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.
Analysis of isothermal two-layer blown film coextrusion
The mechanics of a bicomponent two-layer blown film coextrusion is studied theoretically. As a first step for the modeling of this complex process, we adopt a simple system in which the flow is assumed to be isothermal and the two layers are a Newtonian and an upper-convected Maxwell fluid (UCM), respectively. The two fluids are chosen to investigate the relative influence of viscous and viscoelastic forces on the flow mechanics of the process. For a given total flow rate, blow-up ratio, freeze-line height, and film gage, the radius and the melt thickness profiles of the blown film are determined numerically for various values of the flow rate ratio of the two fluids. When the relaxation time of the UCM layer is small, the flow mechanics including the shape of the bubble (or the radius profile) is not much different from that of a Newtonian single-layer flow. With increasing relaxation time, the viscoelasticity effect of the UCM layer becomes more and more pronounced and eventually dominates the bubble dynamics even though its layer thickness may be smaller than that of the Newtonian layer.
Simulation of Blown-Film Process Taking Account of Cooling-air Effect
Abstract Numerical simulation of blown-film process of plastics provides a tool for air ring designs and suitable polymer properties. Although cooling-air plays a key role in the process ability and film properties, there have been few research works taking correct account of the cooling-air effect. Numerical analyses of heat transfer from the surface of molten polymer to cooling-air were conducted using precise turbulence models and then simulations of bubble shape of blown-film process were performed under the air pressure and the thermal boundary conditions estimated from the numerical analyses of cooling-air. The film bubble shape predicted has coincided well with experimental data and it is confirmed that the bubble stability is strongly dominated by characteristics of cooling-air flow and heat transfer near the bubble surface.