Solidification behavior of high‐density polyethylene during injection molding process: Enthalpy transformation method

Abstract

It was widely established that many end-use properties of the final parts are considerably affected by the macromolecular hierarchical structures formed during the cooling stage of injection molding, especially for crystalline polymers which undergo both solidification and crystallization processes simultaneously. Enthalpy transformation method (ETM) has been demonstrated to give the reasonable descriptions of the temperature profiles for real processing operations (e.g., injection molding, gas-assisted injection molding, and compression molding, etc.) in our previous work. However, to observe the phase-change plateau, where rapid cooling rate is imposed, is not easy in traditional treatments using the enthalpy approach. In this work, ''modified cooling curves'' (i.e., temperature versus time, plotted in logarithmic scale) at various locations in the mold cavity were found to show similar trend with an obvious turning point indicating the occurrence of phase transition. Mold temperature is more effective in controlling the cooling rate than melt temperature. Turing point of the cooling curves in the plot of ln y vs. ln t can be used to estimate the minimum cooling time of the injection molding of crystalline polymers.

A Novel Kinetic Analysis of Crystallization of Polypropylene (PP) in Dynamically-vulcanized PP/Ethylene-propylene-diene Rubber (EPDM) Blends Using an In-situ Measurement Technique

“…As the cooling rate increased, the crystallization took place much faster and hence the time of phasechange plateau lasted got much shorter. 19 Therefore, it can readily be seen that not all the curves displayed three stages. Because of the high thermal conductivity of metallic container and the relatively greater temperature difference between the polymer melt and cooling medium, the latent heat released from the phase-change process can be easily transferred away to the cooling medium.…”

“…In our previous work, we developed a set of apparatus for real-time measurement during the cooling process for studying the phasechange behavior of polymer materials. [16][17][18][19] After several repeated tests, the apparatus showed good stability and reliability. Based upon those results, we here proposed a novel method to study the crystallization kinetics of PP in PP/EPDM blends and attempted to further explore the non-isothermal crystallization process of PP in the blends by using the solidification kinetic analysis coupled with the press-volume-temperature (P-V-T) relationship, as shown in Figure 1.…”

“…The existence of the phase-change plateau was thus caused by the heat liberated from crystallization being equal to the heat transferred through the metallic container during the cooling process. Based on the previous work, [15][16][17][18][19] the crystallization rate was in proportion to the cooling rate. As the cooling rate increased, the crystallization took place much faster and hence the time of phasechange plateau lasted got much shorter.…”

Facile way of dynamically tailoring microporous structures in polyvinylidene fluoride films prepared by thermally induced phase separation

“…From the overall morphologies of the films (Figure 4(a 0 -d 0 )), the microstructure of the films exhibited complex hierarchical structure. In order to further interpret the relationship between microstructures in the films and temperature fields of the polymer solution, enthalpy transformation method (ETM) [49][50][51][52][53][54] was used here to acquire the three-dimensional temperature fields during the film formation process at different TIPS quenching temperatures (cf. Figure 5).…”

The Use of an In-Situ Measurement to Probe the Solidification Kinetics of Injection-Molded HDPE/PP Blends

“…Thus, enormous solution methods (e.g., variational, nodal integral, boundary immobilization, enthalpy, etc.) were developed, among which the enthalpy transformation method (ETM) has recently proved an efficient method to treat the phase-change heat transfer issues of crystalline polymers [3][4][5][6].…”