Abstract:
For a vertical form, fill, and seal machine, the motion profile of the sealing jaws is controlled to reduce turbulence of fluid in the tube thereby promoting a better seal. Additionally, the jaws have an endless stroke so that a consistent sealing pressure may be achieved even after the jaws have worn down with repeated use.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to U.S. patent application Ser. No. 10/625,875, filed on Jul. 23, 2003, naming Stefan Tobolka as its inventor. The related application is hereby incorporated by reference. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to an assembly for sealing containers; in particular, it relates to a novel sealing assembly for sealing a web of thermoplastic film or other materials into a series of containers.  
         [0004]     2. Description of the Related Art  
         [0005]     Sealing machines are used to package fluids, such as granular materials or liquids of various viscosities, from water to syrup in pouches. One type of package or pouch-making machine is a vertical form, fill and seal machine. With vertical form, fill, and seal machines, a heat sealable web material may be supplied from a roll. The flat web material may be unwound and formed into a vertically oriented tube which surrounds a fluid delivery device, such as a spigot. A sealing assembly may be located below the delivery device, sealing across the tube by heat pressing the two layers of web materials together through the use of a moving sealing head reciprocating toward and away from the tube opposite a backstop. Initially, a bottom seal may be made and a quantity of flowable material delivered to the tube. The tube may then be indexed downwardly, and another seal made above the bottom seal so as to form a pouch between the two seals. The second seal will also act as a first seal for the next-to-be formed pouch.  
         [0006]      FIG. 1  illustrates an exemplary vertical form, fill, and seal machine  100 , in which a flat web  112  of film is unwound from a roll  110  and sealed into a tube  114  surrounding a spigot  116  by a sealing head  118 . The spigot is used to deliver a fluid to the tube and a sealing assembly  122  may then be used to seal and cut filled pouches  120  from the tube  114 . The filled pouches may be discharged from the machine onto a conveyor belt  130 . The film of the web may be made of a heat sealable flexible, light-weight material, such as a polyethylene/polypropylene laminate or other similar plastic materials.  
         [0007]     The speed of machine  100  may be increased if, rather than metering out a suitable volume of fluid for each pouch before the upper seal to complete the pouch is formed, the tube  114  is filled above the level where the upper seal will be formed. Sealing devices that form a seal through a liquid-containing tube are known in the art. For example, U.S. Pat. No. 5,038,550 by Wirsig and Perret, discloses a vertical form, fill and seal machine with a pair of transverse heat sealing jaws, a pair of spreader fingers, and at least a pair of detucker fingers to pinch a longitudinal edge of the tubular film. Another example is the vertical form, fill, and seal device described in U.S. Pat. No. 6,164,042 by Tobolka. A drawback with sealing below the fluid level, however, is that if the sealing jaw is reciprocated quickly, it will impart turbulence to the fluid and consequent vibrations to the tube, which can result in an inferior seal being formed. This risks forming pouches that leak.  
         [0008]     Another drawback with known sealing devices is that as the sealing jaws wear down with repeated cycling, the sealing pressure between them reduces. This risks leaking pouches, which when this occurs necessitates re-calibration of the device or replacement of the sealing jaws.  
         [0009]     Therefore, there remains a need for a rapid sealing assembly that will minimize hydraulic turbulence and tube vibration, the occurrence of which may weaken the bond between the layers of the material forming the container, and that will also manage a consistent sealing pressure over many cycles.  
       SUMMARY OF THE INVENTION  
       [0010]     To seal a tube, such as a tube in a vertical form, fill, and seal machine, the motion profile of sealing jaws is controlled to reduce turbulence of fluid in the tube and vibration of the tube, thereby promoting a better seal. Additionally, the jaws have an endless stroke so that a consistent sealing pressure may be achieved even after the jaws have worn down with repeated use.  
         [0011]     According to one aspect of this invention, a device for sealing a tube containing a fluid is provided. This device comprises a pair of opposed jaws disposed about the tube, and a motor for driving each one of the opposed jaws toward the other. An indicator, such as a motor rotation indicator, is provided for indicating position of said jaws. A controller is input by the indicator and outputs to the motor for controlling a motion profile of the jaws.  
