FIELD OF THE INVENTION The present invention relates to the art of packaging devices, and more particularly to L- sealers, hermetic sealers and the operation thereof.

BACKGROUND OF THE INVENTION L-frame sealing and packaging devices, commonly referred to as L-sealers, are used to wrap objects with a film and to seal and sever the film around the object to make a completely enclosed package. Typically in such devices, an object to be wrapped is first placed between the layers of a folded strip of commercially available shrink film. The object with its attendant film is then advanced into a sealing area of the machine. Upper and lower L-shaped seal bars are situated in the sealing area, such that when the upper seal bar is lowered onto the lower seal bar, an L-shaped seal is formed between the two layers of film and the film is severed along the seal.

The front longitudinal seal and the cross seal at the trailing end of the package formed by the L- shaped seal bars are sufficient to completely seal the package; the front side of the package already is sealed because of the rear cross seal from the previous package, and the film is folded around the back of the package. After sealing, the upper

seal bar is raised, and the newly packaged object is moved off the sealing area for further processing, often involving a heat tunnel which shrinks the film around the object. The sealing area is then ready for another package. Any one of the preceding steps, or all of them, may be either manual or automatic.

The seal itself is made by a heated sealing element, mounted in one of the seal bars, which presses the film layers against a seal bed, mounted in the other seal bar, for a predetermined period of time. Typically, the sealing element also severs the film along the seal. In heat sealing, the sealing element is hot during the entire time it is pressed against the seal bed, whereas in impulse sealing, the sealing element is heated and cooled while it is pressed against the seal bed. The sealing element is typically a thermal electrode consisting of a metal wire or blade.

Temperature and pressure are the two factors used in sealing, both of which must be precisely set for a uniformly good seal. The greater the pressure of the sealing element against the seal bed, the lower the sealing element temperature needs to be, and vice versa.

A longstanding problem in L-sealers is non- uniformity of pressure along the seal, caused by misalignment of the upper seal bar against the lower, or warpage of either seal bar as a result of the high temperature and pressure used in sealing. The upper seal bar is typically mounted on a U-shaped member whose free ends pivot on the back of the sealing machine. The free ends can be adjusted up or down to align the upper seal bar.

Such adjustments are delicate and difficult to make precisely, and even if done right, over time adjustments will again need to be made. In addition, such adjustments will not compensate for warpage of the seal bars.

Various attempts have been made to solve the problem of misalignment. Bennett et al., U.S. Pat. No. 4,650,535, discloses an L-sealer in which the upper and lower seal bars are locked together, and in which pneumatic cylinder assemblies push up structural members underlying the lower L-shaped seal bar, at the ends and corner of the L, to compensate for misalignment during sealing. However, such an operation appears to be cumbersome and slow in operation, and moreover, does not fully compensate for warpage. Shanklin, U.S. Pat. No. 3,490,981, discloses an L-sealer in which the upper seal bar is not mounted on a U- shaped member. Rather, the L-shaped seal bar itself is pivoted to the machine at its ends.

Such a configuration still requires precise adjustment, however, and warpage continues to be a problem.

A second problem with L-sealerε is operator safety. The pressure used to make a good seal is typically high, in order to allow relatively low operating temperatures, thereby avoiding fumes produced from burning the film. Therefore, in many prior art devices, the upper seal bar comes down with great force, endangering the operator who inadvertently has his hand or arm in the way. This problem has been addressed by installing sensors or limit switches in the machine, which are not always reliable and which sometimes do not arrest movement of the upper bar until after it

has hit the operator. Another attempt to solve this problem has been to decrease the force at which the upper seal bar is lowered, and once it is in sealing position, i.e., in contact with the lower seal bar with nothing but the film to be sealed between them, the two seal bars are locked together and only then is sealing pressure brought to bear. Patents disclosing this principle include Bennett et al., U.S. Patent No. 4,650,535, and Shanklin, U.S. Patent No. 3,490,981.

Merely decreasing the pressure at which the upper seal bar is lowered into sealing position, however, does not eliminate hazards to the operator. Especially in heat sealing, where the sealing element is heated before being pressed against the seal bed, the operator's hand or arm may be burned by the hot sealing element, exposed on one of the seal bars, before the upper sealing bar has been fully lowered. This problem is exacerbated by the sealing element's usual placement in the upper seal bar, the one which the operator is most likely to contact.

