Helmut Potente

Screw extrusion : science and technology

Nedvojbeno je da su pu`ni ekstruderi najva`niji strojevi, radni konji u polimerstvu. Dva vrhunska stru~njaka na podru~ju strojeva s pu`nim vijcima okupili su vrlo sna`nu skupinu koautora. To je omogu}ilo nastajanje sa`etoga, ali vrlo vrijednoga priru~nika. Autori smatraju da knjiga poput ovih ima malo; navode svega tri, uklju~ivo glasovito djelo Hensena, Knappea i Potentea: Handbuch der Kunststoff-Extrusiontechnik I, Grundlagen iz 1989. Kako je od izdavanja navedene knjige pro{lo punih 15 godina, moralo se pristupiti pisanju novoga djela, ali koje je ~vrsto ukorijenjeno u navedenoj knjizi H. Potentea i suradnika.

SIMULATION OF QUASI-SIMULTANEOUS AND SIMULTANEOUS LASER WELDING

The laser transmission welding of thermoplastics has become increasingly important since it was introduced into industrial series production 15 years ago. The respective influences of laser intensity, warming up time and both the joining pressure and the joining displacement on the quality of the weld have generally been established through experimental studies. In order to be able to predict the temperature, the melt shift profiles and the residual melt layer thickness in the welding zone, calculations were performed on the basis of a simplified mathematical-physical model, employing finite element analysis. The quasi-simultaneous and simultaneous laser welding process was simulated, including the heating and cooling phase, using temperature-dependent data. The convective cooling and emissivity at the surface of the adherends was taken into consideration. The intensity distribution, pressure and warming up time have to be specified as the influencing parameters for the computation. Pressure-regulated welds with time as the termination criterion were simulated, since these are the type of welds generally employed in practice. The simulation results have been continuously adapted for the experimental determination of the melt layer thickness with different process parameters. This paper presents the influence of the process parameters on the heat-affected zone, taking the example of a PA 6.

Description of the foaming process during the extrusion of foams based on renewable resources

Some polymers based on renewable resources like starch can be plasticated by extrusion processing. Foams based on starch have an increasing importance in nonfood applications. Starch contains water, which works as a physical blowing agent for expansion of molten starch. The aim of this work is to investigate how cell nucleation and foaming are affected by process parameters in the case of starch materials. Therefore, the cell nucleation has been investigated by a slit die with transparent inserts in the flow channel. It could be shown that a higher pressure gradient in the die leads to a later cell nucleation. Contrary to this, an earlier nucleation is supported by high shear rates. A later nucleation leads to a high cross-sectional expansion ratio. Furthermore, the rheological properties also have an impact on the foaming behavior of molten starch. It was observed that, besides water content and temperature, the screw speed had a significant impact on the viscosity. In this case, the major effect is the specific mechanical energy input (SME) which increases with increasing screw speed.

Polymer Pellet Flow out of the Hopper into the First Section of a Single Screw

Abstract The flow of plastic pellets into the first section of a single screw is frequently neglected when analyzing the solids conveying process in single screw machines. In order to get a better insight into the complex correlations that exist between pellet properties, barrel and screw geomtry a physical-mathematical model is presented. As a result the pellet inflow behavior and the degree of filling in the first channel of the screw can be determined.

Melting of Polymer Blends in Co-rotating Twin Screw Extruders

Abstract Part II of the publication describes a model for calculating the melting of polymer blends which has been implemented in the SIGMA simulation software for the design of co-rotating twin screw extruders. The model is based on the findings discussed in Part I and makes it possible to calculate the temperature progression in the solids conveying section and during the subsequent melting process for binary incompatible polymer combinations. The two material components are observed in parallel during the melting process, and the respective degrees of melting over the length of the screw are calculated. The properties of the melt phase, which forms from both components, are calculated with mixing rules implemented in the program. The calculations supply the melting profiles for both components, thereby permitting a comprehensive analysis of the melting of binary polymer combinations. The results additionally form the basis of a calculation to estimate the morphology development of polymer blends in the melting section of the extruder. Part III of the report looks into a means of investigating the melting of binary polymer combinations, and the results of experimental investigations into polypropylene/polyamide blends are discussed. The simulation calculations are compared with the results of experimental studies in order to verify the model presented in this part of the publication.

Pressure/Throughput Behavior of a Single-screw Plasticising Unit in Consideration of Wall Slippage

Abstract The mathematical models so far available for describing the throughput behavior of plasticising extruders were developed with the assumption of a wall-adhering melt. There are, however, a series of plastic melts, and also elastomers, polymer suspensions, ceramic materials and food products that display wall slippage during processing. A mathematical model has been developed for this material behavior, which describes the flow behavior for the unidimensional, Newtonian, isothermal case. Apart from the development of the analytical model, the flow behavior of wall-slipping polymer melts was also analysed with the aid of finite element calculations (FEM). A comparison of the results for the pressure/throughput behavior shows that the calculation results tally very well for the two methods. It is thus possible to develop a procedure which makes it possible to describe the phenomenon of wall-slippage for the non-Newtonian, multi-dimensional, non-isothermal case.