         [0012]     According to another aspect of this invention, a method for sealing a flexible tube containing a fluid is provided. This method involves driving opposed jaws disposed about a tube toward each other and controlling the speed and rate of speed of said jaws in order to reduce turbulence in the fluid and vibration of the tube as the jaws deform the tube. The jaws are driven into abutment with the tube interposed between the jaws a sealing pressure is applied for a dwell time, and then the jaws are retracted.  
         [0013]     Other features and advantages of the invention will become apparent from a review of the following description in conjunction with the figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     In the figures which disclose example embodiments of the invention:  
         [0015]      FIG. 1  is a perspective view of a conventional vertical form, fill and seal machine discharging onto a conveyor belt a succession of sealed pouches.  
         [0016]      FIG. 2  is the top view of a sealing assembly made in accordance with an embodiment of the invention.  
         [0017]      FIG. 3  is the front view of the sealing assembly of  FIG. 2 .  
         [0018]      FIG. 4  is velocity v. time diagram illustrating the velocity profile of a sealing head of the sealing assembly of  FIG. 3 .  
         [0019]      FIG. 5  is the top view of an alternative embodiment of a sealing assembly made in accordance with this invention, using a rack and pinion assembly instead of levers to move the sealing heads.  
         [0020]      FIG. 6  is an enlarged breakaway view of the rack and pinion assembly of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0021]     A sealing assembly is employed to clamp together layers of suitable materials, such as two layers of flexible laminate plastic film and to bond the layers by applying heat and a controlled amount of pressure. In one embodiment of the invention, the sealing assembly includes an upper and lower pair of jaws, each slidably mounted on a pair of parallel shafts, with a mechanism for effecting reciprocal movement of the jaws, together and apart, as required to clamp together the container forming material.  
         [0022]     Referencing the top view in  FIG. 2 , from a home position indicated by a rotary encoder  245 , coupled to motor  250  to track the rotational angle of the motor, the motor  250  turns ball screw  260 . The ball screw  260  is received by a threaded sleeve of a driving element, namely plate  255 . Thus, when the ball screw turns, this moves plate  255  either toward, or away from, stationary frame members  212 ,  214 . The stationary frame members  212  and  214  anchor the various moving assemblies of reciprocating upper jaws  205  and  215 .  
         [0023]     Upper sealing head (jaw)  205  is bearing mounted on smooth shafts  210  and  220  which are supported by frame members  212 ,  214  so that the sealing head  205  slides along the shafts  210  and  220  when pushed. Sealing head  205  incorporates an elongate heating ribbon  222  that is heated by a power source (not shown).  
         [0024]     Plate  255  is attached to the upper sealing head  205  through push rods  265  and  275 . A rocker arm  235  is medially pivotably joined to frame  212 . An inboard link arm  270  is pivotably mounted to the inboard end of the rocker arm and to plate  255 . Movement of plate  255  toward frame  212  pushes the inboard link arm  270  toward frame  212  which, in turn, pushes the inbound end of a rocker arm  235 . The rocker arm  235  is pivotably joined, at its outboard end, to an outbound link arm  230  which is, in turn, pivotably joined to a back support  225 . The back support  225  is fixed to an end of shaft  220 . An identical arrangement of arms  230   a ,  235   a ,  270   a  links plate  255  to back support  225  at smooth shaft  210 . A backstop (jaw)  215  is fixed to the two smooth shafts  220  and  210  so as to move with the smooth shafts  210  and  220 . Consequently, when plate  255  advances toward frame  212 , pusher arms  265 ,  275  push sealing head  205  toward the tube  200  and toward backstop  215 . Simultaneously, outbound link arms  230 ,  230   a , pull the smooth shafts  220 ,  210 , and therefore the backstop. In consequence, then the sealing head  205  advances toward the backstop, the backstop simultaneously advances toward sealing head  205 . The backstop acts as a back stop for sealing head  205  and may, for example, be fabricated of a high density plastic material.  
