Abstract
The main shrinkage and dimensional stability impact factors are processing temperature and pressure. For that reason, the pvT-behaviour is often used to estimate plastic part shrinkage. As there are still differences between predicted and actual part dimensions, a closer look was taken at these influencing factors. During processing, temperature changes occur due to heat conduction to the mold walls, and the pressure varies in the filling stage and holding time along the flow path. However, these pressure gradients are typically not taken into account in plastics processing. Hence, the influence of pressure was examined in detail and further investigations on pvT-behavior of polymers were performed.
For that purpose, fundamental examinations of the behavior of amorphous thermoplastics during cooling and compression were made. These include analysis in the different phases and at different compression speeds as well as variation of the pressure-temperature-cycles and their succession. It was found that the specific volume is not defined by one value of p and T, but is dependent on the pathway of the process. That applies for both thermal expansion and compressibility. The adiabatic compression heating was also examined and a numeric solution was found to easily adapt the pvT-results on the base of compressibility measurements. These findings are discussed by means of the free volume theory. Additionally, a choice of these investigations was performed with semi-crystalline thermoplastics. It was found, that compression heating is even more important for these polymers because it can superpose crystallization heating at appropriate parameters. Furthermore, it can be seen that the density, morphology and lamellae thickness is influenced by pressure. In the outlook of this paper, the impact of these results on the injection molding process is discussed in detail.
References
Breuer, H., Rehage, G., “Zur Thermodynamik der glasigen Erstarrung”, Kolloid Z. Z. Polym., 216–217, 159–179(1967), DOI: 10.1007/BF01525081Search in Google Scholar
Cohen, M. H., Crest, G. S.: “Liquid-Glass-Transition – a Free Volume Approach”, Phys. Rev. B, 20, 1077–1098(1979), DOI: 10.1103/PhysRevB.20.1077Search in Google Scholar
DIN 53 479:1990, „Prüfung von Kunststoffen und Elastomeren – Bestimmung der Dichte“, Deutsches Institut für Normen, Berlin (1990)Search in Google Scholar
Dlubek, G., et al., “The Free Volume in Two Untreated, Pressure-densified, and CO2 Gas Exposed Polymers from Positron Lifetime and Pressure-Volume-Temperature Experiments”, e-Polymers, 108(2007)10.1515/epoly.2007.7.1.1244Search in Google Scholar
Ehrenstein, G. W., et al.: Thermal Analysis of Plastics, Hanser Gardner Publications, Cincinnatti(2004)10.3139/9783446434141Search in Google Scholar
Engelsing, K.: „Einfluß des freien Volumens auf das verarbeitungsabhängige Deformationsverhalten spritzgegossener amorpher Thermoplaste“, PhD Thesis, University of Technology, Chemnitz, Germany (2000)Search in Google Scholar
Gilmour, I. W., et al., “The Thermoelastic Effect in Glassy Polymers”, J. Polym. Sci., 16, 1277–1290(1978)Search in Google Scholar
Hellwege, K. H., et al., „Die isotherme Kompressibilität einiger amorpher und teilkristalliner Hochpolymerer im Temperaturbereich von 20–250°C und bei Drücken bis zu 2000 kp/cm2“, Kolloid Z. Z. Polym., 183, 110–120(1962), DOI: 10.1007/BF02657207Search in Google Scholar
Hirai, N., Eyring, H.: “Bulk Viscosity of Polymeric Liquids”, J. Polym. Sci., 37, 51–70(1959), DOI: 10.1002/pol.1959.1203713104Search in Google Scholar
Hirschfelder, J., et al., “A Theory of Liquid Structure”, J. Chem. Phys., 5, 896–912(1937), DOI: 10.1063/1.1749960Search in Google Scholar
ISO 17 744:2004, “Plastics – Determination of Specific Volume as a Function of Temperature and Pressure (pvT – Diagramm – Piston Apparatus Method”, International Organization for Standardization, Geneva(2004)Search in Google Scholar
ISO 11 357-1:2009, “Plastics – Differential Scanning Calorimetry (DSC) – Part 1”, International Organization for Standardization, Geneva(2009)Search in Google Scholar
Kazmierczak, T., et al., “Plastic Deformation of Polyethylene Crystals as a Function of Crystal Thickness and Compression Rate”, Polymer, 46, 8926–8936(2005), DOI: 10.1016/j.polymer.2005.06.073Search in Google Scholar
van Krevelen, D. W., “Cristallinity of Polymers and the Means to Influence the Crystallization Process”, Chimia, 32, 279–294(1978)Search in Google Scholar
Kuenkel, R., “Auswahl und Optimierung von Kunststoffen für tribologisch beanspruchte Systeme”, PhD Thesis, Erlangen, University of Erlangen-Nuremberg, Erlangen, Nuremberg, Germany (2005)Search in Google Scholar
Menges, G., et al., „Das physikalische Verhalten von Thermoplasten bei der Aufnahme von pvT-Diagrammen unter verschiedenen Messbedingungen“, Plastverarbeiter, 28, 632–637(1977)Search in Google Scholar
Moneke, M., “Die Kristallisation von verstärkten Thermoplasten während der schnellen Abkühlung und unter Druck”, PhD Thesis, University of Darmstadt, Darmstadt, Germany (2001)Search in Google Scholar
Rudolph, N., et al., “Pressure Solidification of Amorphous Thermoplastics”, Polym. Eng. Sci., 49, 154–161(2009)10.1002/pen.21234Search in Google Scholar
Rudolph, N., “Druckverfestigung amorpher Thermoplaste”, PhD Thesis, University of Erlangen-Nuremberg, Erlangen (2009)Search in Google Scholar
Rudolph, N., Kuehnert, I.; “High Precision Parts by Compression Induced Solidification (CIS)”, Proceedings of the Polymer Processing Society 25th Annual Meeting, Goa, India (2009)Search in Google Scholar
Schmidt, M., et al., “Macroscopic Pressure–Volume–Temperature Properties versus Free Volume Characteristics of Isotropic Pressure-densified Amorphous Polymer Glasses”, J. Chem. Phys., 112, 11095–11106(2000)., DOI: 10.1063/1.481748Search in Google Scholar
Turnbull, D., Cohen, M. H., “Molecular Transport in Liquids and Glasses”, J. Chem. Phys., 34, 1164–1169(1961)Search in Google Scholar
Vretas, J. S., Duda, J. L., “Diffusion in Polymer – Solvent Systems. I. Reexamination of the Free-Volume Theory”, J. Polym. Sci., Part B: Polym. Phys., 15, 403–416(1977)10.1002/pol.1977.180150302Search in Google Scholar
Vretas, J. S., Duda, J. L., “Diffusion in Polymer – Solvent Systems. II. A Predictive Theory for the Dependence of Diffusion Coefficients on Temperature, Concentration, and Molecular Weight”, J. Polym. Sci., Part B: Polym. Phys., 15, 417–439(1977)10.1002/pol.1977.180150303Search in Google Scholar
Vretas, J. S., Duda, J. L., “A Free Volume Interpretation of the Influence of the Glass Transition on Diffusion in Amorphous Polymers”, J. Appl. Polym. Sci., 22, 2325–2339(1978), DOI: 10.1002/app.1978.070220823Search in Google Scholar
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