Abstract:
An apparatus for sealing films together along a path includes a sealing shoe and a counter shoe defining a clearance through which the films pass. The sealing shoe includes an elongated optical energy source having a length dimension oriented generally parallel to the sealing path; a reflector for focussing light emitted by the energy source; a window transparent to the light and having an outer surface adapted to be oriented toward the counter shoe for sealing the films by the light; and a firing arrangement for activating the energy source. The window and the counter shoe are urged toward one another.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the priority of Swiss Application No. 1999 0161/99 filed Jan. 29, 1999, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Packing machines using plastic film wrappers frequently include sealing shoes with cooperating counter shoes for providing sealed seams on superposed plastic films (sheets) forming a packing hose. The sealing shoe or both the sealing shoe and the counter shoe are heated to a temperature above the melting temperature of the thermoplastic packing sheet, and the shoes are pressed to one another to seal the superposed plastic films positioned between the two shoes. 
     The output capacity of the above-outlined systems is necessarily limited. The heat is conducted through the film into the sealing zone. If the sealing temperature is too low, insufficient heat is transferred to the sealing layer. If, on the contrary, the sealing tools are too hot, the film tends to adhere to the contact faces of the tools. Dependent on the film thickness and the operating cycle, the parameter range in which a reliable operation is ensured might be extremely narrow. In case of rotary transverse sealing shoes, the sealing period depends from the feeding speed of the film. At high feeding speeds the sealing period is too short to produce a stable sealed seam. Such a boundary speed may be increased by providing that the transverse sealing shoe co-travels along a linear trajectory with the traveling film as described, for example, in International Application WO 96/17720. For this purpose, however, a complex mechanical system is required which often leads to vibrations, wear and operational disturbances. 
     In general, the sealed seam of a thermoplastic material may be exposed to stresses only after the temperature has dropped below the melting temperature. Since, because of the contact with the hot sealing tool, the entire seam volume is heated, in addition to the speed of the energy supply, the cooling phase also limits the minimum required period to ensure that the sealed seam may be exposed to stresses. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved apparatus of the above-outlined type with which a rapid sealing may be performed and which is of simple construction. 
     This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the apparatus for sealing films together along a path includes a sealing shoe and a counter shoe defining a clearance through which the films pass. The sealing shoe includes an elongated optical energy source having a length dimension oriented generally parallel to the sealing path; a reflector for focussing light emitted by the energy source; a window transparent to the light and having an outer surface adapted to be oriented toward the counter shoe for sealing the films by the light; and a firing arrangement for activating the energy source. The window and the counter shoe are urged toward one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional end elevational view of a preferred embodiment of the invention. 
     FIG. 2 is a sectional side elevational view of the construction shown in FIG.  1 . 
     FIGS. 3 and 4 are sectional side elevational views of two further preferred embodiments of the invention. 
     FIG. 5 is a fragmentary enlarged sectional elevational view of a variant of FIG.  4 . 
     FIGS. 6 a  through  11   a  are cross-sectional views of superposed plastic films of various properties depicted during irradiation with optical energy for sealing the films to one another. 
     FIGS. 6 b  through  11   b  are cross-sectional views of the superposed plastic films shown in the respective FIGS. 6 a  through  11   a , depicted in a sealed state after irradiation with optical energy. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus shown in FIGS. 1 and 2 includes a sealing shoe  10  and a counter shoe  11  which are rotatable in synchronism in opposite directions about two respective, parallel spaced axes  12 ,  13  in the direction of respective arrows  14   a  and  14   b . The packing hose  15  which is advanced in the conveying direction A between the sealing shoe  10  and the counter shoe  11  contains uniformly spaced products  16  to be packaged. The hose  15  is composed of a thermoplastic film  18  and has a longitudinal sealed seam (not shown). By means of transverse severing through the middle of the transverse sealed seams, individual packages  17  are obtained. 
