轮胎生产中挤出工艺的实验研究和教值模拟

发布时间：2017-12-27 18:22

本文关键词：轮胎生产中挤出工艺的实验研究和教值模拟　出处：《中国科学技术大学》2016年博士论文　论文类型：学位论文

【摘要】：挤出是聚合物材料的基本成型工艺之一,子午线轮胎的一些重要半成品部件,例如胎面,便是通过挤出成型的。由于流体弹性的存在,轮胎胶料在离开挤出口模后会出现明显的胀大,使得挤出物的截面形状与口模出口处的截面形状不同,这样一来便增加了模具设计和工艺控制的难度。挤出工艺分析是轮胎工程界和学术界长期关注的一个具有挑战性的应用基础问题,尽管人们对此已经进行了长达几十年的研究,但至今尚未透彻理解轮胎胶料特殊的流变学行为,某些复杂挤出工艺的分析方法也有待完善。在这样的背景下,本文采用实验研究和数值模拟相结合的方法,开展了一系列的研究工作,试图针对轮胎生产中的挤出成型工艺提出一套完整有效的数值求解策略。利用动态剪切流变仪RPA2000、双料桶毛细管流变仪Rosand RH2200和旋转流变仪Physica MCR 301在宽广的剪切速率(频率)范围内对多种典型轮胎胶料进行了系统的流变学测试,较全面的揭示了胶料的剪切粘度、复数粘度、储能模量和损耗模量等随剪切速率、振荡频率、剪切应变和温度的变化关系。胶料的粘度会随着剪切速率的增加而减小,属于典型的剪切变稀流体,但在低剪切速率下却没有明显的牛顿粘度平台。当测试温度由80℃上升至100℃时,胶料的粘度会出现下降,但随着温度的进一步升高,由温度变化引起的粘度差异越来越小。颗粒填充会影响胶料粘弹性行为的线性,硫化体系则不会。此外,利用Cox-Merz关系对稳态和动态粘度数据进行比较发现,毛细管流变仪和旋转流变仪测得的剪切粘度均低于动态剪切流变仪测得的复数粘度。基于流变学测试结果,先后利用Carreau模型和Phan-Thien-Tanner (PTT)模型对轮胎胶料的流变学行为进行了纯粘性和粘弹性表征,并在通过建立毛细管挤出的有限元模型考察了牛顿流体、剪切变稀流体和粘弹性流体的挤出胀大行为。对于牛顿流体和剪切变稀流体,轻微的胀大现象来自挤出前后速度场的重新分布,牛顿流体的挤出胀大比为常数,剪切变稀流体的胀大比随着体积流量的增加逐渐减小。对于粘弹性流体,挤出胀大现象产生的主要原因是高Weissenberg数下流体的弹性回复。粘弹性模型计算出的挤出胀大现象更为明显,且胀大比会随着体积流量的增加而增加,与实际相符,说明相比纯粘性Carreau模型,粘弹性PTT模型更适合轮胎胶料挤出工艺的数值模拟。此外,为分析壁面滑移对毛细管测量的影响,建立了毛细管流变仪的有限元模型。仿真结果表明,壁面滑移的存在会使毛细管流变仪测得的剪切粘度偏小,从而揭示了实验中动态和稳态粘度曲线间存在偏差的原因。针对计及挤出胀大的共挤出问题的求解,首次提出了一种分步迭代方案,解决了同时计算挤出物自由表面和材料交界面变形所存在的困难。结果显示,受挤出物胀大和弯曲的影响,挤出物下游的材料交界面形状与口模出口处的交界面形状存在明显差异,表明了计及挤出胀大段的必要性。利用任意拉格朗日-欧拉(Arbitrary Lagrangian-Eulerian, ALE)方法建立了轮胎胎面单挤出成型和共挤出成型的有限元模型,模拟得到的挤出物截面轮廓和材料交界面形状均与试验结果符合良好,只在局部存在一定差异。此外,还利用流体体积(volume of fluid,VOF)方法建立了轮胎胎面单挤出成型的有限元模型,该方法能够描述胎面挤出的动态过程并计及传送带牵引对挤出物截面形状的影响,但对比ALE方法的计算结果发现,在同样的网格尺度下,VOF方法的计算精度相对较差。针对矩形挤出物建立了口模数值逆向设计的有限元模型,发现设计出口模尺寸会随着体积流量的增加而减小,随着壁面滑移程度的提高而增大。在仿真结果的基础上,利用数控机床将设计出的口模加工成实物并安装在挤出机上进行了挤出试验,发现在高体积流量下,挤出物的截面形状对挤出速度的变化不敏感。此外,测试获得的挤出物轮廓略小于数值逆向设计中的目标轮廓。为分析差异产生的原因,建立了正向挤出模型并加大了模具壁面的滑移程度。提高滑移程度后,挤出物的轮廓更加接近测试结果,可见在口模的数值逆向设计中,必须选择合理的滑移模型和参数才能获得准确的模具形状。最后对全文工作进行了总结,并对今后的研究工作进行了展望。
[Abstract]:Extrusion is one of the basic forming processes of polymer materials. Some important semi-finished parts of radial tire, such as tread, are extruded. Due to the existence of fluid elasticity, the tire rubber will obviously expand after leaving the extrusion die, so that the cross-sectional shape of the extruder is different from the cross-section shape at the outlet of the die, which will increase the difficulty of mold design and process control. The extrusion process analysis is a long-term concern of tire engineering and academia challenging application foundation problems, although people have done research for decades, but has not yet thorough understanding of tire rubber special rheological behavior, some complex extrusion process analysis methods also need to be improved. Under such a background, a series of research work has been carried out by combining experimental research with numerical simulation, aiming at providing a complete and effective numerical solution strategy for extrusion process in tire production. By using dynamic shear rheometer, RPA2000 capillary rheometer Rosand RH2200 and double barrel rotary rheometer Physica MCR 301 in the shear rate (frequency) wide range of various typical tire rubber were systematically rheological tests, comprehensively reveal the glue viscosity, complex viscosity, storage modulus and loss modulus the change of shear rate, oscillation frequency, shear strain and temperature relationship. The viscosity of the rubber decreases with the increase of the shear rate, which belongs to the typical shear thinning fluid. However, there is no obvious Newton viscosity platform at low shear rate. When the test temperature rises from 80 to 100 degrees, the viscosity of the rubber will decrease, but with the further increase of temperature, the viscosity difference caused by temperature will be smaller. The particle filling will affect the linearity of the viscoelastic behavior of the rubber, and the vulcanization system will not. In addition, the relationship between steady state and dynamic viscosity data is compared by using Cox-Merz relationship. It is found that the shear viscosity measured by