水辅助注塑尼龙6制品的结晶行为研究

利用差示扫描量热法(DSC)对水辅助注塑尼龙6(PA6)制品靠近注水喷嘴(P1)位置和远离注水喷嘴(P2)位置的表层、中芯层和水道层的结晶行为进行了研究,并初步探讨了高压水对PA6制品结晶行为的影响。结果表明:对于P1和P2位置,表层与水道层的结晶度均低于中芯层的结晶度;P1位置表层与水道层结晶度均低于P2位置同层的结晶度;P1位置中芯层结晶度高于P2位置中芯层的结晶度;高压水的冷却作用会降低水道层熔体的结晶度并促使水道层形成较小的晶体。     

水辅注射成型聚丙烯制品的取向结晶

应用小角X射线散射研究了水辅注射成型(WAIM)等规聚丙烯制品中串晶(shish-kebab)结构的形成、分布以及片晶取向行为.结果表明:根据取向度不同,WAIM制品沿壁厚方向可明显分为表层、芯层和水道层.表层和水道层均有shish-kebab结构生成,且在表层生成的数量比水道层多,而芯层则没有这种结构.这种shish...     

水辅助注塑PP/EVOH共混物制品的相形态与阻渗性能

选择3种不同黏度的聚丙烯(PP)与聚乙烯醇(EVOH)共混制备质量比为90/10的共混物,并采用水辅助注塑(WAIM)将这3种共混物成型为中空制品。从WAIM制品靠近浇口(1^#)和末端(2^#)两个位置取出样品,通过扫描电镜观察样品壁厚上3个位置的相形态,并测试所取样品的甲苯渗透率。借助WAIM中高压水作用下模腔内熔体的流场对样品中3个位置的相形态进行了分析。对WAIM的高黏度比共混物制品2^#样品,在外表层和内表层分散相呈粗纤维状,芯层主要呈液滴状,其阻渗性能与相应的WAIM PP样品比有适度提高(约2.4倍);对WAIM的2种低黏度比共混物制品2^#样品,外表层和内表层分散相呈细纤维状,芯层呈粗长纤维状,其阻渗性能与相应的WAIM PP样品比提高幅度较大(其中对黏度比最小的共混物达9.8倍)。1^#位置所取3种WAIM PP/EVOH样品中分散相纤维的平均直径比2^#位置的大,导致1^#位置所取样品的阻渗性能比2^#位置的低。     

水辅助注塑PP/EVA共混物的相形态

通过扫描电镜(SEM)观察了水辅助注塑聚丙烯(PP)/乙烯-醋酸乙烯共聚物(EVA)共混物制品近浇口和远浇口处外表层、芯层和内表层的相形态.结果表明,分散相主要以层状分布在基体中,在两个观察位置外表层和内表层的层状分散相厚度比芯层的大,远浇口处外表层和内表层的分散相厚度比近浇口处对应层的大.从温度场和剪切场两个方面分析了分散相的形变过程.     

水辅助注塑制品水穿透长度和残留壁厚的研究

介绍了自主研发的水辅助注塑设备以及一款经过大量实验后得到的新型喷嘴,并通过单因素实验法研究了短射法成型聚丙烯弯管时工艺参数对制品水穿透长度和沿水道的残留壁厚的影响。结果表明,熔体注射量对水穿透长度影响最大,当其增加13．5％时水穿透长度减小了153mm；沿水道上制品注水后充填部分比注水前充填部分的残留壁厚减小约0．5～1．0 mm；提高注水压力与增加注水延迟时间时注水后填充部分的残留壁厚略有减小。     

车用聚丙烯复合材料结晶行为研究

以聚丙烯为基体,滑石粉、乙烯/辛烯共聚物为添加成分,制备了聚丙烯复合材料。采用广角X射线衍射表征了复合材料注塑样板皮层和芯层,以及注塑样板不同区域表层的结晶行为。结果表明:聚丙烯复合材料注塑样板皮层具有显著的b轴结晶取向趋势;复合材料注塑样板芯层的b轴结晶取向趋势较弱;注塑样板正中位置表层b轴结晶取向趋势最明显。     

水辅助注塑制品残留壁厚和穿透长度影响因素的研究

为能水辅助注塑成型壁厚较小且较均匀的聚丙烯弯管,设计了一种自锁式喷嘴,既满足高流量的要求,又可防止熔体堵塞喷嘴。在相同的工艺条件下,分别使用自锁式喷嘴和孔形喷嘴,对成型的弯管壁厚进行比较,结果表明:与孔形喷嘴相比,使用自锁式喷嘴时制品的残留壁厚更小更均匀。此外,使用该自锁式喷嘴,研究了水辅助注塑中注水压力和延迟时间对制品残留壁厚和水穿透长度的影响.     

