Paul J. Gramann

Design of Dispersive Mixing Devices

Abstract Mixing is one of the main functions of screw extruders. In the analysis of mixing, there are two mixing mechanisms that need to be considered: distributive and dispersive mixing. It is well known that single screw extruders generally have poor dispersive mixing capability, even when dispersive mixing elements are incorporated into the screw design. This paper will discuss the requirements for dispersive mixing, current dispersive mixing elements used in single screw extruders will be analyzed, and the lack of efficient dispersive mixing will be explained. A new generation of dispersive mixing elements will be introduced for use in both single and twin screw extruders and internal mixers. A two and three dimensional flow simulation will be used to analyze the mixing performance of these new mixers. Experimental work on the new mixing elements will be presented in a follow-up paper; the results demonstrate that it is possible to achieve dispersive mixing on single screw extruders that is as good as what can be achieved on twin screw extruders. The implications of these results on the compounding industry will be briefly discussed.

Cell Development in Microcellular Injection Molded Polyamide-6 Nanocomposite and Neat Resin:

The effects of nanoclay addition into polyamide-6 (PA-6) neat resin and processing parameters on cell density and size in microcellular injection molded components are investigated. The analyses are performed on the sprue section of standard ASTM D 638-02 tensile bars molded based on a fractional four-factorial, three-level, L9 Taguchi design of experiments (DOE) with varying melt temperature, injection speed, super critical fluid (SCF) concentration, and shot size. It is found that the presence of nanoclay greatly reduced the cell size and increased the cell density when compared to neat resin processed under identical molding conditions. In addition, cell size distribution at the sprue center was, in general, the largest, gradually decreasing toward the skin for both the neat resin and the nanocomposite. Finally, in contrast to neat resin, in which shot size and injection speed were important to cell density and all molding parameters affected cell growth, the cell size and density for nanocomposite only depended strongly on shot size.

Flow Analysis in Screw Extruders-Effect of Kinematic Conditions

Abstract Recently, several investigators have challenged the simplifying assumption of using a stationary screw with a rotating barrel when analyzing the flow in single screw extruders. The investigators claim that there are significant differences between the realistic and simplified cases, such as a 12 percent difference in throughput [1]. This paper presents an analytical derivation for the flow in a laid out system and one which includes the curvature of the screw. An experimental set-up was built which allows the rotation of the barrel with a stationary screw and the rotation of the screw with a stationary barrel, and a three dimensional flow simulation program was used to model the flow in the extruder using the two conditions. The analytical, experimental and numerical analyses show that it is justified to use the stationary screw assumption when modeling the flow in single screw extruders.

Simulating Polymer Mixing Processes Using the Boundary Element Method

Abstract The mixing of plastic into filled and unfilled polymer blends has been an important issue in the polymer industry. Processing difficulties with these polymers have been encountered in the mixing quality as well as in the thermal degradation due to viscous heating. Mixing often occurs as an element of the processing step, e.g., inside single and twin screw extruders used in the fabrication of final parts or sheets, and inside internal mixers such as the Banbury type mixer. Quantifying the mixing inside an extruder or an internal mixer and predicting the thermal degradation due to viscous heating is an extremely difficult task. A better understanding of the mixing process and control of viscous heating will lead to optimal parts and may eventually allow us to increase the relative amount of fillers within the material. This paper presents a boundary element simulation of the flow of filled and unfilled polymer blends inside extruders and internal mixers. First, the general equations that govern such flows are shown. The boundary integral equation and the fundamental solutions are then formulated followed by the numerical implementation and logistics of the simulation. After the theoretical background is presented, a numerical example is given. The paper then shows how the simulation was used to analyze the flow inside a single screw extruder and internal batch mixers. Such a simulation can be used to eliminate some of the tedious trial-and-error tasks that are typically performed in the early stages of material synthesis and can also be used by manufacturers of mixing equipment when optimizing the geometries of cavities and mixing heads to achieve optimum mixing with reduced viscous heating. This research will significantly increase our knowledge of the behavior of filled and unfilled polymer blends and expand our understanding of the complex phenomena that take place during their mixing process.

The dual-reciprocity method for heat transfer in polymer processing

Abstract This paper presents a boundary-element simulation of the heat transfer during polymer processing. The governing equations, boundary integrals and fundamental solutions, with their numerical implementation for heat transfer and creeping flows, are presented. Viscous dissipation and internal-heat generation for curing polymers is included in the equations. In order to calculate the transient-heat transfer and internal-heat generation, the dual-reciprocity boundary-element method is applied. The heat-transfer-simulation results are in good agreement with analytical and other numerical results.

Flow and Heat Transfer Simulation of the Mixing of Polymer Blends Using the Boundary Element Method

Abstract In most polymer processes the quality of the final part stems back in great part to the mixing of the polymer blends and the viscous heating that occurs during the process. To date, these processes have been simulated using the finite difference or finite element methods which rely on cumbersome remeshing techniques, restricting the analysis to simple geometries. However, mathematical manipulation of the governing equations results in boundary integrals, allowing the solution of complicated moving boundary problems without domain meshing. The resulting boundary element model and simulation results of flow with heat transfer and the blending of two or more fluids of different viscosity effects during mixing of polymer blends is presented in this paper. The results give insight to the complex phenomena that take place during mixing processes.

Experimental and Numerical Study of Rhomboidal Mixing Sections

Abstract The rhomboidal mixing section is becoming very popular among processors to provide distributive mixing. Currently, several different designs are used but the details of the flow behavior and mixing efficiency is not well understood. This information is needed to be able to design and find the most efficient rhomboid geometry. In this investigation nine different geometries with various pitches (helix of rhomboids) were analyzed using a 3-dimensional boundary element method (BEM). The geometries were compared according to mixing efficiency, pressure and energy consumption. The results were compared to experiments performed with a conventional single screw extruder that was fitted with three different rhomboidal mixing sections. The investigation led to the conclusion that the most effective distributive mixing sections were those with neutral rhomboids (pine-apple mixer). However, the neutral rhomboidal mixing section consumes the most pressure in the extruder. It was also concluded that rhomboidal mixing sections deform the material by shear, making them poor dispersive mixing sections.

Comparative study of mixing in corotating twin screw extruders using computer simulation

Twin screw extruders are common for mixing and compounding of different polymers and additives in the production of materials with enhanced properties. However, the flow field and resulting mixing that occur inside these extruders are not well understood. For gaining a better understanding of the flow phenomena that take place during these operations, computer simulation can be used; however, due to the complexity of the moving boundaries in the twin screw extruder this is not an easy task. Using the boundary element method (BEM), which is the method of choice for moving boundary problems, the flow field, pressures, stress, and strains can be computed at every time step. With this information, the quality of mixing can be determined quantitatively and allows the engineer to modify the geometry of the screws for optimal mixing conditions. In this article, the boundary element method is used to analyze and compare mixing effectiveness of various geometries of the twin screw extruder.

Simulating the non-isothermal mixing of polymer blends using the boundary element method

In most polymer processes the quality of the final part stems back in great part to the mixing of the compound and the viscous heating that occurs during the process. To date, these processes have been simulated using the finite difference or finite element methods which rely on cumbersome remeshing techniques, restricting the analysis to sim ple geometries. However, mathematical manipulation of the governing equations results in boundary integrals, allowing the solution of complicated moving boundary problems without domain meshing. The resulting boundary element model and simulation results of the flow and heat transfer effects during mixing of polymer blends is presented in this arti cle. The results are in good agreement with experimental data and give insight to the com plex phenomena that take place during mixing processes.