Colour and press chemistry - The Avinash Kawadkar blog

So, let us see how chemistry directly affects colour reproduction.

Color fidelity and chemistry
An important factor that impacts the color reproduction is a stability of various parameters. Following is the list of variables that need stability.

The stability of:

• Operators skill
• Press conditions
• Hydrophilic nature of non-image areas
• Hydrophilic nature of dampening rollers
• Water
• Ink-water emulsion
• Acid levels, hardness of water
• Conductivity range of fount solution
• Roller hardness and dimension

Any one of the above parameters on its own or with others has the potential to adversely impact color reproduction in offset printing. Therefore it is all about the stability of these parameters that is of critical importance.

To achieve the consistency in color reproduction, one needs a well-maintained press, cleaned with the correct solvents, pastes etc. A well-buffered fount. Good biocidal protection and treated (therefore) consistent water, further treated to hardness standards recommended by GATF/Ifra.

From the chemistry perspective, a stability of colour output is affected primarily by the stability of the emulsion, which is directly related to the fount. Choosing the correct fount for the raw water quality in use, the percentage of IPA etc is important in this respect. Also, keeping metering rollers clean and keeping them hydrophilic is important. Maintenance products, therefore, are the need and not a luxury.

Much work goes into ensuring that the colour gamut, accuracy and tonality of print is as close to the clients’ wishes or specifications as possible. This is largely done at the origination stage of the process as stated earlier in this article. Once the plates are mounted the operator sets the press parameters to produce sheets that are as close to the proofs as possible.

Once printing commences, the most important aspect of producing print that is consistent is stability of press conditions:

–      Water and ink feed – Ink/water balance
–      Dampening water pH and conductivity
–      Consistent transport of water and ink films to the plate
–      Condition of plate image and non-image areas

Factors affecting printing from dampening perspective are:

–      Conductivity
–      pH
–      Total Hardness (Calcium/Magnesium etc.)
–      Total Alkalinity
–      Corrosive ingredients (Chlorides)

Standard water quality acceptable for offset printing can be summarised as under:

	pH	Conductivity	Total Hardness	Total Alkalinity	Chlorides
Standard	7.0 – 8.0	300–360 µS	< 150 ppm	<70 ppm	<25 ppm

Press maintenance steps:

• Efficient removal of wet ink
• Removal of  embedded ink pigments and glaze
• Removal of accumulated ink from roller shoulders
• Maintain the blanket specifications

Understanding terminologies:

•       Cleaning

–      Removal of wet ink: Solvent soluble
–      Removal of water soluble components

•       Deglazing

–      Removal of dried and embedded Ink vehicle and pigments
–      Removal of dried and compressed gum deposits

•       Rejuvenating

–      Replasticise the roller surface
–      Restoration of surface profile
–      Optimize the sheet release and transfer properties