Finishing assembly and method for a vertical form, fill and seal machine

Abstract

A stripping/sealing assembly and method for a vertical form, fill and seal machine. The stripping assembly comprises a rotatable master axis including a pair of oppositely rotating primary axes. A stripper may be associated with each of the primary axes, each stripper being located in a mount. A slave axis, comprising a pair of rotatable cranks mounted on central axes, is provided for displacement of the mounts in unison toward and away from the primary axes during rotation. By judicious retarding and then accelerating of the rotation of the cranks in relation to the primary axes, the stripper/sealer dies traverse linear paths along the tubular film to strip and seal the tubular film. The method provides adjustable seal dwell time, stripping and seal cooling time all programmable in software.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to vertical form, fill and seal (VFFS) machines, and in particular to utilization of two coaxial axes with a compound motion to generate a “D” path for the purpose of stripping, sealing and staging product handled by the VFFS machine.

[0002] Continuous motion VFFS machines have been used for years for high speed packaging of various products, such as potato chips. During the bagmaking cycle, plastic film is constantly moving through the VFFS machine. The motion of the stripper, sealer and stager in the VFFS machine is generally referred to as a “D” motion for rotary motion sealing systems. The “D” motion is a circular motion with one flat side. The flat side is the area in which the stripping, sealing and staging generally takes place. The sequence of operation is:

[0003] 1. Strip the product from the seal area. Stripping is always done at a rate which is faster than the film velocity. In this manner, any product in what becomes the end seal area is moved downwardly in the film and out of the sealing area. Stripping occurs either before the flat side in the “D” motion or during a portion of the “D” motion, depending on the design of the machine.

[0004] 2. Seal the film. Sealing takes place by matching the film speed with the speed of the sealing jaws along the flat side in the “D” motion. Severing packages generally takes place during the sealing phase of the bag making cycle.

[0005] 3. Stage the product. Staging takes place at any point along the flat side of the “D” motion to prevent product being inserted in the tubular film from impinging on a molten seal area that has yet to cool. This is especially important when films such as polyethylene are used in the packaging process. If the seal has cooled inadequately, the seal can be damaged by falling product impinging on the molten seal area.

[0006] U.S. Pat. No. 4,663,917 discloses one type of packaging apparatus for high speed packaging of various products. In one form disclosed in the '917 patent, a combination stripping, sealing and staging head is driven in a rotational motion by a single prime mover which rotates at a substantially constant velocity. Stripping occurs by using spring-loaded extensible arms to support the stripping, sealing and staging means. Stripping is achieved by changing the length of the extensible arms supporting the stripping head. The center of rotation of the head is fixed and the radius is varied while the heads are guided in a cam track. In this embodiment of the '917 patent, the stripping and sealing areas are fixed, since the axis of rotation is fixed and cam tracks define the paths of motion of the stripping, sealing and staging heads.

[0007] U.S. Pat. No. 5,753,067 teaches achieving a “D” motion by using a single prime mover which runs with a variable velocity profile which has an average velocity proportional to the desired package per minute output of the VFFS machine. In the '067 patent, the stripping, sealing and staging elements are located in a head which rotates about a central axis with a constant radius. The central axis is shifted by a complex turnbuckle mechanism which results in the “D” motion. The turnbuckle mechanism can be operated by a servo motor or stepper motor to determine the amount of displacement of the axes of rotation, and therefore the length of the flat side of the “D”. The device of the '067 patent can be programmed to change velocity of the head in relation to the film for the stripping and sealing operations. During stripping, spring-loaded stripper bars are moved along the tubular film at a rate faster than the film to strip the sealing area. After stripping has been completed, the speed of the head is matched to the speed of the film tube and a sealer engages the film during the lower portion of the “D” motion. During and following sealing, a stager is kept in contact with the film over the seal to protect the seal area from falling product.

[0008] There are several disadvantages in the apparatus of the '067 patent. First, because of the use of a complex turnbuckle mechanism, the apparatus is expensive and cannot operate at high speeds without rapid deterioration of the apparatus. Second, because of the geometry of the arrangement, the stager does not provide adequate staging time before the head begins its return rotation. Third, due to a constantly moving radius, the head never actually matches the speed of the film tube without also modifying the speed of the film tube to match that of the head. This introduces another degree of complexity into the apparatus, leading to breakdowns and unreliability.

