Gas-assisted Injection Molding Research Articles

Numerical Simulation of Gas-assisted Injection Molding Process for A Door Handle

A unified mathematical model is proposed to predict the short shot, primary and secondary gas penetration phenomenon in gas-assisted injection molding (GAIM) process, where the Cross-WLF model and two-domain modified Tait equation are employed to simulate the melt viscosity and density in the whole process, respectively. The governing equations of two-phase flows including gas, air and polymer melt are solved using finite volume method with SIMPLEC technology. At first, two kinds of U-shaped gas channels are modeled, where the shape corner and generous corner cases are studied. At last, as a case study, the short shot, primary and secondary gas penetration processes of a door handle are simulated. The numerical results show that the mathematical model can successfully depict the race-tracking and corner effects, which are very helpful for the design of GAIM.

The Cooling Time of Gas Assisted Injection Molding

The mathematical model of solid-liquid interface is obtained by making an idealized hypothesis for parameters of the cooling process in gas-assisted injection molding, and simplifying the mathematical model. And then the cooling time of plastic products in the process is obtained by using two ways of solving the solid and liquid phases. And verified by example the conclusions that some plastic products of gas-assisted injection molding, when the materials and workmanship conditions are constant, cooling time of the airway is proportional to the square of the wall thickness is obtained.

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.

Cooling Simulation of External Gas-Assisted Injection Molding

This paper present a cooling simulation method for External Gas-assisted Injection Molding (EGAIM). To simulate the cooling process of EGAIM, a Finite Element Method model with heat transfer processing with isotropic materials containing inner heat source was established, and pressure was loaded on the interface of mold and part as gas packing pressure. The simulation was used to explore the effect of process parameters on the quality of an example with different thickness ribs with ANSYS. The results showed the method is feasible and reliable.

Self-Organizing Fuzzy Controller for Gas-Assisted Injection Molding Combination Systems

This study designed and constructed a gas-assisted injection molding system and incorporated it into a traditional injection molding machine. This combined system was referred to as the “gas-assisted injection molding combination system” (GAIMCS). The GAIMCS has complicated and uncertain dynamics, so it is impractical to design model-based controllers for this kind of system. To address this problem, this work developed a model-free self-organizing fuzzy controller (SOFC) to control the GAIMCS and evaluated its control performance. The SOFC has learning ability, which can regulate fuzzy control rules in real time during the control process. The initial fuzzy control rules for the SOFC can be set to zero before the onset of learning. The SOFC achieved better control performance than the fuzzy logic controller or the proportional-integral-derivative controller for high-pressure gas control in the GAIMCS, as confirmed by the experimental results.

Stress relaxation behavior of high density polyethylene (HDPE) articles molded by gas-assisted injection molding

The effect of gas pressure, delay time and short-shot size on the stress relaxation behaviour of high density polyethylene (HDPE) parts molded by gas-assisted injection molding (GAIM) was examined using a universal tensile testing machine. The results showed that resistance to stress relaxation of the specimens molded by GAIM is better at higher gas pressure, smaller short-shot size and an appropriate delay time. The stress relaxation behavior of the parts was found to be dependent on the crystallinity, the grain size and grain number of the crystals.

Hybrid Optimization Approach for Gas-Assisted Injection Molding Based on Metamodeling and Particle Swarm Algorithm

As a new plastic process technique, Gas-assisted injection molding has many advantages comparing to the traditional injection molding. Meanwhile, Optimization of Gas-assisted injection molding is more complex since many additional parameters have been introduced to the process. In this paper, a hybrid optimization approach based on metamodeling and particle swarm optimization algorithm is proposed and applied for Gas-assisted injection molding. Moreover, the validation of the approach will be illustrated through the optimization process of a real panel.

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.

Residual stresses in gas-assisted injection molding

Residual stresses are a major issue in the mechanical and optical behavior of injection-molded parts. In this study, we analyze their development in the case of gas-assisted injection molding (GAIM) of amorphous polymers. Flow-induced residual stresses are computed within a decoupled approach, in which elastic effects are neglected in the momentum balance, assuming a generalized Newtonian material behavior. In a staggered procedure, the computed viscous flow kinematics are used to calculate normal stresses employing a compressible version of the Rolie-Poly model. For the computation of thermally and pressure-induced residual stresses, a linear thermo-viscoelastic model is used. A 3-D finite element model for GAIM is employed, which is able to capture the kinematics of the flow front and whose capabilities to predict the thickness of the residual material layer have been validated by Haagh and Van de Vosse (Int J Numer Methods Fluids 28:1355–1369, 1998). In order to establish a clear comparison, the development of residual stresses is analyzed using standard injection molding and GAIM for a test geometry.

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

AbstractThe 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 nrr − nθθ, the variation of the average birefringence, 〈nzz − nθθ〉, 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

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.

Surface Model Based Modeling and Simulation of Filling Process in Gas-Assisted Injection Molding

Present gas-assisted injection molding simulations are all based on either a midplane model or a 3D model, in which second modeling is unavoidable for a midplane model, and a 3D simulation needs a full-scale three-dimensional discretization of parts leading to unsustainable computing time and unstable numerical analysis. In this paper, surface model based modeling and numerical simulation of gas-assisted injection molding are proposed. By taking the influence of gas penetration on melt flow as boundary conditions of the melt-filling region, a hybrid control-volume finite element/finite-difference method (CV/FEM/FDM) similar to conventional injection molding simulation is employed. The gas penetration interface within the gas channel is solved by applying the matching asymptotic expansion method, which educes an analytical model of the gas penetration thickness ratio. A technology of generating gas-channel mesh semiautomatically is put forward, which combines selecting the path of gas channel manually and calculating the parameters of gas nodes automatically. The second modeling is thus avoided. The proposed model and simulation are verified by comparing with the experiment.

