Non-destructive packaging seal strength analysis and leak detection using ultrasonic imaging

This study investigated the effectiveness of ultrasonic imaging for non-destructive assessment of seal strength in 335 ml polymeric trays. In the first part of this study, the experimental procedures involved filling sample trays with beef enchilada, then sealing at temperatures of 170-260degreesC for 5 sat 3 bar. Ultrasonic measurements were conducted using an immersion technique in pulse/echo mode. The captured signal amplitudes of the reflections from the samples were digitized and analysed to construct high-resolution C-scan ultrasonic images of the seals. The acquired data were statistically analysed for each sample and used as a basis for comparison with the seal (peel) strength, measured as the energy at break. The energy at break was obtained from destructive peel strength analysis, performed using a tensile testing frame. In the second part of this study, ultrasonic imaging was used to predict the performance of the heat sealer used to fabricate the test trays by comparing the seal quality of each of the four quadrants of the samples trays. This study also investigated the minimum channel leak size that could be detected using this ultrasound system. The results of the peel strength analyses showed that a direct correlation exists between seal strength and sealing temperature (170-227degreesC). Results of the ultrasonic imaging showed that the statistical scatter in the signal amplitude decreased as the sealing temperature increased. This reduction in the scatter of the signal amplitudes indicated a trend of increasing uniformity in the fusion of the two seal faces. Between 170degreesC and 193degreesC, sharp changes in the scatter were observed, however, little change was noted for sealing temperatures greater than 193degreesC, indicating that 204degreesC could be a minimum recommended sealing temperature. To confirm this result, a drop test analysis was performed on trays filled with beef enchilada and sealed at temperatures of 170-260degreesC for 5 s at 3 bar. Results of this drop test confirmed that 204degreesC was the minimum critical sealing temperature. Based on these results, ultrasonic imaging appeared to offer a promising non-destructive method for the prediction of seal quality in polymeric trays, sealing equipment monitoring and identification of channel leaks greater than or equal to 20 mum in diameter, and is comparable with the more established destructive peel strength analytical method. Copyright (C) 2003 John Wiley Sons, Ltd.