SUMMARY OF THE INVENTION

[0009] The invention is directed to a relatively simple apparatus with which it is possible to match the speed of the head to the speed of the film for the entire “D” with relatively easy programming of a master axis. The invention includes a stripping assembly for a vertical form, fill and seal (VFFS) machine. The VFFS machine includes a source of plastic film, a product delivery head, a device for forming the film into a tubular configuration and a drive mechanism for advancing the tubular film as product is packaged therewithin. The stripping assembly comprises a rotatable master axis including a pair of oppositely rotating primary axes. A drive is provided for rotating the primary axes in unison. A stripper is associated with each of the primary axes, with a mount being provided for each stripper. Each mount is connected to one of the primary axes. A slave axis is also provided for the mounts, the slave axis including a displacement mechanism connected to each mount for selectively moving the mounts in unison toward and away from the primary axes.

[0010] In accordance with the preferred form of the invention, the master axis includes a support mounted on each primary axis. Each support has a slot formed therein, with one of the mounts being located in each of the slots. Each displacement mechanism comprises a rotatable crank mounted on a central axis and an arm is connected to the crank at an offset position. The arm is connected to one of the mounts. Preferably, the primary axes and the central axes are coaxial. A drive is provided for rotating the central axes in unison, with the drive preferably comprising a servo motor or a stepper motor.

[0011] In an alternative form of the invention, the drives can comprise a differential gear box, with a prime mover connected to the gear box for driving the primary and central axes. A further drive is connected to the gear box to provide a variable differential speed for the axes.

[0012] In all forms of the invention, a programmable control is provided for the drives. In this manner, the paths traversed by the strippers, as the primary axes rotate, can be altered to provide any variation of the “D” motion.

[0013] In the preferred form of the invention, each stripper comprises a stripper bar. Each stripper bar is mounted in a die which is connected to one of the mounts. Each die also includes a sealing mechanism for sealing the stripped tubular film, and each die also preferably includes a staging mechanism for protecting the molten seal.

[0014] In the operating of the invention, the master axis is activated to rotate the strippers in unison about the primary axes. The strippers are then displaced toward and away from the primary axes during a predetermined portion of each revolution of the primary axes to cause the strippers to follow generally linear paths along the tubular film at a speed faster than the tubular film in order to strip the tubular film. With the strippers being mounted in the slots, the displacing of the strippers necessarily is along the slots. Following stripping, sealing and severing of the sealed tubular film occurs in a conventional fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention is described in greater detail in the following description of examples embodying the best mode of the invention, taken in conjunction with the drawing figures, in which:

[0016] FIG. 1 is a representation of a simple “D” path traversed using a 5 inch radius of the master axis and a one inch radius of the slave axis, resulting in a crank throw of 2 inches and yielding an 8 inch linear leg in the “D” motion,

[0017] FIG. 2 is a view similar to FIG. 1, and showing the effective changing the radius of the slave axis on the “D” motion,

[0018] FIG. 3 represents the position of the crank arm as the path of the “D” motion is traced for each revolution of the apparatus according to the invention,

[0019] FIG. 4 illustrates a typical velocity profile for the master and slave axes showing the differential velocity change required to produce a simple “D” motion,

[0020] FIG. 5 is a top plan view, with portions eliminated, of one form of the invention,

[0021] FIG. 6 is a front view of the apparatus illustrated in FIG. 5,

[0022] FIG. 7 is a view similar to FIG. 5, taken along lines 7-7 of FIG. 6 with portions in cross section to illustrate detail,

[0023] FIG. 8 is a schematic system organization for the preferred embodiment of the invention,

[0024] FIG. 9 is one form of a spreadsheet which shows the relative position of the master axis in relation to the slave axis during the linear portion of a simple, straight “D” motion,

[0025] FIGS. 10A through 10E illustrate the ability of the invention to change the profile of the linear portion of the “D” motion to incorporate the requirements of stripping, sealing and other functions,

[0026] FIG. 11 is a top plan view, partially in cross section to illustrate detail, of a second form of the invention employing a differential gearbox,