Transcrystallinity in a Polycarbonate(PC)/Polyethylene(PE) Blend Prepared by Gas-Assisted Injection Molding: A New Understanding of Its Formation Mechanism

A PC/PE(20/80) blend was molded by gas-assisted injection molding and, after being etched, its crystalline morphology was investigated. Microscopy results indicate that transcrystallinity is favorably formed on the surface of the thinner PC fibrils. However, transcrystallinity cannot be observed on the PC droplets' surface. In addition, in the non-isothermal crystallization process, there was no transcrystallinity formed around the PC fibrils under the condition without shear. Combining the characteristics of gas-assisted injection molding and the PC droplet's deformation, the transcrystallization origin is proposed to be the local shear stress, which is brought by the relative motion of the two components during PC droplet's deformation and solidification.

Banded spherulites of HDPE molded by gas-assisted and conventional injection molding

Crystal morphologies of high density polyethylene (HDPE) with low molecular weight obtained by gas-assisted injection molding (GAIM), conventional injection molding (CIM), and spontaneous cooling, respectively, were studied by scanning electronic microscopy (SEM). It is found that banded spherulites are generated in the inner zone of GAIM parts and the outer zone of CIM parts but are absent in quiescent parts. According to the results, the representative morphologies of crystal change with gradual increment of instantaneous flow field in crystallization from non-banded spherulite to banded spherulite and then to oriented lamellae. This morphological evolution indicates that banded spherulites could be induced by flow field with certain intensity, which is confined by both an upper critical value and a lower one.

A study on gas-assisted injection molding filling simulation based on surface model of a contained circle channel part

A practical model of cavity filling in gas-assisted injection molding (GAIM) based on the surface mode has been presented. In this model, according to the flow field at the gas front, gas penetration thickness was educed by solving the theoretical equations governing the behavior of the gas penetration through the matching asymptotic expansion method. In addition, a technology of generating gas channel mesh semi-automatically based on maximum inscribed circle of arbitrary polygon was to proposed. This method combined selecting the path of gas channel manually and calculating the parameters of gas node automatically. Finally, an experiment was employed to verify this proposed model by comparing the results of the experiment with the simulation.

Influence factor analysis of gas fingering defect in gas-assisted injection molding of automobile handle

A frequent problem encountered in gas-assisted injection molding(GAIM) is gas fingering,i.e.gas bubbles penetrating outside designed gas channels and forming finger-shaped branches.Severe gas fingering will result in reduction of part strength, failure of gas-assisted injection molding or cancellation of the advantages of GAIM.Gas fingering defects in GAIM parts are investigated with the methods of numerical simulation and orthogonal experiment.Six process parameters that have the most important influence on gas fingering defect are studied,i.e.shot size,gas delay time,gas injection pressure,mold temperature,melt temperature and gas injection time.The results show that the most important process parameters that influence gas fingering are gas delay time,melt temperature and gas injection pressure successively.Therefore,to choose reasonable gas delay time,melt temperature and gas injection pressure is particularly important.For reducing the degree of gas ring ering,it is suggested that at the same melt temperature the gas delay time should be appropriately prolonged.

Hierarchical crystalline structure of HDPE molded by gas-assisted injection molding

To understand the crystalline morphology of the parts molded by gas-assisted injection molding (GAIM), in this work, the hierarchical structures and the crystalline morphology of gas-assisted injection molded high-density polyethylene (HDPE) were investigated. According to the comparison between the results of the GAIM part and those of the conventional injection molded counterpart, it is found that gas penetration can remarkably enhance the shear rate during GAIM process and oriented lamellar structure, shish–kebab structure and common spherulites arise in the skin, subskin and gas channel region, respectively, owing to the different shear rate in these regions. Meanwhile, cooling rate also plays an important role in the formation of the oriented crystalline structure.

The role of gas penetration on morphological formation of polycarbonate/polyethylene blend molded by gas-assisted injection molding

Polycarbonate (PC)/polyethylene (PE) blend was molded respectively by short shot (SS) and gas-assisted injection molding (GAIM). In order to investigate the origin of the “skin-core” structure during GAIM process, the morphology of the two parts molded by SS and GAIM, far from skin, was studied. The results indicate that the structure of the SS part (SSP) is similar to that molded by conventional injection molding (CIM), while the structure of the GAIM part (GAIMP) shows an unusual gradient structure. Many coarse, short PC fibrils arise in sub skin, while such fibrils become more well-defined and reduce in number towards core layer. And the PC phase at the non-gate end, experiences more severe deformation than that at the gate end, which is also different from that in CIM parts. In addition, Moldflow 5.1, a commercial simulation package, was employed to determine the flow behaviors during SS and gas penetration processes. The experimental and simulated results indicate that shear rate and cooling rate are significant for the gradient structure formation during GAIM.

Gas Assisted Injection Molding of an Industrial Part: 3D Simulation Provides Virtual Molding Trials

Abstract Techniques developed for three-dimensional simulation of gas assisted injection molding of thermoplastics are here extended to a complex industrial application, a car door handle, providing uniquely detailed insights into the molding process. Handling of the three stages of the process – plastic filling, and primary and secondary gas penetration – is described in terms of the boundary and initial conditions applied. Predictions for plastic filling patterns are compared successfully with molding results, and provide detailed forewarning of potential problems due to weld surfaces and air-traps. Particular emphasis is placed on accurate modeling of secondary gas penetration, which is also validated against practical results. A further novel feature is the export of an STL model of the gas core, providing a CAD model of the finished molding for subsequent in-service analysis.