The screw is the component of the feeder that provides the “feeding.”

The agitation ensures that the screw flights fill uniformly. Screws are selected to achieve maximum and minimum feed rates. The screw diameter and pitch are sized to achieve the feed rate at an optimal screw rotational speed. At too low a screw speed the “pulsing flow” may cause an undesired variation in the final product. Too high a screw speed may cause an incomplete filling of the screw. Below are discussed three common screw types.

Spiral Screw: The single spiral screw is the most common selection. It is “open” for the ingredient to freely flow into the flights of the screw. It has a smaller surface area that helps reduce adherence of the ingredient that changes the screw volumetric geometry. Ingredients are transferred with low shear. On the negative side, aerated ingredients can easily flood past the screw flights.

Blade Screw: The single blade screw offers more resistance to aerated ingredients however, because there is more surface area, adhesive ingredients can adhere reducing the volumetric geometry. Blade screws are often preferred for heavy powders with bulk densities over 80 lb/cu ft.

Twin Screw: Twin concave screws can provide superior performance for poor-flowing powders. Twin concave screws are solid and the screw flights have a low volume compared to spiral and blade screws. The screws are intermeshing and as such are self-wiping. The negative effects of some adhesive powders are reduced with the self-wiping of the two screws. Twin concave screws are co rotating. The powder flows around the flights of both screws. The shear on the powder is high, particularly in the screw tube. The screw tube is typically short to reduce negative effects of high shear.

Twin concave screws are ideally selected for powders with feed rates below 5 cu ft/hr. The cross section of the two screws is wider than an equivalent single screw reducing the tendency to bridge in the transition from the screw trough into the screw. Also, screw speeds for twin concave screws can be higher than single screws for the same feed rate because of the larger inlet and low volume of screw flight. Higher screw speeds reduce the negative effects of pulsation.