Band 10, Heft 4

On modern extrusion plants, polyolefins are generally processed using barrels that have a grooved feed section. Accurate calculation of the melt throughput is of decisive importance for the process engineering layout of these extruders. Up to the point at which the limit speed that gives rise to the conversion from solid friction to melt film friction is attained, the conveying capacity is determined by the feed zone that is filled with granules. This paper sets out a method of calculating the throughput for speeds below this limit, paying consideration to the melt flow components in both the screw channel and the grooves. The calculation method was verified experimentally with the aid of throughput measurements for different polyolefins. The results generally fall within a tolerance range of +/−10 %.

A model for representing polymer devolatilization behavior in a screw extruder with multi-vent sections has been proposed. In this model, an overall film-interface geometry factor (OFIGF) comprised of several local film-interface geometry factors (LFIGF) is used as a measure for the devolatilizing performance of a screw extruder. Moreover, the OFIGF is related to screw geometries and open angles of open areas in vent sections, and dependent on the degree of filling or the flowrate of the fluid and the screw speed. An intermeshing co-rotation twin-screw extruder popularly used was taken as an example to discuss the effect of the OFIGF on its devolatilizing performance. For this purpose, a solution of how to calculate the local film-interface areas and their exposure time in a venthole unfitted zone (VUfZ) and a venthole fitted zone (VFZ) with an open area was presented. Furthermore, several interesting results about how the OFIGF is affected by flowrate, screw speed, lead, and number of tips were predicted and discussed. The results suggested that the devolatilizing performance of the VFZ is lower than that of the VUfZ due to decrease in the surface renewal effect there, and a plot of ln {(w out – w*)/(w in – w*)} against (N 1/2 /Q) in generally used flowrate and screw speed ranges for a given screw geometry does not strictly satisfy a linear relationship due to the filling-degree dependence of the OFIGF.

An experimental and theoretical study of the behavior of a List/Buss Kokneter is presented. The experimental study which uses polypropylene considers (i) the interaction of the Kokneter with a crosshead screw extruder, and (ii) distribution of polymer in the modular screw. A modeling study considers the dynamics of flow including the influence of screw oscillation. Calculations are made for various individual Buss elements. A model is developed for the total system including the crosshead screw extruder. It is shown that an oscillating length of fill in the crosshead screw extruder can account for the throughput oscillation stabilization characteristics of the composite machine. Predictions of pressure profiles and positions of starvation in a starved flow machine are made.

Counterrotating twin screw extruders are widely used for compounding, devolatilization, blending, and reactive extrusion. A fluid dynamics analysis package-FIDAP, using the finite element method, was implemented to simulate the 3-D, isothermal flow patterns in intermeshing and tangential counterrotating twin screw extruders. The rheology of the fluid was described by a power-law model. We developed a framework to evaluate the distributive mixing efficiency. The dynamics of distributive mixing were studied numerically by means of tracking the evolution of particles originally gathered as clusters. The extent or goodness of mixing was characterized in terms of length stretch, pairwise correlation function and volume fraction of islands. The length stretch reflects the capability of the mixer to spread particles away from their neighbors initially present in the same cluster. The pairwise correlation function characterizes the global distribution of particles in the mixing region. The volume fraction of islands quantifies the regions void of minor component and provides the local analysis of mixing. This framework can then be used to differentiate among various operating conditions or various designs of the same type of equipment.

Capillary rheometers have been installed on-line very successfully for real time control of the quality of polymers in production and processing. A substantial extent of the real time information about the state of material and process is reached using an on-line rheometer with a wedge-shaped slit working under pressure controlled conditions. True viscosity, flow exponent and melt flow index can be measured simultaneously and changes in average molecular weight and molecular weight distributions are indicated immediately.