Description of the Pressure/Throughput Behavior of a Single-screw Plasticating Unit in Consideration of Wall Slippage Effects for Non-Newtonian Material and 1-D flow

Abstract Up to now, existing mathematical models can only describe the throughput behaviour of compounding extruders assuming the condition of a wall adhering melt. However, there is a variety of polymer melts, as well as polymer suspensions, ceramic materials and food stuffs, which in the course of manufacturing are wall-slipping. A mathematical model describing the behaviour of the isothermal flow of non-Newtonian materials is developed. In addition to the development of the analytical model, which is based on Flumerfelt et al. [1] and Hirshberger [2], the flow behaviour of wall-slipping polymer melts is also analysed using Finite-Element-Simulations (FE). Comparison of the results for pressure, throughput and velocity profiles along the channel depth shows a very high degree of agreement.

Two Dimensional Description of Pressure-Throughput Behaviour of Newtonian Materials Considering Wall Slippage Effects

Abstract The description of pressure-throughput behaviour of wall adhering melts in plastifying machines has been issued in a number of publications, e. g. [1 to 4] in details. Nevertheless there are a variety of polymer melts as well as elastomers, polymer suspensions, ceramic materials and food stuff, which do not behave wall adhering in the course of manufacturing. The theoretical discussion of the pressure-throughput behaviour has not sufficiently been described up to now. Worths [5] case studies include the isothermal one-dimensional flow of a Newtonian medium in an unwinded screw channel. He assumed that after exceeding a critical shear rate the slipping process starts on the barrel wall only – but not on channel wall and screw root surface. Mennig, [6 to 9] took up this statement again and assuming a shear rate on the barrel wall as well as on the screw root surface he derived simple relations for the radial velocity profile from it. Lawal and Kalyon [10 to 12] have developed an analytical model that describes the behaviour of visco-plastic liquids in flat channels for different quotients of wall slippage, which are derived from the wall slippage quotients on screw root surface and on barrel wall. A mathematical model describing the flow behaviour for the two-dimensional Newtonian, isothermal case has been developed for wall slipping materials in order to describe the pressure-throughput behaviour multi-dimensionally. In addition to the development of the analytical calculating model the flow behaviour of wall slipping polymer melts has been analysed by using the finite-element-calculation (FEM). Comparison of the pressure-throughput behaviour results show a high degree of conformity in the calculation results.

Physico-Mathematical Model for the Description of the Temperature Development and the Power Consumption in Co-Rotating Twin Screw Extruders

Abstract Tightly intermeshing, co-rotating twin-screw extruders are commonly employed for tasks requiring good mixing. The modular constitution of both barrel and screw makes it possible to optimise the extruder configuration for a given task. Even today this optimisation is frequently done by applying the “Trial and Error”-method. Physico-mathematical models enable the process engineer to predict the process behaviour of a chosen extruder configuration and to optimise existing extrusion processes. Increasing demands in mixing quality and efficiency of the processing unit result in efforts to optimise existing extruders with respect to those factors. For this reason knowledge concerning the temperature profile along the screw and the power consumption is essential. We present a new physico-mathematical model for the description of the temperature development. At stages where the analytical solution itself would result in a not satisfying degree of accuracy we used descriptions based on finite element simulation results to achieve the desired exactness. As a result we got a model to describe the temperature development in the screw channel and a model to describe the power consumption in different screw sections. Applying these models it is possible to optimise the process parameters and the screw configuration with only a minimum of preceding experimental investigations.

Polymer Blends in Co-Rotating Twin-Screw Extruders

Abstract The properties of polymer blends are determined to a decisive extent by the morphological structure of the polymer combinations employed. The design of extruders thus calls for models to calculate the estimated morphology development over the length of the extruder screws in the melt-conveying section. Since the most significant morphological changes are observed in the melting section, however, it is also necessary for the morphology formation and development to be analyzed in this section of the extruder. The melting process of binary material combinations is thus important too. In the context of this research, experimental investigations were conducted using polypropylene/polyamide 6 (PP/PA 6). In the tests, the degree of melting and the morphology development were determined over different screws and compared with calculations. In order to analyze a range of relevant influences, the extruder size, screw configuration, screw speed, weight components and also the viscosity ratio were varied by using different PP types. Apart from the model for calculating the melting of polymer blends, a formulation was developed that can be used to estimate the morphological changes occurring in the melt-conveying section. The model is based on the assumption that morphological changes can be estimated by calculating the probabilities of different drop breakup mechanisms and the coalescence process. The investigations of the blend morphology in the melting section and the melt-conveying section reveal key findings that have to be taken into account for modeling the formation and development of the morphology. First of all, in the melting section, it is very clear that a kind of melt film removal occurs at the surface of the granules of the second component, which melts at higher temperatures, as these granules melt. The drops of second component in the melting section, which are directly adjacent to fractions that have not yet fully melted in some cases, have already assumed dimensions (in the μm range) similar to those seen at the end of the extrusion process. This means that, in the melting section of the twin-screw extruder, small volumes are broken or worn off the already-molten granule surfaces. An evaluation of scanning electron micrographs also shows that, in the melting section of co-rotating twin-screw extruders, virtually all the breakup mechanisms that can essentially be distinguished take place in parallel, such as quasi-steady drop breakup or supercritical breakup, folding, end pinching, tip streaming and breakup through capillary instabilities. Alongside the breakup mechanisms, there are also drops that clearly unite to form bigger drops through coalescence. When comparing the calculations for the melting of polymer blends, relatively good agreement is obtained with the experimental test results. The calculations display a satisfactory level of accuracy, particularly for polymer combinations with similar viscosities and also for bigger extruders. The calculations with the morphology model also show the same trends as the experimental investigations. Hence, for the design or optimization of twin-screw extruders, it is now possible not only to calculate the fundamental process variables (such as pressure, temperature, melting) but also to estimate the morphology that has a decisive influence on the resultant material properties.