Abstract:
A process and an apparatus for the ultrasonic testing of welds between plastic packaging like food trays and cover foils. The welded joint between the tray and cover foil is periodically exposed to pulses of ultrasonic radiation. The pulses pass through the weld and are picked up or received by a receiver. The amplitude of the received pulses is evaluated. The cross-section of the beam of ultrasonic radiation is so small that the radiation only passes through the weld itself. The packages and the ultrasonic tester are moved relative to each other in order to examine the whole circuit of the weld. The pulse rate is such that the weld areas covered by successive pulses overlap.

Description:
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&#39;s cycle time. In a further modification with respect to the process, it is proposed that the ultrasonic testing be performed with its own cycle control running independently of the cycle time of the packaging plant itself. This eliminates the need for connections or coordination with the packaging plant itself, and the testing can be started and stopped relatively quickly. In accordance with the process, the heat-sealed packages are conveyed by a conveyor belt to a baffle plate, which diverts the packages one by one into the ultrasonic testing unit, in which the heat-sealed joints are tested. The entire passage takes only a few seconds, e.g., three seconds. The packages are then lifted back onto the conveyor belt. Untight packages are sorted out after they have been placed back on the convevor belt, while packages with intact heat-sealed joints remain on the conveyor belt. The acoustic coupling of the transmitting and receiving ultrasonic probes is effected through a water advance interval. Either both probes are located in a water bath, into which the heat-sealed joint or at least the portion of the heat-sealed joint currently being tested is also immersed, or the coupling is effected by water jets, i.e., for example, probes of the type described in U.S. Pat. Nos. 3,255,626, 3,485,088, 3,908,455 and 4,403,510 or in European Patent 119 096. In both cases (immersion method or coupling by water jets), it has been found to be advantageous to work in a tank to catch or hold the water. 
     In regard to the equipment, a so-called &#34;bypass system&#34; has proven to be very effective. It can be connected to existing packaging plants. The only thing needed is a short stretch of conveyor belt, up to which the system can be pushed. 
     With respect to the equipment, the testing of circular plastic packages is especially simple and advantageous because in this case only the traylike package must be turned about its own axis, while the ultrasonic test line can remain stationary. However, the process of the invention also allows the testing of noncircular packages. In this case, the package to be tested is placed in a matching mount; this mount ensures geometric coordination. The mount with the package is turned about a quasi-center. In addition, the ultrasonic test line is moved transversely to the transport direction 44, so that deviations from the circular shape can be eliminated. Finally, rectangular packages can also be tested by covering heat-sealed joint areas running at right angles to the linear direction of transport by probes which are carried along in the direction of transport but which are simultaneously moved in the transverse direction. 
     Transmission of the ultrasonic radiation through the heat-sealed joint was wound to be crucially important; testing on the basis of reflected sound signals does not yield satisfactory results. 
     The ultrasonic testing of packaged foods is basically already known from EP-A-269 185. In that process, ultrasonic radiation is transmitted through the food itself for the purpose of detecting gas bubbles caused by fermentation before the fermentation has progressed so far that the package shows noticeable bulging. There is no preventive test that can be performed immediately after the food has been packaged, but rather deterioration of the packaged food due to fermentation must already have occurred for the previous ultrasonic test to be able to sort it out. Testing of the heat-sealed joint is not performed. 
     Overlapping of the joint areas covered by the individual ultrasonic pulses is understood to mean the overlapping of a first ultrasonic pulse by the following ultrasonic pulse by at least 2% and preferably at least 20% of the ultrasonically covered area of the heat-sealed joint. The overlapping is usually less than 100%. 100% overlapping occurs when exactly the same area covered by a first pulse is covered again by a second pulse. 
     Noncircular, e.g., oval, ultrasonic coverage areas, in which the cross-sectional length is greater than the cross-sectional width, have been found to be very advantageous. In this case, the greater length dimension of the ultrasonic coverage area should lie essentially parallel to the course of the heat-sealed joint line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Additional features and advantages of the invention are described in the other claims and in the following description of a specific embodiment of the invention, which is explained in detail with reference to the drawings. It is understood that the invention is in no way limited by this example. 
     FIG. 1 shows a lateral cross section of a test device along line I--I in FIG. 2. 
     FIG. 2 shows a cross section along line II--II in FIG. 1. 
     FIG. 3 shows a front view of the test device with the conveyor belt and a water tank. 
     FIG. 4 shows a cross section along line IV--IV in FIG. 1. 
