Gradient issue

[tbuck]

We have a phenomenon happening to our gradients on press. Has anyone seen this before? The films we use to shoot the screens are perfectly smooth but we get a ragged look near the midpoint and again in the lighter tones. We have tried multiple dot shapes, angles, resolutions with no luck. Although difficult to capture in a photo my hope was that someone has seen this. The red arrows point to the issue-the green checkmark is just part of the gradient. Any ideas?

 

[Bobby H]

What software is being used to produce the gradient? Is the gradient made only with Black ink settings or are the levels of gray made using CMYK formulas? You might get less banding with the latter. It could also be a matter of settings in the fill dialog box, such as a lower default level of gradient steps in the fill.

 

[tbuck]

ESKO. Whether generated natively or imported from Illustrator, these gradients are baby smooth. NO BANDING! These artifacts occur during the printing process. What do you think causes this? Mesh? Squeegee angle? Sharpness? Ink viscosity? Peel?

 

[tbuck]

Gradients are typically one color. Custom inks. So frustrating. I know what banding is-this is not that.

 

[Bobby H]

I don't know what the cause could be. But something is causing the dot sizes in that halftone to go from big to small in a suddenly uneven shift. If it's not something related to original art or software settings then it has to be something involved in the hardware. That's the only other factor left to explain it.

 

[Johnny Best]

If you are making screens with this are you using a single light source or UV multiple lamps. And are you coating screens or indirect film.

 

[tbuck]

The photo is of a test print off press which is when this problem happens. The films are output at 2540ppi and are beautiful. The screens are exposed and washed out and are smooth as well. This problem is at press. My hope was that someone would see that ragged appearance in the gradient and say hey I've seen that before! I'm an old offset prepress guy so screen printing is like black magic to me.

 

[Johnny Best]

I think you have sawtoothing happening from your exposure. Light coming from different light sources are creating that. A single light source will elimate that. You cannot see it from just looking at it but a good loupe will show it and when the ink goes through the screen it lays it down with those problems you see.

 

[tbuck]

Thanks JB I will look into this on Monday and report back. Management is convinced prepress can image a film with a magic dot shape, screen angle or ruling. We've tried dozens with no luck. So many variables beyond prepress-screen mesh, exposure, washout, stretch, squeegee sharpness, angle, ink viscosity. So frustrating.

 

[bob]

Add a bit of noise to the gradient. A 'bit' is somewhere less than 10%.