CN113990413A - Simulation method and system for establishing relation between polyethylene chain structure and extrusion processing performance - Google Patents
Simulation method and system for establishing relation between polyethylene chain structure and extrusion processing performance Download PDFInfo
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Abstract
The invention discloses a simulation method and a system for establishing a relation between a polyethylene chain structure and extrusion processability, wherein the simulation method comprises the following steps: distributing charges to the designed polyethylene molecular chains and carrying out structural optimization; constructing an initial periodic boundary amorphous model by using the optimized polyethylene molecular chains; optimizing the initial periodic boundary amorphous model; sequentially carrying out structural optimization, a dynamic balance process and parameter calculation on the optimized periodic boundary amorphous model; and calculating the processing characteristics of the homogenizing section of the single-screw extruder corresponding to the designed polyethylene molecular chain by utilizing the simulation parameters of the polyethylene chain structure in the finite element simulation model of the homogenizing section of the single-screw extruder with pre-established and preset conditions. The invention avoids unnecessary experiment consumption and errors by means of a computer simulation technology, and provides a new method for improving the extrusion processing performance of the low-density polyethylene cable insulating material from the perspective of molecular chain structure design.
Description
Technical Field
The invention belongs to the technical field of computer simulation of high polymer insulating materials, and particularly relates to a simulation method and a simulation system for establishing a relation between a polyethylene chain structure and extrusion processability.
Background
With the rapid development of advanced technologies such as extra-high voltage transmission, clean energy and the like, higher requirements are put forward on the insulation performance of the high-voltage cable. Polyethylene, especially low density polyethylene, is used as a main base material for preparing high voltage cable insulation, and the extrusion processability of the polyethylene determines the final insulation performance of the material. Due to the complexity of the manufacturing process of low density polyethylene, the chain structure, including the molecular weight and the degree of branching, has a distribution of varying degrees. However, the molecular chain structure is a key factor affecting the processability of low density polyethylene. At present, the characterization modes of the polyethylene molecular chain structure at home and abroad are numerous, but the relationship between the chain structure and the extrusion processability cannot be accurately characterized only by using the traditional experimental method, so that the research and development of high-performance polyethylene are severely limited.
Disclosure of Invention
In order to solve the problems in the prior art, the invention aims to provide a simulation method and a simulation system for establishing a relation between a polyethylene chain structure and extrusion processability.
The technical scheme adopted by the invention is as follows:
the simulation method for establishing the relation between the polyethylene chain structure and the extrusion processing performance comprises the following processes:
distributing charges to the designed polyethylene molecular chains and carrying out structural optimization to obtain the optimized polyethylene molecular chains;
constructing an initial periodic boundary amorphous model using the optimized polyethylene molecular chains, wherein the density of the initial periodic boundary amorphous model is set to be lower than the actual density of the polyethylene material;
optimizing the initial periodic boundary amorphous model, wherein the optimization sequentially comprises structural optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain an optimized periodic boundary amorphous model;
sequentially carrying out structural optimization, a dynamic balance process and parameter calculation on the optimized periodic boundary amorphous model to obtain simulation parameters of a polyethylene chain structure;
calculating the processing characteristics of the homogenizing section of the single screw extruder corresponding to the designed polyethylene molecular chain by utilizing the simulation parameters of the polyethylene chain structure according to a finite element simulation model of the homogenizing section of the single screw extruder with pre-established and preset conditions; wherein the condition setting of the finite element simulation model of the homogenizing section of the single-screw extruder comprises the following steps: selecting a constitutive equation which respectively represents the dependence of the fluid viscosity on the shearing rate and a constitutive equation which represents the dependence of the fluid viscosity on the temperature in a finite element simulation model of a single-screw extruder homogenizing section, and setting the flow boundary condition and the temperature boundary condition of a flow passage part in the pre-established finite element simulation model of the single-screw extruder homogenizing section according to the actual processing condition of the polyethylene single-screw extruder.
Preferably, when the initial periodic boundary amorphous model is subjected to dynamic equilibrium, a regular ensemble is selected, the temperature is set, the temperature and the volume are kept unchanged, and the preset time is executed.
Preferably, when annealing the initial periodic boundary amorphous pattern, an annealing temperature and pressure are set, an isothermal and isobaric ensemble is selected, and the number of annealing cycles and the execution time of each annealing are set.
Preferably, when the initial periodic boundary amorphous model is subjected to secondary dynamic equilibrium, a regular ensemble is selected, temperature and pressure are set, temperature and volume are kept unchanged, and preset time is executed.
