Certain aspects of film blowing of low-density polyethylene

This article considers two topics: (1) Cooling of the film bubble and (2) Production of shrinkable film. Heat is transported by conduction, convection, and radiation, but only convection and radiation are relevant for the cooling results. Heat radiation can be calculated with known surface area, temperatures, and emission coefficient. Here we learn that the emission coefficient depends on the film thickness. Heat transfer by radiation amounts to about 15 percent of the total heat transfer. Heat transfer by convection normally can be calculated by known methods assuming ideal flow conditions. Because the flow conditions of the cooling air differ considerably from ideal conditions, known heat-transfer laws give incorrect results. Therefore, a new formula was set-up by determining the heat transfer conditions in real experiments. Each blown film is shrinkable, but in most cases the influence of the die on the shrinkage is disregarded. The die causes an important anisotropy in the elastic behavior of the film. Further on, shrinkage is influenced by the degree of bubble deformation, the temperature profile, and the deformation velocity between die exit and freeze line. This is discussed qualitatively. A calculations is not yet possible because of the unknown relaxation behavior of the melt. The shrinkage of an LDPE blown film depends on time and temperature. Time and temperature are exchangeable variables.