Band 6, Heft 2

This paper deals with the finite element modelling of non-isothermal flow of a power-law fluid past an internal mixer rotor. The model uses a set of boundary constraints which correspond to stressfree surfaces at the inlet and the outlet of the domain. At the walls of the domain the conditions of no-slip, controiled-slip and perfect-slip are alternatively incorporated in the simulations. A stream-line upwinding technique used in the solution scheme enables it to yield convergent results for high Peclet No. flow, even if relatively coarse meshes are used. Comparison of these results with the available experimental evidence confirm the general validity of the methods used.

Experiments aimed at detecting agglomerate dispersion were performed in a rotating concentric cylinder device mounted in a light scattering monophotometer. The dispersion of carbon black agglomerates suspended in various media (e.g. water, squalene, poly (dimethyl siloxane)) was inferred by measuring changes in turbidity under the application of a controlled simple shear field. In a given suspension the turbidity was observed to be constant for shear rates less than a threshold value. At shear rates in excess of this value, the turbidity was observed to increase monotonically in time to a steady state value. The effects of particle size and fluid viscosity on the critical shear rate necessary to induce agglomerate breakage were examined. Also, the change in the nature of the agglomerate size distribution upon shearing was studied. The results obtained indicate that the breakup of highly structured agglomerated solids such as carbon black does not proceed by a halving process. Also, the initial breakup process was observed to be irreversible.

In previous work we have applied hydrodynamic lubrication theory to simulate 3-dimensional flow of a polymer melt or elastomer in an internal mixer. That study presumed isothermal Newtonian flow in a flattened out mixing chamber. In the present paper, we take into consideration both non-Newtonian flow properties of the melt/elastomers and increase in temperature due to viscous dissipation heating. We consider the influence of non-Newtonian viscosity on flow patterns in the internal mixer. Viscous dissipation heating in the internal mixer is also modelled. It is shown that large scale internal mixers tend to operate adiabatically while small scale machines are close to isothermal. Temperature-time profiles are predicted for laboratory and large scale internal mixers.

Internal mixers are commonly used for rubber compounding in industry. However, the relations among the designs of equipments, mixing conditions and the properties of the resulting compound are little known. In this paper we tried to reveal some manners of the relations concentrating, particularly, on rotor designs. A specially designed internal mixer (Banbury type) was developed. A number of double winged rotors of various shapes and replaceable mixing chambers were prepared. In developing the equipment, eight parameters were taken into consideration concerning the shape of rotors, that is, lengths, twist angles and overlap ratio of the wings, width and clearance at the rotor tip, angles of flow-in and flow-out at the gap area and total bulkiness of the rotor. Measured properties concerning mixing are: unit work, Mooney viscosity of dumped compound, the amount of bound rubber, weight average molecular weight of the extracted portion of the compound, and peaks in a torque curve, i.e., values of first peak torque and second peak time. In order to estimate the degree of influences of the design parameters mentioned above and mixing conditions, each measured properties were expressed as functions of these parameters by means of multiple regression analysis.

Dynamical modelling techniques are presented that facilitate the derivation of transfer function models between key inputs and outputs of internal mixing processes. Tests to model the mastication of natural rubber in a Francis Shaw Intermix at two levels of fill-factor are described. The derived models will form the basis for closed-loop control strategies.

Barrier screws used in conjunction with grooved-barrel extruders are gaining increasing importance on account of their high throughput and melting performance as well as their good homogenizing action. Although a number of calculation formulae now exist for the barrier screw, no comprehensive, logical design system for power law melts has been compiled to date. This paper takes a simple melting model and shows how the geometry of the barrier section can be designed (flight depth and screw channel width of the channels for the solid and the molten material and the length of the barrier section). A method of calculating the pressure and temperature profile in the melt channel is also presented. The different variables affecting the melting process are discussed on the basis of parameter studies.

In this paper, an analysis for a whole section of the List KoKneader is presented. The analysis is restricted to the isothermal flow of a Newtonian fluid and uses a modified Flow Analysis Network method of simulation. This paper continues our efforts to analyze the flow of polymer melts in screw extruders containing pins.

Staggered arrays of kneading paddles in co-rotating intermeshing twin screw extruders have drag and pressure characteristics similar to those in conventional screws. The pressure flow term was determined by measuring pressure drop as a function of flow rate in a non-rotating configuration. The drag flow per revolution was estimated as half of the product of lead length and open barrel cross section. The preliminary determinations were subsequently confirmed in a flooded operating extruder.

The extent of feeder error to survive a plasticating compounding extruder is addressed in this theoretical and experimental study. Results of unit step and cyclic variation of tracer concentration were used to define the transfer function and damping ratio respectively. The frequency dependent damping ratio of cyclically varying tracer content in the feedstream was predicted for three screw configurations with fair agreement to experiment.

A theory of distributive mixing in a modular intermeshing corotating twin screw extruder is described. This is based on the Spencer – Wiley premise of representing mixing through interfacial area increase and its relationship to shear strain. The increase of interfacial area in both screw elements and kneading disc elements is analyzed. The flow in screw elements has a figure eight configuration but basically is analogous to flow in a single screw extruder. The primary mixing mode in these machines is within the kneading disc elements. A model for increase of interfacial area, which includes backflows through kneading disc tips, is developed. The kneading disc and screw element models are combined to compute interfacial area development in modular twin screw machine. We consider that modular twin screw extruders often have regions of starvation. To apply this model it is necessary to know flow configurations such as may be determined by a composite model of the total flow such as has been developed by the authors and their coworkers in earlier papers.

In conventional processing, extruders have been used, mainly, for continuous compounding and processing of polymers. Yet, for a long time they have been serving as chemical reactors. It has been 50 years since first developments were made in the field of extruder application as a reactor. In the following, we will offer the reader a review of research and development activities of the Institut für Kunststoffverarbeitung (IKV) in the field of reactive extrusion.

Twin Screw extruders are operated typically in the starve-fed mode, with regions of channels partially-filled with polymeric materials. Although much has been done to understand and characterize the process of devolitalization and polymerization, many of the models have been based on simple mixers like batch mixers or continuous stirred reactors, not on the complex flow patterns found in twin screw extruders. Accurate models of the entire process requires appropriate modelling of the nature of mixing in a partially-filled channel. This paper presents results of a visual study of the counterrotating, non-intermeshing twin screw extruder operated with partially-filled channels. The experiments characterize the nature of mixing and the effect due to screw flight stagger and fluid viscosity.