Research On The Numerical Simulation Of Two-layer Polymer Profile Coextrusion Process

Coextrusion is the simultaneous extrusion from a single die of two or more homogeneous melts. This is a polymer processing technique that was developed to combine different physical properties of different polymeric materials into a single product. This technique creates layered structures and benefits from properties of several plastic grades in combination, and forms stratified structures with desirable or economic benefits. During the 1980s the polymer processing industry has seen a large increase in the technology of polymer melt coextrusion, both in terms of the sophistication of the technology and in the breadth of its application. Coextrusion technique was applied widely in making complex film, board, pipe, profile, wire and cable. Coextrusion die design is one of the key points of the process. While using the numerical simulation, people can get optimize coextrusion dies and technique parameters based on the rheology theory.In this paper, the finite element simulation technique of three-dimension non-Newtonian polymer coextrusion process was studied. The step to solve the finite element equations was described. Based on the rectangle die, the coextrusion flow and die swell rules were investigated comprehensively. The effect of material characteristic, processing parameters and die structure on the coextrusion interface and die swell were also investigated. The coextrusion process of five typical composite profiles was analyzed. Combined the numerical simulation technique and production practice, the CAD/CAE integration system of polymer coextrusion die design were developed.Firstly, the finite element model of three-dimension (3D) viscoelastic steady coextrusion flow was founded according to the flow character of the polymer melt coextrusion. The key points of finite element simulation for polymer coextrusion process were studied. The whole course to solve the finite element analysis of 3D viscoelastic coextrusion flow was presented. During the solution of viscoelastic equation by coupling format, the stress was divided into purely-viscous component and elastic component, the Streamline Upwind method was used to correct the convection term in momentum equation. The kinematics and dynamics condition of moving free boundary and die swell boundary were analyzed. The solution step of the free boundary problem was presented. The non-linear equation was solved by the evolution arithmetic method. It was shown that this technique is an effective method to solve the non-linear problem in polymer melts coextrusion simulation. The rule of polymer coextrusion flow inside the rectangle die was investigated comprehensively. The material properties of polymer were characterized by Power-law fluids and viscoelastic fluids PPT model respectively, and the slippage between the polymer and die wall was considered. The distribution of velocity, shear stress, pressure, temperature and viscosity inside the extrusion die path were obtained by numerical simulation. The coextrusion interface influenced by the material characteristic (viscosity, Power-law index, relaxation time, etc.), processing parameters (inflow flux ratio, temperature and pull rate) and die structure (die angle, die length and slippage status) were analyzed based on the simulation results. The results indicated that the elastic character of polymer represents insufficiency inside the die flow owing to the restraint effect of die well. The viscosity and the Power-law index of polymers are two key parameters to influence the coextrusion interface. Thus, the flow of the polymer in extrusion dies can be appropriately described using Generalize Newtonian fluid model. In process parameters, the total flow flux and flow flux ratio are the decisive factors for the change of interfacial location. But the length of coextrusion flow path and the slippage status are two important die structure parameters to the interface. To get the steady interface, the ratio of coextrusion flow length to height is at least l-1.5._The role of smooth flow zone is important to reduce the pressure-fall. The higher surface quality of the fluid path can make the velocity field more uniform and the interfacial offset less. However, the layer interface position was almost unaffected by the convergence angle, so for the convenience of die manufacture and extruder's collocation, it is fit to make the contact angle as 90°with smooth corner.The die swell of coextrusion process was also studied systematically using the rectangle coextrusion flow model. The distribution of velocity, shear stress and viscosity of the die swell zone were obtained by numerical simulation. Based on the finite element simulation data, the die swell rate and recoverable elastic strain were calculated. The simulation results show that the die swell rate of height direction is greater than that of width direction. Due to the difference of material characteristic of the two polymers, the end section shape of coextrusion is dissymmetric drum. The coextrusion interface would be change during the extrusion swell process. The effects of material characteristic, processing parameters and die structures on the die swell rate were quantitatively analyzed based on the simulation results. The results reveal that the shear viscosity and relaxation time affect the die swell obviously. The influence of the flow flux to the die swell is related to the length of extrusion path. While flow flux is certain, die swell reduces with increasing of length of extrusion path. However, the length-height ratio up to 10, the die swell rate is approach to an invariable value. In order to get steady section, the drawing velocity must accord with the velocity of flow end. With increasing of surface quality of the fluid path, the ratio of die swells of the two fluid decrease. In addition, the contact angle has little effect on the die swell rate. The molding zone of extrusion die conduces to the attenuation of the recoverable elastic strain, and at the same time, it can also leads to greater pressure loss. So the appropriate geometric figure of coextrusion flow path is the matching result of pressure loss and recoverable elastic strain.Traditionally, trial-and-error iteration approach is adopted for die designers and process engineers to tune or refine the design and process parameters. However, it is time-and-money costing. Recent years, numerical simulation has been gradually replaced the traditional approach. In this paper, the coextrusion processes of five typical composite profile, which include composite pipe, half-club, eccentricity club, surface coating hollow profile and coextrusion profile with inlay interface were simulated using finite element method. The fields of flow velocity, pressure and shear stress were explained in detail. The molding processes of complex coextrusion interface such as column, rectangle, hatch trapezia and inlay shape, were analyzed. It is showed that the interfacial appearances obtained by the simulation results are quite matched with the results of experiment. Based on the simulation, the technique parameters of extrusion processing parameters can be confirmed reasonably.To combine the numerical simulation technique and production practice, the CAD/CAE integration system of polymer coextrusion die design were developed. The main technique and the functions of coextrusion die design CAD system were studied, and two-layer pipe coextrusion die design CAD system was founded. The system can execute polymer coextrusion die designing and modeling according to the product's information, give the three-dimension assembly structure and engineering drawing. In the CAD/CAE compositive system, the designers can get parameterized structure of die structure, and optimize the flow path according to the numerical simulation results. At the end, the die design of LDPE/PS two-layer pipe and the optimized of coextrusion profile flow path were execute to prove the feasible of the system.