型腔截面对水辅助注塑成型水穿透的影响

为考察型腔截面对溢流法水辅助注塑中水穿透的影响,基于装备有自行研发的水辅助注射系统的注塑机对几种具有代表性的型腔截面进行溢流法水辅助注塑实验,探究了型腔截面形状、大小对水穿透截面形状、中空率及残留壁厚等方面的影响规律,并分析了影响机理。实验发现：在溢流法水辅助注塑中,水的穿透截面形状会趋向型腔截面形状;在型腔截面积相近时,水穿透的中空率随截面圆率增大而增大;在型腔截面圆率一定时,水穿透的中空率受截面大小影响不大;型腔截面恒定,水的穿透截面也较恒定,残留壁厚均匀;圆截面型腔的残留壁厚最均匀,且残留壁厚随截面的增大而变厚;型腔截面的最大残留壁厚和最小残留壁厚与壁面到截面内切圆中心的距离有关。这些发现为水辅助注塑制品的截面设计提供了参考。     

Residual wall thickness of water‐powered projectile‐assisted injection molding pipes

Water‐powered projectile‐assisted injection molding (W‐PAIM) is an innovative molding process for the production of hollow shaped polymer parts. The W‐PAIM utilizes high pressure water as a power to drive a solid projectile to displace the molten polymer core to form the hollow space. The residual wall thickness (RWT) and its distribution are the important quality criteria. The experimental and numerical investigations were conducted. Experimental specimens showed that the RWT of a W‐PAIM pipe was much thinner than that of a water‐assisted injection molding pipe. The cross‐section size of the projectile defined the basic penetration section size. The software FLUENT was used to obtain the instantaneous distributions of the flow field, which revealed the forming mechanism of the RWT. The experiments indicated that the processing parameters, such as melt temperature, melt injection pressure, mold temperature, and water injection delay time had obvious effects on the RWT, while the water pressure had little effect on it. The RWT of curved pipes was thin at the inner concave side while thick at the outer convex side. The RWTs at the bend portion are influenced by the deflection angle and bending radius, which is due to the pressure difference between the two sides. POLYM. ENG. SCI., 59:295–303, 2019. © 2018 Society of Plastics Engineers     

方管短玻璃纤维增强聚丙烯高压水穿透行为研究

采用方形截面管件,以短玻璃纤维增强聚丙烯为原料,通过溢流法水辅助注射成型实验探究了熔体注射温度、注水延迟时间和注水压力等工艺参数对制件宏观现象的影响机理,并分析了高压水在方形管道中的穿透行为。结果表明,当熔体温度升高时,方管的直角边和斜边残余壁厚都呈减小趋势,但温度过高时会出现管件收缩现象,管件截面中空面积增大且截面形状与高压水的穿透前沿形状一致,偏圆形,但截面的圆率逐渐减小;当注水压力增加时,管件残余壁厚减小,截面中空面积增大,其截面形状随着注水压力的增加逐渐与型腔结构一致,偏方形;当注水延迟时间增加时,管件残余壁厚增大,中空截面减小且管件截面形状也与高压水前穿透前沿一致,偏圆形,但相较另外两个参数,注水延迟时间对方管件的影响程度更小,因而对截面的圆率影响不大。     

基于响应面法的水辅助共注塑管件的工艺参数优化

以最小总壁厚及内层壁厚为目标,基于响应面法（RSM）对成型工艺参数进行优化。由单因素实验确定总壁厚和内层熔体壁厚的主要影响因素;由Plackett Burman试验确定关键因素;再通过Box Behnken试验设计和响应面法分析与优化,获得最小总壁厚和内层熔体壁厚的工艺条件为：注水压力7.5 MPa,注水延迟时间2s,内层熔体温度215℃;在优化条件下,利用Design-expert模型预测总壁厚和内层壁厚与实验结果吻合较好,表明响应面法能够优化水辅助共注塑管件最小壁厚的工艺参数。     

气辅成型过程气体穿透行为与工艺参数优化的试验研究

基于带平板的回形管式气体辅助成型样件对气体穿透行为和残余壁厚与气辅工艺参数的关系进行了研究分析.试验结果表明,气辅工艺参数如气体延迟时间、气体注射压力和熔体预注射量是影响气体辅助成型中气体穿透长度和残余壁厚的重要因素,而各参数间的交互作用致使气体穿透与残余壁厚的变化趋势更为复杂.     