Preferably, the condition for performing structural optimization on the optimized periodic boundary amorphous model is the same as the condition for performing structural optimization on the initial periodic boundary amorphous model; the conditions for performing the dynamic balance process on the optimized periodic boundary amorphous model are the same as the conditions for performing the dynamic balance and the secondary dynamic balance on the initial periodic boundary amorphous model.
Preferably, when the optimized periodic boundary amorphous model is subjected to parameter calculation, a reverse non-equilibrium dynamics method is used for parameter calculation, and the obtained simulation parameters of the polyethylene chain structure comprise a heat conductivity coefficient, an isobaric heat capacity, a density, a zero-shear viscosity and a relaxation time.
Preferably, the constitutive equation which represents the dependence of the fluid viscosity on the shear rate in the finite element simulation model of the homogenizing section of the single-screw extruder adopts a Bird-Carreuu model, and the constitutive equation which represents the dependence of the fluid viscosity on the temperature in the finite element simulation model of the homogenizing section of the single-screw extruder adopts an Approximate Arrhenius law;
when the flow boundary condition of a flow channel part in a finite element simulation model of a homogenizing section of the single-screw extruder is set, the inlet of the flow channel is set to be free flow, the outlet of the flow channel is set to be normal stress and is controlled by pressure, an evolution algorithm is adopted, the inner wall of the flow channel is at Cartesian rotating speed and is consistent with the rotating speed of a screw, and the outer wall of the flow channel does not slide;
when the temperature boundary condition of the runner part in the finite element simulation model of the homogenizing section of the single screw extruder is set, the inlet of the runner is set to be at a fixed temperature, the outlet of the runner is free fluid, the inner wall of the runner is heat insulation, and the outer wall of the runner is at a fixed temperature.
The invention also provides a system for establishing the relation between the polyethylene chain structure and the extrusion processability, which comprises the following steps:
a first optimization module: the method is used for distributing charges to the designed polyethylene molecular chains and carrying out structural optimization to obtain the optimized polyethylene molecular chains;
initial periodic boundary amorphous model building blocks: for constructing an initial periodic boundary amorphous model using the optimized chains of polyethylene molecules, wherein the density of the initial periodic boundary amorphous model is set to be lower than the actual density of the polyethylene material;
a second optimization module: the method is used for optimizing the initial periodic boundary amorphous model, and the optimization sequentially comprises structural optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain the optimized periodic boundary amorphous model;
an optimization calculation module: the system is used for sequentially carrying out structural optimization, a dynamic balance process and parameter calculation on the optimized periodic boundary amorphous model to obtain simulation parameters of a polyethylene chain structure;
a simulation calculation module: the finite element simulation model of the single screw extruder homogenizing section which is pre-established and has preset conditions is used for calculating and obtaining the processing characteristics of the single screw extruder homogenizing section corresponding to the designed polyethylene molecular chain by using the simulation parameters of the polyethylene chain structure; wherein the condition setting of the finite element simulation model of the homogenizing section of the single-screw extruder comprises the following steps: selecting a constitutive equation which respectively represents the dependence of the fluid viscosity on the shearing rate and a constitutive equation which represents the dependence of the fluid viscosity on the temperature in a finite element simulation model of a single-screw extruder homogenizing section, and setting the flow boundary condition and the temperature boundary condition of a flow passage part in the pre-established finite element simulation model of the single-screw extruder homogenizing section according to the actual processing condition of the polyethylene single-screw extruder.
The present invention also provides an electronic device, comprising:
one or more processors;
a storage device having one or more programs stored thereon;
when executed by the one or more processors, cause the one or more processors to implement the simulation method of establishing a polyethylene chain structure versus extrusion processability relationship as described above.
The invention also provides a storage medium on which a computer program is stored, wherein the computer program, when executed by a processor, implements the simulation method for establishing the relationship between the polyethylene chain structure and the extrusion processability as described above.
