PE-LLD和PE-LD熔体高速挤出特性的对比研究

采用恒速型双毛细管流变仪对比研究了PE—LLD和PE—LD熔体在高速挤出时的挤出压力振荡现象、流动曲线和挤出物外观的变化，探讨了两种PE发生挤出畸变的机理。结果表明，高速流场中PE—LLD熔体在整个毛细管上的挤出压力降发生规律性振荡，振荡幅度为1．5MP8左右，此时流动曲线发生断裂，断裂前曲线斜率为10^-1数量级，而断裂后变为10^-2数量级；而PE—LD熔体在口模入口处的压力降发生振荡，振幅为0．2MP8左右，流动曲线不发生断裂。综合分析认为，PE—LLD的挤出畸变是由于熔体／毛细管壁的边界条件变化引起的，而PE-LD的挤出畸变是由于毛细管入口处熔体的应力集中引起的。     

塑料异型材挤出口模设计的数值模拟概述

从挤出口模设计原则出发,综述了数值模拟技术在塑料异型材口模流道的曲面构型,入口角和压缩比、降低或消除挤出胀大和挤出口模成型段的设计、熔体流动平衡和压力降的研究进展,以及口模设计的发展趋势.     

气体辅助挤出的实验研究

对一种新型挤出技术-气辅挤出进行了实验研究,建立了气辅挤出实验装置并进行了实验研究,实验中在口模内壁和聚合物熔体之间建立了稳定的气垫膜层。研究了气体压力、气体温度和实验过程对气辅挤出中气垫膜层的影响。形成稳定的气垫膜层,应使气体压力低于熔体压力,但在0.2MPa以上,气体温度控制在与口模温度接近,实验时应先打开压力控制阀再进行挤出,并分析了气辅挤出对口模压降、挤出机产量及挤出胀大的影响。     

PTT熔体在塑料异型材挤出口模内黏弹流动的数值模拟

采用PTT黏弹模型,对聚合物熔体在T型异型材挤出口模内的三维等温流动进行数值模拟,得出口模内外速度、剪切速率、压力和应力分布.结果表明:由于出口效应,在口模出口处,速度、剪切速率、压力分布均发生突变.这种突变有可能造成负压,从而引发熔体破裂.口模内二次流动产生的剪切应力甚至会超过因挤出流动产生的剪切应力.应力易集中发生在口模截面的拐角处,随着挤出流量的增加,口模内最大剪切应力和最大第一法向应力差几乎线性地增加,而不会急剧增大.     

聚合物熔体在异型材三维动态挤出口模内的停留时间分布

利用ANSYS软件对受口模入口处周期性振动的压力驱动的聚合物熔体在“Y”形和“L”形异型材挤出口模内的停留时间分布进行了数值模拟。设熔体流动为三维等温广义牛顿幂律流动，在求得速度场后，应用路线示踪法跟踪流场中任一粒子的流动轨迹，进而求得熔体在口模中的停留时间分布。通过改变振动参数再重复计算，总结出振动参数对停留时间分布的影响规律。结果表明：压力的振动能有效地缩短熔体在口模内的停留时间，振幅和频率越高则停留时间越短，而且在靠近口模流道壁面处停留时间的缩短最明显，表明振动的引入可以降低壁面附近的流动阻力，使截面上的流速更平稳，从而改善挤出制品的表观质量。     

基于Polyfow的吹膜模头熔体流动二维数值模拟

对共挤复合吹膜工艺所采用的螺旋芯棒式模头内部熔体流动进行二维数值模拟,简化分析模型并得到挤出过程中熔体流动的流速及压力变化的相对趋势。模拟结果表明:熔体进入模头螺旋部分后具有一定的环向速度,随着螺旋槽深度变浅,流动的方向逐渐由螺旋环向变为挤出方向的轴向,且流速变得均匀;熔体在螺旋槽旋转处静压力较大,接近挤出方向后压力逐渐变小,口模出口处熔体流动速度仍大于流入模头的入口速度。     

入口速度振动对单管挤出口模熔体流动的影响

研究了塑料挤出过程中入口速度振动对挤出口模内熔体流变性能的影响规律。基于计算机流体力学理论,利用FLOTRAN软件对单管口模成型段熔体流动过程进行数值模拟,分析了振动力场作用下与稳态加工成型中口模成型段熔体压力场、速度场及温度场的分布情况。研究表明,沿熔体流动方向上叠加一个正弦振动的入口速度时,挤出压力、温度是周期性变化的;振动速度的引入可以提高挤出速度、降低挤出压力、加剧黏性生热,同时也能提高口模体积流率,而且随着振动幅值的增大,效果越来越明显。     

马鞍型异型材挤出成型过程三维等温粘弹性的数值模拟

基于PTT粘弹性本构模型,通过马鞍型异型材挤出成型过程的全三维稳态等温有限元数值模拟,系统研究了聚合物粘弹性流变性能参数和成型工艺参数对异型材口模挤出成型过程的影响规律,并揭示了其影响机理.研究结果表明,聚合物异型材口模挤出离模膨胀是由口模出口处的二次流动引起,离模膨胀比随着口模出口处的二次流动强度增加而增大.聚合物异型材口模挤出离模膨胀随着进口流量和聚合物熔体松弛时间的增加而增加,而随着聚合物熔体材料常数和粘度比的增大而减小.     

