Effect of particle size on precision dosing of screw feeder

Abstract: Screw feeder is a traditional device for particulate materials feeding, and associated with many advantages such as simple structure, environmentally sealed case, controllable output and ease of regulation. But, its application in precision dose feeding is restricted by the cyclical fluctuations and unstable phenomenon commonly found during the feeding. Structure and operation parameters are assumed to be two important factors affecting the feeding stability and accuracy of screw feeders. The possible influence of these two factors on precision dosing has been confirmed by many researchers using numerical and experimental method. However, most of the researches focus on a specified particle size, and do not take into account of the impacts of particle size on precision feeding. In this study, the influence of different particle sizes on feeding was investigated by analyzing feeding stability and accuracy. To achieve this objective, common spherical cereal (millet, soybeans, etc.) in diameter of 3-5 mm was selected, and a screw feeder designed for 3-5 mm particles with lap feeding quantity of about 40 g were utilized. A numerical simulation model was established by discrete element method based on the soft ball with a constant stiffness coefficient and linear elastic constitutive relations. The model had an inner diameter of 40 mm, thread pitch of 40 mm and inlet size of 40 mm × 60 mm. The feeding processes of different cereal sizes (3, 4 and 5 mm) were simulated, and force chains, particle group motions and feeding quantity were determined. Furthermore, an experiment of particle group motions and feeding was carried out to verify the effects of particle sizes on feeding stability and accuracy. The PFC3D (Particle Flow Code in 3 Dimensions) software simulation and feeding experiment showed that: 1) When feeding at a fixed speed of 60 rpm and an angle of 60o, the packed number and the average contacts number of 3 mm particles were larger than those of 4 mm and 5 mm particles. As a result, the force chain network in 3 mm particles was distributed more evenly than that in 4-5 mm particles; 2) As the particle sizes decreased, the distribution of force chain was more uniform, the movement between 3 mm particles was more consistent, and the trend of the mixed movement was weaker. Accordingly, the 3 mm particles could keep the shape better than the 4 mm and 5 mm particles during transportation, while the stability in 5 mm particles was worst and the mixing phenomenon in 5 mm particles was most obvious; and 3) The average feeding quantity of 3 mm particles was higher than that of 4 mm and 5 mm particles. With decreasing particle sizes, the standard deviation and the maximum relative error of feeding quantity decreased and the stability and accuracy of feeding were improved. When feeding at angle of 60o, the average feeding quantity, the standard deviation and maximum relative error of 3 mm particles were 6.477 g, 0.537 and ±0.153%, respectively.