Simulation of crystallization evolution of polyoxymethylene during microinjection molding cycle

Abstract

A mathematical model coupled with a numerical investigation of the evolving material properties due to thermal and flow effects and in particular the evolution of the crystallinity during the full microinjection molding cycle of poly (oxymethylene) POM is presented using a multi‐scale approach. A parametric analysis is performed, including all the steps of the process using an asymmetrical stepped contracting part. The velocity and temperature fields are discussed. A parabolic distribution of the velocity across the part thickness, and a temperature rise in the thin zone toward the wall have been obtained. It is attributed to the viscous energy dissipation during the filling phase, but also to the involved characteristic times for the thermal behavior of the material. Depending on the molding conditions and the locations within the micro‐part, different evolution of crystallization rates are obtained leading to at least three to five morphological layers, obtained in the same part configuration of a previously work, allowing a clear understanding of the process‐material interaction.

Experimental investigation and numerical simulation of the microinjection molding process through an expanding flow configuration

“…However, a free slip condition between the air and the mold walls allowed the air to escape the mold cavity by the outlet boundary ξ out , and this is instigated by the Robin boundary condition, as detailed before by Anass et al 8 For pressure, no viscous stress is expected at the outlet interface, ξ out : …”

“…(b) Sensitive and very rapid heat transfer on the melt/mold interface because of the wide surface‐to‐volume ratio. (c) Numerical simulation problems due to meshing, instability, and low convergence, and finally (d) complex simulation of multi‐physics 8 . Furthermore, standard numerical schemes and commercially available computational packages are not yet adequate to satisfactorily describe the complex conditions experienced by the melt during the microinjection molding.…”

An investigation of crystallization kinetics of polyoxymethylene in processing conditions

“…On the contrary, the crystallization behavior of immiscible blends of POM and high density polyethylene was not altered by the shearing. Two studies 19,20 were recently conducted. In both cases, the numerical simulations of the microinjection molding process are carried out by considering that the POM crystallization kinetics is described by adopting the Kolmogoroff approach.…”

“…The model correctly describes the phenomenon of crystallization in a wide range of cooling rates that also covers the cooling rate characteristics of the polymer processing. In the literature, 19,20,22 the models for POM crystallization that use the Kolmogoroff approach were defined considering isothermal or quasiisothermal conditions (assuming cooling rates up to a few K/s). Therefore, they adopted only homogeneous nucleation and a growth mechanism.…”

“…However, in Reference 20 parameters of the POM Kolmogoroff model are taken according to Krevelen and Nijenhuis 21 which adopt standard calorimetric tests. In Reference 19 parameters of nucleation and growth of crystalline entities are taken from a different paper, 22 and also in this paper, parameters are determined by adopting standard calorimetric test at low cooling rates.…”