Nondestructive evaluation of seal strength of heat sealing using instantaneous frequency of ultrasonic pulse

Abstract

Heat sealing is a packaging technique for hermetic sealing of foods and medical supplies etc. Evaluation of seal strength is important to guarantee both airtightness and easiness to open which are conflicting requirements. There are few studies of nondestructive evaluation although conventional spot check with destructive evaluation costs a great deal. This study proposes a new nondestructive evaluation method for the seal strength with ultrasonic pulse. The proposed method focuses on instantaneous frequency of the ultrasonic pulse which is time varying frequency in waveform. This paper demonstrates that the instantaneous frequency is more effective to evaluate the seal strength than conventional indexes such as amplitude and arrival time of the pulse through experiments. The transmittance and reflectance on the welded boundary surface are supposed to vary depending on the seal strength because the welded condition does not affect only the seal strength but the characteristics on the welded boundary surface. We also analytically discuss the mechanism how the instantaneous frequency senses the change of the transmittance and reflectance on the welded boundary surface. Simulated instantaneous frequencies also vary according to the change of the transmittance and reflectance. This result indicates that our proposed method effectively senses the variation of the welded surface characteristics pertaining to heat strength.