Printing News
Dampening Solution and Dampening Units in Offset Printing

Dampening Solution

The dampening solution is made generally of water. Experience has been shown that in conventional offset printing the wetting solution needs to have a pH value among the 4.8 and 5.5 and the water applied in the dampening solution should have a hardness grade of between 8 and 12° DH.

Dampening solution commonly has plate preservative ingredients, wetting agent, isopropyl alcohol (IPA), buffer items, and anti-microbe ingredients. Gum Arabic is utilized as plate preservative. Wetting components and IPA are used to minimize surface tension. Admixed buffer components are used to stabilize the pH value. Antimicrobial ingredients are absolutely essential if the solution is prepared for a couple of offset printing presses in a central component metering device. Without these items the pipelines would become blocked as a result of the expansion of algae.

Alcohol-free dampening solutions have alcohol substitutes such as glycol, rather then isopropyl alcohol.

Dampening Units

Conventional offset printing needs a dampening device to deliver an extremely thin film of solution for dampening (approx. 2 microns) into the non-printing components of the printing plate. Since part of this solution is printed using the ink, plate, and blanket and another part evaporates, it is required to have a consistent supply of dampening solution.

Dampening units have developed from the "dampening roller" used to dampen the lithographic stone. Vibrator-type dampening systems and non-stop flow dampening systems are systems with contact between the dampening solution pan, the dampening vibrator, and the printing plate. The problem with these dampening systems is in the fact that compounds (e.g., particles of ink, paper dust) can get from the printing plate into the wetting solution pan and can result in contamination.

This problem does not happen in dampening systems that are contact-free or where there can be no feedback from plate or ink flow. The amount of solution for wetting should be metered very correctly, as excess dampening solution cannot flow back from the printing plate into the dampening unit with these systems. The systems are classified as "brush-type and centrifugal dampening systems"

Printing inks absorb wetting solution to a certain point. Printers talk about this as an "emulsion". In physical/ chemical terms it is an ink/water dispersion. The solution for wetting is contained in the ink like droplets and several 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 transport of ink into the plate is no longer even or in harmony with the image. The very small droplets of solution mean that separation of the printing and non-printing plate areas is no longer possible. Scumming will be the end result, that is, the non-printing places on the plate also print.



? 1999-2015 All rights reserved.