Abstract:
The problems associated with the prior art have been overcome by the present invention, which describes a tubular heating element having a more reliable and accurate temperature monitoring system. The tubular heating element has a thermocouple attached to its exterior surface, in close proximity to the cutting and sealing portion of the tubular heating element. In addition to improving the accuracy of the temperature measurement, this technique reduces electromagnetic interference caused by the interaction between the heating wire and the thermocouple.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/558,472, filed Nov. 11, 2011, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Machines used to wrap and seal articles and packages in thermoplastic film are well known in the art. Two types of machines are commonly referred to as side-sealing and lap-sealing machines. In the typical side-sealing configuration, an article or set of articles travels, typically via a conveyor belt, toward the machine. A sheet of center-folded plastic film, having two layers, is fed from a direction, which is preferably perpendicular to the direction of the conveyor. The two layers of the film are then separated such that the article is placed between the lower layer and the upper layer. On one side of the article is the center-fold, while on the other side, there is an open edge where the two layers are not attached. The machine has several sets of belts to hold and guide the film, and a side sealing mechanism, which typically comprises a heating/sealing element that fuses or welds the two layers together and a cutting element that removes the excess material. In some embodiments, the heating element serves to cut the film as well. These elements, whether a unitary element or separate components, are referred to as the heating/sealing/cutting element throughout this disclosure. Thus, as the article passes by the side sealing mechanism, this open edge is sealed by welding the two layers together, the plastic is cut and the waste is removed and discarded. At this point, the plastic film resembles a tube, with openings at both the leading and trailing ends of the article, but sealed along both sides. As the article continues to advance, an end sealing mechanism is then employed to seal the film at the leading end of the article. The article is then advanced and the end sealing mechanism then seals the film at the trailing end of the article. 
         [0003]    Incomplete, inconsistent or sloppy welds can be problematic with these types of machines. The choice of heating/sealing/cutting element, film thickness and film speed are all factors in determining the quality of the seal. It is possible that different types of side sealing mechanisms may optimize seals for certain configurations. For example, tubular heating elements may optimize seals for high speed and/or thick films, while heated cutting blades may optimize seals for lower speed and/or thinner films. 
         [0004]    One potential issue associated with any side sealing mechanism is the ability to accurately control the temperature of the heating element. For some devices, such as hot wire heaters, the temperature is measured indirectly by monitoring a change in the length of the wire. For other heating elements, the temperature may be monitored directly, typically at a location away from the cutting surface to minimize the chance of damaging the thermocouple. This approach may work well where the heating element has a high thermal capacity, such as a cutting blade having a substantial mass. However, tubular heating elements have much less mass, and therefore more instantaneous temperature change and localized temperature variation. 
         [0005]    In addition, some tubular heating elements dispose the thermocouple within the tube adjacent to, or proximate the heating element. This proximity of the heating element introduces a source of error between the temperature being monitored and the actual temperature at the cutting surface. In addition, the proximity of the heating element to the thermocouple may also cause electromagnetic interference which affects the reliability of the data received from the thermocouple. 
         [0006]    Therefore, a tubular heating element for which the temperature of the cutting surface may be more accurately measured would be beneficial. 