Another problem in L-sealers is that the seal quality might not be acceptable if there is any tension in the film layers during sealing. This can be a particular problem if the machine is automatic, since the film must be made taut in order to, ensure smooth movement of the film through the stages of the machine. Further, in the case of manually operable machines, there is no assurance that the operator will reduce tension in the film. Adams et al., U.S. Patent No. 3,653,177, proposes to solve this problem in a cross sealer apparatus by arresting the forward movement of the film beyond the seal area while

allowing the film behind the seal area to continue forward, thereby relieving film tension. This method, however, only works in automatic machines, and only with cross seals, not longitudinal seals. It is also expensive. Prior art methods of relieving film tension also slow the operating speed of the sealer.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to overcome the shortcomings associated with prior art sealers.

It is a further object of the invention to provide a sealer that produces a uniform pressure of the sealing element.

It is a further object of the invention to provide a sealer which compensates for any misalignment or warpage of the seal bars.

It is a further object of the invention to provide a sealer that is safer to operate than prior art sealers.

It is a further object of the invention to provide a sealer that automatically relieves the tension of the film in the immediate area of the sealing element, thus ensuring a high quality seal and increasing operating speed.

It is a further object of the invention to produce a hermetic sealing apparatus and method having all of the just-described advantages. In accordance with a first aspect of the invention, a sealing technique, for sealing a plastic film through the application of heat and pressure, employs a sealing assembly, having a sealing element thereon, adapted for generally vertical movement within a housing and a linearly

disposed pneumatic source operatively connected to the sealing assembly along substantially the entire length of the sealing assembly, for imparting the vertical movement to the assembly upon actuation of the pneumatic source. An abutting surface is provided and adapted to provide a reactive force against the vertical movement of the sealing assembly, and a source of heat for the sealing element is also provided. When the film is disposed between the sealing assembly and the abutting surface, the sealing element applies heat and pressure to the film, upon actuation of the pneumatic source.

In accordance with a second aspect of the invention, a sealing technique, for sealing plastic film through the application of heat and pressure, employs a sealing element disposed with a recess in a first housing and adapted for generally vertical movement relative to the first housing, and an abutting surface protruding from a second housing and adapted for generally vertical movement relative to the second housing. A first pair of clamping surfaces is provided on the first housing on either side of the recess, and a second pair of clamping surfaces is provided on the second housing on either side of the abutting surface. Means are provided for moving the first and second housings into an abutting relation¬ ship, such that when a plastic film is placed between the first and second housings, the abutting surface pulls the film into the recess and the first and second pairs of clamping surfaces clamp the film on either side of the recess, when the first and second housings are moved into abutting relationship, such that slack

is created in the film when the vertical movement is effected by the sealing element and abutting surface.

In accordance with still a further aspect of the present invention, a sealing technique employs a first housing, a seal bed in the first housing, and a device for biasing the seal bed outwardly from the first housing to cause it to protrude therefrom. A second housing, adapted to come into substantial contact with the first housing, has a sealing element recessed within it. The first and second housings' are locked together and the sealing element is moved in a direction leading out of the second housing, whereby the sealing element is pressed against the seal bed.

Preferably, the sealing element is segmented to thereby allow uniform pressure despite any warpage or misalignment that may occur.

^• In accordance with a further aspect of the present invention, a hermetic sealing technique employs pairs of opposed nichrome ribbons on either side of a cutting element, or a single pair of nichrome ribbons on either side of the cutting element, to effect hermetic sealing of plastic film, in connection with the above described sealing techniques.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, aspects, and embodiments of the present invention will be described with reference to the following drawing figures, of which:

FIG. 1 is a perspective view of an example of a sealing apparatus according to the present invention.

FIG. 2 is a side view along line 2-2 in FIG. 1.

FIG. 3 is a cross sectional view of the upper and lower seal bars of the sealing apparatus of FIG. 1.

FIG. 4 is a cross sectional view of the upper and lower seal bars of FIG. 3, with the bars in sealing (closed) position.

FIG. 5 is a cross sectional view of the upper and lower seal bars of FIG. 3, with the actuator activated.

