Volume 12, Issue 2

The dominant distributive mixing mechanism in polymer processing is convection. Convection, involving movement of fluid elements from one spatial location in the system to another, results in the creation of interfacial area. Length stretch distributions can be used as a measure of distributive mixing efficiency. We have studied the dynamics of distributive mixing in single and tangential twin screw extruders by means of tracking the evolution of particles originally gathered as clusters. Length stretch distributions and the dynamics of the average length stretch indicate that the tangential twin screw extruder is a better mixing device by comparison with the single screw extruder. The chaotic features of flow in these devices, studied by means of Poincaré sections and Lyapunov exponents, correlate well with the distributive mixing efficiency of the equipment.

The Variable Intermeshing Clearance (VIC) mixer possesses the unique feature of the ability to change the inter-rotor clearance during the compounding process. To improve the mixing performance of the VIC mixer new designs are proposed. A fluid dynamics analysis package-FIDAP, using the finite element method was employed to simulate the flow behavior in the VIC mixer. The problem of time dependent flow boundaries was solved by selecting a number of sequential geometries with 20 degree increments to represent a complete mixing cycle. Dispersive mixing was evaluated in terms of both the shear stress distribution and the elongational flow components generated in the flow field. Distributive mixing was studied numerically by means of tracking the evolution of particles originally gathered as clusters. Our studies indicate that the new design of an enlarged chamber gives overall better dispersive and distributive mixing performance than the traditional VIC. The VIC mixer with both an enlarged chamber and wider rotor blades design shows poorer dispersive mixing but better distributive mixing capability. The effect of inter-rotor clearance on mixing efficiency is also discussed.

Non-isothermal flow is simulated in a modular Buss Kneader including crosshead screw extruder. A method of calculating cycle averaged mean temperature rises for individual elements is discussed and results are presented. A method is then described for making calculations in a modular multi element Buss Kneader. Finally we compared the calculations with experiment.

Blends made of one linear low density polyethylene (LLDPE) and either of two low density polyethylenes (LDPE 1 and LDPE 2) were tested in elongational modes of deformation using elongational viscosity and melt strength test. In addition to rheological measurements the melt miscibility of the blends was ascertained by the differential scanning calorimetry (DSC) test on melt quenched samples. From the results, the relation between melt morphology of the blends and their performance under elongation, as well as the influence of molecular structure in particular long chain branching, on elongational response were determined. It was observed that high resistance to elongational deformation of the blends is linked to the immiscibility of the blend components in the melt. On the other hand the influence of long chain branching is to increase elongational viscosity for the parent polymers, as well as promoting the blend immiscibility and therefore altering elongational performance of the blends.

Glass fiber-reinforced polyamide-6 (GFPA-6) was compounded using a HYPERKTX46 twin screw extruder with three different types of screw segments (rotor segments, kneading disc segments and turbine segments). Based on the ‘neural network’ regression analysis, we investigated the effects of compounding conditions, such as screw configurations, screw rotation speed, and production rate on mechanical properties, such as tensile, flexural and impact strength of GFPA-6. It was found that the mechanical properties of GFPA-6 changed in different ways with compounding conditions (screw rotation speed and production rate), depending on the type of screw segments. It was also found that the rotor segment exhibited good mechanical properties in wider ranges of compounding conditions than the kneading disc segment and the turbine segment, probably due to its mild and uniform mixing capability.

The traditional method of mathematical modeling of the die flows treats the die as a stand alone tool which is independent of the extrusion conditions which feed the polymer melt into the die. Here, we demonstrate that especially under the conditions where a breaker plate is not used the flow and the deformation which occur in the die are dependent on the prevailing velocity and the stress conditions of the fluid at the extruder which feeds the die. A finite element method based technique is used to solve the conservation equations using a mesh which covers both the die and the rotating conical screw tips of the extruder. The presented methodologies provide a more realistic representation of the thermo-mechanical history experienced by the polymer melt or structured fluids including emulsions or suspensions in the die and thus provide better tools for die design and process optimization. The results also suggest that significant improvements in die design and hence die performance could ensue by using the geometry of the screw tips, the distance between the screw tips and the die and the rotational screw speed as additional parameters for design and optimization.

This work covers a new model to compute flat dies for the production of biaxially oriented polypropylene film. The computational model was developed in the late eighties and is particularly suited for dies of high throughput and width. In the development of new dies for our production lines, it was realised that all models known at the time we began this work had failed. These models only accounted for viscous flow in the die channels under ideal conditions. Temperature differences in the melt and the elastic behaviour of the die walls were not accounted for in those models. The new model allowed us to design dies for high throughput with no difficulties. With the model we were now able to provide our production with dies of optimal design. Before the model is explained in detail, the production process for biaxially oriented polypropylene film is sketched and the market demand for such films is explained.

Blown film properties depend on the thermo-mechanical history experienced by molten polymer during biaxial deformation. In this study on-line birefringence measurements along the length of the bubble in film blowing of a linear low density polyethylene (LLDPE) were carried out in order to assess the stress level in the melt zone and total orientation in the solid zone. Bubble temperature measurements were carried out to find out the onset and the end of crystallization. Strain rates were also determined from bubble diameter and axial velocity measurements. We have focused on the effects of key processing parameters on the thermo-mechanical history of polymers. The relations between the birefringence and temperature profiles are described. The birefringence value is shown to be very small in the molten zone and increases rapidly as crystallization proceeds. The birefringence of the solidified film is strongly dominated by the crystalline phase contribution. Stresses in the molten blown film were calculated using the data of birefringence and pressure inside the bubble. The birefringence technique appears to be a promising but limited tool to determine stresses occurring in film blowing.

In this contribution the various rupture mechanisms which can occur during the elongation of polymer melts will be briefly reviewed, with the focus on the cohesive brittle melt rupture. Observations of melt rupture phenomena during polymer processing operations will be discussed. The occurrence of brittle melt rupture is characterised by a constant value of the rupture stress in the order of 1 MPa. Published rupture stress data on various polymer melts are critically discussed, in particular those on polyethylenes and other polyolefins. Using the Rheotens technique a newly developed method will be described to measure the melt rupture stress of polypropylene (PP) melts. The measurement of the rupture stress of PP melts is hampered by the occurrence of the instability draw resonance. This problem was overcome by applying more pronounced cooling conditions. Initial data on the rupture stress of PP melts will be presented. It is shown that the rupture stress of PP melts is not a constant but increases with increasing molecular weight.

The injection mold filling behavior of a blend of low density polyethylene (LDPE) and barium sulphate (BaSO 4 ) was studied experimentally and compared with that of pure LDPE. The method of short shots was used to analyze the mold filling behavior. An experimental study using a full factorial design was implemented with the following factors: mold temperature, injection speed and injection time. Experimental results and computer simulation predictions of the mold filling stage were found to be in acceptable agreement. The produced short shots with the LDPE/BaSO 4 blend have significantly lower percentages of volume filled than those of the corresponding short shots with the pure LDPE. The increase in the melt viscosity and density of the polymer resin with the addition of BaSO 4 cannot be neglected when producing injection molded articles with LDPE/BaSO 4 blends. To study the deviations between the experimental results and the computer simulation predictions, a factorial numerical study was completed. The results show that the simulation predictions are significantly affected by the accuracy of the polymer material properties.

The objectives of this work are to assess the rotomoldability of rubber toughened polypropylene resins and to investigate how resin properties can influence the process of rotational molding and the final part quality. Four commercially available polypropylene/polyethylene copolymers were tested. The chemical composition and the powder, rheological and thermal properties of the resins were determined. Rotational molding experiments were carried out in a laboratory scale uniaxial machine which is capable of measuring internal mold temperature and which facilitates the visual observation of various transitions in the cycle. Impact and environmental stress cracking resistance tests, as well as thickness distribution and density measurements were performed on the roto-molded parts. It was found that increasing rubber content results in an improvement of the impact properties which is counteracted by difficulties in processing, presence of bubbles and uneven surfaces.

The effects of injection-molding parameters on the mechanical-physical properties of a liquid silicone rubber (LSR) injection-molded ring were studied through an experimental design. The effects of the changeover point (volume or pressure driven), runner (hot or cold), hold pressure, mold temperature and injection flow rate variations were evaluated based upon the responses related to both material properties (ultimate elongation and tensile stress in a tensile test, swelling in hexane, hardness, density) and ring properties (cross sectional diameter, weight, volume). It was determined that ring density is not dependent on the parameters studied. The mold temperature appeared to be a very influential parameter, especially concerning material characteristics: it affects the swelling ratio, hardness, and ultimate properties, but weight and volume are only affected when the cold runner is used. The hold pressure has an effect on both the weight and volume of the ring. The type of changeover used (volume or pressure) has no effect when a cold runner is used, but affects the weight, volume, and ultimate properties of the ring, when a hot runner is used. The injection flow rate did not affect the observed characteristics.

This paper describes a fully three-dimensional transient finite element method for calculating the flow behavior and fiber orientation during filling of injection molded parts. The fiber-fiber interaction is taken into account. The momentum and continuity equations are first solved with the viscoelastic stress treated as a fixed body force. The kinetic equation for the orientation tensor is then integrated with known kinematics using the standard Galerkin method. The calculation is performed on a time-dependent flow domain. Since the method is truly three-dimensional, singular regions such as the flow front or near injection gates and solid boundaries, where decoupled approximations are not valid, are naturally dealt with. The material anisotropy behavior is modeled by using the Doi-Doraiswamy-Metzner model. Numerical results, involving the Poiseuille flow and the filling of an end-gated plate, emphasizing the importance of the three-dimensional coupling calculations between the flow and orientation are presented.

Circular disks (280 mm diameter, 2 mm thickness) were injection molded from two grades of isotactic polypropylene under systematic variation of processing parameters (melt temperature, mold temperature, flow rate, packing pressure, and packing time) according to a two-level fractional factorial design. The disks showed ring-shaped zones (“halos”) of enhanced turbidity. The wide-angle X-ray scattering of the disks was measured as a function of the distance from the gate at 13 positions along the radius, using a diffractometer and reflection geometry. These measurements yielded radial profiles of the orientation of α-PP and of the distribution of β-PP crystallites. Both kinds of profiles revealed clear correlations with the polymer grade and with the melt and/or mold temperature. Radial profiles of optical turbidity of the disks were obtained by means of a microdensitometer. Pronounced maxima of turbidity were found to coincide with the maxima observed in the radial distribution of β-PP. An analysis of variance (ANOVA) of the experiments showed that the mass temperature has the greatest influence on the radial distances of the maxima of turbidity. Increasing mass temperature led to decreasing radial distances of the turbidity maxima, whereas the mold temperature as well as the flow rate only had a small influence on the radial positions of the turbidity maxima. The investigation of cross-sections, which were taken from some disks at selected distances from gate, by means of optical microscopy using polarized light revealed significant changes in the morphology with increasing distance from gate, e.g., changes in the size and distribution of β-PP spherulites.