注射成型中聚合物剪切诱导结晶行为的三维模拟

在考虑剪切导致分子链取向并升高其平衡熔点的基础上,建立了基于Nakamura方程的剪切诱导结晶动力学模型。在WLF-Cross黏度模型中引入结晶对黏度系数的影响,构建了考虑结晶的注射成型过程模型。采用改进的有限体积法对聚合物剪切诱导结晶行为进行了三维数值模拟,模拟中耦合了流动场、熔体压力、温度、诱导时间与结晶度。结果表明,本方法可清晰模拟出注射成型过程中聚合物的三维"喷泉"流动行为以及3层"皮-芯"结晶结构,同时,诱导结晶时间指数与相对结晶度的模拟结果与理论及实验结果吻合。     

HDPE微注射成型过程非等温动态结晶模拟

采用布拉本德实验仪模拟微注射成型过程，研究结晶型聚合物微注射成型过程的剪切诱导结晶。结果表明，其他工艺参数不变时，当班伯里转子转速从40r／min增加到80r／min，试样的结晶度从74．43％增大到84．36％，结晶度随着剪切速率的增加而增大，剪切速率促进剪切诱导结晶的形成；当熔体初始温度从145℃升至185℃时，试样结晶度从74．96%增大到79．43％，结晶度随着熔体初始温度的上升而增大；当剪切时间从10min增加到20min时，试样结晶度从77．48％增大到80．17％，结晶度随着剪切时间的增加而增大，剪切作用可以促进结晶。比较动态与静态DSC的实验结果，动态结晶度（79．43％）高于静态结晶度（77．48％）。在同样的热历史影响下，剪切等外力场作用会促进HDPE的结晶过程。将上述结晶过程的研究结果应用于微注射成型生产实际，可从结晶度的角度来优化微注射成型熔体温度、注射压力和注射速率等工艺参数，提高成型零件的质量。     

注塑成型中PA46的结晶行为及数值模拟

以结晶型聚酰胺46(PA46)注塑样条制件为研究对象,采用X射线衍射和差示扫描量热分析了注塑制件不同位置的表层和芯层的结晶行为,使用Moldflow软件对PA46注塑样条制件的剪切速率、结晶过程和尺寸收缩进行了模拟仿真,并与实际注塑成型制件进行比较。在注塑成型的剪切诱导下,PA46在受限区域显示出复杂的结晶行为,其表芯层显示完全不同的晶型结构,且不同位置处的结晶行为也存在很大差异。模拟结果表明,不同位置处的剪切速率和表/芯层的剪切速率梯度对相对结晶度和晶粒尺寸有着直接的影响。基于结晶模拟的PA46注塑制件体积收缩率和沿流动方向的收缩率明显较高,但在厚度和宽度方向上收缩率降低,且小于实测收缩率。通过注塑成型中PA46结晶行为的实验和模拟研究,有助于理解结晶型聚合物在成型过程中的结构演变,实现材料-成型-性能的一体化控制。     

注射成型工艺参数对制品结晶度的影响机制

以一种塑料水杯为研究对象,通过设定不同熔体温度、模具温度、注射时间及保压压力作变量,对不同条件下聚合物熔体剪切速率分布情况及其对结晶度的影响机制进行研究。结果表明,在充模流动方向上剪切速率随着距离浇口位置越远而逐渐减小,厚度方向上剪切速率呈现对称的"V"状分布,分别在中心层和距模具0.1 mm处达到最小和最大值;不同熔体温度与模具温度下熔体剪切速率分布规律相似,影响制品结晶性能的关键因素是二者的温度差;在成型过程中缩短注射时间或增加注射速率将有利于分子链发生结晶,而保压压力则对制品结晶度影响不大。     

结晶型聚合物注射成型数值模型优化

以聚丙烯为研究对象,分析了结晶型聚合物注射成型过程中晶态结构对成型工艺的影响,提出了结晶项和黏度变化因子的计算模型,优化了聚合物宏观本构方程。通过实例的模拟分析和对比,证明所提出的基于结晶微结构特征的优化模型是行之有效的。该模型为注射成型的准确模拟和制品性能预测提供了依据。     

聚合物剪切流变特性及其结晶行为研究

通过旋转流变仪对熔融态聚丙烯的流变特性进行研究,首先得到不同剪切流动条件下聚丙烯黏度及应力随剪切速率变化曲线,待剪切后的熔体冷却至室温时,再对经历不同速率流动后的聚丙烯试样进行X射线扫描,对其衍射图谱进行分析,最终得到聚合物剪切历史对其结晶行为的影响规律。结果表明,聚丙烯熔体在剪切流动时呈现明显的剪切变稀特征,其黏度随剪切速率的增大而降低并最终趋于一个恒定值,且剪切应力与剪切速率两者关系符合幂律模型;聚丙烯的结晶行为表现出明显的剪切历史依赖性,其结晶度、晶粒尺寸和晶粒取向均与剪切速率呈正相关性。     

