Gas-assisted Injection Molding Process Research Articles

FEM of Gas-Assisted Injection Molding Based on 3D Model

The gas-assisted injection molding (GAIM) process is so complicated that increasing reliance has been placed on CAE (Computer Aided Engineering) as a tool for both mold designers and process engineers. In this paper, a 3D theoretical model and numerical scheme is presented to simulate the GAIM process, in which an equal-order velocity-pressure formulation method is employed to eliminate the pressure oscillation. In addition, the whole flow field including the gas and melt regions is calculated using a uniform momentum equation with the viscosity of gas raised to a certain order of magnitude, and a 3D control volume scheme is employed to track the flow front of the melt and gas. Finally, the validity of the model has been tested through case studies and experimental verification.

Macroscopic and mesoscopic simulation of viscoelastic free surface flow in gas‐assisted injection moulding process

AbstractIn this work, an improved simple coupled level‐set and volume of fluid (S‐CLSVOF) method is proposed to trace the moving interfaces in gas assisted injection moulding (GAIM) process based on the viscoelastic FENE‐P model. Firstly, the Kreisselmeier‐Steinhauser (KS) function constructed by means of Boolean operations is adopted as the shape level set (LS) function to precisely represent the complex mould cavities. Then, the benchmark problem of two‐dimensional deformation is used to verify the ability of the S‐CLSVOF method for capturing the moving interfaces. The stress birefringence is calculated and compared with the experiment result. Finally, the proposed method is further applied to the mould filling and gas penetration processes in GAIM. The macroscopic bubble appearance, temperature distribution, and the behaviour of the mesoscopic molecular orientation are shown and analyzed in detail. Due to the complexity of the gas‐liquid interaction, the phenomenon of asymmetrical gas flow is clearly observed in the gas penetration process. The influences of the macroscopic parameters on the macroscopic gas flows and mesoscopic molecular orientation are also discussed, such as insert positions, melt temperatures, and gas delay time. The numerical results illustrate that the coupled method can be applied to the multiscale numerical simulation of viscoelastic flows with complex free surfaces and provide significant guidance for the GAIM process.

Three‐dimensional simulations of non‐isothermal flow for gas penetration in complex cavity during gas assisted injection moulding process

AbstractIn this paper an R‐function is employed to construct the shape level set (LS) function for representing the complex mould cavity in a gas‐assisted injection moulding (GAIM) process. With the shape LS function, the continuity, momentum, and energy equations can be extended for solving the gas‐liquid two‐phase flows using the finite volume and immersed boundary methods. Firstly, an improved simple coupled level set and volume of fluid (S‐CLSVOF) method is proposed to track the moving interfaces of gas‐liquid interface and polymer melt front. Then the benchmark problems of the two‐ and three‐dimensional deformation tests are carried out to verify the ability of the improved S‐CLSVOF method. At last, the gas penetration processes are simulated in two different complex mould cavities. The numerical results show that the improved S‐CLSVOF method has higher accuracy than the S‐CLSVOF method, and the coupled numerical framework can successfully depict the asymmetry phenomenon of gas flow, racetracking, and fountain phenomena, which has great significance in determining processing conditions and designing mould cavities.

Visualization of counter pressure mechanism in gas-assisted injection molding process

Gas-assisted injection molding (GAIM) refers to injecting gas into the short shot melt during the filling stage. Compressed gas is used as the medium to push the melt and to provide the packing pressure. In GAIM, the hollow area and penetration length are the main factors that will affect the quality of molded parts. This study has applied a Gas Counter Pressure (GCP) mechanism and has discussed the effect of GCP in the GAIM process with in-mold visualization of this complex molding flow. This study introduces a counter pressure mechanism in a thick paper-clip-shaped cavity design. The flow field under different counter pressure conditions is observed by high-speed photography, the fiber orientations are analyzed with SEM, and the affected penetration length and hollow area are measured relatively. The experimental results show that when the GCP is applied to GAIM, although the hollow area is reduced, the penetration length will be increased, so as to make the quality of molded part more uniform and reduce the shrinkage. And a quantitative measuring method of two-stage penetration time span is proposed to get more in-depth discussion about the interactions between GCP and GAIM.

