Coextrusion and calendering are two fabrication techniques for polymeric materials in which flow through converging sections is dominated by elongational flow. Several mathematical models have been reported in the technical literature for the extrusion and calendering processes from which the velocity, temperature and pressure distributions can be predicted. In this thesis mathematical models were developed for both coextrusion and calendering processes, with special attention paid to the rheological properties of the materials in the converging section. For the calendering process, particular attention is paid to the relationship between the elongational rheological properties (newtonian, strain thinning, and strain hardening) and stress and strain in the converging section. An other theoretical model based on a power law fluid was used for the three- layered coextrusion system to analyze the velocity profile, stream function, and the draft interface shape between the layers in a converging section. The assumption of the velocity with or without rotation in the è direction was also considered.