不同截面型腔的溢流法流体辅助注塑工艺的实验分析

基于自行构建的流体辅助注塑实验平台,对5种不同截面型腔管件的溢流法气体辅助注塑(GAIM)和水辅助注塑(WAIM)进行了实验研究。研究了辅助介质对管件流体穿透截面形状、残余壁厚大小和流体穿透率的影响规律以及工艺参数对流体穿透率的影响规律,并分析了其影响机理。实验发现:气体的穿透截面趋于型腔截面形状,水的穿透截面趋于圆形;GAIM的五截面管件的最小残余壁厚均大于WAIM,最大残余壁厚均随着内切圆圆心到壁面的最大距离的减小而减小;气体的穿透率较水的穿透率小,均随着圆率的增加而增加,随气体/水注射压力的增加而增加,随气体/水注射延迟时间的增加而减小,随熔体注射温度的增加而有所增加。这些发现为GAIM和WAIM制品截面设计及工艺参数调节提供了参考。     

水辅助注塑中高压水穿透过程的数值模拟

对水辅助注塑(WAIM)中高压水的穿透过程进行有限元模拟,研究熔体的充模行为,并分析其拉伸场和剪切场。结果显示,高压水推动熔体充模的过程可分为填充初期、快速填充期和填充末期3个阶段;较明显的拉伸应变速率仅出现在高压水前沿和熔体前沿区域;高压水前沿区域存在分布较宽且较为强烈的剪切速率,而高压水对已穿透区域的熔体几乎没有剪切作用。此外,模拟所得WAIM制品的穿透长度和掏空率与实验结果较吻合。     

水辅助注射成型中水穿透行为的可视化研究

基于自主研发的注水系统,利用具有矩形变截面和弯道模腔的水辅助注塑可视化模具,采用聚苯乙烯(PS)材料,对不同水压下的水辅助熔体流动充模的过程进行了观察,着重研究了水的穿透行为。研究发现：注水喷嘴的冷却使其周围熔体黏度增大,水穿透高黏度熔体区后产生紊动射流,射流穿透长度随注水压力的增大而增长。注水压力较低时,水的穿透方向容易发生改变,注水压力越高,水道越光滑。水在弯道入口和收敛过渡区的穿透过程中前缘逐渐缩小,在弯道出口和扩散过渡区的穿透过程中前缘逐渐扩宽。     

The hierarchical structure of water-assisted injection molded high density polyethylene: Small angle X-ray scattering study

High density polyethylene (HDPE) was molded by a new polymer processing method, that is, water-assisted injection molding (WAIM), and its hierarchical structure was studied by two-dimensional small angle X-ray scattering (SAXS). For comparison, the hierarchical structure of HDPE molded by conventional injection molding (CIM) was also characterized. The result shows that the WAIM part exhibits a distinct skin-core-water channel structure which is different from the skin-core structure for the CIM part. In the skin layer of both WAIM and CIM parts, the shish-kebab structure was formed due to the shear stress brought by melt filling, but the lamellar orientation parameter of CIM part is smaller than that of WAIM part. The spherulites with random lamellar orientation are dominant at the core of both parts owing to the low cooling rate and feeble shear stress therein. Interestingly, the shish structure and the lamellae with low level of orientation can be found at the water channel layer of WAIM part. They are attributed to the shear stress brought by water penetration. Moreover, the lamellar orientation parameter in water channel layer is smaller than that of skin layer. In addition, the long period of WAIM part first increases and then decreases with the elevating distance from the skin surface, while that of CIM part tends to increase monotonously. In a word, one can conclude that the rapid cooling rate and shear brought by the injected water have significant influence on the structural evolution for the WAIM part. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Morphology of gas‐assisted and conventional injection molded polycarbonate/polyethylene blend

The skin‐core structure of the gas‐assisted and conventional injection molded polycarbonate (PC)/polyethylene (PE) blend was investigated. The results indicated that both the size and the shape of the dispersed PC phase depended not only on the nature of PC/PE blend and molding parameters, but also on its location in the parts. Although the gas‐assisted injection molding (GAIM) parts and conventional injection molding (CIM) part have the similar skin‐core structure, the morphology evolution of PC phase in the GAIM moldings and the CIM moldings showed completely different characteristics. In the section perpendicular to the melt flow direction, the morphology of the GAIM moldings included five layers, skin intermediate layer, subskin, core layer, core intermediate layer as well as gas channel intermediate layer, according to the degree of deformation. PC phase changed severely in the core layer of GAIM moldings, as well as in the subskin of CIM moldings. In GAIM parts, PC phase in the core layer of the nongate end changed far more intensely and aligned much orderly than that in the gate end. The morphology of PC phase in the GAIM part molded with higher gas pressure changed more severe than that in the GAIM part molded with lower gas pressure. In a word, PC phase showed more obvious fibrillation in the GAIM moldings than that in the CIM moldings. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3069–3077, 2006     

