| Plug assist thermoforming is a widely used process for fabrication of cups, containers, and other high volume items. In this study the influence of plug design, namely plug volume, plug taper, plug depth and plug temperature on the wall thickness distribution, weight and compression strength in thermoformed polypropylene cups was investigated. The variables studied included plug volumes of 60%, 75% and 90% of the mold volume, plug tapers of 0, 3, and 6 degrees, plug depths of 77%, 85%, and 92% of the mold depth, and plug temperatures of 75°C, 90°C, and 105°C. The Box-Behnken design was selected for design of experiments. Both vacuum and pressure were used in the thermoforming process. It was observed that the plug volume was the most importance factor for the formed part. As plug volume was increased, the part thickness, part weight, and the compression strength increased. Plug depth had a significant effect on the bottom and corner thicknesses and part weight. When plug depth was increased, the bottom, corner thickness and the part weight increased. However, the plug depth had no effect on the wall thickness and the compression strength of the formed part. Plug temperature and plug taper had a significant effect only on the compression strength. The interaction between plug volume and plug depth had a significant effect on the bottom and corner thicknesses. Good correlation between the experimental and a computer simulation results were obtained at plug temperatures 90°C and 105°C, while the experimental and simulation results did not match at a plug temperature of 75°C. |
