热定形对聚碳酸酯性能的影响

众所周知,塑料注射制品总是处于复杂的应力状态下。制品中的应力有时相当大,从而导致制品在使用过程中,产生翘曲、开裂、性质显著变坏等现象。制品在各种介质里和各种温度——时间条件下进行热定形,是降低应力的一种有效方法。利用偏振光方法,可评价聚碳酸酯试样在不同的热定形条件下的内应力。园柱     

聚碳酸酯气体辅助注射成型制品内应力分布及改善工艺

聚碳酸酯(PC)注塑制品出模后在无外加约束条件下内部应力得不到松弛释放,易导致制品产生开裂,严重影响制品的服役性能。为探究PC制品内应力分布规律,找到有效降低内应力的工艺措施,基于气体辅助注射成型(GAIM)工艺和自行设计制造的带有半圆形加强筋的平板试验模具,结合光弹法和溶剂检测法,研究GAIM制品内应力的分布特点,讨论注气压力和气体保压压力两种工艺参数对GAIM制品内应力的影响规律,提出两种改善制品内应力的工艺方法。研究发现：通过光弹法观测到的应力条纹分布与溶剂浸泡后的裂纹开裂区域一致,即GAIM制品末端及边缘处内应力较大,沿气道两侧应力分布较均匀;较高的注气压力和较低的气体保压压力可以更加有效减小GAIM制品的内应力;通过对GAIM制品24 h即时退火和延长二次注气的卸压时间可以有效降低制品内应力。     

一种聚合物制品内应力新型测量方法的研究

提出了利用超声波来测量高分子聚合物制品表面内应力的新方法,并且建立超声波波速和内应力间的关系模型,通过聚合物拉力实验方法也验证了此模型的正确性,最后有效地测量了通过注塑成型后聚碳酸酯(PC)薄片制品上内应力的大体分布情况,证明了用表面波测量聚合物表面残余应力值切实可行,且该方法快速、有效。     

工程用高聚物注塑成型制品服役应力优化设计

近年来,航空航天、汽车等工程领域大量使用工程用高分子聚合物材料制备结构零部件。在复杂的工作环境下,工程用高聚物注塑成型零部件承受机械荷载、热荷载等服役条件,其服役应力的大小直接关系到工程设备的结构安全。将半球形聚碳酸酯（PC）制品的翘曲变形和残余应力作为服役的初始条件,建立考虑制品结构、模具结构以及工艺条件的服役应力优化模型,利用基于Kriging代理模型和EI加点法的序列优化方法,有效地降低了半球形聚碳酸酯制品的服役应力。结果表明,制品厚度、熔体温度和保压压力对服役应力影响较大。     

工程用高聚物注塑成型制品服役应力优化设计

近年来,航空航天、汽车等工程领域大量使用工程用高分子聚合物材料制备结构零部件。在复杂的工作环境下,工程用高聚物注塑成型零部件承受机械荷载、热荷载等服役条件,其服役应力的大小直接关系到工程设备的结构安全。将半球形聚碳酸酯(PC)制品的翘曲变形和残余应力作为服役的初始条件,建立考虑制品结构、模具结构以及工艺条件的服役应力优化模型,利用基于Kriging代理模型和EI加点法的序列优化方法,有效地降低了半球形聚碳酸酯制品的服役应力。结果表明,制品厚度、熔体温度和保压压力对服役应力影响较大。     

退火对注射成型PC制品力学性能的影响

注射成型的塑料光学制品应用日益广泛,但注射成型加工的聚碳酸酯制品通常有较大的残余应力,会对制品的光学性能、力学性能有负面的影响,而退火可减少/消除制品的残余应力。本文考察了退火对不同工艺条件下注射制品残余应力和力学性能的影响。研究的工艺条件包括三水平的变化的模具温度、熔体温度、保压压力、冷却时间等;残余应力的变化通过光弹实验的应力干涉条纹表示,力学性能的变化以拉伸强度、延伸率的变化表示。实验发现,退火前后,不同工艺条件下注塑PC制品拉伸强度平均提高4.5%,最大达9.0%;同时,断裂延伸率平均降低3%,最多减少14%。     

模具结构和熔体温度对PC透明制品性能的影响

用光弹实验测试不同浇口类型、不同熔体温度下聚碳酸酯透明注塑制品的应力分布,并测试了制品的拉伸强度和透光率。研究发现,模具结构对残余应力的分布形式影响显著,高应力区的拉伸强度较低,相对于无应力区最高降幅达2.69MPa;在同一成型条件下扇形浇口试样的透光率普遍要高于潜伏式浇口试样;熔体温度对试样的残余应力分布影响不大,但随着温度的升高,拉伸强度缓慢提高;要满足最佳透光率效果,不同模具结构的试样成型温度不同,试样的透光率最高达89.93%,浇口高应力区的透光率明显较低。     