       SUMMARY OF THE INVENTION 
       [0007]    The problems associated with the prior art have been overcome by the present invention, which describes a tubular heating element having a more reliable and accurate temperature monitoring system. The tubular heating element has a thermocouple attached to its exterior surface, in close proximity to the cutting and sealing portion of the tubular heating element. In addition to improving the accuracy of the temperature measurement, this technique reduces electromagnetic interference caused by the interaction between the heating element and the thermocouple. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates a representative side-sealing machine of the prior art; 
           [0009]      FIG. 2  illustrates a view of the side-sealing mechanism in accordance with the present invention; 
           [0010]      FIG. 3  illustrates a top view of the side-sealing mechanism shown in  FIG. 2 ; 
           [0011]      FIG. 4  illustrates the shape of a tubular heater and its relationship to the film; 
           [0012]      FIG. 5  illustrates a tubular heating element and thermocouple assembly according to one embodiment; 
           [0013]      FIG. 6  shows an isometric view of the tubular heating element of  FIG. 5  in the stowed position; 
           [0014]      FIG. 7  shows a front view of the tubular heating element in an operative position; 
           [0015]      FIGS. 8A-8B  illustrate the relationship of the tubular heating element to the film when in various positions; 
           [0016]      FIGS. 9A-9B  illustrate how the tubular heating element responds to a foreign material in its path; 
           [0017]      FIG. 10  shows another embodiment of the tubular heating element; and 
           [0018]      FIG. 11  shows the placement of a thermocouple in a tubular heating element according to the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  illustrates a representative side-sealing machine used to encapsulate or wrap an article in thermoplastic film, as described in U.S. Pat. No. 6,526,728. The machine  10  utilizes a conveyor belt  12  operating at a relatively constant speed to deliver articles  8  that are to be encapsulated. The thermoplastic film  1  is center-folded, such that the side with the fold is closed, while the opposite side  6  is open. On this opposite side, there are two layers of film  4 , 5 , which will later be sealed. This center-folded thermoplastic film  1  is fed from a reel (not shown) that is preferably mounted such that the film is fed perpendicular to the direction of travel of the conveyor belt  12 . The film is then inverted and separated by an inverter  13  such that the article is enveloped between the two layers  4 , 5 . At this point, the film  1  on one side of the article is closed, while the opposite side  6  remains open. Also, the film at both the leading and trailing ends of the article are not sealed. Downstream from the inverter is the side-sealing mechanism  20 . After proper relative positioning of the article between the layers of the film  4 , 5 , the enveloped article approaches the side-sealing mechanism  20 . 
         [0020]    The side-sealing mechanism  20  is located on the open side  6  of the enveloped article. The mechanism holds the two layers of film  4 , 5  together, and guides the layers through the heating and cutting means. It then welds the two layers together, and cuts off the surplus material. The surplus material is pulled away so as not to reattach to the film while it is still at an elevated temperature. 
         [0021]    As shown in  FIG. 2 , to perform these actions, the mechanism  20  preferably comprises two sets of cooperating pulleys, an upper set  101  and a lower set  102 . These sets work in unison to pull the two layers of film  103  into the mechanism and hold the layers in place. In the preferred embodiment, each of the pulleys has teeth  110  in its channel so as to accept one or more, preferably two, timing belts  120 . The presence of teeth  110  ensures that the timing belt does not slip relative to the pulleys. However, V belts can also be utilized with this invention, as well. The first set of pulleys  101  is located above the layers of film, while the second set  102  is located below the layers. Each set comprises a drive pulley  101   a  ,  102   a  and a tail pulley  101   b,    102   b.  There may optionally be one or more idler pulleys (not shown). Each of these pulleys also may have one or more O-rings mounted in the channel where the belts are located, so as to provide individual channels for each of the timing belts. 
         [0022]    Each of the timing belts preferably has a special gripping outer surface, that is bonded to a truly endless steel or Kevlar reinforced timing belt. Each corresponding set of belts has upper and lower pressure plates that are preset to insure good contact between the pair of belts. 
