Extrusion 8-2016

The history of the polypropylene polymers started in 1954 with the researchers Giulio Natta and Karl Ziegler. This plastic material produced for the first time in the Italian industry by Montecatini (then Montedison) has known a huge success since the very beginning. A mong the many applications of polypropylene, today this product has become increasingly more important for the food packaging industry and particu- larly for disposable cups. Before the thermoforming process, the material is ex- truded, calendered, cooled and transformed into rolls. The width and thickness of the sheet may range from 0.2 mm to 2 mm. During the production process, the material is being unwind and then fed into the thermoforming machine through guides with toothed chains passing the infra- red heating oven in order to obtain the necessary softness be- fore entering the forming station. During the transformation of the Polypropylene, it is necessary to take into account the two main mechanical characteristics of this polymer such as the: • Coefficient of thermal expansion • Specific heat coefficient of the material The polypropylene has a thermal expansion coefficient three times bigger and a specific temperature two times higher than APET. Consequently, to process the polypropylene, it is impor- tant to pay attention to the heating process and slackening of PH Series Pre-Heaters for Polypropylene Sheet the internal tensions of the material in order to obtain a stable and qualitative molding process. As we deal with a material coming from an extrusion and calendering process, the poly- propylene re-heated for a second time, has the tendency to slack. The material, which is retained in the chains teeth, fre- quently may show undulations that may be difficult to uneven and therefore creating problems in distributing the material during the forming process affecting the esthetics of the final product. In order to avoid these drawbacks, it is necessary to preheat the foil to allow the "stress relaxation" of the material before placing it on the transport chains of the thermoforming machine and consequently obtaining a homogenous tempera- ture in the core of the sheet through a slow but uniform heat- ing process. 44 Thermoforming Extrusion 8/2016 Picture 1 Pictures 2 to 4