基于两相模型的聚合物流动诱导结晶数值模拟

基于两相流动诱导结晶模型,采用谱方法分别计算无定形相的构象张量分布函数和半晶相的取向张量分布函数,进而根据Avrami方程和晶核成核速率与第一法向应力差的关系计算成核速率和结晶度。预测剪切对体系结晶速度的影响,并模拟了活化晶核数目和晶体取向的演化。计算结果表明,剪切对聚合物的结晶动力学性能有显著的影响,但是剪切对聚合物结晶的加速作用不是无限制的,随着剪切强度的增加,对结晶加速作用会变得不再明显。     

聚合物流动诱导结晶取向模型及数值模拟

流动使结晶性聚合物的结晶行为发生很大变化,不仅加速了整个结晶发展过程,还使结晶形态发生变化,形成不同的取向结构。将晶体假设为刚性哑铃,介绍描述晶体取向的数学模型,并结合模型根据闭合近似法模拟稳态简单剪切流场下聚合物的取向行为。     

微层共挤出过程层叠单元流道三维流场分析

采用Cross-WLF本构方程,建立了层叠单元流道短纤维填充聚合物注塑成型充填阶段三维黏弹数值模型,运用有限元法,对聚合物熔体在层叠单元流道中注塑流动过程进行数值模拟。研究了层叠单元流道中聚合物熔体的流动过程、剪切场分布以及微层剪切流场对短纤维填料的取向作用。结果表明:分流道结构的微小差异会引起聚合物熔体流动波前的不一致,但最终趋于平稳流动状态;聚合物熔体进入扭转、展宽、变薄的区域时,由于流道结构和尺寸突变,剪切速率急剧增大,影响了流动的稳定性;层叠单元流道的结构设计有利于聚合物熔体中短纤维的取向。     

振动注射成型技术研究

对本课题组多年来进行的动态保压注射成型、剪切振动注射成型和压力振动注射成型进行了简要总结，研究表明，振动注射成型不仅能改善聚合物熔体的高黏度、难流动、压力传递难等工艺问题，同时能部分挖掘出聚合物材料的力学潜能，达到自增强。     

Injection molding of semicrystalline polymers. II. Modeling and experiments

The injection molding of an isotactic polypropylene was computer-simulated with both quiescent and shear-induced crystallization taken into account. A one-dimensional finite difference model was used to simulate the filling, packing, and cooling stages of the injection-molding cycle. The Spencer-Gilmore equation was used to relate the density variations to the pressure and temperature traces in the packing simulation. The quiescent crystallization kinetics was modeled by the differential form of the Nakamura equation. The theory developed by Janeschitz-Kriegl and co-workers was used to model the shear-induced crystallization kinetics. The pressure traces during the filling and packing stages of the molding cycle, the thickness of the shear-induced crystallization layer, and the crystallinity profile throughout the thickness of the part were measured and compared with predicted values. © 1995 John Wiley & Sons, Inc.     

Effect of flow rate and temperature on crystallization kinetics,crystallinity index,and elastic modulus of PEEK

Dynamic, in situ wide angle X-ray scattering (WAXS) studies of the melt crystallization of injection-molded poly(ether ether ketone) (PEEK) have been carried out using an X-ray diffractometer and a position-sensitive detector. A test cell has been fabricated to fit inside the diffractometer and yet work as a complete injection molding apparatus. The rate of crystallization has been shown to increase with decreasing crystallization temperature and/or increasing flow rate in the mold. The crystallization rate decreases dramatically with increase in melt soak time at 400°C. The crystallinity index, which affects the stiffness, toughness, and fracture behavior of PEEK, has been measured under various processing conditions, by wide angle X-ray scattering, so as to optimize the process parameters: molding time, mold temperature, melt temperature, soak time at melt temperature, and flow rate. It has been shown that the crystallinity and hence the elastic modulus increase with increase in crystallization temperature and/or flow rate. Chain orientation has been shown to be absent in the bulk of the injection-molded specimens under normal molding conditions.     

Modeling/simulating the injection molding of isotactic polypropylene

A unified formulation is presented for modeling the injection molding of isotactic polypropylene (i‐PP). The crystallization kinetics are based upon the differential Nakamura equation in which the characteristic time is dependent upon temperature, pressure and flow‐induced shear stress, without any explicit need for an induction time. A Cross/WLF model is used to represent the shear viscosity in which η_O is dependent upon temperature, pressure and crystallinity. Use is made of a recent correlation for the PVT behavior of i‐PP with an explicit dependence upon crystallinity. A finite‐difference implementation of the modeling is applied to two independent molding experiments available from the literature, with notable results concerning the late‐time cavity pressure traces and time‐dependent gapwise shrinkage prior to ejection.     