         [0023]    In one embodiment, as shown in  FIG. 3 , one set of O-rings  200  is positioned such that the movement of the outermost belt  210  is made to be parallel to the direction of the film movement. The outer wall of the pulley  240  and this first set of O-rings  200  provide the guides for the outermost belt  210 . As shown in  FIG. 3 , O-ring  200   a  and O-ring  200   b  are equidistant from the outer wall of their respective pulleys. A second set of O-rings  201  is used to guide the innermost belt  220  in a path that diverges away from the direction of the film and the outermost belt. This can be accomplished in a number of ways. For example, a combination of one O-ring and the inner wall of the downstream pulley  250   b  can be used to define the channel for the innermost belt  220 , as shown in  FIG. 3 . Similarly, two O-rings may be inserted on the upstream pulley to define a channel for the innermost belt. Alternatively, a single O-ring  201   a,  as shown in  FIG. 3 , can be used to define the inner wall of the channel for the innermost belt  220 . Because of the divergence angle, there are no forces pushing the innermost belt  220  toward the outermost belt  210 , thus the second O-ring may be eliminated. In other words, in the channel associated with the upstream pulley  240   a,  the O-ring  201   a  provides the inner guide for the belt  220 . In the channel associated with the downstream pulley  240   b,  the O-ring  201   b  provides the outer guide for the belt  220 . As a result, the innermost belt  220  is closest to the outermost belt  210  at the upstream pulley, and farthest away from it at the downstream pulley. The tubular heating element  230  is preferably located between the upstream and downstream pulleys. Thus, as the film passes the upstream pulley, it is still intact; however, it is cut before it reaches the downstream pulley. By introducing this divergence angle, the innermost belt  220  helps guide the unwanted surplus away from the film after it is cut. In one preferred embodiment, the innermost belt  220  is guided in the channel of the downstream pulley a distance further away from the film than on the upstream pulley sufficient to force the surplus plastic away from the film. One such suitable distance is about ¼ inch, although other distances may be used. This ensures that the surplus material does not reattach itself to the film while still at an elevated temperature. This surplus material is then held under tension and fed into a reel, which is later discarded. While the use of multiple belts, with a divergence between them is preferred, the use of a single belt, or multiple parallel belts is also within the scope of the present invention. 
         [0024]    The side-sealing mechanism  20  includes the tubular heating element  230 . As described above, this element is preferably located between the upstream and downstream pulleys, so that it can seal and cut the film before it is separated by the downstream pulley. The tubular heating element  230  may be a tube, having a circular cross-section. The tubular heating element  230  may be formed into an open oval, such as is shown in  FIG. 4 . The leading edge  231  of the tubular heating element may be semi-circular, and the first end  232  may contain the connections, such as electrical wires, which supply current allowing the tubular heating element  230  to be heated. The tubular heating element  230  may have an extended straight portion  234 , which terminates in a curvilinear trailing end  235 , which may be mounted or supported, such as by one or more brackets. In some embodiments, the tubular heating element  230  may be rigidly mounted to the machine  20 , and positioned such that a portion of the tubular heating element rests beneath the plane of film  237 . In other words, plane  237  is the horizontal level at which the tubular heating element  230  passes through the film. Thus, the leading edge  231  is responsible for initially heating and cutting the film. Although not shown in  FIG. 4 , the tubular heating element  230  may include an electrically actuated heater located within a cavity in the tubular heating element  230 . 
         [0025]    One issue with this tubular heating element  230  is the potential difference between the actual temperature of the leading edge  231  and the measured temperature. To measure the temperature of the tubular heating element  230 , a thermocouple  300  may be installed within a hollow cavity in the tube, as shown in  FIG. 11 . In this embodiment, the tubular heating element  230  has a connector  305  at first end  232 . This connector may have four connections; two of which provide current and a return path to the heating element  310 ; while the other two are for use with the thermocouple  300 . The heating element  310  extends a significant amount of the way into a hollow cavity in the tube  230 . The thermocouple  300  is preferably positioned near the cutting surface  231 . As best shown in the expanded view on  FIG. 11 , the heating element  310  and the thermocouple  300  are in close proximity as they extend into a hollow cavity within the tubular heating element  230 . In some embodiments, after the thermocouple  300  and heating element  310  have been added, a filler material (not shown) is used to fill the cavity. This filler material may have a high thermal conductivity such that the thermocouple  300  measures a temperature close to that of the tubular heating element  230 . The filler material serves to fix the heating element  310  and thermocouple  300  in place. 
         [0026]    One issue associated with this approach is that the current passing through the heating element  310  may affect the thermocouple  300 , or the transmission of data from the thermocouple  300  back to connector  305 . For example, the proximity of the heating element  310  to the thermocouple  300  may affect the temperature observed by the thermocouple, such as providing a reading higher than the actual temperature of the exterior of the tubular heating element  230 . Another issue is that the passage of current through the heating element  310 , which may be a coil having an inductance, may cause interference with the reading being transmitted by the thermocouple  300 , as these wires are positioned near to each other within the tube  230 . 
