Doctoral Dissertations

Date of Award

8-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Committee Members

Donald C. Bogue, Edward S. Clark, Marion G. Hanson

Abstract

The literature on tubular film blowing is contradictory, even on such basic matters as to whether increasing inflation pressure increases or decreases the bubble radius. To provide a solid experimental base for physical understanding and theoretical modelling, detailed on-line measurements of the tubular film blowing process were made using three polyethylenes (a low density, a linear low density and a high density material, all of melt index 1.0). The measurements included blow-up ratio as a function of inflation pressure and take-up ratio; the extrusion temperature and the air flow rate were also varied. On-line distributions of radius, thickness, velocity and temperature along the machine direction were made in some runs. For the most part these data showed an "intuitive" effect of inflation pressure on blow-up ratio; that is, increasing the pressure caused the final radius (the blow-up ratio) to increase. However, at high blow-up ratios (typically values of 3 - 4, depending on the material and processing conditions), regimes having a "counter-intuitive" relationship were observed in some cases. There were not substantial differences among the three materials except that the low density material was less prone to instabilities at high blow-up ratios. The detailed on-line data indicates that at a fixed take-up ratio all the deformation rates in the three principal directions increase with an increase in blow-up ratio. This phenomenon is used as a means of explaining the deformation-thinning associated with "counter-intuitive" pressure effect. The validity of the two existing theoretical models (the earlier model of Pearson and Petrie and the newly proposed model in the author's M.S. thesis) are examined by making qualitative comparisons with experimental data. It is found that prediction of the new model agrees with the basic trend of data but that of Pearson-Petrie does not. Furthermore, the newly proposed theoretical model based on treating the bubble as "quasi-cylindrical" at each point (neglecting curvature in the axial direction) and incorporating a deformation-thinning viscosity equation (which is motivated by experimental results) explained all of the essential features of the data. The viscosity function required to fit the blown film data were in plausible agreement with viscosity data (complex viscosity data) measured independently. The predicted passage from an "intuitive" regime to a "counter-intuitive" regime, qualitatively in agreement with the data, comes about from the thinning behavior of the viscosity model. Comparisons with the earlier model of Pearson and Petrie were also made. This model, based on thin-shell theory, disagrees with the data in several essential ways when the analysis is done as a problem in fluid mechanics: it predicts "counter-intuitive" results as regards the effect of inflation pressure and melt viscosity on bubble radius. These unusual effects come about from an axial curvature term in the thinshell formulation. In a free-boundary fluid mechanics analysis, the theory predicts the shape of the bubble. If, however, one does the analysis as one would in solid mechanics — in which the shape of the bubble is known a priori — there are not substantial differences between the predictions of the Pearson-Petrie model and that of the present paper. The reasons for this difference are not clearly understood.

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