T形异型材挤出口模的逆向数值模拟

采用Polyflow软件对3种过渡段形状不同的T形异型材口模进行逆向挤出数值模拟,得到T形挤出口模的口型以及聚合物熔体在口模内的速度场和压力场分布。模拟结果显示了T形挤出口模出口口型与口模流道形状结构的变化无关,而适当加宽流道比直角形流道的挤出速度和压力分布更均匀。     

基于灵敏度分析的挤出平缝口模优化设计

给出了以直接微分法计算稳态非线性系统设计灵敏度的方法。在基于挤出平缝口模熔体流动机理分析的基础上，建立了口模内熔体流动数学模型。并把设计灵敏度分析法和数值成型模拟技术相结合，运用于口模流道优化设计中，降低挤出平缝口模的出口速率变化率，从而改善了产品质量。通过实例验证，提出的口模优化设计方法非常有效。     

Self-reinforcement of polypropylene by flow-induced crystallization during continuous extrusion

Self-reinforced polypropylene (PP) sheets have been prepared from melt flow-induced crystallization through a conical slit die fed by a conventional extruder. Their structure and properties, influenced by the die pressure ranging from 20 to 50 MPa and die outlet temperature, are studied by scanning electron microscopy observation, differential scanning calorimetry analyses, tensile strength, and light transmittance measurements. At a die outlet temperature of 162°C and a pressure above 30 MPa, conspicuous increases in the melting peak, tensile strength, and light transmittance (they can be used to characterize the self-reinforcement degree of sheet) are observed. The self-reinforcement degree, however, increases only slightly with increasing pressure as it exceeds 40 MPa. Raising the die outlet temperature from 162 to 172°C results in a further increase in the self-reinforcement degree (for example, a highest tensile strength of 288 MPa) while keeping the pressure at 40 MPa, so bulk PP materials with high properties can be produced from continuous melt extrusion under pressures lower than 40 MPa. Furthermore, the melt temperature plays an important role in determining the properties of self-reinforced polymeric materials. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2111–2118, 1998     

Improved die assembly for gas-assisted sheet extrusion using different up and down gas layer thicknesses

Gas-assisted sheet extrusion is a promising and environment-friendly method used in polymer processing. Insufficient gas intake due to the adherence of the polymer melt to the inlet gap is a disadvantage of the method. Therefore, in this study an improved die assembly with parallel gas intake was designed. As the melt is prone to falling at the die exit during the horizontal extrusion process, different thickness settings are used for the up and down gas layers (0.25 and 0.75 mm, respectively) to avoid this. For the improved die assembly, the gas inlet pressure may be varied systematically to study the mechanism of stable sheet extrusion. When the gas inlet pressure is 0.1 MPa, the melt can be extruded smoothly, and the extruded sheet surface is transparent and flat. However, when the optimum pressure is exceeded, the extruded sheet becomes twisted or even broken.     

Impact of organoclay and maleated polyethylene on the rheology and instabilities in the extrusion of high density polyethylene

The impact of organoclay on the rheology and extrusion of high density polyethylene (HDPE) was studied. Organoclay effect was studied at very low clay loading (≤0.1 wt %) while serving as a processing aid. A special design slit die with three transducers was used in the study of the extrusion melt instabilities. The rheological results showed that normal stress difference of HDPE was reduced during steady shear rate and stress growth tests when organoclay (≤0.1 wt %) was added. The extensional strain and stress growth of HDPE reduced with the addition of organoclay. So, organoclay (≤0.1 wt %) has an effect on the shear and extensional rheology of HDPE. The intensity of the melt instability was characterized with both a moment analysis and distortion factor (DF) from an advanced Fourier transform analysis. Both showed the same trends in the characterization of the pressure fluctuations in the die. Generally, addition of organoclay (≤0.1 wt %) to HDPE led to the reduction in DF. The ratio of first and second moment analyses became reduced as well. The results quantified the extent of elimination of gross melt fracture in HDPE by organoclay. Also, the extrusion pressure was reduced with organoclay (≤0.1 wt %) inclusion hence more throughput. There was a good correlation between rheology and extrusion. Both showed that the platy‐like organoclay streamlined the melt flow. However, the maleated polyethylene added as a compatibilizer did not give substantial synergistic effect. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

