4 color process printing is proving to be more difficult than I initially expected and for all the reasons I thought it would't be. I'm struggling to get a job printed by 9am (tomorrow) and I'm getting moire on my positives when i line them up to each other.
Is it acceptable to get moire on your positives, or is that simply a sign that your actual artwork will print with moire?
My setup:
Original artwork LPI is 137.5
Epson 1400
Photoshop CMYK channels used to print the separations
Design printed with a 45 LPI
Angles:
C - 15
M - 45
Y - 75
K - 75
I also tried printing at 55 lpi with the following settings
C - 7.5
M - 67.5
Y - 22.5
K - 37.5
Any help/comments/suggestions would be more than appreciated.
Those angles willmoiré, they're pretty much guaranteed to do so. The problem with digital angles is that they have to correspond to a square - i.e. the cell must be something like 10 dots x 10 dots, it can't be 10.5 x 10.5.
You need a decent RIP (I recommend Wasatch) or a decent techie (modesty forbids). You could try printing all plates at the same angle, I've seen that done.
Dave
Photoshops "Print with Preview" menu can calculate the angles/frequencies, except it will not accept the low resolution used in screen printing.
Just double the resolution from 137.5 dpi to 275 dpi and set it to auto calculate.
The angles Photoshop uses are always the same, it will just change the frequencies to match the hole-number condition Dave mentions. The angles on cyan and magenta are chosen so that tangens to the angles are 3 and 1/3 respectively.
We run all our 4 color process at 55lpi and 61 angle, learned this angle last year at LB ISS, tried it and have never looked back, it will hold a complete page od half tones and never pattern one bit.
Thanks again for all your input! The job was a last minute one and I was doing my best to get it out to the customer to meet one of their deadlines but it wasn't feasible. I had carefully detailed the process to them before we begun (and he has worked in a shop before) so it (extending the time) wasn't a big deal.
This dialogue has brought up a question that I sincerely hope we can, in this thread, put to rest once and for all.
Do halftones in four color process need to have separate screen angles and why / why not?
Some have said they do and increasingly, people have said that they don't. Moire is from what i've learned thusfar a function of noise/interaction of the mesh with the screened image as well as possibly interaction between different color/screen angles amongst themselves.
I understand that the popular school of thought is that "if it works, it works and I won't question it" but this only flies when things are working properly and is exactly what keeps people cutting the christmas turkey before cooking year after year without questioning it...only to find that the only reason they were doing it was because someone's great grandmother started the tradition out of necessity (her oven wasn't big enough to fit a whole uncut turkey).
Naga took the initiative and posted the excel worksheet with some of the math behind choosing specific LPI / screen angles. Lets keep this going so we can once and for all get to the bottom of this.
Not quite sure how to summon some of the big dogs of the forum (Ridgely has already chimed in thusfar), but here goes (tongue in cheek): ghostscript doesn't work (ImageIt), my exposure calculator is reading out backwards (Richard Greaves), is it possible to get white down with a single stroke? (Bill Hood) and customers are always right (TShirtGuru and everybody else that has every had a customer )
(to avoid any confusion and to clarify the humor, Ghostscript works great, exposure calculators are absolute necessities with every screen you expose, single stroke white is easy (thanks Bill) and customers are always right )
BTW: NAGA - You need to convert those angles to radians before running them through a sin/cos or any other type of trigonometric function. Excel expects these values as such by default (assuming you haven't changed that default somewhere).
BTW: NAGA - You need to convert those angles to radians before running them through a sin/cos or any other type of trigonometric function. Excel expects these values as such by default (assuming you haven't changed that default somewhere).
Oh yes, I just left out some details in the column headings. I should have mentioned that.
Another thing is that there is also texture in the textile, that can interfere and which should be taken into account. But I am not experienced enough to go into that discussion.
Moire on positives before print... 
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