Numerous problems may appear during the extrusion process in the production of film, sheet, pipe, profiles and other products. In single-screw extruders surging (output and pressure fluctuations), screw and barrel wear,unmelts in the extrudate, transverse thickness variations in extrusion of film or sheet, flow lines in machine direction, flow imbalance in profile extrusion and flow instabilities (sharkskin/Melt fracture) are common.Computer simulation is an excellent tool for pinpointing where such problems may originate and for deciding on what remedial action to take. It is imperative that before any simulations are undertaken the resin properties be accurately measured. Of course, viscosity as a function of shear rate, is the most important property, but other properties may also have significant influence in the thermo-mechanical processes taking place. If a suitable software package is available for simulation of solids transport, melting and melt pumping in a single screw extruder problems relating to surging and unmelts can easily be identified. Similarly software for simulating flow through the die is necessary for fault diagnosis in this part of an extrusion line.A significantly lower actual output from what the simulations predict, would suggest the possibility of serious screw and barrel wear, which would necessitate visual screw inspection to pinpoint screw damage and barrel boroscoping for detecting possible misalignment.Also, when the solids transport rate of the screw is higher than the melting rate, one side of the screw may get plugged, pushing a section of the flights against the barrel.Theoretical estimates indicate that several tons of force can be developed resulting in damage to flight edges and the barrel.The metal burrs coming off the screw edges may cause scars on the screw root and drop in output rate. In the absence of burrs the screw root scarring will be due to abrasive material in the feed.Computer simulation should always be used in combination with material characterization and careful inspection of the equipment and monitoring of the extrusion process.