     The sealing shoe  10  includes a cross-sectionally rectangular rotor  20  rotatable about the axis  12  and an optical unit  21  which includes a carrier  22  radially displaceable on the rotor  20  and biased by a spring  23  radially outwardly against a non-illustrated stop. A prismatic housing  24  made of an insulating material such as a plastic is secured to the carrier  22 . Further, in the housing  24  an aluminum reflector  26  is mounted, having a cylindrical reflecting surface  27  which is cross-sectionally elliptical. A cylindrical gas discharge flash lamp such as a xenon lamp is arranged coaxially with the focal axis  28  of the surface  27 , extending parallel to the rotary axis  12 . The two electrodes  30 ,  31  of the lamp  29  are connected to a high-voltage pulse generator  32  which has a condenser switching circuit, setting elements  33  for setting parameters such as voltage, current intensity, duration of pulse and pulse shape as well as indicator elements  44  for displaying the set parameters. The space  35  between the lamp  29  and the reflector  26  is closed by a transparent window  36  which is preferably of a scratch proof material, such as sapphire glass. The approximately cylindrically curved outer surface  37  of the window  36  has a central flattened portion  38  which is oriented perpendicularly to the plane containing the axes  12 ,  28 . The space  35  is connected to a coolant circuit  39 ; the coolant may be air or a transparent, electrically insulating liquid such as de-ionized water. 
     The counter shoe  11  has a rotor  42  having a rectangular cross section. A holder body  43  is radially displaceably mounted on the rotor  42  and is biased radially outwardly by a spring  44  against a stop. The arcuate (convex) counter face  45  of the holder body  43  has a central flattened portion  46 . 
     In the description which follows, the operation of the above-described apparatus will be set forth. 
     The sealing shoe  10  and the counter shoe  11  run in synchronism in opposite directions. The circumferential speed of the two surfaces  37  and  45  is approximately the same as the advancing speed of the tubular hose  15  at least when the surfaces  37  and  45  press together the two film layers of the hose  15  running between the sealing shoe  10  and the counter shoe  11 . The rotary angle of the rotors  20 ,  42  is synchronized with the longitudinal feed in such a manner that the window  36  of the sealing shoe  10  and the holder body  43  of the counter shoe  11  at all times engage the hose  15  between two products  16 . The flash lamp  25  is fired at the moment when the axes  12 ,  13  and  28  lie in a common plane, that is, the flattened portions  38  and  46  press the hose  15  together. By means of a pulsed electrical field between the two electrodes  30 ,  31  in the discharge volume, a gas is converted into an electrically conducting plasma by impact ionization, and the plasma is heated up by the electric current. The light emission consists of a black body radiation with a color temperature of up to approximately 10,000 K which is superposed by the characteristic spectral lines of the ionized gas; this corresponds to a wide spectral emission of 160-2500 nm. The emission proceeds from the upper surface of the ignited, light-impervious plasma. 
     The energy radially emitted by the lamp  29  is reflected by the elliptical reflecting surface  27  on the second focal point of the ellipse. Such second focal point is situated approximately on the flattened portion  38  of the window  36  or, stated differently, at a location which is at a distance from the hose  15 , corresponding to a single or dual thickness of the film  18  of the hose  15 . In this manner more than one-half of the energy radiated by the lamp  29  is concentrated on the focal line at the surface in the middle or on the underside of the hose  15  so that on the focal line an energy density of more than 2 J/cm 2 , up to 30 J/cm 2  is obtained, resulting in a very high degree of efficiency. Dependent on the thickness of the film of the hose  15 , a pulse duration between 50 microseconds and 10 milliseconds is required. The desired spectrum of the emitted radiation depends from the type of the film  18  because the absorption coefficient of the material is dependent from the wave length. The radiation spectrum is relatively wide; it has, however, a maximum which depends from the current intensity. In case of 1,000 A/cm 2  the maximum is, for example, in the visible spectral range and shifts to the ultraviolet range upon an increase to 10,000 A/cm 2 . 
