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The die optimization design is an effective way of overcoming the extrusion problems of plastic tube. Due to the obvious influence of the rheological properties of melt on the extrusion shape of the plastic tube, the reasonable cross-section shape of the die is one of most important factors in the die design. In this study, the effects of rheological properties of polymer melt on the cross-section shape of plastic tube were investigated. Numerical results indicate that the inner radius and the outer radius of the die’s cross-section shape increase with the increasing of the viscosity, and the parameter of ε and ξ , but decrease with the increasing of the relaxation time. For the gap thickness of the die, the gas thickness increases with the viscosity, and the parameter of ε and ξ , but decreases with the increasing of the melt’s viscosity. Introduction The extrusion forming [1] is one of the most important methods of producing the plastic tube in the process of the plastic products. The plastic tube with different cross-section types can be obtained by using the method of the molten polymer extruded from a metal die with a fixed cross-section type. However, due to the properties of elastic energy recovery for the melt, the triggering of the high shear stress and shear rate in the flow channel, the plastic extrudates are easy to generate the some unstable phenomena, such as extrudate swell [2], extrudate fracture [3], and extrudate distortion [4]. Up to now, many methods have been studied to overcome these above mentioned problems, such as the fluoride additives [5], die surface modification [6], process optimization [7], and die design optimization [8]. where the method of die design optimization has the advantages of low price, high efficiency and short development time etc. However, there are some problems should be solved for the die design of the plastic tube extrusion forming, such as the effect of the physical properties for the melt, the effect of the process parameter, the effect of the die length, etc. In this study, the effect of the rheological properties of the melt on the reverse design of the plastic tube extrusion forming was numerically investigated by using the computed fluid dynamics software POLYFLOW [9]. This study has an important guidance value in the die reverse design of the plastic tube with different physical properties. Numerical Simulation Numerical Model. The geometric model of the die for the plastic tube extrusion forming is shown in Fig.1(a). 0414 4th Annual International Conference on Material Science and Engineering (ICMSE 2016) © 2016. The authors – Published by Atlantis Press Fig.1 Geometric model of the die for the plastic tube extrusion forming In Fig.1 (a), the 1/4 part structure was used due to the axis-symmetry property of the plastic tube. The section of ABDC-A’B’D’C’ is the convergent zone of the die, the section of CDFE-C’D’F’E’ is the shaping zone of the die, the section of EFHG-E’F’H’G’ is the free zone of the die. The length of the convergent zone is 50mm, the length of the shaping zone is 50mm, the length of the free zone is 30mm, the thickness of the plastic tube is 3mm, the inner radius of the tube is 7mm, outer radius of the tube is 10mm. The finite element mesh of the Fig.1(a) is shown in Fig.1(b). In Fig.1(b), the meshes are properly refined near the inlet face, the inlet face of the shaping zone, die outlet face, exit face, the inner wall, and outer wall of the tube. The mesh number of the Fig.1(b) is 1800. Hypotheses. In order to conveniently study the effect of rheological properties of melt on the die design of the plastic tube, some reasonable hypotheses should be set as follows, 1) The melt is regarded as the isothermal, incompressible, steady, laminar Non-Newtonian fluid. 2) The inertia force, gravity force and thermal irradiation effect of the melt are neglected. Governing Equations. According to the above mentioned hypotheses, the mass and momentum conservation equations are given as follows, 0 v ∇⋅ = (1) 0 p τ ∇ −∇⋅ = (2) where ∇Hamilton operator, v is the velocity vector of the melt, p is the pressure, τ is the extra stress of the melt. In this study, Phan-Thien and Tanner(PTT) constructive equation [10] was used as the constructive equation of polymer flow in the micro-channel due to the wide range of shear rate. PTT constructive equation can be shown as follows,

Study on the Effect of the Rheological Properties of Melt on the Die Cross-Section Shape for the Plastic Tube Extrusion Forming