Verarbeitungsinduzierte Veränderung der physikalischen Struktur und daraus resultierend der mechanischen Eigenschaften von spritzgegossenen Bauteilen aus teilkristallinen Thermoplasten
Final Report Abstract
The processing-structure-property response of injection molded semicrystalline thermoplastics has been investigated. The effect of the complex thermorheological history during processing has been studied on three semicrystalline thermoplastics, viz. PP, PBT and sPS. The results show that processing conditions have a strong influence on the creep behavior of the injection molded semicrystalline thermoplastic parts. The deformation behavior can be successfully explained by the influence of free volume fraction as well as crystallinity content of the finished part that results from different rates of cooling during injection molding process. Thus a change in the mold temperature shows the largest influence on creep behavior of the injection moldings because it influences the rate of cooling remarkably and hence free volume as well as crystallinity, whereas a change in melt temperature shows less pronounced effect as it does not influence the cooling rate significantly. A change in the injection flow rate showed a negligible effect on creep which indicates that the frozen- in molecular orientations have no significant effect on the creep behavior of the injection moldings. A difference in the holding pressure also shows the same effect as that of the injection flow rate. But the results of structure analysis showed that the semicrystalline polymers showed differences in the specific volume when the holding pressure was changed although the creep behavior varied negligibly. This indicates that the specific volume measurement alone is not adequate to characterize the molecular packing density in the case of injection molded items. But this difference in specific volume could be attributed to the effect of packing pressure on the densification of the moldings that hardly affected the free volume fraction. By performing the water absorption tests, it was affirmed that the change in holding pressure has only a negligible effect on the free volume fraction. This suggested that the pressureinduced densification has a very small effect on the creep behavior. An effort to study the anisotropic behavior of the injection molded items showed a variation in the creep strain across the cross-section of the molded part which could be attributed to the gradient in the internal structure produced as a result of injection molding. This further supports the fact that injection molding results in anisotropic structures and properties which varies not only along the length of the molded part but also along the part thickness. The results also showed that when no significant effect is observed in terms of short-term mechanical properties by changing the processing conditions, but while considering the long-term behavior they show a significant effect. Moreover, the results also indicate that the effect of physical aging cannot be neglected in the case of injection molded semicrystalline thermoplastics because it affects the viscoelastic properties of the thermoplastic materials by reducing the tendency to creep as a result of the decrease in free volume with increase in aging time. Thus annealing below Tg showed minimum creeping as the material tries to reach its thermodynamically equilibrium state faster after annealing. Finally, it can be concluded that the deformation behavior of an injection molded semicrystalline thermoplastic part is governed by the free volume as well as by crystalline fraction and it is very difficult to distinguish between the effects of these two factors in determining the influence of processing conditions on the deformation behavior since both of them influence the relaxation time of the polymer in the same way which ultimately determines the deformation behavior of the molded part.
Publications
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G. Mennig; K. Banik: Physical Ageing of Injection Moulded Thermoplastics, Thiruvantapuram, India, 16.12. 2004
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K. Banik: Kriechverhalten von spritzgegossenen teilkristallinen Kunststoffen, Institut für Allgemeinen Maschinenbau und Kunststofftechnik, Technische Universität Chemnitz, Chemnitz, Germany, 22. 08. 2005
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K. Banik: Verarbeitungsinduzierte physikalische Struktur und daraus resultierend das Deformationsverhalten von spritzgegossenen Bauteilen aus teilkristallinen Thermoplasten, Lehrstuhl für Polymerwerkstoffe, Universität Erlangen- Nürnberg, 10. 06. 2005
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K. Banik; G. Mennig, Influence of physical aging and processing conditions on the mechanical properties of Semicrystalline PBT, Proceedings of conference on Polymeric Materials, Halle, Germany (2004)
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K. Banik; G. Mennig, Physical ageing of injection moulded thermoplastics, Proceedings of "Macro 2004" Conference, Thiruvantapuram, India (2004)
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K. Banik; G. Mennig: Influence of Physical Aging and Processing Conditions on the Mechanical Properties of Semicrystalline PBT, Halle, Germany, 30.09.2004
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K. Banik; G. Mennig: Influence of the injection molding process on the creep behavior of semicrystalline PBT during aging below its glass transition temperature, Mechanics of Time-Dependent Materials, 9 (2006) 247 - 257
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K. Banik; G. Mennig: Kriechverhalten von teilkristallinen Kunststoffen, Institut für Allgemeinen Maschinenbau und Kunststofftechnik, Technische Universität Chemnitz, Chemnitz, Germany, 24.05 2004
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K. Banik; G. Mennig: Process-induced long-term deformation behavior of semicrystalline PBT, Polymer Engineering and Science, 46 (2006) 882-888
