Volume 19, Issue 1

Tightly intermeshing, co-rotating twin-screw extruders are commonly employed for tasks requiring good mixing. The modular constitution of both barrel and screw makes it possible to optimise the extruder configuration for a given task. Even today this optimisation is frequently done by applying the “Trial and Error”-method. Physico-mathematical models enable the process engineer to predict the process behaviour of a chosen extruder configuration and to optimise existing extrusion processes. Increasing demands in mixing quality and efficiency of the processing unit result in efforts to optimise existing extruders with respect to those factors. For this reason knowledge concerning the temperature profile along the screw and the power consumption is essential. We present a new physico-mathematical model for the description of the temperature development. At stages where the analytical solution itself would result in a not satisfying degree of accuracy we used descriptions based on finite element simulation results to achieve the desired exactness. As a result we got a model to describe the temperature development in the screw channel and a model to describe the power consumption in different screw sections. Applying these models it is possible to optimise the process parameters and the screw configuration with only a minimum of preceding experimental investigations.

Prediction of residence time distribution (RTD) in the modular type screw extruders has always been one of the most attractive subjects for predicting. In the present work an attempt was made to establish a method with functionality similar to additive rule for predicting RTD of the modular screw extruders with different configurations such as Buss Ko-Kneader. Results predicted from this method, which was named as cluster model, were compared with the results predicted based on our previously reported trajectory method as well as experimental results. A reasonability good agreement was found between these results justified reliability of the cluster model for being used as a new approach to predict the RTD of the modular screw extruders.

A dielectric sensor, mounted in an extrusion die, was used to measure the dielectric properties of PVC melt during extrusion process. The dielectric permittivity and loss factor, of the polymer melt, were used to calculate the ionic viscosity and a dielectric viscosity index, which were correlated to the melt viscosity measured by slit rheometry equations. The viscosity of the PVC melt was varied by varying the level of chemical blowing agent (AZO) and processing aid (K400) in the polymer compound. The extrusion conditions; screw speed and heating zones, were fixed throughout the experiments. Strong correlation between the measured dielectric parameters and the melt viscosity was found to be consistent with established theoretical models. Therefore, this technique is capable of monitoring the viscosity of polymer melts accurately and reliably, in realtime, without disturbing the flow stream.

An experimental study of the influence of carboxylic acid additive in polyethylene/filler and polystyrene/filler compounds in slit die is described. Slit die extrusion experiments and contact angle measurements are reported. A series of fillers of varying particle size including carbon black, calcium carbonate, zinc oxide, and talc are compared. Aliphatic fatty acids, especially octadecanoic acid were used as additives. The greatest viscosity reduction was found in polyethylene/(carbon black) and polystyrene/(calcium carbonate) compounds. Smaller effects are found with the other polyethylene/filler compounds and none in polystyrene/filler compounds. Contact angle experiments indicated carboxylic acid layers on the die surfaces for the polyethylene and not the polystyrene compounds. Mechanisms for this behavior are discussed.

Crystallization of binary blends of semi-crystalline polyolefins (i-polypropylene and HD-polyethylenes) and amorphous cycloolefin copolymers (COC) was followed by Differential Scanning Calorimetry (DSC). Blend morphologies were investigated by Scanning Electronic Microscopy (SEM). The thermal behavior and morphology of iPP–COC blends are characteristic of immiscible blends. The two crystallization exotherms observed on cooling from the melt were explained in terms of fractionated crystallization. This phenomenon occurs when iPP inclusions are finely dispersed in the COC matrix so that their number is much greater than the number of heterogeneities originally present in the bulk polymer, resulting in the suppression of the heterogeneous nucleation. A quite different behavior is observed in PE–COC blends. DSC and DMTA results show a significant decrease of both PE Tm and COC Tg, observations usually associated to some polymer miscibility in the amorphous phase. The crystallization mode and the resulting morphologies also appear to be largely dependent on the molecular weights and polydispersity indices of the dispersed phase.

Structural transformations of semicrystalline polymers form an important field of current scientific research. The preparation of controlled supermolecular structure may lead to significant improvements in mechanical, thermal and optical properties. As for isotactic polypropylene (iPP), its properties essentially depend on its crystalline phase composition, which is due to polymorphism of iPP. Thus, the aim of this work is to analyze the influence of the β-phase content in polypropylene and of its spatial arrangement on mechanical properties, in particular on its impact strength. Commercially available isotactic polypropylene (iPP) was modified by various amounts of a specific β-nucleating agent (N, N'-dicyclohexylnaphtalene-2,6-dicarboxamide). From the prepared mixtures standard impact testing bars were injection molded. The structure of the specimens was studied by X-ray diffractometry, and impact properties were determined using Charpy impact tester. It was found that addition of the nucleator led to β-phase content increase, especially in the core region of the injection molded samples. The skin (surface layer of the samples) contained lower amount of β-phase. Furthermore, notch impact strength dramatically increased with increasing concentrations of the β-nucleating agent. The concentration of 0.03 wt.% of the nucleator can be considered a “boundary value” due to the fact that the maximum notch impact strength value was reached. With further addition of the nucleating agent the impact strength slowly decreased. Therefore it should be noted that the improvement of impact properties is not only due to the presence of β-phase, but also there are some synergy-applied factors – such as the structural heterogeneity and spatial arrangement of the β-phase in injection molded samples.

Transient extensional rheological behaviour of several polypropylene (PP) homo- and co-polymer grades was studied at different temperatures. In addition, shear rheological characterisation was completed. The structure of PP grades was characterised by using gel-permeation chromatography (GPC), differential scanning calorimetry (DSC) and hot-stage optical microscopy (HS-OM). It was found that the extensional viscosity of the polymers followed the three-fold transient shear viscosity curves when the tests were conducted in fully molten state, as expected for linear polymer melts. However, the strain hardening behaviour was evident when PP was stretched in the semi-molten state, i. e. below its melting temperature (T m ). Similar to results of a recent study by Rauschenberger, three temperature zones of different extensional behaviour were found: linear viscoelastic behaviour and uniform drawing in the molten state (180 °C), unstable drawing just above T m (170°C) and stable drawing with strong strain hardening below T m (165°C). The strain hardening behaviour was quantified and related to molecular structure and morphology of PP. It was found that none of the investigated parameters alone was sufficient to predict the extensional behaviour of PP. The definition of Extensional Viscosity Index (EVI) as the ratio of extensional viscosities at the Hencky strain of 3 and 1, respectively, was found to be practical for the evaluation of the polymers' processability for the important industrial processes, such as BOPP film manufacturing, where extensional deformation predominates.

It is known that while the double bubble tubular film process gives better impact strength and more uniform shrinkage balance than the tenter process, yet it is relatively poor in film thickness uniformity. The film thickness uniformity of the PA 6 produced by the double bubble tubular film process was investigated. The optimum stretching stress during the double bubble tubular film process exists. The bubble breaks over stress 130 MPa and bubble is unstable below stress 60 MPa. In the optimum condition, which is process temperature 310 °C and stretching ratio 3.2, stretched film uniformity was twice worse than non-stretched film one. The thickness uniformity of the film stretched in the optimum condition was better than that of the film stretched at higher process temperature and at lower stretching ratio. In the observations of the bubble sample through the polarizing plates, deformation pattern showed the equality of deformation. Thickness uniformity of non-stretched film significantly affects the thickness uniformity of biaxially oriented film. Local stretching ratio, which varies along the film width, influences tensile modulus and tensile strength at break. In order to obtain the uniform physical properties of film, it is important to produce the film with uniform thickness. Film thickness uniformity can be improved when stretching stress is high and bubble stability is good. These factors are influenced by stretching temperature, stretching ratio and air velocity from air ring.

This report is to discuss the bowing phenomenon and the sag elimination tension for biaxial oriented PA 6 film process, which consists of double bubble tubular film process and the thermosetting tentering. The bowing phenomenon does not occur in the heat set of the triple bubble tubular process. However, there is a serious problem of the bubble stability when the sufficient thermosetting by the tubular process is carried out, so the heat set by the tenter process is required. The bowing phenomenon arises when the annealing is carried out by the tenter method. The bowing distortion becomes large when the annealing temperature is high and film relaxation ratio is large. It is possible to reduce the bowing distortion when thermosetting is carried out in a two-step tenter process. The shrinkage stress of the stretched film was relaxed in the heat treatment during the second step whose temperature is higher than the first one. A two-step heat treatment was able to decrease the bowing distortion compared with the one-step heat treatment. When the bowing phenomenon occurred, shrinkage percentage in hot water changes along the cross-direction of a film. The humidity expansion coefficient in the film which increases as the increasing shrinkage percentage in hot water is related to the molecular orientation. The sag also increased with increasing the bowing distortion. The bowing distortion and the sag elimination of tension have proportional connection.

Reaction injection molding involves the injection of two or more kinds of liquid polymers into a mold at relative high speed. Such high inertia effect may result in air pocket inside the mold or non-uniform velocity distribution. How to design the mold entry geometry to provide uniform velocity distribution becomes one of the major topics in reaction injection molding process. Thus a new and simple method was developed in this work to optimize the reaction mold entry design. Several regulation islands were located in a fish tail entrance to help distribute the flow more even. Dimensions and locations of the islands will affect the flow uniformity and become the factors to be optimized. Taguchi's design of experiments coupled with computer flow simulation was used to achieve die geometry optimization. Effects of different dimensions of the islands on the flow uniformity can also be evaluated through statistical analyses in the flow simulation. Flow visualization technique was done by capturing flow images with high speed CCD. The experimental results showed very good agreements with the Fluent simulation results. The design methodology proposed here can help the molders in designing different reaction injection mold entries for better uniformity and performance.

The shape deformation of optical pickup lens in injection molding was investigated in this study. It was realized that residual stresses and warpage deformation in injection molding parts were affected by different process conditions such as melt temperature, mold temperature, packing pressure and time. A two dimensional analysis in injection molding only provides information on the thickness shrinkage for optical lens. In order to simulate the surface contour variations, a three-dimensional warpage analysis was conducted in this study using the finite element method. Moldings under different packing pressures and mold temperatures were numerically investigated to study the effects on the residual stresses and deformation of a molded pickup lens. The packing pressure and mold temperature were found to be the dominant factors that affect the shrinkage of lens in the thickness direction, resulting in surface contour deviation.

This paper presents the development of a hybrid solid-microcellular co-injection molding process that combines aesthetic and processing advantages of injection molding with benefits and property attributes of microcellular plastics (MCPs). A two-color injection molding machine has been modified and equipped with an interfacial platen and a supercritical fluid (SCF) unit for co-injection molding with regular resins and MCPs. Co-injection molded polystyrene (PS) parts with a microcellular core encapsulated by the solid skin layer have been successfully produced. Systematic experiments were carried out with solid-microcellular co-injection molding, conventional solid-solid co-injection molding, and regular microcellular injection molding processes to study the effects of process conditions and core/skin volume ratios on the penetration and morphology of the microcellular core. Light microscopy and scanning electron microscopy (SEM) of the solid-microcellular co-injection molded specimens reveal a microcellular core with fairly fine and uniform cell size of 8 to 12 microns and a cell density of up to 3 × 10 cells/cm. Under similar process conditions, microcellular cores were found to penetrate longer and generate a more uniform and thicker skin layer than do solid cores. While improving the surface finish with solid skin layers, this process is capable of producing parts with reduced sink marks, lighter part weights, and shorter cycle times.