Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Patent
1992-10-05
1994-11-01
Noland, Tom
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
73592, 73 52, G01M 300
Patent
active
053598954
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to a process for the ultrasonic testing of heat-sealed joints between plastic packages (especially trays for ready-to-eat meals) and cover films. The invention also pertains to equipment for performing this process.
2. Description of the Related Art
Dishlike plastic packages in various shapes for packaging foods are well known. After they have been filled, they are sealed with a cover film, which is not removed until immediately before the food is to be eaten. The stability of foods packaged in this way depends, among other factors, on the tightness of the heat-sealed joint between the cover film and the dishlike package. When the film is applied to the edge of the package, it frequently happens that wrinkles develop, plastic particles become heat-sealed in the joint bulges form, or food particles are trapped between the cover film and the package. If no bond forms between the plastic package and the cover film in such places, the package is not sealed, and then oxygen and contaminants can reach the food through these defects. Furthermore, the food may be able to run out etc. Therefore, to improve the probability that the packaged food will remain safe in accordance with applicable food laws until the expiration date that is printed on the package, it is necessary to make sure that the heat-sealed joint is completely closed all around the package, i.e., that there are no defects in the heat-sealed joint.
Accordingly, the goal of the invention was to develop a process and equipment for testing heat-sealed joints of the type described above. A further goal of the invention was to be able to perform this process with both a high degree of accuracy and reproducibility and with as much speed as possible.
SUMMARY OF THE INVENTION
Surprisingly, it was found that by focusing a beam of ultrasonic radiation of relatively small cross section exclusively on the heat-sealed joint, very reliable information can be obtained about the quality of the joint. Since the ultrasonic radiation passes exclusively through the heat-sealed joint, boundary effects of the regions on either side of the heat-sealed joint, where air bubbles, pieces of food or other undesirable elements may be present between the cover film and package, do not enter into the measuring result. Due to the overlapping of the areas along the heat-sealed joint through which the ultrasonic radiation passes, continuous testing is achieved. The overlapping can be relatively broad, e.g., more than 50%, so that one and the same point of a heat-sealed joint is tested several times. Perpendicular transmission of the ultrasonic radiation, which is ensured by suitable measures in the course of the process and in the operation of the equipment, makes it possible to clearly distinguish signals caused by defects in the joint from signals caused by normal fluctuations in the area of the test system, e.g., in the thickness of the edge of the tray etc. In this connection, it should be noted that defects produce abrupt changes in the ultrasonic signal, even though the changes may be quite small, whereas geometric deviations cause no spontaneous change in the signal amplitude. Fluctuations in the amplitude of the transmitted ultrasonic pulse can be averaged out by sufficiently broad overlapping of the areas covered by the pulse. Finally, the plastic package and ultrasonic test device must move relative to each other in such a way that the entire heat-sealed joint, which forms a closed curve, is scanned once with complete certainty.
The process of the invention and the equipment used to perform it allow relatively fast testing of a heat-sealed joint. For example, the testing of a heat-sealed joint between the tray and cover film of a ready-to-eat meal can be performed in less than a second. This has the advantage that the equipment of the invention can be integrated in an existing packaging plant without having to reduce the plant's cycle time. In a further modification with respect to the process,
REFERENCES:
patent: 3384733 (1968-05-01), Burbank et al.
patent: 4295375 (1981-10-01), Ganglbauer
patent: 4697452 (1987-10-01), Prakken
patent: 4821573 (1989-04-01), Nagata et al.
IBM Technical Disclosure Bulletin, vol. 32, (1), "Inspection of Adhesion in Magneto-Optic Thin Film Structure". Jun. 1989, Armonk, N.Y., US, pp. 258-259.
Althaus P. Gerhard
Isenberg Heinz H.
Vogt Goran
Kwok Helen C.
MPV MEB-und Pruftechnik Vogt GmbH
Noland Tom
Schmalbach-Lubeca AG
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