疲劳／蠕变复合作用下聚碳酸酯的时温等效关系

研究了疲劳／蠕变复合作用下聚碳酸酯的时温等效关系。试验条件随加载最大应力、最大应力保持时间和温度而变化，但是都得到一致的试验结果：随最大应力保持时问延长或试验温度升高，聚碳酸酯断裂寿命下降；最大应力保持时间t-断裂寿命Nf的关系曲线与试验温度T-断裂寿命Nf关系曲线十分相似；疲劳／蠕变复合作用下的聚碳酸酯时温等效关系仍然适用。     

热处理对聚碳酸酯环境应力开裂性能的影响

对聚碳酸酯(PC)的耐环境应力开裂性和拉伸性能进行了研究。结果表明，在不同溶剂和热处理温度条件下，PC的耐环境应力开裂性差异很大。较大的二甲苯含量容易降低PC发生环境应力开裂的临界拉伸应力及材料的拉伸强度、断裂伸长率。玻璃化转变温度以下的热处理可以提高PC的拉伸强度，但是对改善材料的耐环境应力开裂性不利。     

基于Moldfl ow的注塑光学厚壁透镜工艺优化

利用Moldflow数值模拟分析并结合正交试验法,以光学厚壁透镜制品的光相移角度(双折射)为参照,对注塑光学厚壁透镜的工艺条件进行优化,且模拟分析了分层注射对注塑光学厚壁透镜残余内应力的影响规律。结果表明:优化注射工艺参数能够降低制品的残余内应力,减小光学透镜的双折射缺陷,其最优注射条件为:模具温度90℃、熔体温度240℃、注射时间9 s、保压压力60 MPa。此外,分层注射可以有效减小制品的残余内应力,并使其分布更加均匀,该效果与分层方式以及层厚、层数有关。     

降温模式对甲烷水合物形成的影响

在定容条件下,以两种不同的降温模式(缓慢降温和快速降温)进行甲烷水合物在沉积物中的形成实验. 结果表明,甲烷水合物在沉积物中的形成过程包括气液溶解、核化、生长、稳定4个阶段. 在相同的初始条件下,降温模式对水合物生成的热力平衡影响较小,但对水合物生成动力学有显著改变. 快速降温下水合物生长速度明显快于缓慢降温,随着水合物初始条件不同,缓慢降温比快速降温水合物形成时间约增加21.4%~28.8%.     

Residual stresses and toughness of polycarbonate exposed to environmental conditions

The role of the size and type of residual stresses and their distribution in the interior of polycarbonate mold-injected test bars was studied, in view of the interrelationship between toughness and thermal or environmental history. The large thermal gradient during solidification of the polymer in the mold builds up compressive stresses near the wall and tensile stresses in the core. Annealing followed by slow cooling may reverse the type of stresses near the surface, while quenching augments the compressive stress. The latter stress near the wall is responsible for the extraordinarily high impact strength of polycarbonate. Exposure to the atmosphere and immersion in hot water may affect the distribution of residual stresses and thus contribute to the embrittlement of the originally tough polymer. The importance of molecular weight and polymer stabilization is elucidated.     

Modelling the rapid cooling and casting of chocolate to predict phase behaviour

In recent years, rapid cooling processes have been developed to manufacture complex, three-dimensional shapes for confectionery products (e.g. chocolate Easter eggs). These processes are similar to injection moulding and casting processes used in the metallurgic and polymer industries. Although these manufacturing routes are commonly used, they are still poorly understood and thus not optimised. In traditional chocolate processing, cocoa butter is cooled at a low rate (<2 °C/min) to obtain the desired polymorph (form V), and higher cooling rates lead to undesired products. It is thus a paradox that rapid cooling processes which generate very high cooling rates (>100 °C/min) are used to manufacture acceptable products. To obtain an understanding of the occurring phenomena, a mathematical model of the phase change of cocoa butter and heat transfer has been developed, solved using the finite element method and validated against X-Ray diffraction data.During this rapid cooling step, only a small fraction of the cocoa fats is solidified. This fraction increases the viscosity and the yield stress of the material locally. This allows maintaining the shape prescribed during the rapid cooling throughout the rest of the process. Numerical experiments over a range of processing conditions indicated that the Planck solution of the Stefan problem could be used to design processes. The numerical simulation shows that the rest of the material does not crystallise during the rapid cooling step but when subsequently passing through cooling tunnels.If a filling is injected at high temperature into the egg shells, local melting of the chocolate shells will occur. It was calculated using the model that for polycarbonate moulds a deposition at temperatures above 36 °C would compromise the structure of the chocolate shell. The model predicted that metal moulds of higher thermal conductivity would allow better cooling efficiency and raise the critical filling deposition temperature up to 45 °C.     

