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.
Correlating the Plastic Strain With the Properties of the Low Density Polyethylene Blown Film
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.
Bodies Politic : Disease , Death and Doctors in Britain , 1650-1900
In spite of its intellectual, literary and comic brilliance, this book contains a dark and disturbing, but revealing, message. In some ways, my melancholy reading of Bodies Politic has inevitably been shaped by Roy's recent untimely death. Roy Porter was without doubt the finest social historian of medicine this country, or indeed the world, has produced. Expansive, humorous, intelligent, and incisive, his broad range of historical works has excited and educated several generations of students of history over the past thirty years. His death leaves an unfillable void within the global history of medicine. At the same time, however, this book itself, completed well before his death, is haunted by elements of despair and uncertainty that should make us carefully reconsider not only the purpose and direction of our professional inquiries but also our own mortality.