The sieving process is carried out in Plan Sifter. The Plan sifter is a machines consisting of a vertical nest horizontal sieves, the whole assembly gyrating in a horizontal plane. The mixture to be sieved is fed in at the top and falls from sieve to sieve by gravity. A single Plan sifter may incorporate sieves of four or five different mesh sizes thereby delivering five or six fractions of differing particle size.
A certain depth of stock on the sieving cloth is required to obtain a satisfactory sieving effect. Owing to the movement the stock resting on sieve becomes stratified, the finest particles work down until they are adjacent to the cloth, whilst the largest ride up-to the surface of the stock. The overlying layers of medium and coarse particles thus tend to keep the finest particles. In contact with the sieve cloth and assist their passage through the sieve apparatuses.
