The Dampening system applies a thin film (approx. 2 μm) of water or the dampening solution to the lithographic plate. It is necessary to keep the non-image areas of the plate free of ink. The dampening system consists of a fountain which holds the dampening solution. In a conventional dampening system a fountain roller rotates in the fountain and takes the fountain solution or dampening solution and supplies them to feed rollers or supply rollers, which supplies the solution to the plate. This conventional dampening system is always criticized for improper ink-water balance. Hence the type of dampening system varies greatly between different types of presses.
In conventional form rollers cotton or cloth coverings were used. But when the operator wants to reduce the fountain solution flow the effect is not immediate as cotton acts as a reservoir of water which take time to get dried. In all modern presses a new type of damper is used without any covering. It has a grained surface and many times rubber coverings are put on them.
Direct dampening systems are systems with contact between the dampening solution pan, the dampening vibrators, and the printing plate. The disadvantage of these dampening systems lies in the fact that substances (e.g., particles of ink, paper dust) can get from the printing plate into the dampening solution pan and can lead to contamination. Dampening systems acting directly on the plate use dampening form rollers to transfer the dampening solution onto the plate.
Vibrator-type dampening systems often have dampening rollers covered with absorbent mate- rials (e.g., molleton, plush). The inertia of these systems with respect to necessary changes with the amount of dampening solution to be fed is very great because the cover absorbs a great amount of dampening solution. The fabric covering also has other disadvantages with regard to the printing process:
• high maintenance,
• high paper waste rate because the required
ink/dampening solution balance is reached very slowly,
• frequent malfunctions due to coverings becoming fluffy (mainly with new covers),
• often uneven distribution of the dampening solution,
• high risk of excessive supply of dampening solution.
Continuous-type dampening systems work without ductor/vibrator roller and fabric-coverings yet do usually need alcohol additives or special dampening solution additives.
Indirect dampening systems are contact-free or where there is no feedback from plate or ink flow. The amount of dampening solution has to be metered very accurately, as excess dampening solution cannot flow back from the printing plate into the dampening system with these systems.
In the case of indirect dampening through inking systems the dampening solution is fed to an ink form roller which then transfers ink and dampening solution to the plate in dispersed form (dampening solution also in the form of “surface water”).
Contact-free dampening systems are known as centrifugal, turbo or brush-type systems.
Here, dampening solution is catapulted onto the printing plate as finely dispersed droplets. Adequate surface tension ratios must then ensure a rapid spreading of the droplets. Droplets have essential disadvantages compared to the continuous-type application and it is therefore continuous-type dampening systems that prevail in the commercial printing sector. The main advantage of catapult (centrifugal) dampening systems is that the quantity of dampening solution can be metered in axial direction of the roller/plate (this is an advantage since different quantities of ink and dampening solution are needed to achieve an even emulsion throughout the ink zones).
Printing inks absorb dampening solution to a certain extent. Printers refer to this as an “emulsion”. In physical/ chemical terms it is ink/water dispersion. The dampening solution is contained in the ink in the form of droplets and some of it also sits on top of the ink film.
If the dampening solution droplets fall below a certain size, the offset printing process immediately breaks down, that is, the transfer of ink onto the plate is no longer even or in accordance with the image. The tiny droplets of dampening solution mean that separation of the printing and non-printing plate elements is no longer possible. Scumming is the result, that is, the non-printing areas on the plate also print. As with inking units, there are dampening system designs that allow the path and the distribution of the dampening solution application to be changed. This can be achieved using intermediate rollers that can be engaged and disengaged from the inking unit. The quantity transferred can be changed and the cleaning effects achieved at the printing plates by using variable drive ratios of the dampening rollers (slippage).
Types of rollers in the dampening system
- Forms: covered with absorbent paper or cloth called molleton.
- Pan (fountain) roller: aluminum, stainless steel, or chrome plated steel (best)
- Ductor: covered with molleton
- Vibrator: aluminum, stainless steel, or chrome plated steel (best)
All rollers must be clean--free from ink--to prevent an interruption of water flow (ink is hydrophobic).
Molleton covers soak up water in much the same way as a bath towel--once they're too wet, it takes a long time from them to dry. This phenomenon results in the "gets-wet, stays-wet" problem and causes changes made to the water flow by the press operator to to take effect slowly.