The invention has the following beneficial effects:
in the simulation method for establishing the relation between the polyethylene chain structure and the extrusion processability, the designed polyethylene molecular chain has the characteristic of flexible and accurate structure, the design of the polyethylene molecular chain, the establishment and optimization process of the periodic boundary amorphous model, the calculation method of the intrinsic parameters of the model and the finite element simulation conditions are basically consistent with the reality, and the accuracy, the reliability and the universality of the calculation are ensured. The molecular simulation technology and the finite element simulation method are coupled through the intrinsic characteristic parameters of the material, so that the relationship between the polyethylene molecular chain structure and the extrusion processing performance can be accurately provided, and the consumption and the error of an actual experiment are avoided. The invention can accurately describe the relationship between the molecular chain structure and the material performance by utilizing molecular simulation, can describe the relationship between the material characteristics and the extrusion processing performance by utilizing finite element simulation, and can effectively avoid unnecessary experimental loss and errors by utilizing a simulation technology. The invention can be applied to the field of polymer insulating computer simulation, and has great use value for accurately establishing the relevance between the chain structure of the polymer material and the extrusion processing performance.
Drawings
FIG. 1 is a polyethylene molecular chain of example 1 of the present invention, having a degree of polymerization of 60 and a degree of branching of 0.2;
FIG. 2 is a periodic boundary amorphous model of polyethylene having a degree of polymerization of 60 according to example 1 of the present invention;
FIG. 3 is a temperature profile of polyethylene of example 1 of the present invention in the homogenizing zone of a single screw extruder;
FIG. 4 is a shear viscosity profile of polyethylene of example 1 of the present invention in a single screw extruder homogenization section.
FIG. 5 is a polyethylene molecular chain of example 2 of the present invention, having a degree of polymerization of 90 and a degree of branching of 0.2;
FIG. 6 is a periodic boundary amorphous model of polyethylene with a degree of polymerization of 90 according to example 2 of the present invention;
FIG. 7 is a temperature profile of polyethylene in the homogenizing section of a single screw extruder according to example 2 of the present invention;
FIG. 8 is a shear viscosity profile of polyethylene in the homogenizing section of a single screw extruder according to example 2 of the present invention.
Detailed Description
The embodiments of the present invention will be described in further detail with reference to the accompanying drawings and examples:
the invention relates to a simulation method for establishing a relation between a polyethylene chain structure and extrusion processability, which takes polyethylene molecular chains with different molecular weights as an example, designs two polyethylene molecular chains with the same branching degree and different molecular weights through molecular simulation software, strictly controls the structure optimization and dynamic balance processes of a model, calculates material parameters of periodic boundary amorphous models corresponding to the two polyethylene molecular chains, takes the material parameters calculated by molecular simulation as input parameters of finite element simulation, and ensures that the geometric parameters and the boundary conditions of a homogenizing section of a single-screw extruder are consistent.
Specifically, the simulation method for establishing the relationship between the polyethylene chain structure and the extrusion processability comprises the following steps:
1) designing a polyethylene molecular chain by using Materials Studio (MS) software, distributing charges to the polyethylene molecular chain and carrying out structural optimization on the polyethylene molecular chain, and constructing an initial periodic boundary amorphous model by using the optimized polyethylene molecular chain, wherein the density of the initial periodic boundary amorphous model is set to be lower than the actual density of a polyethylene material; the chain structure of the polyethylene molecular chain, including the molecular weight and the branching degree, can be accurately regulated by using MS software.
2) Optimizing the initial periodic boundary amorphous model in the step 1) in MS software, wherein the optimization comprises structure optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain an optimized periodic boundary amorphous model; the method comprises the following steps of controlling the structural optimization and the dynamic optimization processes of different initial periodic boundary amorphous models by using MS software, wherein the simulation temperature and pressure settings are the same as the processing conditions of a homogenizing section of an actual single-screw extruder;
3) introducing the periodic boundary amorphous model optimized in the step 2) into Large-scale atomic/molecular mapping parameter simulator (LAMMPS) software, then performing structure optimization and dynamic balance processes on the model in the LAMMPS software, keeping the optimization conditions consistent with the step 2), performing parameter calculation on the optimized periodic boundary amorphous model by using a reverse non-equilibrium dynamic method, wherein the calculated parameters comprise a heat conductivity coefficient, an isobaric heat capacity, a density, a zero-cut viscosity and a relaxation time;
4) respectively establishing a geometric model of a screw and a flow channel of a homogenizing section of the single screw extruder, respectively carrying out grid unit division on the geometric models of the screw part and the flow channel part by using a finite element method, and then carrying out grid unit combination on the screw and the flow channel part of which the grid units are divided by using a grid overlapping technology; the length of the runner part is 5-15mm greater than that of the screw at the inlet and the outlet, the screw part uses tetrahedral units to divide the grid units, and the runner part uses hexahedral units to divide the grid units;
5) inputting the periodic boundary amorphous model parameter calculation result in the step 3) as a material parameter of finite element simulation, selecting a constitutive equation to respectively express the dependence of fluid viscosity on a shearing rate and the dependence of fluid viscosity on temperature, and setting a flow boundary condition and a temperature boundary condition of a flow channel part of a homogenizing section of a single-screw extruder according to the actual processing condition of the polyethylene single-screw extruder, wherein the flow boundary condition and the temperature boundary condition of the flow channel part and the rotating speed and the rotating direction of the screw part are consistent with the actual processing condition of the polyethylene single-screw extruder; and defining the screw part of the homogenizing section of the single screw extruder as a rotating area, setting the material parameters, the rotating speed and the steering direction of the screw, and obtaining the processing characteristics of the homogenizing section of the single screw extruder corresponding to the polyethylene with different molecular chain structures after the simulation is finished.