FIG. 6 is a partial perspective view of the upper seal bar of the sealing apparatus of FIG. 1.

FIG. 7 is a partial perspective view of the lower seal bar of the sealing apparatus of FIG. 1.

FIG. 8 is a partial cross sectional view of the upper and lower seal bars in a hermetic sealing embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to a semi-automatic L-sealer 10, shown in Fig. 1, but it will be appreciated that the present invention can be used in other sealing apparatuses as well.

Fig. 1 depicts a semi-automatic sealing machine 10, supported by legs 12, and divided into a tabular packaging area 14 on the right and a tabular sealing area 16 on the left, as viewed by an operator standing in front of the machine.

A film dispenser 18 is affixed to the rear of the packaging area 14, and is used to dispense a roll of pre-folded film 20. The film 20 is oriented such that its fold is to the right as it approaches the packaging area 14.

A surface plate 22 is situated on the packaging area 14 directly in front of the dispenser 18, and between the upper and lower tri¬ angular prongs 24 and 26 of a film forming plow 28. The film forming plow 28 positions the film

20 for placement of an object to be packaged (not shown) between the upper and lower layers of film 20. The height between upper and lower prongs 24 and 26 may be adjusted by an adjustment bar 23 at the corner of the plow 28. Devices similar to the film forming plow 28 are known in the art, e.g., Shanklin, U.S. Patent Re. 30,010. A crank 25 is disposed on the front of packaging area 14 to adjust the area 14 forward or back to compensate for package size.

A U-shaped member 32 is mounted at its free ends 34 at the rear of the sealing area 16 on a pivot bar 36, Fig. 2, which in turn is pivotally secured at its ends to machine 10. An upper L-shaped seal bar 38, formed from a right side cross member 40 and a front longitudinal member 42, is mounted on the front and right side of the U-shaped member 32 by a support beam mounted on the member 32 and fitted into a slot 44 in the upper seal bar 38 (Fig. 3) . The front longitudinal member 42 abuts the right side cross member 40 at a point 41 behind the front end of the cross member 40, creating a protrusion 43. The left side of the member 32 is curved up and away from the sealing area 16 so that it will not contact the surface of the latter when the upper seal bar 38 is in lowered sealing position. Two metal magnet mates 46 protrude from the front of the member 32.

A lower L-shaped seal bar 48, formed of a right side cross member 50 and a front longitudinal member 52, is mounted on the surface of the sealing area 16 by a support beam mounted on the sealing area 16 and fitted into a slot 54 in the lower seal bar 48 (Fig. 3) . The front longitudinal member 52 abuts the right side cross member 50 at a point 51 behind the front end of the cross member 50, creating a protrusion 53. The lower seal bar 48 comes into substantially direct, continuous, and uniform contact with the upper seal bar 38 when the latter is in sealing position. Two electromagnets 56 are situated in front of the lower seal bar 48 in order to come into contact with the magnet mates 46 and lock the upper and lower seal bars 38 and 48 together when the upper seal bar 38 is in sealing position. Other locking means can readily be used, as will be apparent to those skilled in the art. A height-adjustable outfeed conveyor 58, adjusted by a crank 55 at the front of the sealing area 16, is situated on the sealing area 16 to remove newly sealed packages to the next stage of processing. Two buttons 60 are mounted in front of the sealing area 16 for initiation of sealing action. A lever 57 is located at the front the of sealing area 16 to adjust the height of the U- shaped member 32, as will be described below. Finally, a control panel 62 is placed at eye level above the sealing area 16 for easy viewing, which is used, e.g., to select impulse or heat sealing, temperature of the sealing element, length of seal time, and to select and display package count and other information, in a well known manner.

As shown in Fig. 2, a pneumatic cylinder 64 is secured to a support beam 66 on a bracket 65 at the rear and below the surface of the sealing area 16. The position of the bracket 65 may be horizontally adjusted along beam 66 to change the maximum height at which the member 32 will stop when in a fully raised position, to allow for larger or smaller objects to be packaged. The piston 68 of the pneumatic cylinder 64 is secured to a bracket 70 protruding rearwardly from the pivot bar 36. As will be described in more detail below, two tubes 71, each having an adjustable valve 73, lead from a pneumatic source 72 into the lower seal bar 48. Fig. 2 also shows a rear protrusion 74 from the U-shaped member 32, which is located so as to trigger a limit switch 76 when the member 32 is fully lowered into sealing position.

