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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Gas-assisted injection molding (GAIM) is an innovative plastic processing technology, which was developed from the conventional injection molding, and has currently found wide industrial applications. About 70% of the whole gas-assisted injection molding cycle is actually occupied by the cooling stage. The quality and production efficiency of molded parts are considerably affected by the cooling stage. Hence, it is necessary to study the solidification behaviors during the cooling stage. In this work, a simple experimental method was designed to simulate the solidification behaviors of high-density polyethylene during cooling stage of GAIM. The enthalpy transformation approach, coupled with the control-volume/finite difference techniques, was adopted to deal with the transient heat transfer problems with phase change effects. In situ measurements of the temperature decreases in the cavity were also carried out. Reasonable agreements between the experimental values and the simulated results such as cooling time, cooling rates, and temperature curves were obtained, which proved that this simple experimental method was effective.

In this study, an increase in the cooling rate of high-density polyethylene parts was carried out via a change in the fluid flow pattern to introduce gas cooling under a gas-assisted injection-molding process; this was conducive to the retention of orientation chains shaped during the injection stage and further developed into much more oriented crystals. Morphological observation showed that the parts without gas cooling (WOGC) were composed of oriented crystals except the gas channel zone, whereas the parts with gas cooling (WGC) were full of oriented crystals, especially much more interlocking shish-kebab structures in the subskin zone. The WGC parts had a higher degree of orientation than the corresponding zone of the WOGC parts. Although the lower crystallinity, the wider orientation regions, and much more interlocking shish-kebab structures led to considerable increases from 32 and 990 MPa in the WOGC parts to 36 and 1150 MPa in the WGC parts for the yield strength and elastic modulus, respectively. V C 2014Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40349.

Recently, a melt penetration process in which one kind of polymer melt severely penetrated by a second polymer melt is achieved in our home-made multimelt multi-injection molding instrument. The morphologies of polystyrene (PS)/polyethylene blends are observed to study the characteristics of melt fl ow during melt penetration. For comparison, the morphologies of dispersed phase in injection molded samples with only the "fi rst" melt injection process (FIM) and only the "second" melt (SIM) are also examined. In the penetrated layer, the dispersed phases PS deforms much more in comparison to those in the molding parts by FIM and SIM processes, which can be ascribed to a larger deformation induced by the second fl ow via melt penetration. In the penetrating layer, the deformity degree of PS is less than those in the molding parts during SIM processes, owing to the resistance of the "fi rst" melt.

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