Online Acoustic Wave Sensing for Paperboard Score Quality Control
Inline acoustic wave sensing detects sub-surface paperboard delamination during scoring, stopping un-creased board defects before downstream cartoning line jams.
Lamb Wave Propagation
Wave Mechanics
Acoustic energy travels through thin plates or cylindrical geometries by displacing particles in a direction parallel to the surface boundary. Lamb wave propagation occurs when these elastic oscillations interact with the top and bottom faces of a substrate, creating a bounded waveguide effect. The resulting motion consists of symmetrical or antisymmetrical modes that vary in velocity according to the frequency and material thickness.
Shear forces combined with longitudinal compression generate these movements throughout the cross section of the media.
Conversion Sensitivity
Ultrasonic transducers excite specific modes within paperboard or thin metallic films to detect discontinuities like micro-cracks or voids in the internal structure. Inspectors monitor the phase velocity of these waves to identify changes in the acoustic impedance of a stock. Variations in board density or local fiber orientation alter the signal travel time, providing a tool for quality control in high speed converting lines.
Mill operators adjust the input frequency to optimize signal penetration through opaque or coated surfaces. Sensor alignment relative to the machine direction determines the accuracy of the detection across the width of the moving web.
Production Boundary
Signal attenuation limits the usable range of these waves when the material under test exhibits high porosity or irregular surface roughness. Fibrous substrates scatter the acoustic energy because the internal voids prevent efficient modal development over long distances. High frequency noise from factory equipment interferes with the detection of subtle echoes within the structure.
Reliable measurements require a stable coupling medium to transfer energy from the probe to the sheet. Quantitative analysis remains effective only when the wavelength matches the physical dimensions of the specimen being evaluated.