Viscoelastic Material Models of Polypropylene for Thermoforming Applications

Polypropylene (PP), a semi-crystalline material, is typically solid phase thermoformed at temperatures associated with crystalline melting, generally in the 150° to 160°Celsius range. In this very narrow thermoforming window the mechanical properties of the material rapidly decline with increasing temperature and these large changes in properties make Polypropylene one of the more difficult materials to process by thermoforming. Measurement of the deformation behaviour of a material under processing conditions is particularly important for accurate numerical modelling of thermoforming processes. This paper presents the findings of a study into the physical behaviour of industrial thermoforming grades of Polypropylene. Practical tests were performed using custom built materials testing machines and thermoforming equipment at Queen′s University Belfast. Numerical simulations of these processes were constructed to replicate thermoforming conditions using industry standard Finite Element Analysis software, namely ABAQUS and custom built user material model subroutines. Several variant constitutive models were used to represent the behaviour of the Polypropylene materials during processing. This included a range of phenomenological, rheological and blended constitutive models. The paper discusses approaches to modelling industrial plug-assisted thermoforming operations using Finite Element Analysis techniques and the range of material models constructed and investigated. It directly compares practical results to numerical predictions. The paper culminates discussing the learning points from using Finite Element Methods to simulate the plug-assisted thermoforming of Polypropylene, which presents complex contact, thermal, friction and material modelling challenges. The paper makes recommendations as to the relative importance of these inputs in general terms with regard to correlating to experimentally gathered data. The paper also presents recommendations as to the approaches to be taken to secure simulation predictions of improved accuracy.

The Perils of Moviegoing in America, 1896-1950

The Perils of Moviegoing in America is a film history that examines the various physical and (perceived) moral dangers facing audiences during the first fifty years of film exhibition.

Chapter 1: “Conflagration”
As early as 1897, a major fire broke out at a film exhibition in San Francisco, with flames burning the projectionist and nearby audience members. From that point until the widespread adoption of safety stock in 1950, fires were a very common movie-going experience. Hundreds of audience members lost their lives in literally thousands of theatre fires, ranging from early nickelodeons to the movie palaces of the thirties and forties.

Chapter 2: “Thieves Among Us”
Bandits robbed movie theatres on hundreds of occasions from the early days of film exhibition through the end of the Great Depression. They held up ticket booths, and they dynamited theatre safes. They also shot theatre managers, ushers, and audience members, as a great many of the robberies occurred while movies were playing on the screens inside.

Chapter 3: “Bombs Away”
Bombings at movie theatres became common in small towns and large cities on literally hundreds of occasions from 1914 to the start of World War II. Some were incendiary bombs, and some were stench bombs; both could be fatal, whether due to explosions or to the trampling of panicked moviegoers

Chapter 4: “It’s Catching”
Widespread movie-going in the early 20th century provoked an outcry from numerous doctors and optometrists who believed that viewing films could do irreparable harm to the vision of audience members. Medical publications (including the Journal of the American Medical Association) published major studies on this perceived problem, which then filtered into popular-audience magazines and newspapers.

Chapter 5: “The Devil’s Apothecary Shops”
Sitting in the dark with complete strangers proved worrisome for many early filmgoers, who had good reason to be concerned. Darkness meant that prostitutes could easily work in the balconies of some movie theatres, as could “mashers” who molested female patrons (and sometimes children) after the lights were dimmed. That was all in addition to the various murderers who used the cover of darkness to commit their crimes at movie theatres.

Chapter 6: “Blue Sundays”
Blue laws were those regulations that prohibited businesses from operating on Sundays. Most communities across the US had such legislation on their books, which by the nickelodeon era were at odds with the thousands of filmgoers who went to the movies every Sunday. Theatre managers were often arrested, making newspaper headlines over and over again. Police sometimes even arrested entire film audiences as accomplices in the Blue Law violations.

Chapter 7: “Something for Nothing”
In an effort to bolster ticket sales, many movie theatres in the 1910s began to hold lotteries in which lucky audience members won cash prizes; by the time of the Great Depression, lotteries like “Bank Night” became a common aspect of the theatre-going enterprise. However, reception studies have generally overlooked the intense (and sometimes coordinated) efforts by police, politicians, and preachers to end this practice, which they viewed as illegal and immoral gambling.

The Films of Joseph H. Lewis