Continuous ultrasonic inspection of extruded wood-plastic composites

Nondestructive evaluation (NDE) techniques are needed for in-line monitoring of wood-plastic composite (WPC) quality during manufacturing for process control. Through-transmission ultrasonic inspection is useful in characterizing stiffness and detecting cracks and voids in a range of materials; however, little is documented about ultrasound propagation in WPC materials. The objectives of this research were to 1) determine applicable ultrasonic transducer frequencies, coupling methods, configurations, and placements for wave speed monitoring and web defect detection within an extrusion process; 2) quantify the effects of temperature on ultrasonic parameters; and 3) develop a prototype ultrasonic inspection system for a full-size extrusion line. An angled beam, water-coupled ultrasonic inspection system using a pair of 50-kHz narrowband transducers was adequate for monitoring wave speed parallel to the extrusion direction. For locating internal web defects, water-coupled 500-kHz broadband ultrasonic transducers were used in a through-thickness transmission setup. Temperature compensation factors were developed to adjust ultrasonic wave speed measurements. The prototype inspection system was demonstrated in a 55-mm conical twin-screw extrusion line.