Ink emulsification
In the offset printing process, emulsification of the ink is an inevitable phenomenon, and offset printing relies on moderate ink emulsification to achieve ink transfer. However, if the ink is over-emulsified, a thick layer of meshed, burr-like floc-like ink accumulates on the surface of the roller when it is stopped.
Over-emulsification of ink (sometimes the surface of the ink core roller is not inked, and some of the water is collected at the contact of the ink groove and the ink core roller) will cause the newspaper image to have a light color, a dull appearance and a serious distortion of the dot. The drying speed of the ink is slow, and the back of the print is seriously dirty.
The factors that cause the ink to be heavily emulsified are the properties of the ink, the properties of the printing paper, the concentration of the fountain solution, the temperature and humidity in the workshop, and the printing speed.
Ink characteristics
Inverse relationship between ink viscosity and emulsification, and the relationship between fluidity and emulsification. The ink has high viscosity, strong water resistance, and low emulsification value. On the contrary, the ink has small viscosity, strong fluidity, reduced cohesion between molecules, and is easy to emulsify. The acid value of the ink should not be too large. The free fatty acid in the ink is a typical surfactant. The higher the content, the stronger the polarity of the ink and the more serious the emulsification. At the same time, the ink with high acid value needs a thicker fountain solution. Will intensify ink emulsification. In addition, the amount of auxiliary materials having emulsification factors such as resins, diluting agents, and driers are controlled in ink manufacturing.
2. Paper performance
The surface of the newsprint contains certain surface active substances and fillers. During the printing process, due to dampening of the fountain solution and peeling of the blanket, these substances are transferred to the ink roller through the printing plate, and then transferred back to the ink fountain by the ink roller. Mixing with the ink reduces the viscosity of the ink and destroys the ink's water resistance, resulting in increased emulsification of the ink. Therefore, it is necessary to select newsprint with tight paper, good water resistance, less powder on the surface, and less hair loss.
3. Fountain solution concentration
Common dampening fluids include ordinary fountain solution, non-ionic surfactant fountain solution, and alcohol fountain solution. Alcohol fountain solution is a non-electrolyte solution, alcohol volatilizes quickly, greatly reducing the emulsification value of the ink. The ordinary fountain solution in the use of the process must control the concentration, the conductivity of the dampening solution in the tank should be controlled between 900 ~ 1400μs, the pH value is controlled between 4.8 ~ 5.5, if the fountain solution concentration is too high Will increase ink emulsification.
4. Workshop temperature and humidity and printing speed
The temperature and humidity in the workshop and the printing speed can also cause ink emulsification. The higher the shop temperature, the greater the ink flowability, the smaller the viscosity, the weakened water resistance, combined with the high printing speed, the increase of the peeling force, and the increase of the surface temperature of the ink roller, which could easily cause over-emulsification of the ink. In addition, when the humidity in the workshop is high, especially in the rainy season in the south, the phenomenon of “cold sweat†often occurs on the surface of the machine. These dripping waters dripping along the surface of the machine to the ink fountain will inevitably lead to ink emulsification. Therefore, the temperature and humidity of the workshop and the printing speed must be controlled during the production process.