A three roll mill or triple roll mill is a machine that uses shear force created by three horizontally positioned rolls rotating in opposite directions and different speeds relative to each other, in order to mix, refine, disperse, or homogenize viscous materials fed into it. The three adjacent rolls of a three roll mill (called the feed roll, centre roll and apron roll) rotate at progressively higher speeds.
Material, usually in the form of paste, is fed between the feed roll and the center roll. Due to the narrowing space between the rolls, most of the paste initially remains in the feed region. The part that makes it through the first in-running nip experiences very high shear force due to the different rotation speeds of the two rolls. Upon exiting, the material that remains on the center roll moves through the second nip between the center roll and apron roll.
This subjects it to an even higher shear force, due to the higher speed of the apron roll and typically, a smaller gap than between the feed and centre rolls. A knife blade then scrapes the processed material off the apron roll and the paste rolls down the apron. This milling cycle can be repeated several times to maximize dispersion.