The present study alms at improving the state of art physical modeling in the numerical simulation for a precision injection molding to take into account the shrinkage behavior observed by experimental measurements. In Part I, new lateral motion models have been developed to predict the birefringence, residual stress profiles, and effects of various processing conditions on the shrinkage altogether. In conventional models, the lateral motion is either allowed too early or totally prohibited in the cooling stage. With the new modeling, more general description of lateral motion has been achieved either by allowing the lateral motion if an averaged lateral residual stress reaches a certain value or by letting the lateral motion develop continuously when a shearing strength is equal to an estimated shear stress.