1. Field of the Invention
The present invention relates to an ultrasonic sealing apparatus for use in a filling/packing machine for producing liquid beverages or the like, which are packed in paper containers so that they can be stored for a long time, and more particularly to an ultrasonic sealing apparatus provided with an opposing jaw having an action face of a specific shape for improving sealability by controlling molten thermoplastic resin flow.
2. Description of the Related Art
Hitherto, there has been known a filling/packing machine, as a whole shown in FIG. 1, in which a sterilized packing material web is used for a rectangular parallelepiped packing container filled with a liquid, such as a juice. In brief, such a filling/packing machine as a whole comprises a reel 2 supporting a laminated packing material web 1 in a rolled state; a sterilization device 3 for sterilizing the tubular laminated packing material web successively unwound from the reel; a longitudinal sealing section 4 for sealing both longitudinal end portions of the sterilized laminated packing material web and thereby forming a tubular laminated packing material web; a liquid supply tube 5 for filling a fluid content into the web formed into the tubular form; a transverse sealing apparatus 7 for transversely sealing the tubular laminated packing material web in the direction perpendicular to the longitudinal direction thereof in the presence of a fluid while feeding downward the tube filled with the content by the length substantially corresponding to one package, and at the same time, continuously forming a pillow-shaped container 6 which is rectangular in section; and a container shaping device for forming the pillow-shaped containers 6 into rectangular parallelepiped containers 8 by folding the end portions to make the final form. The transverse sealing device 7 is composed of a sealing unit, a sealing jaw for fixing the sealing unit, and a driving means for driving the sealing jaw.
Such a sealing apparatus may comprise an ultrasonic sealing device in the form of a piezoelectric driving unit with a piezoelectric ceramic plate or a magnetostriction driving unit, which is connected to an AC supply source, and a horn with an elongated sealing surface, with one or more reaction bodies forming a half wavelength together with the horn (Japanese Laid-Open Patent Application No. 2231/1995). In addition, other ultrasonic sealing apparatuses are known, for example as disclosed in Japanese Patent Publication No. 22784/1987, Japanese Patent Publication No. 2544450/1996, Japanese Laid-Open Patent Application No. 33121/1995, Japanese Laid-Open Patent Application No. 92046/1981, Japanese Laid-Open Patent Application No. 15741/1994, and the like).
Among the other ultrasonic sealing apparatuses, there has been known an ultrasonic sealing apparatus for liquid containers that is characterized by the shape of the action face of the opposing jaw for pressing the laminated packing material in cooperation with the sealing face of the horn. In this apparatus, a band-shaped ridge is disposed nearly at the center of the face of the horn butting against the portion to be sealed. A concave groove perpendicular to the ridge is disposed nearly at the center of the face of an anvil butting against the portion to be sealed. A bottom portion of the concave groove is made into a two-step structure and its step portion and both end portions are tapered. A protruded and inclined face of an acute angle extending sideward is provided at the end portion of the center of the bottom portion. The open portions at the top and bottom ends of a cylindrical liquid container are sealed with an overlapping bonding portion approximately at the center of the body consisting of the horn and opposing jaw or anvil (see Japanese Laid-Open Utility Model Application No. 147408/1988 and Registered Utility Model No. 3004786).
On the other hand, there is known a high-frequency heat sealing apparatus (Japanese Laid-Open Patent Application No. 230834/1996), wherein the molten resin remains in grooves without flowing out of the sealed zones, even if the resin on the packing material is melted and pushed by a ridge. In this arrangement, the ridge is formed in the longitudinal direction of an action face and high-frequency coils are used with the grooves formed parallel to the ridge on both sides of the ridge on the high-frequency heat sealing apparatus. There is additionally known a high-frequency heat sealing apparatus (Japanese Patent Publication No. 244728/1996), wherein the high-frequency coils are formed to incline gradually toward the farther direction as the sealed zone forming portion goes to the outer side (i.e., to the container's interior side). In this arrangement, the heat sealing is done such that liquid and the molten resin are discharged smoothly from the sealed zones to the container's interior side when the tube is pressed together, in the presence of a fluid, on the high-frequency heat sealing apparatus.
In order to achieve excellent heat sealing when transversely heat sealing a tubular packing material filled with contents such as liquid beverages, i.e. in the presence of the liquid, it is necessary to exclude the liquid from the sealed zones of the tube as much as possible when the tube is sealed with heat and under pressure. However, the tube inner face in contact with the liquid is not necessarily completely flat and has small asperities, if observed in detail. In the high-frequency heat sealing apparatus disclosed in Japanese Laid-Open Patent Application Nos. 134744/1983, 269854/1993, 164523/1995, 240607/1997 and 230834/1996, liquid or dirt enters the small asperities of the tube inner face and cannot be discharged to the outside of the sealed zones, so that the sealing property cannot be said to be adequate, as compared with a sealing method in which the molten resin can be prevented from flowing out of the sealed zone and thus is left in the sealed zones.
On the other hand, the inventors of the present invention have discovered that the liquid in the sealed zones may be guided to flow out of the sealed zones together with the molten resin so as to eliminate the liquid or dirt entered into the asperities when sealed, as disclosed in Japanese Laid-open Patent Application No. 244728/1996. The method using this heat sealing apparatus can completely eliminate liquid or dirt entering into the asperities to provide excellent sealing properties, but the molten resin flowing out to the side in contact with a fluid, which is the container's interior side, may not homogeneously extrude. As shown in FIG. 2, undulated molten resin beads 9 are formed on the edge portion of the container's interior side. At the secondary step using a shaping machine after the sealing step, it has been found out that cracks start from crests 10 of the undulations of the beads 9 and allow liquid leakage when pressure is applied to the container, although such leakage is not continuous. Problems with the high-frequency sealing apparatus are expected to occur almost similarly with the ultrasonic sealing apparatus, even considering the particularity of the ultrasonic sealing apparatus.
As to the particularity of the ultrasonic sealing apparatuses, i.e. the distinction from the high-frequency sealing apparatuses, the inventors of the present invention gained an insight as follows by preliminary experiments. In pressing the tubular packing material with an ultrasonic horn and an opposing jaw working together on the ultrasonic sealing apparatus, it was found that keeping the uniformity of the pulsation would be difficult, if configurations such as the grooves and an incline which increase the sealability are added on the sealing face of the ultrasonic horn, due to the additional asperities. Thus it would be impossible to apply an appropriate pressure on the intended sealing area of the tubular packing material. Further, in the case of ultrasonic sealing, as opposed to high-frequency sealing which can heat even the unpressed area, it was determined that a portion of the molten thermoplastic resin inevitably flows out to the side in contact with the fluid which is on the container's interior, because only the pressed area, which infallibly transmits the ultrasonic pulsation, is heated.
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