Pressure on ducts keys

[NikNak]

Has anybody come across this issue, or am I imagining things? When the duct is filled to the brim (usually for a job with lots of coverage on a particular colour) it applies slight pressure on the keys and opens them ever so slightly allowing more Ink to feed out. And as the duct empties out the pressure reduces thus bringing down the density on the sheet? My manager thinks I’m pulling his leg. This isn’t Heidelberg specific by the way.

 

[Adil]

Hi sir
You're not imagining things this is a well-known phenomenon in offset printing. When the ink duct is filled to the brim, the weight and pressure of the ink can slightly push down on the ductor roller and keys, causing them to open marginally and allow more ink to flow. As the ink level drops, this pressure decreases, leading to a reduction in ink flow, which can cause a drop in density on the sheet.

It’s more noticeable with higher viscosity inks and when there’s a large ink demand in a specific zone. Some printers counteract this by adjusting ink keys slightly after the initial fill or monitoring density changes throughout the run. If your manager isn’t convinced, you could demonstrate the effect by measuring density shifts at different ink levels or running a test with a half filled duct vs a full one.