Modeling of flow-induced crystallization of colored polypropylene in injection molding

We present a study on flow-induced crystallization of an injection-molded isotactic polypropylene (iPP) mixed with colorant additives. Two types of blue colorants were used in the study: one is the ultramarine Blue composed of Sodium Alumino Sulpho Silicate (UB) and the other is the PV Fast Blue composed of Cu-Phthalocyanine (CuPc). The CuPc colorant exhibits increased nucleation of both quiescent and flow-induced crystallization, and results in more oriented microstructures, causing a high degree of anisotropy in material properties and shrinkage of the injection molded parts. In contrast, the nucleating effect of the UB colorant is negligible under the quiescent condition, but becomes notable in the case of short-term shear treatment at high shear rates. The UB colorant does not significantly increase the degree of anisotropy as compared with the virgin iPP. A flow-induced crystallization kinetic model is used in the numerical simulation for the injection molding process, and predicted the colorant effects as observed in experiments.