Abstract:
A system and method for cutting and heat sealing polypropylene film and/or other separator material around individually shaped cathode, anode or other active components, for use in a battery or capacitor and/or other implantable medical device.

Description:
FIELD OF INVENTION  
         [0001]    The present invention pertains generally to a system and method for manufacturing components for implantable medical devices such as batteries or capacitors.  
         BACKGROUND OF THE INVENTION  
         [0002]    Devices for heat sealing thermoplastics are generally known. Examples of heat sealing apparatus are disclosed in the patents described below.  
           [0003]    U.S. Pat. No. 4,268,338 to Peterson shows a pressure die  17 , preferably of Delrin, having a profiled face  17   a  that provides a greater amount of pressure at the outer edge  17   b  of the die. In that manner, the outer edge  17   b  of the pressure die sinks into the thermoplastic material sheets P 1 , P 2  a distance sufficient to form a parting line while the remaining profiled face  17   a  applies pressure sufficient to achieve a weld between the two layers of thermoplastic material.  
           [0004]    U.S. Pat. No. 5,028,294 to England relates to a die for heat welding a plastic motif to a textile base and comprising a welding ledge  12   b  and a cutting edge  14 , best shown in FIG. 5.  
           [0005]    U.S. Pat. No. 3,577,820 to Silverstein pertains to a die for sealing or cutting thermoplastic material. Heated die  45  is machined from ribbon or bar stock of a material such as nichrome and inlaid in a groove  30  formed in a rigid plate  20  of insulated material. FIGS. 13 and 14 illustrate dies having a longitudinally extended rib  48  in the form of a raised portion.  
           [0006]    U.S. Pat. No. 4,055,456 to Carnegie, Jr. pertains to an impulse heat-sealing machine having an etched metal foil heater element  127  having a TEFLON® non-stick layer provided thereon. The heater element serves to seal two thermoplastic sheets together at a seam while a proximate knife blade  122  severs the joined sheets from the stock material.  
           [0007]    U.S. Pat. No. 3,614,383 to Watts, Jr. pertains to an apparatus for cutting and/or sealing plastic film, and includes an impulse-heated cutting device and a pressure pad provided on opposite sides of the film. The cutting device includes a support frame member and a resistance-heated element formed by a corrugated ribbon supported in the frame by a dielectric material. An edge of the resistance ribbon projects from the support frame towards a pressure pad  73  for cutting and/or sealing a film sheet when the film is compressed between the pressure pad and the resistance element.  
           [0008]    The problem with the prior art is that while it describes various heat sealing apparatus, the material to be heat sealed is not precisely cut to form. This can lead to waste with sealed envelopes of too little or too much material being used. In the former case, the envelope can be easily compromised, while the latter situation can lead to an improper fit as the excess material takes up space intended for other components. This is especially the case in electrochemical cells.  
           [0009]    What is needed is a system, for precisely cutting and sealing separator materials around electrode assemblies, that is reliable, economical and user friendly.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention meets the above-described need by providing a system and method for cutting and heat sealing polypropylene film and/or other separator material around individually shaped cathode, anode or other active components, for use in a battery or capacitor and/or other implantable medical device.  
           [0011]    The system includes a laser material cutting and perforating mechanism and a thermo-sealing apparatus.  
           [0012]    The laser cutting and perforating system provides for cutting guide holes and other openings in the separator with a high degree of accuracy. The guide holes in the separator are used to position the separator in the thermo-sealing apparatus for sealing.  
           [0013]    The thermo-sealing apparatus includes a fixture base that provides a mounting surface for a dielectric platform, acts as a heat sink, and provides a mounting surface for the necessary electrical connections.  
           [0014]    The dielectric platform acts as a precision track for retaining a heating element during repeated heating and cooling cycles. The dielectric platform also acts as an insulator during the sealing process.  
           [0015]    The track on the dielectric platform receives a heating element. The heating element is the portion of the fixture that does the actual cutting and sealing of the film. The geometry of the element is important to the process. The element is machined in two thicknesses. The raised or thicker portion provides the precise cutting dimension while the lower or thinner area provides a precision seal width. The element is chemically or mechanically machined on a thin metallic plate. The element design is unique to each individual sealing operation in size and shape.  
