When to choose volumetric and loss-in-weight feeders

The loss-in-weight feeder can achieve and maintain the set flow by detecting the actual flow and automatically adjusting the screw speed of the feeder. The volumetric feeder is to sample and calibrate the corresponding flow value of each point within the whole screw speed range for the raw materials to be transported before operation. If another raw material is replaced, it needs to be recalibrated, and then infer the screw speed according to the calibrated value and the required operation flow, and operate the feeder at this speed.

Volumetric feeder is an open-loop control feeding device, which has the advantages of considerable economy, simple maintenance and low requirements for the installation environment. However, when the bulk density of raw materials changes greatly, it will make the feeding accuracy of volume screw difficult to control. The volumetric screw feeder is generally used in the processing technology of which the accuracy is not the most critical technical index. There are three factors that affect the accuracy of the scheme: the geometric and mechanical structure of the screw (such as the combination of single screw and twin screw); the screw thread structure (Spring, Auger and Concave); the feeding speed of the screw and the physical state of the raw material (e.g. bulk density, fluidity, viscosity of material). Through the understanding of the state characteristics of raw materials, the accuracy of the volumetric feeder can be improved and guaranteed by choosing the appropriate screw combination and screw structure.            

For example: for most granular raw materials, especially for new materials, single screw volumetric feeder is generally selected. Because of its uniform particles and good fluidity, the bulk density is relatively stable and consistent during the transportation process, and the volume screw feeder can achieve high repetition and linear accuracy. It only needs off-line sampling correction and other tedious operations to accurately achieve the set flow to be run. For powder raw materials, due to the poor fluidity and high viscosity of most powders, we usually choose the twin-screw volumetric feeder. Although the twin-screw can overcome the problem of raw materials staying in the screw groove and bridging, it can not guarantee the constant volume of raw materials in each screw groove. Therefore, the volumetric screw feeder should be selected carefully for its poor precision performance for such raw materials.