Volume 17, Issue 3

The additiveness of equivalent residence times method of deconvolution/convolution was used to determine the RTD curves of individual screw elements in the melt region of a twin-screw extruder. Initially RTD curves were obtained injecting a tracer (TiO 2 ) at two different ports in the extruder barrel, using several screw configurations and operation conditions. Both ports are downstream, after the melt seal, well inside the molten polymer screw section. Then using the deconvolution method the RTD curve referent to the portion between the two ports was calculated. Finally, using again the deconvolution method and the data from several screw configurations the RTD of each individual screw element was obtained. The independence of screw element positions was also evaluated, where specially designed screws (e. g. changing the position and type of screw elements) were tested, showing no significant effect in the RTD curves. Kneading elements of 45 degrees (KB45) showed an important contribution of conveying, while kneading elements of 90 degrees (KB90) practically do not show any conveying, being characterized mainly by a pressure flow. With RTD curves of individual elements is possible, using the convolution method, predict the RTD curve of any screw configuration. Experimental measurements using screw configurations specially designed were used to check this analytical method. Finally it was applied to data published in the literature, obtained using other deconvolution methods and the results compared and discussed.

The effects of zinc surfactant (ZB) on the TESPT (bis(triethoxy-silylpropyl)tetrasulfide)-silica mixture in NR (isoprene rubber) and S-SBR (solution butadiene-co-styrene rubber) were investigated, and their processability, dynamic, mechanical, heat build-up, blowout properties, and silica dispersion characteristics were compared. In the NR composite, the addition of ZB increased the reversion resistance time, tensile modulus, and blowout time and lowered the viscosity, heat build up, and tanδ values. In the S-SBR composite, the addition of ZB increased the heat build up, and tanδ values and lowered the reversion resistance time, tensile modulus, blowout time, viscosity, and tanδ values of the compound. Over all, in the NR compounds, addition of the ZB significantly increased the processability and mechanical property; however, in the S-SBR compounds, it increased the processability but it was not effective in improving mechanical property.

The problem of setting the best operating conditions of a co-rotating twin-screw extruder is solved using an optimization approach based on Genetic Algorithms. Two possibilities are considered: i) the maximization of an objective function; ii) the simultaneous optimization of the individual criteria and the use of Pareto plots. The flow conditions inside the machine are simulated using the Ludovic© software. Various practical case studies are discussed, among them a reactive extrusion application. Generally, the methodology is sensitive to the influence of the main process criteria and provides solutions that are apparently sound.

The characteristics of sharkskin surface instability for linear low density polyethylene are studied as a function of film blowing processing conditions. By means of scanning electron microscopy and surface profilometry, is it found that for the standard industrial die geometry studied, sharkskin only occurs on the inside of the film bubble. Previous work suggests that this instability may be due to critical extensional stress levels at the exit of the die. Isothermal integral viscoelastic simulations of the annular extrusion process are reported, and confirm that the extensional stress at the die exit is large enough to cause local melt rupture. However, the extensional stress level at the outer die wall predicts melt rupture of the outside bubble surface also, which contradicts the experimental findings. A significant temperature gradient is expected to exist across the die gap at the exit of the die, due to the external heating of the die and the low conductivity of the polymer melt. It is shown that a gradient of 20°C is required to cause sharkskin to only appear on the inner bubble surface.

Two different methodologies were employed to determine tensile strength properties of two LDPEs and two metallocene LLDPE resins through capillary and Haul-off (Rheotens-like) measurements. It has been shown that the tensile strength properties obtained by both methods yield almost the same values. Moreover, these values can be correlated to the drawability in tubular film blowing for corresponding materials.

Biaxially Oriented Polypropylene (BOPP) film has become an increasingly important commodity over the last 30 or so years, offering an alternative to more traditional cellulose films with much improved mechanical and optical properties. The process is similar to the traditional bubble process for manufacturing films from a polymer melt, and the main difference in modelling the two processes lies in the constitutive equation employed. As the manufacturing process involves large strain deformations, a convected Standard Linear Solid constitutive equation was used in conjunction with the Pearson and Petrie force balance equations and a suitable energy balance to arrive at a mathematical model of the double bubble process. The non-linear ordinary differential equations forming the model require simultaneous solution, carried out using numerical techniques. An experimental program was conducted to obtain large-scale biaxial tensile test data. This data was used to determine the necessary material properties required to allow the model to be applied to a particular commercial process. Material data not obtained thus far has been estimated using previously published data for similar materials. Operating data was obtained from a fully instrumented full-scale industrial bubble process to enable the model to be validated. Initial comparisons between the model results and process data suggest that the model developed is a good starting point for a process simulation but that more accurate material data is required.

We investigated the effects of tacticity on crystallization and orientation development in melt spinning isotactic polypropylene (iPP) through air. Melt-spun fibers were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The melt-spun fibers usually exhibited the monoclinic α-crystalline form, but the mesomorphic form was found under certain processing conditions. The mesomorphic structure was formed more readily in low tacticity fibers. We found certain amounts of hexagonal β-crystalline form in all of the α-form fiber samples. The amouns were generally negligibly small, but the low tacticity fibers spun at high draw-down ratios exhibited significant amounts of β-form crystals. At low spinline stress levels, some low tacticity fibers exhibited lower birefringence and chain orientation than high tacticity fibers due to a less perfect crystal structure. At certain high spinline stress levels, low tacticity fibers exhibited much higher birefringence and chain-axis orientation levels than expected. This was attributed to a reduction in epitaxial lamellar branching (a*-axis oriented group in bimodal orientation). However, the extent of lamellar branching in low tacticity fibers appeared to increase again at higher stress levels. This led to a significant decrease in birefringence and crystalline orientation.

