Y. Demay

Investigation of Bubble Instabilities in Film Blowing Process

Abstract This paper describes an original on-line video device developed in order to study bubble instabilities occurring in the film blowing process, taking into account their three-dimensional behavior. For a linear low-density polyethylene, two forms of instabilities and combination have been observed: draw resonance and helical instability. These instabilities could be quantitatively described and differences in behavior could be assessed using real objective measurements and criteria. The influence of key processing conditions was investigated and the results showed that the instabilities are enhanced by increasing the draw ratio, blow up ratio and frost line height. These first results are in agreement with the majority of the results reported in the literature, but allow for a more accurate analysis of the phenomena.

Finite element simulation of flow and director orientation of viscous anisotropic fluids in complex 2D geometries

A simplified model of the Leslie–Ericksen theory is used to solve the flow of liquid crystalline polymers, seen as viscous anisotropic fluids, making the assumptions of high viscosities and neglecting the director elasticity in the viscosity terms. The resulting equations, known as Ericksen’s ‘‘Transversely Isotropic Fluids’’ equations involve four rheological parameters. Axisymmetrical isothermal steady flows are considered and the problem is divided into two parts using the method of the fixed point. The velocity calculation is performed by a variational method and a functional minimization technique and the equations are discretized with a finite element method. The director computation is made using a method of characteristics. The results of the computations describe the behavior of anisotropic viscous fluids as a function of flow geometries and rheological coefficients. Complex geometries including converging and diverging channels, expansion and contraction flows, as well as flow around spherical o...

Cast Film Problem: A Non Isothermal Investigation

Abstract An important problem arising in the cast film process is the so-called neck-in phenomenon, which may induce a drastic film width reduction as well as an inhomogeneous film thickness distribution (with the so called “dog-bone” defect on each side of the film). This neck-in phenomenon is highly influenced by the polymer rheology (increasing polymer elasticity leads to decrease markedly the neck-in), the stretching distance, the cooling conditions and the draw ratio. In this paper, we present a 2D Newtonian non-isothermal model based on a membrane approximation for the film. Stress balance, mass balance and temperature balance equations are solved iteratively using various finite element methods adapted to free surface problems and to transport equations. Coupling between mechanical and thermal equations is achieved introducing an Arrhenius temperature dependance for the viscosity. The influence of the heat transfer coefficient has been carefully investigated.

Cast Film Extrusion

Abstract Cast film extrusion is a widely used technique to produce polymer films for packaging or coating applications. The final width of the film may be smaller than the initial die width (necking phenomenon) and therefore is the final film thickness larger than what one could expect. Moreover it is heterogeneous (in terms of showing a larger thickness along the borders, also called dog bone defect). Above a critical draw ratio between the chill roll and the die both width and thickness fluctuations are observed (Draw Resonance Instability). All these phenomena limit the productivity of the process. This paper provides an overview of the processing and material parameters governing the development of these geometrical defects and instabilities. Numerical models capturing the main experimental features are proposed, allowing to investigate process modifications in order to limit the development of these defects.

Complex transients in the capillary flow of linear polyethylene

The spurt flow behavior of a linear high–density polyethylene was carefully studied in capillary experiments. Depending on the flow conditions, new types of complex transients were observed. The study pointed out a second oscillating area, described by a second hysteresis cycle, close to or inside the classical one. In one case, the second oscillating area was situated between two parts of the stable second branch, and pressure oscillation patterns for the two distinct oscillating areas were clearly reported. In another case, the smaller secondary oscillations occur within the cycle of the primary oscillations, showing double instability pressure patterns. The transients in the capillary flow of linear polyethylene put in evidence a complexity, which cannot be explained only in terms of melt compressibility associated with a simple relationship between slip and shear stress.

Evaluation of stresses in a two-layer co-extruded LDPE melt blown film

Despite the fast growth of co-extruded film production, little research work has been devoted to this process. Films of LDPE, ULDPE, LDPE/ULDPE and ULDPE/LDPE were produced at various processing conditions. A continuous experimental set up was used to evaluate the in-plane birefringence during bubble formation. The data collected were used to derive the axial and transverse stresses. A non-isothermal Newtonian model that allows the determination of the kinematics and dynamics of the two-layer film blowing process was then used to calculate the axial and transverse stress. Generally the calculated stresses are in agreement with the experimental values.

Thermomechanical modeling of polymer processing

Publisher Summary This chapter discusses thermomechanical modeling of polymer processing as well as considers the flow of a homogeneous molten polymer. In a continuum mechanics approach, the different equations to solve are the mass, momentum, and thermal balances, linked by a constitutive equation and appropriate boundary conditions are presented. It is possible to compute the flow of molten polymers in most of the complex geometries encountered in polymer forming processes, in both stationary (extrusion) and unstationary (injection molding - blow molding) conditions. This requires first precise volume meshing methods, starting, for example from a CAD surface meshing of the tools. This necessitates robust numerical finite element methods, which are mixed velocity/pressure Galerkin method for the mechanical problem, Taylor-Galerkin method for the thermal problem, and discontinuous Galerkin method for convection problems (viscoelasticity, for example). Iterative solvers and parallel computing reduce storage requirement and computation time. This allows performing numerical simulations in complex industrial geometries, with refined meshing. These direct numerical methods do not necessitate any geometrical or kinematics assumptions, but they remain limited, to purely viscous constitutive equations (in 3D). In addition, computation time is important and die or mold optimization, which necessitates many successive numerical simulations, remains a difficult task. In that sense, approximation methods, which require however sophisticated mechanical and thermal treatments based on kinematics, heat transfer, and geometry assumptions, are still useful.