Computer simulation of stress‐induced crystallization in injection molded thermoplastics

Injection molding of semicrystalline plastics was simulated with the proposed stress‐induced crystallization model. A pseudo‐concentration method was used to track the melt front advancement. Stress relaxation was considered using the WFL model. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. The simulation results reproduced most of the experimental results in the literature. Comparison is made between the slow‐crystallizing polymer (PET) and fast‐crystallizing polymer (PP) to demonstrate the effect of stress on the crystallization kinetics during the injection molding process for materials with different crystallization properties. The results show that for fast‐crystallizing plastics, stress has little effect on the final crystallinity in the injection molded parts.     

Crystallinity and microstructure in injection moldings of isotactic polypropylenes. Part II: Simulation and experiment

The injection moldings of isotactic polypropylenes with various molecular weights were simulated using finite difference method. In the simulations, the unified crystallization model proposed in our previous paper was applied. The prediction of crystallinity and microstructure development in the moldings was based upon the crystallization kinetics and the “competing mechanisms” for introducing various microstructure layers in the moldings. Extensive injection molding experiments were carried out. The pressure traces during the molding experiments were recorded. The crystallinity distribution in the moldings was determined using differential scanning calorimetry. The measurements on the microstructure embedded in the moldings were performed, including the thickness of the highly oriented skin layer and the gapwise distribution of the spherulite sizes. The measured data for the crystallinity and microstructure in the moldings were compared with the simulated results. The effects of molecular weight and processing conditions on the development of crystallinity and microstructure in the moldings were elucidated. Theoretical predictions were found to be in a good agreement with experimental measurements.     

Injection molding of semicrystalline polymers. I. Material characterization

Various material data for an isotactic polypropylene were acquired for the simulation of the injection molding of this material. Viscosity as a function of shear rate and temperature was measured using a capillary rheometer at high shear rates and a cone-and-plate rheometer at low shear rates. Heat-flow properties, characterizing kinetics and induction time of quiescent crystallization, were obtained from DSC measurements. Material data characterizing shear-induced crystallization were obtained from extrusion experiments through a slit die with subsequent quenching of the material in the die after various rest times. The thickness of the shear-induced crystallization layer was measured along with the birefringence in this layer. A model of shear-induced crystallization developed by Janeschitz-Kriegl and co-workers was used to fit the kinetic data. Thus, kinetic parameters such as the limiting shear rate below which no shear-induced crystallization can occur and the characteristic time for the relaxation of birefringence were obtained. © 1995 John Wiley & Sons, Inc.     

Effect of polymer properties on the structure of injection-molded parts

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection-molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear-induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear-induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.     

Flow‐induced crystallization in the injection molding of polymers: A thermodynamic approach

The prediction of the crystallinity and microstructure that develop in injection molding is very important for satisfying the required specifications of molded products. A novel approach to the numerical simulation of the skin‐layer thickness and crystallinity in moldings of semicrystalline polymers is proposed. The approach is based on the calculation of the entropy reduction in the oriented melt and the elevated equilibrium melting temperature by means of a nonlinear viscoelastic constitutive equation. The elevation of the equilibrium melting temperature that results from the entropy reduction between the oriented and unoriented melts is used to determine the occurrence of flow‐induced crystallization. The crystallization rate enhanced by the flow effect is obtained by the inclusion of the elevated equilibrium melting temperature in the modified Hoffman–Lauritzen equation. Injection‐molding experiments at various processing conditions were carried out on polypropylenes of various molecular weights. The thickness of the highly oriented skin layer and the crystallinity in the moldings were measured. The measured data for the microstructures in the moldings agree well with the simulated results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 502–523, 2005     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of semicrystalline polymers

A novel approach to predict anisotropic shrinkage of semicrystalline polymers in injection moldings was proposed using flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from amorphous and crystalline contributions. The amorphous contribution was based on the frozen‐in and intrinsic amorphous birefringence, whereas the crystalline contribution was based on the crystalline orientation function, which was determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with temperature‐ and crystallinity‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs on polypropylene of various molecular weights were carried out by varying the packing time, flow rate, melt temperature, and mold temperature. The anisotropic shrinkage of the moldings was measured. Comparison of the experimental and simulated results indicated a good predictive capability of the proposed approach. POLYM. ENG. SCI., 46:712–728, 2006. © 2006 Society of Plastics Engineers     

Heat transfer in injection molding of crystallizable polymers

A model is proposed for the treatment of heat transfer with crystallization during plastics processing in general, and injection molding in particular. The model incorporates experimentally determined crystallization kinetics parameters. It permits the calculation of the distribution of both temperature and crystallinity in the molding. Theoretical predictions are in good agreement with experimental measurements in both injection molding and a prototype apparatus.