Gas‐assisted injection molded polypropylene: The skin‐core structure

In this article, gas penetration‐induced skin‐core structure of isotactic polypropylene(iPP), which is molded by gas‐assisted injection molding at different gas pressures, was investigated. For comparison, the counterpart was also molded by conventional injection molding (CIM) using the same processing parameters but without gas penetration. They were characterized via PLM, DSC, and SEM. And the crystal morphology at different gas pressures was principally concerned. For the GAIM parts, highly oriented structure is formed in the skin zone, and much less oriented structure in the inner zone (near the gas channel surface). Furthermore, it is suggested that the naked shish structure can be developed in the skin zone of GAIM part, which is molded at higher gas pressures, and shish‐kebab structure is mainly formed in the skin zone of that, which is molded at lower gas pressure. However, for the CIM part, from the skin to the core zone, the dominant morphological feature is spherulite. In a word, the presence of gas penetration notably enhances the oriented structure formation and gives rise to the skin‐core structure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Formation mechanism of crystal morphologies in LLDPE/HDPE blends investigated via water‐assisted and conventional injection molding

The LLDPE/HDPE blends with two different weight ratios as well as pure LLDPE were molded by means of water‐assisted and conventional injection molding (WAIM and CIM) in terms of their different thermal fields. The formation of the crystal morphology in the molded parts was investigated by a scanning electron microscope. The results showed that banded spherulites formed in the WAIM and CIM pure LLDPE parts. Banded spherulites of LLDPE coexisted with the randomly oriented lamellae of HDPE for LLDPE/HDPE blend parts with lower HDPE content at higher cooling rates, whereas a banding to nonbanding morphological transition occurred for LLDPE component (particularly for blend with higher HDPE content) at lower cooling rates. The heterogeneous nucleation effect of HDPE component on LLDPE component was responsible for the banding to nonbanding morphological transition by hindering the twist of LLDPE lamellae. It was interesting to find that the thermal effect, rather than the shear effect, was the main factor for the formation of crystal morphologies in both CIM and WAIM blend parts. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Tailoring the crystalline structure of polypropylene parts molded through water‐assisted injection molding: Effects of melt temperature and polymeric nucleating agent

The melt temperature and a special polymeric nucleating agent [acrylonitrile–styrene copolymer (SAN)] were investigated to find an effective way for tailoring the crystalline structures of the water‐assisted injection‐molded polypropylene (WAIM PP) parts. The results showed that lowering the melt temperature led to the formation of a small amount of β‐form crystals in both outer and core layers of the WAIM PP parts. Nevertheless, the melt temperature had little effect on tailoring the crystalline structures of the WAIM PP parts. The addition of a low content (6 wt%) of the SAN was interestingly found to gradually influence the crystalline structures as lowering the melt temperature. WAIM PP/SAN blend parts with high contents of β‐form in both outer and core layers (30.7 and 18.4%, respectively), and high contents of transcrystals in the inner layer were molded at relatively low melt temperature (180°C), whereas the SAN had little influence on the crystalline structures at higher melt temperature (230°C). The formation of the transcrystals was ascribed to the in situ fibrillation of the SAN, which was resulted from high shear and cooling rates caused by high‐pressure water penetration during WAIM. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

Process‐induced phase and crystal morphologies in water‐assisted injection molded polypropylene/polymeric β‐nucleating agent blend parts

By adding a polymeric β‐nucleating agent (acrylonitrile–styrene copolymer, SAN), in situ microfibril reinforced isotactic polypropylene (iPP)/SAN blend parts with high contents of β‐form crystals and transcrystals were molded via water‐assisted injection molding (WAIM). Thanks to the unique stress and temperature fields occurring during the WAIM, SAN microfibers formed across the whole residual wall of iPP/SAN blend parts with relatively large thickness. Numerical simulations on high‐pressure water penetration and cooling stages of the WAIM were carried out to reveal the stress and temperature fields. Comprehensive analysis of both experimental and simulated results showed that not only the shear flow field but also elongational flow field occurring during the WAIM was responsible for the formation of SAN microfibers and unique crystal morphology distribution in the WAIM iPP/SAN blend part. Moreover, during the WAIM, the high cooling rate also played an important role in the formation of both phase and crystal morphologies. The preferential formation of transcrystals in the inner layer of WAIM iPP/SAN blend part could be ascribed to the strong elongation, rather than the strong shear. It was believed that the quantification of stress and temperature fields of the WAIM via numerical simulation could provide a guidence for molding high‐performance products. POLYM. ENG. SCI., 55:1698–1705, 2015. © 2014 Society of Plastics Engineers     

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

Molten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009