Application of the hottest reverse extrusion in th

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Application of reverse extrusion in the design of extrusion die for wood plastic profile

in the field of polymer processing, extrusion molding is a very important processing method, and the extrusion die has a great impact on the quality of products. The traditional trial and error method to design the extrusion die mainly depends on the experience and intuition of designers, which is lack of scientific basis and has great blindness, making the mold production cycle long, the cost high, and the quality difficult to guarantee. At present, the domestic research on the design of extrusion die mostly focuses on the profiles with simple cross-section shape. For the profiles with complex cross-section shape, the design theory of die is still very immature because of the diversity of die and the complexity of die configuration and melt flow in die after extrusion in the 21st century. In recent ten years, more and more researchers abroad have applied computational fluid dynamics (CFD) software to the numerical simulation of extrusion die, and its research results have greatly promoted the development of polymer industry. POLYFLOW is one of the commonly used CFD simulation software

polyflow is a CFD software that uses the finite element method to simulate the flow of viscous and viscoelastic fluids. Its basic program structure is shown in Figure 1. As a component of fluent, it has strong ability to solve non-Newtonian fluid and nonlinear problems, and it has many kinds of flow models, which can solve various isothermal/non isothermal, two-dimensional situations/three-dimensional Steady/unsteady flow problems can be used to simulate various molding methods, such as extrusion, blow molding, hot listening, and pressure pressing for more than 2100 college students

because POLYFLOW software can deal with the flow problem with free surface, it is widely used to study the extrusion swell of polymer melt. In addition, given the cross-section shape of the extrudate, POLYFLOW can simulate and analyze the size of the die through its "reverseextrusion" function. In the process of die design, in order to get the desired product shape, the die shape should be designed in advance, rather than the product shape for a given die, which involves the problem of reverse extrusion. The basic process is to set the fixed part and adaptive part of the free surface of the product and the die with the function of regenerating lattice of poly-flow and the gradient method after determining the shape of the product, and repeatedly iterate and calculate it to obtain the appropriate shape and size of the die. Taking the extrusion die design of I-shaped products as an example, the application of POLYFLOW reverse extrusion in die design is introduced

1. Establishment of mathematical model

according to the characteristics of the flow channel of the head and the characteristics of the polymer, the following assumptions are made: the fluid is a non-Newtonian viscous fluid, and its rheological properties meet the power-law equation; The Reynolds numbers of polymer melts are relatively small, so it can be considered that the flow of melt in the die is laminar flow; It is considered as a stable flow field, that is, the distribution of the flow field is independent of time; The volume forces such as inertia force and gravity are far less than viscous force, which can be ignored; The fluid is viscous and incompressible; The flow field is isothermal

according to the continuity and momentum conservation of polymer melt in the processing process, combined with the above assumptions, the mathematical model of internal flow in the die can be established

2. Establishment of geometric model and lattice division

the finite element lattice required by POLYFLOW is usually established by its preprocessor gambit. For the model with simple geometry, its geometric model can be directly established in gambit and lattice division can be carried out. For the model with complex geometry, you can first use CAD modeling software (such as solid-works, pro/e, etc.) to establish the geometric model, then save it as a file in a certain format (such as IGES file), and finally import it into gambit to modify and divide it. When establishing the geometric model, the model can be simplified according to the structural shape of the model and the requirements of the analysis results, such as the axisymmetric three-dimensional model. If the analysis results allow, only the axial section can be analyzed, so as to simplify the three-dimensional problem into two-dimensional problem, which can greatly save the analysis time and cost; For a symmetrical model, only half of it can be analyzed, and the result of the whole face can be obtained through symmetry from half of the result. The section size and shape of the simulated I-shaped die runner are shown in Figure 2

because the structure of the part in Figure 2 is relatively simple and there are two symmetrical planes, its geometric model is directly established in gambit and its quarter is divided into lattices, 7552 hexahedral units are obtained, and its boundary and area type are set

3. Establishment of analysis task

after dividing the lattice of the model, save it as a mesh file and import it into polydata. In polydata, set and analyze the physical model, material properties and boundary conditions of the task

3.1 task selection

in polydata, task selection includes the selection of task nature, geometric model and simulation type. Among them, the nature of the task includes finite element task and hybrid task. The mixing task is mainly used to analyze the fluid trajectory and make statistical analysis of the flow. Except for mixed tasks, others can be selected as finite element tasks. The types of geometric models include 2-dimensional plane symmetric model (describing 2-dimensional velocity field in 2-dimensional Cartesian coordinate system), 2-dimensional axisymmetric model (describing 2-dimensional velocity field in 2-dimensional cylindrical coordinate system), 2.5-dimensional plane symmetric model (describing 3-dimensional velocity field in 2-dimensional Cartesian coordinate system), 2.5-dimensional axisymmetric model (describing 3-dimensional velocity field in 2-dimensional cylindrical coordinate system) and 3-dimensional geometric model. The types of simulation include steady-state, time-dependent and gradual models. The gradual model is applied to the model with convergence problems due to the nonlinearity caused by flow parameters. It makes the flow parameters gradually approach from an initial value to a satisfactory value through a continuous setting function. In this example, because the sliding boundary conditions of the die wall are set, the sliding coefficient should be changed gradually. At the same time, because the model is a three-dimensional model, a three-dimensional gradual finite element model is used here

