The particle grade of screening depends on the screen surface and is divided into three types: grate screen, plate screen and mesh screen. Grate is suitable for screening large granular materials. The grate gap is 1. 1 ~ 1.2 times of the particle size of the undersize, and generally it should not be less than 50 mm The flat screen is made of stamped steel plate, with circular, square or rectangular holes, and the aperture is generally10 ~ 80 mm. Long service life, not easy to block, suitable for screening medium particles. The mesh screen is made of steel wire woven or welded, and the hole is square, rectangular or long, and the commonly used hole diameter is generally 6 ~ 85 mm The long screen hole is suitable for screening wet materials, and the advantage of the mesh screen is large effective area. The principle of screening machine is divided into rolling screening and vibration screening. Roller screen includes screen shaft screen, roller screen, three-dimensional screen, spiral roller screen, vane roller screen and so on. Vibrating screen basically includes banana screen, tall clothes screen, slack screen and so on. The principle of rolling screening equipment adopts forced transportation, which is not easy to cause materials to block the screen surface. Vibration screening of sticky and wet materials (generally, the moisture content is greater than 6%) is easy to cause blockage due to its principle and characteristics, and its power is high.