Melting is one of the most important process in plastics processing. Admit the extensive use of plastic injection moulding machine in industry, most studies were on extruders which were simpler to model. This paper studies the melting process and derives its transient (dynamic) model for predicting the characteristics of melting in feeding stage of injection extruder. Based on this transient model, factors affecting the melting speed in injection extruders are exposed. Simulation results agree with the results from experiments.