Dielectric studies of PVDF crystallization. Application to in-situ monitoring in injection molding

In recent years, experimental techniques based on microdielectrometry have presented an attractive and increasing interest for continuous monitoring in a nondestructive way of the advancement of thermoset resin reactions under cure. A new application of this technique has been carried out also for in situ monitoring of thermoplastic crystallization. Therefore, a dynamic rheometer equipped with a dielectric cell, as well as an instrumented slit die, were designed. The crystallization process is depicted by a conductivity drop as well as by the occurrence of a maximum in the real permittivity. The decrease of the specific conductivity results from a modification of the conductive paths whereas the increase in permittivity is due to an interfacial polarization phenomenon between the amorphous phase and the growing crystalline entities. As a second step of development, microdielectrometry has been applied for in situ monitoring of the crystallization of thermoplastics during an injection molding process. The electric sensors were located at the walls of the mold cavity so that an analysis of bulk dielectric properties during the filling, post-filling and the cooling phases could be performed. The dielectric method detailed herein provides a new non-invasive technique and could be applied to a closed-loop control of the injection molding process.