Simulation of dynamic gas penetrations on fingering behaviors during gas-assisted injection molding

Abstract The gas-assisted injection molding (GAIM) process is often adopted to compensate for shrinkage and improve part quality. However, gas penetrations consisting of primary and secondary penetrations will usually induce “fingering” behaviors, in which gas penetrates unevenly inside the parts and may lead to significant reductions in part stiffness. Due to the dynamic instability and complex material properties of fingering behaviors, it is desirable to be able to predict the behaviors with the help of simulation tools. In order to simulate the fingering behaviors, it is necessary to calculate the gas penetrations with a sharp interface between melt and gas, and to incorporate the compressibility of materials with high accuracy. In this study, we present a three-dimensional finite volume method that considers the non-Newtonian compressible flow with a high-resolution interface scheme for simulating the fingering behaviors during the GAIM process. A thin plate case is done to validate the accuracy and the capability of our proposed method to predict the sharp interface for primary and secondary gas penetrations. Then, we present a fish-bone plate with branches to investigate the gas penetrations on fingering behaviors through the dynamic simulation results and the specific volume changes on a pressure-volume-temperature diagram.

Strong shear-driven large scale formation of hybrid shish-kebab in carbon nanofiber reinforced polyethylene composites during the melt second flow.

The formation of a hybrid shish-kebab (HSK) structure with different degrees of lamellar orientations was first observed in the solution crystallization of polyethylene (PE) in the presence of carbon nanofibers (CNFs). In this study, PE crystal lamellae were periodically decorated on the surface of CNFs and were aligned approximately perpendicular to the long axes of the CNFs, forming aligned hybrid shish-kebab nanostructures. More importantly, the fascinating structure was directly formed in all regions of the injection molded bars of HDPE/CNF composites, via a gas-assisted injection molding (GAIM), instead of the shell-core structure. In the GAIM process, an intense shear was imposed onto the melt during the melt second flow and drove PE chains to orient along the axes of the CNFs. Then the entropy penalty for PE chains deposited on the CNF surface was drastically decreased. Although the attractive van der Waals interactions were weak, the oriented PE chains could successfully adsorb on the CNF surface due to the decrease of the entropy penalty, therewith the underlayer coating was formed along the axis based on a two-dimensional mode for early nucleation on the CNF surface. Subsequently, subglobules appeared on the ordered structure, which could be regarded as the crystal nucleus. Finally, the oriented PE chains began to epitaxially grow from the subglobules with a folded-chain shape to decrease the polymer surface energy and grew perpendicular to the CNFs long axis, abiding by the "soft epitaxy" crystallization mechanism regardless of strict lattice matching.

A cleaner production of rice husk-blended polypropylene eco-composite by gas-assisted injection moulding

Abstract Gas-assisted injection moulding processes are widely applied in the plastics industry to improve moulding quality by using fewer polymers. This process is achieved by hollowing out the internal section of the mould to reduce the usage for conventional polymers and is particularly good for thick, moulded products. The eco-composites polymers, such as rice husk-filled polymers are environmental friendly materials to replace polymers. A cleaner production can be achieved by using rice husk-blended polypropylene RH/PP eco-composite in the gas-assisted injection moulding process. No successful case is reported in literature, as the increased shear viscosity of the non-petrochemical and natural-based polymers make it difficult for the eco-composite to flow inside the moulds. In this study, gas-assisted injection moulding technology was successfully applied to the rice husk-filled polypropylene eco-composites polymers in different compositions. Different stages of injection pressure and delays of gas pressure were applied in order to improve the flow characteristics. In addition, the internal wall surface of the mould must be polished to ‘Mold-Tech’ SPI A2, an industry standard textured finishes. The new approach uses fewer petrochemical polymers with improved moulding quality, especially for thick, moulded parts. The new method is also an environmentally-friendly approach as it uses less injection pressure and clamping force. This has created a good foundation for further research in cleaner production of different kinds of eco-composites material by gas-assisted injection moulding.

