Volume 11, Issue 2

Knowledge of the plasticizing behavior in tightly intermeshing, co-rotating twin-screw extruders is of crucial importance for assessing the compounding process. Experiments were conducted on a laboratory extruder in order to establish the parameters that influence the melting process. Solidified samples of material were taken from the screw and microtome cuts produced from the plasticizing section. These were then assessed on the basis of image analysis. This visualization of the solid and molten phases which are present simultaneously together with the distribution of the two phases made it possible to quantify the reduction that occurs in the amount of solid material over the length of the screw configuration employed. A model is additionally presented for the calculation of the melting lengths and the melting profiles. This works on the basis of individual particles which are uniformly dispersed in the polymer melt. Allowance is made for the temperature increase in the solid material conveying section. A comparison of calculated and experimentally determined melting lengths revealed a satisfactory level of agreement.

In this paper we simulate the flow of a Newtonian fluid in a tangential counter-rotating twin screw extruder using a cylindrical coordinate lubrication theory model. Special attention is given to the material flows leading to mixing specially to inter-screw and backward leakage between the screws, and how they vary with process conditions.

The Cavity Transfer Mixer (CTM) was primarily designed as a distributive mixing device to be used as an add-on unit to existing extruders. Understanding the flow patterns and characteristics within the CTM is of fundamental importance for a proper design and optimum processing. Due to the complex geometry and the transient characteristics of the flow, solving for the flow patterns in the CTM is not an easy task. However, some of the design features of the CTM can be reproduced in a simplified device which will allow more detailed experimental and numerical investigation. In this work such a simplified device was built and used to reveal some of the features of the flow patterns and the influence of one design parameter (cavity shallowness) on the dispersive mixing efficiency.

Closely intermeshing, co-rotating, twin-screw extruders are increasingly being used for polymer preparation and reactive extrusion. For a theoretical description of these processes, not only the mechanism of reaction kinetics but also aspects of heat transfer and in particular the conveying behaviour of modular twin-screw extruders must be taken into account. Using practical examples, this work sets out to evaluate the possibility of predicting process parameters such as melt temperature, drive power, degree of filling and pressure build-up as functions of screw geometry. To this end, trials with two twin-screw extruders of different size were carried out.

A method of modeling chemical reactions in a modular inter-meshing co-rotating twin screw extruder is described. This is applied to simulate the anionic polymerization of caprolactam. Calculations are made for specific modular screw configurations. The screw configurations included both screw elements and kneading disc blocks. Reaction conversion and temperature rise were calculated along the screw axis. The results are compared to experiment.

We studied the morphology of a reactive blend, constituted by an EVA/EMA phase dispersed in a polypropylene matrix. The dispersed phase was crosslinked in-situ during the extrusion through a transesterification reaction, catalysed by dibutyltin oxide. Rheological studies allowed to define the changes in rheological behaviour during the crosslinking reaction. Using a flow model developed for twin screw extruders, we computed the reaction extent along the screws and the corresponding blend morphology using a very simple approach. Theoretical results underestimate the extent of the reaction, but explain qualitatively the general tendencies when varying operating parameters.

Numerical simulations have been performed of viscoelastic flow in double-layer coextrusion of polymer melts. The constitutive equation used is a realistic integral model of the K-BKZ type, which can describe the viscoelastic memory of polymer melts. In particular, coextrusion of a typical PS/HDPE system through a slit die has been simulated. The model constants have been determined using experimental data available in the literature. The simulation results reveal bending of the extrudate. The occurrence of bending can be justified using the relative values of the rheological properties of the melts. Bending can be eliminated for certain extrusion rate regions while it is dominant in others.

Melt spinning of polymer melts is a formation process making extremely high demands on the material's continuous deformation ability by highest deformation speeds. The structural conditions of a thermoplastic polymer which are necessary for spinnability have not been exactly cleared up yet. But this knowledge gap must be closed especially with regard to the many efforts to spin fibers with new properties made from modified polymers or polymer blends. The article is begun with a pragmatic definition of the spinnability of a polymer melt. The discussion of a structural rheological model shows a correlation between dynamic rheological measurement datas of polymer melts and experimental experience about their spinnability. Proceeding from a comprehensive description of possible failure cases (fiber beak problems), results of rheological measurement of a number of different polymers, which are spinnable well, not as well or not at all, are compared. The article is concluded by some comments on open problems which have to be solved by basic research in the near future.

To a homo-isotactic polypropylene (PP) was added 0 to 10 ppm of quinacridonequinone which was a β-phase nucleating agent for PP. The structure and properties have been studied on the inflation films processed from the PPs by the water-cooling or air-cooling methods. The effect of cooling conditions on crystallization has been also studied for reference. Even PPs containing β-phase nucleating agent do not form the β-phase but the smectic or α-phase at low cooling temperature. However, compared with PP without β-phase nucleating agent, they form the α-phase at lower cooling temperature and form the β-phase at high cooling temperature. Even PPs containing β-phase nucleating agent form only the smectic phase in the water-cooled inflation films but form the β-phase in the air-cooled inflation films if β-phase nucleating agent contents are high. Although the film properties are generally not so much affected by the addition of the β-phase nucleating agent, the dart impact strength increases for the air-cooled films which contain the β-phase.

The dimensional stability and mechanical and optical properties of injection molded articles are affected to a large extent by the distribution of residual stresses. The distribution of residual stresses varies as a result of the temperature and pressure history during the process. In this paper, two thermoelastic models were used to study the effect of processing conditions on generation of thermal stresses. Then, based on these models, two inverse models were developed to calculate the unknown pressure history and initial melt temperature from a known residual stress distribution in the final product. The models were successfully tested with stress data simulated by the forward models. The effect of experimental errors on the inverse methods was investigated by adding random errors to the stress data. Finally, experimentally measured stress profiles were used to further test the inverse models. The results are presented and discussed.

Thermal residual stresses in injection mouldings result from the non-uniform cooling of the melt in the mould. Normally, compressive stresses develop at the surface and tensile ones within the core of the mouldings. The magnitude of these residual stresses was examined for an acrylonitrile-butadienestyrene copolymer (ABS) under various injection moulding conditions. Although the occurrences of thermal stresses are inevitable, the warpage results only when the residual stresses are out of balance. The imbalance of the residual stresses are caused by the non-uniform temperature distribution through the thickness of the mouldings which may be caused by variation in cross section, part geometry and temperature difference between the two surfaces. The magnitudes of thermal residual stresses in injection moulded ABS flat plaques were determined using a layer removal technique. The development of warpage in a 3D component (an L-shaped bracket) was modelled using the residual stresses obtained for the flat plaques and the predictions were compared with the experimental warpage measurements.

The thermomechanical history of the melt under injection compression molding follows a quadrilateral-shaped path, rather than conventional triangular-shaped path, over the P-v-T diagram. The control capacity and flexibility over post-filling is significantly enhanced. It thus can produce parts with better dimensional control with less frozen-in orientation than those from conventional injection molding. This paper is devoted to study its technological application on the precision molding of parts with precision contours. A modified injection molding machine incorporating compression mechanism, and a mold with movable wall and shut-off mechanism, are used. The quality of injection-compression-molded polysytrene convex lenses is defined and compared with those injection molded. With the same cooling time, injection compression can produce lenses with better quality than those without compression