Relationship between the crystallization behavior and the warpage of film‐insert‐molded parts

The dimensional variation of an injection‐molded, semicrystalline polymer part is larger than the variation of an amorphous polymer part because the shrinkage of a crystalline polymer is generally greater than the shrinkage of an amorphous one. We investigated the warpage of film‐insert‐molded (FIM) specimens to determine the effect of the crystallization behavior on the deformation of FIM parts. More perfect crystalline structures and higher crystallinity developed in the core region of the FIM specimens versus other regions. Relatively imperfect crystalline structures and low crystallinity developed in the adjacent regions of the inserted films, whereas a thin, amorphous skin layer developed in the adjacent regions of the metallic mold wall. The crystallizable substrate in the FIM specimens caused very large warpage because nonuniform shrinkage occurred in the thickness direction of the specimens. Therefore, the warpage of an experimentally prepared FIM poly(butylene terephthalate) specimen was greater than that predicted numerically because of its complex crystallization behavior. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

基于CAE的厚壁塑件注塑工艺参数优化

利用Moldflow软件对某厚壁塑料制件的注射成型过程进行分析,选取反映制品收缩与翘曲的多个评价指标,结合正交实验法,优化充填时间、熔体温度、保压时间、保压压力、冷却时间等工艺参数,通过均值分析与极差分析研究各因素对各评价指标的影响,并通过综合评分法得到一组最佳的工艺参数。结果表明,增加保压时间与保压压力能减小产品的收缩和翘曲,且得出的最佳工艺组合为注射时间为2.5 s,熔体温度为280 ℃,保压时间为130 s,保压压力为110 MPa,冷却时间为40 s,该工艺下产品的质量疏松度、体积收缩率、平面误差、翘曲分别降低了6.66 %、7.90 %、12.5 %、20.83 %,产品整体成型品质得到有效提高。     

Optimization and numerical simulation analysis for molded thin‐walled parts fabricated using wood‐filled polypropylene composites via plastic injection molding

Plastic injection molding is discontinuous and a complicated process involving the interaction of several variables for control the quality of the molded parts. The goal of this research was to investigate the optimal parameter selection, the significant parameters, and the effect of the injection‐molding parameters during the post‐filling stage (packing pressure, packing time, mold temperature, and cooling time) with respect to in‐cavity residual stresses, volumetric shrinkage and warpage properties. The PP + 60 wt% wood material is not suitable for molded thin‐walled parts. In contrast, the PP + 50 wt% material was found to be the preferred type of lignocellulosic polymer composite for molded thin‐walled parts. The results showed the lower residual stresses approximately at 20.10 MPa and have minimum overpacking in the ranges of ?0.709% to ?0.174% with the volumetric shrinkage spread better over the part surface. The research found that the packing pressure and mold temperature are important parameters for the reduction of residual stresses and volumetric shrinkage, while for the reduction of warpage, the important processing parameters are the packing pressure, packing time, and cooling time for molded thin‐walled parts that are fabricated using lignocellulosic polymer composites. POLYM. ENG. SCI., 55:1082–1095, 2015. © 2014 Society of Plastics Engineers     

Molded geometry and viscoelastic behavior of film insert molded parts

Virgin injection‐molded tensile specimens without any inserted film and four kinds of film insert molded (FIM) tensile specimens were prepared. They were annealed at 80°C to investigate the effect of residual stresses and thermal shrinkage of the inserted film on thermal deformation of tensile specimens. The FIM specimens with the unannealed film were bent after ejection in such a way that the film side was protruded and the warpage was reversed gradually during annealing and the film side was intruded. Warpage of the FIM specimen with the film annealed at 80°C for 20 days was not reversed during annealing. Processing of the FIM specimens have been modeled numerically to predict thermoviscoelastic deformation of the part and to understand the warpage reversal phenomenon (WRP). Nonisothermal three‐dimensional flow analysis was carried out for filling, packing, and cooling stages. The flow analysis results were transported to a finite element stress analysis program for prediction of deformation of the FIM part. The WRP was caused by the combined effect of thermal shrinkage of the inserted film and relaxation of residual stresses in the FIM specimen during annealing. It is expected that this study will contribute towards the improvement of the FIM product quality and prevention of large viscoelastic deformation of the molded part. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