Taking polyethylene molecular chains with the same branching degree and different molecular weights as an example, two polyethylene molecular chains with the same branching degree and different molecular weights are designed through molecular simulation software, the structural optimization and the dynamic optimization processes of a strict control model are the same, the material parameters of periodic boundary amorphous models corresponding to the two polyethylene molecular chains are calculated, the material parameter calculation result of the periodic boundary amorphous models is used as the input parameter of finite element simulation, and the structural parameters and the boundary conditions of a homogenizing section of a single-screw extruder are ensured to be consistent with the actual processing conditions. Compared with a model with small polyethylene molecular weight, the model with large polyethylene molecular weight has obviously high temperature and viscosity distribution range in a single screw extruder, and has large local temperature and viscosity difference. The invention provides a simulation method for establishing the relation between the polyethylene chain structure and the extrusion processability by means of a computer simulation technology, avoids unnecessary experimental consumption and errors, provides a new method for improving the extrusion processability of the low-density polyethylene cable insulating material from the perspective of molecular chain structure design, and can be popularized and applied to other similar high polymer materials.
Example 1
In this example, the polyethylene molecular chain has a polymer content of 60, a branching degree of 0.2, and a truncation distance of the long-range interaction force in molecular dynamics calculation is
In the dynamics optimization process, a Berendsen method is selected in a pressure control mode, an Andersen method is selected in a temperature control mode, a COMPASS II force field is set as a force field, electric charge is set as current distribution electric charge, an EWald method is selected as an electrostatic force calculation method, and an Atom based method is selected as a van der Waals force calculation method; in the finite element simulation, a Pacard iterative convergence method is adopted for a flow field, and an Upwinding interpolation method is adopted for a temperature field.
The simulation method for establishing the relationship between the polyethylene chain structure and the extrusion processability in the embodiment comprises the following steps:
1) designing polyethylene molecular chains with the polymerization degree of 60 and the branching degree of 0.2 by using MS software, distributing charges to the polyethylene molecular chains and performing structure optimization, setting a structure optimization algorithm to Smart as shown in figure 1, and selecting 10 polyethylene molecular chains to construct an initial periodic boundary amorphous model with an initial density of 0.4g/cm as shown in figure 23Lower than the actual density of the polyethylene material;
2) optimizing the initial periodic boundary amorphous model in the step 1) in MS software, wherein the optimization comprises structure optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain an optimized periodic boundary amorphous model; the method specifically comprises the following steps:
(a) performing structure optimization on the initial periodic boundary amorphous model in the step 1) in MS software, and setting a structure optimization algorithm as Smart;
(b) performing dynamic balance on the periodic boundary amorphous model after structure optimization in MS software, selecting a regular ensemble, setting the temperature to be 393K, keeping the temperature and the volume unchanged, and executing 200 ps;
(c) annealing optimization is carried out on the balanced periodic boundary amorphous model in MS software, the temperature is set to 393-593K, the pressure is 20MPa, an isothermal and isobaric ensemble is selected, the annealing cycle number is 15 times, and the execution time is 100ps each time;
(d) selecting the periodic boundary amorphous model with the lowest energy in the annealing process in the step (c), performing dynamic relaxation on the model in MS software, setting the temperature to be 393K and the pressure to be 20MPa, selecting an isothermal and isobaric ensemble, and executing for 500 ps;
(e) performing secondary dynamic balance on the relaxed periodic boundary amorphous model in MS software, selecting a regular ensemble, setting the temperature to be 393K and the pressure to be 20MPa, keeping the temperature and the volume unchanged, and executing for 500 ps;
3) introducing the periodic boundary amorphous model optimized in the step 2) into LAMMPS software, then performing structural optimization and dynamic balance processes on the model in the LAMMPS software, keeping the optimization conditions consistent with the step 2), and calculating the parameters of the optimized periodic boundary amorphous model by using an inverse nonequilibrium dynamic method, wherein the parameters comprise the heat conductivity coefficient of 0.196W/(m.K), the isobaric heat capacity of 2190J/(kg.K) and the density of 0.802kg/m3A zero-cut viscosity of 2.46X 107mPa · s and a relaxation time of 9.7 s;
