By: Rolf Hansson
There are several sources of variation in the printing process such as mechanical wear, press settings, properties of blankets and rollers, heat release during production, pollution of the dampening water and differences in paper properties.
Target values for process control
Controlling a process assumes the possibility to measure it. All registered measurements results from the dynamics in the process. By registering several static measurements at fixed and frequent intervals it is possible to catch up the variations of the process. All measurements require appropriate and sufficiently precise instruments. The target values applied to the process could either be values taken from a general standard – such as ISO 12647 for offset printing – or values elaborated from the process itself – as conductivity value for the dampening water and cylinder nip pressure. All target values are based on distinct and precise conditions.
The target values specified in ISO 12647-2 or 3 could be applied provided that:
- Printing is carried out on the paper quality the target values are based on, and standardised inks are being used
- Printing sequence is according to the standard (C+M+Y)
- Conditions for measuring are according to the standard
As soon as these conditions are fulfilled, the standard values are applicable and general for all printing independent of country, company or printing press.
An important parameter in controlling the dampening water is its conductivity that measures the concentration of dampening solution additive. The conductivity value is influenced by:
- Quality of water
- Type of dampening solution additive used
- IPA concentration
These specific preconditions make the conductivity reference value process-dependant, not general, and cannot therefore automatically be used by other printing shops. A reference value can only be measured when mixing a fresh solution.
The conductivity value should be read and registered regularly. When the value oversteps a permitted limit (± 50 mS/cm) that is probably caused by contamination from paper, ink or blanket wash. The dampening system then has to be cleaned and a fresh solution mixed.
In the cylinder nips ink and water are transferred from the plate via the blanket to the paper. The contact between blanket and paper also creates the necessary preconditions for feeding the web or sheet through the nip. The conditions in this nip are complex and blankets reacts in varying ways, dependent on their different constructions. There will arise tiny movements in the blanket, both in the warp (radial) and the weft (axial) directions, which have a great impact on the blankets dynamic performance in the nip.
By measuring the pressure in the cylinder nips we can get an understanding of the best conditions for the blanket. The pressure levels, expressed in N/cm2, are dependent on:
- Packing and mounting
- Blanket construction and its specific properties
- Cylinder diameter
- Cylinder positions
The nip pressure is process dependent and applies only to one single printing press. Therefore it cannot be transformed to the conditions in other presses. It is still applicable as a reference value when the blanket has to be shifted to a new one as well as when controlling underpackings and cylinder parallelism.
Certain instruments measure the nip when the sensor is fed into the cylinder nip. By measuring the pressure in the nip in slow motion all factors influencing the function of the blanket will be included. That is why this method reflects the performance of the blanket in a better way than a static method, like measuring the height of the blanket on the cylinder.