[0027] FIG. 12 represents a velocity profile for the differential motor of the differential gearbox of FIG. 11 to provide a matched velocity of the output shafts with zero differential velocity, resulting in a simple linear portion of the “D” motion,

[0028] FIG. 13 illustrates another method of achieving position control with the differential motor being driven only in one direction,

[0029] FIG. 14 represents a velocity profile of the differential motor of the differential gearbox of FIG. 11 which provides a matched velocity of the two output shafts with a constant differential velocity,

[0030] FIGS. 15a through 15d schematically represent the motion of the apparatus illustrated in FIGS. 5 through 8, without the stripping, sealing and staging dies being illustrated,

[0031] FIGS. 16a through 16d are identical to FIGS. 15a through 15d, but with the stripping, sealing and staging dies in place,

[0032] FIG. 17 illustrates an alternate operation of the mechanism of FIGS. 5 through 8, without the stripping, sealing and staging dies,

[0033] FIG. 18 is identical to FIG. 17, but with the stripping, sealing and staging dies being illustrated, and

[0034] FIG. 19 is a schematic assembly drawing illustrating a complete sealing and stripping assembly consisting of front and rear jaws and the guide rods associated with this assembly.

DESCRIPTION OF EXAMPLES EMBODYING THE BEST MODE OF THE INVENTION

[0035] FIG. 1 illustrates a simple “D” motion which is the subject of the apparatus and the method of the present invention. Only one path is illustrated, and it will be evident that, since two sealing jaws and two stripper bars are employed, two “D” paths are traversed with the tubular film between them, the second path being the mirror image of that shown in FIG. 1.

[0036] Dimensions are provided in FIG. 1 for reference only. The size of the “D” can be changed depending on the mechanics of the machine and programming to provide a “D” configuration from a near semicircle to a complete circle. This is achieved by changing the radius of the slave axis having the crank arm, as described in further detail below.

[0037] The effect of changing the radius of the crank arm is schematically illustrated in FIG. 2. It should be obvious that a change in the radius of the master axis from the five inches illustrated results in far more possible combinations, but for the purposes of illustration, the radius of the master axis is fixed at five inches, and illustrated are modification of the crank radius in half inch increments between one half inch and two inches. FIG. 2 illustrates the wide variation in the linear portion of the “D” motion, which is depicted by the dimension “B” . One of the advantages of the present invention, as will become apparent from the apparatus when described below, is that a large linear dimension B can be achieved with a relatively small motion of the crank. This is illustrates in the sector degrees labeled C, which range from 74° to 156° in a crank range of ½ inch to 2 inches. As the linear portion of the “D” motion is expanded, greater stripping, sealing and staging time and length is possible, an advantage of the present invention not shared by the prior art.

[0038] As will become obvious from the description of the apparatus, in order for the crank to run constantly in the forward direction, that is, in the same direction as the direction of rotation of the master axis, the sum of the crank radius and the length of the crank arm must equal the radius of the master axis. For example, if the crank radius is 1 inch and the length of the connecting arm is 4 inches, the range of movement of the dies carrying the stripping, sealing and staging apparatus varies from a minimum of 3 inches to a maximum of 5 inches, assuming a 5 inch radius of the master axis.

[0039] FIG. 3 schematically illustrates the motion generated by the present invention between the extremes of 3 inches to 5 inches. Since the length of the crank arm is always constant, the “D” motion is formed by the motion of the slave axis, relative to that of the master axis. In the illustration of FIG. 3, the dimensions of FIG. 1 are assumed, with a 5 inch radius of the master axis, a 1 inch radius of the slave axis, and a 4 inch length of the crank arm.

[0040] In the majority of a full revolution, before the linear leg of the “D” motion occurs, the crank arm, depicted by the reference 10, is perpendicular to the circular path 12 traversed by the connection of the crank arm 10 in the slave axis. The outer end of the crank arm 10 thus traverses the circumference 14 of a 5 inch radius circle of the master axis.

