Correlating the Plastic Strain With the Properties of the Low Density Polyethylene Blown Film

Abstract

Experiments were carried out to verify a theory developed to cor relate the plastic strain history in the film blowing to the ultimate physical properties of the blown film. The theory predicts that the amount of strain put in the blown film once it starts to freeze, defined as plastic strain here, has the prominent effect on the film's ultimate physical properties. A grade of low den sity polyethylene was used to produce the blown film runs yielding a 1.25 mil thin film at the outlet. Film surface and bulk temperatures, along the machine direction, were measured using the infra-red techniques to identify the points where film starts to crystallize. A video digitization technique was used to measure the bubble kinematics. Film's physical properties were measured us ing standard ASTM methods. The results indicate a correlation between the amount of plastic strain, or its derivatives, and the ultimate physical properties of the blown film. This principal applies equally well to the film properties in the cross or transverse direction. The results obtained can be exploited by de signing the blown-film equipment and processing conditions which result in op timal bubble stretching once the melts begin to crystallize.