Screening

[vaugh6288]

We have been noticing that when we print gradients from light to dark that there are typically hard lines and a general grainy look to the printed image. We are currently printing (on coated papers) 175lpi elliptical dot with standard angles from Prinergy.

We are fairly certain that it isn't the paper or the press. We have tested numerous papers including premium coated sheets, changed blankets/rollers, etc.

Has anyone else had this problem before? I need some guidance on which screen lines, shapes or angles for testing purposes.

Thank you,
Mitchell

 

[gordo]

It's hard to comment on your issue without actually seeing the problem presswork so I'll just make some general comments.

when we print gradients from light to dark that there are typically hard lines and a general grainy look to the printed image.

The Elliptical dot shape is not optimal in a CtP environment because the dot shape varies at different screen angles which can cause single color moiré and uneven dot gain. The dot shape is directional, so at low lpi frequencies the “chaining” of the dots as two points touch can cause lines to appear as artifacts. At high lpis slight slur or doubling can exaggerate the visibility of the chaining. Directional problems on press such as slur and doubling can also cause strong tone and color shifts depending on the angle of orientation of the dots relative to the angle of the paper as it travels through the press.
Graininess can result from poor dot formation in the presswork - you'd have to take a photo micrograph** to see if that is the case. Graininess will also appear wherever there are rosettes. That is because rosettes are moiré structures that have a frequency about half that of the screen itself. I.e. if the screen is 175 lpi the rosette frequency is about 87.5 lpi which is easy to see. Also, slight misregistration will cause the rosette to drift from clear-centered to dot-centered which can also make them more visible as graininess.
As far as dot shapes are concerned, if you're running AM screening in a CtP workflow, the non-transforming Round dot is the optimal dot shape. (BTW, it was the dot shape used for the GRACoL 7/G7 pressruns)

More info on dot shapes here:
The Print Guide: AM Screening Halftone Dot Shapes

**Every printshop should have one of these (or one similar):

A relatively inexpensive (<$100) QC tool.