Experimental And Theoretical Study On Gas-assisted Co-extrusion Of Plastic Profile With An Irregular Cross-section

Polymer gas-assisted co-extrusion is a new technique in the plastics processingindustry. It can effectively diminish the die swell, encapsulation and interfacialinstability phenomenon and make the cross-section profile of the products in keepingwith that of the die because the stable gas layer between the die wall and the surfaceof the polymer melt can make the melt flow by means of the fully slip non-adhesiveshearing co-extrusion instead of the no-slip adhesive shearing co-extrusion. In thispaper, the plastic profile co-extrusion technique was combined with the gas-assistedmolding technology to form the new plastic profile gas-assisted co-extrusiontechnique. The research on the new technology was carried out through experimentsand numerical simulations. The main study work is summarized as follows.1. The L-shaped profile gas-assisted co-extrusion experimental system wasestablished, and the L-shaped profile gas-assisted co-extrusion dies were designedand manufactured. Then the experimental research was carried thoroughly, and theexperimental studies reveal that:(1) In addition to gas pressure, gas temperature andthe startup sequence of the extruder screw and gas valve, the gas layer thickness, thepolymer melt flow rate (ratio), the polymer characteristics, the external force and soon are very important to form the stable gas layer between the die wall and thepolymer melts surface.(2) The output of the gas-assisted co-extrusion is more thanthat of the traditional co-extrusion under the same energy consumption, and theoutput growth rate increases with the increase of the polymer melt viscosity, forexample, the output growth rate of two kinds of PP melts co-extrusion is1.76%～4.65%, while the output growth rate of PP melt and the more viscous HDPE meltco-extrusion is as high as12.95%.(3) The technological parameters have a greatimpact on the traditional co-extrusion and a little effect on the stable gas-assistedco-extrusion.(4) It has little difference between the surface quality of traditionalco-extrusion products and that of gas-assisted co-extrusion products. What’s more,the surface quality of gas-assisted co-extrusion products is best when the two meltsflow rate are equal and the gas pressure and temperature are both close to the melts pressure and temperature.2. The three-dimensional non-isothermal multiphase gas-assisted co-extrusionmathematical models were established based on the gas layer was simplified as theincompressible fluid or the full-slip boundary condition. The numerical simulationswere presented using the finite element method and the results were compared andanalyzed. The simulation results show that:(1) The gas-assisted co-extrusion with afive-millimeter traditional co-extrusion flow can make the two polymer melts sticktogether and the interface between the polymer melts isn’t ready to separate under thegreater gas pressure, but the gas-assisted co-extrusion without a traditionalco-extrusion flow makes the interface ready to separate under the greater gas pressurebecause of the small interface adhesive force.(2) The smaller gas pressure and thethinner gas layer can improve the quality of products in the gas-assisted co-extrusion.(3) The flow field distribution of making gas layer as the incompressible fluidcoincides with that of making gas layer as the full-slip boundary condition, but themelt boundary and interface profile are different.3. The three-dimensional viscoelastic numerical simulation was developed fortwo-layer co-extrusion through an irregular cross-section channel using the finiteelement method. The Phan-Thien and Tanner (PTT) model was considered asviscoelastic constitutive equations and the Generalized Naviers’ law was adopted tofound the slip boundary condition. The simulation results display that:(1) In thetraditional co-extrusion, the secondary flow at the die exit leads to the die swell andthe unbalanced velocity distribution along the melts flow direction results in thedeflection deformation of the products. In the gas-assisted co-extrusion, it doesn’texist the secondary flow at the die exit and the velocity distribution along the meltsflow direction is uniform, so polymer melts have little die swell and little deflectiondeformation after leaving the metal die.(2) The polymer melts pressure linearlyreduces to zero from the die entrance to the die exit and the pressure drop increaseswith the increase of the melt viscosity in the traditional co-extrusion, for example, thepressure drop of more viscous HDPE is0.20MPa, and that of lower viscous PP is0.11MPa. But the melts pressure in the die is close to zero and there has little pressuredrop in the gas-assisted co-extrusion.(3) In the traditional co-extrusion, the distribution of the local shear-rate and the stress are uneven and concentrate in theinside right-angle, the line and arc junction and arc and arc junction of the irrelugarcross-section die. The concentration of the shear-rate and the stress will result indeforming the extrudates. In the gas-assisted co-extrusion, there has little localshear-rate and stress in the die, so melts have no deformation after leaving the die.(4)The simulation results also indicate that the process parameters and the polymercharacteristics have a great influence on the traditional co-extrusion and have littleeffect on the gas-assisted co-extrusion. The above results well explain theexperimental phenomena, such as the output of gas-assisted co-extrusion is greaterthan that of traditional co-extrusion under the same energy consumption, it exists dieswell and deformation phenomenon on the traditional co-extrusion products, and noton the gas-assisted co-extrusion ones.