Referring now to Fig. 3, the upper seal bar 38 comprises a seal bed housing 78 substantially inverted U-shaped in cross section, and a seal bed assembly, indicated generally at 80, located in the housing 78, and adapted for vertical movement within the housing 78. The lower ends of the housing 78 have an inwardly larger cross section to form abutments 82. The cross section of the bottom most portions of the lower ends of the housing 78 are further increased inwardly, as shown at 84, and the lower tip of the assembly 80 is tapered to complement the smaller opening of the housing 78. Rubber grippers 94 are mounted on the lower tips of the housing 78. A stopping block 86 is situated at the top of the inside of the housing 78, with uniformly spaced springs 88 recesεively mounted on the stopping block 86,

between the stopping block 86 and the seal bed assembly 80. The seal bed assembly 80 has a wider cross section at its upper end to form shoulders 90, which are urged into contact with the abutments 82 by the springs 88.

A seal bed 92 is mounted at the lower tip of the assembly 80, and protrudes out of the housing 78. The seal bed 92 preferably comprises a dovetail shaped fiberglass substrate 93, a layer of silicon rubber 95, preferably on the order of

1/8" in depth, cemented to the bottom of substrate 93, and a teflon tape 97 disposed on the silicon rubber layer 95. The teflon tape 97 helps release the seal bed 92 after the sealing of sticky film, and the silicon rubber layer 95 contours the seal bed around irregularities. Other combinations of material may be used, as will be apparent to those skilled in the art.

Lower seal bar 48 comprises a sealing element housing 96 substantially U-shaped in cross section, and a segmented sealing element assembly, indicated generally at 98, located inside the housing 96. As shown, the housings 78 and 96 are constructed with substantially identical cross sections, and the assemblies 80 and 98 are constructed with substantially identical cross ' sections, to facilitate interchangeability of parts, but other configurations can be used, as will be appreciated by those skilled in the art in view of this discussion. Like the seal bed housing 78, the sealing element housing 96 has abutments 100, a narrowed opening 102, and grippers 104, intended to engage the grippers 94 when ^" in sealing position. Similarly, the assembly 98 has shoulders 106, a tapered upper end, and is

adapted for vertical movement within the housing 96. The assembly 98 is segmented as further detailed below. A continuous mylar tape 101 is secured to the bottom of assembly 98. A pneumatic 5 lifter 108 is situated at the bottom of the housing 96, between the housing 96 and mylar tape 101. The pneumatic lifter 108 can be of the "Pneuma-Seal" type manufactured by Presray Corporation of Pawling, New York. Other types of

10 actuators, e.g., electrical or mechanical lifters, also may be used.

The assemblies 80 and 98 also contain cooling passages 110 and 112, respectively, through which gas or liquid can be run. In the preferred

15 embodiment, cooling fluid only needs to be run in the passage 112 of assembly 98, but it can also be run through the passage 110 if desired. Also, fine channels 114 can be drilled or otherwise formed between the outer surface of the assembly

20 98 and passage 112, through which fluid (a liquid or gas) can be forced to the exterior of the seal bar 48.

A segmented sealing element mount 116 is mounted at the upper tip of the assembly 98. The

25 element mount 116 is preferably made of teflon coated ceramic, but fiberglass or other suitable material could be used. Like the assembly 88, the mount 116 is segmented, as detailed below. A sealing element 118 is mounted in the

30 element mount 116. The sealing element 118 is preferably a narrow nichrome sealing blade, on the order of 1/8 inch in width and 10-20 thousandths of an inch thick, a thickness of 18 thousandths of an inch having been found to work satisfactorily.

35 The edge of the blade meeting the film may be

shaped to a sharp edge, or otherwise, to make microfine seals in a particular film. A nichrome wire or other sealing element could also be used. The seal bed 92 protrudes outwardly from the bottom tip of the housing 78 as a result of the downward force imparted to the assembly 80 by the springs 88. Conversely, the sealing element 118, on assembly 98, is recessed within the gap between the upper ends of the U-shaped housing 96, since the pneumatic lifter 108 is normally in its deflated condition. The size of the gap between the upper ends of the U-shaped housing 96, typically on the order of about 3/8", is such that the operator's fingers or hands will not contact the sealing element 118, which may be hot, even if the operator accidentally places his hand on the lower seal bar 48.