         [0025]     Referencing the front view of  FIG. 3 , the mechanism for reciprocating the sealing head and the backstop is repeated with a lower assembly comprising a lower motor  350 , lower ball screw  335 , lower sealing head  305  and lower backstop  315 . Also shown are the rotary encoder  345  of motor  350 , lower plate  355  threaded to lower ball screw  335 , and one of the lower smooth shafts  310 . Fluid delivery spigot  316  is shown here for delivering flowable material into the film tube (which is not shown in  FIG. 3 ).  
         [0026]     A processor  370  receives an output from each rotary encoder  245 ,  345  and outputs to a control input of each motor  250 ,  350 . The processor is loaded with software from computer readable media  372  which may be for example, a disk, a read only memory, or a file downloaded from a remote source. Additionally, the processor may receive input from a user interface  374  which may be, for example, a keyboard.  
         [0027]     In operation, considering the upper assembly, the advancement of plate  255  toward frame  212  moves the upper sealing head  205  and the upper backstop  215  toward one another to conduct sealing of the tube. Under control of the processor  370 , with input from the rotary encoder  245 , the motor rotates the ball screw  260  through an initial pre-set number of rotations, to advance the sealing head  205  and backstop  215  toward one another. The motor  250  is then decelerated, possibly to a momentary stop, in which case it is thereafter re-started. Once the motor  250  stalls out, indicating that the sealing head and backstop have been pushed against each other with the tube  200  sandwiched between them, the processor controls the motor voltage to generate a selected torque that provides a selected sealing pressure between the sealing head and backstop.  
         [0028]     The processor may maintain the sealing pressure for a pre-determined dwell time. Thereafter, the motor  250  is reversed to draw the sealing head  205  and backstop  215  away from each other, back to the home position. The encoder  245  has kept a count throughout so that the processor can accurately reposition back to the home position.  
         [0029]     The lower assembly works in exactly the same way. Consequently, both an upper and a lower seal may be applied to the film tube. It will be apparent that the upper and lower sealing assemblies are controlled independently by separate motors with no mechanical connection between the upper and lower sealing assemblies. Because of the independent operation of the upper and lower sealing heads  205  and  305 , the sealing heads  205  and  305  may be co-ordinated in any desirable manner through software control of the processor.  
         [0030]     The processor  370  may control each motor according to a stored motion profile to attain a desirable velocity versus time motion profile for each pair of jaws. For example, where the flowable material is water, the profile illustrated in  FIG. 4  may be used. Considering the lower sealing assembly, the movement of the lower jaws (sealing head  305  and backstop  315 ) toward each other can be divided into several phases. In the first phase, the processor controls the motor  350  to progressively accelerate in order to drive the lower jaws, from opposing directions, towards the film at a high rate of speed. This is indicated in  FIG. 4  between roughly 0 ms to 150 ms. When the jaws impact the tube, they impart hydraulic turbulence to the fluid in the tube and vibrations to the tube. To reduce this turbulence and vibration, the motor  350  decelerates in a time window during which tube impact is expected. This window roughly spans 150 ms to 360 ms. The most likely time of impact is 220 ms, in consequence, the motor is decelerated more rapidly prior to 220 ms and more gently after 220 ms. To further reduce this turbulence and vibration, the jaws stop, roughly between 430 ms to 500 ms as shown on the graph of  FIG. 4 , maintaining a gap between the two layers of film for a fixed period of time. The delay and gap are programmable through the motion controller to suit the liquid being packaged, preparing for the most critical moment when the two layers of film touch and the bonding process begins.  
         [0031]     The motor is then re-started to close the minimal gap between the jaws (between about 500 and 710 ms). As illustrated in  FIG. 4 , at about 710 ms the velocity of the jaws returns to zero as the motor  350  stalls when the jaws move into abutment with each other, with the tube sandwiched between them. At this point, the processor controls the motor&#39;s voltage in order to apply a selected torque to the motor. This selected torque is translated by the rotary-to-linear mechanism to result in the application of a selected force between the jaws. This force results in a selected sealing pressure on the film tube along the heated ribbon  222  of sealing head  205 . Once the sealing pressure has been maintained for a desired dwell time (710 to 980 ms), the motor is reversed to return the jaws to a home position (980 to 1190 ms).  
