Volume 17, Issue 4

A single-screw-extruder is presented, consisting of a barrel with grooved feed zone (helical grooves), a grooved melting zone (axial or helical grooves) and a barrier-screw with mixing and shearing elements. Compared with extruders with a smooth melting zone, significant process improvements are achieved regarding mass throughput, pressure build up, melt temperature, melt homogeneity, feed-zone cooling, processing wall-slipping polymers, abrasive wear, residence time, total degree of efficiency and economy. The processing principles of the new extruder are explained. Experimental results confirm the theoretical predictions. The system has proven to be very worthwile in more than 100 extrusion blow molding machines since 3 years.

The flow of a silicone oil of high viscosity along a block of staggered kneading discs of a co-rotating twin screw extruder was investigated with the use of an acrylic transparent barrel. A digital video camera recorded the flow path of a red dye, dispersed in the silicone oil, along the screws and in the intermeshing zone. Different geometrical configurations were studied, namely various staggering angles (+30°, 90° and −30 °) and thicknesses (7.5, 15.0 and 22.5 mm) of kneading blocks. The effects of these geometrical parameters on the pressure generation and filling ratio were investigated. It was observed that, even for negative staggering, the filling ratio of the kneading block was lower than one. Based upon these experiments, analytical expressions describing the filling ratio as function of geometry (staggering angle, thickness and number of discs) and operating conditions (feed rate and screw speed) were derived.

In this paper, we discussed how unmixed and well-mixed zones are determined by the practical design parameters of the Chaos Screw (CS) system for the single-screw extruder, e. g. the helix angle, the magnitude of adverse pressure gradient, and the fraction of no-barrier zone. The key concept is the frequency ratio distribution of the unperturbed system. Through the extensive three-dimensional numerical simulations with finite element methods and particle tracing techniques, successive creation and breakup of resonant orbits and KAM tori in terms of design parameters are presented. The results show that one can predict the spatial configuration of well-mixed and unmixed zones from the frequency ratio distribution and the amount of the perturbation strength. Finally, the exit time distributions caused by dynamical structures are discussed.

The development of heterophasic PP-copolymers, which are necessary in many applications because of their specific property profile, requires large capacities in pilot plants (or at least in bench-scale reactors) for a systematic variation of the matrix and elastomer properties. A possible alternative are model compounds, where the different components are blended in an extruder. The possibility to evaluate three main influence factors on mechanics of such systems, namely the viscosity ratio between matrix and elastomer phase, the phase compatibility and the composition of the elastomer phase, is demonstrated in a series of investigations. Morphology investigations based on transmission electron micrographs have proven to be an indispensable tool for obtaining systematic information about such blend systems, which can be further used for the development of reactor-based systems.

A Computational Fluid Dynamics analysis of the flow through the distributionplate of a fibre spinning pack has been performed. The features of the flow that are relevant to polymer processing are addressed, such as the wall shear rate, residence time and dead zones. An analyticalmodel, that agrees very well with the calculations, is proposed. It also yields insight into the role played in the flow by different design parameters of the distribution plate. This model, primarily based upon a Newtonian fluid, is also extended to a shear thinning one.

In this study, various effects on the properties of the material, such as free-radical (FR) processes, oxygen induction time (OIT), melting temperature and melting heat are examined during several stages of the polyethylene (PE) pipe extrusion process. The spectra in the Electron Spin Resonance (ESR) are researched between 77–293 K and in a wide range of magnetic fields. At 77 K a triplet signal in ESR is observed. The parameters of the signal are ΔH m1 ≅ 1:4 – 1:6 mT; ΔH m2 ≅ 1 – 1:2 mT; ΔH m3 ≅ 0:6 – 0:8 mT and g 1 ≅ 2:020; g 2 ≅ 2:011; g 3 ≅ 1:99 and Hyper Fine Structure constant ΔH 12 ≅ 2:1 mT; ΔH 23 ≅ 5:3 mT. As the temperature increase from 77 K up to 293 K, the signal for g 4 ≅ 2:02 (ΔH m 4 ≅ 0:7 – 0:8 mT) appears and intensity of signals increase. It is also thought that the triplet observed at 77 K belongs to macroradicals of alkyl (–CH 2 –CH–CH 2 ), alyl(~CH 2 C HCH = CHCH 2 ~), carboanions and the acyl (H C O) radicals of the fourth signal. At room temperature, all the FR's transform into peroxide radicals. The total concentration of the FR's is minimum at 463 K and increases at the outlet of the extruder and the pipe sample. It is observed that the OIT (Oxygen Induction Time) values decrease as one approaches the outlet of the extruder (from 20–30 to 10–15 minutes), while they increase for the processes of 433 K to 463 K. This event proves that the peroxide radicals form as one passes to room temperature. These results support those of the effects of the extrusion stage on the dynamic-mechanical characteristics [2]. Finally, FR's appear as a result of the breaking of the C-C bonds in the main polymer chains starting from the first step in the extrusion process (433 K) and in the later stages, due to the oxygen in the polymer structure, oxidation processes take place.

