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screen angle issue

Posted on Thu, 11 Nov 2004 at 13:44



we've been having a problem lately with moire in our cyan screens. the angles we are using for 100lpi art is c 37,m 67,y 82 k 7. everytime time i shoot the cyan film on a 390 mesh, there is a moire pattern, no matter how i place the art on the screen. the strange thing is if i shoot it on a 355 mesh it cures the moire. been thinking about getting some 420 mesh to see how that responds since the angles listed above were from the sgia and said that we might get "localized moire" issues with those angles. does anyone have any ideas's on what might put us in the correct direction with this issue.
thanks for reading....
craig

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Anonymous says: You might want to check you EOM's on your screens. We were getting the same problem, spent weeks reworking out angles only to have Autotype (our emulsion vendor) come in, check our EOM's (emulsion over ...

You might want to check you EOM's on your screens. We were getting the same problem, spent weeks reworking out angles only to have Autotype (our emulsion vendor) come in, check our EOM's (emulsion over mesh) and found we were only geting 1 to 1.5. We've bought a meter of our own, checking every couple of weeks and are up to EOM's of around 4 consistently. We've also started to face coat and are getting RZ's down around 6-7 (from around 13-14). Our moire problem has completely dissapeared and we've been able to footprint our presses and are getting phenominal consistency in our pre-press.

Get your screen supply vendor in and have them discuss the way you can measure your screen making to eliminate moire problems (don't just blame your film output, like we did for years).

Good Luck
Rocky

posted on: Fri, 11/12/2004 - 2:07pm
Anonymous says: Most of the time moire is not a film issue. It's mesh. Rocky is right using the face coat, and keep the EOM down to stay away from the topographical profile of the dots. Mesh manufactures cannot hold ...

Most of the time moire is not a film issue. It's mesh. Rocky is right using the face coat, and keep the EOM down to stay away from the topographical profile of the dots. Mesh manufactures cannot hold to a given thread count. A 380 has a plus/minus of 3%, thats 11 threads. Bolt "A" purchased today could be a 390 and you might see no moire. Bolt "B" purchased two months from now might be a 370. Both are within the 3% of manufactures specs but twenty threads different, which is a significient variation, and more then enough to cause moire.

When we get moire, and we do, we switch mesh counts. A 380/34 and a 355/34 and a 330/34 deposit the same height (amount think hockey pucks) of dot (within a micron or two) and the diameter (percentage of dot) is controled by the exposure. We do it all the time and it works well. We teach that concept in the SPTF workshop on four color process.

On high lpi (118 plus) we use 406/27 and 420/27 fabric, mostly the 460/27 with Autotypes CP emulsion. Has about a 2.5 EOM and a fablous Rz value as it is a film, not a direct coated emulsion.

Watch the inks and look at the dots under a high power loop. Make sure the dots are dots and not breaking up because this will give the the visual effect of moire also. If you are not getting a clean transfer of ink it can give the same visual effect as moire.

Bron Wolff

posted on: Fri, 11/12/2004 - 2:47pm
Anonymous says: Hi Craig, You mention the word "lately" in your post. If you have not had the moiré problem before with this angle set, and then it has started to show up, ask your self what has changed? As Bron had ...

Hi Craig,

You mention the word "lately" in your post. If you have not had the moiré problem before with this angle set, and then it has started to show up, ask your self what has changed? As Bron had stated, the mesh would be the first place to look. Mesh manufactures have a very hard time weaving a symmetrical bolt of mesh (equal weft & warp in thread count). The spec that most of them have is +-3% in tolerance from weft to warp, BUT ALSO from 3 to 10 percent elongation after stretching, all depending upon the mesh manufacturer that you buy from. Are you counting your threads weft and warp before stretching, during stretching, and after stretching? If your 390 comes in to your shop one day as a 389 weft x 381 warp, and you have no moiré. But the next bolt comes in as a 401 weft x 379 warp and moiré shows up. Then you can see right away what has changed. Both bolts would be to "spec” as far as the manufacture is concerned, but the later will give you unacceptable results for printing half tones. You want the numbers to be as close as possible to each other after stretching. The tolerance should be 7 to 10 threads from weft to warp, (I like to try to get it to around 4 to 5 threads difference) after stretching. Some mesh manufactures also do not have a balanced elongation weft and warp. You can see this in a heartbeat with roller frames that are large format. You will turn the one set of rollers only half as far as the other set to get the same tension, or you will max out the rollers to get the desired tension that you need. Balanced weft and warp thread counts are the key to moiré free prints. Not balanced tension. It doesn’t matter what the numbers are to start out with, (389, 369, 399, etc.)
They need to be as close as possible to give you a square mesh opening and not a rectangle. If you have a rectangle mesh opening, that means that your mesh will have to different algorithms that your line count will have to deal with, or in other words, it’s like trying to shoot a half tone on two different mesh counts at the same time. This will usually always cause a moiré pattern that looks like a wood grain or wavy lines.
Is your screen tensioning procedure consistent? How many people in your shop stretch screens? Do you label your stretched screens with the date they were stretched, the manufacturer, the mesh count, the thread diameter, operators initial, etc.? What is your format size for your screens? Are your screen tensions consistent from screen to screen? Different screen tensions lead to different percentages of elongation, thus making different mesh counts from screen to screen.
If you have no moiré with the 355,34 micron mesh, then stick with it. Find out what works in your shop, and document everything. If anything changes you will then be able to see it. If your bolt of mesh comes in the shop with a different mesh count than the previous bolt did, send it back. Make sure that your mesh supplier is aware of what you will and will not accept as far as tolerance in the bolts goes. That way they should not send you any thing that is to far out of “spec”.

On a different note. What kind of film are you using for your separations? If you are using an Oyo or an aspect unit to make your films, the image setter its self will create a moiré like pattern in the film out put. Depending upon what angle you choose. Check the film before you shoot to make sure that this is not the problem. If you are using an inkjet type of plotter to make your films (Quadra, Epson, Mimaki Jv-4, etc.) the films might look good before shooting on the screen, but you can get a moiré after exposing the screen. This is due to not having a "hard" dot on the positive. The centre of the dot will be very opaque, but the edges will not and the light will burn through the lower density areas of the dot. This will cause a different image to be made in the screen than what you though you were getting. And what do you know... Moiré will pop up, usually in the lighter areas like the highlights. You also should be checking to see what the density of your film is as well. If your Dmax of your film (the dark areas) are not at a density of at least a 3.5, then the above mentioned problem will happen every where.

I hope this helps you.

Let me know if there is anything else that you need.

Best regards, Peter Duff.

posted on: Sat, 11/13/2004 - 1:01am
Anonymous says: try using the SAME lpi and angles on your next 4CP job- you'll be suprised how well it works (assuming you use same tension/mesh screens) have your art department put a test square of say 25% gray in ...

try using the SAME lpi and angles on your next 4CP job- you'll be suprised how well it works (assuming you use same tension/mesh screens)

have your art department put a test square of say 25% gray in multiple linescreens to the side of the print- you will sometimes get moire just changing shirt brands and a little test square on the side can show you what to re-output at.

posted on: Thu, 11/18/2004 - 6:08pm

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