| Rapid heat cycle molding (RHCM) is a new developed injection molding process based on rapid heating and rapid cooling technologies. Compared with conventional injection molding (CIM), the most notable feature of RHCM is that the injection mold should be rapidly heated to a high temperature before melt filling and rapidly cooled to a low temperature after melt filling. The high cavity surface temperature during melt filling can effectively prevent the premature cooling of the polymer melt and eliminate the frozen layer completely. Therefore, RHCM process can significantly improve the fluidity of the polymer melt and hence increase its transferability of the mold cavity geometry. RHCM process can effectively solve the part surface defects, such as flow mark, jetting mark, weld mark, floating fiber, low glossy, etc., usually appearing in CIM process. In addition, RHCM can be used to mold the plastic part with super-high flow length ratio, the plastic part with micro structure. Besides, RHCM can also significantly reduce injection pressure, injection velocity, packing pressure and clamping force of the molding process. This is of great significance to reduce the dependence of the molding process on large tonnage injection molding machines. Low injection pressure and low injection velocity can reduce the inner stress of the molded plastic part, which is very helpful to reduce shape distortion, improve dimension accuracy and optical performance. Since RHCM parts has extremely high surface appearance and the secondary processing operations for CIM parts, such as polishing, painting and finishing, are not needed any more, it can significantly shorten the production process, effectively reduce production cost, energy consumption and environmental pollution. Altogether, RHCM is a type of high-quality, high-accuracy, energy-saving and also environment-friendly green injection molding process, which has broad application prospects and huge market potential. In this paper, RHCM will be given a systematic and in-depth study in the aspects of technology principle, technological process, dynamic mold temperature control system, mold design and manufacture, equipment and production lines construction, mold thermal response analysis, mold heating and cooling system optimization design, mold fatigue life analysis and optimization, process optimization and experimental research.By analyzing the process principle and dynamic mold temperature control principle of RHCM, two types of new RHCM processes are presented. One based on steam heating is the so-called rapid heat cycle molding with steam heating (S-RHCM) and the other one based on electric heating is the so-called rapid heat cycle molding with electric heating (E-RHCM). The reasonable process steps for the two RHCM processes are presented by analyzing their molding cycle compositions. The corresponding dynamic mold temperature control system for the two RHCM processes are developed and manufactured based on programmable logic controller and touch panel techniques. S-RHCM mold and E-RHCM mold for a type of large LCD TV panel are also designed and manufactured. The thermal response efficiency of the two RHCM molds is evaluated by heat transfer analysis. In order to achieve uniform heating and cooling the mold cavity surface with complex geometry, a new S-RHCM mold structure with conformal heating and cooling channels and a new E-RHCM mold structure with conformal heating elements are presented. With the developed dynamic mold temperature control systems and RHCM molds, S-RHCM production lines and E-RHCM production lines for LCD TV panels are constructed. The test production results show that the developed RHCM processes can significantly improve the surface appearance of the plastic part by eliminating weld marks and increasing surface gloss, and at the same time the molding cycle time of RHCM is very close to that of CIM. Several types and series of dynamic mold temperature control systems and RHCM molds are developed and manufactured and a large application of the developed RHCM processes are achieved.Heat transfer in the molding systems of S-RHCM and E-RHCM are investigated and the corresponding thermal balance equations are presented and deduced. Based on the developed thermal balance equations, the factors affecting the heating and cooling efficiency of the RHCM mold are analyzed and hence the design guidelines for S-RHCM mold and E-RHCM mold are proposed. The thermal response analysis models for S-RHCM mold and E-RHCM mold are constructed. Heat transfer analysis based finite element method (FEA) is performed to investigate the thermal response of the mold cavity surface. The effect of the insulation layer and mold materials on thermal response efficiency and temperature uniformity of the mold cavity surface are also investigated. According to the thermal response analysis results, the heating efficiency, cooling efficiency and energy consumption of the two RHCM processes are calculated and compared. Some useful guidelines are presented for mold optimization design and application of the two RHCM processes. A new E-RHCM mold structure with a floating cavity block or cooling plate is developed to reduce the thermal inertia of the cavity block that has to be rapidly heated and cooled. Thermal response analysis results show that the new developed E-RHCM mold structure can significantly improve the thermal response efficiency of the mold cavity surface. Based on the new E-RHCM mold structure, a new large E-RHCM mold for a type of LCD TV panel is developed. The tricks in design, manufacture and assembly of the new E-RHCM mold are also discussed and some useful guidelines are presented. The effect of different types of heating/cooling medium, heating/cooling medium temperatures, heating and cooling channels distribution, mold material and plastic part thickness on thermal cycle efficiency and temperature uniformity of S-RHCM process are also investigated by heat transfer analysis. Based on the analysis results, some guidelines for improving heating system design, cooling system design and mold design of S-RHCM process are presented. Finally, the effectiveness of heat transfer analysis is verified by comparing the analysis results with the theoretical results.The effect of the heating/cooling channels distribution on thermal response efficiency, temperature uniformity and fatigue life of the S-RHCM mold and the effect of the electric heating elements distribution on thermal response efficiency, temperature uniformity and fatigue life of the E-RHCM mold are both systematically investigated based on response-surface experimental design and thermo-structural coupling analysis. With the experimental design and analysis results, least squares regression analysis is used to fit the response surface models for the three objective variables including the required mold heating time, the maximum temperature difference of the cavity surface and the maximum von mises stress. The significance and effectiveness