Numerical Simulation And Experimental Study Of Gas-assisted Extrusion Die Design For Precision Extrusion

Gas-assisted extrusion (GAE) is a new polymer molding technique. In the extrusion processing, a stable gas layer is established at the interface of metal die and molten polymer by injecting gas into the extrusion die, which makes the molten polymer is extruded out with a full slip mode. GAE technology can significantly reduce die swell ratio, so it has potential applications in the manufacturing of precise extrusion. Researchers have been studying gas-assisted extrusion for many years, and get many research achievements in numerical simulation and experiments. While for achieving industrial applications, it is also necessary to further study in the following three areas:(1) the stable process of gas-assisted extrusion is not perfect; (2) the extrudate's radius is smaller than the die's radius; (3) the die entrance is difficult to design; (4) no predicting or judging methods can be applied to the slip length of GAE die and the process conditions under the precise extrusion. Thus, more attention is required to be paid to the deep study of the four problems mentioned above, and further improvement on the radial dimensional accuracy of extrusion products is of great theoretical and practical significance for gas-assisted extrusion process optimum design and intelligent control.Based on the finite volume method and Generalized Navier's slip law, a numerical simulation system describing the flow of visco-elastic polymer when it passes through the gas-assisted extrusion die is established; at the same time, the finite element discrete forms of the governing equations are built, and the mixed finite element methods of EVSS/SUPG (Elastic-Viscous Stress Split elastic/Streamline Upwind Petrov-Galerkin), the Evolution method, and Thompson transformation are utilized to resolve the problem that high weissenberg often made computations difficult.Drawing on an experimental assemble system of gas-assisting extrusion, we can find the effect which the technical parameters have on the stability of gas-assisting extrusion. These experiments show that the gas can be injected into the GAE die when the gas pressure is higher than the melt pressure at the gas injection point. The GAE process could not be operated when the pressure of melt is beyond the pressure of gas. When the gas pressure is equal to the flowing polymer' s pressure, then a stable, uniform gas layer is formed at the interface between the molten polymer and the metal die. In addition these experiments also indicate that the control of gas mass flow appears to be an important part of obtaining a stable extrusion. When the gas flow is small, the melt extrudate in the form of laminar flow. When the gas mass flow is large, a periodic product deformation termed 'blistering' is observed and the gas generated turbulence forced the melt vibration extrusion.By using the model and method developed in this study, the temperature profile and viscocity profile were numerically simulated for non-isothermal heat exchange from cool-gas to hot- melt. The results reveal that when the temperature of the gas is below the temperature of the melt, when the polymer melt flow activation energy is larger and when the slip length is longer, it is easier to form a stationary polymer layer attached to the die wall and to leave a "semisolid" layer of polymer at the wall, which blocked the die and caused the diminishing of the extrudate. To raise the temperature of gas near the melt might eliminate semi-solid membrane. The simulation results are verified though this experiment and the diminished problem of extrusion products in previous research is successfully solved.In view of the uneven junctions in the right-angle shape, the conical shape, or the arc shape entrance structure, two stream-lined entrance geometric configurations is constructed, which are reverse-tangent and coped with three supra-precision interpolations respectively. Numerical simulations are conducted with the melt flow in five different entrance gas-assisted dies. The features of the pressure field, the velocity field, the stress field, and the extrusion swell in each die are also investigated. The analysis showed that the stream-lined dies had smaller extrusion pressure, bigger flow velocity, smaller concentration degree, and smaller die swell ratio although it is difficult to design and process them. As for those gas-assisted dies whose formed segments' L/D was equal to or beyond 1 and whose slip segments is half of formed segments, the extrusion swell ratio released from the die had nothing to do with the die's entrance because the elastic strain stored in the entrance could be released completely in the gas-assisted section.By simplifying the gas-assisted extrusion dies into rod models, a numerical simulation is conducted to investigate the flow of melts of different lengths of slip parts in the gas-assisted dies. The effects of shear rate and relaxation time on die swell ratio are investigated with the aid of this simulation. The interrelation among the slip length on the drop pressure, die swell and the stress concentration is also examined. It is found that drop pressure could be largely reduced while surface quality could be greatly raised with the increase of slip length. From the simulation, an equation about the die swell ratio is proposed which can be applied to the design of GAE die, relating the residence time of melt in slip part and relaxation time. The mathematical relationship can be written in B = 1 + Ae^-t/λ, which essence is that molten polymer at slip part in deformation attenuation process. With reference to the available documents, the validity and reliability of the formula mentioned above was proved experimentally, thus providing a solid theoretical basis for the design of slip part and the control of technical parameters.In this simulation, GAE experiments on HDPE, PP and ABS were carried out; The major parameters of extrusion products, such as the pressure drop, radial dimension, and the surface quality, are measured or examined. The numerical simulation results are validated. Constructed on the basis of simulation and experiments, it has been verified that the formula can be used to predict the radial dimension of extrusion products and be used to guide the optimum design of the slip length as well as the design of the length of slip parts in the gas-assisted extrusion dies.