4) respectively establishing a geometric model of a screw and a flow channel of a homogenizing section of the single screw extruder, respectively carrying out grid unit division on the geometric models of the screw part and the flow channel part by using a finite element method, and then carrying out grid unit combination on the screw and the flow channel part of which the grid units are divided by using a grid overlapping technology; the method specifically comprises the following steps:
(a) establishing a geometric model of a screw and a flow channel of a homogenizing section of a single-screw extruder, wherein the length of the flow channel is 100mm, the diameter of the flow channel is 60mm, the length of the screw is 80mm, the diameter of the screw is 46mm, the depth of a screw edge is 12mm, the width of the screw depth is 10mm, and the screw pitch is 30 mm;
(b) using a finite element method to divide a grid unit into a geometric model of a homogenizing section of a single-screw extruder, using a 3mm tetrahedral grid unit for a screw part and a 2mm hexahedral grid unit for a runner part, and combining the divided screw and runner of the grid unit by using a grid overlapping technology;
5) inputting the periodic boundary amorphous model parameter calculation result in the step 3) as a material parameter of finite element simulation, selecting a constitutive equation to respectively express the dependence of the fluid viscosity on the shearing rate and the dependence of the fluid viscosity on the temperature, and setting a flow boundary condition and a temperature boundary condition for a flow channel part of a homogenizing section of a single-screw extruder according to the actual processing condition of the polyethylene single-screw extruder; and defining the screw part of the homogenizing section of the single screw extruder as a rotating area, setting the material parameters, the rotating speed and the steering direction of the screw, and obtaining the processing characteristics of the homogenizing section of the single screw extruder corresponding to the polyethylene with different molecular chain structures after the simulation is finished. The method specifically comprises the following steps:
(a) inputting the parameter calculation result of the periodic boundary amorphous model in the step 3) as a material parameter of finite element simulation, selecting a Bird-Carreau model to express the dependence of the fluid viscosity on the shearing rate, and selecting an Approximate Arrhenius law to express the dependence of the fluid viscosity on the temperature;
(b) setting flow boundary conditions of a flow channel part of a homogenizing section of the single-screw extruder in the step 4), wherein an inlet is free flow, an outlet is normal stress and is controlled by pressure, an evolution algorithm is adopted, the inner wall is at a Cartesian rotating speed and is consistent with the rotating speed of the screw, and the outer wall has no slippage;
(c) setting temperature boundary conditions of a runner part of a homogenizing section of the single-screw extruder in the step 4), wherein the inlet is at a fixed temperature 383K, the outlet is free fluid, the inner wall is heat insulation, and the outer wall is at a fixed temperature 393K;
(d) the screw portion of the homogenizing section of the single-screw extruder was defined as a rotation region, the coordinates of the axes of rotation were (0, 0, 0) and (0, 0, 1), and the rotation speed was set at-60 r/min and the density was 7870kg/m3The thermal conductivity coefficient is 40W/(m.K), and after the simulation is finished, the processing characteristic corresponding to the molecular chain structure of the polyethylene with the polymerization degree of 60 is obtained.
As an example, the present invention utilizes the above steps to obtain the temperature distribution diagram of the polyethylene with the molecular chain polymerization degree of 60 in the single-screw extruder homogenization section in FIG. 3, and the shear viscosity distribution diagram of the polyethylene with the molecular chain polymerization degree of 60 in the single-screw extruder homogenization section in FIG. 4.
Example 2
In this example, the polymer content of the polyethylene molecular chain was 90, the degree of branching was 0.2, and the distance of truncation of the long-range interaction force in the molecular dynamics calculation was
In the dynamics optimization process, a Berendsen method is selected in a pressure control mode, an Andersen method is selected in a temperature control mode, a COMPASS II force field is set as a force field, electric charge is set as current distribution electric charge, an EWald method is selected as an electrostatic force calculation method, and an Atom based method is selected as a van der Waals force calculation method; in the finite element simulation, a Pacard iterative convergence method is adopted for a flow field, and an Upwinding interpolation method is adopted for a temperature field.
