| Wood-plastic composite(WPC)materials are produced by adding 20%-85% of wood powders and other additives to the plastic substrate,and ultimately abtained by mixed granulation and extrusion.Comparing to wood and plastics,WPC materials have more advantages in quality,processability,corrosion resistance,flame retardancy,etc.Domestic WPC materials are processed by using ordinary plastic extruders which have been improved according to the technical personnel's experience.WPC materials are much more different from general plastics in processability,diathermancy and rheological property,also the basic issues of WPC rheological properties are lacking of in-depth study,which resulting in the production of wood plastic products showing a poor plasticizing,surface texture is not good,rice weight is not up to standard,broken tendons and many other shortcomings.At present,special machining equipment design and processing technology of WPC materials are lacking of theoretical guidance.Thus,taking the extrusion process of PVC wood-plastics materials as the research object,the rheological properties of WPC extrusion process have been studied systematically through theoretical modeling and numerical simulation based on the basic theory of polymer rheology and numerical simulation technology,and the correctness of the theoretical model and numerical simulation have been verified by experiments.The research results of this dissertation have provided theoretical basis and guidance for the single-screw extrusion equipment design and processing technology of WPC materials machining.The main research contents are as follows:(1)Based on the basic theory of polymer rheology,the rheological properties of wood-plastics two-phase polymer mixture and its important characterization parameter of viscosity have been focused on,which showing that the wood powder volume fraction is the main factor which affecting the melt viscosity in the WPC extrusion process.Based on the theoretical basis of mathematical models between polymer melts viscosity and different impact factors,combing with experimental phenomena and results,a viscosity mathematical model of the PVC wood-plastics two-phase mixture in the melt conveying zone of WPC extrusion process have been established.The variation trend of melt viscosity during extrusion process can be predicted by using the viscosity mathematical model,so as to provide theoretical guidance for numerical simulation and actual production of the PVC wood-plastics extrusion process,and avoid blind configuration of extrusion process parameters.(2)Melting zone and melt conveying zone flow model of PVC wood-plastics single-screw extruder have been established based on the complex rheological properties of melt in the WPC material extrusion process,also considering the factors such as material properties,fluidity of the melt,the geometry of the screw and channel,the processing conditions,etc.A rheological properties test of some formula PVC wood pellets have been carried out,the viscosity data of material at different shear rates and temperatures revealing that the material is a typical non Newtonian fluid,also certifying the correctness of the viscosity mathematical model.Then the fixed flow model of power-law model combined with Arrhenius approximate model have been used,and finally the rheological parameters of the formulation of wood pellets have been gotton through fitting by using POLYMAT module of POLYFLOW software,so as to provide flow models and rheological parameters of the material for the subsequent numerical simulation.(3)Based on the established mathematical model of melt viscosity,combing with experiments,the relevant process parameters have been configured.Results and variation of pressure field,temperature field,velocity field,liquid fraction,and enthalpy in melting zone of PVC wood-plastics single-screw extrude under different screw parameters and process conditions have been obtained by changing screw parameters or process conditions and using FLUENT software to solve.The variation laws of the flow field indexes such as enthalpy and viscous dissipation have been analyzed and discussed.(4)Based on the established mathematical model of melt viscosity and motion state equation of wood-plastics melt,combing with the relevant process parameters of experiments,boundary condition parameters such as volume flow have been configured.Results and variation of pressure field,temperature field,velocity field,and viscosity field in melt conveying zone of PVC wood-plastics single-screw extruder under different screw parameters and rotating speeds have been obtained by changing screw parameters or rotating speeds and using POLYFLOW software to solve.The variation law of the relationship between shear rate and melt viscosity under the condition of different rotating speeds(reflecting in shear rate)has been focused on.The qualitative and quantitative results of numerical simulation are in good agreement with the mathematical model,which have fully shown the theoretical value of the mathematical model of melt viscosity.(5)On the basis of the numerical simulation,melt temperature experiment,wood-plastics products forming experiment and rheological test of the PVC wood-plastics extrusion process have been carried out.PVC wood-plastics material temperature of different spiral section shapes at different time and location,the influence of different rotating speeds on the quality of wood-plastics products,the influence of different initial temperatures and rotating speeds on the PVC wood-plastics material melting process have been studied.It is found that the numerical simulation results are in good agreement with the experimental results,the model of partial relationship between PVC wood-plastics extrusion process and influencing factors have been certified,the reasonable values of some structure and process parameters have also been presented under the condition of a wood-plastics formula,which has provided exact theoretical guidance for processing equipment design and processing technology of WPC materials extrusion process. |
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