Experimental And Numerical Study Of Extrusion Process In Tire Manufacturing

Extrusion is one of the basic forming processes of polymeric materials. Some important parts of radial tires, such as the tread, are processed by extrusion. Due to fluid elasticity, the rubber compound swells obviously after leaving the extrusion die, which means the section shape of the extrudate is different from that of the die exit. Therefore, relevant die design and process control become more difficult. The analysis of extrusion process is a challenging fundamental subject which has drawn the attention of both tire industry and academic circle for a long time. Although the research in this field started decades ago, the singular rheological behavior of rubber compound still has not been fully understood and the analysis method of some complex extrusion process also needs to be improved. In such a condition, this dissertation presents a series of experimental and numerical studies, with the aim of proposing a complete analysis strategy for the extrusion process in tire production.By using a dynamic shear rheometer RPA2000, a twin bore capillary rheometer Rosand RH2200 and a rotational rheometer Physica MCR 301, the rheological properties of several typical rubber compounds are tested systematically over a wide range of shear rates or oscillatory frequencies. The dependence of shear viscosity, complex viscosity, storage modulus, loss modulus, etc., on shear rate, oscillatory frequency, shear strain and temperature is revealed comprehensively. The rubber compounds exhibit a typical shear-thinning behavior but the Newtonian viscosity plateau is not captured at low shear rates. When the testing temperature rises from 80℃ to 100℃, the viscosities of the compounds decrease. With the further increase of testing temperature, the variations of viscosities become not significant. Filler reinforcement has an effect on the linearity of viscoelastic behavior, while the vulcanization system does not. Besides, with the use of the Cox-Merz rule, it can be found that the steady shear viscosity obtained from the capillary and rotational rheometers is lower than the complex viscosity measured by the dynamic shear rheometer.Based on the testing results, the Carreau model and the Phan-Thien-Tanner (PTT) model are used to characterize the viscous and viscoelastic behaviors of the rubber compounds, respectively. In order to investigate the die swell behaviors of Newtonian, shear-thinning and viscoelastic fluids, a capillary extrusion model is built. For Newtonian and shear-thinning fluids, the slight swelling is caused by the rearrangement of velocity field. The swell ratio of Newtonian fluid is constant and the swell ratio of shear-thinning fluid decreases with the increase of volumetric flow rate. For viscoelastic fluid, the main source of extrudate swell is the elastic recovery at high Weissenberg numbers. The die swell phenomenon simulated by the viscoelastic model is more obvious and the swell ratio increases with volumetric flow rate, which is consistent with the reality. Thus, the PTT model is more suitable than the Carreau model for the numerical simulation of extrusion process. Moreover, a finite element model of the capillary rheometer is built to analyze the effect of wall slip on capillary test. Numerical results reveal that, in the presence of wall slip, the capillary rheometer tends to underestimate the shear viscosity. An explanation to the deviation between dynamic and steady viscosity curves is proposed.For the coextrusion problem including extrudate swell, a two-step iteration algorithm, which resolves the difficulty of updating extrudate free surface and layer interface simultaneously, is proposed for the first time. Numerical results reveal that the swelling and bending of the extrudate have a significant influence on the distortion of layer interface and make its final shape different from that at the die exit, showing the necessity of including the extrudate section in coextrusion simulations. Finite element models of tire tread extrusion and coextrusion are constructed using the Arbitrary Lagrangian-Eulerian (ALE) method. The numerical predictions of extrudate profile and interface shape are generally in agreement with the experimental results while small differences do exist. Based on the volume of fluid (VOF) method, another finite element model of tire tread extrusion is constructed. The VOF method is able to analyze the time-dependent process of tread extrusion, together with the traction of conveyor belt. From the comparison of the VOF results and the ALE results, it is found that the accuracy of VOF method is relatively poor.An inverse extrusion model is built to obtain the die geometry for a rectangle profile. Numerical results reveal that the shape of the die exit shrinks with the growth of volumetric flow rate and expands with the increase of wall slip level. Based on the numerical design, an actual extrusion die is fabricated using computer numerical control (CNC) machine tools and then installed on an extruder to perform an extrusion experiment. Experimental results reveal that, at high Weissenberg numbers, the extrudate profile is not sensitive to the variation of extrusion speed. Besides, it is found that the extrudate profile obtained from the experiment is slightly smaller than the target profile. To analyze the reason of the deviation, an ordinary extrusion model is constructed with higher levels of slippage at the die wall. After increasing the wall slip level, the simulated extrudate profiles are more close to the experimental results. Thus, for the numerical design of extrusion die, the accurate die geometry can only be obtained by using appropriate slip models and parameters.The final part concludes all the works of this dissertation and presents an outlook to the future research.