This paper presents the development of a nonlinear mathematical model and a microcomputer controlled servo-pump system for the study of the dynamics of a servo-pump controlled injection molding machine (IMM). The model is formulated by the Reynolds transport theorem which is applied to describe the polymer flow dynamics in the nozzle. The comparison between the simulation results and experimental investigations indicated that the nonlinear dynamic model is a reasonable representation of a servo-pump controlled IMM. The output flow rate and working pressure of a servo-pump controlled IMM can be reasonably represented by the nonlinear model when the acrylonitrile-butadiene-styrene (ABS) is injected.