Abstract:
Wrapping material utilized by a packaging machine is advanced along a set feed path through a station where an accessory material, typically adhesive, is applied in spots to selected areas of the surface, whereupon the material is charged electrostatically at a first station, and thereafter sensed at a second station to reveal the distribution of the charges; the electrostatic charges register more strongly on and around the spots, so that by measuring any variation in strength relative to the remainder of the surface, identifiable with faulty application of the adhesive, substandard material can be detected before it reaches the wrapping stations of the machine.

Description:
This application claims priority to Italian Patent Application No. BO 2002A 000529, filed Aug. 8, 2002 which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of checking wrapping material in a packaging machine. 
     In particular, the invention relates to wrapping material of the type decoiled from a roll and fed to a processing station in the form of a continuous strip, or of discrete lengths separated previously at a cutting station, also to wrapping material procured in the form of blanks taken from a stack. 
     It is normal for such materials, before entering a first station of a machine where they are wrapped around the respective products being packaged, to undergo an operation in which accessory materials are applied to selected areas. 
     Depending on the particular requirements, these accessory materials can take the form of adhesives, inks, and metallic powders utilized for example in combination with adhesive substances. 
     In accordance with conventional techniques, the accessory materials in question must be applied to selected areas of the wrapping material and can be distributed in different ways. They can appear for example as spots, or as continuous or discontinuous stripes, or in other specific patterns. 
     Clearly, the distribution of accessory materials in this manner must be repeated cyclically on each length of wrapping material destined to provide a single wrapper, and similarly, the geometry of the distribution must remain identical for each such length. 
     If this is not the case, the steps of wrapping and packaging the finished product could give rise to structural or visual defects necessitating the rejection of the substandard products at a point downstream of the wrapping stations. 
     The object of the present invention is to provide a method of checking wrapping materials, generally considered, whereby the correct distribution of accessory materials can be verified upstream of the wrapping stations and the above noted problems duly overcome. 
     SUMMARY OF THE INVENTION 
     The stated object is realized according to the present invention with the adoption of a method for checking wrapping material in a packaging machine, comprising the steps of advancing the wrapping material along a predetermined feed path, applying an accessory material to selected areas of the wrapping material, charging the wrapping material electrostatically as it passes through at least one first charging station, and subsequently verifying the distribution of the electrostatic charge on the wrapping material as it passes through at least one second checking station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: 
         FIG. 1  shows a portion of a packaging machine equipped with a checking device embodied according to the present invention, illustrated schematically and in perspective with certain parts omitted; 
         FIG. 2  shows a second embodiment of the device seen in  FIG. 1 , illustrated schematically and in perspective with certain parts omitted; 
         FIG. 3  shows a third embodiment of the device seen in  FIG. 1 , illustrated schematically and in perspective with certain parts omitted; 
         FIG. 4  shows a fourth embodiment of the device seen in  FIG. 1 , illustrated schematically and in perspective with certain parts omitted; 
         FIG. 5  is a graph illustrating a control signal relative to the device of  FIG. 1 ; 
         FIG. 6  is a detail of  FIG. 1 , shown enlarged and in a schematic side elevation view. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIGS. 1 and 2  of the drawings,  1  denotes a portion, in its entirety, of a packaging machine in which a wrapping material  2  is caused to advance along a feed path  3 , decoiling from a roll  4  and proceeding toward a cutter device  5  by which it is divided up transversely into discrete lengths, and passing also through a checking unit denoted  6  in its entirety. 
     As illustrated in  FIG. 1 , the roll  4  is carried by a pivot  7  rotatable about a horizontal axis  7   a  and driven by a relative motor  8  in such a way that the wrapping material  2 , which takes the form of a continuous strip  9 , can be decoiled with the aid of conventional traction means (not illustrated) and directed between guides  10  toward a device  11  by which an accessory material, denoted  12 , is applied to selected first areas of the strip  9 . 
     The applied accessory material  12  appears in the example of  FIG. 1  as a continuous succession of spots  13  occupying predetermined positions on the strip and consisting, for example, in an adhesive substance such as cold and/or hot melt glue. 
     In this way, downstream of the cutter device  5 , each of the discrete lengths  14  separated from the strip will present a given number of spots  13  of adhesive occupying positions predeterminable by suitably timing and interlocking the operation of the applicator device  11  and the cutter device  5 . 
     In the example of  FIG. 1 , proceeding upstream to downstream along the feed path  3 , the checking unit  6  comprises a device  15  by means of which to charge the advancing strip  9  electrostatically, and thereafter, a device  16  by means of which to sense the electrostatic charges applied previously to the strip; the devices  15  and  16  in question coincide respectively with a first operating station A and a second operating station B. 
     In particular, the charging device  15  comprises a pair of plates  17  offered one to each face of the continuous strip  9  and connected to an electric field generator  18  such as will create a potential difference between the plates  17  and thus charge at least the upwardly directed face  9   a  of the strip  9  electrostatically. 