         [0032]     Where the flowable material is more viscous than water, the motion profile may cause the motor to decelerate in the time window of expected impact, but not stop thereafter. Instead, after decelerating during the window of expected impact, the motor may simply continue to advance the heads at a constant, or decelerating, rate, until the motor stalls.  
         [0033]     It is important to recognise that the selected sealing pressure is achieved without the processor having direct information on the distance through which it must advance the jaws so that they abut. Instead, the processor simply keeps advancing the jaws until they do abut. Thus, the jaws have a variable (endless) stroke: they will keep advancing until they abut. This has the advantage that even with the jaws wearing down over repeated cycles of operation, they will still achieve the selected sealing pressure. For example, considering  FIG. 4 , it may be that, after a million cycles, the motor will not stall out until 720 ms, due to wear at the face of the jaws. In such instance, the processor will begin to control the motor voltage to achieve the selected sealing pressure at 720 ms, rather than at 710 ms. This result is in contrast to, for example, a sealing assembly having fixed stroke cylinders. In this fixed stroke arrangement, wear on the jaws will result in a reduction of the sealing pressure. This will increase the number of seals that leak. Once the quality of the seals degrades to unacceptable levels, it would be necessary to take the machine out of service for re-calibration or replacement of the jaws.  
         [0034]     As aforenoted, the motion profile of the jaws reduces hydraulic turbulence before a seal is formed. This helps ensure the tube has a predictable shape when clamped by the jaws. In consequence, the chance of wrinkles at the seal are reduced. Furthermore, reducing the hydraulic turbulence allows the containers to have a precisely consistent volume. Coordinated motion profiles between the upper and lower sealing heads can be modified and stored to the processor to suit a range of tube diameters.  
         [0035]     The heat of sealing may also sever the tube at the bond. Alternatively, a knife associated with the jaws, or a knife below the jaws, may be used. It will be apparent that segments of the tube which have been filled, sealed, and severed form containers, which may be in the nature of pouches.  
         [0036]      FIGS. 5 and 6  show a different implementation of this invention, using racks and pinions, instead of link arms, to drive the jaws. Turning to  FIGS. 5 and 6 , wherein like parts to those of  FIG. 2  are given like reference numerals, plate  255  of the upper sealing assembly is joined to push rods  565 ,  575 . A rack  566  extends from an end of each push rod, which end is joined to the plate  255 . Similarly, a rack  522  extends from the end of each of shafts  510 ,  520 , which end is proximate plate  255 . Frame  512  supports two pinions  580 ,  590 ; each pinion meshes with both a rack  566  of one of the push rods  565  or  575  and a rack  522  of one of the shafts  510  or  520 . In operation, when the motor rotates ball screw  260  so as to advance plate  255  toward frame  512 , the push rods  565 ,  575  advance sealing head  205  toward the tube film (not shown) and the backstop  215 . Advancement of the push rods causes the rack of each push rod to rotate the pinion that meshes with it (in a counterclockwise sense). Because each rotating pinion also meshes with a rack  522  of a shaft  510 ,  520 , the pinion pulls each shaft  510 ,  520 . Since backstop  215  is joined to shafts  510 ,  520 , the result is that the backstop is pulled toward the tube and sealing head  205 .  
         [0037]     As an alternative to a rotary encoder, a position sensor may be used to sense the position of one of the linearly moving parts, such as the push rods. In such instance, the processor would learn the motor had stalled when the push rods were not moving despite the fact that the processor was applying a motivating signal to the motor. As an alternative to a rotary motor and ball screw, a linear motor may be used. In such case, the rotary encoder may be replaced with a motor position sensor or a sensor sensing the position of one of the other linearly moving parts. Obviously, the device could comprise only a single seal sealing assembly rather than the upper and lower sealing assemblies described.  
         [0038]     Other advantages and modifications within the scope of the invention will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.