| As a polymer molding technology based on rapid mold heating and cooling, rapid thermal cycling molding(RTCM) is capable of molding polymer parts with high gloss and no weld mark. Therefore, it has attracted more and more attention in both industry and academia. This dissertation aims at developing an electric-heating rapid thermal cycling molding(E-RTCM) technology with low cost, simple structure, good adaptability and high heating efficiency. To achieve this goal, this work deeply investigates the E-RTCM technology from three aspects: mold structure design, mold heating system optimization, and the technology application. This dissertation can lay the foundation for industrial application of the E-RTCM.According to the RTCM process principle, the heat transfer for E-RTCM mold is analyzed based on the basic theory of heat transfer. By deducing the thermal balance equations in both mold heating and cooling stages, some efficient ways to improve the mold heating and cooling efficiencies are obtained, which can provide theoretical guidance for E-RTCM mold structure design. The results show that, under the condition of adequate mold structural strength and stiffness, reducing the mold volume heated as much as possible is the most direct and effective method to improve the mold heating and cooling efficiencies.In order to achieve rapid and uniform heating of mold cavity surface, a hybrid optimized strategy for E-RTCM mold heating system design is proposed by integrating finite element method(FEM) and particle swarm optimization(PSO) algorithm, and the corresponding optimization program is also developed.The optimal design parameters of heating system can be obtained rapidly and effectively by using this strategy to design the electric heating system of a rapid thermal cycling blow molding mold for automobile spoiler. After optimization, the heating efficiency and temperature uniformity of cavity surface are improved by 14% and 91%, respectively.By applying the RTCM process into the extrusion blow molding, an electric-heating rapid thermal cycling blow molding technology is developed, and the corresponding molding system is also constructed. Thermal cycling blow molding experiments of automotive spoilers are conducted. The results show that the spoilers molded using the developed technology exhibit high gloss, which can be used directly for the final assembly process. The secondary operations that are usually employed in the conventional extrusion blow molding can be eliminated completely, thus shortening the produce process and increasing the productivity. Meanwhile, the molding cycle is also maintained at a reasonable range. In addition, the mechanisim for the improved surface quality of spoiler is disclosed. This is mainly attributed to the fact that higher mold temperature in RTCM can effectively lower the cooling rate of inflated parison after it contacts with cavity surface, so the temperature of its skin layer can be maintained higher than the polymer thermal deformation temperature for a longer time. This makes the parison melt to flow on cavity surface to some extent during inflation and thus precisely replicate glossy cavity surface. Therefore, the surface defects(e.g., sunken pits, flow marks, etc.) can be eliminated and hence part with high-gloss surface appearance can be blow molded.In order to adapt the characteristic of high efficiency for polymer injection molding, a RTCM technology with electric heating and water impingement is proposed, and a corresponding RTCM mold is developed. The mold thermal response efficiency is evaluated by using experimental measurement and numerical simulation. The results show that, in comparison to the conventional RTCM mold with electric heating and water cooling, the cavity surface heating and initial cooling rates for the developed RTCM mold are enhanced by 1.6 and 4.0 times, respectively. Considering the fact that the rapid thermal cycling injection mold needs to suffer relatively large alternative thermal stress during molding process and thus the fatigue crack can be easily induced in mold, a new mold cavity plate-fixing mode with thermal compensation gap is proposed. By three-dimensional finite element thermo-structural analysis, the thermal stress distribution in the cavity plate during mold heating process is obtained. The analyzed results indicate that the thermal stress in cavity plate can be greatly reduced by using this new mode, which is expected to significantly improve the durability of rapid thermal cycling injection mold. Moreover, by using the rapid thermal cycling injection molding process with electric heating and water impingement cooling developed in this work, the parts with high gloss and no weld marks can be injection molded directly, meanwhile the molding cycle time can not be increased greatly in comparison to the regular injection molding process. In addition, the mechanisim for the improved surface quality of injectin molded parts is disclosed.To solve the problem that the parts molded with the regular microcellular injection molding process usually exhibit poor surface appearance, a rapid thermal cycling microcellular injection molding process combining microcellular injection molding and RTCM technology is developed. Based on the constructed electric-heating rapid thermal cycling microcellular injection molding system, the effect of cavity surface temperature on surface roughness of microcellular injection molded parts is investigated quantitatively and its influence mechanism is disclosed. The results show that the surface roughness of microcellular injection molded parts is gradually reduced with increasing cavity surface temperature, and finally stabilized at a low level comparable to that of solid injection molded parts. Increasing the cavity surface temperature can delay the cooling rate of filled melt, which facilitates the gas trapped between the skin layer of filled melt and cavity surface to be redissolved into melt under high cavity pressure. Therefore, the silver streaks and swirl marks appearing on the microclleluar injection molded parts can be greatly reduced or completely eliminated, and hence significantly decreasing the surface roughness and improving the surface quality of molded parts. |
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