Injection molding of delamination‐free ultra‐high‐molecular‐weight polyethylene

When ultra‐high‐molecular‐weight polyethylene (UHMWPE) in powder form is injection molded, the so‐called delamination layering occurs near the skin of the parts. This layering defect hampers UHMWPE's superior wear resistance property and part surface quality. The delamination layer was caused by a combination of excessive shear stress near the part surface and high degree of molecular entanglement of UHMWPE. A mold insulation method that delays the rapid cooling of UHMWPE to reduce the shear stress and improve the polymer chain “interdiffusion” across the entangled chain bundles was used to eliminate the delamination layer. When the insulation layer thickness and mold temperature were optimized, the delamination layer was eliminated completely while still maintaining a reasonable cooling/cycle time. The delamination‐free parts were found to regain UHMWPE's superior impact resistance and tensile properties. POLYM. ENG. SCI., 59:2313–2322, 2019. © 2019 Society of Plastics Engineers     

Development of rapid heating and cooling systems for injection molding applications

The injection molding process has several inherent problems associated with the constant temperature mold. A basic solution is the rapid thermal response molding process that facilitates rapid temperature change at the mold surface thereby improving quality of molded parts without increasing cycle time. Rapid heating and cooling systems consisting of one metallic heating layer and one oxide insulation layer were investigated in this paper. Design issues towards developing a mold capable of raising temperature from 25°C to 250°C in 2 seconds and cooling to 50°C within 10 seconds were discussed. To reduce thermal stresses in the layers during heating and cooling, materials with closely matched low thermal expansion coefficient were used for both layers. Effects of various design parameters, such as layer thickness, power density and material properties, on the performance of the insert were studied in detail with the aid of heat transfer simulation and thermal stress simulation. Several rapid thermal response mold inserts were constructed on the basis of the simulation results. The experimental heating and cooling response agrees with the simulation and also satisfies the target heating and cooling requirement.     

Reducing residual stresses in molded parts

Means of reducing the flow-induced residual stresses in injection molded parts through optimization of the thermal history of the process are presented. An approach through the use of a passive insulation layer with low thermal inertia on the cavity surface was investigated. The passive insulation layer prevents the polymer melt from freezing during mold filling and allows the flow-induced stresses to relax after the filling. The criteria for the optimal thermal properties and the required thickness of the layer are presented. A numerical simulation model of non-isothermal filling and cooling of viscoelastic materials was also used to understand the molding process and to evaluate this approach. This model predicts the stress development and relaxation in the molding cycle. Both simulation and experimental results show that the final stresses in the molded parts can be reduced significantly with the use of an insulation layer. This technique can also be applied to other molding or forming processes in order to decouple the material flow and cooling process for minimum residual stresses in the molded parts.     

High‐frequency proximity heating for injection molding applications

High‐frequency proximity heating was used to rapidly heat injection molds. The principle is based on the proximity effect between a pair of mold inserts facing each other with a small gap and forming a high‐frequency electric loop. Because of the proximity effect, the high‐frequency current will flow at the inner surfaces of the facing pair, thus selectively heating the mold surface. With this method, the electrical insulation layer beneath the mold surface can be eliminated, resulting in a mold insert made of a single metal. A mold with a cavity of 25 × 50 mm² was constructed with careful design on its electrical, structural, and thermal performance. Air pockets with reinforcing ribs were embedded right beneath the mold surface for enhancing the heating performance. The resulting mold cavity can be rapidly heated from room temperature to about 240°C in 5 s with an apparent heating power of 93 W/cm². The new mold heating method was applied to thin‐wall molding and micromolding, and in all testing cases, short cycle times less than a minute were achieved. POLYM. ENG. SCI. 46:938–945, 2006. © 2006 Society of Plastics Engineers     

蒸汽加热变模温注射成型数值模拟与结果分析

采用Moldflow软件对变模温注射成型过程进行数值模拟。利用蒸汽加热和冷却水冷却的变模温注塑工艺,研究不同蒸汽加热时间下注塑位置处压力以及制件冷凝层的变化规律,同时分析了制件表面和模具型腔表面的热响应规律。结果表明,相比于传统注射成型工艺过程,变模温注射成型通过提高注塑充填过程中模具温度,使得制件冷凝层出现在充填阶段之后;随着模具加热时间从10、15、25 s增加到40 s,注塑位置处最大注射压力从87.0608、84.6064、79.6863 MPa减小到74.4342 MPa,大大提高了熔体注塑充填过程中的充填能力;通过不同的蒸汽加热时间,制件表面和模具型腔表面可以获得不同的温度值,同时通过模拟获得了传热系数对制件表面温度的影响。     