         [0027]    To overcome these problems and provide an improved method of measuring the actual temperature of a tubular heating element  230 ,  FIG. 5  shows a view of a tubular heating element  230  that uses an external thermocouple assembly  330 . One end of the thermocouple assembly  330  is a sheath  335  that attaches to, or is in contact with, the exterior of the tubular heating element  230  at a connection point  340 . This attachment at connection point  340  may be a physical connection, such as one caused by the heating of a material to thermally bond the sheath  335  to the tubular heating element  230 . Examples of this type of thermal bonding include welding, braising or soldering. In other embodiments, a mechanical coupling device, such as strapping, is used to hold the sheath  335  to the tubular heating element  230 . In other embodiments, the sheath  335  contacts the tubular heating element  230  without being attached or coupled thereto. For example the sheath  335  may be biased toward the tubular heating element  230 , such that a bias force serves to hold the sheath  335  in close proximity to the tubular heating element  230 . As demonstrated above, any method can be used to position the sheath  335  close to the tubular heating element  230  at the connection point  340 . While a permanent connection (such as a mechanical coupling or a heat related bond) may be preferable, other methods which hold the sheath in close proximity or in contact with the tubular heating element  230  may be used as well, such as bias force. 
         [0028]    A thermocouple (not shown) is positioned within the sheath  335 , preferably at the connection point  340 . If the thermocouple is positioned away from the connection point  340 , the sheath  335  may be filled with a thermally conductive material such that the thermocouple receives a temperature nearly identical to that of the exterior of the tubular heating element  230 . 
         [0029]    In another embodiment, the thermocouple is not enclosed in a sheath  335 . In this embodiment, the thermocouple itself may be soldered, welded, braised or mechanically coupled to the tubular heating element  230  at a connection point  340 , as described above. In addition, the thermocouple may be biased to remain in close proximity to the exterior of the tubular heating element  230 . Thus, while the use of a sheath  335  is disclosed, it is not required to be present in the present invention. 
         [0030]    The connection point  340  may be located anywhere on the tubular heating element  230 . However, in some embodiments, the connection point  340  is the location where the tubular heating element meets the plane of the film  237  (see  FIGS. 8B ,  9 A and  10 ). This location may give the truest indication of the tubular heating element at the point where it is actually contacting the film. 
         [0031]    In the embodiment shown in  FIG. 5 , a junction box  331  may be used to change the thickness and type of wires that are used. In some embodiments, the thermocouple uses a simple two wire connection. In some embodiments, it may be advantageous to use braided wires that are routed toward the side sealing machine. The conversion from simple wires to braided wires may be performed in junction box  331 . In other embodiments, the junction box  331  may be completely unnecessary and may be eliminated. These braided wires may be placed in a conduit  337 , and are in communication with a controller, which is responsible for monitoring and adjusting the temperature of the tubular heating element  230 . While the present invention utilizes braided wires, the conduit  337  may include one or more wires, which may be optionally bundled and encased in a tube, such as a plastic or metal liner. It should be noted that the position of the junction box  331  relative to the sheath  335  and conduit  337  is for illustrative purposes only. The exact position of the junction box  331  may be changed as desired. In other embodiments, the junction box  331  may not be present. 
         [0032]    In some embodiments, the controller receives data from the thermocouple, such as via conduit  337 . In response to this data, the controller determines the power to be applied to the heating element  310  within the tubular heating element  230 . In some embodiments, the controller may utilize a closed loop control system to maintain the exterior of the tubular heating element  230  at a desired temperature. Various types of closed loop control systems may be utilized, including proportional, derivative, integral, or a combination thereof, such as a PID control loop. 
