Three Roll Mill - Theory of operation

A three roll mill is composed of three horizontally positioned rolls rotating in opposite directions and at different speeds. The material to be milled is placed between the feed and center rolls and gets transferred from the center roll to the apron roll by adhesion. The shear forces between adjacent rolls generate the dispersion. Decreasing the distance between rolls produces greater shear but also reduces throughput. Gap settings in the range of 0.001” are common. Milled material is scraped from the apron roll by a take-off knife. The cycle can be repeated to improve dispersion or until particle size equilibrium is reached. The three roll mill is a classic technology with inherently low throughput and requires a skilled operator but it remains to be one of the best methods for preparing very fine particle dispersions in paints, inks, cosmetics, ointments, dental composites and other viscous materials.