Dampening solution analysis
for offset printing
Dampening solution analysis
The dampening solution is made up of fresh water, additives and alcohol. Ideally, the dampening solution will have a water hardness of 8° dH to 12° dH and a pH value of 4.8 to 5.5. The usual dampening solution temperature is 10°C to 15°C. The printer needs to be aware that temperatures that are too low can result in condensation forming in pipes and dampening solution tanks, and therefore also lead to the formation of water droplets.
The function of the individual substances
can be grouped into four main categories:
Plate protection component
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Requirements for dampening solution additives:
1. Effect on the water
- Adjustment and long-term stabilisation of the pH value
- Targeted adjustment of the surface tension
- Partial bonding of hardening agents
2. Effect in the machine area
- Can be used for a wide range of dampening unit types
- Protection of machine steel, rollers and plastics
- Safeguarding of optimised productivity
3. Effect on printing plates
- Good wetting of water-carrying parts
- Fast, free running of the plates
- Good protection against corrosion
4. Effect on offset inks
- Formation of a stable dampening solution-ink emulsion
- Rapid establishment of the ink/dampening solution equilibrium
- No impairment of ink drying
5. Effect on the printing stock
- No loosening of the paper coating
- Leaves behind no separating layer that could have a negative effect on ink adhesion
The wide variation in water qualities means that the correct additive needs to be chosen.
The basis: water
The water hardness must always be determined before the addition of any additives. This is because it is not possible to use simple means to determine the hardness of prepared dampening solution. Test strips are helpful for the simple determination of overall hardness. The carbonate hardness is determined using an indicator solution (measuring tools are available from Gebrüder Heyl, Myron L, Merck, Neukum, etc., for example). It should be borne in mind, however, that any measurement represents only a
“snapshot”, and that the water quality can fluctuate quite considerably.
A precise water analysis can be carried out on request.
The lime content of water can cause problems during printing. For example:
- Bare running of the ink rollers (limescale deposits)
- Deposits on the rubber sheet
- Influence on the pH value
- Fluctuations in the pH value
Excessively high amounts of chloride, sulphate or nitrate also encourage corrosion.
Test strips can be used to easily determine the overall hardness of the water: briefly dip the hardness testing strips into the water and read off the results around two minutes later.
“pH” comes from the Latin Potentia hydrogenii and represents the logarithmic representation of the hydrogen ion concentration. The pH value is therefore a measured value for determining the acidic or alkaline behaviour of aqueous solutions. It is not possible to determine which type of acid or alkali is involved. A liquid with a pH value of 5 has ten times more acid in it than a liquid with a pH value of 6. Generally speaking, dampening solution additives are buffered in order to neutralise external influences as much as possible
pH value and buffer
Modern dampening solution additives are put together so that the correct pH value is ensured when the prescribed dose is used. Buffering prevents paper and ink from changing the pH value.
The indicator strip for determining pH should be dipped in briefly and then compared with the colour scale.
Conductance value = µS/cm
The conductance value describes how current is conducted in a liquid. Impurities in the dampening solution will cause the conductance value to rise. The conductance value varies, depending on the water and additive. The conductance value must be tested and serves as a guide parameters. The temperature and alcohol concentration also influence the conductance value. The higher the isopropanol (IPA) content, the lower the conductance value. Modern conductance value measuring equipment factors the temperature into the measurement. It is important that the conductance probe in the central dampening solution system is cleaned and calibrated on a regular basis.
The conductance value should be determined with “freshly prepared dampening solution”, so that it can serve as the “yardstick” when changing the dampening solution. If the conductance value in the dampening solution has risen by around 1,000 µS/cm, this should be regarded as the trigger to change the dampening solution. To prevent printing problems, it is recommended that the dampening solution be replaced at regular intervals. The use of optional dampening solution filters (e.g. Softflow) allows the shelf life of dampening solutions to be increased significantly.
A universal testing device allows the pH value, temperature and conductance value to be measured. All electronic measuring devices must be calibrated regularly.
The surface tension of the water can be reduced by using gum Arabic, glycol, glycerine or alcohol. The constituents of the dampening solution additives are listed by the manufacturer in the product safety data sheet.
- Poor wetting, large droplet volume, large edge angle
- High surface tension
- Good wetting
- Small droplet volume, small edge angle
- Low surface tension
Alcohol is an excellent wetting agent. Isopropanol, also known as IPA, reduces surface tension, increases the viscosity of the dampening solution and therefore promotes film formation in the dampening unit. This produces even dampening. IPA evaporates quickly. As a result, the ink dries faster. At the same time, the printing units are cooled by evaporative cooling. The addition of IPA increases the pumping volume and supports the uptake of dampening solution. IPA has a foam-inhibiting effect.
Checking the alcohol
The alcohol used should be very pure. This can be checked with a simple test: add equal parts of water and alcohol to a clean glass. After 30 to 45 minutes, the liquid should be clear. If the liquid is cloudy, the alcohol should not be used.
Reading off the hydrometer
The measuring spindle must always be able to float freely in the glass cube or similar. There must be no air bubbles in the liquid or on the measuring
spindle. Transparent liquids are read “at the bottom”. Read off the measured value at Vol. % and the temperature, and calculate with the Vol. % value. The calculated value is then verified against the table.
The hydrometer can be used to determine the IPA content in water. The device shows the percentage by volume and weight. Essentially, percentages by volume should be measured. Temperature has an important role to play in IPA determination. Therefore please note the temperature comparison. Include the specific weight (density) of the dampening solution additive in the calculation.
Alcohol is conventionally measured in the dampening solution central system using the density of the dampening solution (float). The density of the dampening solution is now determined by the IPA content, however, but also by the temperature, the nature of the additive and the degree of contamination. Regular cleaning is essential in this context. Modern measuring methods such as infra-red or ultrasound measurement are largely unaffected by external influences.
Disadvantages of alcohol:
- IPA encourages bare running, especially if the water hardness is high, since IPA reduces the solubility of calcium salts.
- Too much IPA can destroy the binding compounds in the ink, release the protective covering around metal pigments and therefore lead to dullness.
- IPA can attack the paper coating and lead to build-up on the rubber sheet.
- IPA can also cause health problems.
A selection of common printing problems caused by incorrectly set dampening solutions:
Deposits on the rubber sheet:
pH value too high, too much water, water too soft, too much additive, roller adjustment incorrect, too much IPA, inadequate ink acceptance
pH value too low, incorrect printing stock and ink combination, pH value of the printing stock too low
Poor free running
Feeding damping distributors
The plate is oxidising, plate protection from the additive is insufficient
Dampening solution analysis in printing
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How to improve the print quality in offset printing