Control of Dampening Solution in Offset Printing

Posted on 04-18-2024 by

In offset printing, the dampening solution is applied to the printing plate by the dampening rollers, wetting the non-image areas of the plate while repelling water from the image areas. The image areas subsequently attract ink when ink is applied to the plate by the inking rollers. Through the printing process, both water and ink transfer from the plate cylinder to the blanket cylinder and then onto the paper.

The challenge in this process lies in achieving a proper balance between water and ink, with the dampening solution playing a crucial role.

Composition and Functions of Dampening Solution:

The selection of dampening solution should be done carefully, considering its composition. It should ideally include buffering agents, plate protectors, wetting agents, preservatives, drying agents, biocides, and anti-foaming agents.

Buffering agents primarily stabilize the pH value of the dampening solution. Since the original solution tends to be acidic, variations in the amount of buffering agent added can cause significant fluctuations in pH value, which can affect the lifespan of the printing plate (due to corrosion). Moreover, inappropriate amounts can hinder ink drying, especially with metallic pigment inks.

During the printing process, the dampening solution continuously contacts the plate, resulting in the dissolution of the non-image areas (inorganic salt layer), leading to reduced hydrophilicity of the plate and dirtying of the plate. Plate protectors continually replenish the dissolved inorganic salt layer, maintaining dynamic equilibrium and ensuring stable hydrophilicity of the plate. Additionally, plate protectors contain plate care agents, eliminating the need to clean the plate during temporary shutdowns.

Minimizing the use of dampening solution during printing, while ensuring the plate remains clean, is essential. Wetting agents play a crucial role in this aspect by reducing the surface tension of the dampening solution, facilitating uniform distribution on the plate. With less dampening solution, satisfactory wetting effects can be achieved. Traditional dampening solutions often contain alcohol in concentrations above 15%. However, modern formulations incorporate wetting agents, significantly reducing the need for alcohol.

Preservatives protect metal components as the dampening solution is mildly acidic and corrosive to the printer's metal parts. Drying agents promote ink drying, while biocides prevent algae and microbial growth in the water circulation system, preventing pipe blockages. Anti-foaming agents prevent foam formation in the water tank, as excessive foam is detrimental to plate wetting.

Modern dampening solutions should include these components. Moreover, users should consider the water quality and other conditions of the printing plant, testing water hardness, conductivity, and pH. Dampening solution manufacturers should provide suitable formulations and related parameters based on the user's water quality conditions, including the amount of dampening solution concentrate, alcohol content, pH value, and conductivity.

Usage of Dampening Solution:

The addition of dampening solution concentrate should typically be controlled within 2% to 3%. However, without proper measuring tools or in cases of malfunctioning automatic dispensers, operators often rely on experience, leading to various issues in controlling the amount of dampening solution used.

Insufficient addition of dampening solution concentrate can result in dirty plates during printing, prolonged machine setup times, difficulty achieving ink-water balance, and increased risk of machine corrosion. Excessive addition can lead to ink over-emulsification, difficulty controlling ink density, ink slinging, ink contamination of the fountain and dampening rollers, and inadequate ink transfer to the plate.

To address these issues, standard measuring tools should be used for manual dampening solution addition to ensure precise measurement. For automatic dispensing systems, regular inspection and maintenance are necessary to ensure proper functioning.

When using modern dampening solutions, alcohol content should be kept below 10%. Operators often mistakenly believe that higher alcohol content prevents plate dirtying. However, excessive alcohol content can intensify paper surface dissolution, leading to paper lint and dust on printed materials. Additionally, it can cause over-emulsification of ink, affecting print quality.

pH value and conductivity are two parameters commonly used to monitor the condition of the dampening solution. pH values in the range of 4.8 to 5.3 are optimal for stable printing, extended plate lifespan, improved print quality, and easier ink-water balance control. The pH value is not only influenced by the amount of dampening solution concentrate but also by the acidity or alkalinity of the paper. Conductivity should generally be maintained within (1400±200) μS. However, it can be affected by water hardness, alcohol content, dust, and ink contamination. While conductivity within 800 to 2000 μS is acceptable, contaminated dampening solution must be replaced promptly.