Response of a sequential‐valve‐gate system used for thin‐wall injection molding

Sequential injection molding using a valve‐gate‐controlled hot runner system has attracted attention for industrial applications in recent years. Because of the complexity of the operation mechanisms, a commercial valve gate usually delays for about 0.3–0.5 s once the valve‐opening command is given. The signal‐to‐operation delay is acceptable for the conventional injection molding of large parts. However, this operation delay limits its application to thin‐wall molded parts for computer, communication, and consumer electronics, for which the required filling time is very short. In this study, a gas‐driven fast‐response sequential‐valve‐gate system was developed for thin‐wall injection molding by the adoption of valve‐gate control performance. The characteristics and verifications of the valve‐gate opening were monitored with a charge‐coupled device (CCD) camera (nonmelt condition) and cavity pressure transducers and an accelerometer (melt‐filled condition). The influence of the tolerance between the inner piston and cylinder and the gas pressure on the valve‐gate opening was investigated in detail. Tensile bar parts 1 mm thick were used for the molding experiments. The delay time has been found to be intimately related to the response of the gas‐pressure delivery controlling the valve‐gate movement. In a nonmelt environment, the delay time of the valve‐gate opening decreases with increasing driven gas slightly. In a melt‐filled environment, the delay time is quite sensitive to the operating gas pressure because of the extra resistance between the shaft and the melt. A threshold pressure as high as 100 bar is required to keep the delay time below 15 ms. With the proper choice of the piston size and driven gas pressure, the delay time can be reduced to about 8 ms in a nonmelt environment and to about 12 ms in a melt‐filled environment. Molding using this improved system for sequential valve opening can provide thin‐wall injection parts without a weld line, and good cosmetic quality and better tensile strength require a lower injection pressure than molding using single‐gate and concurrent‐valve‐gate opening. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1969–1977, 2005     

不同水针口径下的水辅助注射成型数值模拟

使用流体力学软件,对使用溢流法的三维120 °~150 °弯曲圆管件进行了水辅助注射成型可视化研究。分别改变水针口径尺寸、注水延迟时间、注水压力与熔体温度,分析其对制件内部水穿透行为的影响。结果表明,水针口直径为7 mm时,能显著增加制件的内部穿透长度并得到残余壁厚更薄的制件;受水针结构影响,注水延迟时间为1 s、注水压力为8 MPa、熔体温度为250 ℃时,水穿透长度最优能增长400 %,壁厚减少20 %;在注水延迟时间为1 s、注水压力为10 MPa、熔体温度为230 ℃时,穿透长度最大达到298 mm;注水延迟时间为1 s、注水压力为8 MPa、熔体温度为250 ℃时,比熔体温度为210 ℃和230 ℃的实验组受水针影响严重;缩短注水延迟时间、增加注水压力、升高熔体温度都能有效增大制件的中空率,成型出更薄的管件,但是水针对水辅助注射成型的影响不容忽视,其微小变化能极大地改变成型制件的内部型腔,有效提高水穿透行为的效率。     

