| Rapid heat cycle molding (RHCM) is a new plastic processing technology raised in recent years. In RHCM process, mold cavity is rapidly heated to a high temperature before injection, usually higher than the glass transition temperature of the polymer. Since the elevated mold temperature can eliminate the unwanted premature melt freezing during filling stage, melt flow resistance is greatly reduced and the filling ability of the polymer melt is also significantly improved. This heated mold cavity needs to be rapidly cooled after filling and packing to maintain a short cycle time. RHCM new technology can produce plastic parts with glossary surface and without weld lines. The re-processing operations including spraying and coating in conventional injection molding (CIM) process are eliminated in RHCM technology, which makes truly green injection molding process to be available.RHCM technology is a very complex physical process, involving polymer physics, rheology, heat transfer, fluid mechanics and other related disciplines, its forming mechanism needs further exploration. Traditional theoretical analysis and pure experimental study are not enough to solve this complex physical problem. With the development of calculation method and computer technology, numerical analysis method becomes a powerful tool to solve complex multidisciplinary physical and engineering problems. However, traditional injection molding simulation method meets many difficulties and probles when analylizes the RHCM process. Thermal analysis results by business software can not be applied to melt filling simulation in RHCM process. Conventional injection molding simulation methods ignore the effects of temperature varations in RHCM mold on the temperature and filling state of polymer melt in filling and packing processes. And the analysis results will lead to distortions of melt temperature and flow fields. Traditional cooling methods usually use the cycle averaged temperature instead of transient mold temperature, and varations of mold temperatue during different molding cycles are neglected. As a result, it is diffcult to reflect the effects of transient mold temperature on RHCM rapid cooling process. Mold and melt are isolated in conventional injection molding simulation methods. Mold temperature varations in multi-cycle RHCM process can not be predicted. Therefore, it is impossible for conventional injection molding simulation methods to achieve transient analysis in multi-cycle RHCM process. Developing numerical methods considering the transiently changed mold temperature in RHCM process has great significance in theory and practice. It is helpful to reveal the mechanism of RHCM technique. Numerical analysis can predict irrational mold structures and forming parameters prior to actual productions, and provide valuable guidance for actual RHCM productions.This paper focus on numerical methods and its key techniques for RHCM processes. A simulation method coupled with mold heat transfer was proposed accorded to the changeable mold temperature in RHCM process. The mathematical models for rapid heating, melt filling, packing and rapid cooling stages in RHCM process were established. And the corresponding numerical analysis programs were developed. Principles for the distributions and variations of melt velocity, melt pressure, melt temperature, mold temperature and other important characteristics were gained. A rapid heat cycle molding transient simulation system was developed by integrating the various stages RHCM mathematical model and numerical analysis procedures. Three-dimensional transient simulation for multi-cycle RHCM process was achieved. An RHCM experimental system with electric heating and water cooling was established. Multi-cycle electric heated RHCM forming processes were numerically and experimentally studied. And the princples of mold temperature distributions during different RHCM cycles were gained. Effects of electric heating time before injection on melt filling ability, filling state, weld line micro structure and its mechnical properties were invesgated. Numerical results were consistent with the experimental results in RHCM process with electric heating. And the accuracy and reliability of the simulation system developed in this paper were verified.The open source C++liberies OpenFOAM on Linux system and finite volume method were used in current paper to discrete the governing equations and solve the subsequent algebric equations. Basic theories of computational fluid dynamics and finite volume method were described. Discretization processes for the transient, convection, diffusion and source terms in generic transport equation were derived in detail. And solution methods for the algebraic equations after discretization were discussed. Discretization and solution methods for general governing equation lay a theoretical foundation for the development of numerical simulation program for the rapid heating, melt filling, packing and rapid cooling stages in RHCM molding process.Rapid heating process plays an important role in RHCM technology. Mold temperature field after heating is the initial condition for melt filling simulation. Developing rapid heating simulation procedure will supply a program platform for mold temperature analysis in filling, packing and cooling stages of RHCM process. The transient thermal response model for RHCM mold was established and its numerical procedure was developed. Natural convection heat transfer on mold-air boundary, fixed heat flux on mold-electric rod boundary and forced convection heat transfer on mold-steam boundary were established according to the characteristics of electric heating and steam heating processes. Mold temperature distributions and its variations in electric heating and steam heating processed were gained by transient thermal response analysis. Numerical results agreed well with the thermal analysis results by ANSYS.Traditional middle-plane and dual domain techniques, in which the velocity component in the gapwise direction is neglected, can not analyze melt flow process in thick-walled work piece or plastic part with variable thickness. In order to solve this problem, the velocity component in the gapwise direction was considered in the current paper. Mathematical model and its numerical procedure for three-dimensional, non-isothermal, impressible melt filling flow process were presented. The pressure implicit with splitting of operators (PISO) method was adopted to solve the coupled velocity and pressure field. The volume of fluid (VOF) method was used to capture melt flow front. For the purpose of avoiding the migration of volume fraction, an artificial compression term was introduced in VOF transport equation. The imposing of slip or no-slip boundary condition at the mold boundaries will result in unrealistic interface predictions. A dynamic boundary condition was presented in the current research to solve the aforementioned problem. No-slip (for melt) and traction free (for air) boundary conditions were switched dynamically according to the filling status of the boundary cells. Temperature, velocity and pressure distributions in melt filling process were numerically researched. Melt flow characteristics with different mold temperatures were compared, and the influence of mold temperature on melt flow process was accessed. Numerical results show good agreement with literature and analytical results by commercial software Moldflow, which validates the feasibility and correctness of the developed program.Conventional filling simulation methods usually apply a constant temperature boundary condition on mold cavity. Ignoring the effects of variable mold temperature on melt flow will lead to distorted melt temperature and flow field predictions in RHCM filling process. An idea coupled with mold heat transfer in melt flow simulation was proposed in this paper. Mathematical model and its numerical procedure were developed in RHCM filling process. Mass and momentum conservation equations were solved in cavity region to gain melt velocity and pressure distributions, while the heat transfer equation for mold and energy conservation equation for melt were solved in a coupled manner on the mold and cavity domains, and temperature distributions for both mold and melt were calculated. Boundary conditions on the coupled mold-melt surface were established. Two-dimensional electric heated and three-dimensional steam heated RHCM filling processes were numerically investigated by implying analytical results in rapid thermal response as initial mold temperature conditions. Melt flow fronts, flow field distributions and temperature field distributions for mold and melt in RHCM filling process were revealed. Differences between analytical results by conventional filling simulation with constant boundary conditions and coupled filling simulation were compared. The results show that the coupled method considering mold temperature variations is more suitable for melt filling simulation in RHCM process.Compressibility of melt is great and can not be ignored in packing process. Mathematical model and its numerical procedure were developed for RHCM packing process coupled with mold heat transfer. Single-domain Spencer-Gilmore state equation was used to describe the compressibility of polymer melt in RHCM packing stage. The coupled velocity and pressure in compressible melt flow equation were solved using PISO method. Flow field and density field of polymer melt were gained. The VOF method with artificial compression term was used to capture melt flow front. Temperature distributions for both mold and melt were solved in a coupled manner. Two-dimensional electric heated and three-dimensional steam heated RHCM packing processes were numerically investigated by implying analytical results in filling simulation as initial conditions. Temperature, density distributions for polymer melt and temperature distributions for mold in RHCM packing process were revealed.It is difficult for conventional simulation methods to simulate the transient cooling stage in RHCM process by using cycle-averaged mold temperature. Heat transfer models between mold and melt, mold coolant, mold and air were presented in this paper. Mathematical model and its numerical procedure were developed for RHCM cooling process coupled with mold heat transfer. Mass, momentum, energy conservation equations for polymer melt and heat transfer equation for mold were established. The body force (gravity) and latent heat during melt solidification were considered. In order to investigate melt solidification and shrinkage phenomena in rapid cooling process, relations between solidified polymer and polymer melt were established based on Darcy’s law. Two-dimensional electric heated and three-dimensional steam heated RHCM rapid cooling processes were numerically investigated by implying analytical results in packing simulation as initial conditions. Solidification, shrinkage, temperature and density distributions for polymer melt and temperature distributions for mold in RHCM rapid cooling process were revealed.Conventional injection molding simulation methods, in which the changes of mold temperature are ignored, can not be used to simulate multi-cycle RHCM process. In order to solve this problem, a rapid heat cycle molding simulation system was developed by integrating the various stages RHCM mathematical model and numerical analysis procedures presented in this paper. Numerical results in mold thermal response analysis were implied as initial boundary conditions for melt filling and packing simulations. And the distributions of melt flow filed, temperature and mold temperature after filling and packing were used as intal conditions in cooling stage. The numerical results for former RHCM cycle were used as initial boundary conditions in simulation of next RHCM cycle. As a result, three-dimensional transient simulation for multi-cycle RHCM process was achieved.Temperature control system and electric heating mold developed by our lab were linked with plastic machine to construct RHCM injection system. Multi-cycle electric heated RHCM processes were investigated using the developed transient RHCM simulation system combined with experimental study. Mold temperature distributions during different heating and cooling cycles were founded. Effects of electric heating time before injection on melt flow ability and its filling state were revealed. Influences of electric heating time on morphology and mechanical properties of weld line in double gated plastic parts were experimentally investigated. Numerical results by the developed transient RHCM simulation system were consistent with the experimental results in electric heated RHCM process. Therefore, the accuracy and reliability of the simulation system developed in this paper were verified. |
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