Closure integrity testing of heat sealed aseptic packaging using scanning acoustic microscopy
The objective of this study was to determine the possible application of ultrasonic inspection for non-destructive, online evaluation of the integrity of heat sealed, flexible package structure commonly used in packaging of aseptic and shelf-stable food products.
A scanning acoustic microscope (SAM), Olympus UH-3, and image analysis system were used to establish the operational parameters to ultrasonically inspect the heat seal closure of various flexible packages. The frequency range, attenuation, and focal length (Z-value) were determined respectively for paper laminate containers, plastic and plastic/aluminum pouches and plastic trays with plastic or plastic/aluminum lidding materials.
The SAM images of channel leakers, blisters and wrinkles were sufficiently characteristic to allow their identification. The same should be possible in an on-line, ultrasonic testing device through proper design of the transducers and scanning mechanism of the inspection system and by monitoring of the ultrasonic signal. Channel leakers of 20 μm diameter were successfully detected in all package structures with the exception of the paper laminate which scattered the ultrasonic waves. The frequency used for inspection ranged from 30 to 100 MHz and best results were obtained when focussing at the seal bottom surface. As a general rule, lower frequencies were used for inspection of relatively thick seals or laminates containing an aluminum layer. Geometry, thickness, surface characteristics, and laminate composition of the seal to be inspected were found to affect SAM's performance. The SAM was able to detect defects as small as 20 μm when working in the pulse mode, using focussed transducers of frequency ranging between 30 and 100 MHz. However, because smaller defects could not technically be manufactured at the seal interface, this value is not definite and it is believed that smaller defects could be successfully detected, especially in the higher frequency range. For optimum results, seals to be inspected should be free of embossment, flat, and should remain parallel to the surface of the transducer during inspection. Finally, biotests showed that a 20 μm channel leaker in a seal of 5 mm width was of sufficient size to cause post-process contamination in Meal Ready to Eat (MRE, plastics/AI structure) pouches.