         [0033]    The coupling between the sheath  335  and the tubular heating element  230  (at connection point  340 ) is sufficiently strong so as to tolerate the temperatures associated with sealing. The sheath  335  and connection point  340  are also rugged enough to tolerate the possibility of melted plastic becoming attached thereto. In some embodiments, the coupling at the connection point  340  is rigid such that any movement of the tubular heating element  230  results in a corresponding movement of the sheath  335 . 
         [0034]    In this configuration, the heating element  310  is still within the hollow cavity within the tubular heating element  230  (as illustrated in  FIG. 11 ). As described above, this hollow cavity may be filled with a filler material after the heating element has been installed to insure the stability and placement of the heating element  310 . Thus, in the embodiment shown in  FIG. 5 , the heating element  310  and the thermocouple have been physically separated from each other, thereby alleviating the problems associated with the configuration of  FIG. 11  described above. 
         [0035]      FIG. 6  shows a front view of a modular heating/sealing/cutting assembly  600  using a tubular heating element  230 . This tubular heating assembly  600  may also be used with a universal side mechanism, as disclosed in U.S. patent application Ser. No. 13/195,132, the disclosure of which is incorporated herein by reference in its entirety. The assembly  600  may have a round tubular heating element  230 , which is made of a metal, such as stainless steel. The tube  230  is heated through the application of power to a heating element, such as a coiled wire, located within a hollow cavity in the tube  230 . The power applied to the heating element may be a constant voltage and a variable current. In other embodiments, this power is a variable voltage. The power from the sealing machine  10  passes to the tube via a power connector (not shown). 
         [0036]    In some embodiments, the external thermocouple assembly is held in place, or supported by thermocouple bracket  350 . The thermocouple bracket  350  adds rigidity and support to the thermocouple assembly  330 . In some embodiments, the tubular heating element  230  is connected to an air cylinder  650 , as disclosed in U.S. patent application Ser. No. 13/195,117, the disclosure of which is incorporated herein by reference in its entirety. In one embodiment, the piston  660  (shown in  FIG. 7 ) is attached to the trailing edge  235  of the heating element  230 . The air cylinder  650  allows the tube  230  to be pushed downward toward the film, or pulled upward away from the film. The leading edge  231  of the tube  230  is pivotably attached to a point  651  on the tubular heating assembly  600 . This point serves as a hinge. In one mode, there is no or little air in the air cylinder  650 , and the tubular heating element  230  is in a stowed position.  FIG. 8A  shows the tubular heating element in the stowed position, where the tubular heating element  230  is raised above the plane of the film  237 . 
         [0037]    Air can then be introduced to the air cylinder  650 , so as to force the piston  660  to extend downward from the air cylinder  650 , as shown in  FIG. 8B . The air cylinder  650  causes the tubular heating element  230  to pivot about the hinge  651 . This causes at least a portion of the extended straight portion  234  to extend below the plane of the film  237 .  FIG. 8B  shows one active position for the tubular heating element  230  where the element extends beneath the plane of the film  237 . In this embodiment, the sheath  335  is rigidly attached to the tubular heating element  230 , so that it pivots as well, as shown in  FIG. 8B . 
         [0038]    In one embodiment, the air cylinder  650  is an adjustable stroke air cylinder. In this embodiment, the amount of extension, or stroke, allowed by the piston  660  is limited by an adjustable mechanical stop. Thus, the portion of the tubular heating element  230  which is intended to contact the film can be changed by adjusting the mechanical stop. In another embodiment, a cylinder having multiple stop positions may be used, thereby allowing different portions of the heating element  230  to contact the film. 
         [0039]    The thermocouple assembly  330  and the sheath  335  may be configured to pivot with the tubular heating element  230 . The conduit  337  and/or wires may be designed to accommodate some amount of rotation between the junction box  331  (if present), which pivots, and its connection to the side sealing mechanism, which does not pivot. In addition, referring to  FIG. 5 , the sheath  335  may be of any shape required. For example, as seen in  FIG. 6 , the sheath  335  may pass between other components located on the side sealing mechanism. In other embodiments, the sheath conduit  335  may have a different shape, and therefore the shape or size of the conduit is not limited by the embodiment shown in  FIGS. 5 and 6 . 
