The mathematical model of polymer plastication in a reciprocating screw injection molding machine is presented. It takes into account all characteristic features of working of a real injector, such as periodical action of the three zones screw, to-and-from screw motion with controlled stroke, static and dynamic melting, etc. Methods for description of time changes of the solid bed profile in the screw channel during injection cycle as a function of operating parameters were developed. Ways of determination. of pressure and temperature profiles in the screw channel were also discussed. Methods of caculation of the most important flow characteristics such as plasticizing capacity, power requirements, screw torque, energy consumption were analyzed. Results were presented in a form, which is most convenient for numerical applications in computer simulation programs.