A secondary step in the manufacturing of printing ink, following mixing, in which particles of pigment, or coloring matter, are further ground and dispersed in the fluid ink vehicle. In the mixing stage, the solid pigment is mixed with the fluid vehicle, and in the production of many types of ink only one or more mixing stages is necessary. In some types of ink, however, the particles of pigment are large and/or hard enough to warrant further refining and dispersion in one of several types of milling devices, which vary in method and configuration, but all are based on the principle of applying shearing and/or frictional forces to the ink slurry (the mixture of vehicle and pigment) to break up and disperse the particles. Depending on the nature of the ink produced, one of several types of mills may be utilized. Flexographic and gravure inks, due to their thin, highly-volatile solvent-based vehicles, need to be milled in closed systems, such as a ball mill or a closed sand/shot mill. Heavy letterpress inks may be milled in ball mills, but their heaviness may necessitate the use of more powerful mixing and milling systems, such as high-speed dispersers. (See Three-Roll Mill, Ball Mill, Sand/Shot Mill, and Colloid Mill.)
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