Theoretical And Experimental Study On Polymer Gas-assisted Co-extrusion

Polymer co-extrusion is a widely used molding technique due to environmental protection, low cost and high performance products. However, there are still some problems, such as co-extrusion swell, viscous encapsulation and interfacial instability. These problems seriously restrict the further development of co-extrusion. The primary causes are the viscoelasticity difference between multiphase melts and no-slip adhesive shearing extrusion mechanism. To solve these problems, the new gas-assisted co-extrusion technique is proposed in the paper by combining the co-extrusion and gas-assisted technique. The paper makes a thorough research on this new processing technique through experiments and numerical simulations.Firstly, the paper designs a rectangular profile gas-assisted co-extrusion die with gas inlet, and establishes the experimental system of gas-assisted co-extrusion. Then, the stable gas-assisted co-extrusion conditions, the factors affected the stability of the gas layer and the effects of processing parameters on co-extrusion swell and encapsulation in gas-assisted co-extrusion are investigated experimentally. The results show that:(1) In gas-assisted co-extrusion process, gas temperature and pressure are the two most critical parameters. Only when the gas temperature and gas pressure are equal to those of melts in the die can the stable gas-assisted co-extrusion be developed. (2) Extrusion swell and viscous encapsulation in gas-assisted co-extrusion are enormously decreased. The die swell ratio in traditional co-extrusion is over 35%, but in gas assisted co-extrusion, the die swell ratio is less than 3%. The degree of viscous encapsulation in gas-assisted co-extrusion is 30% less than that of traditional co-extrusion. On the same condition of low screw speed(less than 10rpm), the output of gas-assisted co-extrusion is 1.8%-3.8% more than that of the traditional co-extrusion. There is no obvious difference between gas-assisted co-extrusion and traditional co-extrusion in appearance quality. (3) In gas-assisted co-extrusion, the die swell ratio and the degree of viscous encapsulation increases slightly as the flow rate or its ratio increases. The viscosity ratio brings relatively great influence on the viscous encapsulation, the degree of encapsulation also increases with the viscosity ratio, and the growth rate is the same as traditional co-extrusion.Secondly, the method of building viscoelastic numerical model for full-slip flow in co-extrusion die is discussed. The boundary conditions are analyzed, and the shearing stress on the wall of co-extrusion die is set to be zero to replace the function of gas layer, which is used as the kinetic conditions of the wall. The key solution techniques of FEM adopted by the paper are studied, and the whole routine to solve the finite element analysis of 3D viscoelastic co-extrusion flow is presented. Then, the paper performs FEM simulations for the flow of polymer melts inside and outside the gas-assisted co-extrusion die with rectangle and L shaped section, obtains the physical fields such as velocity field, pressure field and stress field. Based on the penetrating analysis of the physical fields, the paper reveals the fundamental mechanism that explains why gas-assisted co-extrusion can eliminate die swell, reduce the degree of encapsulation and improve the stability of the interface. The influence of process parameters and material parameters on die swell, viscous encapsulation and shear stress of interface in gas assisted co-extrusion is simulated and analyzed. The main conclusions are presented as follows:(1) In gas-assisted co-extrusion, the distribution of two melts'velocity field is uniform and the melts are extruded like a rod; The shear stress, tangential stress and normal stress of melt surface in die outlet are all zero, the pressure drop in the die channel is almost zero, so gas-assisted co-extrusion can eliminate co-extrusion swell, reduce the pressure drop and energy loss in the die effectively. The extrusion speed can be improved greatly and the "sharkskin" on the surface of the polymer parts can be prevented effectively. Thus, gas-assisted co-extrusion can improve quality and efficiency of the co-extruded parts comprehensively. (2) In gas-assisted co-extrusion the die swell is not affected by the viscosity or viscosity ratio of melts. The swell ratio of gas-assisted co-extrusion is zero, and there are no extrusion swell outside of the die, which agrees with the experimental results. The co-extrusion swell is also not affected by relaxation time and parameter a of Giesekus constitutive equation. (3)The interface stability of gas-assisted co-extrusion is superior to that of traditional co-extrusion, the primary reasons are presented as follows:a. On the gas-assisted co-extrusion interface the velocity of in Y and Z axis direction is less than that of traditional co-extrusion, and is distributed uniformly, so that the co-extrusion interface advances and developes gently; b. In gas-assisted co-extrusion, the distribution of shear rate in interface can be improved effectively and the peak value of shear rate can be decreased; c. In the gas-assisted co-extrusion shear stress in most area of interface approaches to zero, although there is shear stress at the die entrance, the value is far less than that of traditional co-extrusion; d. The normal stress difference at the interface can be decreased effectively, peak value of N1 decreases by 27%. (4) In the gas-assisted co-extrusion the viscous encapsulation is almost completed at the entrance of co-extrusion die where two polymers converge. The viscous encapsulation is affected by polymer viscosity ratio,ξvalue of PTT constitutive equation parameter and the ratio of two melts'flow rates. It is almost not affected by relaxation time and flow rate. The peak of shear stress of the interface appears at the entrance surface, then, decreases rapidly. The value ofτyz tends to be stable when the melts flowed as far as about 10mm. The influence of material parameters on peak value ofτyz, which increases with viscosity ratio but decreases with relaxation time, occurs at the entrance. For viscous encapsulation or the shear stress, the sensitivity to material parameters or process parameters are both far less than that of traditional co-extrusion. (5)With the gas inlet moving towards die entrance, the viscous encapsulation degree at the die entrance decreases gradually. At the die entrance and the gas inlet, the peak of shear stress on the co-extrusion interface also decreases gradually. That means the stability of the interface improves as the length of the gas-assisted co-extrusion increases.Finally, the process conditions for gas-assisted co-extrusion and the principles for designing the gas-assisted co-extrusion die are proposed. It is pointed out that the stable gas layer can be propitiously formed when the pressure difference between gas and melts is within 0.1MPa and the temperature difference is within 10℃. Reducing the flow rate of melts and keeping them same can decrease the co-extrusion swell and viscous encapsulation and improve the stability of the interface. For gas-assisted co-extrusion die, the proper thickness of gas inlet is 0.1mm-0.2mm, the proper length of gas-assisted sect is 30mm-40mm.