Having trouble printing the spot color on duotones

[robbg439]

First, I'd like to thank all the helpful folks that contribute to this forum. I am a self-taught printer making the best of minimal equipment, and your help has been invaluable.

I'm having a weird problem printing duotones. I print the black first, then the spot (different pantone mixes). The problem is that the spot color prints with uneven density. It has strange horizontal (as in, perpendicular to the printing direction) bands where the spot color is less dense or completely absent from the printed halftone. The location and intensity of the banding varies from sheet to sheet. At first I thought the problem might be that I was using an oil based black ink followed by rubber based pantone mixes, so I switched to oil based spots and the problem didn't go away.

I'm working with a one color Hamada duplicator press, laser plates, and zipset inks, alcohol fount solution with a glycerin based additive (anchor ARS-X) to help with scumming problems. I usually let the black dry for a day or two before printing the spot color. I have absolutely no problem printing 4-color stuff, which is why this is so perplexing. I'd like to know if anybody has any ideas about what would cause this, and how to prevent it? When I pull a print of the spot plate on a new, white sheet, it looks perfect, so I know its something to do with the interaction of the black and the spot. I suppose my next step is to try laying the color down before the black, but I don't know if the black will have the same banding problem (which would look way worse).

I've attached a scan representative of the problem.

Thanks

[Ajax]

Whoops. Wrong problem.

[dabob]

I don't know if this would be relevent . .. but what screen angles and lpi are you using????

[Boneck]

Sounds like Screen angles would be your issue... they are laying on top of each other when you are in perfect register... should be able to tell with a loupe if that is your issue.
Change spot color to magenta angle.

[pdan]

Yep, the screen angles.

[DCurry]

Boneck ...chaging the color will not resolve the problem ...

He means change the angle to the Magenta angle, not changing the spot color.

[mtlprint]

Usually spot colors have the angle of Black, try to change it to the anglwe of Magenta or Cyan.

[Dan Roll]

Try adding some varnish to the color you put down second.
Anchor's ARS-X does not contain any glycerine, it is mostly propylene glycol.

[robbg439]

Screen angles! That's it. My platemaking system automatically sets spot color angles to 45 degrees, same as it sets black. Thank you so much.