As mentioned above, the element housing 96 and bed housing 78, as well as the element assembly 98 and bed assembly 80, are of identical construction in cross section to decrease the cost of manufacturing. Additionally, each of the housings 96 and 78, assemblies 98 and 80, stopping block 86, seal bed 92, pneumatic lifter 108, element mount 116, and grippers 94 and 104, may be economically manufactured by extrusion techniques, as can be seen by the perspective views of the upper seal bar 38 in Fig. 6 and the lower seal bar 48 in Fig. 7. As shown in Fig. 7, the sealing element assembly 98 and mount 116 are divided into segments, indicated generally at 99, each approximately one inch in length and closely fitted together. The segments 99 allow the assembly 98 and mount 116 to compensate for

misalignment or warpage in the seal bars by contouring to any irregularity. The seal bed assembly 80 can also be segmented, if desired. The construction of the upper and lower seal bars 38 and 48 has been described above with reference to the cross section views of Fig. 3. As shown in Fig. 1, to make a complete upper L- shaped seal bar 38, two extrusion-manufactured bed housings 78 and seal bed assemblies 80 will be used, one for the right side cross member 40, and one for the front longitudinal member 42. A section of the cross member 40, including the housing 78 and assembly 80, is cut away at 41 so that the seal bed of the front member 42 smoothly abuts the seal bed of the cross member 40, making the seal bed 92 continuous throughout the upper seal bar 38.

The lower seal bar 48 is constructed in a similar fashion. A section of the right cross member 50, including the housing 96, assembly 98, and element mount 116, is cut away at point 51. To ensure that the corner of the seal is continuously sealed and cut, the sealing element 118 of the front member 52 makes a right angle turn at 51 and runs alongside the sealing element 118 of the cross member 50 into the protrusion 53, at the end of which the sealing elements 118 terminate. In addition to making a continuous seal at the corner, the protrusions 43 and 53 also sever scrap film.

The front member 52 and cross member 50 of the lower seal bar 48 each contain a separate pneumatic lifter 108. As shown in Figs. 2 and 7, a tube 71 leads from the pneumatic source 72 into each pneumatic lifter 108. The pneumatic source

72 pressurizes and expands the pneumatic lifters 108 upon activation, the level of pressurization being regulated by the adjustable valves 73. In order to lengthen the useful life of the pneumatic lifters 108, the vertical displacement of the assembly 98 should be such that the pneumatic lifters 108 do not expand to their maximum extent when inflated by the pneumatic source 72. Although the pneumatic source 72 will ordinarily be a source of pressurized gas, such as air, a liquid can also be used if desired, and it is intended that the term "pneumatic source" as used herein cover both gas and liquid pressurization techniques. In operation of the sealing machine 10, as the operator pushes the object and its surrounding film from the packaging area 14 to the sealing area 16, as described below, film 20 is pulled from the dispenser 18. The top layer of the film 20 is pulled around the top of the upper prong 24 of the film forming plow 28, and the bottom layer is pulled around the bottom of the lower prong 26. This action turns the film 20 inside out and turns it a right angle, with the fold now to the rear of the machine 10 as the film 20 is advanced toward the sealing area 16. The operator places an object to be packaged on the surface plate 22, between the top and bottom layers of the film 20, and moves the object and surrounding film into the sealing area 16 and situates it so that the L- shaped seal will be made in the desired place. The leading edge of the film 20 is already sealed by previous sealing action.

Since the sealing element 118 is recessed into the lower seal bar 48, and the gap between the

edges of the element housing 96 is small relative to the operator's fingers and hands, as mentioned above, even if the operator touches the lower seal bar 48 during manipulation of the package, he is unlikely to touch the sealing element 118 and be burned by it.

