With DHC Dynamic Head Compensation these mechanical tolerances are compensated so that the printed densities of all ribbons are all according to specification. Each cylinder, and each ribbon of each colour, is 'tailor made' compensated.
Nowadays you will find engraving machines using 'volume Test Cuts' or 'CellEye', and that's also a method of optimising tolerances. DHC goes a bit further, and also compensates Engraving Head- and Amplifier linearity compensation, and for Copper Engravability.
In Magazine Gravure you sometimes will see Ribbon Imbalance; two opposite pages printing differently because of mechanical tolerances that added up and become visible.
Especially in pictures that run on both pages like in the example in Fig. 1 where the left page is slightly green and the right page is more magenta.
Three process colours in Fig.2 with the printed densities deviating from the target (grey line)
They are all individually within specs, but while Yellow and Cyan are 3% low in the mid-tone and three-quarter tone area, Magenta is 4% high.
That is a difference of 7% between the colours in a critical area, and that is likely to be visible in print.
Because of the different behaviour of the colours over the tonal range in each ribbon, it is impossible to correct this with ink parameters in the press.
The mechanical tolerances are not negligible, especially in multi-ribbon gravure.
In Publication Gravure, DHC makes sure that all ribbons on all cylinders print the same, regardless of
- Stylus Wear (increasing Stylus Angle over time)
- Stylus Chips (damaged cutting edge)
- Copper Quality or Engrave-ability (influences the cell symmetry)
- Test Cut Tolerance (a tolerance of a few micron from the target test cut can be compensated)
- Head Characteristics (Engrave Heads can be profiled for linearity)
- Amplifier Linearity (machine characteristics are compensated as well as differences between Test Cut measurements and production gravure)
- For large cylinder widths, the pressure roller has an influense, with more pressure at the edges.
When one cylinder of a set needs to be re-engraved, that replacement cylinder will print slightly different because all mechanical tolerances on all ribbons will be different.
Most printers will try to engrave replacement cylinders on the same engraving machine to limit mechanical differences as much as possible. DHC will give you the possibility to control and compensate all these tolerances.
Each ribbon on each cylinder will be compensated differently, resulting in printing each page according to specs, always.
If you have a SigmaGraph Copper Conductivity measuring device, you can use that in DHC too, and you can use DHC as a cylinder management system.
This is the DHC user interface on the iPad.
The cylinder- and job-parameters get transferred by scanning a QR code on the iPad (Fig 4 Cyan Outer Cylinder as an example) and the operator only needs to measure the test cut sizes and 'star rate' the cell quality.
The DHC system calculates the compensation-gradations for each ribbon and the ribbon data is compensated dynamically and automatically.
Here is a short movie on how to operate DHC.
Drop me an E-mail if you want more information.