Paper: The whiteness of paper is fundamental to vibrant printed colors. Ideal paper reflects almost all colors of light, while paper containing gray, black, bluish, reddish, or other off-colors absorbs some incident light, affecting the hue, brightness, and saturation of the printed image, ultimately resulting in color differences. This is equivalent to adding other inks to the ink, making the ink impure and giving a dark, muddy, and off-color impression.
The main components of paper are cellulose, sizing agents, and fillers. The flowing part of the ink is the binder; the ink pigments adhere to the paper surface through the formation and drying of a film by the binder. Both paper and ink are primarily composed of asymmetric molecules; when they approach each other, secondary bonding forces allow the molecules to adhere to the paper. Currently, packaging and decoration printing companies mostly use coated white cardboard, wood pulp coated white cardboard, ordinary linerboard, wood pulp coated linerboard, kraft paper, and white cardboard. Due to differences in the production processes of different paper mills, there are significant differences in the surface whiteness and smoothness of the paper. Therefore, at the moment of printing, the highly fluid low-molecular-weight portion of the ink binder is forced into the larger pores of the paper, and the binder begins to separate from the ink. When the pigment content of the ink is high, numerous tiny capillaries can form within the ink film. These numerous tiny capillaries have a much greater capacity to retain the binder than the fiber gaps on the paper surface. When the pigment content is low, the ink adheres to the surface of the paper, causing most of the binder to enter the paper's pores, resulting in a thinner ink film on the substrate, exposed pigment particles, and ultimately, a less vibrant color.
If the ink fountain and ink rollers are not clean during ink application, and are mixed with other colored inks, color deviation will inevitably occur, also resulting in a less vibrant color. Therefore, when changing colors, it is essential to thoroughly clean the ink fountain, ink rollers, and water rollers, especially when changing from dark to light colors. The usual practice is to clean the dark ink thoroughly, then apply a portion of the light ink to be used, mix it evenly for a period of time, and then clean it again. For two-color and multi-color offset printing presses, especially those where the upper and lower units share a single impression cylinder, extra care must be taken to prevent color mixing. Color mixing will inevitably cause color deviation, resulting in dull colors.
Additives: Traditional offset printing relies primarily on ink-water balance to complete the printing process. Based on the ink transfer process, ink emulsification is unavoidable. However, excessive emulsification can cause the printed colors to fade and appear aged, reducing color vibrancy.
Offset printing also requires the addition of various additives, such as thinners and drying agents. Excessive addition of these additives can sometimes affect the color vibrancy of the printed product. Thinners include white ink and white oil. White oil is mainly an emulsion composed of magnesium carbonate, stearic acid, ink thinner, and water. This emulsion promotes ink emulsification, resulting in dull colors. Drying agents are mainly metallic soaps and are also strong emulsifiers. A small amount of drying agent can stabilize ink emulsification, but excessive addition will cause severe ink emulsification.
In addition, for printed materials with a small area of text and images, the amount of new ink added to the ink roller in each cycle is also less. As a result, the ink that remains on the ink roller for a long time has to undergo mixing, squeezing and rolling of dampening solution several times, causing severe emulsification of the ink and resulting in dull colors.