The operator presses the buttons 60 to initiate sealing action. In response, the cylinder 64 is activated, which pushes the piston 68 and bracket 70 up, rotating the pivot bar 36 and lowering the U-shaped member 32. The cylinder 64 uses only enough pressure to lower the member 32 reasonably quickly, and is not used to provide sealing pressure. The member 32 thus is not likely to injure the operator if his hand or arm is in the way. If the limit switch 76 is not triggered within a specified time, i.e., if the member 32 has been obstructed before reaching sealing position, the cylinder 64 will discontinue lowering the member 32, in a well known manner, and return it to its original position.

If the member 32 is lowered into sealing position without obstruction, the limit switch 76 is triggered by the rear protrusion 74, activating the electromagnets 56. The electromagnets 56 then lock the magnet mates 46 into position, locking the upper and lower seal bars 38 and 48 together.

As shown in Fig. 4, the protruding seal bed 92 causes the film 20 to be pulled into the sealing area between the edges of the housings 78 and 96, as indicated by the inwardly directed arrows. The grippers 94 and 104 abut one another and grip the film 20 between them. The pneumatic source 72 is then activated, and supplies air or other fluid at

a selected pressure to the pneumatic lifters 108, through the tubes 71. As shown in Fig. 5, the pneumatic lifters 108 expand, pushing up the assembly 98, which in turn causes the sealing element 118 to press the film 20 against the seal bed 92. Under this pressure, the assembly 80 is forced upwardly, compressing the springs 88 into their recess in the stopping block 86, until the assembly 80 abuts against the stopping block 86. The retreat of the protruding seal bed 92 causes the excess film brought into the sealing area to create slack in the film 20, thus ensuring that there will be no tension in the film during sealing. The sealing element 118 then seals the film 20 under heat and pressure.

It will be appreciated that the segmented sealing element assembly 98 will compensate for any misalignment or warpage in the upper or lower seal bars 38 and 48, by conforming itself to any irregularity. The pressure exerted by the pneumatic lifter 108 will be uniform even while expanding more in some areas than in others, since gas in a single container exerts the same pressure at every point of the container. After the film 20 is sealed, the pneumatic source 72 is deactivated, causing the pneumatic lifters 108 to deflate and return to their original size. This action causes the assembly 98 to lower, releasing the pressure on the seal bed 92 by the sealing element 118. The seal bed 92 returns to its original position under force of the springs 88. The electromagnets 56 are then deactivated, and member 32 is returned to its original elevated position by compression of the cylinder 64, lowering the piston 68 and bracket

70, and rotating the pivot bar 36. The outfeed conveyor 58 is then activated, which removes the newly sealed package from the sealing area 16. At this point, the sealing area 16 is ready to seal another package.

The present invention also includes a hermetic sealing embodiment as shown in Fig. 8. In this embodiment, upper and lower seal bars 210 and 212 are similar in construction to the upper and. lower seal bars 38 and 48 in Figs. 1-7, except for the addition of nichrome ribbons 214 and 216, placed on the seal bed 220 and on the tip of the sealing element assembly 221, respectively, on opposite sides of a cutting element 218 which is mounted on the tip of the sealing element assembly 221. This construction carries out hermetic sealing through the abutment of the nichrome ribbons 214 and 216 when the upper and lower seal bars 210 and 212 are in sealing position. In such position, the nichrome ribbons 214 and 216 are heated and effect sealing on either side of the cutting element 218. The cutting element 218 is preferably a blade which merely severs the film against the seal bed 220. Alternatively, the nichrome ribbons may be used only on the sealing element assembly 221, or only on the seal bed 220, but faster sealing will be achieved when the nichrome ribbons are used on both assembly 221 and bed 220.

Although described in terms of an L-sealer, and a hermetic sealer, it will be appreciated that the present invention can be employed in other sealing apparatuses as well. Numerous other embodiments of the present invention are possible. For example, the springs 88 may be any type of resilient means, such as a pneumatic lifter

similar to the pneumatic lifter 108. Additionally, the sealing element and its adjacent components may be placed in the upper seal bar, while placing the seal bed and its adjacent components in the lower seal bar. If operator safety is not a concern, e.g., in fully automatic machines, film tension could be relieved by causing the sealing element to protrude and recessing the seal bed, and by placing the pneumatic lifter whereby it moves the seal bed instead of the sealing element. Other embodiments and variations of the invention will be apparent to those skilled in the art.