Learning how paint is manufactured

The ingredients that are used to make paint are combined by a process called grinding. The grinding machines disperse the pigment and wet it with the vehicle, but do not reduce the size of the component particles.

The grinding machines that are most used today are the roll, ball, and pebble mills. Another type, the stone mill, is an earlier form of grinding machine. It consists of a rotating stone which rubs against a stationary stone.

Roll-Mill Grinding

The procedure of combining the paint ingredients varies slightly with the type of grinding machine used. With the roll mill, the powdered pigments are first premixed with a small amount of vehicle to form a paste. This is done in small mixing tanks equipped with rotating paddles.

The paste then flows directly into a three- or five-roll mill. The steel rollers may be arranged horizontally or banked one above the other. Each roller turns at a different speed and in the opposite direction from the adjacent rollers. This produces a shearing and pressing action that makes a paste of uniform consistency. The paste is scraped off the last roller and is transferred to mixing tanks with capacities of up to about 600 gallons.

The remaining vehicle is then added, and the mixture is stirred by high-speed agitators. The paint is then tinted to the required color, and thinner is added. The paint is strained and transferred to hoppers from which it is passed to filling machines that pump it into cans and drums for shipment. Each batch of paint is also tested at intervals during the manufacturing process to make certain that it meets standards of quality.

Ball and Pebble Mills 

When ball or pebble mills are used in the grinding process, the powdered pigments and the vehicles are loaded directly into the mills without premixing. Ball and pebble mills are essentially rotating drums which vary in capacity from about 35 gallons to more than 1,200 gallons. They are partly filled with steel balls or white pebbles. As the drums turn, the grinding action of the balls or pebbles causes the pigment and vehicle to be combined.

Pebble mills are used primarily for grinding white paints and enamels which might be discolored by steel balls. The length of time a paint is ground varies with its intended use. Some paints and enamels may be ground for up to 48 hourse.