杂环芳香族聚酰胺纺丝原液孔口胀大效应的研究

采用摄影法研究了杂环芳香族聚酰胺纺丝原液的孔口胀大效应。结果表明:杂环芳香族聚酰胺纺丝原液在喷丝头挤出时会发生孔口胀大现象,在纺丝原液比浓对数黏数为4.20 dL/g,温度20～65℃,挤出压力0.25～0.45 MPa,喷丝孔长径比为1～7,滤布层数为1的条件下,纺丝原液的孔口胀大比为1.089 0～1.344 8;纺丝原液的孔口胀大比随着原液温度的上升、毛细管长径比的增大和滤布层数的增加呈逐渐减小趋势,随着聚合物相对分子质量的增大和挤出压力的增加呈逐渐增大趋势;纺丝原液的孔口胀大活化能随挤出压力的增加而增大。     

Solid-State Extrusion of Isotactic Polypropylene Through a Tapered Die. I. Phenomenological Analysis

Abstract

The solid state extrusion of isotactic polypropylene through a tapered die was conducted under various extrusion conditions using a piston-cylinder type extruder. The equation for evaluating the extrusion pressure was derived based on the plasticity theory as a function of die geometry and frictional coefficient between die wall and material. The integrand of the equation includes the yield stress as a function of true strain, which was given by the tensile properties of the same polymer. The data of lower degree of processing were fairly well described by the equation, but the estimation was not so accurate at high extrusion ratio. The molecular weight dependence of extrusion pressure was very small in solid state extrusion compared with the case of melt viscosity. This fact showed that the local inter-molecular interaction is a predominant factor in solid state extrusion. The effect of frictional resistance on a cylinder part was found to be negligible in our experimental conditions.     

Morphological and mechanical property studies of polymer extrudates obtained by the rotational extrusion process

The orientation of the reinforcing fibers in glass fiber filled polypropylene tubular extrudates has been controlled effectively by the superposition of the linear flow in a melt extruder with torsional flow generated by rotating the capillary portion of the extrusion die of the extruder. The so produced extrudates have mechanical properties which can be balanced along the hoop and extrusion direction by adjusting the extrusion rate and the rotational speed of the die for example, the breaking load of 75N along the hoop direction increases by ～40% when the capillary of the die was rotated at 80 rpm. At the same time the Young's modulus in the extrusion direction decreased from 1100 MPa under conventional extrusion conditions to 800 MPa.     

Converging flow of polymer melts in extrusion dies

Practical extrusion processes often involve geometrically complex dies. Such dies are usually tapered, or streamlined, to achieve maximum output rate under conditions of laminar flow. These converging flows may be analysed in terms of their extensional and simple shear components to calculate the relationships between volume flow rate, pressure drop, and post extrusion swelling. The analysis can also be extended to cover the free convergence as fluid flows from a reservior into a die. Comparisons between predicted and observed data for a series of coni-cylindrical dies suggest that using this approach the pressure drop/flow rate relationship can be predicted within ±20% and the swell ratio/flow rate relationship within ±10%. Similar treatments have been in use for the last three years in solving such complex flow problems as radial flow in injection moulding and two-dimensional annular convergence in blow moulding dies.     

气辅挤出过程中挤出胀大的实验和模拟研究

气体辅助挤出是一种新型挤出工艺,可以显著减小挤出胀大,减小口模压力降。对气辅挤出过程中的挤出胀大现象进行了实验研究和数值模拟,在实验研究中,对传统挤出和气辅挤出进行了对比实验,分析了螺杆转速、辅助挤出气体压力和流量对挤出胀大的影响,并采用CFD有限元通用软件FIDAP分析了气辅挤出过程中口模内的速度场和压力场。     

The effect of teflon™ coatings in polyethylene capillary extrusion

Two LLDPE resins were used in this work to determine the critical conditions for the occurrence of wall slip and melt fracture in capillary extrusion. It was found that the polymer-metal interface fails at a critical value of the wall shear stress of about 0.1 MPa and, as a result, slip occurs. At values of wall shear strees of about 0.18 MPa the extrudate surface appears to be matte, while small amplitude periodic distortions (sharkskin) appear on the surface of extrudates at wall shear stresses above 0.25 MPa. Using a special slit die, the polymer–wall interface was coated with Teflon? in order to examine the effect of this coating on the processability of polyethylenes. It was found that use of Teflon? promotes slip, thus reducing the power requirement in extrusion and, most importantly, eliminates sharkskin at high extrusion rates. © 1995 John Wiley & Sons, Inc.