聚合物气体辅助口模挤出成型的理论及实验研究
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摘要
挤出是聚合物加工的一种重要成型方法，塑料制品中40％是通过挤出加工成型。由于聚合物熔体的高粘性、粘弹性，传统挤出成型时易出现挤出胀大，由于挤出熔体和挤出口模的非滑移粘着导致挤出口模压降大，当挤出速度超过材料的临界挤出速度时挤出物表面产生“鲨鱼皮”和挤出破裂等现象，以上三大难题严重影响挤出制品精度、限制挤出速度提升，导致挤出能耗高。产生以上问题的根源在于传统挤出技术采用非滑移粘着剪切口模挤出方式，使得口模壁面对挤出熔体的阻力大。气辅挤出技术通过气体辅助控制系统和气体辅助挤出口模，使挤出时在挤出口模壁面产生一稳定的气垫膜层，挤出由非滑移粘着剪切口模挤出方式转换为完全滑移非粘着剪切口模挤出方式，从而将口模壁面对挤出熔体的摩擦阻力降到最低限度，熔体在口模中呈柱塞状流动。本文从实验和数值模拟两方面对气辅口模挤出成型进行了系统而深入的研究，主要研究工作概述如下：
1．对有关挤出胀大、减粘降阻、熔体破裂及壁面滑移的研究进展进行了综述，指出气辅挤出技术是解决以上问题的有效方法。
2．建立了气辅挤出实验装置。根据气辅挤出对气体压力和温度控制要求，建立了气辅挤出成型压力控制系统和温度控制系统，设计加工了气辅挤出口模，完成了整套实验装置的安装和调试。
3．进行了深入的实验研究。研究了工艺参数(气体压力、气体流量、气体温度、挤出流量)、口模结构参数(口模有气辅段长度L_2、环形缝隙δ)以及操作顺序(气阀和挤出螺杆的开启顺序)对气垫膜层的建立及其稳定性的影响；分别研究了气体压力、气体流量、环形缝隙δ、有气辅段长度L_2对挤出速度(熔体从口模中挤出的线速度)、挤出流量和挤出物直径的影响；采用和传统挤出相对比的方式研究了气辅挤出对口模压降、挤出胀大和挤出物表面质量的影响。
4．基于流体力学三大方程和Rivlin-Ericksen粘弹本构方程建立了气辅口模挤出成型的机理模型，根据气辅挤出过程的特点，通过合理的简化与假设和方程的无量纲化，形成了有限元方程，以壁面剪切应力为零的边界条件代替气垫膜层作用，自由表面采用脊线法更新，应用Successive Substitution和Quasi-Newton Updates迭代技术，利用通用的CFD有限元软件FIDAP实现了对有限元方程的求解。分别对轴对称结构口模、方形截面口模和L形截面口模的气辅挤出过程进行了数值模拟，得到气辅挤出成型口模内外的速度场、温度场、压力场、应力场和剪切速率的分布，并和传统挤出过程进行了对比；研究了不同口模截面形状的气辅挤出中挤出流量、气辅挤出口模有气辅段长度L_2、本构方程中反映挤出熔体粘性和弹性的参
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 L_2 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
1．对有关挤出胀大、减粘降阻、熔体破裂及壁面滑移的研究进展进行了综述，指出气辅挤出技术是解决以上问题的有效方法。
2．建立了气辅挤出实验装置。根据气辅挤出对气体压力和温度控制要求，建立了气辅挤出成型压力控制系统和温度控制系统，设计加工了气辅挤出口模，完成了整套实验装置的安装和调试。
3．进行了深入的实验研究。研究了工艺参数(气体压力、气体流量、气体温度、挤出流量)、口模结构参数(口模有气辅段长度L_2、环形缝隙δ)以及操作顺序(气阀和挤出螺杆的开启顺序)对气垫膜层的建立及其稳定性的影响；分别研究了气体压力、气体流量、环形缝隙δ、有气辅段长度L_2对挤出速度(熔体从口模中挤出的线速度)、挤出流量和挤出物直径的影响；采用和传统挤出相对比的方式研究了气辅挤出对口模压降、挤出胀大和挤出物表面质量的影响。
4．基于流体力学三大方程和Rivlin-Ericksen粘弹本构方程建立了气辅口模挤出成型的机理模型，根据气辅挤出过程的特点，通过合理的简化与假设和方程的无量纲化，形成了有限元方程，以壁面剪切应力为零的边界条件代替气垫膜层作用，自由表面采用脊线法更新，应用Successive Substitution和Quasi-Newton Updates迭代技术，利用通用的CFD有限元软件FIDAP实现了对有限元方程的求解。分别对轴对称结构口模、方形截面口模和L形截面口模的气辅挤出过程进行了数值模拟，得到气辅挤出成型口模内外的速度场、温度场、压力场、应力场和剪切速率的分布，并和传统挤出过程进行了对比；研究了不同口模截面形状的气辅挤出中挤出流量、气辅挤出口模有气辅段长度L_2、本构方程中反映挤出熔体粘性和弹性的参
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 L_2 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
引文
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