[0041] When the position 16 is reached by the crank 10, the linear leg 18 of the “D” motion begins. In order for that to be accomplished, the slave axis begins to run slower than the master axis by appropriately retarding the motion of the crank arm 10 relative to the motion of the master axis, until the horizontal position 20 is reached. At the horizontal position 20, the velocity of the crank arm 10 must exactly equal that of the master axis.

[0042] As rotation continues, in order to continue to trace the linear leg 18, the slave axis must run faster than the master axis, in order to “catch up”. This continues until the crank reaches the position 22 at the bottom of the linear leg 18. At the position 22, the velocity of the crank arm 10 must again exactly match the velocity of the master axis. The crank arm 10 then continues with the master axis through the remainder of the revolution until the position 16 is reached, at which time the retardation and acceleration process of the slave axis in relation to the master axis begins once again.

[0043] FIG. 4 is a diagram of relative speeds involved in the simple “D” motion illustrated in FIG. 3. In FIG. 4, Vm represents the velocity of the master axis, and because of the retardation and acceleration of the motion of the slave axis, the relative velocity of the slave axis assumes the shape of a sinusoid for the length of the linear leg 18, in this example 106° (see the table in FIG. 2). The actual velocities and the profile of the velocity curve for the slave axis are dependent upon the actual motion desired, as well the mechanics and the geometry of the apparatus producing the “D” motion.

[0044] One form of the apparatus according to the invention is shown in FIGS. 5 through 7. The figures are truncated, not showing the opposite ends of connection for the dies, nor is the other half illustrated. There must be a pair of sealing dies, a pair of stripper bars and a pair of stagers employed, as will be evident to one skilled in the art, and as disclosed in greater detail in U.S. Pat. Nos. 5,377,474 and 4,391,079, which are assigned to the assignee of the present invention, and whose disclosures are incorporated herein by reference. For the purposes of description and illustration, only the “left” side is illustrated in FIGS. 5 through 7, although the orientation obviously is relative.

[0045] As illustrated in FIGS. 5 through 7, the apparatus 30 according to the invention has a master axis, operated by a prime mover 32, and slave axis, operated by a prime mover 34. The prime movers 32 and 34 are preferably servo motors or stepper motors, which are capable of being operated by a motion controller (see FIG. 8). Because of the geometry illustrated, the master axis and the slave axis are coaxial, with the slave axis being supported within the master axis.

[0046] The prime mover 32 of the master axis in connected to a drive gear 36 which in turn drives a mating gear 38. The gear 38 is fitted on a cylindrical spool 40 as shown in FIG. 7 extending from a master slide block 42. The spool 40 is mounted for rotation and supported by an inner bearing 44 and an outer bearing 46. The bearings 44 and 46 are appropriately supported, such as by a frame 48 which is schematically depicted in FIGS. 5 through 7. The prime mover 32, therefore, is situated to rotate the master slide block 42.

[0047] The prime mover 34 of the slave axis is attached to a shaft 50 by means of a coupling 52. The shaft 50 is supported within the spool 40 by means of an inner bearing 54 and an outer bearing 56.

[0048] The shaft 50 is secured in a crank shaft 58. The crank shaft 58 is therefore rotated by the prime mover 34, independent of rotation of the master slide block 42.

[0049] A journal pin 60 extends from the crank shaft 58 into a bearing 62 at one end of a crank arm 64. The opposite end of the crank arm 64 is connected by a pin 66 to a slidable mount 68. The mount 68 is located in a carrier slot or track 70 formed in the master slide block 42. The slot 70, while not shown in detail, supports the sliding motion of the mount 68 for each revolution of the master axis, and is appropriately formed of a friction-reducing material to reduce wear and provide a long life.

[0050] The mount 68 extends into a bearing 72 at one end of a stripping, sealing and staging die 74. The die 74 can be formed in any conventional manner, such as described in U.S. Pat. No. 5,753,067, the disclosure of which is incorporated herein by reference, and therefore the die 74 is not described in greater detail.

[0051] The prime mover 34 of the slave axis therefore is formed to drive the mount 68 back and forth in the slot 70 by means of the offset of the crank shaft 58. The radius from the center of the shaft 50 to the center of the journal pin 60 determines the extent of movement of the mount 68. For example, if the radius in one inch, then the total displacement of the mount 68 in the slot 70 is two inches.

