Volume 16, Issue 3

The effects of the main processing conditions on the dissolution behavior of CO 2 in polystyrene melt were studied using a novel in-line method in connection with both single and twin-screw foaming extruders. Apparent solubility data obtained from the twin-screw extrusion experiments showed better consistency compared with those obtained from the single screw experiments, indicating the importance of enhanced mixing in affecting the gas dissolution and solubility behavior during an extrusion foaming process. Gas dissolution inside a twin-screw foaming extruder was further studied for CO 2 in the polystyrene melt. The measured gas up-take was compared with the values calculated assuming equilibrium conditions in the gas injection zone in an effort to elucidate possible mechanisms for gas dissolution inside a foaming extruder during the actual foaming process.

The design of a screw for plasticating polymer extrusion based on scientific principles is still a challenging task, which has received surprisingly little attention in the literature. In the present work the design of a screw is considered as an optimization problem. The corresponding design methodology is discussed. Two alternative approaches are considered. In the first case, the aim is to maximize the value of an objective function that describes quantitatively the process performance in terms of pre-selected important process criteria. A multiobjective optimization scheme resorting to the use of optimal or non-dominated Pareto frontiers is also implemented, where the set of feasible individual solutions in terms of the various criteria is correlated, thus evidencing the compromise between them. The relevance of the solutions and the sensitivity of the method to changes in the criteria considered are demonstrated with a case study.

Analytical expressions for the isothermal, two-dimensional flow of an Ellis fluid in rectangular channel have been derived. The obtained solution describes also the flow of Newtonian and power law fluids as the specific cases. This solution was further adapted for the theoretical analysis of the temperature and pressure effects during the non-isothermal flow of Ellis fluid. The results obtained both for isothermal and non-isothermal flow conditions have been verified numerically.

Analytical model for the isothermal and non-isothermal, two-dimensional flow of an Ellis fluid in curved channel has been proposed. It describes also the flow of Newtonian and power law fluids as the specific cases. The solution basing on cylindrical coordinates can be treated as a generalization of a model in rectangular coordinates to account the effect of channel curvature on the flow. The results obtained for isothermal and non-isothermal flow conditions have been verified numerically.

An experimental study is presented of the melting mechanism in a starve fed closely intermeshing counter-rotating twin screw extruder of a modular Leistritz design. Polypropylene and low density polyethylene as well as different screw configurations of thick flighted and thin flighted elements were investigated, at various operating conditions. A “Screw Pulling-out Technique” was used to characterize polymer behavior along the screw axis. In particular, the solid conveying, melting positions, the extent of starved character along the screw, and the fully filled regions were observed. Polymer samples were stripped off from each screw which was removed from the machine to investigate melting mechanism. Generally, it was concluded that the melt mechanism consists of pellets being dragged into the calendering gap where they are melted due to calendering action. In the case of thick flighted screws, the molten polymer is expelled from the gap and pushes against the pellet bed which is continuously dragged into the gap.

The existence of multiple stationary values of the fully filled length in reactive extrusion is studied on the basis of a mathematical model describing dynamic extruder behavior. The extruder is modelled as a plug flow type of reactor. Through the introduction of travel time for a fluid element in the extruder, time dependent viscosity models following from rheokinetics can be used. Special attention is paid to ionic and radical polymerization reactions. Hysteresis behavior for stationary values of the fully filled length exists when die resistance and rotational speed are varied. The effects of reaction rate, viscosity levels and degree-of-fill on stable operation at high conversions are studied. The mathematical dynamic model presented here can account for some of the experimentally observed dynamic phenomena in the reactive extrusion of radical polymers.

Biaxially oriented Polyamide 612 (PA612) films made by double bubble tubular film extrusion were characterized. The processing characteristics of the polymer in the double bubble process were investigated. The development of crystallinity and orientation in the films was clearly considered. First bubble PA612 films showed a low level of crystallinity (X = 7.7%). Differential scanning calorimetry (DSC) indicated that the first bubble PA 612 films experienced a considerable increase in glass transition temperature (T g ), cold crystallization temperature (T c ) and crystallinity during aging at room conditions. The second bubble films were more highly crystalline (X = 22 to 28%) with poorly grown β unit cells. The WAXD and SAXS patterns showed that the crystallites of double bubble PA612 films became more perfect by annealing in a boiling 20% formic acid solution, resulting in a well-developed triclinic α crystals coexisting with minor proportion of β phase. The WAXD and SAXS patterns indicated that crystallography planes and lamellae in the second bubble films were oriented depending on SBUR × SDDR (second blow-up ratio x second draw down ratio). White-Spruiell biaxial orientation factors were calculated with the WAXD pole figure data. The hydrogen bonding (0k0) planes of biaxial double bubble films were found by WAXD to be aligned parallel to the sheet surface before and after annealing.

Recently great efforts have been made to increase the data access speed and the data storage density of the optical memory disks such as CD-ROM, DVD, ASMO disk, etc. In the viewpoint of increasing the data access speed of the optical disks, it is of great importance to characterize accurately the vibration phenomena of the rotating disk, which is related to the critical speed and the characteristics of the frequency response. In this paper, towards the goal of increasing the data access speed of the optical disks, we have proposed a novel method for improved characteristics of disk vibration without deteriorating any other specification for the optical memory disks such as geometrical and optical characteristics. A possible idea to improve the vibration characteristics of the optical disk is that the tensile initial stress in the radial direction is to be imposed within the optical disk on purpose. By the synergy effect from the residual (initial) tensile stress frozen within the optical disk combined with the stress induced by the centrifugal force occurring in rotating the optical disk, the stiffness of the disk would increase to have a stronger resistance to the disturbance resulting in increase of critical speed. With such a synergy effect in mind, modifications related to the optical disk substrate design, injection mold design and processing conditions of injection molding were presented and partially evaluated. The computer simulation and experiments indicate that the suggested ideas clearly improve the characteristics of disk vibration without deteriorating any other specification for the optical memory disks.

This paper presents a finite element solution algorithm to solve polymer injection molding problems. The methodology solves the momentum, mass and energy conservation equations in three-dimensions. Packing and cooling stages of the injection molding process are modeled by considering compressibility effects. The procedure is aimed at processes in which three-dimensional effects are important. The method is also shown to be effective for thin parts. The performance of the proposed approach is measured on the injection of a plate for which experimental data are available. The procedure is also applied to a thick 3D part. The method results in accurate solutions and it is a useful tool to quantify the material behavior on cases otherwise difficult to investigate.

A constitutive equation for cold-rolled polypropylene is presented based on a modified two-phase model for semicrystalline polymer, which combines the crystalline network that obeys the Langevin based theory and the entangled network with slip-linked character. The microstructure of the rolled semicrystalline polymer consists of packed crystalline inclusions which are separated by adjacent amorphous regions. A theoretical description of the stress strain behavior is obtained by the sum of the stress effects from crystalline and entangled networks. The microstructural observations by scanning electron microscopy X-ray pole figures and differential scanning calorimetry show that polymer chains are aligned along the rolling direction. The stress-strain equation can be obtained from the modified two-phase model and verified by experimental results.