Different lpi & angle readings in one plate

[sandpiper]

Hi all. While printing one job we noticed that a moire effect was very evident for the whole sheet. Both press and prepress guys agreed that it was the magenta plate causing the moire.

We print our plates from our old Apogee v3.0 with 175lpi/2400 dpi, Elliptical Dots. Our angles used are C15 M75 Y0 K45 (Agfa Balanced Screening). Also in the Apogee, under the Screen-Render tab the "Use Single Settings for All Content" is Checked and under the Screen tab the "Fixed Screen" is Checked. Our CTP is Galileo and we use positive Silver plates.

We got our ccDot plate reader and measured the color bar of the Magenta plate, as well as some screen tints in the image area. Measurements revealed 163lpi and the angle is 15 degrees.

We're confused since the Agfa Balanced Screening setting for Magenta is 75 degrees and we printed the plate at 175lpi...and not 163lpi. Now we have both the Cyan and Magenta plates both at 15 degrees.

We're hoping someone could shed some light.

Thanks.

 

[gordo]

While printing one job we noticed that a moire effect was very evident for the whole sheet. Both press and prepress guys agreed that it was the magenta plate causing the moire.
[SNIP]
We got our ccDot plate reader and measured the color bar of the Magenta plate, as well as some screen tints in the image area. Measurements revealed 163lpi and the angle is 15 degrees.
We're confused since the Agfa Balanced Screening setting for Magenta is 75 degrees and we printed the plate at 175lpi...and not 163lpi. Now we have both the Cyan and Magenta plates both at 15 degrees.

The 163 lpi seems a bit off, but might be within the error tolerance of the ccDot. For 175 lpi I would have expected about 169.706 lpi. Due to the math involved with digital screening, you seldom get the precise screen frequency you ask for, but convention tends to be that being within around 5 lpi of the requested traditional frequency is acceptable.

You may want to go back and check other jobs to try and determine whether the screen angle issue only occurred with this job or if the screen angle error has been there for awhile and it's just this particular job that had content that made the problem visible.

I'm not familiar enough with your RIP to comment on the settings you've selected, however, tracking down the cause of these kinds of problems and then solving them is made a great deal easier if there is an effective plate checking and documentation procedure in place.

best, gordon p

my print blog here: Quality In Print

 

[meddington]

If your rip has an event log, you may be able to check to see what frequency/angle your separations are ripping in at. Its possible that ppd settings are overriding the rip settings (Esko plugins allow for such).

If you have the ability to check the ripped elements prior to plating (a 1-bit tif viewer), this could save you from wasting plates.

 

[Lukas Engqvist]

Are you doing eliptical or ABS screen? Don't know much about the scaling that Gordo refers to (never been interested in the exact maths) but I know that the Agfa system has possibility to map screens. This means that a you can configure the system so that no matter what screen you request you will get the same output, or if you like a number of different "wrong" screens would be over ridden by the "right" screen (shape, lpi curve). Ofcourse such intelgence can be reversed in making various acceptable screens produce the wron result. If I remember correct the mapping also includes angles for the first screens, and I rember adjusting first spot colour to magenta and secon spot to Cyan angles (by default).

 

[sandpiper]

Thanks a lot for your suggestions. The problem is put now. We deleted the whole pipe and recreated a new Hot Ticket.

Gordo, you're correct with your calculations since majority of our readings appear to be in that area (169lpi, 173lpi, etc.). But at times it also gives a different value.

Thanks once again.