工艺参数对聚丙烯/聚酰胺6共混材料WAIM管件性能及微观形态的影响

基于自行搭建的水辅助注射成型(WAIM)实验平台对聚丙烯/聚酰胺6（PP/PA6）共混材料进行WAIM实验,探究工艺参数对PP/PA6共混材料的WAIM管件的残余壁厚、表观质量及力学性能的影响,并进行了微观形态观测。结果表明,随着熔体温度、模具温度和注水压力的增加,塑件残余壁厚逐渐减小;随着注水延迟时间的增加,塑件残余壁厚逐渐增大;随着模具温度、注水延迟时间的增加,管件拉伸强度逐步增大;随着熔体温度、注水压力的增大,管件拉伸强度逐步减小;工艺参数通过温度场的变化影响熔体的成核和晶体长大从而影响结晶度,结晶度越高,管件的拉伸强度越高。     

工艺参数对水辅注塑弯管壁厚的影响

利用华南理工大学自主研发的注水系统和水辅注塑弯管模具,研究了熔体温度、模具温度、注水延迟时间、熔体注射量、注水压力、注水温度、熔体注射速率和熔体注射压力等8个水辅成型主要工艺参数对聚丙烯制品壁厚偏差率的影响,并分析了影响机理。结果表明,部分工艺参数对于制品弯曲段的壁厚偏差率有影响;增加注水延迟时间,降低注水压力和模具温度,短射填充区的制品壁厚的偏差率有所减小;提高熔体温度,壁厚偏差率的波动幅度增大。     

注水参数对水辅助注射成型充填过程影响的数值模拟

建立水辅助注射成型二维、瞬态、非定常流动模型,采用黏度幂律模型,在k ω湍流模型下,充分考虑注射水的湍流特性以及熔体前沿的喷注效应,采用有限体积法（VOF）对充填过程中的注水速度、注水温度和注水延迟时间等注水控制参数的影响进行数值模拟。结果表明,注水速度的增加会增加水在熔体中的穿透长度,并且会减小残余壁厚;注水温度对水的穿透长度和残余壁厚的影响均不显著;随着注水延迟时间的增长,水的穿透长度和残余壁厚均有增加的趋势。     

气体辅助注射成型中气道截面对气体穿透的影响

将各种不同气道截面等效于圆形,引入形状因子来衡量它们偏离圆形的程度。利用moldflow软件对气体在不同构型气道内的穿透过程进行了数值模拟。结果表明:形状因子越大则气体穿透深度越大,对气体辅助注射成型有利。但由于形状因子越大则气道截面偏离圆形越远,不利于形成均匀的熔体壁厚,因此在设计气道截面时,应合理选择形状因子以便获得恰当的气体穿透深度和熔体壁厚。最后还给出了气道设计的一般原则。     

Numerical simulation on residual thickness of pipes with curved sections in water‐assisted co‐injection molding

The residual thicknesses of the skin and the inner layers are important quality indicators of water‐assisted co‐injection molding (WACIM) process or overflow WACIM (O‐WACIM) parts. At the curved section, the residual thicknesses change significantly. A numerical simulation program based on the computational fluid dynamics method was developed to simulate the O‐WACIM process. After the numerical simulation program was validated with the experimental results, it was used to study the effects of the bending radii and bending angles on the residual thicknesses of the skin and inner layers of O‐WACIM parts. The results showed that the penetration of the inner melt and water was always close to the inner concave side due to the higher local pressure gradient and temperature. The effects of processing parameters on the residual thicknesses of the skin and inner layers were investigated using the orthogonal simulation method. It was found that the residual thicknesses of the skin/inner layer at the inner concave/outer convex side are mainly influenced by different parameters. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42468.     