     FIG. 5 shows a cross section along line V--V in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Successively filled plastic packages 22 that have been sealed with cover films are located on a conveyor belt 20 moving perpendicularly to the plane of the drawing, i.e., into the plane of the drawing. These untested packages are conveyed towards an obliquely positioned baffle plate 24, which diverts them from their path and, as is shown for package 23, causes them to slide into the ultrasonic testing device, which will be described below. If the ultrasonic testing is not to be performed, the baffle plate 24 is swung completely out of the path of the packages 22. 
     As FIG. 3 shows, the ultrasonic testing device forms a feed hopper, which is bounded by a plate 26 and a U-shaped sheet-metal guide 28. As will be described below, the plate 26 holds all parts of the device. It is inclined at an angle of 30° from the vertical. The packages 22 slide on their bottoms on the plate 26, so that the cover film remains a certain distance from the surface of the plate 26 and is thus protected. The heat-sealed joint, whose tightness is to be tested, is located at 32. 
     The package 22 to be tested is guided between guide jaws 34 by parts of the sheet-metal guide 28 that are positioned transverse to the direction of conveyance of the conveyor belt 20 and falls into the area between a conveyor belt 38 and the plate 26. The conveyor belt 38 has catches 40 arranged at distances that are adjusted to the package 22. The system is designed in such a way that the package can only slide between two catches 40 in the space between the conveyor belt 38 and the plate 26. The catches 40 extend so far towards the plate 26 that a package that is sliding down first strikes them and then slides further only when a sufficiently large gap becomes available for it between two catches 40. This situation is shown in FIGS. 1 and 2. The package 22 is supported towards the bottom by a support ledge 36. It is arranged in such a way that the center of the packages 22, which are circular in the specific example shown here, lies on the center line of the conveyor belt 38. 
     In a modification of the process, the conveyor belt 38 can also be stopped in the position shown in FIGS. 1 and 2 until a package 22 is located in the position shown at the far left. 
     The conveyor belt 38 runs around in a closed loop. It is guided over two rollers 46, 47, at least one of which is driven. The rollers rotate about axes 42 that run parallel to the surface of the plate 26. As FIGS. 1 and 2 show, the distance between the respective axes 42 of the two rollers 46 and 47 is slightly greater than three times the diameter of the packages 22. 
     The conveyor belt 38 conveys the packages to the right in FIGS. 1 and 2 in the direction of arrow 44. The packages arrive at an intermediate position, which is shown in FIG. 1 as the middle position of the packages 22, and then finally reach the actual test position, which is shown on the right in FIGS. 1 and 2. Thus the conveyor belt 20, the baffle plate 24, the feed hopper and the conveyor belt 38 cooperatively supply unrested packages successively to the test position. They are held at the top by suitably extended, now horizontally running guide jaws 34 and are supported at the bottom on the support ledge 36. 
     The ultrasonic test position has respective axes 42 of the two rollers 46 and 47 three driven, grooved rollers 46, 47, one of which is shown in cross section in FIG. 5. As FIG. 1 shows, two lower rollers 47 are located in a continuation of the support ledge 36 at a distance from each other that is selected so narrow that the ultrasonic test device, which will be discussed later, has just enough room between them. They are mounted at the same height. 
     A single roller 46 is arranged centrally above the two lower rollers 47. The centers of the rollers 46, 47 form the vertices of an isosceles triangle, whose apex angle (at roller 46) is selected as acute as possible. 
     The purpose of the rollers 46, 47 is to tune the packages 22 about their centers and at the same time to guide them exactly. During the turning movement of the package 22, the conveyance of the packages 22 by the conveyor belt 38 is momentarily stopped. 
     As FIG. 1 shows, in the test position, the packages 22 are positioned somewhat lower than before, i.e., as long as they are supported by the support ledge 36. To allow them to enter the space between the three rollers 46, 47, the upper roller 46 is briefly raised, for which purpose a cylinder (shown schematically) is provided above this roller. As a package 22 arrives at the test position, it drops down slightly onto rollers 47 and triggers a control pulse, which in response thereto the roller 46 drops back down and the ultrasonic test is performed with timing that is determined by the ultrasonic test device itself. After the roller 46 drops back down, the turning movement is performed. 
     The three structurally identical rollers 46, 47 consist essentially of three disks, namely, two outer, beveled guide disks 48 and a middle, driver disk 50, which is knurled on the outside. The outer edge of the latter disk grips the edge of the tray by friction. The driver disk 50 is selected with such a thickness that the edge of the tray with the cover film on it is maintained between the two guide disks 48 with as little play as possible. 
