Abstract:
For sealing off overpressure valves of packaging containers, a small quantity of liquid sealant is introduced into a valve. To check whether such valve has been equipped with sealant, the sealant is heated, prior to introduction into the valve, to a temperature that is elevated compared with ambient temperature. After that the valve is checked for a temperature difference with a heat sensor, which causes valves not equipped with sealant to be rejected. The sealant is heated immediately prior to its application to a valve, in a nozzle delivering it.

Description:
BACKGROUND OF THE INVENTION 
     The invention is directed to improvements in a method for sealing off an overpressure valve for a packaging container. 
     It is known for overpressure valves on packaging containers to allow gas produced by the product in the container--such as carbon dioxide in the case of coffee--to escape from the packaging container at a certain overpressure yet to prevent air and thus oxygen from coming into contact with the product. Such valves are generally equipped with a liquid sealant, such as silicone oil. The sealant disposed between the valve elements does not impair the opening of the valve, but in the closed position of the valve it prevents oxygen molecules from passing through the valve into the packaging container through tiny channels formed between uneven portions of the contacting surfaces of the valve elements. By a method disclosed in European Patent Document B 12 874, the liquid sealant is applied with a nozzle to an inlet or outlet end of the valve channel formed by foil elements; from there, it penetrates the channel by capillary attraction. It may sometimes happen, for instance if the applicator is defective or if the sealant supply is exhausted, that valves may not be equipped with sealant, so that their tightness does not meet requirements. Packaging containers thus provided with unsealed valves do not assure the necessary protection for the product, so that the product prematurely loses its quality or even spoils as a result of oxidation. Until now, it was not possible to check whether a valve was activated with sealing fluid. Although it has already been proposed that the presence of sealant in a valve be ascertained optically with the aid of a photoelectric cell, the results were not satisfactory. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is a principal object of the invention to provide a method as defined hereinafter having the advantage that the presence of sealant in a packaging container overpressure valve can be checked simply, yet with high reliability. By evaluating the outcome of the check, valves not equipped with sealant can then be precluded from being used. 
     A particularly simple provision is to heat only a small quantity of sealant at a time and to do so immediately prior to applying it to a valve. The presence of sealant can be checked particularly simply and reliably in a valve by purposefully monitoring the site where sealant is supplied in the valve; this can be done with an infrared sensor. In an apparatus for performing the method of the invention, in which the sealant is delivered by a nozzle, it is particularly advantageous if the nozzle is thermally conductively connected to a heater element. 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole figure of the drawing shows an apparatus for introducing sealant into valves and for checking for the presence of sealant in the valves. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the method according to the invention, sealed off with a liquid sealant, for instance silicon oil. Such valves 1, which are for instance described in European Patent Document B 23 703, can substantially comprise a stiff base plate 2, which has an aperture 3, and a flexible backing foil 4 that covers the base plate and its central aperture; this foil is glued to the base plate 2 at two parallel zones on the edge, defining a valve channel 5 in the middle. The sealant is introduced into the valve channel 5, for instance in accordance with European Patent Document B 12 874, by application in the form of a droplet 6 onto an outer end on the edge of the valve 1 or onto an inner end in the region of the aperture 3 of the base plate; from there it is drawn into the channel 5 by capillary attraction. Equipping such valves 1 with sealant is preferably performed before or after the valve is joined to a packaging container. Instead of applying sealant to one end of the valve channel 5, it is also possible to introduce the sealant directly, through a thin nozzle introduced into the valve channel. 
     In the exemplary embodiment shown, valves 1 are supplied in a row, sticking to a carrier strip 7; are equipped, in this arrangement, with a sealant; then are peeled off from the carrier strip 7 at a deflection edge 11 of the carrier strip 7 and transferred with a suction die 12 to a packaging container. The carrier strip 7, which has openings 8 each coinciding with one aperture 3 of the base plate 2 of each valve 1, is advanced incrementally in a horizontal plane; the valves are disposed on its underside, so that the inner ends of the valve channel 5 are accessible from above through the opening 8 and through the aperture 3 in the base plate 2, and the backing foil 4 forms a bottom. 
     A sealant delivery device 20 is disposed at one station along the delivery route of the carrier strip 7; it has a nozzle 21 in the form of a hollow needle, pointing downward and ending just above the carrier strip 7 in the region of an opening 8. Preferably the nozzle 21 ends not centrally, but rather near the edge of the opening 8 of the carrier strip 7 or of the aperture 3 in the base plate 2 of a valve 1, so that a droplet 6 of sealant expressed by the nozzle 21 and dropped through the opening 8 and the aperture 3 onto the backing foil 4 of a valve 1 comes at least partly into contact with the edge of the aperture 3 of the base plate 2 that defines one end of the valve channel 5. For this purpose it may be useful to advance the carrier strip 7 on an incline, so that the droplet 6, dropped on the then likewise inclined backing foil 4, will flow to the edge of the aperture 3 of the base plate 2 and from there into the valve channel 5 or parts of it. 
     For dosage of the necessary quantity of sealant for a volume of approximately 4 to 6 m 3 , the nozzle 21 is connected to a metering device 30 and a sealant supply container 31. In the exemplary embodiment shown, the metering device 30 comprises a flexible tube 32 that connects the supply container 31, which is at slight overpressure, with the nozzle 21 and a clamping jaw 34 pressing the tube 32 against a fixed stop 33; the clamping jaw is pulled incrementally away from the stop 33. Each time the clamping jaw 34 is briefly retracted, a small quantity of sealant flows in the tube 32 to the nozzle 21, emerges from its lower end in the form of a droplet 6, and is received by the valve 1 furnished there. 
     To check whether a quantity of sealant has been introduced into each valve 1, a monitoring apparatus 40, is provided at the station following the sealant delivery apparatus 20. This apparatus has a heat sensor 41, which responds to a temperature difference between a location being homed in on and its surroundings. The monitoring apparatus 40, like the sealant delivery device 20, is disposed above the delivery plane of the carrier strip 7, and its heat sensor 41 is aimed at the point on the carrier strip 7 or on the valve 1 onto which a droplet 6 of sealant was previously deposited by the nozzle 21. 
     In order for the monitoring apparatus 40 to respond upon the presence of sealant in a valve 1, the sealant is supplied to the valves 1 at a temperature that is elevated compared with the ambient temperature. To this end, the metal nozzle 21 is thermally conductively received in a protrusion 23 of a heating element 22 containing a temperature-regulated electric heating cartridge 24. The nozzle 21 is heated by the heating element 22 such that the small quantity of sealant located in the nozzle is heated to a temperature in the range from 40° to 100° C., preferably 60° to 80° C. As a result of this arrangement, only a small quantity of sealant at a time is heated to a temperature that is elevated compared with the ambient temperature and compared with the valves 1 as delivered, and the sealant is not heated until just before it is introduced into a valve 1. 
     If the monitoring apparatus 40 ascertains a temperature difference for the valve 1 brought within its scanning range at a certain time, this means that sealant is present in the monitored valve 1, and that this valve 1 can be joined to a packaging container. Contrarily, if the monitoring apparatus 40 ascertains no temperature difference, then it controls the sealant delivery device such that although the die 12 does pick up the applicable valve once it comes within range of the sealant delivery device, it will not secure it to a packaging container but instead rejects it. 
     The monitoring apparatus 40, which preferably comprises an infrared temperature difference measuring instrument, can be disposed on the side of the valve on which the sealant is supplied, as shown in this exemplary embodiment; or it may be disposed on the opposite side, in which case it does not scan the applied sealant directly but instead scans the thin backing foil at the point at which the sealant has been applied. 
     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.