工艺参数对聚丙烯/聚酰胺6共混物水辅助共注塑管件壁厚及性能的影响

基于自行搭建的水辅助共注塑实验平台,通过正交实验制备了系列水辅助共注塑管件,探究工艺参数对各层壁厚、拉伸性能及各相结晶的影响。结果表明,外层壁厚随着外层熔体温度、注水压力、内层熔体注射压力、模具温度增大而逐渐减小,随着熔体注射切换延迟时间、注水延迟时间增大而逐渐增大;内层壁厚随着注水延迟时间、内层熔体注射压力增大而逐渐增大,随着注水压力、模具温度增大而逐渐减小;管件拉伸强度随着外层熔体温度增大而逐渐减小,随着熔体注射切换延迟时间、注水延迟时间增大而逐渐增大;工艺参数会影响到成型壁厚及冷却进程,进而影响各相结晶度,最终影响管件性能。     

Primary and secondary gas penetration during gas‐assisted injection molding. Part II: Simulation and experiment

Simulation and experimental studies have been carried out on the transient gas‐liquid interface development and gas penetration behavior during the cavity filling and gas packing stage in the gas‐assisted injection molding of a spiral tube cavity. The evolution of the gas/melt interface and the distribution of the residual wall thickness of skin melt along with the advancement of gas/melt front were investigated. Numerical simulations were implemented on a fixed mesh covering the entire cavity. The residual thickness of a polymer layer and the length of gas penetration in the moldings were calculated using both the simulation and model developed in Part I of this study and commercial software (C‐Mold). Extensive molding experiments were performed on polystyrene at different processing conditions. The obtained results on the gas bubble dynamics and penetration behaviors were compared with those predicted by the present simulation and C‐Mold, indicating the good predictive capability of the proposed model. Polym. Eng. Sci. 44:992–1002, 2004. © 2004 Society of Plastics Engineers.     

Numerical simulation on residual wall thickness of tubes with dimensional transitions and curved sections in water‐assisted injection molding

Residual wall thickness is an important indicator which aims at measuring the quality of water‐assisted injection molding (WAIM) parts. The changes of residual wall thickness around dimensional transitions and curved sections are particularly significant. Free interface of the water/melt two‐phase was tracked by volume of fluid (VOF) method. Computational fluid dynamics (CFD) method was used to simulate the residual wall thickness, and the results corresponded with that of experiments. The results showed that the penetration of water at the long straight sections was steady, and the distribution of the residual wall thickness was uniform. However, there was melt accumulation phenomenon at the dimensional transitions, and the distribution of the residual wall thickness wasn't uniform. Adding fillet at the dimensional transitions could improve the uniformity of the residual wall thickness distribution, and effectively reduce water fingering. Additionally, at the curved sections, the residual wall thickness of the outer wall was always greater than that of the inner wall, and the fluctuations of the residual wall thickness difference were small. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Primary and secondary gas penetration during gas‐assisted injection molding. Part I: Formulation and modeling

A theoretical study has been carried out on the transient gas‐liquid interface development and gas penetration behavior during the cavity filling and gas packing stage in the gas‐assisted injection molding (GAIM) of a tube cavity. A mathematical formulation describing the evolution of the gas/melt interface and the distribution of the residual wall thickness of skin melt along with the advancement of gas/melt front is presented. The physical model is put forward on the basis of Hele‐Shaw approximation and interface kinematics and dynamics. Numerical simulation is implemented on a fixed mesh covering the entire cavity. The model and simulation can deal with both primary and secondary gas penetrations. The predicted and measuredresults are compared in Part II of this study to validate the theoretical model. Polym. Eng. Sci. 44:983–991, 2004. © 2004 Society of Plastics Engineers.     

Experimental study of kerosene–water two‐phase flow in a vertical pipe using hot‐film and dual optical probes