The simulation method for establishing the relationship between the polyethylene chain structure and the extrusion processability in the embodiment comprises the following steps:
1) designing polyethylene molecular chains with the polymerization degree of 90 and the branching degree of 0.2 by using MS software, distributing charges to the polyethylene molecular chains and performing structure optimization, setting a structure optimization algorithm to Smart as shown in FIG. 5, and selecting 10 polyethylene molecular chains to construct an initial periodic boundary amorphous model with an initial density of 0.4g/cm as shown in FIG. 63Lower than the actual density of the polyethylene material;
2) optimizing the initial periodic boundary amorphous model in the step 1) in MS software, wherein the optimization comprises structure optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain an optimized periodic boundary amorphous model; the method specifically comprises the following steps:
(a) performing structure optimization on the initial periodic boundary amorphous model in the step 1) in MS software, and setting a structure optimization algorithm as Smart;
(b) performing dynamic balance on the periodic boundary amorphous model after structure optimization in MS software, selecting a regular ensemble, setting the temperature to be 393K, keeping the temperature and the volume unchanged, and executing 200 ps;
(c) annealing optimization is carried out on the balanced periodic boundary amorphous model in MS software, the temperature is set to 393-593K, the pressure is 20MPa, an isothermal and isobaric ensemble is selected, the annealing cycle number is 15 times, and the execution time is 100ps each time;
(d) selecting the periodic boundary amorphous model with the lowest energy in the annealing process in the step (c), performing dynamic relaxation on the model in MS software, setting the temperature to be 393K and the pressure to be 20MPa, selecting an isothermal and isobaric ensemble, and executing for 500 ps;
(e) performing secondary dynamic balance on the relaxed periodic boundary amorphous model in MS software, selecting a regular ensemble, setting the temperature to be 393K and the pressure to be 20MPa, keeping the temperature and the volume unchanged, and executing for 500 ps;
3) introducing the periodic boundary amorphous model optimized in the step 2) into LAMMPS software, then performing structural optimization and dynamic balance processes on the model in the LAMMPS software, keeping the optimization conditions consistent with the step 2), and calculating the parameters of the optimized periodic boundary amorphous model by using an inverse nonequilibrium dynamic method, wherein the parameters comprise the heat conductivity coefficient of 0.201W/(m.K), the isobaric heat capacity of 2346J/(kg.K) and the density of 0.806kg/m3A zero-cut viscosity of 3.84X 107mPa · s and a relaxation time of 12.5 s;
4) respectively establishing a geometric model of a screw and a flow channel of a homogenizing section of the single screw extruder, respectively carrying out grid unit division on the geometric models of the screw part and the flow channel part by using a finite element method, and then carrying out grid unit combination on the screw and the flow channel part of which the grid units are divided by using a grid overlapping technology; the method specifically comprises the following steps:
(a) establishing a geometric model of a screw and a flow channel of a homogenizing section of a single-screw extruder, wherein the length of the flow channel is 100mm, the diameter of the flow channel is 60mm, the length of the screw is 80mm, the diameter of the screw is 46mm, the depth of a screw edge is 12mm, the width of the screw depth is 10mm, and the screw pitch is 30 mm;
(b) using a finite element method to divide a grid unit into a geometric model of a homogenizing section of a single-screw extruder, using a 3mm tetrahedral grid unit for a screw part and a 2mm hexahedral grid unit for a runner part, and combining the divided screw and runner of the grid unit by using a grid overlapping technology;
5) inputting the periodic boundary amorphous model parameter calculation result in the step 3) as a material parameter of finite element simulation, selecting a constitutive equation to respectively express the dependence of the fluid viscosity on the shearing rate and the dependence of the fluid viscosity on the temperature, and setting a flow boundary condition and a temperature boundary condition for a flow channel part of a homogenizing section of a single-screw extruder according to the actual processing condition of the polyethylene single-screw extruder; and defining the screw part of the homogenizing section of the single screw extruder as a rotating area, setting the material parameters, the rotating speed and the steering direction of the screw, and obtaining the processing characteristics of the homogenizing section of the single screw extruder corresponding to the polyethylene with different molecular chain structures after the simulation is finished. The method specifically comprises the following steps:
(a) inputting the parameter calculation result of the periodic boundary amorphous model in the step 3) as a material parameter of finite element simulation, selecting a Bird-Carreau model to express the dependence of the fluid viscosity on the shearing rate, and selecting an Approximate Arrhenius law to express the dependence of the fluid viscosity on the temperature;
(b) setting flow boundary conditions of a flow channel part of a homogenizing section of the single-screw extruder in the step 4), wherein an inlet is free flow, an outlet is normal stress and is controlled by pressure, an evolution algorithm is adopted, the inner wall is at a Cartesian rotating speed and is consistent with the rotating speed of the screw, and the outer wall has no slippage;
(c) setting temperature boundary conditions of a runner part of a homogenizing section of the single-screw extruder in the step 4), wherein the inlet is at a fixed temperature 383K, the outlet is free fluid, the inner wall is heat insulation, and the outer wall is at a fixed temperature 393K;
(d) the screw portion of the homogenizing section of the single-screw extruder was defined as a rotation region, the coordinates of the axes of rotation were (0, 0, 0) and (0, 0, 1), and the rotation speed was set at-60 r/min and the density was 7870kg/m3The thermal conductivity coefficient is 40W/(m.K), and after the simulation is finished, the processing characteristic corresponding to the molecular chain structure of the polyethylene with the polymerization degree of 60 is obtained.
