A screw feeder is a device used in various industries to accurately meter or feed amorphous or loose grained materials from one part of a process to another. Screw feeders are very similar to screw conveyors in their basic structure, both of which are based on the principles of the Archimedean screw for their operation. A screw feeder is capable of delivering dense slurries and dry granular products with great accuracy at a range of operational speeds. These feeders are also capable of maintaining the high torque values required to feed these types of materials across their entire speed range. There are two basic types of screw feeder.
Screw feeders make use of the Archimedean Screw principle to transport materials. The Archimedean Screw is one of the oldest positive displacement machines known to man; it is a beautifully simple and efficient method of moving substances as diverse as sand and water. All machines based on this design have a helical screw mounted on a central shaft driven by various methods. This assembly rotates inside a tube fitted with a feeder mechanism at one end. Material is added at the feed end of the device and is transported to the discharge end by a process of positive displacement.
Materials suited to screw feeder delivery are typically of loose grained or dense slurry consistencies. These include granulated injection molding thermoplastics, cereal grains, and suspended chemicals or pharmaceutical ingredients. Screw feeders are capable of delivering the high torque levels at all operational speeds required for transporting these types of materials. The relatively slow speeds involved in the process are also beneficial when feeding sensitive materials.
A screw feeder's drive is controlled by servo motors capable of precise stop-start and speed control. In a volumetric screw feeders, the amount of material transported is carefully calculated and controlled by adjusting the speed at which the screw turns. This variable, in conjunction with the known volume of material within the tube, allows for very accurate feed values to be maintained. The rotational speed may be preset or constantly adjusted according to process requirements.
The delivery rate of a gravimetric screw feeder, one of the basic types of screw feeder, is controlled by adjusting the rate at which the material is introduced into the machine. This closed-loop system of control is more accurate than the volumetric feeder, the other type, and is commonly used in applications requiring the highest levels of feed accuracy. Both types, however, are ideally suited for applications that require constant, accurate control of process feed rates of granular materials.