Over the past few years a technique for relating the quality and properties of sub-components to those of real injection moldings has been developed. A major aspect of this is the correlation of microstructure in the two situations. This paper describes the computer prediction of structure development in injection molded polypropylene. This is done on a personal computer using a finite difference method and it is shown that accurate predictions can be made in relatively low computing time. The characteristic skin in polypropylene moldings is shown to be controlled by the filling phase and is dependent on the shear stress and temperature. The computer model is able to predict the onset of skin formation and hence the skin thickness. These predictions have been shown to agree well with experimental observations. The skin-core boundary which has been found to have a major effect on the mechanical properties of molded polypropylene has also been shown to lie between the no-flow isothermal and the maximum shear rate locus. This has important implications for computer simulations of the injection molding process.