Control of coating properties of LDPE through melt strength measurements

Control of melt strength is proposed to indirectly regulate the neck-in in a twin screw extrusion process producing pellets of low density polyethylen e. The melt strength is measured by passing the polymer strand leaving the extruder, through a system of pulleys connected to a balance. Two types of controllers were used, a conventional proportional integral (PI) and an mod el predictive controller (MPC). Both the simulation and experimental result s indicated that the MPC outperformed the PI controller. The reason is that the model relating the manipulated variable (screw speed) to the controlle d variable (melt strength) is of high order. MPC is especially suitable for controlling these types of systems. (C) 2001 Elsevier Science Ltd. All rig hts reserved.