Volume 5, Issue 4

Advanced fiber and film extrusion is a term used to describe high speed spinning of low denier filaments and stretching of thin films at high output rates and high efficiencies. Using the advantages of separated process steps, extrusion systems have been developed in which the main process steps, such as feeding, melting, homogenizing, filtering, mixing and metering, take place in special separate equipment aggregates. In comparison to standard extruders with combined process steps, these extrusion systems give better melt quality as measured by the uniformity of melt flow, drawability of the fibers or films and the dispersion of additives. Examples of extrusion systems for PET or PP with appropriate operating conditions and output rates are described. The different applications of these systems and their main details are listed in a survey.

In this study, the melting behavior and melt homogeneity in a single screw extruder have been investigated to obtain better products in quality and higher productivity for extrusion and injection molding. For the analysis of the melting behavior, so-called “cooling experiments” were used. For the melt homogeneity evaluation, furthermore, molten polymer temperature fluctuations at the exit of a single extruder and injection molding machine were measured. As a result of these analyses, we have concluded that the melting process in extrusion and injection molding played an important role on improving product quality and its productivity. Based on experiments and analyses on the melting mechanism, we have developed a mixing and kneading screw configuration with a new melting process, in which the compulsory circulation flow in solid bed accelerates rapid melting of raw materials and melt homogeneity.

The flow and pumping capability of extruder screws with slices in their flights and pin barrel extruders have been modelled with special consideration on its single or multi-flighted character and the periodicity of the positions of the slices and rows of pins. This is done using an analytical method developed by the authors. The method of analysis is extended to non-Newtonian fluids.

In past numerical simulations of polymer melt flows, the flow field has been visualized through velocity vectors and streamlines providing a steady-state Eulerian viewpoint. This work shows that enhanced understanding of flow kinematics can be achieved by calculating the residence time distribution (RTD) of material points through the integration of the known velocity field and by plotting isochrones and the deformation field of fluid elements. Such kinematic patterns provide a Lagrangian viewpoint of the flow and fully describe the history of the elements of the system as they move through the field. In cases with zones of recirculation the period for one revolution is also calculated. Typical examples are shown for such polmyer processing operations as entry flows in contractions, exit flows from dies, calendering and wire-coating coextrusion processes.

Flow history-morphology interactions have been studied for three crystallizable polymers: high density polyethylene, poly(vinylidene fluoride), and polypropylene. Materials were extruded through an impinging channels film die which produces a mixed extensional and shearing flow field. Extrudate morphology was characterized by optical microscopy and x-ray diffraction. Stress fields during die flow were measured from flow birefringence and compared to calculations based on the power law model. Good quantitative agreement was found for the shearing stresses, however, large differences were observed between the measured and computed normal stresses along the symmetry axis of the flow where extensional gradients are also the highest. Extensional stresses on the order of 2.8·10 6 , 1.4·10 6 , and 3.9·10 5 dynes/cm 2 were required to induce row-nucleated crystallization in the HDPE, PVF 2 , and polypropylene respectively. The respective stresses were found to decrease with increasing relaxation time in a non-linear fashion. An explanation for this behavior is presented based on the opposing tendencies of chain relaxation and oriented crystallization.

The material functions, viscosity η, the first normal stress function N1, dynamic shear moduli G′ & G″ from sinusoidal oscillatory measurements and the transient responses of stress build up and stress decay in step flow were investigated experimentally for solutions of poly-p-phenylene terephthalamide in H 2 SO 4 at concentrations from 0.5 to 10 wt.-% covering the phase transition from isotropic to nematic LCP solutions at 8 % where all mentioned properties change abruptly. General analytical forms have been found for η and η* functions. LCP solutions show high value of N 1 which builds up at shear strain larger than the yield value of 2. The structural origin for the observed rheological properties are discussed.

This paper deals with instability phenomena in PET film casting. An experimental investigation makes it possible to confirm the occurrence of draw-resonance above a critical draw ratio, and to show interesting coupling effects between thickness and width fluctuations. A theoretical model is developed on the basis of some important assumptions: constant width, vertical film, isothermal regime and Newtonian behavior. It allows to calculate the response to a sudden increase of the chill roll velocity and to predict the critical draw ratio (D R = 20.3). Other types of perturbations of the roll velocity (sinusoidal oscillation, random fluctuations) are also treated using this model.

In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was to study the effect of the roll temperature on the polymer crystallization in the thickness of extruded films. A thermomechanical model of the whole process taking into account the polymer crystallization was used to determine the mean stretching stress in air, local temperature, local cooling rate and local crystallization development. The morphologies in the thickness of the films were observed by optical microscopy on microtomed slices and the crystalline structures were investigated by wide-angle X-ray diffraction and DSC measurements. From DSC melting curves of thin sections it is possible to know the local crystallization temperature and to check the thermomechanical model. The model is validated and explains the influence of the roll temperature on the formation of structures and morphologies. Nevertheless neither the existence of three different zones in the thickness nor the high p phase concentration are predicted by the model, which needs new developments.

Melt strength data are presented on the three major classes of commercial polyethylenes (LDPE, LLDPE and HDPE). The LDPE samples studied were polymerised by both tubular and autoclave reactor technology and the LLDPE samples were produced with different α-olefins as comonomer. Melt strength was assessed from uniaxial tensile experiments with a Rheotens apparatus. The melt strengths of both LLDPEs and HDPEs are much lower than those of LDPEs when compared at equal melt index or zero-shear viscosity level. Among the LDPEs the highest melt strength is observed for the LDPEs made by autoclave technology. The type of comonomer present in LLDPEs does not significantly affect its melt strength. The molecular factors controlling the melt strength of pure polyethylenes are discussed. A high melt strength is identified with a good bubble stability in the film blowing process.

A double bubble tubular film process for nylon 6 was investigated. This report discussed three topics: Relationship between process conditions and film stretching stresses; deformation rate during double bubble process, birefringence and film thickness uniformity; and scale-up rule for double bubble tubular film process. The optimum stretching stress during the double bubble tubular film process exists. The bubble break occurs over stress of 1300 Kg/cm 2 and bubble stability occurs below stress of 600 Kg/cm 2 . The cross point of stretching stress, which means stress in the machine direction is equal to stress in the transverse direction, is influenced by a deformation rate which is closely related to birefringence and bubble stability. Bubble stability and film thickness uniformity are improved with increasing deformation rate. It is found that the scale-up rule which was set up theoretically is applicable to predict the physical properties and bubble stability for large scale double bubble tubular film process once the same process is carried out by using the same scale machine and a small amount of resin.

A numerical model has been developed to simulate the polymeric flow and heat transfer in calendering processing. In the flow calculation, the polymer is treated as a thermal-dependent power-law fluids; in the coupled heat transfer calculation, both the roll and the polymer are modeled in order to assess the performance of various designs of calendering rolls. The numerical predictions agree very well with the on-line measurements. The generalized finite difference scheme was selected to cope with the numerical difficulties involved in the modeling of the process, especially the curved boundary and the free surface conditions. The computational aspects of the finite difference scheme in modeling of polymer behavior are also discussed.