Identification of cell‐nucleation mechanism in foam injection molding with gas‐counter pressure via mold visualization

The mechanisms of cell nucleation and growth are investigated in foam injection molding (FIM) using gas‐counter pressure (GCP). An in‐situ mold visualization technique is employed. The application of GCP suppresses cell nucleation, and prevents the blowing agent from escaping during mold‐filling. The inherent structural heterogeneity in the regular FIM can be improved because of the uniform cavity pressure when employing GCP. The cavity pressure profiles show much faster pressure‐drop rates using GCP, because the single‐phase polymer/gas mixture has a lower compressibility than the two‐phase polymer/bubble mixture. Therefore, both the cell nucleation and growth rates are significantly increased through a higher pressure‐drop rate on the removal of the GCP. The effect of GCP magnitude on the cell morphology is explored. When the GCP is lower than the solubility pressure, bimodal foaming occurs. As the GCP increases above the solubility pressure, the cell density increases because of the higher pressure‐drop rate. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4035–4046, 2016     

Bubble growth model and its influencing factors in a polymer melt under nonisothermal conditions

Traditionally, in order to simplify the bubble growth process in a polymer melt, an isothermal model is typically used. In fact, the temperature of the polymer melt is changing during the foaming process. In order to accurately study the growth mechanism of bubbles in polymer melts, we build a physical and mathematical model of bubble growth in a polymer melt under nonisothermal conditions. The parameters of pressure, zero-shear viscosity, relaxation time, Henry's constant, diffusion coefficient, and surface tension were determined. The fourth-order Runge–Kutta method was used to solve the nonisothermal bubble model in the polymer melt. A computational program is developed to find the dimensional change during the bubble growth process, and the correctness of the model is verified. The nonisothermal growth mechanism of and factors influencing bubbles in the polymer melt are analyzed. Combined with the design of experiment (DOE) analysis method, the transfer function of the bubble radius and the maximum growth rate of bubbles with the process parameters were obtained, such as cooling rate, system pressure, and gas concentration. The results show that system pressure has the most significant effect on bubble growth. At the same time, a bubble growth prediction model is built, which can be used to predict the growth of bubbles. Through optimization analysis, it can be used to control the growth of bubbles. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47210.     

Studies on structural foam processing II. Bubble dynamics in foam injection molding

An experimental study was carried out to gain a better understanding of the dynamic behavior of gas bubbles during the structural foam injection molding operation. For the study, a rectangular mold cavity with glass windows on both sides was constructed, which permitted us to record on a movie film the dynamic behavior of gas bubbles in the mold cavity as a molten polymer containing inert gas was injected into it. The mold was designed so that either isothermal or nonisothermal injection molding could be carried out. Materials used were polystyrene, high-density polyethylene, and polycarbonate. As chemical blowing agents, sodium bicarbonate (which generates carbon dioxide), a proprietary hydrazide and 5-phenyl tetrazole, both generating nitrogen, were used. Injection pressure, injection melt temperature, and mold temperature were varied to investigate the kinetics of bubble growth (and collapse) during the foam injection molding operation. It was found that the processing variables (e.g., the mold temperature, the injection pressure, the concentration of blowing agent) have a profound influence on the nucleation and growth rates of gas bubbles during mold filling. Some specific observations made from the present study are as follows: an increase in melt temperature, blowing agent concentration, and mold temperature brings about an increase in bubble growth but more non-uniform cell size and its distribution, whereas an increase in injection pressure (and hence injection speed) brings about a decrease in bubble growth but more uniform cell size and its distribution. Whereas almost all the theoretical studies published in the literature deal with the growth (or collapse) of a stationary single spherical gas bubble under isothermal conditions, in structural foam injection molding the shape of the bubble is not spherical because the fluid is in motion during mold filling. Moreover, a temperature gradient exists in the mold cavity and the cooling subsequent to mold filling influences bubble growth significantly. It is suggested that theoretical study be carried out on bubble growth in an imposed shear field under nonisothermal conditions.     

