Band 25, Heft 4

Microinjection molding is one of the methods used to fabricate micro scale plastic components. Addition of reinforcement in the thermoplastic materials can enhance the mechanical properties of the molded components. Due to the small dimension in micro features, the filling of reinforcements into those features is more difficult than into the main body of the component. This study is to investigate the filling efficiency in micro features for different reinforcements. The filling efficiency in the micro feature is almost the same as that in the main body of the component for using the reinforcement in nano size. For milled glass fibers, which have a distribution of the particle size, higher fiber content has better filling efficiency in the micro features. On the other hand, the filling efficiency is not affected by the volume fraction for using short glass fibers.

The microcellular foamed polypropylene (PP)/mica composites were prepared through chemical microcellular injection to investigate the relationship between the mechanical behavior and the cell size. The mechanical behavior model of the microcellular foams is built based on the elastic stress/strain field. The mechanical behavior of the microcellular composites coincides well with the theoretical model. Under loading the large and non uniform cells are in a state of plane strain, which leads to low tensile and impact strengths, whereas small and uniform cells are in a state of plane stress, which contributes to the transition from brittle to tough fracture behavior in the microcellular PP/mica composite.

This work is aimed at the numerical modeling of the flow inside single and twin-screw extruders. Numerical solutions are obtained using a recently developed immersed boundary finite element method capable of solving the flow in the presence of complex non-stationary solid boundaries. The method is first validated against the solution obtained on a body-conforming grid for a single screw extruder and then applied to a twin-screw mixer. The time dependent single screw problem can also be solved in a rotating reference frame for which a steady state solution can be obtained. This allows the evaluation of time integration errors of the moving immersed interface algorithm. For instance the flow is considered isothermal and the material behaves as a Generalized Newtonian fluid. Because the viscosity depends on the shear rate, solutions will be shown for various rotation velocities of the screw and compared with solutions obtained on body-conforming grids. The method is shown to give very accurate results and can be used for a more in-depth investigation of the material behavior in extruders.

The excess pressure losses due to end effects (mainly entrance) in the capillary flow of several types of polyethylenes were studied both experimentally and numerically under slip and no-slip conditions. These losses were first measured as a function of the contraction angle ranging from 15° to 90°. It was found that the excess pressure loss attains a local minimum at a contraction angle of about 30° for all types of polyethylenes examined. This was found to be independent of the apparent shear rate. This minimum becomes more dominant under slip conditions that were imposed by adding a significant amount of fluoropolymer into the polymer. Numerical simulations using a multimode K-BKZ viscoelastic model have shown that the entrance pressure drops can be predicted fairly well for all cases either under slip or no-slip boundary conditions. The clear experimental minimum at about 30° can only slightly be seen in numerical simulations, and at this point its origin is unknown. Further simulations with a viscous (Cross) model have shown that they severely under-predict the entrance pressure by an order of magnitude for the more elastic melts. Thus, the viscoelastic spectrum together with the extensional viscosity play a significant role in predicting the pressure drop in contraction flows, as no viscous model could. The larger the average relaxation time and the extensional viscosity are, the higher the differences in the predictions between the K-KBZ and Cross models are.

In this study, polyethylene degradation is related to its thermo-mechanical history from processing in a batch mixer. A focus is made on the rheological and molecular properties of the polyethylene subjected to different processing conditions by changing the temperature, the rotor speed and the time of process. The results indicate that while shear rheological properties changed slightly, the elongational properties were substantially modified, especially the maximum stress and the elongation at break. Due to the formation of macroradicals while processing, macromolecular changes were characterized in terms of molecular weight distribution and the results represent a balance between rates of chain scission and chain branching. It was found that the main parameter to characterize the degradation behaviour was the input power.

The crystallization of semi-crystalline polymers is often analyzed by rheometry and calorimetry. By rheometry the viscosity evolution during crystallization can be followed, whereas from a calorimetric measurement, the evolution of the degree of crystallinity can be calculated. The time evolution of these material properties is valuable input for polymer processing simulation software and in order to combine the data in a reliable manner, hardening curves are used as a characterization tool. Such a hardening curve correlates the relative increase of the viscosity resulting from crystallization, to the advancing degree of crystallinity. In this study, these are extracted from simultaneous measurements on one sample using a RheoDSC device. The RheoDSC technique allows for the direct combination of the rheological and calorimetric signal without the need of combining separate stand-alone measurement results. In this study, isothermal crystallization experiments are used to discuss the benefits of this approach. This will lead to the recommendation that measuring the hardening effect in steady shear measurements at very low shear rates in a direct combined RheoDSC setup is the most reliable method to compile unambiguously a material specific hardening curve for semi-crystalline polymers.

Low density, easy of availability, less abrasive and biodegradable nature of lignocellulosic materials emphasized their usage for development of thermoplastic composites based on Rice Husk and Polypropylene (RHPP). Mechanical properties of RHPP composites such as tensile, impact strength decreases by the addition of rice husk, preheating of rice husk and its proper size is essential for its effective use as filler in polypropylene composites. Improvement in tensile, impact, hardness and abrasion resistance is possible by adding compatibiliser and reduction of cooling time. Impact strength of the composite increase with temperature, optimum temperature for obtaining good tensile, hardness and abrasion is 200°C. RHPP Composites thus prepared have potential for applications in building and automobile industries.