3.2 selection of subtasks

in the selection of polydata subtasks, it includes the selection of subtask model, the selection of subtask area, the setting of material parameters, the setting of performance characteristics of horizontal tension testing machine for boundary strip insulator, the setting of regeneration grid, etc

3.2.1 selection of subtask models

available subtask models in polydata include generalized Newtonian flow problems, Darcy flow problems (porous media), heat conduction problems, and viscoelastic flow problems. Select the corresponding model according to the corresponding fluid and the problem to be solved. Because the generalized Newtonian fluid is mainly analyzed, the isothermal generalized Newtonian flow model is adopted

3.2.2 selection of subtask area

in order to apply the regenerating lattice technology to the free surface and the sliding wall of the die respectively, so as to realize the accurate description of the die shape, the calculation area is divided into two sub areas, namely, the extrusion area with free surface outside the die and the fluid area with sliding wall inside the die. Because both areas are involved in the problem under investigation, these two areas are selected as the areas of the subtask

3.2.3 setting of material parameters

common options for material parameters include the dependence of viscosity on shear rate, the dependence of viscosity on temperature, porous media and flow viscosity, differential viscoelastic model, integral viscoelastic model, density, inertia term, coefficient of thermal expansion, heat conduction, heat capacity per unit mass, viscous heating, gravity and average temperature, etc. Because the flow field is assumed to be isothermal and the influence of gravity and inertial force is not considered, it is only necessary to define the dependence of viscosity on shear rate, select the power-law constitutive equation, and set its consistency as 30 and power-law index as 0.75

3.2.4 setting of boundary conditions

the commonly used boundary types include contact surface, applied normal velocity and tangential velocity, applied normal force and tangential force, applied normal force and tangential velocity, applied normal velocity and tangential force, sliding boundary, symmetry surface, population flow, outlet flow, free surface, applied volume force, and skardikal velocity. The volume flow rate is adopted at the inlet of the calculation area, q=80cm3/s, and the speed curve is set to be automatically determined with POLYFLOW; Set the tangential force and normal force on the exit area as 0; The wall of the die is set as the sliding boundary, because the nonlinearity introduced by the dynamic equation makes the simulation process difficult to converge, so the gradual change method is adopted for the sliding coefficient of the sliding boundary to promote the convergence of the simulation process; The wall outside the die in contact with the air is set as a free surface; Finally, set two symmetry planes

3.2.5 setting of regenerating lattice

regenerating lattice technology is applicable to the problem of free surface or moving interface. Part of the lattice will be affected by the change of the position of free surface or moving interface. The function of this technology is to redefine the internal points according to the change of the position of each point on the boundary. Commonly used lattice generation techniques in POLYFLOW include Thompson transformation method (applicable to 2D and 3D flow problems), spines method (applicable to extrusion, injection and other problems), euclidi an method (applicable to 2D flow), thin shell method (applicable to blow molding and thermoforming) and optimized lattice (applicable to 3D extrusion problems), etc. Because there is a free surface in the model, its location is unknown, so we need to use the re lattice technology for the free surface. At the same time, because the simulation task is the reverse extrusion problem, the position of the die wall near the exit area is changed, and it needs to be set as the dynamic wall, so the regenerating lattice technology should also be used for the die wall. Here, the lattice regeneration technology for three-dimensional extrusion is used for the two locally regenerated lattice regions

after completing the above settings, save and exit to generate the data file required to solve the problem

4. Problem solving

run POLYFLOW to solve the set problem, and check the list file of the analysis process to determine whether the analysis converges and meets the predetermined accuracy requirements. If it does not converge or does not meet the predetermined accuracy requirements, analyze the reasons. At the same time, refer to the suggestions made by the expert system, reopen the data file to modify the defined problem, and then run poly-flow again to solve it

5. View the analysis results

after the analysis is successful, run fluentpost to view the analysis results. Using two symmetrical planes to mirror the simulation results, the simulated die shape and its corresponding product shape are obtained. The simulated die shape can provide a valuable reference for producing qualified dies. If you need to design the next step according to the simulation results, you can view the coordinate value of the die shape in the graphic display, or save it as a text document for viewing. If you want to carry out further computer-aided design and NC machining on the obtained die, you can select the output IGES format in the output setting of task setting, and then import the IGES format file into CAD software for processing

6. Conclusion

describes in detail the process of POLYFLOW simulating the die shape of I-shaped products, and shows the role of POLYFLOW reverse extrusion function in the process of polymer extrusion die design, which can provide a scientific basis for die shape design, greatly shorten the die production cycle and reduce its cost, while it is strict for profile and deformation with complex shape

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