Visual observation and numerical studies of polymer foaming behavior of polypropylene/carbon dioxide system in a core‐back injection molding process

Thermoplastic foaming within a mold cavity was visualized as it was conducted in an 85‐ton core‐back injection‐molding machine. The core‐back molding process moved a section of the mold just after injecting a molten polymer into the cavity, quickly reducing the pressure to enhance the bubble nucleation. The foaming behavior during core‐back was observed directly through the glass windows of the mold. In the experiments, impact copolymer polypropylene was foamed with carbon dioxide. The effects of the gas concentration and the core‐back rate on bubble nucleation and growth were investigated. It was experimentally confirmed that the bubbles disappeared when the cavity was fully packed and that bubble nucleation occurred when the mold plate was moved and the cavity pressure dropped. Faster core‐back rates and higher gas concentrations increased the number of bubbles while decreasing their size. To analyze the experimental results, a bubble nucleation and growth model was employed that was based on batch foaming. The numerical results were a reasonable representation of the experiments, and this study demonstrated the applicability of the conventional free foaming model to the industrial core‐back molding process. Many aspects of the foaming in the core‐back molding aresimilar to the behaviors observed by batch foaming. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Dynamically enhanced membrane foaming

Foamed food products like chocolate mousse, ice cream or fresh cheese are increasingly popular due to their soft and creamy sensory properties. Their perception, stability and flow behavior strongly depend on gas fraction and bubble size distribution. Foam processing research focuses on developing new optimized processes and material systems to achieve small mean bubble size and narrow size distribution.In this work, we present a new dynamically enhanced membrane foaming process. This foaming device basically consists of two concentric cylinders: the inner cylinder is rotated with circumferential velocities up to , the outer cylinder is fixed. Thus, a shear field is created in the narrow annular gap. The membrane can either be mounted to the inner or outer cylinder. Gas is pressed through the membrane and is detached as small bubbles by the acting flow shear stresses. The comparison of rheological and microstructural analysis of foams to results on bubble breakup in simple shear flow and on detachment of bubbles from the pore of a rotating membrane proved that the detachment of small bubbles from the membrane is the dominating bubble formation process in the dynamically enhanced membrane foaming process. Compared to conventional rotor-stator foaming devices, the dynamically enhanced membrane foaming process leads to significantly smaller mean bubble sizes at higher gas volume fractions and to reduced size distributions widths.     

Studies on structural foam processing. IV. Bubble growth during mold filling

An experimental and theoretical study was carried out to achieve a better understanding of bubble growth during the filling of gas-charged molten polymers into a rectangular mold cavity. For the experimental study, a rectangular mold cavity (15.24 × 4.55 × 0.64 cm) was constructed, with glass windows on both sides to permit recording on a movie film of the growth of gas bubbles in the mold cavity as a molten polymer containing inert gas was injected into it. Sodium bicarbonate (generating carbon dioxide) was used as a chemical blowing agent, and the polymer used was a general purpose clear polystyrene. All experimental runs were made at isothermal molding conditions, and the injection rate was varied. It was found that, at and above a certain injection rate, little bubble formation was observed in the mold cavity during injection except at and near the moving melt front. For the theoretical study, the growth of a single gas bubble in a viscoelastic medium (represented by the DeWitt model), subjected to high injection rates, was considered by including the effects of diffusion from the liquid phase to the gas phase, interfacial tension between the liquid and the gas phases, and stress relaxation of the melt upon ejection. It was found that the level of stresses, built up in the met during injection, has a profound influence on the formation and growth of gas bubbles during the initial stage of mold filling. Also, a multichannel mold cavity was employed in order to observe the effect of processing variables on the cell size and its distribution in molded specimens. A uniform cell structure was obtained at higher injection pressures, at an optimum injection melt temperature, and with an optimum combination of blowing agent and nucleating agent concentrations.     

Experimental and numerical studies on bubble dynamics in nonpressurized foaming systems

This work explores the influence of rheological properties on polymer foam development in nonpressurized systems. To understand the complex contributions of rheology on the mechanism of bubble growth during different stages of foam processing, visualization studies were conducted by using a polymer‐foaming microscopy setup. The evolving cellular structure during foaming was analyzed for its bubble surface density, bubble size, total bubble projected area, and bubble size distribution. Morphological analysis was used to determine the rheological processing window in terms of shear viscosity, elastic modulus, melt strength and strain‐hardening, intended for the production of foams with greater foam expansion, increased bubble density and reduced bubble size. A bubble growth model and simulation scheme was also developed to describe the bubble growth phenomena that occurred in nonpressurized foaming systems. Using thermophysical and rheological properties of polymer/gas mixtures, the growth profiles for bubbles were predicted and compared to experimentally observed data. It was verified that the viscous bubble growth model was capable of depicting the growth behaviors of bubbles under various processing conditions. Furthermore, the effects of thermophysical and rheological parameters on the bubble growth dynamics were demonstrated by a series of sensitivity studies. POLYM. ENG. SCI., 54:1947–1959, 2014. © 2013 Society of Plastics Engineers     

