Investigation of Polymer Stretching Instabilities: Application to Film Blowing

Abstract

International audienceFilm blowing, as other elongational polymer forming processes, may present marked drawing instabilities leading to unacceptable products. But in film blowing, these instabilities are much more complex than for example in fibre spinning: there is no stabilizing effect of the polymer cooling, and the symmetry of the process may be broken, leading in some processing conditions to so called helical instabilities. Stability of the process has been investigated using a strategy inspired from shell or homogeneisation theory: as the classical approach uses a frame locally affixed to the membrane, the equations of the problemare now written in the cartesian laboratory frame. Making the equations dimensionless introduces naturally a small parameter defined as an aspect ratio (ratio of the film thickness to the bubble radius). Kinematic and stress variables are expanded as a function of this small parameter and introduced in the equations. It leads classically to a sequence of equations at successive orders. This strategy is used to obtain a time dependent membrane model. The stationary solution is equivalent to the one obtained using the classical approach. This model allows to develop a stability analysis, first in the axisymmetric case and then in the non axisymmetric one. Even a crude Newtonian temperature dependent rheology allows to capture qualitatively the observed instability phenomena

Improvement of blown film extrusion of poly(Lactic Acid): Structure–Processing–Properties relationships

“…By comparing the bubble stability of different types of PE, Minoshima and White concluded that long‐chain branched PEs are the most stable. The most unstable are PEs with narrower molecular weight distributions (MWD) . However, there are no articles dedicated to the blown extrusion processing of PLA and the improvement of its stability window.…”

An Overview of Molten Polymer Drawing Instabilities

“…The same type of phenomenon as in cast-film may be expected, that means a periodic fluctuation of thickness and diameter (figure 6) (Kanai and White, 1984;Minoshima and White, 1986;Ghaned-Fard et al, 1996;Laffargue et al, 2002Laffargue et al, , 2010. In fact a non-axisymmetric instability may also be observed, which corresponds to a helical shape of the bubble between the die exit and the film freezing line (figure 7).…”

“…On the contrary, for a Bur of 3.25, the bubble radius remains constant when increasing Dr from 6.6 to 7.1, but at the same time the centre of the bubble rotates (figure 9) in an horizontal plane giving a helical shape to the bubble. Laffargue et al (2010) performed film blowing experiments on various extrusion lines but with the same LLDPE. They point out the existence of a stability hill splitting stable and unstable blowing conditions (for both axisymmetric and helical instabilities (figure 10)).…”

“…Following Housiasdas and Tsamopoulos (1998), Laffargue et al (2010) wrote and solved the film blowing equations in the frame affixed to the die geometry. This leads to complex developments which are detailed in Laffargue (2003) and Laffargue et al (2010) but this allows capturing both axisymmetric and helical instabilities. Each variable (bubble radius,…”

A new model for the blown film process

“…Component σ n is small compared to σ t and σ φ . It can be proven, using an expansion according to the small parameter , that σ n is of order while σ t and σ φ are of order 1 (see [5] and [6]). Using σ n = 0 we have p = 2η V e de ds and using (3) we obtain finally:…”