The influence of thermal history on internal stress distributions in sheets of PMMA and polycarbonate

The through-thickness birefringent patterns (n_x – n_z) of PMMA and polycarbonate (PC) are measured as a function of thickness, cooling rate, and annealing time. A method is demonstrated for separating the elastic stress from the residual birefringence. The residual birefringence is explained as the residual orientation induced when cooling through the galss transition. The shape of the elastic (tempering) stress distribution is apparently independent of the cooling rate and annealing time, the choice of polymer, and the sheet thickness. The magnitude of the tempering stres is dependent on cooling rate but nearly independent of choice of polymer and sheet thickness. The decays of both the tempering stress and the residual orientation are measured as a function of annealing time at 120°C for polycarbonate. The measured birefringence decays only slightly; however, the elastic tempering stress reverts, on annealing, to permanent (anelastic) orientational strain. The tempering stress at the sheet surface is not an adequate criterion for defining toughness in polycarbonate.     

The effect of injection molding process parameters on mechanical and fracture behavior of polycarbonate polymer

In this work, the mechanical and failure behavior of injection molded aviation standard optical grade polycarbonate (PC) was investigated through uniaxial tensile testing. The effect of different injection molding process parameters including injection velocity, packing pressure, cooling time, mold temperature, and melt temperature were determined to observe their effect on yield and postyield behavior of PC. Out of these examined parameters, the mold and melt temperature show significant effect on mechanical behavior of studied polymer. The yield and flow stresses in polymer increase with the increase in mold and melt temperature during injection molding. However, other process parameters i.e., packing pressure, injection velocity, and cooling time showed little effect on mechanical performance of the polymer. The molded specimens were annealed at different temperatures and residence time to evaluate its effect on mechanical behavior and fracture morphology. The yield stress increases gradually with the increase in annealing temperature and time. The annealing treatment also changed the failure mode of PC specimens from ductile to brittle. In addition to process parameters, the effect of increased loading rate was also undertaken which shows substantial effect on mechanical and failure behavior of PC. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44474.     

注塑件残余热应力的数值计算

注塑件残余应力主要有流动残余应力和残余热应力 ,文中主要考虑注塑件的残余热应力 .利用文中建立的二维积分型热黏弹本构方程的递推公式 ,推导了单元的有限元求解公式 .数值算例考察了冷却效率、熔体注射温度对注塑件残余应力的影响 ,得到的结论与文献中结论一致 .     

Thermal residual stress development for semi‐crystalline polymers in rotational molding

In rotational molding, polymer powders are subjected to heating, melting, cooling, and subsequent crystallization processes. Because of the asymmetrical cooling condition, thermally induced residual stresses are created inside molded parts leading to part warpage. A detailed theoretical heat transfer model is presented for the entire rotational molding process including the consideration of endothermic and exothermic transitions. At the same time, the development of residual stress inside the molded parts is simulated with thermoelastic model. The warpage values are calculated under different processing cases, and the generated numerical results are in good agreement with data reported in the literature. Our results show that both crystallinity and temperature gradients developing within the polymer during the cooling process greatly affect the dimensional stability of ethylene copolymers typically processed in rotational molding. The latter is found to be the determining factor in evaluating the effect of cooling conditions on the warpage generated in a molded product. Our results also demonstrate the importance of the crystallization kinetics, the material stiffness, and its evolution during the solidification process on the dimensional stability of the molded products. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Thermal and strain rate sensitive compressive behavior of polycarbonate polymer - experimental and constitutive analysis

The mechanical behavior of polycarbonate (PC) polymer was investigated under the effect of various temperatures and strain rates. Characterization of polymer was carried out through uniaxial compression tests and split Hopkinson pressure bar (SHPB) dynamic tests for low and high strain rates respectively. The experiments were performed for strain rates varying from 10 ^?3 to 10³ and temperature range of 213 to 393 K. By conducting these experiments, the true stress–strain (SS) curves were obtained at different temperatures and strain rates. The results from experiments reveal that the stress–strain behavior of polycarbonates is different at lower and higher strain rates. At higher strain rate, the polymer yields at higher yield stress compared to that at low strain rate. At lower strain rate, the yield stress of the polymer increases with the increase in strain rate while it decreases significantly with the increase of temperature. Likewise, initial elastic modulus, yield and flow stress increase with the increase in strain rate while decreases with the increase in temperature. The yield stress increases significantly for low temperature and higher strain rates. On the basis of experimental findings, a phenomenological constitutive model was employed to capture the mechanical behavior of polymer under temperature and loading rate variations. The model predicted the yield stress of polymer at varying strain rate and temperature also it successfully predicted the compressive behavior of polymer under entire range of deformation.