[0052] The position illustrated in FIGS. 5 and 7 corresponds, in relation to FIG. 3, to the horizontal position 20. At this position, the crank shaft 58 has withdrawn the crank arm 64 to the fullest possible extent. The positions 16 and 22 (and elsewhere on the circumference 14) would be depicted, in relation to FIGS. 5 and 7, by the journal pin 60 being located at the opposite end of the crank shaft 58 in relation to that shown in FIGS. 5 and 7, with the mount 68 therefore extended to the right in the slot 70.

[0053] A preferred form of the invention is shown schematically in FIG. 8, and most portions, except for the particular master and slave axes according to the invention, are shown and described in greater detail in incorporated U.S. Pat. No. 5,377,474. Further detail is thus omitted.

[0054] Measure rolls 76 are preferably driven by servo motor 78. The measure rolls 76 meter the film from a supply roll (not illustrated), and therefore comprise an overall master axis for the packaging apparatus. Pull belts 80 and 82 are used to advance the film after it has been formed over a forming shoulder (not illustrated) into a tubular configuration. The pull belts 80 and 82 follow the measure rolls 76 in a predetermined ratio, indicated schematically by the ratio 84. The pull belts 80 and 82 are driven in opposite directions to one another, as indicated by the changed direction 86.

[0055] The master axis according to the invention drives the master slide block 42. It again follows the measure rolls in a predetermined ratio, indicated by the ratio 88. Film registration is maintained by a registration eye 90 in a conventional fashion.

[0056] The slave axis follows the master axis, and is controlled by a cam table 92 to provide the required linear leg in the “D” motion. The cam table 92 is described in further detail below.

[0057] A 360° revolution, or one cycle, of the master axis is that necessary to produce one package. That package cycle is now described for the simple “D” motion of FIG. 3 in relation to the apparatus of FIGS. 5 through 8.

[0058] For the majority of a single revolution, before the linear leg of the “D”, the angular velocities of the master axis and the slave axis are exactly matched, and the mount 68 is at its furthest-most outward excursion in the slot 70. The circumference 14 is therefore being traversed by the die 74. This is represented by the master axis positions 106° through 360° in FIG. 4.

[0059] At the position 16, the prime mover 34 begins to decelerate the rotation of the crank shaft 58 in relation to the rotation of the master slide block 42, commencing the sinusoidal velocity curve illustrated in FIG. 4. The speed differential continues until the position 20, at which the angular velocities of the crank shaft 58 and the master slide block 42 are again matched, and the apparatus configuration is at the position depicted in FIGS. 5 and 7, with the journal pin 60 having retracted the mount 68 to its inner most excursion. At this point, the prime mover 34 begins to accelerate the crank 58, returning the mount 68 to its outer most excursion at the angle 106°. The crank arm 64 is then fully extended, and the rotational speeds of the crank arm 58 and the master slide block 42 are matched for the remainder of the cycle, therefore tracing the curved portion of the “D” motion.

[0060] The motion is preferably controlled through a cam table which is incremented by the master axis. FIG. 9 is a compilation of the positions of the mount 68, identified as slide position, in degrees to the position of the crank arm 64, also expressed in degrees. This is a typical table which would be generated by a motion control program.

[0061] The slide position and the crank position start at 0°, which represent the start of the linear portion of the “D” motion at point 16 of FIG. 3. What is depicted in FIG. 9 represents the positions in the range illustrated from position 16 through position 20 in FIG. 3. A similar table, the inverse of the table shown in FIG. 9, would be generated for the positions 20 through 22.

[0062] When the “D” motion begins, an acceleration occurs from 0° to 27°. During this time, the crank length changes from +1 inch to 0inches as the crank rotates 90°. At this time, deceleration begins when the crank goes from 90° to 180°. When the crank is fully retracted at 180°, the slide axis is at 54°. This is the point where there is instantaneously a matched velocity between the master axis and the slave axis with the crank length at 1 inch, which is in the fully retracted position as illustrated in FIGS. 5 and 7. The remaining portion of the chart, not illustrated in FIG. 9, would be an exact duplicate, with the exception that the crank position would go from 54° to 108° and the slider would be fully extended at position 359° of the cam or 108° of the slide axis. Programming of this nature is well known to one skilled in the art, when given the parameters as expressed above.

