Simulation of phase‐change heat transfer during cooling stage of gas‐assisted injection molding of high‐density polyethylene via enthalpy transformation approach

Gas‐assisted injection molding (GAIM) is one of the significant fabricating technologies of plastics in modern industry, mainly owing to the light weight of products, good structural rigidity and dimensional stability, as well as shorter molding cycles. The objective of this article is to explore the temperature profiles during the cooling stage of gas‐assisted injection molded high‐density polyethylene (HDPE) parts using a transient heat transfer model of the enthalpy transformation method, which could always be utilized for the numerical studies of the phase‐change heat transfer issues. The simulated results were validated by the in situ measurement of temperature decay, and good agreement has been observed. The comparison between GAIM and conventional injection molding (CIM) reveals that it is the rapid cooling rate (because of thin wall‐thickness) and the inner gas cooling effects that together lead to the shortening of molding cycles. As cooling rate plays a part in the stabilization of the crystalline structure during the GAIM process according to our previous studies, this work is of significance for the operational designs in GAIM industrial applications and further investigation on the detailed mechanisms of various crystalline structures in GAIM parts. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Simulation of a mold‐cooling process for gas‐assisted injection molded parts designed with a top rib on the gas channel

The same CAE model used for the filling and packing stage in the gas‐assisted injection molding (GAIM) process simulation was also applied to simulate the cooling phase. This was made possible by using the line source method for modeling cooling channels. The cycle‐averaged and cyclic transient mold cavity surface temperature distribution within a steady cycle was calculated using the three‐dimensional modified boundary element technique similar to that used in conventional injection molding. The analysis results for GAIM plates of a semicircular gas channel design attached with a top rib are illustrated and discussed. It was found that the difference in cycle‐averaged mold wall temperatures may be as high as 10°C, and within a steady cycle, part temperatures may also vary by about 15°C. The conversion of the gas channel into equivalent circular pipe and further simplification into two‐node elements using the line source method not only affects the mold wall temperature calculation very slightly but also reduces the computer time by 93%. This indicates that it is feasible to achieve an integrated process simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application.     

Extension of the orientation region of high density polyethylene molded by gas‐assisted injection molding: control of the thermal field

Wider zones with close‐knit orientation crystals in high density polyethylene (HDPE) parts prepared via the gas‐assisted injection molding (GAIM) process were obtained under high cooling gas pressure. In this study, compressed nitrogen, as a cooling medium, was introduced to retain a high cooling rate of the polymer melt. The high gas pressure leads to fast cooling of the polymer melt, which contributes to the stability of more oriented and stretched chains during the cooling stage. Then many more oriented structures are formed. SEM shows that many more oriented structures and interlocking shish‐kebab structures are achieved in parts under highest cooling gas pressure (P3). The P3 parts possess a higher degree of orientation than the corresponding regions of parts under lowest cooling gas pressure (P1). Moreover, tensile testing indicates that, compared with P1 parts, although P3 parts have lower crystallinity, the mechanical properties are improved because of the wider orientation zone and many more interlocking shish‐kebab structures. Combining the HDPE molecular parameters with the characteristics of the GAIM flow field and temperature field, the stability of oriented or stretched chains and the formation of orientation structures in various zones of the parts were analyzed. © 2014 Society of Chemical Industry     

Temperature and shear rate distributions of gas-assisted injection molded (GAIM) polypropylene and the application in forecast of cooling time

An umbrella handle product of polypropylene molded by gas-assisted injection molding (GAIM) was studied from both aspects: theoretical modeling and simulation as well as in situ temperature measurement. The simulation was primarily through the use of the commercial software Moldflow (version 6.1) coupled with enthalpy transformation method (ETM) in an attempt to investigate the shear rate and temperature fields during GAIM process. A four-parameter model (FPM) was used to nonlinearly fit the temperature decays during the GAIM cooling stage on the basis of a three-parameter model (TPM) raised previously in our group. The FPM showed perfect fitting effect as well as presented fairly acceptable cooling time (t_c) prediction in comparison to experimental data, which could better reflect the nature of crystalline polymers during melt crystallization process. The understanding of the shear rate and temperature fields would be of practical importance to the further research on relationship of “processing–structure–property” as well as the optimization of cooling parameters for industrial GAIM operations of crystalline polymers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47390.     

A 3‐D finite element model for gas‐assisted injection molding: Simulations and experiments

To gain a better understanding of the gas‐assisted injection molding process, we have developed a computational model for the gas assisted injection molding (GAIM) process. This model has been set up to deal with (non‐isothermal) three‐dimensional flow, in order to correctly predict the gas distribution in GAIM products. It employs a pseudo‐concentration method, in which the governing equations are solved on a fixed grid that covers the entire mold. Both the air downstream of the polymer front and the gas are represented by a fictitious fluid that does not contributeto the pressure drop in the mold. The model has been validated against both isothermal and non‐isothermal gas injected experiments. In contrast to other models that have been reported in the literature, our model yields the gas penetration from the actual process physics (not from a presupposed gas distribution). Consequently, it is able to deal with the 3‐D character of the process, as well as with primary (end of gas filling) and secondary (end of packing) gas penetration, including temperature effects and generalized Newtonian viscosity behavior.     