A unified K-BKZ model for residual stress analysis of injection molded three-dimensional thin shapes

The flow-induced and thermally induced residual stresses during injection molding of a thin part with complex geometries are predicted. The injection molding precess was considered to consist of a filling and a post-filling stage (packing coupled with cooling). Additionally, the analysis were applied to successive stages of the process. The model takes into account the viscoelasticity of the molding polymer, which has been neglected in most previous works, because of the complexity of its inclusion. A unified K-BKZ viscoelastic constitutive model, capable of modeling both the fluid-rubbery state and the glass state of amorphous polymers, was employed for simulating this problem. For the flow-induced residual stress predictions of the filling stage, a quasi-steady state approximation was employed for each element of the part, for the calculation of stress profile and subsequent stress relaxation after cessation of flowf. Stress calculations were provided for the thermally induced residual stress predictions of the post-filling stage. These explicit calculations led to the results of pressure and temperature distributions of the part during the post-filling stage into the viscoelastic constitutive model. Additionally, the pressure and asymmetric temeprature profiles of the post-filling stage were based on finite element packing analysis coupled with a boundary element cooling analysis of the molding process. Finally, the total residual stress in the part was obtained via superposition of the flow-induced and thermally induced residual stresses. An example is provided to demonstrate the entire concept. The results indicate that thermally induced residual stress is higher than the flow-induced residual stress by one to two orders of magnitude.     

Flow‐induced warpage of injection‐molded TLCP fiber‐reinforced polypropylene composites

The most common belief is that warpage in injection‐molded fiber‐reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow‐induced stresses generated during mold filling do not play a significant role. Injection‐molded plaques of polypropylene (PP) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) microfibrils were generated in order to investigate the role of residual flow‐induced stresses relative to that of thermal stresses on the warpage. In an effort to relate the material parameters to warpage, the rheological behavior of these fiber‐filled systems was investigated. The shrinkage and the thermal expansion of the TLCP/PP composites, and hence, the thermally induced stresses decreased with an increase in fiber loading while the flow‐induced stresses increased. The increase in the flow‐induced stresses was attributed to increased relaxation times (this is not the only cause, but is a significant factor) with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber‐reinforced thermoplastics, the flow‐induced residual stresses must be accounted for. It is expected that the results reported here can be extended to glass‐reinforced PP composites as well. POLYM. COMPOS., 27:239–248, 2006. © 2006 Society of Plastics Engineers     

Warpage–crystallinity relations in rotational molding of polypropylene

The authors present a study of the crystalline properties of selected series of polypropylene (PP) materials in relation to their warpage at conditions relevant for rotational molding. The PP materials have different crystallization temperatures and kinetics. The authors study the crystalline features of the materials using hot‐press experiments and differential scanning calorimetry (DSC). In the DSC, the authors do constant cooling rate runs as well as isothermal crystallization and subsequent heating. A multimode crystallization kinetics model was fitted to the DSC cooling runs. Regression analysis was done to relate the results from the crystallization experiments to warpage. The derived empirical model shows that the crystallization temperature, crystallization half‐time, and heat of fusion are the most significant parameters influencing warpage. Materials with warpage deviating from the model average are discussed taking aspects of the multimode kinetics into account. The present work could represent a basic platform for understanding and predicting the warpage during rotational molding process. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Residual stress and viscoelastic deformation of film insert molded automotive parts

Complex automotive parts were produced by film insert molding and the ejected parts were annealed to investigate the viscoelastic deformation. Warpage of the part was predicted by numerical simulation of mold filling, packing, and cooling stages with non‐isothermal three‐dimensional flow analysis. The flow analysis results were transported to a finite element stress analysis program and the stress analysis was performed by using time‐temperature superposition principle to investigate viscoelastic deformation. Predicted residual stresses, viscoelastic deformation, and warpage showed good agreement with experimental results. Thermal shrinkage of the inserted film and relaxation of the residual stress affected the viscoelastic deformation of the part significantly during annealing. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Comparative study of warpage,global shrinkage,residual stresses,and mechanical behavior of rotationally molded parts produced from different polymers

A comparative study of warpage, global shrinkage, and residual quench stresses developed in rotational molding is made for a series of thermoplastics including various polythylenes, polypropylene, polyamide-6, polycarbonate, and polystyrene. The influence of rate of quench on uniaxial stress strain and impact behavior of rotomolded parts was also studied. Generally, warpage, global shrinkage, and residual stresses increase with increasing quench rate for all the polymers. Further, the levels of warpage and global shrinkage increases with extent of crystallization, i.e., products from glassy polymers exhibit little warpage and those from highly crystalline polymers are highly warped. Increasing rate of quench tends to increase elongation to break and impact strength.     