           [0016]    The lower platen of the thermo-sealing apparatus includes the dielectric material and the heating element. The dielectric material has a pair of openings for a set of retractable pins that provide for precise positioning of the cathode or anode assembly being sealed. The pins are retractable such that the assemblies can be removed from the device after sealing without damaging the assembly. Also, the pins can be retracted for storage. The lower platen also includes a set of locating pins for positioning the separator material.  
           [0017]    The upper platen includes a vulcanized, molded silicone rubber pad. The silicone rubber acts as a pressure pad for sealing.  
           [0018]    Both the upper and lower platens are covered with a pressure-sensitive adhesive tape to provide an insulating layer between the heating element and the rubber pad. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which:  
         [0020]    [0020]FIG. 1 is a front elevational view of the material cutting and perforating mechanism;  
         [0021]    [0021]FIG. 2 is a front elevational view of the thermo-encapsulating device of the present invention with an electrode assembly disposed therein;  
         [0022]    [0022]FIG. 3 is a cross-sectional view taken along lines  3 - 3  of FIG. 2;  
         [0023]    [0023]FIG. 4 is a detailed partial cross-sectional view of the heating element of FIG. 3 disposed in the track in the platform;  
         [0024]    [0024]FIG. 5 is a detailed partial cross-sectional view of the retractable locating pins shown in FIG. 3;  
         [0025]    [0025]FIG. 6 is a detailed partial cross-sectional view of the locating pins of FIG. 5 in the retracted position;  
         [0026]    [0026]FIG. 7 is a view taken along lines  7 - 7  of FIG. 2;  
         [0027]    [0027]FIG. 8 is a view taken along lines  8 - 8  of FIG. 2;  
         [0028]    [0028]FIG. 9 is a top plan view of the sealed electrode assembly of the present invention after sealing but prior to removal of the skeleton;  
         [0029]    [0029]FIG. 10 is a cross-sectional view taken along lines  10 - 10  of FIG. 9;  
         [0030]    [0030]FIG. 11 is a top plan view of the sealed electrode assembly of the present invention with portions removed to illustrate the screen; and,  
         [0031]    [0031]FIG. 12 is a top plan view of the sealed electrode assembly of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    Referring to FIGS.  1 - 12  and initially to FIG. 1, a continuous web of separator material  20  is stored on a film reel  23 . The separator material  20  may comprise polypropylene or other material suitable for use as a separator material for an electrode assembly. As known to those of ordinary skill in the art, for a battery having anode and cathode electrodes, the separator material is disposed between the anode and cathode materials to prevent contact between the active electrode materials to prevent a short circuit from occurring.  
         [0033]    The material  20  from reel  23  is fed into a cutting system  26 . The cutting system  26  is preferably a CO 2  laser galvanometer beam scanner system. A central processing unit  29  having a PLC controlled encoder is used to control the cutting device  32 . The system  26  also includes a reel to reel polypropylene material conveyor system that conveys the film from the supply reel  23  to a take-up reel  35  disposed on the opposite side of the system  26 . The material on the take-up reel  35  is capable of being separated into individual sheets for the sealing process described below. The laser cutting system  26  has the ability to mechanically position and accurately perforate the insulating material with a precise pattern of holes and slots that are unique to the individual design of the component being sealed and are used as locating features in subsequent operations.  
         [0034]    In FIG. 2, the heat sealing fixture  40  of the present invention includes an upper platen  43  that includes a first aluminum block  46 , a second aluminum block  49 , and a molded silicone rubber pad  52 . The molded silicone rubber may comprise silicone rubber having a durometer between  50  and  70 . Other materials and hardnesses may also be suitable. The silicone rubber acts as a pressure pad and is precisely positioned over the heating element in the lower platen. The shape of the die  52  will be described in greater detail below.  
         [0035]    The die  52  is covered by a pressure-sensitive adhesive tape  55  to provide protection from the heat associated with the heating element. The pressure-sensitive tape may comprise KAPTON TEMP-R-TAPE brand model number K250 protection tape. This protection tape is available from CHR Industries in New Haven, Conn. Other tapes having similar properties to this tape may also be suitable.  
         [0036]    An electrode assembly  60  (shown in FIG. 11 with the active material removed for clarity) sealed between two layers of separator material  20  is illustrated in its position between the upper and lower platens  43 ,  70 . The electrode assembly  60  includes a screen  63  having active material pressed thereto. The active material is disposed on both sides of the screen  63 . The separator material  20  is heat sealed around the electrode assembly  60  as described in greater detail below.  
