No abstract

On modern extrusion plants, polyolefins are generally processed using barrels that have a grooved feed section. Accurate calculation of the melt throughput is of decisive importance for the process engineering layout of these extruders. Up to the point at which the limit speed that gives rise to the conversion from solid friction to melt film friction is attained, the conveying capacity is determined by the feed zone that is filled with granules. This paper sets out a method of calculating the throughput for speeds below this limit, paying consideration to the melt flow components in both the screw channel and the grooves. The calculation method was verified experimentally with the aid of throughput measurements for different polyolefins. The results generally fall within a tolerance range of +/−10 %.

The simulation method is able to relieve the engineer of experimental work during the planning of extrusion plant and thus clearly reduces the development time. Sufficiently precise determination of the calculable parameters is possible with the process models developed so far. The empirical knowledge of the engineer is, of course, necessary as well in order to estimate the impact of influencing parameters that are not taken into account in the calculation models. The presented simulation program REX can perform these complex calculations in a comprehensive manner, with its simple and comfortable user interface. The program, however, can be employed as a valuable tool only if it is in the hands of an expert operator who can, and indeed must, be able to interpret the results as well.

Wave-dispersion screws have been used industrially in many types of extrusion processes, injection molding, and blow molding. These high-performance screws are constructed by replacing the metering section of a conventional screw with a melt-conveying zone consisting of two or more parallel flow channels that oscillate periodically in-depth over multiple cycles. With the barrier flight between the screw channels being selectively undercut, the molten resin is strategically forced to flow across the secondary flight, assuring repeated cross-channel mixing of the polymer melt. Despite the industrial relevance, very few scientific studies have investigated the flow in wave-dispersion sections in detail. As a result, current screw designs are often based on traditional trial-and-error procedures rather than on the principles of extrusion theory. This study, which was split into two parts, was carried out to systematically address this issue. The research reported here (Part A) was designed to reduce the complexity of the problem, exclusively analyzing the pressure-induced flows of polymer melts in wave sections. Ignoring the influence of the screw rotation on the conveying characteristics of the wave section, the results could be clearly assigned to the governing type of flow mechanism, thereby providing a better understanding of the underlying physics. Experimental studies were performed on a novel extrusion die equipped with a dual wave-channel system with alternating channel depth profiles. A seminumerical modeling approach based on network theory is proposed that locally describes the downchannel and cross-channel flows along the wave channels and accurately predicts the pressure distributions in the flow domain. The solutions of our seminumerical approach were, moreover, compared to the results of three-dimensional non-Newtonian CFD simulations. The results of this study will be extended to real screw designs in Part B, which will include the influence of the screw rotation in the flow analysis.

In a smooth barrel extruder, the throughput is generally dominated by the metering sections pumping ability. Within the feed zone, more granules can always be conveyed than can melt in the compression zone and be discharged in the metering zone. At high screw speeds, this effect will only occur if the granules flow quickly enough out of the hopper into the screw channel and this is fully filled. In this case, the solids throughput up to maximum screw speed shows an approximately linear relationship to the speed. These linear solids conveying characteristics necessitate constructional changes to the hopper opening. In this article, solids conveying processes with approximately linear conveying characteristics up to speeds of 2,000 min−1 (peripheral velocities up to 3 m/s) are simulated with various feed hopper geometries. The simulations are carried out by means of DEM (Discrete Element Method) in which the granules are approximated by single spherical particles. From the simulation results, via dimensional analysis a model is derived for describing the throughput as a function of the geometry and process parameters of the solids feed zone.