基于CAE的注塑件翘曲变形优化研究

运用Moldflow软件进行某后盖产品的翘曲变形分析,结合Moldflow软件的翘曲变形分析理论,将影响注塑件的翘曲变形因素分为3类:冷却不均、收缩不均及取向因素。然后针对翘曲变形不同的影响因素提出冷却优化、结构优化、保压优化及反变形设计等优化方案。分析结果表明:提出的这几种优化方案能极大地改善注塑件的翘曲变形,为注塑制品企业在控制产品尺寸、满足装配质量要求、改善翘曲变形等方面提供优化思路及工艺参考。     

双色电器外壳的注射成型

利用专业模流分析软件Moldflow/MPI,对双色电器外壳的注射成型填充过程进行了模拟分析。结果表明,内、外层壳体的最佳浇口位置均位于底部外表面圆心处;气穴易出现在制品边缘;翘曲主要表现为径向翘曲。还对内外层壳体的填充时间、冷却时间、体收缩率等进行分析比较,确定了最大注射压力等主要工艺参数,从而使双色电器外壳的注射成型工艺和模具结构得到优化。     

Development of warpage and residual stresses in film insert molded parts

Residual stresses, bending moments, and warpage of film insert molded (FIM) parts were investigated by experimental and numerical analyses. Thermally induced residual stresses in FIM parts were predicted by numerical simulations with both commercial and house codes. Bending moments and warpage of FIM tensile specimens were calculated numerically and compared with experimental results. Thermally induced residual stresses were predicted by utilizing a one‐dimensional thermoelastic model where constant material properties are assumed. The residual stress distribution depended remarkably on the Biot number and the heat was removed rapidly through the surface resulting in high residual stresses. Asymmetric residual stresses generated by nonuniform cooling of the part provoked nonuniform shrinkage and warpage of the molded tensile specimen. It was found that the numerically calculated bending moment is in good agreement with the experimental results. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Exfoliation of Kaolinite Nanolayers in Poly(methylmethacrylate) Using Redox Initiator System Involving Intercalating Component

Injection molding is one of the most widely employed methods for the fabri- cating of polymer articles, being characterized by high production rates and accurately dimensioned products. The process includes the flow of polymer melt through a runner system and gates followed by injection into a cold mold, packing under high pressure, and subsequent cooling to solidification. Accordingly, during the injection-molding process the polymer undergoes simultaneous mechanical and therma! influences while in fluid, rubbery, and glassy states. Such effects introduce residual stresses and strains into the final product [1,2], resulting in highly anisotropic mechanical behavior [3–9] and warpage and shrinkage [10–13]. Thus, understanding the factors governing the residual-stress development during molding is of great importance.     

注塑成型CAE

利用ＣＡＥ能有效提高注塑成型的质量。文中给出了充填、冷却、保压残余应力、纤维取向、应力翘曲分析的数学模型及求解方法，并概述了注塑成型ＣＡＥ的发展趋势。     

A study on the distinguishing responses of shrinkage and warpage to processing conditions in injection molding

A challenging task in injection molding industry is to minimize shrinkage and warpage (S&W) through optimal setting of molding conditions. In determining the relationship between molding conditions and product dimension, most existing literature considered S&W as a whole entity or focused on only one of them. The intention of this study was to distinguish these two terms, and perform a thorough analysis on the effect of operative conditions on S&W during injection molding process through a combination of experimental and numerical methods. Six process parameters with five levels were examined on a box-shaped product, and the single factor analysis of variance (ANOVA) was adopted in identifying the significance of each variable in the experiment. Results indicated that the effect of processing conditions on shrinkage is different from that on warpage. Specifically, packing pressure affects shrinkage most while packing time is the dominant factor in determining warpage. The reaction of shrinkage to packing pressure is monotonic, whereas the plot of warpage shows a U-shaped variation. A differential treatment of S&W can therefore help to enhance product quality. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

