The flow of fluid in the screw
A screw extruder comprising a screw having a screw flight for conveying a fluid and rotatably housed in a cylinder, at least one plug installed in a portion of the screw and a circumferentially continuous leakproof portion formed on at least one part of the plug with only a small clearance provided between the outer periphery of the leakproof portion and the inner surface of the cylinder, the plug being formed with a plurality of through connecting holes therein, interconnecting a space upstream of the leakproof portion and another space downstream thereof, with said connecting holes having inlet and outlet portions in the form of a plurality of auxiliary inlet flow passages circumferentially arranged in a definite order on a part of the leakproof portion of said plug and a plurality of corresponding auxiliary outlet passages circumferentially arranged on another part of the leakproof portion of said plug spaced downstream of said inlet flow passages on said plug, wherein the order of arrangement of the inlets of each of the connecting holes is different from the order of arrangement of the corresponding outlets designed to communicate with said inlets sequentially in a circumferential direction about said screw plug.
The flow of fluid in the screw channel of a single screw extruder is produced by the relative movement between the screw channel and the inner surface of the cylinder comprising part thereof. Such flow element is known to include a drag flow flowing through the screw channel toward the discharge outlet and a transverse flow element circling in the cross sectional plane of the channel.When a backpressure is applied to the distal end of the screw in the vicinity of the discharge outlet, a backpressure flow element occurs in the reverse direction relative to the drag flow to elongate the melt in the screw channel and prolong the resident time of the same in the channel, consequently subjecting the material to a more intensive kneading action per unit amount of discharge.
However, it is to be noted that sheet production extruder which contributes to the kneading operation is a helical flow which advances while slowly circling in the cross sectional plane of the screw channel and which therefore does not cause the material to move in the cross sectional plane of the screw channel outward from a center portion or inward from an outer portion.Single screw extruders of the standard type produce strong shear force element along the screw channel but are relatively ineffective in moving the material transversely of the screw channel as stated above. Thus the mixing action, which may take place locally, fails to achieve a satisfactory result throughout the whole material, giving a product which is not fully homogeneous.