As an example, the present invention utilizes the above steps to obtain the temperature distribution diagram of the polyethylene with molecular chain polymerization degree of 90 in the single screw extruder homogenization section in FIG. 7, and the shear viscosity distribution diagram of the polyethylene with molecular chain polymerization degree of 90 in the single screw extruder homogenization section in FIG. 8.
Compared with the two embodiments, the polyethylene molecular chain structure can be accurately regulated and controlled through molecular simulation software, the single-screw extrusion processing finite element simulation conditions are basically consistent with the actual processing conditions, the temperature and viscosity distribution range of a polyethylene model with large molecular weight is wider than that of polyethylene with small molecular weight, the local distribution difference is obvious, the relevance between the polyethylene molecular chain structure and the extrusion processing performance is accurately and visually established, and unnecessary consumption and errors in actual experiments can be avoided. From the above, the invention can accurately design polyethylene molecules with different chain structures through molecular simulation software, strictly control the calculation methods of molecular model structure optimization, dynamic balance process and parameters, use the molecular simulation calculation result as the input parameter of finite element simulation, and use the same single screw extruder to homogenize segment structure parameters and boundary conditions. The corresponding extrusion processing performance of polyethylene with different molecular chain structures can be obtained.
Claims (10)
1. The simulation method for establishing the relation between the polyethylene chain structure and the extrusion processing performance is characterized by comprising the following steps of:
distributing charges to the designed polyethylene molecular chains and carrying out structural optimization to obtain the optimized polyethylene molecular chains;
constructing an initial periodic boundary amorphous model using the optimized polyethylene molecular chains, wherein the density of the initial periodic boundary amorphous model is set to be lower than the actual density of the polyethylene material;
optimizing the initial periodic boundary amorphous model, wherein the optimization sequentially comprises structural optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain an optimized periodic boundary amorphous model;
sequentially carrying out structural optimization, a dynamic balance process and parameter calculation on the optimized periodic boundary amorphous model to obtain simulation parameters of a polyethylene chain structure;
calculating the processing characteristics of the homogenizing section of the single screw extruder corresponding to the designed polyethylene molecular chain by utilizing the simulation parameters of the polyethylene chain structure according to a finite element simulation model of the homogenizing section of the single screw extruder with pre-established and preset conditions; wherein the condition setting of the finite element simulation model of the homogenizing section of the single-screw extruder comprises the following steps: selecting a constitutive equation which respectively represents the dependence of the fluid viscosity on the shearing rate and a constitutive equation which represents the dependence of the fluid viscosity on the temperature in a finite element simulation model of a single-screw extruder homogenizing section, and setting the flow boundary condition and the temperature boundary condition of a flow passage part in the pre-established finite element simulation model of the single-screw extruder homogenizing section according to the actual processing condition of the polyethylene single-screw extruder.
2. The simulation method for establishing relationship between polyethylene chain structure and extrusion processability as claimed in claim 1, wherein when the initial periodic boundary amorphous model is subjected to dynamic equilibrium, a regular ensemble is selected, the temperature is set, the temperature and the volume are kept constant, and the execution is performed for a preset time.
3. The simulation method for establishing relationship between polyethylene chain structure and extrusion processability according to claim 1, wherein when annealing the initial periodic boundary amorphous model, the annealing temperature and pressure are set, isothermal and isobaric ensemble is selected, and the number of annealing cycles and the execution time of each annealing are set.