The local parameters for kerosene–water upward flow are measured in a vertical pipe of 77.8 mm inner diameter at 4200 mm from the inlet (L/D = 54) using hot‐film and dual optical probes. The effect of superficial water velocity and volumetric quality on radial distribution of two‐phase flow parameters is investigated. The results show the following: (i) the profiles of volume fraction and drop frequency are very similar, and increasing superficial water velocity at low volumetric qualities (<18.6%) change the profile from a convex shape with peak at the pipe centreline to uniform then to concave shape with peak near the wall; (ii) the profiles of drop cut chord change from a parabolic shape with peak at centreline for low superficial water velocities to a flat shape at higher superficial water velocity, and the area‐averaged drop diameter decreases with higher superficial water velocities for all volumetric qualities; (iii) velocity profiles for both phases have shapes similar to single phase flow, flatter at higher values of superficial water velocity and volumetric quality and centreline peaked at low superficial water velocities and volumetric qualities; (iv) the slip velocity decreases with radial distance having a peak at centreline and zero values near the wall; (v) introducing kerosene drops into single phase water flow results in a sharp increase in turbulent intensity, particularly at low water velocity, and the difference between the single phase and two‐phase flow turbulence intensities decreases with higher superficial water velocities and (vi) the results show that interfacial area concentration increased with higher volumetric quality and higher number of bubbles thereby increases the contact area between the two phases. © 2012 Canadian Society for Chemical Engineering     

Three‐dimensional simulation on multilayer parison shape at pinch‐off stage in extrusion blow molding

We tried to predict the multilayer parison shape at pinch‐off stage in extrusion blow molding by nonisothermal and purely viscous non‐Newtonian flow simulation using the finite element method (FEM). We assumed the parison deformation as a flow problem. The Carreau model was used as the constitutive equation and FEM was used for calculation method. Multilayer parison used in this simulation was composed of high‐density polyethylene (HDPE) as inner and outer layers and low‐density polyethylene (LDPE) of which viscosity is five times lower than HDPE as a middle layer. We discussed multilayer parison shape in pinch‐off region. The results obtained are as follows; the parison shape of each layer was clearly visible in the pinch‐off during the mold closing. In addition, the distribution of parison thickness ratios for each layer was located for a large deformation near the pinch‐off region. The melt viscosity for each layer has an influence on the melt flow in the pinch‐off region. In a comparison with an experimental data of parison thickness ratios, the simulation results are larger than the experimental data. These simulation results obtained are in good agreement with the experimental data in consideration of the standard deviations. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Gas assisted injection molding of a handle: Three‐dimensional simulation and experimental verification

Methods implemented in a three‐dimensional finite element code for the simulation of gas assisted injection molding are described, and predictions compared with the results of molding trials. The emphasis is on prediction of primary gas penetration and plastic wall thickness, including the effects of cooling during a delay before gas injection. For the latter, time dependent heat transfer coefficients at the cavity surface are used, determined in a separate analysis of transient heat conduction through the plastic and the mold tool to the circulating coolant. This shows how the initial value of 25,000 W/m²K falls by about an order of magnitude during the first few seconds of cooling, and also how values vary from cycle to cycle as steady periodic conditions are approached. For a tubular handle molded in polystyrene, with melt flow modeled by a Cross WLF model, comparisons of simulations with sectioned parts show excellent prediction of wall thickness and its variation circumferentially and in bends. The increase in wall thickness due to cooling during a gas delay is accurately modeled, as is the occurrence of a blow out. POLYM. ENG. SCI. 45:1049–1058, 2005. © 2005 Society of Plastics Engineers     

Enhanced orientation of the water‐assisted injection‐molded ipp in the presence of nucleating agent

The nucleated isotactic polypropylene (iPP) was molded by water‐assisted injection molding. The crystalline morphology and orientation distribution were studied. The results show that shear brought by melt filling and pressurized water penetration can separately induce the formation of oriented structures in skin region (i.e., the region near mold cavity wall) and the water channel region. For virgin iPP, slightly oriented lamellae appear exclusively in the above aforementioned regions. However, shish‐kebab structure occurs not only in skin and water channel region of the iPP containing moderate content of nucleator (0.2 wt%) but also in the whole region of the iPP containing a higher content of nucleator (1 wt%). It is well known that nucleator cannot directly induce the development of shish‐kebab in the absence of shear, thus the results indicate: shear flow actually distributes over a much broader range than expected; in shear field, nucleator is significantly helpful for the shear which is not sufficient to solely induce oriented structure to promote the formation of the oriented structure. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers