Band 10, Heft 2

The paper is devoted to theoretical and experimental investigation of hydrodynamics, heat exchange and kinetics of dispersing of carbon black with a rubber mixture in a chamber of an internal mixer. Modeling of the process is based on use of a rheological equation of a “power” type. A possibility of slipping along a wall is taken into consideration in formulation of boundary conditions. Results of model calculations gave ground for the engineering method of calculations of optimal values of the size of the gap between a wedge of a rotor blade and a wall of a chamber. Comparison of such principal characteristics of a mixing process as average temperature of a stock and the degree of dispersion demonstrated quite good correlation of calculated and experimental data.

A flow visualization study of starve fed solids conveying in single screw and a modular self-wiping co-rotating twin screw extruder is observed inflow visualization experiments. Single screw extruders exhibit forward Archimedean transport of material on the undersides of the screw. Self-wiping co-rotating twin screw extruders conveying forward in the intermeshing region and in the region near the apex between the screws. Observations are reported of the melting of solid pellets and possible melting mechanisms are discussed.

The residence time distribution (RTD) of a counter-rotating non-intermeshing twin screw extruder has been studied by a radioactive tracer technique. RTDs were measured at both the die exit and at the first compounding zone of the extruder. The operating parameters investigated were: screw speed, screw flight stagger, and barrel temperature. These parameters have an effect on the mixing efficiency. The mixing efficiency was characterized by fitting measured response data with models comprised of different combinations of ideal reactor elements. The empirical flow models tested were: plug flow and tanks-in-series (with and without recirculation), plug flow and axial dispersion, and the Wolf-Resnick model. The fitted model parameters characterized the observed changes in the RTD due to the variation of the operating parameters.

A new mixing device, to be mounted on the tip of a single screw extruder, is equipped with a freely rotating ring. Its homogenising action is clearly understood and explained. In order to understand its dispersing action experiments have been performed in order to find proof of the presence of elongational flow. They regard visualisation of the flow in a transparent model and blending of PS into PE with various viscosity ratios. Aspects of design and applications are discussed.

By monitoring the development of morphology a novel dual extrusion process from the point that the thermotropic liquid crystalline polymer (TLCP) stream meets the matrix material to the final drawn strand, a clear mechanism for the development of the TLCP reinforcement is presented. It was found that under appropriate mixing configurations the fibrillar TLCP morphology seen in strands produced from this method originate at the onset of mixing in the phase distribution system and are subsequently refined as the melt passes through the static mixing elements. The axially continuous morphology seen at the entrance of the die is maintained and further developed as the blend passes through the die and is drawn. In contrast, strands produced using the single extruder blending or the dual extrusion method with excessive mixing produce a droplet morphology prior to the die. At a very low concentration of TLCP, the droplets are too small and well dispersed to sufficiently coalesce and deform into fibrils in the elongational flow field of the die and drawing process. These phenomena result in an undesirable morphology for self reinforcement which clearly has been seen in the measured mechanical properties of the strands.

Simulations of the elastic deflection of a 1.2 m slit die under load during the extrusion process have been performed, using the commercial finite element package MARC. The melt pressure distribution needed to assign the loads to the MARC package was previously obtained by a Hele-Shaw based finite element program, and was thus not coupled with the deflection analysis. Existing analytical calculation methods for deflection are also examined. Experiments using production scale machinery have been conducted to record the die deformations under different melt pressures and compared with the computed data. Analytic deflection predictions are shown to be inadequate. The numerical results are fair, but it is clear that coupling of flow and deflection analyses is essential.

Fabrication conditions and material rheology control the process of film blowing and limit the possible shapes the bubble may take as well as the ranges and ratios of stresses within the bubble. In order to understand bounds on the stresses during the blowing process, we explore the constraints imposed by force balances, the thin shell approximation, and thermodynamics. We show that the ratio of axial stress to hoop stress is independent of the explicit terms for film thickness, velocity, heat transfer, and the rheology of the material. In addition, we demonstrate that the ratio of the axial stress to the hoop stress can be calculated at any point in the blown-film requiring only values for the pressure drop across the bubble, the shape of the bubble, and the force pulling up the bubble. Experimental results, as well as certain theoretical evidence, strongly indicate that the ratio of axial to hoop stress is everywhere greater than 1 in the standard blown-film process. If this is generally true, then the stress relations we derived can be employed to test the applicability of various theoretical models of the blown-film process.

A one-dimensional purely-viscous model that includes slip at solid boundaries is proposed to study the flow of polymeric materials when compressed between two approaching parallel surfaces. A squeeze-flow rheometer was designed and built to compress polymer melts under controlled conditions of constant closing speed and constant applied force for different conditions at the polymer-solid interface. One of the purposes of this study was to investigate how the interface conditions influence the flow field during squeezing. The experimental findings revealed that the model can adequately predict the squeezing force for slow squeezing of relatively thin samples. For fast squeezing, however, the experiments appear to suggest the need of including elongational effects in the modelling of the material behavior.

Polyamide (PA) was compounded with carbon fiber (CF) with various contents (0, 5, 15, 20 vol.-%) using an elastic extruder. The effect of both the carbon fiber contents and the compounding method on thermal behavior and characteristics of crystallization are studied using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and polarizing optical light microscopy (POL). Thermal investigation was carried out under programmed cooling and isothermal conditions. The following results were obtained: (1) The crystallization temperature became higher with the increase of carbon fiber content, while the melting temperature and enthalpy were scarcely affected by the carbon fiber content. (2) The compounding methods affected the thermal behavior of composite systems. The samples that have been extruded twice have higher crystallization and melting temperatures as well as higher melting enthalpy. (3) Isothermal crystallization was observed using DSC. From the rate constant k and Avrami's index n, it was found that crystallization was accelerated by the presence of carbon fiber without changing the type of nucleation. (4) The polarizing micrographs showed that the nucleation of PA occurred on the surface or close to the CF.

An isotactic polypropylene homopolymer (PP) was added with 0 to 10000 ppm of γ-quinacridone, a β-crystal nucleator of PP. Flexural test specimens were injection-molded from the PP at cylinder temperatures of 200 to 320 °C and the structures and properties have been studied. The results can be summarized as follows: The crystallization temperature increases with γ-quinacridone content. The β-crystal content decreases with increasing cylinder temperature and shows a maximum at a γ-quinacridone content of about 100 ppm. The thickness of the skin layer scarcely depends on γ-quinacridone content. The orientation of β-crystals decreases with increasing cylinder temperature. It is high at the surface regions and decreases toward the interior. The Izod impact strength increases with γ-quinacridone content. The degree of the increase is higher as the β-crystal content is higher and reaches more than threefold in the highest case. The flexural modulus and strength decrease by about 10 % in the highest case. The heat distortion temperature increases with γ-quinacridone content. The mold shrinkage increases with γ-quinacridone content. From these results, it is concluded that the injection moldings of PPs containing γ-quinacridone are superior in impact strength and heat resistance.

Previous work has shown that numerical simulation does not provide a correct description of compression experiments if the rheology of the polymer in the solid state is deduced from tensile tests. Therefore, a new method is proposed for the determination of the coefficients of G'sell's constitutive equation, in order to simulate the forging process. It also gives access to a parameter characterizing the friction conditions between the polymer and the tool. It is based on isothermal compression of polymer cylinders with an appropriate geometry and on its complete interpretation by means of computer simulation using the finite element code FORGE2. This original procedure is applied to polyoxymethylene.

During gas-assisted filling, gas penetrations in symmetrical rib channels are found never symmetrical. The development of this unsymmetry is observed and its cause is analyzed. The system of gas penetrations in melt in symmetrical rib channels is found unstable. Symmetry is upset upon perturbation. Symmetrical gas-channel ribs in gas-assisted injection molded parts should be avoided.