The feeding function of transferring the feed polymer from the hopper into the screw channel occurs outside of the screw, and it essentially does not depend on the screw design. The screw performs three basic functions: (1) solid conveying function, (2) melting function, and (3) metering function or pumping function. The three screw functions occur simultaneously over most of the screw length and they are strongly interdependent. The geometric name of a screw section such as feeding section, shown in Chapter 1; Fig. 1.3, does not necessarily indicate the only function of the screw section. For example, the feeding section not only performs solid conveying function, but also melting and metering functions. The screw also performs other secondary functions such as distributive mixing, dispersive mixing, and shear refining or homogenization. Distributive mixing refers to spacial rearrangement of different components, and dispersive mixing refers to reduction of component sizes as described in Chapter 2; Section 2.6.4. Shear refining refers to homogenization of polymer molecules by shearing. A single-screw extruder is a continuous volumetric pump without back-mixing capability and without positive conveying capability. What goes into a screw first, comes out of the screw first. A polymer, as solid or melt, moves down the screw channel by the forces exerted on the polymer by the rotating screw and the stationary barrel. There is no mechanism to positively convey the polymer along the screw channel toward the die. The rotating screw grabs the polymer and tries to rotate the polymer with it. Suppose the barrel is removed from the extruder, or perfectly lubricated, such that it gives no resistance to the polymer movement. Then the polymer simply rotates with the screw at the same speed and nothing comes out of the screw. The stationary barrel gives a breaking force to the rotating polymer and makes the polymer slip slightly on the screw surface. The polymer still rotates with the screw rubbing on the barrel surface, but at a slightly lower speed than the screw, because of the slippage. The slippage of the polymer on the screw surface along the screw channel results in an output rate. A lubricated screw surface increases the output rate, but a lubricated barrel surface detrimentally reduces the output rate. It is clearly understandable why commercial screws are highly polished, and why grooved barrels in the feeding section are preferred. Although many commercial practices were developed empirically rather than based on theoretical analyses, they certainly agree with the underlying theoretical concepts. The mechanisms inside a single-screw extruder are studied by examining the polymer cross-sections along the screw channel taken from “screw-freezing experiments”. In a screw-freezing experiment pioneered by Maddock , the screw is run to achieve a steady-state operation. Then, the screw is stopped and water cooling is applied on the barrel (and also on the screw if possible) to freeze the polymer inside the screw channel. The barrel is heated again to melt the polymer, and the screw is pushed out of the barrel as the polymer starts to melt on the barrel surface. Then, the solidified polymer strip is removed from the screw channel and cut at many locations to examine the cross- sections along the screw channel. Some colored pellets are mixed in the feed to visualize the melting mechanism and the flow pattern. The colored pellets retain their shapes if they remained as solid inside the solid bed before the screw stopped, but they asheared and become streaks inside the melt pool if they were molten before the screw stopped.
Overall Functions of a polymer Single-Screw Extruder
Comprehension of the physical descriptions presented in this chapter alone may prove to be sufficiently beneficial for many readers, and help them to improve their processes and products. An polymer extruder is used to melt a solid polymer and deliver the molten polymer for various forming or shaping processes. The screw is the only working component of the extruder. All other components (motor, gear-box, hopper, barrel and die, etc.) merely provide the necessary support for the screw to function properly. The overall functions of an extruder are depicted below.