Rheological Modeling for Production of High Quality Polymeric
The fundamental defect inherent to the thermoforming technology is wall-thickness variation of the products due to inadequate thermal processing during production of polymer. A nonlinear viscoelastic rheological model is implemented for developing the process model. This model describes deformation process of a sheet in thermoforming process. Because of relaxation pause after plug-assist stage and also implementation of two stage thermoforming process have minor wall-thickness variation and consequently better mechanical properties of polymeric articles. For model validation, a comparative analysis of the theoretical and experimental data is presented. Keywords—High-quality polymeric article, Thermal Processing, Rheological model, Minor wall-thickness variation.
Wall thickness distribution in thermoformed food containers produced by a Benco aseptic packaging machine
Effects of process parameters such as forming temperature, forming air pressure and heating time on wall thickness distribution in plug-assist thermoformed food containers using multilayered material were investigated. Multilayered rollstock-base material formed into containers by thermoforming process using a Benco aseptic packaging machine. Forming temperatures in the range of 131-170°C, air-forming pressures of 2, 3, 3. 5 and 4 bars, and heating times of 66, 74, 84, 97 and 114 seconds were used in the thermoforming process. Analysis of wall thickness data obtained for the thermoforming parameters used in this study showed that wall thickness was significantly affected by forming temperature, pressure and heating time at 0.05 significance level. Besides the processing parameters, wall location, container side, and their interactions significantly affected wall thickness. Forming temperature was found to be the principle parameter influencing wall thickness distribution in a plug-assist thermoforming operation. The optimum operating conditions of the packaging machine for the thermoforming process are: 146-156°C for forming temperature, 2-4 bars for air-forming pressure and 74-97 seconds for heating time.
Elongational flow of polymer melts
Elongational flow experiments with viscous polymeric material were conducted at a constant elongation rate on a modified tensile testing machine. Steady elongational and shear viscosity data and elongational stress growth data are presented for an isobutylene-isoprene copolymer at 100°C. S-shaped elongational stress growth curves are reported for cis-1,4-polyisoprene at 80°C. 
 
The stresses necessary to maintain a cylindrical specimen in steady elongational flow are indicated by a calculation which includes the effects of inertia, gravity, and interfacial tension. 
 
Interrelations among several elongational flow material functions are given for a constitutive equation based on the theory of linear viscoelasticity.