Factors of influence of cooling systems on plastic films and boards
Stefan Seibel, Andreas Limper
Institut fuer Kunststofftechnik
NRW

Keywords: cooling, calibrating, film quality

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The cooling processes have a decisive influence on the quality of thermoplastic products. This is especially true of extruded flat films and boards, where the thermal history of the extrudate during the cooling process strongly influences quality-critical morphological parameters (degree of crystallinity, structure of partially crystalline materials and internal strain). This is why it is important for machine manufacturers and for plastics processors to analyse and understand exactly how the material cools down as it moves through the calibrating and cooling system. The effects of anisotropic material structures in the sheets affect the subsequent process lastingly.
For this reason we want to present our theoretical and experimental investigations, of the process behaviour in different cooling systems in dependence of processing and material parameters.

As basis for the theoretical description a one-dimensional finite differences model is used. Input for the calculation consists of data for the material (density, thermal conductivity and specific heat capacity) and temperature data that describe the existing or planned cooling zone (e.g. temperature of cooling roll surfaces, heat transfer coefficient). By varying thickness coordinates, it is possible to compare cooling profiles at the extrudate surfaces and in the core.
The experimental attempts were accomplished on a planet polishing stack with a steel belt unit. By the variability of this plant it was possible to reproduce most diverse calibrating units and cooling systems, to analyse them. The developed settings are experimentally examined and compared with the theoretical description.

The results can help to clarify the effect of individual processing parameters, machine settings and environmental factors (e.g. cooling roll temperature and position, heat transfer, ambient temperature) on the cooling process.