Continuous in-line melt flow rate control system

A method for continuously controlling the molecular weight of a polymer exiting a polymer mixing and extruding system by altering, in a forward control manner, the mixture of degradent and polymer fed to the extruder relative to a target melt flow rate related to the molecular weight of polymer desired, said method comprising the steps of:

• monitoring the viscosity of a slip stream of melt from the extruder upstream of an extruder die head with a rheometer having a positive pressure displacement pump;
• sensing a difference in the rheometer pump speed from the pump speed required to maintain a desired rate of flow of polymer at constant temperature and pressure through an orifice;
• altering the amount of degradent added to the polymer supplied to the mixing and extruding system in response to the sensed difference in speed of the pump;
• subsequently comparing the pump speed to the pump speed value required to maintain the desired rate of flow of the polymer at a constant temperature and pressure through an orifice after the altered ratio mixture of polymer and degradent has worked its way through the mixing and extruding system;
• and, if a difference in pump speed is sensed, repeating the above until the rate of flow of polymer at constant temperature and pressure through an orifice is substantially the desired rate of flow of polymer at constant temperature and pressure through an orifice related to the desired molecular weight of the polymer.

An apparatus for controlling the molecular weight of the polymer being extruded through a die head at the outlet of an extruder by altering, in a forward control manner, the mixture of degradent and polymer fed to the extruder relative to a target melt flow rate related to the molecular weight of polymer desired, said apparatus comprising:

• means for monitoring the viscosity of a slip stream of melt from the extruder upstream of the die head;
• means for converting the monitored viscosity value to a process signal;
• means for comparing the process signal with a set point value related to a target melt flow rate;
• means for generating a correction signal when there is a difference between the process signal and the set point value;
• means for supplying said correction signal to a degradent additive feed control system for adjusting the amount of degradent added to the polymer fed to the extruder in response to the deviation of the process signal from the set point value;
• means for comparing the actual process signal with an approximated process model signal which includes at least one model for the process transition and a model for dead time;
• means for comparing the process signal with the set point value after the process transition and dead time periods have elapsed and a steady state condition has been reached, and if there is still a difference between the process signal and the set point value, said apparatus being operable to generate a further correction signal thereby to maintain the melt flow rate as close as possible to the target melt flow related to the desired polymer molecular weight.