4. The simulation method for establishing relationship between polyethylene chain structure and extrusion processability as claimed in claim 1, wherein when performing secondary dynamic balance on the initial periodic boundary amorphous model, selecting a regular ensemble, setting temperature and pressure, keeping temperature and volume unchanged, and performing for a preset time.
5. The simulation method for establishing relationship between polyethylene chain structure and extrusion processability as claimed in claim 1, wherein the condition for performing structure optimization on the optimized periodic boundary amorphous model is the same as the condition for performing structure optimization on the initial periodic boundary amorphous model; the conditions for performing the dynamic balance process on the optimized periodic boundary amorphous model are the same as the conditions for performing the dynamic balance and the secondary dynamic balance on the initial periodic boundary amorphous model.
6. The simulation method for establishing relation between polyethylene chain structure and extrusion processability as claimed in claim 1, wherein the parameter calculation is performed by using inverse non-equilibrium dynamics method when performing parameter calculation on the optimized periodic boundary amorphous model, and the obtained simulation parameters of the polyethylene chain structure include thermal conductivity, isobaric heat capacity, density, zero-shear viscosity and relaxation time.
7. The simulation method for establishing the relationship between the polyethylene chain structure and the extrusion processability as claimed in claim 1, wherein the constitutive equation representing the dependency of the fluid viscosity on the shear rate in the finite element simulation model of the homogenizing section of the single screw extruder adopts Bird-Carreau model, and the constitutive equation representing the dependency of the fluid viscosity on the temperature in the finite element simulation model of the homogenizing section of the single screw extruder adopts Approximate Arrhenius law;
when the flow boundary condition of a flow channel part in a finite element simulation model of a homogenizing section of the single-screw extruder is set, the inlet of the flow channel is set to be free flow, the outlet of the flow channel is set to be normal stress and is controlled by pressure, an evolution algorithm is adopted, the inner wall of the flow channel is at Cartesian rotating speed and is consistent with the rotating speed of a screw, and the outer wall of the flow channel does not slide;
when the temperature boundary condition of the runner part in the finite element simulation model of the homogenizing section of the single screw extruder is set, the inlet of the runner is set to be at a fixed temperature, the outlet of the runner is free fluid, the inner wall of the runner is heat insulation, and the outer wall of the runner is at a fixed temperature.
8. A system for establishing a relationship between polyethylene chain structure and extrusion processability, comprising:
a first optimization module: the method is used for distributing charges to the designed polyethylene molecular chains and carrying out structural optimization to obtain the optimized polyethylene molecular chains;
initial periodic boundary amorphous model building blocks: for constructing an initial periodic boundary amorphous model using the optimized chains of polyethylene molecules, wherein the density of the initial periodic boundary amorphous model is set to be lower than the actual density of the polyethylene material;
a second optimization module: the method is used for optimizing the initial periodic boundary amorphous model, and the optimization sequentially comprises structural optimization, dynamic balance, annealing, relaxation and secondary dynamic balance to obtain the optimized periodic boundary amorphous model;
an optimization calculation module: the system is used for sequentially carrying out structural optimization, a dynamic balance process and parameter calculation on the optimized periodic boundary amorphous model to obtain simulation parameters of a polyethylene chain structure;
a simulation calculation module: the finite element simulation model of the single screw extruder homogenizing section which is pre-established and has preset conditions is used for calculating and obtaining the processing characteristics of the single screw extruder homogenizing section corresponding to the designed polyethylene molecular chain by using the simulation parameters of the polyethylene chain structure; wherein the condition setting of the finite element simulation model of the homogenizing section of the single-screw extruder comprises the following steps: selecting a constitutive equation which respectively represents the dependence of the fluid viscosity on the shearing rate and a constitutive equation which represents the dependence of the fluid viscosity on the temperature in a finite element simulation model of a single-screw extruder homogenizing section, and setting the flow boundary condition and the temperature boundary condition of a flow passage part in the pre-established finite element simulation model of the single-screw extruder homogenizing section according to the actual processing condition of the polyethylene single-screw extruder.
9. An electronic device, comprising:
one or more processors;
a storage device having one or more programs stored thereon;
the one or more programs, when executed by the one or more processors, cause the one or more processors to implement the simulation method for establishing a polyethylene chain structure versus extrusion processability relationship of any of claims 1 to 7.
10. A storage medium having stored thereon a computer program, wherein the computer program when executed by a processor implements a simulation method for establishing a relationship between a polyethylene chain structure and extrusion processability as claimed in any one of claims 1 to 7.
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