A. W. Birley, B. Haworth, A. O. Ola
May 27, 2013
A novel practical approach has been used to study the processability of a commercial grade of poly(ethylene terephthalate) during vacuum forming, in order to derive statistical models to relate some physical properties of containers to the thermomechanical nature of the process. Two-variable and three-variable Factorial Experimental designs were chosen to model the practical vacuum forming, and plug-assisted forming techniques, for process draw ratios of 1:1 and 2:1. Response measurements included container thickness distribution, and high temperature dimensional stability. Coefficients for a simple polynomial model have been experimentally determined: statistical correlations were apparent throughout the study, showing that the process environment determined the properties of vacuum formed containers through the morphological changes which occur by deforming in the thermoelastic region. Thickness distribution is improved when the plug assist mode is utilised. However, further optimisation is apparent when stress-induced crystallisation in PET is promoted (for example, when forming at a relatively low sheet temperature), especially for deep-drawn containers deformed beyond the natural draw regime. Whilst the temperature of the PET sheet primarily determines component thickness distribution, high-temperature stability is promoted by increases in the degree of crystallinity which occur when vacuum forming using super-ambient molding tool and plug temperatures.