塑封成型过程芯片热流固耦合变形机理数值模拟研究

基于Castro-Macosko 固化动力学模型,建立了描述塑封填充过程及其芯片热-流-固多场耦合翘曲变形形成过程的理论模型,并揭示了其变形机理。结果表明,芯片热流固耦合翘曲变形先随熔体充填流动时间的增加而快速增加,达到最大值之后逐渐减小,并趋于恒定;芯片热-流-固耦合综合翘曲变形主要由热-流-固耦合压力场诱发的翘曲变形和不均匀温度场诱发的热变形组成,芯片热流固耦合压力场诱发的变形为向外的翘曲变形,且正比于芯片上下表面熔体充填不平衡流动的流长差和充填流动速度差,并沿轴向呈先增后减的对称抛物线分布,热-流-固耦合压力场诱发的翘曲变形远大于不均匀温度场诱发的热变形,芯片热-流-固耦合综合翘曲变形主要由热-流-固耦合压力场诱发的变形控制。     

Modeling warpage in reinforced polymer disks

A circular disk mold is sometimes employed to test how a reinforced polymer will induce part warpage or distortion after removal from an injection mold. These polymer disks are modeled by developing an analogy to axially symmetric disks that have the reinforcement either in the disk radial or tangential direction and warp to a cup or saddle shaped disk during cooling. The governing mechanisms causing warpage are discussed with analytical and numerical models employed to quantify the impact of in-plane thermal anisotropy on warpage. The cup and saddle warpage formulae that are derived can guide the engineer on estimating the underlying polymer properties as well as evaluating the impact of part design on thermal warpage.     

基于Moldflow变直径管工艺参数优化与模具设计

建立变径管模型,并利用Moldflow对变直径管进行充填、保压、冷却、翘曲模拟分析得出最佳浇口位置与数量,结合正交实验分析了模具温度、熔体温度、充填时间、保压压力、保压时间等工艺参数对变直径管翘曲变形的影响并得到最佳参数组合;利用Moldflow验证最佳参数组合,并在此基础上设计出模具,使得生产的产品品质得到提高。     

Properties of poly(ethylene terephthalate) prepared by high-pressure extrusion in a capillary die

The procedure of Southern and Porter has been used to prepare poly(ethylene terephthalate) segments by high-pressure extrusion in an Instron capillary rheometer at temperatures from 245° to 265°C. X-Ray measurements of crystalline orientation along the axis of long-growth segments showed that segment properties were controlled by the time-dependent crystallization of polymer melt in the rheometer reservoir. During the initial stage of extrusion, a highly oriented, translucent segment was generated by flow-induced crystallization. However, formation of the translucent morphology eventually stopped, and thereafter a poorly oriented, opaque segment was generated by solid-state extrusion. From wide- and small-angle x-ray scattering, differential scanning calorimetry, and density measurements, it was determined that the most perfect morphologies were prepared at an extrusion temperature of 265°C. In addition to having the highest degree of crystalline orientation, translucent segments extruded at 265°C have a peak melting point of 267°C and a crystallinity value of 62%. Hot-stage optical microscopy showed that the translucent segments contained axially aligned fibrous crystals whose birefringence persisted to 290°C. The exceptional thermal stability of the segments was corroborated by the results of shrinkage tests at temperatures near the melting point; even after 1 hr at 260°C, the shrinkage did not exceed 7%. The accumulated evidence suggests that the translucent segments contain an extended-chain component.     

基于 Taguchi 法的注塑制品成型收缩率最小化

运用Taguchi实验设计技术研究了熔体温度、冷却时间、保压压力、注射时间等工艺参数对聚丙烯(PP)碱性蓄电池壳注塑制品成型收缩率的影响,获得了使制品的成型收缩率最小的优化工艺参数。研究表明,在所选择的工艺参数中,对成型收缩率的影响程度按大小排序依次为熔体温度、冷却时间、保压压力、注射时间。所预测的优化工艺组合的最小成型收缩率为0.916%,与验证实验值0.908%较为吻合。