     The sensing device  16  comprises a first sensor  19  and a second sensor  20  carried by a supporting structure  21  (see also  FIG. 6 ) and connected to a comparator circuit indicated by a block denoted  22 . The first sensor  19  is directed toward the face  9   a  of the strip  9  and aligned on the aforementioned first areas occupied by the spots  13  of adhesive, whilst the second sensor  20  is directed toward the same face  9   a  of the strip  9 , though aligned on a selected second area not occupied by the accessory material  12 . Observing  FIG. 6 , it will be seen that the structure  21  also performs the function of a screen, for reasons that will become clear in due course. 
     The block denoted  23  represents a master control unit to which the decoil motor  8 , the applicator device  11 , the electric field generator  18  and the comparator  22  are all connected. 
     It has been demonstrated by practical experiment that when a strip of wrapping material is exposed to the action of an electric field able to generate electrostatic charges on the surface of the strip, these will tend to accumulate and to intensify at the areas treated with accessory materials. In the particular case in point, the electrostatic charges were seen to concentrate predominantly around and upon the spots  13  of adhesive, whereas across the remainder of the face  9   a  presented by the strip  9 , the distribution of the charges was typified by a lower and substantially uniform concentration. 
     In operation, following the step of applying the spots  13  of adhesive, the wrapping material can be checked under the sensing device  16  to verify the distribution of the electrostatic charges on the face  9   a  of the strip  9 , by revealing the pattern of their concentration in the selected first areas, of greater intensity, and in the second area of lesser intensity. 
     In the event of the device  16  returning a signal found to be abnormal when compared with a reference signal supplied to the master control unit  23  by a generator  27 , for example if there are spots  13  of adhesive missing from or not correctly positioned in the first areas, the comparator  22  will relay a signal to the control unit  23  which can then, for example, shut off the motor  8  to stop the movement of the strip  9  and pilot the applicator device  11  to restore the correct distribution of the spots  13  of adhesive. 
     By way of example,  FIG. 5  illustrates a signal emitted typically by the first sensor  19  and the second sensor  20 . In practice, the sampled signal presents a succession of peaks  28  corresponding to the spots  13  of adhesive, where the first sensor  19  detects a greater concentration of charges, whilst the second sensor  20  generates a flatline signal  29  of amplitude lower than the peaks  28 , reflecting the lower concentration of charges on the remainder of the strip  9 , that is, on the part of the face  9   a  where no spots  13  of adhesive are present. A signal of this type is compared with the reference signal supplied by the generator  27 , the characteristics of which will be the same as in  FIG. 5 . 
     Adopting a comparative type of control using two sensors  19  and  20 , it becomes possible to obtain greater accuracy and reliability from the sensing device  16 . In addition, the structure  21  prevents any accumulation of charges around the sensors  19  and  20  by discharging them to earth, so that there will be no spurious signals generated. 
     Advantageously, the adhesive substances applied to the wrapping material can be treated in such a way as will increase their capacity to accumulate electrostatic charges, for example by including additives able to attract such charges. 
     The solution illustrated in  FIG. 2  differs from that of  FIG. 1  only inasmuch as the adhesive is distributed by the applicator device  11  along the first area in the form of a continuous fillet  24 . 
     Similarly, the example of  FIG. 3  differs from that of  FIG. 2  only in that the application of a continuous fillet  24  of adhesive is followed by the application over the adhesive, utilizing a suitable pressure roller  25 , of a fillet  26  identifiable as a second wrapping material consisting for example of an easy-tear ribbon designed to facilitate the operation of breaking open an overwrap on packets or cartons of cigarettes. 
     In the example of  FIG. 4 , spots  13  of adhesive are applied according to a predetermined layout on selected areas of the lateral portions  29  presented by flat diecut blanks  30  taken from a stack  31 . 
     At a given point along the feed path  3  followed by the blanks  30 , two applicator devices  11  are supplemented by a further device  33  operating at the first station A, such as will apply dabs  34  of adhesive to respective central portions  32  of the blanks  30 , and a device  35  dispensing a metallic powder designed to cling to the adhesive dab  34 . 
     The device and the checking method thus described are able to ensure that no defective lengths or blanks of material will reach the wrapping stations downstream, so that stoppages are prevented and rejects avoided. 
     The accessory material  12  is described above as an adhesive substance, by way of example, but might also consist in fluid substances such as inks or colorants. 
     In the case of the embodiments illustrated in  FIGS. 2 and 3 , moreover, the presence and correct placement of the continuous fillet  24  of accessory material  12  and the fillet  26  of second wrapping material can be verified simply by comparing the signals from the two sensors  19  and  20 , and without the need for a generator  27  to supply a reference signal as in  FIG. 5 .