Band 16, Heft 2

The “new-generation” Coflow4™ intermeshing internal mixer uses less motor power peak and specific energy than all traditional intermeshing and tangential internal mixers. The Co-flow4™ mixer provides the following advantages: better quality, 20% to 45% increase in productivity depending on your rotor length-to-diameter ratio, improved heat transfer, over 50% reduction in dust-stop leaks, better for one-pass or multiple-pass mixing, wide range of process materials, better wear life, retrofit capabilities and lower mixer cost. It is more cost effective to retrofit your traditional mixer with a Coflow4™ mixer when your chamber bores and rotors are worn out, and you are ready for a rebuild rather than investing into the same old mixing technologies that yield small return. Owing to the optimal constant rotor length-to-diameter ratio and the superior mixing capabilities of the co-flow mixing technology, it will be beneficial to try and use the Coflow4™ mixer to compound the materials, which could not be compounded before by the old traditional mixing technologies or the two-roll mills.

The compounding of wood flour filled polyethylene is discussed with reference to co-rotating twin-screw extruders from two manufacturers. An acrylic acid-grafted polyethylene (PE-HD-AA) copolymer was used as the compatibilizer in high density polyethylene wood flour (PE-HD/WF) composite system. Special consideration was given to the compounding of the heat- and shear-sensitive wood flour. The relevant screw configuration was found to consist of short mixing length with low intensity of shearing. A suitable combination of processing variables was necessary for limiting the thermal degradation of the wood filler, however, tensile properties of the composites were not affected much. Furthermore, the allowable range of processing temperature was limited, and there was an upper bound of rotating speed and a lower bound of throughput rate within which the darkening of wood filler in the composites was acceptable. The processing window was also experimentally constructed in terms of the degree of darkening of wood composites.

In the study of solid conveying process in plastics screw extrusion, a particle element numerical method enables us to simulate behavior of each solid particle during the process and takes into consideration the parameters of individual particle in the calculation. The melting of pellets in the solid conveying section is caused predominantly by heat from both friction and conduction through close contact. Experiments showed, melting happened before the solid plug was formed. Hence, when a particle element numerical method is used to simulate the conveying and melting process in the injection bellow, the analysis of melting process caused by solid particles sliding against wall of bellow should be included. Searching through the recent literature, no valid analysis of the transient close-contact melting process caused by viscous dissipation was found. This paper attempts to present an analysis of the melting process and derive an expression for describing the transient melting caused by both friction and close-contact heating. The melting model of polymer pellets sliding against the barrel was achieved by assuming the friction against the barrel as friction against an isothermal wall given the high heat conductivity of the barrel. To keep the expression simple, a constant temperature boundary condition is utilized for the wall which concurs with reality.

The previous study on the dynamic processing characteristics of co-rotating twin-screw extruder based on a visible technique (which employed a “transparent” extruder) performed by the authors Wong et al. [1 and 2] had enabled a quantitative treatment to be carried out in the present paper on the melting behaviour of polymer during an extrusion process. The qualitative analysis performed previously [1] had led to the development of a melting model which was capable of revealing some of the fundamental melting behaviour. The model also enabled the determination of the length of screw required for completing melting (Z r ). Melting characteristics in terms of effects of solid feeding rate, temperature, screw speed and channel depth on the predicted Z r were discussed to further our understanding on melting performance of polymer melt in intermeshing corotating twin-screw extruder.

The properties of polymer blends are conditioned to a large extent by the morphological structure of the polymer combinations employed. Since the development of the morphology inside the extruder is essentially influenced by the melting section, it is necessary to be able to estimate the melting process of binary material combinations. Part I of this publication summarises the knowledge that has been acquired in theoretical and experimental studies of melting in co-rotating, tightly-intermeshing twin screw extruders. The experimental studies reveal quite dissimilar melting mechanisms for different materials in some cases. Different models are thus available for calculating the melting process of single-component materials which have been developed on the basis of the findings established in experimental investigations. Only one model is so far familiar for the melting process of multi-material systems. This uses material values for polymer blends that have been averaged out with mixing rules to provide the basis for a calculation to estimate the melting process. Part II of the publication describes a model for calculating the melting of binary, incompatible polymer combinations which has been implemented in the SIGMA simulation software for the design of co-rotating twin screw extruders. This model can be used to calculate the temperature development in the solids conveying section and the subsequent melting process. Part III of the publication explains one possible means of investigating the melting of binary polymer combinations and discusses the results of experimental studies of polypropylene/polyamide blends. For purposes of verifying the model set out in this part of the publication, the simulation calculations are compared with the results of the experimental studies.

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.

Experimental studies of the melting process in extruders generally involve a high outlay and can only be performed with certain restrictions, both in respect of the method employed and with regard to the evaluation of the results. This part of the work describes one possible way of studying the melting of binary incompatible polymer combinations. The method described makes it possible to estimate the degree of melting of the two components over the length of the screws of the twin screw extruder. Experimental studies were performed of polypropylene/polyamide 6 (PP/PA6) blends containing low weight contents of the disperse PA 6 phase. In order to verify the theoretical models set out in Part I, page 124 of this issue, not only were the process conditions of screw speed and throughput varied but also the viscosity ratio. This was achieved by using two different PP grades. In addition, comparable tests were performed on two different sizes of extruders. The tests show that the melting of polymer blends, and particularly the melting of the second component, which melts at a higher temperature, is determined to a significant extent by the screw rotational speed, the throughput, the ratio of the extruder channel depth to granule diameter and also the material combination (viscosity ratio). Comparisons of calculations and experiments show that the melting profiles calculated for different material combinations and process conditions tally well with the experimental studies in overall terms.

The development of the mapping method [1, 2] and its application to viscous flow in a (multiflux) static mixer [3] is discussed. The mapping method describes the repeated transport of fluid from one domain to another and computes the concentration distribution, yielding a unique mixing measure, like e.g. the intensity and/or scale of segregation [4,5], the residence time distribution and structure parameters like e.g. the area tensor [6]. In the multiflux static mixer, the domain is a (two dimensional) cross-section of the flow channels at the beginning and end of a single element. Since static mixers are build of a sequence of similar elements, we only need to determine one mapping matrix (a computationally expensive operation). To simulate the complete mixer, the mapping matrix is repeatedly applied (a computationally cheap operation). Three different geometries are analyzed. It is shown that an approach like the mapping method is a prerequisite in order to obtain meaningful results after a number of mixer elements. Moreover, it enables optimization of industrial mixers.

The application of the mapping method [1, 2] on fully three dimensional flows with dynamic boundaries is presented. The computational domain is a (three dimensional) section of a particular type of screw element in a closely intermeshing corotating twin screw extruder. Screw elements include transport, different kneading, and counterconveying zones, for each of which a mapping matrix has to be constructed. In the future, different screw assemblies could be studied by combining distinct mapping matrices, much like an actual screw is assembled from distinct modules. In this paper, the principle and applicability of the mapping method is demonstrated by the analysis of a fully filled transporting section of the corotating twin screw extruder. Concentration and residence time distributions can be computed straightforwardly and the volume average intensity of segregation is chosen as a characteristic mixing measure to quantitatively describe distributive mixing.

There is a study on the comparison of twin screw extruders and a batch mixer in the polymerization of caproiactam. A sodium metal initiator and 4–4'-diphenylmethane diisocyanate (MDI) as the activator were used. Our attention is given to (i) a modular intermeshing co-rotating twin screw extruder, (ii) a modular intermeshing counter-rotating twin screw extruder and (Hi) a modular tangential counter-rotating twin screw extruder. We consider the variables of screw configuration, temperature profile, screw speed, and throughput on polymer characteristics. It was found that the conversion of twin screw extruders with proper mixing segments is much higher than the internal mixer at low mixing times. This was seen as higher conversion in twin screw extruders than an internal mixer for the same mean residence time considered.

The effect of dichlorocarbene modified styrene butadiene rubber (DCSBR) as a compatibilizer in blends of acrylonitrile butadiene rubber and styrene butadiene rubber (SBR/NBR) has been studied. The cure characteristics, physical properties, crosslink density, FTIR studies and low temperature transitions of the blends were determined as a function of chlorine content of DCSBR. Compatibilizing efficiency depended on the chlorine content of DCSBR and composition of blend constituents. Effective compatibilization was achieved when chlorine content of compatibilizer was 25% and SBR content of blend was either 50% or lower. FTIR studies, glass transition behavior measured by DSC and DMA showed that an appreciable extent of molecular level miscibility has been achieved in SBR/NBR blends by using DCSBR as a compatibilizer. Improvement in mechanical properties such as tensile strength, tear strength, resilience, hardness and compression set were achieved both when DCSBR was added and chlorine content of DCSBR increased up to 25%. The resistance of the vulcanizate towards air and oil aging improved with compatibilization. The change in technological properties correlated with crosslink density of the blends assessed from swelling studies and stress-strain data.

This paper is devoted to the numerical study of the stability of the plane flow for two immiscible fluids obeying Maxwell constitutive equation in a channel of aspect ratio 8. For fixed viscosity and flow rate ratios, the shape and non-linear behavior of interfacial waves induced by small perturbations of the input flow rate are studied using finite element simulations for various elasticity stratifications. The stationary flow, close to a Poiseuille flow and presenting a flat interface is first computed. Then the interfacial wave generated by a pulse-like disturbance on entry flow rates is computed by using time dependent computations. It takes the shape of a wave packet which is amplified or damped while moving towards the die exit. In unstable cases, the maximum deviation increases with elasticity (the Weissenberg number). Interestingly, the elastic stratification between the two layers can counteract unstable waves due to viscous stratification.

Non-isothermal calculation of flow history dependent visco-elastic stresses in a coextrusion die is performed using the 8-mode modified Leonov constitutive equation and the deformation rate field from finite element simulations. It is shown that a heuristic criterion based on the difference of normal stress differences across the layer interfaces in the merging area may be used to potentially detect the onset of interfacial instabilities. The developed technique can be a useful tool for coextrusion die design as well as for proper selection of materials and process conditions for coextrusion.

Tighter tolerances are becoming more prevalent as thermoforming applications are increasingly utilized in multiple component assemblies with load bearing and other functional capabilities. A 1 analytical method is described for shrinkage predictions utilizing a thermoviscoelastic constitutive model to try and avoid costly and time consuming forming trials by determine adequacy of the mold and process a-priori. Validation experiments and sensitivity analyses were performed to understand the process behavior and estimate the accuracy of the shrinkage analysis. Design and manufacturing guidelines were then developed to facilitate tight tolerance thermoforming. While the analysis was generally capable of predicting linear shrinkage, statistical yield models indicated that improved process control and further process development is required to consistently produce thermoformed parts to tight tolerances.