Non-contact detection of heat seal defect using the coincidence effect of airborne ultrasound

Heat sealing is a packaging technique by means of welding of seal materials with heating and compression, and is widely used for hermetic seal of foods and medical supplies. However, a defect of the heat sealing, for example a “pinhole”, caused by small wrinkle on the sealing film or jamming by contaminants occasionally arises. In order to enhance the reliability of heat seal products, this study proposes a nondestructive technique to detect such a small defect in the heat seal by utilizing coincidence effect of airborne ultrasound. In the proposed method, a bending wave with sufficient amplitude is excited by the coincidence effect, and this bending wave appears as a traveling wave on the heat seal. This traveling wave is sensitive to the defect in heat seal and shows apparent phase lags compared with defectless heat seal. Therefore, defect in the heat seal is detectable by observation of the phase lags in the traveling wave. Arrangement of ultrasound transmitter and receiver is important for the proposed technique in order to observe the phase lags efficiently. Traveling wave plays an important role to detect the defect in heat seal, so that the airborne ultrasound inspection without the coincidence effect is unavailable because the traveling wave is not generated. Validity of the proposed technique is demonstrated through experiments and numerical simulations.