Five factors that affect the color difference in spot color printing (2)

4. The influence of the dry density difference

For printed products that have just been printed, the ink is still wet and has a density difference when dry. The phenomenon that the wet color density is greater than the dry color density is referred to as a dry retreat density phenomenon. This is because the ink layer just printed has a certain leveling property, so the surface reflection is dominated by specular reflection, which looks bright and shiny. When the ink layer is in a dry state, the reflection of the surface is dominated by diffuse reflection, and the color is naturally dull and darker than when it was printed.

Since the dry color density is generally measured after printing for 30 to 60 minutes after drying, this results in difficulty in measurement and control of spot color density.

A densitometer with a polarizer device can eliminate light from the surface of the ink layer due to specular reflection, and the measured wet color density is very close to the dry color density so that the measured density value is not affected by the wet and dryness of the ink layer. For coated paper, the measured density difference is 0.05 to 0.15, and the non-coated paper has a measured density difference of 0.1 to 0.2. The color difference is also different for different colors, with the smallest yellow difference, the largest black color, and the blue and red colors in between. Therefore, when measured with such a densitometer, the measured value should be higher than the density value of the standard color sample, so as to play the role of control.

5. Effect of system differences

The process of making a color card with a uniform ink meter and an ink displayer is a "dry printing" process. No water is involved, and printing is a "wet printing" process. A wetting liquid participates in the printing process. Therefore, the ink must be generated in the offset printing. The emulsification phenomenon of the water-in-oil emulsified ink changes the distribution state of the pigment particles in the ink layer, and inevitably produces a color difference, and the printed product also appears to be dark in color and not vivid.

In addition, the stability of the ink used for the deployment of spot colors, the thickness of the ink layer, the accuracy of the weighing ink, the new and old differences in the ink supply area of ​​the printing machine, the speed of the printing press, and the amount of water during printing will also have different effects on the color difference. Impact.