Hierarchical crystalline structures and dynamic mechanical properties of injection-molded bars of HDPE: attributes of temperature field

The relationship among the processing parameters, crystalline morphologies and mechanical properties of injected-molded bar becomes much complicated primarily due to the existence of temperature gradient coupled with the shear gradient along the sample thickness. The effect of thermal gradient field on the microstructural evolution, hierarchical structures and dynamic mechanical properties of high-density polyethylene parts molded via gas-assisted injection molding (GAIM) were investigated using scanning electron microscope, differential scanning calorimetry, dynamic mechanical analysis and two-dimensional wide-angle X-ray diffraction. The three-dimensional temperature profiles during the cooling stage under different melt temperatures of GAIM process were obtained by using a transient heat transfer model of the enthalpy transformation approach, and the phase-change plateaus were clearly observed in the cooling curves. It was found that a variety of melt temperatures could induce considerable variations of the hierarchical structures, orientation behavior and dynamic mechanical properties of the injection-molded bars. With reduced melt temperature, GAIM samples with higher molecular orientation and improved dynamic mechanical properties were obtained. Copyright © 2013 John Wiley & Sons, Ltd.

A novel hierarchical crystalline structure of injection-molded bars of linear polymer: co-existence of bending and normal shish–kebab structure

The hierarchy structures and orientation behavior of high-density polyethylene (HDPE) molded by conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were intensively examined by using scanning electronic microscopy (SEM) and 2D wide-angle X-ray diffraction (2D-WAXD). Results show that the spatial variation of crystals across the thickness of sample molded by CIM was characterized by a typical skin–core structure as a result of general shear-induced crystallization. Unusually, the crystalline morphologies of the parts prepared by GAIM, primarily due to the penetration of secondary high-compressed gas that was exerted on the polymer melt during gas injection, featured a richer and fascinating supermolecular structure. Besides, the oriented lamellar structure, general shish–kebab structure, and common spherulites existed in the skin, sub-skin, and gas channel region, respectively; a novel morphology of shish–kebab structure was seen in the sub-skin layer of the GAIM parts of HDPE. This special shish–kebab structure (recognized as “bending shish–kebab”) was neither parallel nor perpendicular to the flow direction but at an angle. Furthermore, there was a clear interface between the bending and the normal shish–kebab structures, which may be very significant for our understanding of the melt flow or polymer rheology under the coupling effect of multi-fluid flow and complex temperature profiles in the GAIM process. Based on experimental observations, a schematic illustration was proposed to interpret the formation mechanism of the bending shish–kebab structure during GAIM process.

Role of gas delay time on the hierarchical crystalline structure and mechanical property of HDPE molded by gas-assisted injection molding

The relationship among the processing parameters, crystalline morphology, and macroscopic properties in injected molded bar becomes very complicated due to existence of temperature gradient and shear gradient along the sample thickness. To enhance the shear strength, gas-assisted injection molding (GAIM) was utilized in producing the molded bars. The aim of our research was to explore the relationship between processing conditions and the spatial variation of the hierarchy structure as well as the mechanical properties of high-density polyethylene (HDPE) obtained via GAIM. In our previous work [Wang L, Yang B, Yang W et al (2011) Colloid Polym Sci 289:1661–1671], we found that the enhancement of the gas pressure can remarkably increase the degree of molecular orientation in the HDPE samples, which turns out to improve the mechanical performances of GAIM parts. In this work, the hierarchy structure, orientation behavior, and mechanical properties of molder bars under different gas delay time were investigated using a variety of characterization techniques including rheological experiments, scanning electron microscope, tensile testing, differential scanning calorimetry, and two-dimensional wide-angle X-ray scattering. Moreover, the temperature field during the short shot stage of GAIM process was simulated using an enthalpy transformation approach. Our results indicate that these properties were intimately related to each other, and with prolonged gas delay time, GAIM samples with higher degree of orientation and improved mechanical properties were obtained.