[0063] FIGS. 10A through 10E represent a series of deviations to the straight linear leg 18 of the “D” motion illustrated in FIGS. 1 through 3. These deviations permit a series of operations, such as stripping followed by sealing and staging.

[0064] As is well known, stripping is accomplished by either programming the strippers to remain slightly open, or providing them with means to remain open while at the same time allowing the strippers to advance faster than the advancing tubular film. This allows the product to be stripped from the seal area in the bag being formed before the sealing jaws close on the seal area. The profiles illustrated in FIGS. 10A-10E show various lengths and depths of stripper motion which are possible in accordance with the present invention. Unlike the prior art, the length of stripping can be programmed in the cam table, altering the straightness of the linear leg 18 to become a profile, as illustrated in FIGS. 10A-10E. In the various depictions illustrated, stripping occurs in the first portion of the linear leg, followed then by sealing and, when needed, staging above the seal.

[0065] FIG. 10E is shown to illustrate the profile which would normally be used to seal polyethylene. In this case, following the stripping operation, the package is sealed by pressing the film between the sealing surfaces briefly to melt the film. Once melted and fused, the jaws are opened and cooling takes place. The cooling is generally supplied by blowing air onto the semi gelatinous material to solidify the seal before the jaws are opened to release the sealed package. Without this cooling phase, it would be impossible to run polyethylene on these machines at high speeds.

[0066] The profiles are generally shown with sharp corners which indicates a significant change in velocity of the motor. This typically results in extremely violent motion which causes audible noise and excessive war on the machinery. This is traditionally resolved by rounding the corners which allows the motions to be much smoother, quieter, and resulting in less wear. The cam profiles presented illustrate the general concept of gap as a function of distance and are not intended to be indicative of the final design of the cams to allow smooth operation.

[0067] The actual operation of the mechanism which controls motion is schematically illustrated in FIGS. 15a through 15d.

[0068] In these drawings, the master slide block 42 of the master axis is shown driving a second master slide block 142. The two slide blocks 42 and 142 are linked by appropriate gearing (not illustrated) in a 1:1 ratio to rotate in opposite directions as indicated. The master slide block 42 is used to carry the die 74 and the second master slide block 142 is used to carry the mating die (75) of the master slide block 142. The master slide block 42 is driven by the prime mover 32.

[0069] The master slide blocks 42 and 142 each have a crank shaft 58 and 158 which are similarly geared together in a 1:1 ratio and driven by the prime mover 34. As shown in FIGS. 15a through 15d, the prime mover 34 is generally driven in an opposite direction to the primary axes of the slide blocks 42 and 142 with a programmable velocity relative to the master slide blocks 42 and 142.

[0070] The slide blocks 42 and 142 each have a die carrier slot 70 and 170 in which die carrier slides 68 and 168 are driven. Journal pins 60 and 160 are attached to the respective crank shafts 58 and 158, and are synchronized as illustrated by the appropriate gearing (not illustrated) between the crank shafts 58 and 158.

[0071] Attached to the journal pins 60 and 160 are crank arms 64 and 164. The other ends of the crank arms 64 and 164 are connected to respective pins 66 and 166 to control the positions of the slides 68 and 168 in the slots 70 and 170. Rotation of the crank shafts 58 and 158 without rotation of the master slide blocks 42 and 142 would produce a harmonic motion analogous to that of an internal combustion engine.

[0072] The “D” motion is generated by the relative motion between the master slide blocks 42 and 142 and the crank shafts 58 and 158 as controlled by the prime movers 32 and 34.

[0073] FIG. 15a shows the position of the axes which would generally begin at the position 16 at the top of the straight line segment of the “D” motion which provides the sealing profile. At this point, as illustrated in corresponding FIG. 16a, the stripping, sealing and staging die 74, mounted on the slidable mount 68, just begins to touch the stripping, sealing and staging 174 on the slidable mount 168.

[0074] FIG. 15b shows the position of the mechanism as it would appear when halfway down the portion of the “D” motion, which corresponds to the position 20 shown in FIG. 3. The positions of the dies 74 and 174 are illustrated, correspondingly, in FIG. 16b.

