Theoretical And Experimental Study On The Rheological Behaviors Of Polyethylene Melts In Abrupt Contraction Die By Birefringence

Extrusion molding, as one main method of plastic forming, plays an important role inplastic industry, by which almost all thermoplastics can be processed. Electromagneticdynamic plastics extruder introduces the vibration force field into the whole process ofpolymer extrusion for the first time, and the vibration force field has a significant impact onthe rheological behavior of polymer melts in the whole forming process.It is instructive, for the reasonable decision of process parameters and the preparation ofproducts with excellent mechanical performance, to master the knowledge of the rheologicalbehavior of polymer melt in the die. So, to give a real characterization of the flow of polymermelts in the die, to master the knowledge of the response property of polymer melts to thevibration force field are of vital important scientific and practical significance to study theforming mechanism of the electromagnetic dynamic plastics extrusion.First, the basic principles of the method of flow induced birefringence to study therheological properties of polymer melts are introduced, and the scientific significance of thestress-optic coefficient is introduced, the method to calculate the stress-optic coefficient isintroduced as well.Secondly, the rheological properties of the three used commercial polyethylene materialsin the experiments were characterized. The melt rheological properties in steady shear floware measured by a high pressure capillary rheometer and the extensional viscosity of theLDPEs were calculated using the famous Cogswell model; the molecular weight and itsdistribution of the three materials are determined by the application of a high-temperature gelchromatography; the rheological properties of three in the small oscillatory shear flow aremeasured by the use of a rotational rheometer, the discrete relaxation time spectra of fivemodes are calculated through the nonlinear fitting method making use of the Matlab programpackage, which would be used in the next numerical simulation.Thirdly, a mathematical model is built to calculate the entrance length and the entrancelengths are calculated from the steady extrusion experimental data. At the same time, theentrance length at different processing conditions are measured by the application ofbirefringence measurement equipment, useful data will be provide for the optimization of forming parameters and of the designation of the die lip. It is proved by the comparison of thecalculted and measured entrance length that, this mathematical model is able to be used topredict the entrance length from the data of flow rate and the relaxation time of the material.The transient extensional viscosities were obtained from the calculated extensional stress byflow birefringence fringes and the measured velocity by PIV along the symmetry axis of thecontraction die.Subsequently, the flow induced birefringence experiments are performed on the vibrationextruder, which equipped with an abrupt contraction die. The flow birefringence fringes areobtained during the extrusion process, the pressure drops along the die symmetry axis aremeasured by applying pressure sensors, the flow rate are measured by catch and measuremethod. The birefringence fringes are obtained in a variation of vibration conditions, byswitching the vibration frequency and vibration amplitude, the corresponding stress state ofthe flow field are analyzed, including shear stress and first normal stress difference, theinfluence of vibration parameters on the stress distribution of the flow field, on the extrusionpressure and flow rate are discussed semi-quantitatively.In the end, a novel flow induced birefringence method is proposed on the basis of theconventional equipment. The birefringence fringes obtained from the conventional equipmentcan not be used to calculate the shear stress of the flow field for the lack of the information ofthe orientation angle. Here the obtained pictures contain information not only of isoclinic butalso of isochromatic by rearranged the optic components, which can be used to calculate theshear stress directly.