Nip Width Indicator
Pressure Indicator
High Pressure - Low Pressure
 

Rolf Hansson

About ink rollers
The stress put on rollers in modern printing presses is substantial. To be able to maintain high productivity, the presses must be operated almost around the clock. The printing speed is increased, and the start-up time is cut. It is an advantage that the machine temperature variations fluctuate less, but the risk for too high temperatures also increases. The printing presses often are designed more and more compact to safe space, which also locks in the temperature that is produced within the printing unit.

Energy
The energy used to compress the rubber in a soft ink roller transforms into heat. More pressure applied (tighter roller nip), demands more energy and heat generation is increased.

Four variables help to cool off the printing system in an offset press:
1. The roller tempering in the oscillators
2. The fountain solution, which often has an initial temperature of 12 - 15º C
3. The paper, particularly in roll presses. As the paper is heated up in the nip between the blanket and impression cylinders, the printing system is cooled down
4. The air of the printing hall. The inside climate of a print shop seldom is optimal (50% relative humidity at 20–23ºC), complicating the control of the printing press’ temperature.

The operating margins are tight, and all additional heat generation becomes an inconvenience. Consequently, it is important to control heat generation as much as possible.

Ink properties
The ink properties are closely correlated to the temperature of the rollers. A temperature increase lowers the ink viscosity, resulting in larger dots. In addition, the ink will absorb more water and can become destabilized. It becomes difficult to print the colors evenly and with stable dots. See illustration below.

Digital image 40% on the plate is 54.3% printed on the paper. Good ink and water balance 
Good ink and water balance


Digital image 40% on the plate is 52.5% printed on the paper. Too much water in the ink.
Too much water in the ink

Deviation during a print run tends to be too great

Roller settings
There is a connection between roller settings and print results. The temperature in the ink unit is a result of roller adjustment. The offset ink is to be regarded as a semi-manufacture, and it is not usable for printing until mixed with water. The mixing takes place in the nip, where the fountain solution is emulsified into the ink.

An insufficient pressure between the rollers will result in poor processing, and the ink will not be given sufficient time to become “print ready” before reaching the plate.

In addition, the transporting of the ink from the ink fountain to the plate will be disturbed. When this occurs, it is easy to increase the flow of ink so that the print density will achieve correct values. A surplus of ink is formed in the ink unit, which has to be warded off with more dampening. A negative spiral has started, that easily ends with tinting problems and poor print result.

Rollers adjusted with too much pressure will give a substantial increase in temperature, particularly in the centre of the roller. The effects are described above. Furthermore, deformation of the rollers and the wear and tear increases considerably, and the mixing/milling of the ink will be unequal between different sections of the ink unit.

It is worth while to adjust rollers with precision!

Read more!

Offset Printing. System in Balance

"Offset Printing – Controlled Process"
Optirep
ISBN 978-82-998803-0-5 (Hardcover) or
ISBN 978-82-998803-1-2 (Paperback)

You can order the book from:
www.optirep.net or rh.consulting@comhem.se