[0075] FIG. 15c shows the position of the mechanism at the bottom of the straight line segment of the “D” motion, corresponding to the position 22 of FIG. 3. At this point, the stripping, sealing and staging dies 74 and 75 (FIG. 19) will begin to open.

[0076] FIG. 15d illustrates the farthest excursion of each of the slidable mounts 68 and 168. The corresponding positions of the dies 74 and 174 are illustrated in FIG. 16d. This represents the smallest maximum opening between the dies 74 and 174, based upon a counter rotation of the master slide blocks 42 and 142 in relation to their respective crank shafts 58 and 158.

[0077] It is possible to operate the apparatus of the invention with a greater gap between the dies 74 and 174, which may be necessary when packaging extremely large bags. In this case, the crank shafts 58 and 158 can be programmed to position the slidable mounts 68 and 168 in a fully retracted position as illustrated in FIGS. 17 and 18. In this configuration, the length of the flat, vertical portion of the “D” motion is limited by the radius of the crank shafts 58 and 158. This, therefore, limits the motion profile of the dies 74 and 174, and also the transition from the bottom of the D (position 22 shown in FIG. 3) to the top of the D (position 16 shown in FIG. 3) becomes very violent and would not be advisable at high operating speeds of the apparatus 30.

[0078] A second embodiment of the invention is illustrated in FIG. 11. Where elements remain the same as the first embodiment illustrated in FIGS. 5 through 7, identical reference numerals have been used. The nature of, and operation of, those elements remains identical, and will therefore not be repeated.

[0079] In this configuration, a single prime mover 94, again preferably a servo motor, stepper motor or the like, is used to drive both the master axis and the slave axis. Gear ratios between the axes are determined by a differential gearbox 96. The differential gearbox 96 offers the advantage that although two axes continue to be required, one prime mover 94 can be used to drive both.

[0080] Control of the slave axis, and therefore rotation of the crank shaft 58 in relation to the rotation of the master slide block 42, is by means of a second, smaller prime mover 98 which controls the gearing within the differential gearbox 96. For example, a typical configuration would be to have the master axis and the slave axis run at matched angular velocities, if the prime mover 98 is in a stalled or stopped condition. However, if the motor 98 is operated in one direction, the relative velocity of the crank shaft 58 will accelerate in relation to the velocity of the master slide block 42 by an amount proportional to the speed of the prime mover 98. Likewise, if the prime mover 98 is operated in the opposite direction, the relative velocity of the crank shaft 58 will slow in relation to the velocity of the master slide block 42, again by an amount proportional to the speed of the prime mover 98. In this way, the relative position of the slave axis to the master axis can be precisely controlled for a program cycle identical to that previously described above.

[0081] To illustrate the relative velocity and position of motion with the differential gearbox embodiment of FIG. 11, the velocity profiles are as illustrated in FIG. 12. The profiles of FIGS. 12 are similar to those of FIG. 4, but the average velocity of the differential gearbox is now 0 rather than the velocity of the master axis. It will be noted that in this embodiment, the prime mover 98 would actually reverse direction at the midpoint 20 of the linear leg 18 of the “D” motion.

[0082] One way to eliminate the reversal of direction is shown in FIG. 13. This simple change takes advantage of the 180° rotation of the crank shaft 58 and allows motion to be achieved with a constant forward or single motion of the prime mover 98.

[0083] Another method of handling this differential and to avoid reversal of the prime mover 98 is to change internal ratios in the differential gearbox 96 to require a relative motion of the differential to match the velocity of the mount 68. This is illustrated in FIG. 14. In this configuration, the prime mover 98 has a nominal average velocity to ensure that the velocity of the crank shaft 58 matches that of the master slide block 42, from 106° to 360° in the particular embodiment illustrated. During the first portion of the curve, the differential motor first slows to a nominal velocity which is lower than the matched speed, for the first 53°. The remaining 53° are then operated at a higher velocity than matched speed, in exactly the same manner as previously described in relation to the first embodiment of the invention. Of course, as above, the 106° is an example only, and will vary depending on the parameters chosen.

