The process of dividing groups of broken materials with different particle sizes through a single-layer or multi-layer sieve surface with uniformly-distributed holes multiple times and dividing them into several different levels is called screening. The particles larger than the sieve opening remain on the sieve surface and are called the oversieve of the sieve surface. The particles smaller than the sieve opening pass through the sieve opening and are called the undersieve of the sieve surface. The actual screening process is: after a large number of broken materials with different particle sizes and mixed thicknesses enter the screen surface, only a part of the particles are in contact with the screen surface. Due to the vibration of the screen box, the material layer on the screen is loosened, so that large particles exist. The gap is further enlarged, and small particles take the opportunity to pass through the gap and transfer to the lower layer or transporter. Because the gap between small particles is small and large particles cannot pass through, the original disorderly arrangement of particle groups separates, that is, they are stratified according to particle size, forming an arrangement rule with small particles on the bottom and coarse particles on the top. The fine particles that reach the sieve surface, the ones smaller than the sieve hole, can pass through the sieve, and finally the coarse and fine particles are separated and the screening process is completed. However, there is no sufficient separation. During sieving, there is generally a part of the undersize material remaining on the oversize material. When fine particles are sieved, although the particles are smaller than the sieve openings, they have different degrees of difficulty in passing through the sieve. Materials and particles with similar sieve openings are more difficult to penetrate the sieve, and even more difficult to penetrate the particle gaps in the lower layer of the screen.