A new type microporous polypropylene film was prepared simply by biaxial drawing of β-crystalline phase polypropylene The effect of draw ratio on the pore size distribution, ethanol sorption, and porosity, as well as β-α transformation during drawing were studied For the purpose of improving its hydrophilicity ion and solute permeability, the plasma graft-polymerization technique was utilized The resulting film showed much higher permeability toward Na + , Mg ++ and urea, being 3.2 × 10 −6 , 5.8 × 10 −8 and 1.75 × 10 −6 cm 2 /min, respectively, than the original one and had better dimensional stability.

Bowing distortion phenomena of bi-axially oriented film cause serious problems in practical application. Bowing distortion was conveniently defined to carry out the analysis. Bowing distortion profile throughout a tenter machine was observed. This profile was fairly successfully simulated by FEM using suitable anisotropic and heterogeneous elastic constant distributions, and a suitable initial shape of the elastic body. The mechanism of bowing distortion in relation with TD stretching was estimated by the above results and some experimental data. This leads to the forecast that the insertion of a cooling zone between the TD stretching zone and the thermosetting zone will be an effective way for decreasing bowing distortion of film. This forecast was experimentally verified. Despite the insertion of the cooling zone, a certain level of bowing distortion still remained. It was tried to give an explanation for the mechanism of the remaining distortion in terms of heat-shrinkage force in the thermosetting zone.

Circular disks (280 mm diameter, 2 mm thickness) were injection molded from isotactic polypropylene under systematic variation of processing parameters (melt temperature, mold temperature, screw velocity, packing pressure, and packing time). Sections were taken from the disks perpendicular to their surface, at 40 mm distance from gate, in different orientations, and were investigated by means of optical microscopy using polarized light and by spatially resolved wide-angle X-ray scattering. The results from X-ray scans of the cross-sections are presented in terms of several parameters characterizing the state of orientation of α-PP crystallites and the relative amount of β-PP crystallites. The profiles of the various X-ray parameters versus the distance from surface deliver a detailed picture of the layered structure of the cross-sections which is supported and supplemented by micrographs taken with polarized light. The comparison of the results obtained for disks molded under different conditions unveils clear correlations between processing parameters and morphological characteristics of the disks.

The great impact of the injection molding cooling process on both productivity and product quality makes it essential to optimize the cooling conditions as well as the cooling system design. The current practice in injection molding process development, to identify the final operating conditions, which is a time consuming iteration between model design and product line, is costly in both material and time. This defect in the process development also raises the necessity to establish a method, which can provide the manufacturer not only with an optimum but also with a window around the optimum for the operating conditions, so as to avoid this time with consuming process to determine the feasible operating conditions. This paper presents a methodology, which combines an optimization algorithm with a mold cooling analysis, to obtain an optimal cooling processing condition and a cooling window in the injection molding of plastics as an aid for mold designing and manufacturing. The cooling effect, which is characterized through uniform cooling and short cooling time, is quantified in terms of mold cooling simulation output. A mathematical model is then formulated based on semi-empirical criteria and solved numerically by implementing an optimization algorithm. Based upon the resultant final optimum cooling condition, a relaxation scheme scheme is developed by the authors and performed, followed by a sensitivity analysis to determine the cooling window. At last, the application of this method is demonstrated by two industrial molds: a drilling housing and a compact drill battery pack. This approach will help in designing a proper cooling system for a mold and in determining an operating range to achieve better part quality and higher productivity.

A new flow model is developed for the analysis of compression molding of sheet molding compounds (SMC) and a penalty finite element formulation is presented to predict flow front progressions more accurately. In this model SMC is assumed nonisothermal fluid, which has different viscosities in extension and in shear. The flow is allowed to slip at the mold and is resisted by friction force which is proportional to the relative velocity at the mold surface. For the verification of the model, the press force and flow patterns are compared with those of experiments and available results by other works in this field. It is also demonstrated, using the computational procedure described and the proposed model, that optimal initial charge shapes for the filling can be effectively computed.

This work presents four different numerical approaches to analyze heat transfer in the mold halves. The formulations, numerical methods and simulated results of each method are presented and compared to the experimental results based on a metal (aluminum) and a non-metal (reinforced epoxy) molds. Several examples are also presented to show the importance of tool heat transfer analysis in reactive processing.