         [0037]    The lower platen  70  includes a set of locating pins  73  for positioning the separator material. The laser cutting system  26  provides the separator material  20  with corresponding openings for receiving the pins  73  to accurately position the separator  20  for sealing. The lower platen  70  also includes a pair of retractable pins  77  (best shown in FIG. 5) having a very small diameter, for example, twenty-thousandths of an inch in some applications. At least one of the retractable pins  77  engages with an opening  80  (FIG. 11) in a tab  83  (FIG. 11) disposed between two sections of the screen  63 . The retractable pins  77  are spring-biased in the position shown in FIG. 2. By operation of a pivoting lever  86 , the pins  77  may be retracted so that the sealed electrode assembly can be removed from the fixture  40  without the risk of damage from the pins  77 . The retracting mechanism is described in greater detail below.  
         [0038]    The lower platen  70  is comprised of a dielectric material  89  mounted to an aluminum plate  92  that acts as a heat sink. The dielectric material  89  may comprise a machinable glass ceramic material. One suitable material is MACOR brand glass ceramic material available from Corning, Inc. in Corning, N.Y. The material has a precision track  95  (FIG. 7) defined therein for holding the heating element  98  in position (best shown in FIG. 4). The track  95  retains the heating element  98  during repeated heating and cooling cycles, and also acts as an insulator during the sealing process.  
         [0039]    The lower platen  70  is also covered by the pressure-sensitive tape  55  (KAPTON TEMP-R-TAPE brand model number K250 protection tape). Other tapes having similar properties to this tape may also be suitable.  
         [0040]    Opposite ends of the heating element  98  (Fig.7) extend downward through openings  101  in the dielectric plate  89  and terminate in a pair of copper terminals  104  that provide a connector between the heating element  98  and the wires from the power source  105 . The power source  105  is controlled by a rheostat.  
         [0041]    A sliding element  107  provides a means for locking the retractable locating pins  77  in the retracted position for storage. The sliding element  107  is mounted on a set of pins  110 ,  113  by means of a longitudinal slot  116 . The sliding element  107  slides left and right with respect to the orientation of FIG. 2. When the lever  86  is pushed downward and sliding element  107  is slid to the left, the lever  86  is prevented from moving into its spring-biased position and therefore, the pins  77  are held in the retracted position beneath the surface of the dielectric material  89 .  
         [0042]    Turning to FIGS. 3, 5, and  6 , the retracting pins  77  are shown in greater detail. The retracting pins  77  are mounted on the end of a shaft  119  that reciprocates in a bore  122  formed in the apparatus. The shaft  119  has a piston  125  designed to travel inside the bore  122 . A coil spring  128  is disposed around the shaft  119  inside the bore  122 . On the side of the piston  125  opposite the retracting pins  77 . The coil spring  128  biases the piston  125  such that the pins  77  are normally extending through the dielectric plate  89 . The pivoting lever  86  is attached at one end to a pivot  131  and is also attached to the shaft  119 . The opposite end of the lever  86  provides a handle for manually rotating the lever  86  about the pivot point  131 . When the lever  86  is rotated downward, as shown in broken lines in the figure, the shaft  119  is pulled downward against the force of the spring  128  and the pins  77  are retracted to prevent damage to the finished assembly as it is being removed from the lower platen  70  after the sealing has taken place.  
         [0043]    In FIG. 4, a detailed cross-section of the dielectric material  89  illustrates the precision track  95  and the heating element  98  disposed therein. The heating element  98  (best shown in FIG. 7) is a stainless steel, Ni-chrome or other metallic element that is heated by the power source  105  and that provides the cutting and sealing of the separator material  20  about the electrode assembly  60 . The element  98  is chemically or mechanically machined on a thin metallic plate. One embodiment of the heating element  98  is a 303 stainless steel element that is photo-etched on a thin metallic sheet. The element design is unique in size and shape for each individual sealing operation. The element  98  is machined in two thicknesses. The raised or thicker portion  134  provides the precision cutting dimension, while the lower or thinner area  137  provides a precision seal width. In some applications, the raised portion may be 8 to 10 thousandths of an inch wide across the top where it is flat.  
