Optical inspection apparatus and method for an extruder

An apparatus for optical inspection of a mass of polymeric material passing through an extruder, wherein the extruder has:

• a hollow extrusion cylinder, elongated in a longitudinal direction and having an inlet for receiving pellets of polymeric material, and an outlet for expelling melted polymeric material;
• an extruder screw connected to a motor to rotate inside the extrusion cylinder and to move the polymeric material from the inlet to the outlet;
• heaters coupled to the extrusion cylinder,
• wherein the optical inspection apparatus comprises an optical sensor which is operatively coupled to the extrusion cylinder and which has an infrared light emitter and a receiver configured to measure a measurement parameter representing a property of the polymeric material inside the extrusion cylinder,
• wherein the optical inspection apparatus comprises:
• a plurality of the optical sensors, which are operatively coupled to the extrusion cylinder in a plurality of measurement sites located in succession and spaced from each other along the longitudinal direction;
• a processor programmed to acquire a plurality of measurement signals containing the measurement parameters measured by the corresponding optical sensors of the plurality of optical sensors and programmed to process the plurality of measurement signals in order to calculate a corresponding plurality of values of a control parameter indicating a physical state of the polymeric material, wherein the plurality of values represents a curve of the changes in the control parameter inside the extrusion cylinder as a function of a longitudinal position;
• for each of the plurality of optical sensors, a filter connected to the optical sensor and adjustable to selectively vary a position of the bandwidth of the receiver, wherein the processor is configured to receive information relating to the polymeric material processed by the extruder, is connected to the filters to adjust them and is programmed to set for each optical sensor, the position of the bandwidth in a zone of an infrared spectrum of the polymeric material intermediate between absorption peaks.