capillary rheometer and rotational rheometer is lower than that measured by dynamic shear rheometer. The rheological results based on the Carreau model and has Phan-Thien-Tanner (PTT) model of tire rubber rheological behavior of pure viscous and viscoelastic characterization, and the Extrusion Swell Behavior of shear thinning fluid, Newton fluid and viscoelastic fluid was investigated by establishing the finite element model of capillary extrusion. For Newton fluid and shear thinning fluid, the slight swell phenomenon comes from the redistribution of velocity field before and after extrusion, and the expansion ratio of Newton fluid is constant. The dilatancy ratio of shear thinning fluid decreases with the increase of volume flow rate. For viscoelastic fluids, the main cause of the extrusion swell is the elastic recovery of the fluid under high Weissenberg numbers. The extrusion swell phenomenon calculated by viscoelastic model is more obvious, and the expansion ratio will increase with the volume flow rate increasing, which is consistent with the fact. It shows that compared with the pure viscous Carreau model, the viscoelastic PTT model is more suitable for the numerical simulation of tire rubber extrusion process. In addition, in order to analyze the influence of wall slip on capillary measurement, a finite element model of capillary rheometer is established. The simulation results show that the existence of wall slip makes the shear viscosity smaller than that measured by capillary rheometer, which reveals the reason for the deviation between dynamic and steady viscosity curves. For solving the problem of CO extrusion with extrusion swell, a step by step iteration scheme is first proposed to solve the difficulties of simultaneous calculation of the deformation of free surface and material interface. The results show that the shape of the interface between the downstream material and the exit interface of the extrusion die is obviously different from that of the extrudate, which indicates the necessity of considering the extrusion swell section. The use of arbitrary Lagrange Euler (Arbitrary Lagrangian-Eulerian ALE) method to establish the finite element single tire tread extrusion and co extrusion model, simulated extrusion profile and interface shape are in good agreement with the experimental results, there are some differences in local. In addition, the volume of fluid (volume of fluid VOF) method was established for tire tread extrusion finite element model, this method can describe the dynamic process of tread extrusion and traction belt and influence on the extrusion section, but the calculation results of the ALE method found in the same grid the scale and accuracy of the VOF method is relatively poor. A finite element model for the reverse design of the rectangular die is established for the rectangular extrusion. It is found that the size of the design outlet decreases with the increase of volume flow and increases with the increase of the wall slip. On the basis of the simulation results, the dies designed by the NC machine were processed into physical objects and installed on the extruder for extrusion test. It was found that under high volume flow, the cross-sectional shape of the extruders was not sensitive to the change of extrusion speed. In addition, the extrusion profile obtained by the test is slightly smaller than the target contour in the numerical reverse design. In order to analyze the causes of the difference, the forward extrusion model was established and the sliding degree of the die wall was increased. After improving the slip degree, the contour of the extruder is closer to the test result. Therefore, in the numerical reverse design of the die, we must choose a reasonable slip model and parameters to get the accurate die shape. Finally, the full text work is summarized, and the future research work is prospected.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O37

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