Extrusion of a polyamide 12 (PA12) material through a capillary die coated with an ethylene butene copolymer (EBM) was studied. The EBM coated die significantly increased the flow rates of the PA12 melt compared to a clean die at the same extrusion pressure. Introducing a maleic anhydride grafted ethylene-octene copolymer (EOM-g-MAH) into the EBM suppressed the effect. This behavior seems only explained by significant interface slippage between PA12 and EBM melts, which could be eliminated by introducing covalent chemical bonds across the interface. A mathematical analysis was carried out to calculate the interface slippage. The shear stress where slippage began to occur was around 0.045 MPa and the slippage velocity was around 15 mm/s at 0.1 MPa. Adding EOM-g-MAH could largely decrease the interfacial tension between EBM and PA12, thus largely decrease the interface slippage. (C) 2009 The Society of Rheology [DOI: 10.1122/1.3198245]