TECHNICAL INFO




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Characteristics of the fount solution
The fount solution is, together with the photopolymer plate and the ink, one of the key parameters in offset printing. It serves to prevent ink from depositing on the non-printing areas of the plate and therefore assures the differentiation between printing and non-printing areas.
The essential characteristics of the fount solution are as follows:
• the pH (hydrogen potential)
• the surface tension
• the isopropyl alcohol content and refrigeration
• temperature
• protection effects
• the conductivity

pH (hydrogen potential)
The pH measures the acidity or alkalinity of an aqueous solution. The pH scale enables solutions to be classified according to their strength and varies from 0 to 14. A pH of 7 is known as neutral.

 The optimum pH for a fount solution is between 4.8 and 5.5. 
Did you know?
The pH is a logarithmic scale and therefore should be measured accurately.
As an example:

- At pH = 5, the solution is 10 times more acid than at pH = 6

- At pH = 4, the solution is 100 times more acid than at pH = 6
 Even slight variations outside of this scale can give rise to severe disruptions:
• Water that is not sufficiently acid (pH > 5.5) gives rise to instability in the water / ink balance, obliging the operator to adjust the fount and inking flow rates. The ink has a tendency to adhere to the non-printing areas. The contrast between solid print areas and half tones decreases and varies throughout the print run. There is also a decrease in gloss.
• An excessively high acidity (pH < 4.8) causes the drying agents in the ink to be attacked and drying can be retarded.

Fount solution additives contain salts that buffer (i.e. stabilise) the pH in the optimal pH range: they prevent the pH of the fount solution from varying during the printing process. Due to this buffer effect, an excessive dosage does not modify the pH. However, the dosage recommended by the manufacturer should be used.

Measuring the pH of the fount solution does not enable the dosage of the fount additive to be checked. This check can only be carried out by a conductivity measurement.

The surface tension

The surface tension enables the surface area of contact between a drop of liquid and a solid surface to be determined. The aptitude of the fount solution to wet the non-printing areas of the plate is an essential factor in the quality and cost effectiveness of the offset process.
The wetting agents contained in the fount additive lower the surface tension of the fount solution, bringing it below the value for normal tap water. This makes it possible to reduce the flow of fount solution and to obtain a fine and stable water / ink emulsion, good dot gain, high gloss and optimised solid print / half tone contrast.

The isopropyl alcohol content and refrigeration

Adding isopropyl alcohol to the fount solution produces three effects:
• An increase in the viscosity of the solution, which facilitates the transport of the fount solution from the fount tank to the plate.
• A decrease in the water / ink interfacial tension, i.e. the natural repulsion between these two bodies. This accelerates the ingestion of water at the moment the offset process is started and the ink / water balance stabilises more rapidly.
• A decrease in the water / plate surface tension: this effect is combined with that of the fount additive, which enables the flow of fount solution to be reduced.
The amount of isopropyl alcohol added should not exceed 12% by volume. Beyond this figure, the water / ink balance is adversely affected and the emulsion can no longer be controlled (too low interfacial tension leads to an excessive emulsion). 
Did you know?
More and more printers are tending to use isopropanol free fount solutions since isopropanol is a Volatile Organic Compound (VOC), toxic to humans and harmful for the environment.
>
See “Alcohol free printing”.
 Temperature
Cooling is obtained by a temperature regulation system, which enables temperature variations due to running on the press and variations in the climatic conditions in the print shop to be compensated. The aim of cooling the fount solution is to:
Reduce evaporation of the alcohol (toxicity, cost)
• Stabilise the viscosity of the fount solution, enabling the water flow rate to be adjusted more easily
• Reduce the proliferation of algae and bacteria
• Reduce the tendency of ink build up and therefore plate tinting.


The conductivity


The conductivity allows the electrical charges transported in the fount solution to be measured. It is expressed in micro-siemens per centimetre (μS/cm).
Conductivity increases in proportion to the quantity of additive, which allows the printer to determine its dosage accurately.

There is no optimal value for conductivity: it can vary from one fount solution to another. The conductivity is only one means of measuring the proportion of fount additive.

Protection effects

The fount solution plays a protective role with regard to the plates and the press:
• Colloidal substances (Arabic gum is the best known example) provided by the fount additive assure protection of the plate during press stoppages and during printing (particularly by evacuating paper dust)
• The fount solution lubricates the blanket, thus extending its working life
• The fount solution contains corrosion inhibitors, which limit the risk of corrosion of the metal parts of the press
• Most fount additives contain biocides that prevent the proliferation of bacteria and yeasts, which can adversely affect the operation of the press (however, the system must nevertheless be cleaned regularly, using a detergent for the fount solution circuits)
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