Analysis of a liquid‐assisted molding process for coating microchannels with an ultraviolet curable polymer

Injection molding can be altered to form hollow parts by partially pre‐filling a mold with polymer melt and then injecting a gas into the mold before cooling. The gas will core the center section and in the process force melt into the unfilled portions of the mold. This process is called gas‐assisted injection molding (GAIM) and is a thoroughly studied polymer processing technique. Liquid‐assisted molding follows the same principles as GAIM, except the coring fluid is a liquid of low viscosity. Liquid‐assisted molding of an ultraviolet (UV) curable polymer can be used to coat microchannels, the benefit of which being a smooth and circular cross‐section. Presented here are experiments of the controlled microchannel flow of a long, immiscible liquid thread through a viscous UV curable polymer. The roles of channel geometry and bubble velocity are discussed for square, rectangular, and circular microchannels. Finally, a quasi‐analytical model for calculating the Newtonian coating fluid thickness, when the coring fluid is driven by a constant pressure, was developed using the equation for Poiseuille‐like flow within a square channel. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Integrated simulations of structural performance,molding process,and warpage for gas‐assisted injection‐molded parts. III. Simulation of cyclic,transient variations in mold wall temperatures

Whether it is feasible to perform an integrated simulation for structural analysis, process simulation, as well as warpage calculation based on a unified CAE model for gas‐assisted injection molding (GAIM) is a great concern. In the present study, numerical algorithms based on the same CAE model used for process simulation regarding filling and packing stages were developed to simulate the cooling phase of GAIM considering the influence of the cooling system. The cycle‐averaged mold cavity surface temperature distribution within a steady cycle is first calculated based on a steady‐state approach to count for overall heat balance using three‐dimensional modified boundary element technique. The part temperature distribution and profiles, as well as the associated transient heat flux on plastic–mold interface, are then computed by a finite difference method in a decoupled manner. Finally, the difference between cycle‐averaged heat flux and transient heat flux is analyzed to obtain the cyclic, transient mold cavity surface temperatures. The analysis results for GAIM plates with semicircular gas channel design are illustrated and discussed. It was found that the difference in cycle‐averaged mold wall temperatures may be as high as 10°C and within a steady cycle, part temperatures may also vary ∼ 15°C. The conversion of gas channel into equivalent circular pipe and further simplified to two‐node elements using a line source approach not only affects the mold wall temperature calculation very slightly, but also reduces the computer time by 95%. This investigation indicates that it is feasible to achieve an integrated process simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 339–351, 1999     

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     

A proposed technique to improve the filling of the mold cavity by polymer during injection molding

In this study, a technique is proposed to improve the filling process of the injection molding and minimize the solidification during the filling to achieve a complete filling of the mold cavity. Two methods are proposed: stopping the flow rate of the cooling fluid or passing cooling fluid inside the cooling channels at higher temperature during a period of the injection molding cycle. The configuration studied consists of the mold with cuboids‐shape cavity having two different thicknesses. A validation of the numerical model used by an experimental work is presented. The results show that, stopping of cooling fluid on a period of injection cycle has not great effect on the improvement of injection cycle. The results indicate that passing coolant fluid at higher temperature during the ejection stage decreases the solidification of the polymer during the filling stage by about 40%. © POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers.     

聚丙烯气体辅助注射成型冷却过程中温度分布的研究

通过对聚合物气体辅助注射成型冷却过程进行合理的假设与简化,对聚丙烯（PP）气体辅助注射成型冷却过程进行了实验与数值模拟研究。结果表明,注射氮气后,PP的冷却速度显著加快,并在气-熔界面处出现小范围的结晶平台。采用MATLAB软件对氮气辅助注射成型PP冷却过程中的温度分布进行了数值模拟,将计算所得模拟值与温度采集的实验值进行比较,发现在熔体降温阶段温度分布的模拟值与实验值吻合程度很高;在固相冷却阶段由于聚合物本身复杂特性以及气体的渗透效应,PP的模拟值略高于实验值,而氮气的模拟值低于实验值。     

Effect of Melt and Mold Temperatures on the Solidification Behavior of HDPE during Gas‐Assisted Injection Molding: An Enthalpy Transformation Approach

The influence of melt and mold temperature on the solidification behavior of HDPE during the GAIM process is studied using a transient‐heat‐transfer model of the enthalpy transformation approach. An in situ measurement of temperature decay in the mold cavity was carried out to verify the simulated results experimentally, and reasonable agreement was observed. The comparison of the HDPE solidification behavior under various cooling conditions reveals that the rapid cooling rate (due to thin wall‐thickness) is the main reason for the shortening of molding cycles, and that the mold temperature shows greater influence on the controlling of cooling rates than melt temperature during GAIM process.

