Experimental and simulation studies of flow of a slipping polymer melt in a co-rotating twin screw extruder are presented. The experiments involve a wide range of polyolefins (polyethylene, isotactic polypropylene, isotactic polybutene-1, isotactic poly 4 methylpentene-1, and a cyclopolyolefin) with an octadecanoic acid additive. Mooney's capillary method is used to prove the occurrence and magnitude of slippage. In the twin screw extruder experiments, the die pressure is reduced for the compounds with octadecanoic acid compared to the neat polyolefins. Screw characteristic curves (Q vs. dp/dx) were determined experimentally for both the polyolefin compounds and the neat polyolefins. These were found to be rather close. We modeled flow of a slipping non-Newtonian (power law) fluid in a screw channel and calculated screw characteristic curves. These were found to be close to our experimental observations.