Single screw extruder for efficient blending
A single screw extruder is presented which includes a plurality of equidistant undercut barriers disposed in a preselected pattern within the channel of a twin screw extruder. The undercut barriers are disposed in the first metering stage and second metering stage and provide increased dispersive and distributed mixing for highly viscous difficult to blend materials. In another embodiment the sheet production extruder includes a slotted blister ring, having slots and lands disposed thereon, positioned between the first metering stage and the vent zone to further increase the amount of mixing.
A typical two-stage single screw extruder serves three functions: (1) feeding; (2) melting and mixing; and (3) metering. FIG. 3 is a typical two stage single extruder screw of the prior art including a number of zones with zones labeled as feeding, transition, first metering, venting, and second metering. Melting of solids which starts in the transition zone, also called compression zone, should end early in the metering zone. Some degree of mixing between unmelted solids with the melt may occur in the transition zone itself. Many commercially available extruder screws of the prior art are specially designed to perform this specific function and guarantee very early melting, an example being the Barr ET screw as described herein above. Another popular screw, the barrier screw, separates melts from unmelted solid so that the solid is always subjected to high shear forces caused by friction with the barrel wall and therefore melts rapidly. The first metering section simply conveys the melt by drag flow into the venting zone and the second metering zone pumps the melt into a stranding die.
Another screw design of the prior art aimed at improving distributive mixing in a single extruder screw includes a series of undercut baffles placed along the screw channels to disrupt streamlines of the cross-channel flow. The baffle locations are alternated periodically to produced chaotic mixing of the melt. There are several limitations in such an extruder screw. First, if the channels are not completely filled, only a part of the channel cross-section is available for chaotic mixing; therefore reducing the extent of distributive mixing. This is especially true for polymers available in the form of low bulk density powder. Secondly, the aspect ratio of the screw channel must lie between 4 and 6 for realization of appreciable chaotic mixing. This poses serious restriction on the available flow area and throughput rates as the undercut baffles also occupy some physical space. Thirdly, no dispersive mixing mechanisms are provided by the undercut baffles of such a design. As stated herein above it is advantageous for an extruder screw to be able to produce both dispersive and distributive mixing.
It is apparent from the above there exists a need in the art for an extruder which is capable of increased dispersive and distributive mixing. It is a purpose of this single screw extruder to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.