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

AbstractThe 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

Application of Moldflow in Gas-Assisted Injection Molding

Gas-assisted injection molding(GAIM)is a new kind of plastic processing technique. It is one of the most important developments in the injection molding industry. GAIM has many advantages such as lower injection pressure, lower warpage, better surface quality, lower material consumption, and shorten molding cycle time, etc. MPI/Gas module of Moldflow software can be used to simulate the GAIM process to optimize the whole molding process. In this study, the FM new truck interior ceiling handle was analyzed in GAIM process based on the MPI/Gas module. The simulation results showed the gas penetrating time, the contours of plastic layers thickness fraction of the parts and the gas volume fraction changing with time. The results can help technicians to determine the optimum process of the melt injection and the gas injection to ensure the final quality of the parts.

Simulation of the gas-assisted injection moulding process using a viscoelastic extension to the Cross-WLF viscosity model

An approximation to the viscoelastic Maxwell model is developed and combined with a Cross-WLF shear- and temperature-dependent model as a means of introducing aspects of viscoelasticity into the Cross-WLF model at a low computational cost. The main objective of the model is to simulate the gas-assisted injection moulding (GAIM) process with the aspect of a material's strain history included. It is shown that the model gives a transient and steady response comparable to the Doi–Edwards viscoelastic model in constant rate shear and uniaxial deformations, and follows WLF temperature dependence. The model is implemented in a three-dimensional finite element code using the ‘pseudo-concentration’ method to model the polymer and gas phases. The ordinary Cross-WLF model had demonstrated a consistent under-prediction of residual wall thickness (RWT) measurements in comparison to experimental results. It is shown that the viscoelastic extension to the Cross-WLF model gives a marked increase in RWT and exhibits aspects of stress relaxation and history dependence. The model is tested against variations of other process control parameters. It is shown that the simulation gives the correct qualitative response for all control parameters assessed, with quantitative prediction within a factor of 2.

Morphological Study of Linear Low-Density Polyethylene Molded by Gas-Assisted Injection Molding

Position dependence for the supermolecular morphology of linear low density polyethylene (LLDPE) prepared by gas-assisted injection molding (GAIM) was examined using scanning electron microscopy (SEM). Combined morphological feature of both spherulites and those with ring-bands was observed, which differs from our previous studies on the HDPE counterparts. Dynamic rheological measurements were also carried out to interpret the formation mechanism of the morphological feature. The difference in hierarchical crystalline structures between LLDPE and HDPE can be due to their different chain branches as well as the resultant different response to the local shear fields during the GAIM process.

Numerical Simulation of Melt Filling and Gas Penetration in Gas Assisted Injection Molding

The governing equations of two-phase flows including gas and polymer melt are presented, which are solved using finite volume and domain extension methods with SIMPLEC technology. The melt filling and primary gas penetration in gas-assisted injection molding (GAIM) process are simulated, where the Cross-viscosity model is employed to describe the melt rheological behavior, and the CLSVOF(coupled Level Set and Volume of fluid) method is employed to capture the moving interfaces. In order to test and verify the coupled methods, melt filling in a rectangular plate with an insert is simulated, and the numerical results are in good agreement with those of the experiment. As a case study, the melt filling and gas penetration processes in a ring-shaped channel are simulated. The numerical results successfully depict some important phenomena, such as race-tracking effect, corner effect and the flow asymmetry of the gas penetration, which can deepen the understanding and studying of the GAIM process.