     

Birefringence in gas‐assisted tubular injection moldings: Simulation and experiment

The influence of the processing variables on the birefringence and polymer/gas interface distribution is analyzed for polystyrene moldings obtained by gas‐assisted injection molding (GAIM) under various processing conditions. The processing variables studied were: melt and mold temperatures, shot size, gas pressure, injection speed, and gas‐delay time. Measurements and viscoelastic simulations of the radial distribution of birefringence components, Δn and n_rr ? n_θθ, the variation of the average birefringence, 〈n_zz ? n_θθ〉, along the molding and polymer/gas interface along the length of spiral‐shaped tubular moldings are presented. The polymer/gas interface distribution and flow stresses were simulated using a numerical scheme based on a hybrid finite element/finite difference/control volume method. The birefringence was calculated from the flow‐induced stresses using the stress‐optical rule. Simulations qualitatively agreed with measurements and correctly described theeffect of the processing variables on the birefringence andthe polymer/gas interface distribution in GAIM moldings. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Crystallization behavior and molecular orientation of high density polyethylene parts prepared by gas‐assisted injection molding

The dependence of hierarchy in crystalline structures and molecular orientations of high density polyethylene parts with different molecular weights molded by gas‐assisted injection molding (GAIM) was intensively examined by scanning electron microscopy, two‐dimensional wide‐angle X‐ray scattering as well as dynamic rheological measurements. The non‐isothermal crystallization kinetics of the samples were also analyzed with a differential scanning calorimeter at various scanning rates. It was found that oriented lamellar structure, shish‐kebab and common spherulites were formed in different regions of the GAIM samples. The scanning electron microscope observations were consistent with the two‐dimensional wide‐angle X‐ray scattering results and showed that the molecular chains near the mold wall had strong orientation behavior, revealing the distribution of the shear rate of the GAIM process. The differences in crystal morphologies can be attributed to molecular weight differences as well as their responses to the external fields during the GAIM process. The formation mechanism of the shish‐kebab structure under the flow field of GAIM was also explored. Copyright © 2012 Society of Chemical Industry     

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     

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     

Computer simulation of the filling process in gas‐assisted injection molding based on gas‐penetration modeling

Gas‐assisted injection molding can effectively produce parts free of sink marks in thick sections and free of warpage in long plates. This article concerns the numerical simulation of melt flow and gas penetration during the filling stage in gas‐assisted injection molding. By taking the influence of gas penetration on the melt flow as boundary conditions of the melt‐filling region, a hybrid finite‐element/finite‐difference method similar to conventional‐injection molding simulation was used in the gas‐assisted injection molding‐filling simulation. For gas penetration within the gas channel, an analytical formulation of the gas‐penetration thickness ratio was deduced based on the matching asymptotic expansion method. Finally, an experiment was employed to verify this proposed simulation scheme and gas‐penetration model, by comparing the results of the experiment with the simulation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2377–2384, 2003     

复杂壳体类塑料件气体辅助注射成型工艺参数优化研究

以21英寸彩电前壳作为研究对象,将Moldflow 2010作为CAE模拟试验平台,以熔体温度、模具温度、熔体注射时间、气体延迟时间、气体压力为关键工艺因素,考察了复杂壳体类塑料件气体辅助注射成型(GAIM)时制件的翘曲变形量和气体穿透情况。以正交试验设计方法为基础,利用遗传算法并结合径向基神经网络建立GAIM工艺参数优化系统,可用于工艺参数组合的快速确定,为GAIM过程中工艺参数优化提供了一种新的求解思路。     

Numerical prediction of phase‐change heat conduction of injection‐molded high density polyethylene thick‐walled parts via the enthalpy transforming model with mushy zone

Transient heat transfer problems with phase‐changes, also known as the “Stefan problems” or “moving‐boundary problems,” are practically significant in many engineering and technological fields. Injection molding, one of the most widely used plastics processing techniques, mainly consists of filling, packing, and cooling, and the cooling stage is crucial since it considerably affects the productivity and quality of the molded parts. Thus, solutions for transient phase‐change heat conduction problems during injection molding will be instructive. In this article, the enthalpy transforming scheme proposed by Cao and Faghri, which could handle the Stefan problems for generalized multidimensional phase‐change structures, is applied coupled with the control‐volume/finite‐difference techniques. Considering the polydispersity and hierarchical structures, the polymer extended phase change temperature range or mushy zone was included in the two‐dimensional enthalpy formulation to forecast the transient phase‐change heat conduction during the cooling stage for injection‐molded high density polyethylene (HDPE) parts. Experiments were performed and good agreement has been achieved, which reveals that the enthalpy transforming model gives good prediction, especially for the cooling analysis for the injection molding of thick‐walled parts of crystalline polymers. The understanding of the phase‐change heat conduction characteristics may facilitate the optimal designs of polymer injection molding process for industrial applications. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers