Numerical Simulation of the Injection Blow Molding Single Stage Process: Shaping of Two Different Geometries and Comparison with Experimental Thickness Measurements

Abstract

AbstractThanks to the single stage injection blow molding process, specific hollow parts can be produced with standard injection molding machine. A specific mold is designed, enabling the preform injection molding, as well as its blow molding on the same machine. The preform is blow molded right after its injection molding. This particular process does not involve preform storage and reheating before the blow molding stage. This implies that the preform has to remain sufficiently malleable to be blown while being viscous enough to avoid being pierced during the blow molding stage. The polymer used is a polypropylene random copolymer. Therefore the process takes place between the melting temperature and the crystallization one. As the preform is blow molded right after being injected, this single stage process involves high temperature before the blowing. The polymer rheological behavior is investigated through dynamic rheometry in its molten state at various temperatures. A viscous Cross model is used with the thermal dependence assumed by an Arrhenius law. The process is simulated through a finite element code (Polyflow) in the ANSYS Workbench framework. The geometry allows an axisymmetric approach. The transient simulation is run under anisothermal conditions and viscous heating is taken into account. Two geometries are simulated. First, the flask geometry has been used to show the process efficiency. The yoghurt pot geometry has been designed later for industrial application in a more sophisticated mold than the flask mold. Blow molded parts are analyzed by tomography and the thickness has been measured by the tomography data image analysis. The simulation results are compared to the experimental ones. The simulation fits well with the experimental results concerning both geometries. The simple numerical model proposed proves to be robust enough to predict well the blow molded parts final thickness repartitions provided that the relevant temperature and pressure conditions are well defined.