[0084] FIG. 19 depicts a schematic view of the entire sealing and stripping system with the stripping and sealing dies shown in a closed or sealing position. Many of the components have been eliminated for clarity. The front sealing and stripping die 74 and the rear sealing and stripping die 75 are shown being driven by the master axis prime mover 32 through the master slide blocks 42 and 142. These dies 75 and 74 are illustrated as being aligned by guide rods 110 shown mounted to the front sealing and stripping die 74.

[0085] The slave axis driven by prime mover 34 is shown driving the front an rear crank shafts 58 and 59. The operation of the system has already previously been described.

[0086] While two preferred embodiments of the invention have been illustrated and described in detail above, it will be obvious that various changes and modifications can be made to the invention without departing from the spirit thereof or scope of the following claims.

Claims

1. A stripping/sealing assembly for a vertical form, fill and seal (VFFS) machine, the VFFS machine including a source of plastic film, a product delivery head, a device for forming the film into a tubular configuration and a drive mechanism for advancing the tubular film as a product is packaged therewithin, the stripping/sealing assembly comprising

a. a rotatable master axis including a pair of oppositely rotating primary axes,

b. a drive for rotating said primary axes in unison,

c. a stripper/sealer associated with each of said primary axes,

d. a mount for each stripper/sealer, each mount being connected to one of said primary axes, and

e. a slave axis for said mounts, said slave axis including a displacement mechanism connected to each mount for selectively moving said mounts in unison toward and away from said primary axes.

2. The stripping/sealing assembly according to claim 1 in which said master axis includes a support mounted on each primary axis, each support having a slot formed therein, one of said mounts being located in each of said slots.

3. The stripping/sealing assembly according to claim 2 in which each said displacement mechanism comprises a rotatable crank mounted on a central axis and an arm connected to said crank at an offset position, said arm being connected to one of said mounts.

4. The stripping/sealing assembly according to claim 3 in which said primary axes and said central axes are coaxial.

5. The stripping/sealing assembly according to claim 3 including a drive for rotating said central axes in unison.

6. The stripping/sealing assembly according to claim 5 in which each said drive comprises a servo motor.

7. The stripping/sealing assembly according to claim 5 in which each said drive comprises a stepper motor.

8. The stripping/sealing assembly according to claim 5 in which said drives comprise a differential gearbox, said gearbox including a prime mover for driving said axes.

9. The stripping/sealing assembly according to claim 8 including a drive for said gearbox to provide a variable differential speed for said axes.

10. The stripping/sealing assembly according to claim 5 including a programmable control for said drives.

11. The stripping/sealing assembly according to claim 1 in which each stripper comprises a stripper bar, each stripper bar being mounted in a die connected to said mount.

12. The stripping/sealing assembly according to claim 11 in which said dies include a sealing mechanism.

13. The stripping/sealing assembly according to claim 11 in which said dies include a staging mechanism.

14. A method of stripping and sealing an advancing tubular film in a vertical form, fill and seal (VFFS) machine, the VFFS machine including a source of plastic film, a product delivery head, a device for forming the film into a tubular configuration and a drive mechanism for advancing the tubular film as product is packaged therewithin, and the VFFS machine further including a stripping and sealing assembly comprising a pair of stripper/sealer jaws, each stripper/sealer jaw being mounted for rotation about spaced, primary axes with the strippers being located on opposite sides of the tubular film such that the tubular film is contacted between the stripper/sealer jaws for each revolution of the primary axes, the method comprising the steps of

a. rotating the stripper/sealer jaws in unison about the primary axes, and

b. displacing the stripper/sealer jaws toward and away from the primary axes during a predetermined portion of each revolution of the primary axes to cause the stripper/sealer jaws to follow generally linear paths along the tubular film at a speed faster than the tubular film to strip the tubular film.

15. The method according to claim 14 in which the stripper/sealer jaws are mounted in slots in opposite arms rotatably mounted on the primary axes, and method step “b” comprises displacing the stripper/sealer jaws along the slots.

16. The method according to claim 14 in which each stripper/sealer jaw comprises a stripper mounted in a die, the dies further including a sealing mechanism, and including the further step of sealing the tubular film after method step “b”.