     The ultrasonic test will now be described especially with reference to FIG. 4. An ultrasonic transmitter 52 is mounted in a drill hole in plate 26. It transmits short-duration ultrasonic pulses with a frequency of, for example, 10 MHz at a high pulse rate of, for example 1 to 5 kHz. A sound guide tube 54 with an inside diameter of 3 mm is mounted on the transmitter and extends to the immediate vicinity of the edge of the tray. It has an oval shape at its free outlet end, such that the major axis of the oval coincides with the direction of the heat-sealed joint to be tested. In this way, the ultrasonic transmission area on the heat-sealed joint is shaped such that the pulses have a cross-section which includes a dimension of about 2 to 2.5 mm in the direction transverse to the longitudinal course of the heat-sealed joint. The circular package disclosed have heat-sealed joints extended along and defining courses. The shape of the outlet end of the sound guide tube 54 and the exact geometric coordination between the sound guide tube 54 and the edge of the package 22 with the heat-sealed joint ensure that only the actual area of the heat-sealed joint is exposed to the ultrasonic radiation. In this connection, the geometric coordination is guaranteed essentially by the two rollers 47, which are positioned in the immediate vicinity of the ultrasonic test. 
     The ultrasonic pulses which have passed through the edge of the package 22 and the cover film in the area of the heat-sealed joint are picked up or received by an ultrasonic receiver 56. The circumferential speed with which the package 22 is turned and the repeating frequency with which the ultrasonic pulses are transmitted by the transmitter 52 are mutually adjusted in such a way that the oval ultrasonic exposure areas on the heat-sealed joint overlap each other. 
     Apparatus for the generation and evaluation of the ultrasonic pulses are already well known, i.e. state-of-the-art methods can be used. In the practical performance of tests with one type of packages 22, it was found that the ultrasonic signals fluctuate by 2 dB due to deviations from perpendicular transmission through the edge of the package 22 and other factors. Changes greater than 4 to 6 dB can be given as defects in the heat-sealed joint. Another decision criterion that can be used is based on the fact that defects lead to abrupt changes in the ultrasonic signal, while other geometric changes produce slower changes in the ultrasonic signal. 
     In the specific embodiment of the invention shown here, acoustic contact or coupling between the probe and the test specimen is made through water. For this reason, as FIG. 3 shows, the test device is located in a water tank 58, which is filled to a sufficiently high level that at least the lower, currently tested regions of the edges of the trays are submerged in the water. 
     During the test, the package 22 is turned at least once about its own axis. If enough time is available, the package can be rotated a second time. After the ultrasonic test has been performed, the conveyor belt 38 moves forward, and the package 22 advances to the area of a lifting device 60, which lifts the package in the direction opposite the direction in which it previously slid downward. The package 22 is thus lifted back to the plane of the conveyor belt 20 until it tips back onto this conveyor belt. The package then reassumes the position shown in FIG. 3 as the initial position. A baffle plate is then used to sort out those tested packages that were evaluated as defective during the ultrasonic test. In another modification, the lifting device can also be used to sort good and bad packages. 
     The equipment of the invention can be coordinated with an existing packaging line in a bypass operation. Attachment and detachment can be accomplished by a few manual operations. Once the test equipment is connected to the packaging line, a decision can be made for each individual package on the conveyor belt 20 whether that package should be tested or not. This is accomplished by control of the baffle plate 24. 
     To simplify maintenance of the system, the conveyor belt 38 is arranged so that it can be swung out around one of its axes 42. If one wishes not to use water (or another suitable liquid) as the acoustic coupling medium and would rather perform dry measurements, it is also possible to maintain the test probes 52, 56 in rubbing contact with the edge of the package 22 or cover film 30. Mixed forms, i.e., acoustic coupling of the transmitter 52 through water but acoustic coupling of the receiver by rubbing contact, or the like, are also possible. 
     Higher test frequencies, e.g., 15 MHz and higher, have been found to be more suitable for this test than lower frequencies, e.g., 3 MHz. The invention also allows the testing of yogurt containers, drug packages etc. 
     In a modified embodiment of the invention, in which the equipment is otherwise identical, the probes 52, 56 are replaced by probes of types that are already known, for example, from the four US patents and the European patent cited above. The water jets are directed at the heat-sealed joint 32 from both sides in such a way that the degree of overlapping is as complete as possible, i.e., the ultrasonic pulse can pass through linearly. The probes 52, 56 are connected by well-known means to a water reservoir, from which they are continuously supplied with water by a pump. A sound guide tube is unnecessary under these circumstances. Aside from these modifications, the same equipment can be used as described above. 
     However, as far as the process is concerned, when acoustic coupling by water jets is used, in contrast to the immersion method described above, the water in the tank 58 is maintained at such a depth that the water jets issuing from the probes 52, 56 are above the level of the water. 
     The ultrasonic transmission areas are generally circular, but the use of oblong transmission areas is advantageous; in this case, the larger dimension of the transmission area is aligned with the longitudinal direction of the heat-sealed joint.