In such machines, the open end of the filter bag must be sealed (after the product has been filled into the bag): sealing can be accomplished by any of several known methods, for example, using a staple or the string that joins the filter bag to the tag (by knotting the length of string itself or another piece of string) or by sealing the overlaid portions of the web of filter paper.
In the case of sealing, operations may be carried out by traditional sealing means with heating elements or, as in the solutions of the latest generation, by ultrasound devices that use sonotrodes placed in contact with the product to be sealed.
In practice, the sonotrode is the transducer element that converts the power supply into vibrations that are discharged onto the product to be sealed.
For example, one solution for this type of machine is disclosed in patent application BO2002A000798 by the same Applicant as the present.
In that solution, the apparatus for making filter bags for infusion products is equipped with a revolving conveyor drum driven by a first drive shaft to feed a web of filter paper along a path on which there are positioned: a station for feeding the filter paper web; a station for feeding a succession of tags; a station for feeding tie string connecting the filter bags to the respective tags; a station for arranging the tie string according to a defined pattern; and a plurality of sealers facing the feed path and designed to attach the tie string and the tags to the filter paper web. The sealers, which are sonotrodes, perform a defined movement between a sealing position and a waiting position, and this movement, which is performed simultaneously and synchronously by all the sealers, is imparted, through a transmission element shared by all the sealers, by a second cam drive shaft connected to the same source of motion to which the first shaft that drives the conveyor drum is connected.
In other words, the sonotrode is placed in a preferred position relative to the underlying part over which filter paper web moves, this preferred position depending on the inherent properties of the paper itself, on the speed at which it is fed and on the thickness of the components to be sealed (filter paper, tie string and tag) placed one over the other.
The Applicant has found that the relative position between the emitting surface of the sonotrode and the product supporting surface constitutes a fundamental variable for good sealing quality.
In practice, the constancy of sealing quality is directly proportional also to the force of the contact (besides contact time and the frequency of sonotrode activation) between the sonotrode and the product supporting surface, which means that the thrust force applied mist be as constant as possible. This is not always easy to achieve since the components to be sealed (described above) are not always exactly the same in thickness and their total thickness varies within quite a wide range: as a result, the pressure applied to the products that move into the sealing area (under the sonotrode) tends to vary from one product to the other, thus leading to irregular sealing quality.
The present invention therefore has for an object to overcome the above mentioned drawback through a device that enables the sealer sonotrode to apply a substantially constant contact force on the product to be sealed, the application of the force being synchronised with the presence of the product in the sealing area.