Effect of thermal gradient field with phase change on crystal morphologies of HDPE during GAIM process

To understand the effects of thermal gradient field with phase change on the crystal morphologies of high density polyethylene (HDPE) during gas assisted injection moulding (GAIM) process, the combination of the simulation of the temperature field via an enthalpy transformation method and the hierarchical structures and the crystalline morphology of HDPE were investigated in this work. The GAIM parts of HDPE 6098 can be grouped into four layers in the direction of thickness: skin layer, subskin layer, intermediate layer and core layer. Besides, the cooling logarithmic curves were adopted in this experiment, and it is found that they can clearly display the phase change plateaus even at the skin layer zone.

Modelling and simulation of moving interfaces in gas-assisted injection moulding process

The mathematical models of gas–liquid two-phase flow are introduced, in which the multi-mode eXtended Pom–Pom (XPP) model is selected to predict the viscoelastic behavior of polymer melt. The gas-penetration process is simulated using Level Set/SIMPLEC methods, which can capture the moving interfaces at different time, including the gas–melt interface and the melt front. The physical features such as velocity, temperature and elasticity are described at different time. The influences of gas delay time and injection pressure on gas-penetration time and penetration length are analyzed. The numerical results show that the Level Set/SIMPLEC methods can precisely trace the two moving interfaces in gas-penetration process, the fractional coverage increases at very low Deborah numbers, while at higher Deborah numbers the fractional coverage decreases, and the penetration length is affected significantly by gas delay time and injection pressure.

Simulation of Gas-Assisted Injection Molding of High-Density Polyethylene: The Role of Rheological Properties and Physical Fields on the Crystalline Morphology

The shear and temperature fields in the filling process during gas penetration of gas-assisted injection molding (GAIM) of two kinds of high-density polyethylene (HDPE) with different molecular weights (Mw) were simulated numerically via Moldflow 6.1. The simulated results indicated that HDPE with higher Mw showed a relatively low shear rate in both the outer and inner zone of GAIM parts as compared to that with lower Mw, while the shear rate of the inner zone of the parts of the two materials were lower than that of the outer zone, which has a lower temperature and shorter time for crystallization. Considering the remarkable difference between their viscosities, a model of the shear thinning and recovering effects on crystallization during the GAIM process is proposed. Through the simulation, along with the analysis of their viscosities and rheological properties, the influence of molecular weight and shear and temperature fields on two special kinds of crystalline morphology (i.e., shish-kebabs and banded spherulites) during GAIM were interpreted.

Review of Gas-Assisted Injection Molding Process

The forming mechanism and optimization of the gas-assisted injection molding (GAIM) process have been studied for several years, both at home and abroad. Some researches and experiments about GAIM is summarized in this paper. The GAIM's forming mechanism focuses on research of the filling, gas-assisted filling and gas-assisted packing and cooling stages. While the optimization of the GAIM process pays much more attention to one of the parameters affecting the molding quality.

Gas-assisted Injection Molded Polypropylene/Glass Fiber Composite: Foaming Structure and Tensile Strength

Tensile strength of isotactic polypropylene (iPP)/glass fiber (GF) composites and neat iPP molded respectively by gas-assisted injection molding (GAIM) was examined. For comparison, tensile strength of the counterparts, which were molded by conventional injection molding (CIM) under the same processing conditions but without gas penetration, was also examined. Tensile strength of the CIM parts steadily increases with the increase of the GF content. For neat iPP molded by GAIM, as the gas pressure increases the tensile strength increases. However, for the iPP/GF composites, the tensile strength generally decreases when the gas pressure increases. And, at a given content of GF, tensile strength of the parts molded by GAIM is unexpectedly lower than that of the counterparts molded by CIM. At a given gas pressure, the higher the fiber content, the lower the tensile strength. In addition, scanning electron microscope (SEM) results show that foaming structure should be responsible for the poor tensile strength of the composites molded by GAIM. The poor adhesion between the glass fibers and the matrix and the unique properties of the gas used in GAIM process are the substantial factors in the formation of foaming structure.