| In polymer multi-lumen micro tubes extrusion field, such as the manufacturing of medicine interventional catheters, ultra fine coaxial cable and plastic micro optical fiber, unavoidable parison deformation exhibits as a joint result of the cross-scale viscoelastic flow and unconfined cooling shrinkage. The parison deformation increases the design difficulty of extrusion die and leads to product quality defects, thus it is one of the key problems in micro tube extrusion technology. Because the theories of polymer micro tube extrusion flow and cooling are immature, researches on the mechanism and rule of parison shape and size deformation during micro tube extrusion are limited. Due to lack of these understandings of the polymer micro extrusion process, the design of micro extrusion die has no theoretical basis and the quality of micro extrusion products is hard to guarantee, which cannot satisfy the application demands in relative sectors. Therefore, the focus of this thesis is the extrusion process of polymer multi-lumen micro tube. The parison deformation mechanism and regularity during extrusion process are explored on the basis of mathematical models proposed for polymer cross-scale extrusion flow and unconstrained cooling. Pre-compensation design method of micro tube extrusion die is established based on the parison deformation control. The primary items in this work are as follows:Firstly, the theoretical models of cross-scale viscoelastic flow of polymer micro tube extrusion are established based on the characteristics of the micro extrusion flow. The influence of micro-scale effect on the rheological measurement is analyzed. Measuring method for micro-scale viscosity is proposed adopting capillary rheometer, and the viscosity variations of four kinds of polymers are obtained under certain shear rate and dimension ranges. A cross-scale viscosity model characterizing the viscosity variation from macro scale to micro scale is proposed according to the effect of characteristic dimension of the flow channel on polymer rheological property by introducing a size factor. A viscoelastic constitutive equation is proposed to take into account the cross-scale viscosity effect. It is shown that:with the decrease of flow channel characteristic dimension from 1000 ?m to 350 ?m, the average viscosity decrease of four polymers are from 9.9% to 38.3%; the proposed cross-scale viscosity model is capable of describing the viscosity variation within the dimension range involved in micro tube extrusion.Secondly, the fields distribution inside and outside the extrusion flow channel as well as deformation regularity of parison shape and size during micro tube extrusion flow is investigated by numerical analysis method adopting the proposed mathematical models. The mechanism of parison deformation under the conjoint effect of cross-scale viscous flow and elastic recovery is revealed. The effects of material properties, parameters of extrusion die and forming processes on the parison deformation are analyzed. It is shown that, obvious deformation along the axial and radial direction of the parison exhibits in multi-lumen micro tube extrusion. The cross-scale viscoelastic properties of polymer melt have significant influence on the extrudate swell. The obvious swelling outside the die is resulted from elastic recovery; while deformation is due to the viscous flow. Micro-scale effect increases the degree of extrudate swell and decreases the profile deformation. Among the material properties, the increase of second normal stress difference coefficient will restrain the profile swell but enhance the thickness swell, while the branching degree has an opposite effect. The thickness swell ratio decreases with the increase of molecular entanglement degree. The cross section parameters of die land have momentous effects on the parison deformation. Parison deformation parameters show nonlinear variations with axial asymmetry control parameters of the extrusion die. The profile deformation and thickness deformation are caused by the velocity redistribution outside the die leaded by the unbalanced axial velocity inside the die and the radial velocity component resulted from the axial velocity redistribution respectively. Among the process parameters, the condition of slight air injection has an obvious effect on the parison. Reasonable air injection pressure is helpful in improving the parison profile when thicknesses of the parison meet the requirement. The increase of screw speed will enhance the swelling degree and anisotropy. The transient swell plays a leading role at low flow rates; while the contribution of delayed swell to the total swell increases with increasing flow rate. An extrudate swell measuring method combining real-time photography and cooled example measurement is proposed for multi-lumen micro parisons. The reliability of the proposed theoretical models is proved by extrudate swell experiments. The difference between simulation and experimental results is less than 6.3%. It is proved that the proposed mathematical models can quantitatively predict the parison deformation.Thirdly, the mathematical models for the cooling process of multi-lumen micro tube extrusion are established based on the heat transfer theory. Convection coefficients between the parison surface and the cooling medium are calculated. The dependency of thermal parameters of polypropylene on temperature is measured by P-V-T experiments. The temperature distribution and shrinkage of double-lumen and five-lumen micro tubes during cooling stage are analyzed by the finite element method, as well as the effect of haul-off speed on this stage. Results show that the cooling of micro tube parison finishes in a very short time. The cooling length of micro tube parison is much shorter than that is required for conventional tube extrusion. The temperature distribution is uneven during cooling stage and the phase transition of different part of the parison cross section is not synchronized. The cooling time needed changes nonlinearly with the parison thickness, which is also influenced by the parison shape. The increase of haul-off speed reduces the cooling time needed but gives up the temperature distribution uniformity. The impact is weakened with the increase of haul-off speed. Since parison is cooled without external constraint, the cooling shrinkage of the cross section is uniform. However, large residual stress is left in the cooled product because of the constraint of parison internal regions on the connected regions resulted from uneven thermal deformations of connected regions of the parison during the cooling stage.Finally, a pre-compensation design method for micro tube extrusion die is presented on the basis of parison deformation compensation design philosophy and the above obtained parison deformation regularity during micro extrusion flow, swell, cooling and haul-off stages. Design principles, pre-compensation design processes and calculation method of compensation value of micro tube extrusion die are proposed. Double-lumen tube with uniform thickness and five-lumen tube with nonuniform thickness are selected as design examples. The extrusion dies are designed by the conventional method and the proposed method respectively. The extrusion dies are manufactured according to the flow channel design, and extrusion experiments are conducted. Experimental results show that, during the design of die land cross section of micro tube extrusion die, conventional method is lack of theoretical basis. The profile and thickness defects of the product cross section exhibit, which cannot be improved by process parameter adjusting. While the shape and dimension variation of parison during the extrusion process can be controlled through a quantitative calculation of the compensation values of die land cross section geomety. Satisfactory products can be obtained using the extrusion die designed by the proposed method through slight injection process parameter adjustment, which validates the feasibility and effectiveness of the proposed method. |
PDF Full Text Request