Water (H2O) is a polar molecule with excellent solvency. It’s always known as “universal solvent” due to its capability to dissolve a lot of substances compared to any other liquids.
There are plentiful sources of water (tap water, underground water, treated water, etc.) which is easily accessible. However, the source of water plays a very crucial role when we talk about lithography printing. Why?
Water forms one of the major components in entire printing process. Approximately 90 – 95% of working fountain solution is water; and the rest are other printing auxiliaries (such as: alcohols). Quality of water can also significantly influence the quality of the print. Therefore, it is vital to use clean and impurity-free water during printing process.
Water hardness is defined by the amount of dissolved minerals such as calcium and magnesium ions in the water. Usually, water hardness is expressed in German degree (odH). Hardness of water that we use daily from the tap can be unpredictable. It can range from very hard to soft. The most practical method for measuring the relative water hardness for a printer is conductivity. Conductivity increase with ion content, which means hard water will provide higher conductivity readings.
In lithography printing, hard water can cause ink blindness, stripping, ink transfer problems and difficult to control ink balance; while, soft water can cause excessive emulsification on ink and is corrosive against printing press metal parts. Ideally, printers should use treated water for purity and consistency. The optimum water hardness for offset printing is 8 – 12 odH.
Water Treatment System
There are 2 types of water treatment systems which are recommended for printing industry:
Reverse Osmosis (RO) system: is a filtration method that removes many types of large molecules and ions from water by applying pressure to the water when it is on one side of a semi-permeable membrane. The result is that the solute is retained on the pressurised side of the membrane and the pure water is allowed to pass to the other side.
Deionised Water System: is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineral ’ion exchangers’.
This deionised water system is more suitable for use in the printing industry because it is simple, cheaper, and produces greater yield than the RO system.
Using good and consistent water quality for fountain solution dilution is firstly and the most important step in achieving good print quality, high productivity and low wastage.