Series of articles on factors influencing quality in offset printing (Part 3)

Printing engineers are frequently involved in heated debate regarding the factors that influence the results achieved by a particular printing method, especially in offset printing. This debate largely centres on the press and the relevant printing process because this is where all the components needed for printing come together. However, the print result also always reflects the interplay between the materials and resources used and the operating personnel. This series of articles will explain the various factors separately and describe them in detail.

The diagram below shows the most important factors. The pressroom climate is an additional factor, but this will not be considered due to the significant regional differences.

Each of these factors has a significant impact on the print result and is directly linked to the other parameters. The previous two editions of Gallus in Touch focused on how the printing plate and the blanket influence print quality. This article looks at the rollers in the printing unit. In the offset printing unit, this means the rollers in the inking and wetting units.

Printing unit rollers

Overview of the factors influencing printing unit rollers:

Rollers in the inking unit:

There are essentially two different types of rollers in the inking unit:

1. Steel distributor rollers (yellow rollers in the inking-unit layout)
These have an ink-friendly Rilsan plastic coating and also distribute the ink laterally.

2. Rubber rollers
There are two main groups of rubber rollers – transfer rollers (red) and forme rollers (green).
The vibrator roller (brown) transfers the ink from the ink duct to the inking unit. 
The coating thickness of the rubber rollers is normally 6 – 7 mm.

king unit rollers have an ink-friendly design.

Inking-unit layout for the Gallus RCS 430

Rollers in the wetting unit (blue rollers in the inking-unit layout):
A number of different roller materials are used in the wetting unit. The fountain roller (purple) and the forme-damping roller are coated with rubber, the dosing roller (dark blue) is coated with chrome, and the remaining wetting unit rollers can be coated with chrome, rubber and/or Rilsan. All wetting unit rollers have a hydrophilic design.

Use of rubber materials

In the inking unit:

Since only UV inks are used for offset label printing, the rubber rollers are coated with appropriately resistant EPDM and NBR materials. Both have their advantages and disadvantages. EPDM materials tend to shrink, whereas NBR materials tend to swell. NBR materials are also less porous but more hard-wearing.

Depending on the application, a number of different materials can be used e.g. for:

- Better ink transport in the inking unit
- Improved inking up of the printing plate
- Appropriate resistance to ink systems
- Optimised transport of the wetting medium in the wetting unit
- Production speeds
- Service life

In the wetting unit:

Very dimensionally stable and cylindrical rollers are most suitable for the wetting unit because the film of water applied needs to be as even as possible. A material that is around 10 – 15° Shore A softer than for the inking rollers is used for the wetting rollers. EPDM and NBR are once again used as roller materials.

Roller settings

Precise roller settings are all-important for ensuring a constant feed of ink and wetting medium. They should always comply as closely as possible with the machine manufacturer's specifications and must be adhered to. The settings of the wetting rollers are far more important than those of the inking unit rollers.

The setting of all forme rollers in relation to the printing plate and distributor rollers is also vital. The forme rollers play a key role in the quality of printing plate wetting and the inking up of the printing forme.

The roller settings influence the following:

- Ink transfer from the ink duct
- Ink dosing
- Ink transport in the inking unit
- Amount of ink in the inking unit
- Inking up of the printing plate
- Wetting of the printing plate
- Wetting medium dosing
- Ink/water balance in the inking unit
- Constant ink and water feed
- Ink profile setting at the ink duct

Roller care

Roller care is very important for maintaining consistently high print quality. The roller cleaning fluid must clean the rollers quickly and thoroughly but must not attack roller surfaces. The roller surface must maintain a good grip because smooth rollers do not transfer ink as effectively. The cleaning fluid must not cause the rollers to swell or shrink, nor should it attack the printing plate.

The roller manufacturer or machine supplier can offer advice on which cleaning fluids and care products to use.