Pressure drops were measured as a function of time during extrusion of a commercial unplasticized PVC formulation through a specially constructed slit die. For a die with a wire-eroded surface, the pressure drop per unit length is found to be higher upstream than downstream by about 21% at the start of extrusion, falling to 16% after 130 h, owing to a more pronounced reduction in the pressure gradient upstream. Die surface average roughness, measured mechanically, dropped from an initial value of 2.7 mum to 2.2 mum, independent of position in the die. The lower pressure gradient and the lower sensitivity of pressure drop to decreased surface roughness at the downstream position are believed to indicate a thickening of the slip layer in the flow direction. For a die with a sand-blasted surface, roughness fell from 1.05 to 0.95 mum over 130 h, appearing to approach a lower bound of 0.5 mum. Atomic force microscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy was carried out on the surface of inserts removed from the die surface after use. In scans 50, 20 and 10 mum square, the AFM showed roughness on the small scale increasing after use. Raised areas of the surface are smoothed, but covered with parallel striations of approximately 0.5 mum, which, it is believed, are caused by titanium dioxide particles. These raised areas are coated with the lead stabilizer and lubricant. Lower areas remain rough, and trap mineral particles. Very little chlorine is found, indicating that PVC does not adhere to the surface. It is concluded that polishing to below 0.5 mum is unlikely to be worthwhile.