Analysis of asymmetric morphology evolutions in iPP molded samples induced by uneven temperature field

Mold surface temperature has a strong effect on the amount of molecular orientation and morphology developed in a non‐isothermal flowing polymer melt. In this work, a well‐characterized isotactic polypropylene was injected in a rectangular mold cavity asymmetrically conditioned by a thin electric heater specifically designed. The cavity surface was heated at temperatures ranging from 80 to 160°C for different times (0.5, 8, and 18 s) after the first contact with the polymer. Asymmetrical thermal conditions have a strong influence on the melt flow, by changing its distribution along the cavity thickness, and final part deformation. The morphology distribution of the molded samples was found strongly asymmetric with complex and peculiar features. Optical and Electron microscopy confirmed the complete reorganization of the crystalline structures along the sample thickness. X‐rays analysis reveals that molecular orientation of the sample surface decreases with the mold temperature and the heating time. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2699–2712, 2016     

高温绝缘涂料及其应用举例

介绍一种能耐高温和绝缘的涂料及应用于汽车点烟器发热线圈是热丝表面绝缘处理工艺,其工艺简单,高温下绝缘效果良好,符合设计要求。     

电热水器内部结构优化及储能特性研究

将相变储能技术应用于电热水器,并通过添加石墨纳米颗粒改善相变材料的导热特性,对其储能过程进行调节,可以起到"移峰填谷"的作用.建立了四种不同结构的电热水器三维模型,模拟了电热水器内部速度场与温度场分布特性.考察了进出口水管结构、电加热管布置方式、保温层结构等因素对热水器内部流场及传热特性的影响,研究了不同储能层厚度对电...     

Minimum “safe cycle time”: Selecting the frozen layer thickness

Predicting the minimum safe cycle time in injection molding is complicated by the cyclic nature of the process. The mold thermal state changes with continuous molding until a quasi steady state is achieved. In the first part of these series, we presented and experimentally tested a simple approach to predict part and mold surface temperature during continuous molding. In this work, we evaluate the minimum frozen layer thickness required before the part can be demolded and how the growth of this frozen layer is affected as the mold thermal state changes with continuous molding cycles. Experiments are used to relate the minimum frozen layer thickness to dimensional stability. Based on these experiments, statistical tools are used to select an ejection criterion to obtain a minimum safe cooling time. We show that the ejection criteria should be adjusted depending on the precision required from the molded parts. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Design and calibration of a thermodenuder with an improved heating unit to measure the size-dependent volatile fraction of aerosol particles

Separated measurements of volatile and non-volatile fractions of atmospheric aerosols have become more important due to possible effects of non-volatile particle on respiratory diseases. Measurements of non-volatile particle fractions with considerable size and time resolution are therefore needed. The combination of a thermodenuder (TD) with aerosol size spectrometers can provide size distributions of non-volatile particles. TDs usually consist of two units: a heating and a cooling tube. Unfortunately, available TDs have some disadvantages concerning the heated tube, where volatile material should be desorbed from the particles. The residence time in the heating zone might be too short to completely evaporate the volatile material or the temperature at the exit of the heating tube might be below the condensation temperature of the evaporated species. Here, a new TD was developed with a longer residence time at the requested temperature and a more effective insulation to prevent recondensation at the end of the heating unit. Additionally, the TD was calibrated for size-dependent particle losses at different heater temperatures.     

Development and introduction of energy saving materials for steel pouring

A new method is considered for using heat insulation material offered by the OOO Ogneupor Trade Group company during steel pouring into a mold. The technical result of using the new method consists of a reduction in surface defects and those within the macrostructure of an ingot, abandoning use of an exothermic mixture, and a simultaneous reduction of heat insulation material consumption.     

自动脱模半模式橡胶堆模具设计

橡胶堆的传统模具一般被设计成多瓣模结构,手工操作脱模,劳动强度大、生产效率低。文中的新结构模具将型腔设计成半模式对开瓣模,采用托板与楔形块相组合的机构,借助于硫化机的动力实现了硫化产品快速自动脱模。生产实践表明,该模具结构可靠,操作简便,大幅度提升了生产效率。     

