Masters Theses

Date of Award

8-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Committee Members

Donald C. Bogue, John F. Fellers

Abstract

An experimental and analytical study of the process of blown film extrusion was carried out. On-line measuring techniques were used to follow the dynamics and temperature profiles occurring in the process. The applicability of a mathematical model which includes a non-isothermal crystallization rate equation was tested. Subsequently, a new simplified model derived from a modified force balance was proposed and examined.

Linear low density polyethylene, LLDPE, (melt flow index 1.0) provided by Dow Chemical Company was used in the experimental part of the study. On-line measurements for radius, thickness, velocity and temperature as a function of distance from the extrusion die were carried out, and their reliability was examined. The results indicated that these measuring techniques were sufficiently accurate to make the collection of on-line data a useful analytical tool. The measured profiles of radius, thickness, velocity and temperature were used to test the theoretical model for the tubular film blowing process.

The apparent elongational viscosity, a key parameter for the theoretical simulation, was estimated and calculated from experimental data taken on a melt spinline and an inversion procedure developed for obtaining apparent elongational viscosities for melt spinning. This gave a Newtonian, temperature-dependent apparent viscosity equation. The heat transfer coefficient was estimated from measured temperature profiles on the blown film process.

A computer simulation for semi-crystalline materials was carried out using the mathematical analysis for film blowing which appears in the literature plus a non-isothermal crystallization rate equation. The analysis was carried out by using the fourth-order Runge-Kutta method to solve the resulting differential equations. The predicted results were in qualitative agreement only with the experimental data. At the same time, several unexpected phenomena appeared in the simulation. Some of them have also been reported in the previous literature, but still no satisfactory interpretation is available.

A modified physical approach based on a force balance led to the derivation and proposal of a new simplified model. From this modified analysis, an important and useful relationship between the external forces (i.e., the net takeup force and the inflation pressure) and the variation of radius and thickness of the bubble were determined. Based on the same initiaI conditions as that of the original model, the new model gave predictions which were in fair quantitative agreement with the on-line measurements.

Finally, it was also found that the development of crystallinity strongly influences the final values of radius and thickness of the tubular film, two of the important specifications in industrial film processing. In other words, the effect of crystallization is so significant that it should not be neglected in modelling the tubular film blowing process.

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