A device used in the manufacture of ink to further disperse the solid pigment particles in the fluid ink vehicle that utilizes sand or small metal pellets (like steel shot) to chop up the pigment particles and disperse them thoroughly. A sand or shot mill consists of a chambered metal cylinder, which contains the sand or the pellets. The ink slurry is piped in through the bottom of the cylinder, where rotating metal disks force the slurry upward, through the sand or shot, and finally out through a screen at the top. Reducing the disk-rotation speed increases the amount of time the slurry remains in the machine, and the more finely ground the pigment particles are. A metal jacket can be filled with cold water and placed around the cylinder to cool it down, should the heat buildup inside become high enough to compromise the consistency and viscosity of the ink. The advantage of a steel or shot mill is that it can be adapted to a continuous process, rather than remaining strictly a batch process like closed ball mills. Sand and shot mills are also less expensive than ball mills to install, and provide the same high level of milling that can be obtained with a ball mill. Closed shot mills are frequently used in the production of flexographic and gravure inks, which require closed systems to prevent the evaporation and escape of their thin, highly-volatile solvent-based vehicles. Letterpress inks are also produced in shot mills, but due to the heaviness of many types of letterpress inks, other types of more powerful milling systems may be required. (See also Three-Roll Mill, Ball Mill, and Colloid Mill.)
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