     Assuming an advancing speed of 1 m/s in the direction A, a circulating radius of 8 cm of the surface  38  and a flash duration of 0.1 millisecond, there is obtained, for example, a rotary angle of only 3.5 arc minutes of the sealing device  10  during the duration of the flash. The sealing occurs thus extraordinarily rapidly and thereafter the surfaces  38 ,  45  are still pressed together for a sufficiently long period to result in a rapid cooling of the sealed seam. In this manner, very high output rates may be achieved. Large energy quantities may be introduced on purpose into the sealed seam. The pressing components of the shoes remain cold and cool the sealed seam immediately. Thick transparent material may be welded onto any desired absorbing material. The device is adapted also for a contactless sealing without a mechanical contact between the device and the films  18 . 
     The embodiment according to FIG. 3 differs principally from that of FIGS. 1 and 2 in that the reflector  26  and the lamp  29  are stationary and the window  36  is mounted on a carrier  51  which rotates, as indicated by the arrow  14 , about an axis  52  which is parallel to the axis  28  and which lies in a plane which contains the axes  28  and  13 . The space  35  may be closed by a further window  53  shown in a dash-dotted line in FIG.  3 . This arrangement makes possible to provide coolant circuit  39 . This embodiment has above all the advantage that the flash lamp  29  is less exposed to shocks and that the terminals at the generator  32  and the coolant circuit  39  are of simpler construction. 
     In case the hose  15  is intermittently advanced, the counter shoe  11  and the window  36 , instead of being rotated, may be moved linearly in suitable guides in the direction of the arrow  54  perpendicularly to the conveying direction A. 
     The device according to FIG. 4 differs from that of FIG. 3 in that the reflector  26  rotates whereas the flash lamp  29  remains stationary. The holder body  43  has a transparent window  61  and a reflecting, elliptical-cylindrical surface (counter reflector)  62  whose cross section complements the upper surface  27  of the reflector  26  in the focal point to an almost complete ellipse. The focal axis  28  of the reflector  26  is the rotary axis  12  of the reflector  26 . The other focal point  63  lies in the middle of the two films  18  to be welded together. This embodiment is particularly adapted for sealing partially transparent films  18  because the light which is directly radiated from the lamp  29  to the window  36  is concentrated by the upper surface  62  on the focal line  63 . 
     FIG. 5 shows a variant of the reflecting surface  27  of the reflector  26  which is structured in accordance with U.S. Pat. No. 4,641,315 and whose cross section is an involute. Such a cross-sectional configuration is useful mainly in the vicinity of the flash lamp  29  because of its radiation characteristics (opaque surface emitter). 
     The device according to the invention may also be driven in such a manner that a first flash produces a sealed seam and immediately thereafter a second, shorter but more intensive flash severs the hose  15  in the middle of the just-formed sealed seam. 
     The six figure pairs  6   a,b  through  11   a,b  show variants of the films  18  to be sealed. The left-hand illustration of each pair shows the films during application of optical energy, and the right-hand illustration of each pair shows the films provided with a sealed seam. 
     The films of FIGS. 6 a ,  6   b  are weakly absorbing. Particularly the apparatus according to FIG. 4 is adapted to provide them with a sealed seam. 
     FIGS. 7 a ,  7   b  show the sealing of substantially absorbing films  18 . In this case the heat admission to the sealed seam  58  is effected by heat conduction by and through the upper film  18 . 
     In the variant according to FIGS. 8 a ,  8   b  at least one of the films  18  is a compound film having an outer, transparent layer  59  and an inner, light-absorbing layer  60 . 
     According to the variant shown in FIGS. 9 a ,  9   b  the films  18  are transparent and an additional, light-absorbing strip  61  is sealed in between. 
     In the variant according to FIGS. 10 a ,  10   b  the film  18  facing the lamp  29  is transparent while the other film  18  is light absorbing. 
     In the variants according to FIGS. 8 a ,  8   b ;  9   a ,  9   b ; and  10   a ,  10   b  the energy is directly introduced at the location to be welded. In this manner, the sealing process is particularly rapid and efficient. 
     In the variant according to FIG. 11 a ,  11   b  both films  18  are transparent; the holder body  43  and/or its upper surface  45  is then light absorbing. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.