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Many loss in weight feeder incorporate a screw as the feed device due to its wide feed range capability. Each feed device has a range of available screws with different diameters and pitches. Each feeder manufacturer provides a range of models capable of providing feed rates over the entire range of normal applications.

 


Screw feeder. Flexible Walled/External Paddle: The screw trough is constructed of a flexible material, normally polyurethane, in a wedge shape on two sides and near vertical on the other two sides.

 

 

The screw trough is a critical component of a screw feeder. It converges dimensionally from the inlet, attached to the extension hopper, and into the screw. It transitions the mass of dry ingredient contained in the feeder from the larger dimension extension hopper into the smaller dimension screw. Most dry ingredients (powders, granules, pellets, fibers, and flakes) do not flow reliably through converging transitions.

 


There is friction between the particles and an interlocking tendency that restricts or stops the flow. When the flow stops it is called a "bridge." All dry ingredients have a minimum bridging dimension. Flow stops and a bridge forms at this cross section dimension. The reducing cross section dimensions in the screw trough eventually reach the bridging dimension. An agitation device is incorporated in the screw trough in order to prevent the bridge from forming.