THE OFFSET LITHO PRINTING PROCESS
In simple terms offset litho printing relies on the fact that water and oil do not mix.
Printing plates are usually made of high grade aluminium with a surface that is water receptive. An oil receptive polymer layer a few microns thick is applied to this water receptive surface. In the case of plates developed with film this polymer needs to be photo sensitive. The polymer is removed in the non image areas.
The plate is fitted round the plate cylinder of an offset litho press. For each printed sheet or copy the plate must first be dampened, and then inked.
It is possible to use plain water to dampen the plate, but for commercial printing this is not acceptable. A solution known as fount concentrate is added to water to ensure that printing is as trouble free and as high quality as possible.
In many cases, particularly with sheetfed printing it is also necessary to add up to 15% of Isopropanol.
Dampening solution is 80-95% water. It is essential to have a supply of consistent good quality water for all forms of offset litho printing.
Mains water can vary in quality and therefore it is usually necessary to treat it by means of reverse osmosis and dose back a controlled level of hardness.
It is also necessary to filter particulate matter from the water.
Waste materials such as emulsified ink, paper fibres, paper coating, bacteria, and algae, can build up in the dampening solution circulation system. These require filtration, manual cleaning and chemical disposal.
Most dampening circulation systems require refrigeration to avoid evaporation of volatile materials, such as isopropanol.
INK AND DAMP
The statement at the beginning of this document that “water and oil do not mix” is not strictly true. Water and oil can be emulsified. The offset litho process relies on a stable ink and water emulsion being formed. If too much water (dampening solution) is taken up by the ink the result is poor transfer of ink from one roller or cylinder to another, and build up of ink throughout the printing unit. It is therefore necessary for the ink maker to choose all ingredients carefully to ensure the ink has the right ink/water balance. If this is achieved an ink is said to have good litho properties.
There are many new materials that cannot be used due to their negatative effect on on litho properties.
To ensure that a stable ink/water emulsion is achieved and maintained, offset litho presses are designed with many rollers in the inking train. Even the latest anilox inking wet offset litho press has extra distribution rollers fitted to ensure a stable ink/water balance.
Once a stable ink/water balance has been achieved, printing can commence. Ink is transferred from the duct via the roller train to up to four plate inking rollers (forme rollers) to the pre-damped plate. When printing fine halftones the amount of ink and damp applied is extremely critical. Any variations in film thickness can cause quite large variations in dot gain. Because the image area of all offset litho plates is on top of the non-image area, the ink film has to be quite thin in order to print sharp dots.
It is inevitable that some variation in ink/water balance will occur throughout a print run, especially on very long runs. This will result in a variation of print consistency.
When work is designed with very light ink coverage in small areas of the whole sheet it is extremely difficult to maintain good ink/water balance. With any ink type that is not highly press stable (does not increase in tack/viscosity with time) it is essential to create take off bars outside the job area.
INK TRANSFER TO WET SUBSTRATE
The emulsified ink, and some of the dampening solution is always transferred to the blanket cylinder and then to the substrate. With most coated and uncoated papers and boards most of the dampening solution that is transferred to the stock is absorbed before the next printing unit, although with multi-colour presses the stock gradually becomes wetter. Plastic and foil substrates cannot absorb any dampening solution. This remains on the surface and interferes with the transfer of subsequent colours, especially where subsequent images fall in previously unprinted areas.
CONTAMINATION OF SUBSTRATE
A commonly overlooked fact with all offset litho print is the contamination of substrates with chemicals from the fount solution. Fount concentrates can contain acids, pH buffers, algaecides, fungicides, mould growth inhibitors, surfactants, corrosion inhibitors, and alcohol substitutes. Many of these materials are low in volatility and may remain in the substrate for some time.
DISTORTION OF SUBSTRATE
As damping solution is absorbed into papers and boards they change in size in some cases sufficiently to cause misregister.
DAMPENING AND CORROSION
Manufacturers try to ensure that the dampening fluid is circulated in a controlled manner and that it is only applied to areas of the press where it is required. However when rollers and cylinders are turning at high speed, it is inevitable that some dampening fluid is transferred to other parts of the machine. This usually results in corrosion of steel surfaces. Cylinders, cylinder bearers, and roller bearings are particularly vulnerable to attack.
DAMAGE TO ROLLER COMPOUNDS
Dampening solutions contain aggressive chemicals that harden the surface of printing and dampening rollers. They can also leach calcium salts from paper coatings and inks. By the process of evaporation this calcium salt “hardness” forms a glaze on the surface of inking rollers. This calcium salt glaze makes inking rollers prefer water, and once they become damped, the ink is prevented from transferring down the roller train. It is necessary to remove this calcium salt build up regularly with strong acid cleaners.
DAMPENING SYSTEM MAINTENANCE
It is necessary to ensure that all rollers in the dampening system are kept in good, chemically clean, condition, and that they are properly set against one another.
Any appreciable variation in diameter of rubber covered rollers will result in dampening film variation on press. Any build up of ink on dampening rollers should be cleaned off after each job, and sometimes during a print run.
It is necessary to maintain all parts of the dampening system. Pumps, filters, pipes and joints, refrigeration, dosing equipment, etc.
CONDUCTIVITY, pH, HARDNESS, TEMPERATURE, AND DENSITY.
All the above properties should be monitored regularly to ensure that the desired levels of fount concentrate, and isopropanol are dosed. Measurement of pH alone, will not ensure that the right levels of buffered fount concentrate and Isopropanol are being maintained.
It is necessary to purchase indicators and /or equipment to measure these properties.
Make ready for any printing process is essential to ensure that the right colour balance, register, and overall print quality is right for the customer before the actual print run starts. It is an obvious area that can offer cost savings if it can be made as short as possible. Modern offset litho presses have sophisticated systems to ensure that stock size, accurate register, good colour balance, etc., are achieved in the shortest time, with minimum substrate waste. All presses with conventional roller trains need a minimum number of sheets to be printed before the correct colour balance is achieved. Those with anilox inking are claimed to achieve good colour balance in a much shorter time with far less wasted substrate. They still use dampening! It is always necessary to achieve a consistent ink/water balance before starting a print run.
ON THE RUN PRESS STOPS
There will always be a need to put some sheets to waste before good copy returns. On anilox inked presses good copy should be achieved more quickly.