Q Qp Qd Q

where

Qn = net flow, Qd = drag flow, Qp = pressure flow, and Q, = leakage flow.

All extruders produce some or all of these types of flows. Intermeshing counter-rotating screws do not have drag flow. Forward movement occurs due to displacement of volume and this flow replaces the drag flow in the above equation. This same screw system also exhibits a small flow by the "calendaring" action of the parallel shafts. Similar to a roller extruder, it forces the flow to the bottom of the barrel and slightly forward. This is considered to be drag flow and is added in the above equation as such.

Viscosity also plays a important role in the flow of an extruder. Most food substances are thixotropic in nature, that is, they tend to shear thin. As the shear increases on the food substance, the viscosity decreases (Levine, 1988; Rossen and Miller, 1973).

0 0

Post a comment