coater. Always coat out of top of coater, never coat when less than half full.
Same thing happens in a screen on press when ink level allowed to get too low and starts to streak due to ink starvation.
Mesh count, emulsion type and photo magnification please.
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I'd vote for Mr. Kitson's response
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I believe the mesh is an 80 mesh. Possibly a 60.
This is a shot from the side of the screen. Not top or bottom. So the non coated area is about .5"-1" from frame side. The 2nd coat on the other side is slightly off center. Showing a good example of just the one coat and then the next on the other side.
Not sure if it wwas coated with the sharp it the round but I'd guess the sharp side.
I'm wondering how many think this is ok or would do the same thing here. To me, this is grossly too thin. This would cause irregular edges to the image or a waffle like print in what was to be a smooth edge of an image.
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Poor coating technique. Not enough EOM..I agree with Mr. Kitson as well.
Murphy
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How'd they do that? Every other mesh opening on the right side is free of emulsion.
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How'd they do that? Every other mesh opening on the right side is free of emulsion.
Every other cavity filled in this regular fashion has to do with the way the coater edge interacts with the weave. Over thread cell not filled, under thread cell filled.
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The screen in this pic is more like an example of "EIM" (emulsion in mesh) rather than EOM. With a screen coated like this, edges of the imprint are at the mercy of the pattern of the mesh threads themselves. Diagonal edges will probably look saw-toothed (or "stair-steppy", similar to my explanation to custies of their way-too-low resolution of a bitmap causing the "Atari" effect")
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I believe the mesh is an 80 mesh. Possibly a 60.
This is a shot from the side of the screen. Not top or bottom. So the non coated area is about .5"-1" from frame side. The 2nd coat on the other side is slightly off center. Showing a good example of just the one coat and then the next on the other side.
Not sure if it wwas coated with the sharp it the round but I'd guess the sharp side.
I'm wondering how many think this is ok or would do the same thing here. To me, this is grossly too thin. This would cause irregular edges to the image or a waffle like print in what was to be a smooth edge of an image.
Person coating probably tired to use round edge but choose to use sharp because round edge overloaded cell capacity to hold emulsion and created drip medallions due to too much emulsion transfer. Higher viscosity or coat/dry/coat/dry/coat... techniques may improve. Poor emulsion/mesh choice is a strong consideration.
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Too low a screen tension to coat the screen well. Outside edge is supported due to proximity to the frame, the center has little tension and is building EOM too high.
IMO>
Al
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Also, with very coarse mesh it's better to coat 1:1 dry it, then face coat print side to lower RZ value. It takes a high viscosity emulsion to coat this to proper EOM constistently. Better to seal off the mesh with an even coat of 1:1, dry, then come back with multiple face coats.
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If the picture is of the side of the screen, how 'bout the coater was not horizontal, leaving one side low on emulsion, and the other end ok.
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I can't really tell much of anything from that pic without seeing the rest of it. On a similar note, I did some testing a few months back to see if my new screen guy was wasting time or not with his coating technique which is quite frustrating to watch for me. I coated about 50 screens one afternoon and all I did during this particular coating session was document the level of emulsion in the coater and then measured the screens once they were dried. The screens that were coated with extremely low levels of emulsion in the coater were noticeably thinner out on the edges but there was only a difference once you got well below 1/4 full. The thickness gauge couldn't tell which screens were coated with a 3/4 full coater or a 1/3 and everything in between. The results I saw didn't gel with traditional knowledge on the subject and full disclosure I've always kept the coater between 1/2 and 3/4 at all times and was very obsessive with that.
I'd be interested to see if anyone else had the time or want-to to do a similar experiment the next time they do a coating. It would be good to see if the results were similar to mine or if they're different I'd have to go back and get our procedures in order. Let me know if anyone would be willing to do something like that.
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I think as well that too low screen tension is to blame. See this all the time over here, a country were they perfected low screen tension.
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I would think the thinned area indicates the process of the coater rolling out / back at the end of the stroke, OR the beginning of the stroke where the coater is rolled in and possibly started forward too soon before the emulsion fully runs to the edge of the coater.
mooseman
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For most of the '80's we used Advance's Swiss Yellow, 60 mesh, because of all the 60 meshes, it had the thinnest thread, 123 microns (why I remember that when I'm not sure what I had for dinner last night, I don't know) and always coated with the thin edge; one develops the proper speed and pressure after a few, and no more than 2 screens without topping off the coater...
Steve
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Also, with very coarse mesh it's better to coat 1:1 dry it, then face coat print side to lower RZ value. It takes a high viscosity emulsion to coat this to proper EOM constistently. Better to seal off the mesh with an even coat of 1:1, dry, then come back with multiple face coats.
I'm glad you can say that without a bunch of people telling you what a bad idea that is. :)
I can't really tell much of anything from that pic without seeing the rest of it. On a similar note, I did some testing a few months back to see if my new screen guy was wasting time or not with his coating technique which is quite frustrating to watch for me. I coated about 50 screens one afternoon and all I did during this particular coating session was document the level of emulsion in the coater and then measured the screens once they were dried. The screens that were coated with extremely low levels of emulsion in the coater were noticeably thinner out on the edges but there was only a difference once you got well below 1/4 full. The thickness gauge couldn't tell which screens were coated with a 3/4 full coater or a 1/3 and everything in between. The results I saw didn't gel with traditional knowledge on the subject and full disclosure I've always kept the coater between 1/2 and 3/4 at all times and was very obsessive with that.
I'd be interested to see if anyone else had the time or want-to to do a similar experiment the next time they do a coating. It would be good to see if the results were similar to mine or if they're different I'd have to go back and get our procedures in order. Let me know if anyone would be willing to do something like that.
I appreciate this. My SOP for many years has been adding emulsion when I get under half or so.
I've never had a thickness gauge but have done print testing and couldn't find major differences in the results of anything but higher tolerance halftone work. I stay obsessive with those screens. :)