Series of articles on factors influencing quality in offset printing (part1)

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 following example makes this clear.
A specific quantity of ink set by the printer determines the amount of wetting medium required. The rollers need to be correctly adjusted in relation to each other to transport the ink in the inking unit. The inking rollers also need to be sufficiently resistant for the relevant ink system. What's more, the printing plate and blanket must be effective in transferring the ink to the substrate. Depending on the substrate, ink adhesion can also play a role.

In addition to all these aspects of the printing process, further parameters that have a part to play include a printing plate's type, make and resolution, an ink's components, viscosity, resistance and runability, and a press's level of maintenance. The substrate itself also plays a key role. It may be made of paper or foil and its surface may be very smooth or rough and absorbent.

It is possible to optimise each factor individually to achieve the desired print result. The printing process should also be standardised to attain the required print specifications faster and more precisely. Appropriate ISO standards, such as the process standard offset ISO 12647-2, can be used for this purpose.

The first part of this series of articles about factors influencing print quality in offset printing focuses on printing plate parameters.

How the printing plate influences print quality

The diagram below shows the most important printing plate parameters that influence print quality and the print result.

Printing plate resolution

The surface roughness of the anodised layer determines the smallest screen dots that can be imaged on the printing plate. The finer the surface, the finer the screen dots that are possible.

The surface structure also determines the amount of wetting medium that can be stored in the surface pores. In other words, the finer the printing plate surface, the less medium can be stored and the more critical the ink/water balance during the printing process.

Printing plate imaging

Nowadays, most printing plates are imaged with a CtP (computer-to-plate) system. Imaging takes place with laser heads.

The CtP exposing device must be calibrated based on the printing characteristic defined in advance on the press for all major substrate types and this calibration must be checked each day during printing plate manufacture.


Printing plate development process

Depending on the printing plate, different process steps are required for printing plate manufacture.

These steps must always be checked in the manufacturer's instructions and complied with. For example, the developing fluid must always have the appropriate reactivity.

Sometimes, printing plates need to be post-baked in an oven to enable UV inks to be used. 

Printing plate properties

Depending on the intended application, offset printing plates must have a number of properties.
Since label printing presses primarily use UV ink systems, they need to have the following properties:

• Resistance to UV ink
• Resistance to the cleaners and solvents used
• High print run stability
• Good ability to take on ink during proof printing
• Maximum tolerance for ink/water balance