Numerical simulation of bubble growth in viscoelastic fluid with diffusion of dissolved foaming agent

Viscoelastic simulations of bubble growth in polypropylene (PP) physical foaming were performed. A multimode Phan‐Thien Tanner (PTT) model was used to analyze the dynamic growth behavior of spherically symmetric bubbles with the diffusion of a foaming agent (CO₂). Changes in the dissolved foaming agent concentration in the polymer and in the strain of the polymer melt surrounding the bubbles were simulated with the Lagrangian FEM method. The simulation technique was validated by comparison with the bubble growth data, which were experimentally obtained from visual observations of the PP/CO₂ batch foaming system. The simulation results demonstrated that the strain‐hardening characteristic of polymer does not strongly affect the bubble growth rate. The linear viscoelastic characteristic is more influential, and the relaxation mode around 0.01 s is the most important factor in determining the bubble growth rate during the early stage of foaming. A multivariate analysis for the simulation results was also carried out. This clarified that bubble nucleus population density, surrounding pressure, initial dissolved foaming agent concentration, and diffusion coefficient are more important factors than the viscoelastic characteristics. POLYM. ENG. SCI., 45:1277–1287, 2005. © 2005 Society of Plastics Engineers     

Basic study of extrusion of polyethylene and polystyrene foams

A fundamental study of bubble morphology development and apparent rheological properties in foam extrusion is reported. The influence of melt temperature, die length/diameter ratio, and blowing agent level on the morphology are considered. Measurements of the influence of blowing agent on viscosity, extrudate swell, and end-pressure losses are described. The viscosity is reduced, but extrudate swell is increased. End-pressure losses were found to become very large relative to the die wall shear stress at low extrusion rates. These results were interpreted in terms of bubble development. The filling of molds by foaming melts was observed and is described.     

Bubble-scale observations of polyurethane foam expansion

We describe experiments designed to inform computational models of the dynamic filling process of chemically blown, polyurethane foams, especially subgrid models to predict bubble size affecting foam properties. Three experimental methods are used to observe the evolution of bubble sizes during blowing. Magnified views of bubbles at a transparent wall of a channel are recorded during the foaming. The bubble sizes in the final frame after the expansion has stopped are compared to scanning electron microscope images of the interior of the cured samples to determine wall effects. In addition, diffusing wave spectroscopy is used to determine the average bubble sizes across the width of a similar channel during foam expansion. We conclude that the bubble size distribution is dependent on the formulation of foam being tested, temperature, the height in the foam bar, the proximity to a wall, and the degree of overpacking.     

Visualization of bubble dynamics in foam injection molding

This article presents a visualization study on nonisothermal bubble growth and collapse in the foam injection molding process (FIM). Observation study can give more insight to the bubble growth in foaming process, especially in the challenging injection foaming process. In this study, besides the growth of bubbles, collapse of the bubbles was also observed which could provide knowledge to the final foam morphology. Cell growth vs. time was recorded and analyzed using a software‐equipped high speed camera. To investigate the cell collapse, various holding pressure was exerted on the gas‐charged molten polymer. The amount of holding pressure had noticeable effect on the rate of bubble collapse. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

型腔气体反压对微孔发泡泡孔结构与制品表面质量的影响

设计型腔气体反压辅助微孔发泡注塑(GCP–MIM)成型系统、模具及周边设备,利用这一套系统以聚丙烯(PP)材料为研究对象,研究GCP–MIM工艺对注塑PP试样泡孔结构与表面质量的影响。结果表明,随着气体反压持续时间的延长和气体反压压力的增加,试样泡孔尺寸变小,泡孔密度升高,其中反压持续时间从5 s延长到20 s过程中,泡孔平均直径减小了40%,而泡孔密度提高了23%。在相同的气体反压压力下,气体反压持续时间越长,试样表面粗糙度越低;与常规注塑不同,离浇口越远,GCP–MIM试样的表面粗糙度越低;与传统微孔发泡注塑相比,GCP–MIM试样的表面粗糙度明显降低,表面质量明显变好。     

