Bottom Unit Cyan printing image with stripes

Hi everyone,

We are currently running a CMYK color sequence on the DGM430 4-High Tower. The web feeds from the Spicers unit at the back, passes through a turnbar (turning 90° to the left), and enters the bottom A-side Cyan unit. From there, it travels up through the two cyan blanket cylinders, and continues upward through Magenta, Yellow, and finally Black units.

We’ve been experiencing horizontal stripes on the cyan image. These stripes usually don’t appear during make-ready, but they show up once the press reaches production speed(about 20–25K impressions per hour). The problem occurs on all 4-High towers, but only on the bottom cyan units.

I suspect the issue may be related to the wide blanket gap. On the DGM press, the blanket gaps are normally aligned with each other, which means that when the gaps rotate toward one another, the web momentarily loosens. This can cause web bounce and tension fluctuations.

We used a camera system to monitor each impression, and it clearly shows that as each roll transitions to a small butt on the Spicers, the web tension increases sharply, and the cyan registration bounces with every cylinder rotation — resulting in a ghosting effect.

Has anyone encountered a similar issue? If so, what solutions worked for you?

Here are two possible fixes I’m considering (but haven’t yet tested):

1. Replace the drive belt
   
   
2. Retime one of the blanket cylinders so that the cylinder gaps are offset instead of aligned
   (though this may lead to faster plate wear or cracking at the lead edge)
   

Any insights or suggestions would be greatly appreciated.