         [0040]    The above description relates to an air cylinder that is configured to be in the stowed position in the absence of applied air. However, other air cylinders may be used which are in the operative position in the absence of air. In these embodiments, air is introduced to move the heating element to the stowed position. Air is then removed to move the heating element to contact the film. 
         [0041]    The above embodiment discloses a tubular heating element having a pivotable leading edge, with a biasing member on the trailing edge. However, in other embodiments, the leading edge may be attached to the biasing element, while the trailing edge is pivotable. In another embodiment, the pivot point may be located between the leading edge and trailing edge. 
         [0042]    The use of an air cylinder  650  has other benefits as well. For example, the piston  660  is extended due to the force of the compressed air within the cylinder  650 . The force exerted by the air on the piston is not infinite, and can be overcome by an opposing force. For example,  FIG. 9A  shows the tubular heating element  230  in the operative position. However, a foreign material  700  is positioned on the film in the path of the tubular heating element  230 . As described above, with rigidly mounted heating elements, the foreign material may potentially damage the leading edge  231  of the tubular heating element  230 . However, in this embodiment, the force exerted by the foreign material  700  on the tubular heating element is sufficient to overcome the force of the compressed air within the air cylinder  650 . This then causes the piston  660  to retract from its extended position, and allow the tubular heating element  230  to be forced to its stowed position, as shown in  FIG. 9B . 
         [0043]    Thus, the use of an air cylinder  650  attached near the trailing edge, and a rotatable pivot  651  at or near the leading edge of the tubular heating element  230  allows many benefits currently not possible. This air cylinder  650  allows the use of at least two different positions, an operational position (such as  FIG. 8B ) and a stowed position ( FIG. 8A ). In addition, the air cylinder  650  allows the possibility to adjust the angle of the tubular heating element  230 , and therefore, the portion of the tubular heating element that contacts the film. Finally, the air cylinder also allows the tubular heating element  230  to automatically move out of the plane of the film, if confronted with a foreign material, in the path of the heater. 
         [0044]    Furthermore, the use of an air cylinder allows the movement of the tubular heating element  230  to be controlled automatically. For example, the side sealing machine  10  may include a controller. The controller consists of a processing unit, such as a microprocessor, PLC, embedded processor or other suitable device. The controller also includes a memory element adapted to store the instructions that are executed by the processing unit. In addition, the memory element may contain volatile data as required. The memory element may be a semiconductor memory device, such as RAM, EEPROM, FLASH ROM, DRAM or other technologies. It may also include magnetic or optical storage, such as disk drives, CDROMs, or DVDs. In one embodiment, the controller can be programmed to introduce air to the air cylinder prior to starting the pulleys, and programmed to draw air from the air cylinder when sealing is stopped or paused. Thus, the controller can control the position of the tubular heating element relative to the plane of the film prior to, during and after a sealing operation. In addition, in some embodiments, the controller may control the position of the tubular heater based on the type or thickness of the film being used. 
         [0045]    While the air cylinder offers these many benefits, in another embodiment, the only goal may be to create a mechanism that allows the heating element to move out of the plane of the film when confronted with a foreign material. In this case, as described above, the air cylinder may be used. However, other embodiments are also possible. For example, the air cylinder may be replaced with an extendible piston  710 , which is biased downward with a spring  711  or other biasing member, as shown in  FIG. 10 . In this embodiment, the foreign material would push against the downward force of the spring  711  or other biasing member and cause the tubular heating element  230  to rotate so as to be above the plane of the film  237 . This allows the foreign material to pass under the element  230 , without causing any damage to that element. In addition, any suitable biasing member may be used. For example, an electronic solenoid may also be used. 
         [0046]    While the present disclosure describes the use of air cylinders and other biasing members with tubular heating elements, the disclosure is not limited to this embodiment. For example, other heating/cutting/sealing devices, such as heated blades or hot wires may also benefit from the use of biasing members to allow movement relative to the plane of the film. 
         [0047]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.