Reasons for color mixing in offset presses

For two-color or multi-color offset presses, color mixing often occurs. The reasons are as follows.

For two-color or multi-color machines, the overprinting mode is generally a wet-over-wet type, which is different from the wet stacking mode of a monochrome machine. Typical ink fixation speeds are ten minutes or longer. The gap between the two colors of the multicolor machine is only about 1 second during superimposition. In such a short time, it is absolutely impossible to completely fix the imprinting ink on the paper, which is the fundamental cause of color mixing. the reason.

1, the effect of the viscosity of the ink of each color group

Generally, the ink color arrangement should be arranged in descending order of viscosity to prevent the ink of the latter color from absorbing the previous color ink. However, when the manuscript is limited, the manuscript is required to bias the entire color of the ink, for example, cyan. When arranging the color sequence, the cyan color should be printed after printing. At this time, the viscosity of the cyan color should be taken into consideration, and a certain amount of de-bonding agent should be added to the ink. To reduce its viscosity, prevent the appearance of color mixing.

2. Effect of ink depth and coloring rate

General domestic inks in the actual production of its coloring rate than its standard coloring rate index is low, in order to achieve the same field density in the printing value, we must increase the amount of ink, that is, increase the thickness of the ink. Because the ink is not dried, the thicker the ink layer, the easier it is to be disrupted by external forces (printing pressure), and it is also easy to cause color mixing.

3, the impact of ink drying performance

In the actual production, the early drying of the ink is serious, and the sky blue ink is particularly mentioned here. During the operation of the machine, the roller gradually heats up, especially in the summer. The ink starts to change on the roller and the viscosity gradually increases, which is even more serious after transfer to the blanket. This often leads to color mixing. For example, when the J2205 machine prints red and green, the printing is normal, but when printing to a certain number (a few thousand), careful observation shows that the magenta ink gradually becomes darker. To solve this problem, it is necessary for the operator to grasp the drying performance of the ink.

4, must pay attention to the ink balance during printing

If the ink and ink balance is out of control, it will cause a large “ink” of “water”, and the ink staying on the ink roller will also increase correspondingly, which is equivalent to increasing the diameter of the ink roller, increasing the pressure between the rollers, and making the ink roller hotter and hotter. The easier it is to dry, the viscosity of the ink increases accordingly, resulting in a series of vicious cycles that can easily cause color mixing.

5, do a good job of cleaning the layout

When printing, adjust the pressure of the water and ink roller, and pay attention to the pH of the fountain solution to prevent the plates from being dirty on both sides. If the blank parts on both sides are dirty, transferred to the blanket and the impression cylinder, and then transferred to another set of ink roller, it will also cause the phenomenon of color mixing. In addition, when the plate is made, the signal bar and the color mark are also printed on the plate. When the printing is normal, it can be removed to prevent color mixing.