         [0044]    In FIG. 7, the lower platen  70  is shown. As shown, the heating element  98  rests in the precision track  95  that is cut into the dielectric plate  89 . The heating element  98  is designed for an electrode assembly  60  having two sections of rectangular shaped screen  63  connected by a relatively narrow tab  83 . Because the separator material  20  is a single sheet folded over the electrode assembly  60 , the heating element  98  only has to seal three sides of the separator material  20 . The heating element  98  has a flat portion  140  in it where the raised portion  134  (FIG. 4) is discontinuous. The flat portion  140  corresponds to the tab  83  when the electrode assembly  60  is placed into the fixture  40 . The flat portion  140  prevents the separator material  20  from being sealed to the tab  83 . The separator material  20  is not completely sealed to the electrode assembly because the tab  83  is the site of a subsequent welding operation. Also, it is preferable to have a short section of the separator material  20  unsealed in order to provide greater flexibility for the separator material  20  to allow for expansion of the intercalated materials.  
         [0045]    The larger guiding pins  73  for the folded separator materials  20  are disposed at opposite sides of the lower platen  70 . The retractable pins  77  are much smaller and may have a diameter as small as twenty-thousandths.  
         [0046]    Turning to FIG. 8, the vulcanized, molded, silicone rubber pad  52  has a durometer of 50 to 70 and is disposed on the upper platen  43 . The rubber pad  52  acts as a pressure pad during the sealing process. The rubber pad  52  has two rectangular openings  143 ,  147  with rounded edges that correspond to the shape of the electrode assembly  60 . The openings  143 ,  147  are slightly larger than the electrode assembly  60  such that the electrode assembly  60  is received in the openings when pressure is applied to force the upper and lower platens  43 ,  70  together. The rubber pad  52  applies pressure to the heating element  98  and to the periphery of the electrode assembly  60  to aid in the cutting and sealing of the electrode assembly  60 .  
         [0047]    Referring to FIG. 4, the separator material  20  that contacts the top portion  134  of the heating element  98  under pressure is vaporized. The separator material  20  that is disposed just inside the top portion  134  of the heating element  98  is pushed down onto the thinner portion  137  of the heating element where it is sealed against the layer of separator material  20  disposed on the other side of the electrode assembly  60 .  
         [0048]    In FIG. 9, the locating holes  150  in the separator  20  are shown. Also, the central opening  153  that aligns with the tab  83  that connects the two sections of the electrode assembly  60  is shown. The central opening  153  also provides an opening for positioning the locating hole  80  on the tab  83  of the screen  63  (shown in FIGS. 10 and 11) relative to the retractable pins  77  in the center of the fixture  40 . The outline of the screen  63  of the electrode assembly  60  is shown in broken lines. Because the separator material  20  is folded on one side, there are three sides that have to be sealed. Turning to FIG. 10, the separator material  20  is sealed around the electrode assembly  60  but the skeleton (excess material after the separator material  20  has been cut) has not been removed. A first end  156  of the separator material  20  is folded over the electrode assembly  60  and sealed there.  
         [0049]    In FIGS. 11 and 12, the final sealed assembly is shown with the skeleton removed. A uniform seal that is free of stringers, air pockets, cracks or inconsistent seal widths is produced by the present invention. The combination of a precision machined heating element  98  and a precision molded pressure pad  52  provides consistent accuracy.  
         [0050]    In operation, an individual sheet of separator material  20  is placed onto the heat seal fixture  40  such that the locating holes  150  on one end of the sheet are mounted onto the larger mounting pins  73 . Next, the electrode assembly  60  with the screen  63  and the active material pressed onto it is placed on the heat seal fixture  40  and is positioned by the retractable pins  77  which are accessible through the opening  153  in the separator material  20 . Next, the separator material  20  is folded over the electrode assembly  60  and the larger guiding pins  73  are inserted through the second set of locating holes  150  on the separator sheet.  
         [0051]    With the electrode assembly  60  and the separator material  20  precisely positioned on the fixture  40 , the sealing cycle is initiated. During the cycle, the temperature of the heating element  98  is controlled by the rheostat setting, and the pressure pad  52  is pressed against the fixture  40  at a pressure of approximately 80-100 psi for a predetermined time period. The cycle includes a dwell time, where the pressure is applied but the heat is removed. In order to safely remove the sealed electrode assembly from the fixture, the retractable locating pins  77  are retracted by the lever  86 . As known to those of ordinary skill in the art, the variables of the rheostat setting, cycle time, pressure, and dwell time may be varied depending on the size and shape of the electrode assembly  60  and depending on the properties of the separator material.  
         [0052]    While the invention has been described in connection with certain preferred embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.