of the constructed response surface models are then verified by ANOVA analysis and random experiments. Three different optimization design strategies including heating efficiency priority, temperature uniformity priority and fatigue life priority are proposed for optimization design of the S-RHCM mold and E-RHCM mold. The optimization function models for the three optimization strategies are built accordingly. A multi-objective particle swarm optimization algorithm (MOPSO) is then developed to solve the optimization problems. The optimization results show that heating efficiency, temperature uniformity and fatigue life of the mold can be improved significantly. Finally, the developed optimization design method based on MOPSO is used to achieve optimization design of the cavity blocks for a large LCD TV panel S-RHCM mold and E-RHCM mold. Based on the optimization design, the heating efficiency and temperature uniformity of the mold are greatly improved, which is of great significance to increase production yield, production efficiency and service life of the S-RHCM mold and E-RHCM mold.Three-dimensional finite element heat transfer analysis and thermo-structural coupling analysis are used to investigate thermal response, temperature distribution, and thermal stress distribution of the S-RHCM mold cavity block. Thermal response experiment of the S-RHCM mold is performed to verify the effectiveness of the heat transfer analysis. The fatigue analysis based on thermo-structural coupling analysis is further performed to evaluate the fatigue life of the S-RHCM mold. The reason for the difference between the estimated fatigue life and the actual fatigue life is discussed. According to the thermal stress analysis results and the actual failure mode of the S-RHCM mold, the thermal fatigue failure mechanism is proposed. Finally, the effect of the mold cavity surface temperature, clamping force, installation of the cavity block and mold heating system on fatigue life of the S-RHCM mold and E-RHCM mold is investigate by thermo-structural coupling analysis. Some useful guidelines for the process control and mold design are presented to improve the fatigue life of the S-RHCM mold and E-RHCM mold.Simulation technologies for RHCM process are investigated and Moldflow is successfully used to simulate the new molding process. Based on the simulation results of RHCM process and CIM process, the effect of RHCM process on melt filling ability, part shape and dimension accuracy, sink mark, cooling time and birefringence is investigated. The mechanism for part large warpage of the RHCM process in which only the cavity side of the mold is rapidly heated and cooled is proposed. The effect of the packing control and mold cooling control on the part warpage of the RHCM process is investigated by simulation so as to achieve optimization design of the packing process and cooling process. The optimization results show that the warpage and sink depth of the plastic part can be greatly decreased with the optimum packing and cooling control. Based on the optimum packing and cooling control, the effect of RHCM process parameters including injection velocity, melt temperature and the mold temperature of core side and packing pressure on warpage and sink depth of the molded part is further investigated. The quadratic polynomial mathematic models are developed to predicate warpage and sink depth of the plastic part. ANOVA is performed to analyze the significance of the developed mathematic models and also the design variables. Random experiments are used to verify the effectiveness and accuracy of the developed mathematic models. The optimization function model is built with the objective to reduce warpage and sink depth of the part. The developed MOPSO is then used to solve the optimization function model and acquire the optimum process parameters. Finally, actual RHCM production with the optimum parameters on the S-RHCM production line is performed to verify the effectiveness of the optimization design.An E-RHCM production line which can produce standard tensile specimens, impact specimens and heat deflection specimens with/without weld lines is constructed for experimental research of RHCM process. The heating system of the E-RHCM mold is optimized to ensure a uniform heating of the cavity surfaces and the runner system of the E-RHCM mold is also optimized to achieve balance filling of the mold cavities. A cavity surface temperature measurement and acquisition system is developed and built based on thin-film thermocouple, data logger and computer, which is then used to investigate the thermal response of the cavity surfaces. Full factorial experimental design is used to research the effect of the molding heating time and cooling time on the maximum temperature and minimum temperature of the cavity surfaces. Based on the experimental results, regress analysis is applied to develop the mathematical relationships between the design variables of mold heating time and mold cooling time and the objective variables of the maximum temperature and the minimum temperature of the cavity surfaces. The external random experiments are also performed to confirm the accuracy of the developed mathematic relationship models. Thermal response simulation based on FEA is performed to investigate the temperature variety of the cavity surfaces during rapid heating and cooling cycles. The thermal response of the cavity surface acquired by experiments is used to verify the effectiveness of the heat transfer simulation. After verification, thermal response simulation is then used to investigate the effect of power density of the electric heating elements on mold heating efficiency and the effect of cooling water temperature on mold cooling efficiency. With the E-RHCM experimental production line, the effect of injection pressure, injection velocity and mold cavity surface temperature on melt filling ability is investigated by experimental study. The RHCM processes of the high glossy plastics, crystalline plastics, amorphous plastics, nano-particle reinforced plastics and fiber reinforced plastics are systematically and in-depth researched. The effect of the mold cavity surface temperature on surface gloss, surface roughness, weld mark and structural morphology is analyzed. Based on this, the mechanisms for high roughness of the mixed plastics and crystalline plastics with a low cavity surface temperature, the elimination of the floating particles or fibers of the reinforced plastics with a high cavity surface temperature and also the hump-shaped weld mark for the fiber reinforced plastics are proposed. Finally, the effect of the cavity surface temperature in RHCM process on tensile strength and impact strength of the plastic part with/without weld lines for different plastics are systematically studied. The mechanisms for the variety of the plastic part strength with the temperature changes are in-depth analyzed and discussed. |
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