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Transient heat transfer problems with phase-changes, also known as the ''Stefan problems'' or ''movingboundary problems,'' are practically significant in many engineering and technological fields. Injection molding, one of the most widely used plastics processing techniques, mainly consists of filling, packing, and cooling, and the cooling stage is crucial since it considerably affects the productivity and quality of the molded parts. Thus, solutions for transient phase-change heat conduction problems during injection molding will be instructive. In this article, the enthalpy transforming scheme proposed by Cao and Faghri, which could handle the Stefan problems for generalized multidimensional phase-change structures, is applied coupled with the control-volume/finite-difference techniques. Considering the polydispersity and hierarchical structures, the polymer extended phase change temperature range or mushy zone was included in the two-dimensional enthalpy formulation to forecast the transient phase-change heat conduction during the cooling stage for injection-molded high density polyethylene (HDPE) parts. Experiments were performed and good agreement has been achieved, which reveals that the enthalpy transforming model gives good prediction, especially for the cooling analysis for the injection molding of thick-walled parts of crystalline polymers. The understanding of the phase-change heat conduction characteristics may facilitate the optimal designs of polymer injection molding process for industrial applications.

The influence of melt and mold temperature on the solidification behavior of HDPE during the GAIM process is studied using a transient‐heat‐transfer model of the enthalpy transformation approach. An in situ measurement of temperature decay in the mold cavity was carried out to verify the simulated results experimentally, and reasonable agreement was observed. The comparison of the HDPE solidification behavior under various cooling conditions reveals that the rapid cooling rate (due to thin wall‐thickness) is the main reason for the shortening of molding cycles, and that the mold temperature shows greater influence on the controlling of cooling rates than melt temperature during GAIM process.magnified image

Gas‐assisted injection molding (GAIM) is one of the significant fabricating technologies of plastics in modern industry, mainly owing to the light weight of products, good structural rigidity and dimensional stability, as well as shorter molding cycles. The objective of this article is to explore the temperature profiles during the cooling stage of gas‐assisted injection molded high‐density polyethylene (HDPE) parts using a transient heat transfer model of the enthalpy transformation method, which could always be utilized for the numerical studies of the phase‐change heat transfer issues. The simulated results were validated by the in situ measurement of temperature decay, and good agreement has been observed. The comparison between GAIM and conventional injection molding (CIM) reveals that it is the rapid cooling rate (because of thin wall‐thickness) and the inner gas cooling effects that together lead to the shortening of molding cycles. As cooling rate plays a part in the stabilization of the crystalline structure during the GAIM process according to our previous studies, this work is of significance for the operational designs in GAIM industrial applications and further investigation on the detailed mechanisms of various crystalline structures in GAIM parts. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

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The 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

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