Theoretical And Experimental Study Of Polymer Gas-assisted Extrusion From Dies

Extrusion is one of the basic methods in polymer processing. More than 40% of plastic products are produced through extrusion. However, in normal extrusion, several problems appear. Firstly, due to the high viscosity and viscoelasticity of polymer melts, appears in extrusion the die swell which makes it more difficult to enhance the precision of extruded products. The next is the large pressure drop which leads to high energy consumption and the third is the appearing of "sharkskin" and melt rupture when the extrusion speed exceeds the limit of the processed materials, which astricts the efficiency of extrusion. The sticking point about the problems is the no-slip adhesive shearing extrusion mechanism adopted in normal extrusion and big resistance appears at the interface of die wall and polymer melt. In gas-assisted extrusion, a stable gas layer is set up at the interface of the polymer melt and die wall by means of the gas controlling system and gas-assisted extrusion die, so the extrusion process falls in fully slip non-adhesive shearing extrusion in the die, the friction between the melts and the die is thereby reduced greatly and the flowing molten polymer in the die is like a rod. The experimental study and numerical simulation on gas-assisted extrusion have been conducted in this paper. The main works is listed below.1. The research advances about die swell, the reduction of friction between die wall and polymer melt, wall slip and melt fracture in polymer extrusion are described in the paper.2. A gas-assisted extrusion experimental device has been built. A gas pressure controlling unit and a gas temperature controlling unit have been designed to keep the gas pressure and gas temperature under appropriate level and a gas-assisted extrusion die has been designed and manufactured. The device is used successfully in the gas-assisted extrusion experiments.3. The extensive experiments on gas assisted extrusion have been conducted. We have investigated the influence of process conditions (such as gas-pressure, gas-flow rate, gas-temperature, melt flow rate), the dimension of extrusion die (L2, length of gas-assisted extrusion die, 5, size of the gas inlet) and the turning on order of gas valve and extrusion screw on the establishment and stability of gas layer between the die wall and the molten polymer. We have also studied the impact of gas-pressure, gas-flow rate, δ and L₂ on the flow rate of molten polymer, on the diameter of extruded rods. By comparing to the normal extrusion and gas-assisted extrusion, the influence on the die swell and surface quality has been studied too.4. Based on the three equations (continuity equation, momentum equation, energy equation) and Rivlin-Ericksen viscoelastic constitutive equation, through reasonable simplification and...