Injection molding with time modulation of mold surface temperature. Analysis and modeling of pressure and temperature evolutions

Injection molded samples were obtained by a fast evolution of cavity surface temperature system. This technique allows to keep, for assigned time intervals, the cavity surface temperature at intermediate values between injection and cooling channels temperatures. The surface temperature was changed by activating thin heating devices layered below the cavity surface; the devices were activated by the injection molding machine at the starting of screw movement and thus the cavity surface is already heated at the contact with the polymer. The surface temperature reached a plateau after very short time and the small thickness of the heating device allowed a fast cooling at the heater deactivation. Several tests were performed for different levels of the heating powers and heating times. The recorded evolutions of surface temperature and of pressure inside the cavity show that, for high heating power and long heating times, the pressure undergoes two pressure steps down: the first as consequence of the cooling from the injection temperature to the heated surface temperature and the second determined by the cooling to the mold temperature due to the heating deactivation. For small heating times the two cooling steps collapse into a single one. The effects of temperature, pressure and flow on relaxation times, nucleation density, spherulitic growth rate, as well as the interrelation among these quantities were experimentally analyzed and included into an overall injection molding simulation model for the iPP grade developed in the UNISA code. The UNISA code was modified to the purpose of accounting of the surface heating. The simulation results favorably compare with the experimental results of temperature evolutions. Also, simulation results of pressure evolutions reproduce main features shown by the experimental results.