Vibrating Screen

The process of passing a bulk of loose material particles of varying sizes multiple times through a single- or multi-layer sieve surface with uniformly distributed apertures, thereby dividing them into several different grades, is called screening. Particles larger than the sieve openings remain on the screen surface and are referred to as the oversize fraction of that screen, while particles smaller than the openings pass through and are referred to as the undersize fraction.
In the actual screening process, a large quantity of mixed coarse and fine particles enters the sieve. Only some particles come into contact with the screen surface. Due to the vibration of the sieve box, the layer of material on the screen is loosened, which further enlarges gaps around the larger particles. The smaller particles take this opportunity to pass through these widened gaps and move to the lower layer or a conveyor belt. Because the gaps between small particles are too tight, the larger particles cannot pass through. Thus, the originally chaotically arranged particle assembly separates according to size, forming a stratification in which the finer particles are below and the coarser particles are above. When particles reach the sieve surface, those smaller than the openings pass through, achieving coarse–fine separation and completing the screening process.
However, perfect separation is never achieved; during screening, a portion of the undersize particles typically remains with the oversize fraction. Even though all fine particles are smaller than the sieve openings, they vary in how easily they pass through. Particles whose size closely matches the aperture size have more difficulty penetrating, and it becomes even harder for them to pass through to the lower layer of the screen surface.