改性SiO2纳米颗粒沸腾沉积层的形成原理及其沸腾换热

实验研究了改性SiO₂纳米流体液滴蒸发后的沉积图案,以及改性SiO₂纳米颗粒沸腾沉积层对沸腾换热的影响。液滴蒸发实验研究表明：改性官能团会影响改性SiO₂纳米颗粒是否吸附在液-气界面,从而推断出在沸腾过程中改性官能团对纳米颗粒沉积方式的影响。沸腾实验研究结果表明：用聚乙二醇基团改性的SiO₂纳米颗粒沸腾沉积层使加热面的平均粗糙度从160 nm大幅增长到977 nm,且能增强纯水的沸腾传热系数;而用磺酸基团改性的SiO₂纳米颗粒沸腾沉积层对加热面的平均粗糙度的改变不明显,只使其增大了60 nm,且恶化了纯水的沸腾传热系数。通过沸腾换热实验结果较好地验证了通过液滴蒸发实验推断出的沸腾过程中改性官能团对纳米颗粒沉积方式的影响。     

氮化硼基复合陶瓷型壳的抗钛液冲蚀能力

研究了纯氮化硼型壳、氮化硼基复合型壳与熔融钛之间的界面反应,比较了2种型壳的抵抗钛液冲蚀能力.对这2种型壳的高温残余强度、表面形貌、化学成分等进行了分析.结果表明:氮化硼基复合型壳中加入氧化钇,不仅能提高型壳的强度及致密度,还能在型壳内表层形成一层由均匀随机分布的氧化钇和氮化硼粉粒网状链结成的"马赛克"结构的面层,具有显著提高型壳抗钛液冲刷和侵蚀的能力.     

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.     

加热过程的滚塑模具内表面传热系数的计算

给出了2种计算加热过程的圆筒形滚塑模具内表面传热系数的方法。第一种方法是利用模具表面和内部温度的实测值通过模具内部空间的热平衡方程计算出其内表面传热系数。第二种方法是借用回转窑内物料与壁面间传热系数的理论公式来计算滚塑模具内表面的传热系数。在模具内部无粉料和装有0.21 kg粉料的情形下,分别用这2种方法计算了模具内表面瞬时的传热系数和平均的传热系数,并应用流体力学的边界层理论解释了内表面传热系数随时间的变化规律。然后将第一种方法计算所得的平均传热系数整理成努塞尔数与普朗特数的无量纲准则式N_u= 45.73 P_r ^-0.55,其适用条件为绕中心轴转动的轴向长度大于内径的圆筒形滚塑模具。结果表明,随着模具内装有粉料的体积百分比的增大,其内表面的平均传热系数减小,同时上述2种方法计算的平均传热系数也吻合得越来越好;如果粉料的体积百分比不低于62.4 %,那么第二种方法计算结果的相对误差不超过20 %。     

高光无痕注塑成型模具热响应的研究

高光无痕注射成型技术是一种新开发的注射技术,能够消除塑件表面熔接痕等缺陷,表面可以达到镜面效果,免去二次喷涂。通过建立模具的二维几何模型,应用ANSYS模拟了加热、冷却过程,获得了模具表面及熔体中心层的热响应和温度分布情况;对比分析了常规注塑与高光无痕注塑两种成型工艺;通过加热、冷却时间及温度分布等对比,验证了高光无痕注塑的优越性。     

微注射成型中变模温控制技术

阐述了变模温控制在微注射成型中的重要性，介绍、比较了当前出现的电热水冷、感应加热、薄膜电阻式加热、复合模壁绝热-加热、复合模壁绝热-压缩热空气加热几种变模温控制实现方法，展望了未来微注射成型中变模温控制的发展趋势。     

Thermal and cure analysis in sheet molding compound compression molds

Sheet molding compound (SMC) compression molding growth will benefit from faster cycles and more uniform cure so as to reduce in-plane thermal residual stress and resulting warpage in the molded part. These improvements require an in-depth study of the mold thermal design. Here we use a finite element model to analyze the quasi-steady temperature distribution in a plane perpendicular to the heating channels of a representative mold, and a finite difference model to investigate the cure dynamics at critical regions. Several changes in the mold heating system and operating conditions were considered and their effects on the temperature distribution and cure time were studied. It was assumed that the steam condensate is well drained and enough steam is supplied so that the steam tube walls are kept at a constant temperature. An important conclusion of the present study is that better insulation of the mold from the press does not help much in improving the uniformity of cavity surface temperature or cure. It was also found that reducing the distance between two consecutive steam tubes beyond the distance from the steam tube to cavity surface will not yield a significant change. The most practical way to give both more uniform cavity surface temperature and faster cure is to have higher steam temperature for the region where the charge is initially placed.     

LNG管道复合保冷结构保冷厚度的计算与讨论

阐述了最大冷损失法对两种保冷材料的复合保冷结构厚度的计算方法,讨论了复合保冷结构参数的选择。结果表明,内层绝热层外表面温度T1的取值影响复合结构保冷层的总厚度和分层厚度。通过实例计算,采用最大允许冷损失法计算结果经校核结果满足要求。