The effect of dot shape on vignette banding in the silk screening process

[Macmann]

We are testing different screening choices and their effect on vignettes in relation to banding on press. I would like to give the Rugby dot shape a try. We would have to purchase that feature as an add on. We rarely observe the banding on film off the image setter, so it seems as though the interference is manifesting itself in the interaction of the screening with the mesh. We have done extensive testing with different meshes and didn't want to overlook the impact that the dot shape itself would have on our results. Are there any screen printers out there using the Rugby dot shape? How does it work for you? Does it warrant the upcharge from ESKO? Thanks in advance for your input.

 

[markf77]

Rugby dot shape

Rugby dot shape

We are testing different screening choices and their effect on vignettes in relation to banding on press. I would like to give the Rugby dot shape a try. We would have to purchase that feature as an add on. We rarely observe the banding on film off the image setter, so it seems as though the interference is manifesting itself in the interaction of the screening with the mesh. We have done extensive testing with different meshes and didn't want to overlook the impact that the dot shape itself would have on our results. Are there any screen printers out there using the Rugby dot shape? How does it work for you? Does it warrant the upcharge from ESKO? Thanks in advance for your input.

The Rugby shape dot works well for screen. Get a trial licence to evaluate it before you buy..Also check the emulsion build as this has an influence as well..

 

[gordo]

We are testing different screening choices and their effect on vignettes in relation to banding on press. I would like to give the Rugby dot shape a try. We would have to purchase that feature as an add on. We rarely observe the banding on film off the image setter, so it seems as though the interference is manifesting itself in the interaction of the screening with the mesh. We have done extensive testing with different meshes and didn't want to overlook the impact that the dot shape itself would have on our results. Are there any screen printers out there using the Rugby dot shape? How does it work for you? Does it warrant the upcharge from ESKO? Thanks in advance for your input.

Just to clarify what you mean by "banding"

Do you mean shadestepping which looks like this:

Or do you mean the optical bump that happens at 50% which looks like this:

If you mean shadestepping then I do not see how the Rugby dot shape (rounded diamond) would help at all.
If you mean the optical bump (the dark line that can appear at 50% when halftone dots touch each other on all four sides) then it would reduce the effect by splitting its visibility in half by having dots first touch at 35% forming chains (which can look like lines in the printing when using low LPIs) then touching at 65%.

Can you clarify the problem you're seeing?

 

[Macmann]

Thanks Gordo. I think our issue leans more towards the optical bump (which is a term I am not familiar with but I have witnessed the phenomenom). If it were shadestepping I would typically up the resolution or add noise or both. What perplexes me is we can provide the pressroom with baby smooth gradients and we still see a "rattiness" on press with certain gradients-usually shorter ones.

 

[gordo]

Thanks Gordo. I think our issue leans more towards the optical bump (which is a term I am not familiar with but I have witnessed the phenomenom). If it were shadestepping I would typically up the resolution or add noise or both. What perplexes me is we can provide the pressroom with baby smooth gradients and we still see a "rattiness" on press with certain gradients-usually shorter ones.

"Optical bump" "tone jump" "intensity jump" "density jump" is a pretty typical artifact with certain halftone dot shapes. It is most commonly associated with the "transforming Euclidean" dot where the dots transform from a round shape (1%-49%) to square at 50% (with linear output) then go back to round for 51%-99%. This dot shape is arguably the most common shape in use. In Esko's case I believe it's called the Paragon Euclidean (short name: PAREUC).

Several strategies have been developed by vendors to mitigate the issue.

One is Esko's "Fogra-Round dots" (short name: R). This is a modified Euclidean Round/Square/Round dot. The optical bump at 50% is mitigated by elongating the dot shape at that tone so that the dots first touch around 45 % form a chain and touch for the second time around 55%. You end up with two optical bumps that are half the intensity and hence are less visible.

The most common strategy is to use an "eccentric" dot shape, typically Elliptical.

In Esko's case the Elliptical (short name: E) dot uses a more elliptical version of the Round Fogra dots. The first touching point is around 35%. Between 35% and 65%, a chain that looks like lines is formed. If you are using a low lpi screen then the chain may be visible and objectionable.

Esko's Rugby (short name: D) screening uses a modified Diamond dot shape with rounded edges. As with Elliptical dots the first touching point is around 35% and between 35% and 65%, a chain that looks like lines is formed. If you are using a low lpi screen then the chain may be visible and objectionable.
It looks like this:

Another alternative is using a non-transforming Round dot. Esko refers to this as Circular dots (short name: C). This screen design places the optical bump in the shadows at about 75%. Because it's in the shadows it is usually not visible. This dot shape may require a dot gain compensation curve because the small dots in the shadows can fill in without it.

Hope this helps a bit.

 

[Macmann]

Gordo, as usual you are a wealth of knowledge. I too have noticed that the Circular (C) dot begins chaining in the 3/4 tone. This may be our best option as we do not have a license for the Rugby dot at this time. We do print quite a bit at a lower line screen (65 for example). Do you feel that a higher line screen would mitigate the issue or exacerbate it?

 

[gordo]

This may be our best option as we do not have a license for the Rugby dot at this time. We do print quite a bit at a lower line screen (65 for example). Do you feel that a higher line screen would mitigate the issue or exacerbate it?

I doubt that lpi would have any effect on the optical bump unless maybe if the screen is very coarse e.g. 15 lpi where the meniscus effect which results in the appearance of the bump would be smallest relative to the size of the dots. But that's only my speculation.

I would start with Circular dots keeping in mind you'll probably have to apply a tone reproduction curve to the film to avoid plugged shadows.

 

[Macmann]

Thanks for all your input Gordo