Improving the stability of polylactic acid foams by interfacially adsorbed particles

Polymers such as poly(lactic acid) (PLA), which have poor melt strength, are difficult to foam due to severe cell coalescence during foaming. We show that addition of a few percent of polytetrafluoroethylene (PTFE) particles can stabilize PLA foams against bubble coalescence and collapse. The particles and a chemical blowing agent, were dispersed into the PLA by extrusion, and then foamed by heating. The PTFE‐containing foams remained stable even when the foams were held under molten conditions for extended periods. Foam stability is attributed to an interfacial mechanism: due to their low surface energy, the PTFE particles adsorb on the inner surface of the foam bubbles at a high surface coverage, and endow the bubbles with an interfacial “shell” that prevents coalescence. This mechanism resembles the particle‐stabilization of Pickering emulsions in oil/water systems. Particle adsorption at the interface is a necessary condition for using this approach, and hence this approach is most likely to be successful if the particles have a low surface energy and the polymer has a high surface tension. The approach of using interfacially adsorbed particles can be broadly generalized, and offers the opportunity of foaming various polymers with low melt strength, or for expanding the processing window within which foaming can be conducted. POLYM. ENG. SCI., 56:9–17, 2016. © 2015 Society of Plastics Engineers     

Experimental and numerical studies on the effects of pressure release rate on number density of bubbles and bubble growth in a polymeric foaming process

A simulation of simultaneous bubble nucleation and growth was performed for a batch physical foaming process of polypropylene (PP)/CO₂ system under finite pressure release rate. In the batch physical foaming process, CO₂ gas is dissolved in a polymer matrix under pressure. Then, the dissolved CO₂ in the polymer matrix becomes supersaturated when the pressure is released. A certain degree of supersaturation produces CO₂ bubbles in the polymer matrix. Bubbles are expanded by diffusion of the dissolved CO₂ into the bubbles. The pressure release rate is one of the control factors determining number density of bubbles and bubble growth rate.To study the effect of pressure release rate on foaming, this paper developed a simple kinetic model for the creation and expansion of bubbles based on the model of Flumerfelt's group, established in 1996 [Shafi, M.A., Lee, J.G., Flumerfelt, R.W., 1996. Prediction of cellular structure in free expansion polymer foam processing. Polymer Engineering and Science 36, 1950-1959]. It was revised according to the kinetic experimental data on the creation and expansion of bubbles under a finite pressure release rate. The model involved a bubble nucleation rate equation for bubble creation and a set of bubble growth rate equations for bubble expansion. The calculated results of the number density of bubbles and bubble growth rate agreed well with experimental results. The number density of bubbles increased with an increase in the pressure release rate. Simulation results indicated that the maximum bubble nucleation rate is determined by the balance between the pressure release rate and the consumption rate of the physical foaming agent by the growing bubbles. The bubble growth rate also increased with an increase in the pressure release rate. Viscosity-controlled and diffusion-controlled periods exist between the bubble nucleation and coalescence periods.     

PP/LDPE化学交联发泡的研究

聚丙烯(PP)熔融黏度较低,发泡过程中气泡容易从熔体中溢出。在PP中加入低密度聚乙烯(LDPE)和偶氮二异丙苯(DCP),提高PP交联度,从而大大提高PP的熔融黏度。研究了共混聚合物组分的种类和含量对PP交联度的影响。结果表明,在共混过程中,部分PP和LDPE分子在热作用下相互促进,产生了接枝交联;共混物比纯PP的泡孔结构优且发泡效果佳,当LDPE为70%,发泡剂为5%,DCP为0.36%时,PP的发泡效果最好。解决了PP发泡过程中出现的气孔塌陷现象。     

Bubble growth in the microcellular foaming of CO2/polypropylene solutions

This article is concerned with bubble growth dynamics in the CO₂/polypropylene microcellular foaming process. The effect of the melt strength on the bubble growth was thoroughly investigated in theory for the first time. The theoretical results indicate that enhanced melt strength effectively restrains the bubble growth and stabilizes the bubble oscillation. Higher melt strength leads to lower bubble growth rate, shorter growth time, and smaller ultimate bubble size. Compared to the melt strength, the viscoelasticity and the gas pressure have less effect on the microcellular foaming process. The bubble growth varies a little as the viscoelasticity is varied. The bubble oscillation and growth rate are enhanced with increasing gas pressure, which leads to the augmentation of the bubble size. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphological analysis of microcellular PP produced in a core‐back injection process using chemical blowing agents and gas counter pressure