Calender units with rotating rolls are used to produce foils or sheets from polymer material with viscous or elastomeric flow properties. Up to now, the design process of calender units, the calculation of flow and pressure fields in the calender gap and the calculation of resulting deformations of the rotating rolls are implemented with different computer programs. In this work a design concept that integrates design and calculation programs is being developed in order to achieve integrated design and calculation. With this concept, it is possible to design calenders and calculate flow fields and deformations. After designing the rolls with the Computer Aided Design program Pro/Engineer, the flow field in the calender gap is calculated with the Finite Volume Method program CFX4, and the deformation of the rolls is calculated with the Finite Element Method program ANSYS. The iterative process of design, calculation and modification of shape is continued until sheets or foils of constant thickness leave the calender gap. The design concept has an object-oriented operational structure. The data transfer between the programs takes place via a central data base in STEP format. As examples, calenders with combinations of solid rolls are designed. The flow field in the calender gap is calculated for Newtonian and for viscoelastic fluids with use of an integral rheological equation of state. The flow field is compared to a measured flow field. The resulting deformations of the rolls are calculated and represented graphically. Furthermore, a design process for elastomer-coated rolls is implemented and their deformations as well as the interior stress fields are calculated.

This paper describes an original on-line video device developed in order to study bubble instabilities occurring in the film blowing process, taking into account their three-dimensional behavior. For a linear low-density polyethylene, two forms of instabilities and combination have been observed: draw resonance and helical instability. These instabilities could be quantitatively described and differences in behavior could be assessed using real objective measurements and criteria. The influence of key processing conditions was investigated and the results showed that the instabilities are enhanced by increasing the draw ratio, blow up ratio and frost line height. These first results are in agreement with the majority of the results reported in the literature, but allow for a more accurate analysis of the phenomena.

Micro-injection molding has been under development for the mass manufacture of micro parts. This paper presents fundamental research on the filling characteristics during micro-injection molding. Flow visualization was employed to observe the fast filling process. A lab-made impact type micro-injection machine was developed and used in this study. Filling of 4 × 4 × 0.2 mm 3 and 2 × 2 × 1 mm 3 cavities in quartz glass molds was recorded with a high-speed video camera. The effects of the processing conditions on the filling behaviors are systematically studied. The processing parameters include air pressure, mold temperature, and melt temperature. Based on the analysis of flow patterns, it was found that the melt fronts advanced quickly and filled the cavity in less than 0.1 seconds. The filling process can be divided into impact-filling and pressure-filling stages. Mold temperature and air pressure are found to be the dominant processing parameters.

Plug-assisted thermoforming produces a wide range of polymer products through a combination of deformation by air pressure and contact with tool surfaces. In this paper the role of tool/sheet contact in determining the process output is investigated. A combination of thermoforming, friction and heat transfer tests were carried out on common tool and sheet materials. The results show that the typical friction coefficients for the material combinations are within the range 0.1 to 0.3, but the values rise sharply on approaching thermoforming temperatures. Thermal imaging tests demonstrate that all of the plug materials significantly cool the heated sheet on contact, even over very short periods of time. The temperature of the plug is very important. At low plug temperatures heat transfer effects predominate, whereas at high plug temperatures friction effects predominate. A plug temperature of approximately 100°C balances these effects and creates the most effective material distribution.

The elongational viscosities of a low density polyethylene in axisymmetric and planar flows are compared. The experimental data on entrance pressure loss is matched with the corresponding finite element predictions to estimate the parameters in the elongational viscosity model proposed by Sarkar and Gupta. The entrance losses in the capillary and slit rheometers are used to predict the elongational viscosities for axisymmetric and planar flows respectively. The power-law region of the axisymmetric as well as planar elongational viscosity is found to follow the time-temperature superposition principle. The vortices encountered in the flow near the abrupt contraction for the planar and axisymmetric cases are compared. The vortex near the abrupt contraction is found to reduce in size as the flow rate is increased for planar as well as axisymmetric entrance flow.

Much attention has been recently focussed on the melt impregnation of thermoplastic polymers as a consequence of the difficulty involved in driving viscous polymer melt into a tightly packed fiber bundle. This work studies an impregnation process consisting a series of staggered cylindrical pins in a melt pool. A numerical model for evaluating the pressure generation in the wedge region between the roving/pin, the impregnation velocity of the melt through the glass mesh, and the resulting impregnation depth along the wedge is developed using relationships for pressure build-up in foil bearings and Darcy's law. The model results are compared with experimental melt impregnation data. The model adequately predicts the effect of melt impregnation process conditions on impregnation quality.

Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) data is presented and used to characterize the dynamic behaviour of the flow field in a RIM machine, and the main flow patterns and mixing mechanisms are examined and established. A two-dimensional CFD model is used for the simulation of mass transfer and chemical reaction in the mixing chamber of a RIM machine. Some design and operational parameters are considered and its influence on the mixing behaviour of a RIM machine is analysed.