During recent years, the interest in metallocene-catalysed polyethylenes has been steadily increasing. These polymers are generally characterised by a narrow molecular weight distribution and a uniform comonomer distribution. In extrusion coating, the interest in these polymers is mainly related to the enhanced physical performance of the resulting film. However, from the processing point of view, the metallocene-catalysed polyethylenes can present problems. In this study, a number of metallocene-catalysed linear low-density polyethylenes (m-LLDPE) were characterised with regard to their viscous and elastic properties. Conventional grades of LDPE and LLDPE were included as references. The same grades were used in pilot scale extrusion coating experiments. The processability of the polymers was primarily expressed in terms of neck-in (NI), which describes the lateral reduction of the film width as the film is drawn down into the coating nip, and the draw-down ability (DD), which provides a measure of the ability of a thin film to be drawn into the nip at a high machine speed. A lower NI-value, which is desirable, was noted when the polymer melt exhibited a higher elasticity, expressed in terms of the steady state compliance, the extrudate swell and Trouton's ratio. Factors that had a positive effect on the NI-value tended in general to reduce the draw-down ability. A relation between NI and DD was established, which in principle makes it possible to optimise both NI and DD, based on finding the optimal values of the shear and the elongational viscosities. The results obtained indicate that a low shear viscosity and a high Trouton's ratio provide optimal processability with regard to NI and DD. It was noted that m-LLDPE-grades could be chosen to match the processability of more conventional polymers (expressed in terms of NI and DD). Rheology can be an important tool for such a selection.

In this work, we develop a numerical simulation method to optimize the injection molding process using the design sensitivity analysis (DSA). The optimization concerns the filling stage and focuses on the location of gates in the mold cavity as well as the injection pressure profile, in order to minimize the fill time. Since the problem to be solved involves transient flow with free surface (flow front), the direct differentiation method is used to evaluate the sensitivities of the Hele-Shaw, filling fraction and energy equations with respect to the design variables used in the analysis. The search domain parameterization is coped with using B-spline functions. Sensitivity and state equations are solved by means of finite element method. The proposed numerical approach is combined with the sequential linear and quadratic programming method of the design optimization tools (DOT) to find new design variables at the end of each complete filling simulation. Starting from any initial gate locations and injection pressure profile, the iterative optimization procedure enables us to find the optimal gate locations together with the optimal injection pressure profile. Finally, numerical results involving complex mold geometries are presented and discussed to assess the validity and robustness of the proposed method.

In this work, a three-dimensional finite element flow analysis code is used to solve sequential co-injection molding problems. Non-Newtonian, non-isothermal flow solutions are obtained by solving the momentum, mass and energy equations. Two additional transport equations are solved for tracking polymer/air and skin/core polymers interfaces. Solutions are shown for a center gated rectangular plate. The effect of varying the melt/mold temperature and the ratio between the skin and core materials is investigated. The solution obtained for the same skin and core materials is compared with those in which viscosities of core and skin materials are much different. Finally, the solution for the co-injection of a C-shaped plate is presented. The three-dimensional modeling of co-injection molding provides the complete shape of the core polymer as well as the skin polymer thickness at any location. This is a major improvement over the traditional mid-plane approach, which is unable to recover the material behavior in critical regions as near corners, obstacles, and changes in part thickness.

In the postfilling stage of an injection molding process, thermal induced residual stress usually develops during the solidification of the melt with non-uniform through thickness temperature distribution. Furthermore, the packing pressure is applied through the gate, and will be different in different locations. Since the local melt pressure varies with time due to the change of melt viscosity, the material solidifies under different frozen-in pressure. The stress relaxation behavior of the plastic materials also complicates the stress field. This study uses a simple thermoviscoelastic model to calculate the residual stress developed in the postfilling stage and a elastic model to determine the warpage after demolding for three dimensional complex parts. Based on the calculation of the injection molding process, the temperature and pressure histories in the postfilling stage can be determined, and used to calculate the developed stress field. The model divides the thickness into two regions, liquid and solid phases. Residual stress calculation is applied in the solid region, while the liquid region is kept at the melt pressure without any strain. The resulted in-mold stress fields are used to calculate the part warpage after demolding.

Carbonated soft drink (CSD) Poly(ethylene terephthalate) (PET) bottle contains pressurized liquid and its bottom design is petaloid in shape for self-standing. The trouble in this bottle is bottom cracking. In this study the design parameters of the petaloid bottom of the PET bottle have been investigated. Cracking test has been performed for different shapes of petaloid bottles and for several sizes of bottles, too. Through the cracking test, it has been concluded that the design parameters that affect the crack generation are clearance, foot length and valley width in the petaloid bottom. Computer simulation has been performed to seek for the optimum values of the design parameters that can reduce maximum principal stress concentration. Consequently the novel design of the petaloid bottom has been completed.