A complete experimental analysis of the microcellular injection process using Chemical Blowing Agents (CBA) with Gas Counter Pressure (GCP) and core‐back expansion is presented. Three different types of polypropylene, neat and charged, were mixed with two different CBAs and injected into a plate mold with varying process parameters. First, an exhaustive cartographical mapping of the plate morphology is analyzed. In a second step, the relation between injection parameters and the resulting morphology is investigated. The results show that injection time affects the cellular structure. The formulation, especially the type of chemical foaming agent, controls the average bubble radius. Compared with classical injection process, the use of CBAs, combined to Gas Counter Pressure and core‐back process, allows obtaining parts with good surface aspect, more homogeneous cellular structures and smaller bubble radius. POLYM. ENG. SCI., 55:2465–2473, 2015. © 2015 Society of Plastics Engineers     

非对称温度场下微孔发泡模内表面装饰复合成型工艺的翘曲变形

以拉伸试验样条为研究对象,采用正交试验设计与流动仿真计算,对非对称温度场下微孔发泡模内表面装饰复合成型工艺（MIM/IMD）中制品翘曲的影响因素进行分析,获取翘曲变形最小的最佳工艺参数组合。并对4种注射成型工艺（MID/IMD、IMD、MID、CIM）分别在X、Y、Z方向的翘曲变形仿真结果进行对比,分析不同成型工艺的翘曲变形在不同方向上的变化、泡孔半径及密度,探究微孔结构及分布对翘曲变形的影响规律。结果表明,熔体温度210 °C、注射速率55 cm³/s、充填体积98 %、超临界N₂浓度0.2 %（质量分数）、冷却时间40 s时,翘曲最小,翘曲变形最大减少3.414 mm;采用发泡工艺对制品的翘曲有一定的抑制作用;非对称温度场导致覆膜侧的泡孔尺寸及密度大于非覆膜侧,且温度较非覆膜侧高,使样条覆膜侧收缩变形慢于非覆膜侧,从而导致样条两侧产生具有向非覆膜侧内凹卷曲趋势的不均衡翘曲变形。     

矩形薄腔中聚合物熔体非等温注气充填的数值模拟

采用五参数Cross粘度模型,利用有限元和控制体积法对矩形薄腔中聚合物熔体非等温注气充填过程中不同时刻的气泡边界、熔体前沿、熔体压力场、温度场和速度场等进行了数值模拟,结果表明:气泡主体边界平行于模壁方向生长,其前沿总是先向中心线附近移动,再向中心线两边移动;熔体压力在气泡周围某一区域内保持不变且等于气压;对于充模时间很短的情况,除气泡附近外,温度在各处相差不大。     

The effects of gas counter pressure and mold temperature variation on the surface quality and morphology of the microcellular polystyrene foams

In this study, we developed a foaming control system using the Gas Counter Pressure (GCP) combined with mold temperature control during the microcellular injection molding (MuCell) process and investigated its influence on the parts' surface quality and foams structures. The results revealed that under GCP control alone when GCP is greater than 10 MPa, part surface roughness for transparent polystyrene (PS) improved by 90%. When GCP increased, the skin thickness also increased, the weight reduction decreased and the average cell size reduced to about 30 μm. For black PS parts, when GCP is greater than 10 MPa, the part gloss reaches the same value as that molded by conventional injection molding. By increasing gas holding time, the cell density decreased and the cell size distribution became more uniform. The increase in amount of supercritical fluid foaming agent also increased the cell density. Applying mold temperature control alone with temperature in the range of 90–120°C (near T_g), the surface roughness improved by 65%. Increasing mold temperature decreased the skin thickness; however, the cell size distribution became significantly nonuniform. It was found that thin skin, small and uniform cell size as well as good surface quality can be achieved efficiently by simultaneous combining of GCP and mold temperature control. The proposed innovative approach may lead to a significant improvement and a more broad application for MuCell process. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013