Abstract:
A buffer apparatus configured to hold a stack of sheets includes an input, first and second side guides each having a face facing transverse to the travel direction, and a backstop facing the input and spaced from the input in the travel direction. The first side guide, second side guide and backstop define a hopper for holding a plurality of the sheets. First and second retractable supports are shiftable from first positions in which a first portion of the respective retractable support projects a first distance into the hopper and a second position in which the first portion of the respective retractable support projects a second distance into the hopper or does not project into the hopper, the second distance being less than the first distance. The at least one second retractable support is located between the input and the at least one first retractable support.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 62/084,175, filed Nov. 25, 2014, the entire contents of which is hereby incorporated by reference. 
     
    
     TECHNOLOGICAL FIELD 
       [0002]    The present disclosure is directed to a buffer and to a stacking system including the buffer and to a method of buffering a stream of sheets using the same. 
       BACKGROUND 
       [0003]    A conventional stacking apparatus  10  is illustrated in  FIG. 8 . The stacking apparatus  10  is configured for use adjacent to a rotary die cut machine  12  which cuts blanks (not illustrated) from sheets of material, for example, corrugated paperboard. The stacking apparatus  10  includes a receiving or “layboy” section  14  that receives the blanks from the die cut machine  12  and discharges them onto a transfer conveyor  16 . The transfer conveyor  16  carries the blanks to an inclined main conveyor  18 , and the blanks travel along the main conveyor  18  to its downstream end  20  where they are discharged into a hopper  22 . 
         [0004]    After the blanks are discharged from the downstream end  20  of the stacker, they impact a backstop  24  and fall onto a receiving conveyor  26  mounted on a lift table  28 . As the stack  30  on the lift table  28  grows, the lift table  28  drops, either continuously or periodically, so that the sheets are always falling approximately the same distance from the downstream end  20  of the stacking apparatus  10  onto the lift table  28  or onto the partial stack  30  on the lift table  28 . Alternately, the stack may fall onto a fixed-height conveyor and the end of the main conveyor may rise to maintain a constant distance from the top of the stack. 
         [0005]    When the stack  30  has reached a desired height, the lift table  28  lowers the stack  30  to a level even with a secondary conveyor (not illustrated), and the receiving conveyor  26  moves the finished stack  30  away from the stacking apparatus  10 . When the stack  30  has moved off the receiving conveyor  26 , the lift table  28  raises the receiving conveyor  26  to a level for receiving additional sheets from the downstream end  20  of the stacking apparatus  10 . 
         [0006]    The rotary die cut machine  12  operates substantially continuously, and sheets of material therefore continue to traverse the stacking apparatus  10  and reach the hopper  22  even while a finished stack is being removed from the receiving conveyor  26 . During the time that the receiving conveyor  26  is out of position, accumulator shelves  32  are extended to receive sheets as they leave the downstream end  20  of the main conveyor  18 . When the receiving conveyor  26  has discharged a completed stack and returned to a position at the downstream end  20  of the main conveyor  18 , the accumulator shelves  32  retract and drop the sheets that have accumulated thereon onto the receiving conveyor  26 . Additional sheets exiting the downstream end  20  of the stacking conveyor fall onto the stack, and the process repeats until the stack on the receiving conveyor reaches a desired height. 
       SUMMARY 
       [0007]    Stackers as discussed above are well adapted for use with rotary die cut machines. However, it has been found that it is sometimes desirable capture or buffer incoming sheets of material before they reach the accumulator. For example, digital printers now in use that print directly onto sheets of corrugated material. The sheets are generally rectangular and have a long dimension and a short dimension. 
         [0008]    There are two main differences between the output of digital printers and the output of a conventional stacker that receives sheets from a rotary die cut machine: first, the digital printer outputs printed sheets at a lower speed than the rotary die cutter. Second, die cut sheets are generally cut so that they travel in the direction of their shorter dimension. However, printed sheets are generally printed as they travel in the direction of their longer dimension. In other words, printed sheets travel in a direction offset 90 degrees from the travel direction of die cut sheets. The combination of these factors means that the leading edge of a sheet leaving the digital printer may droop and drop onto the printed surface of a lower sheet in a stack as upstream portions of the sheet continue to move through the printer. The leading edge of a sheet is thus pushed in a scraping manner along the printed top surface of the next lower sheet in the stack until the trailing edge of the sheet exits the nip of the printer and drops onto the stack. This scraping may damage the printing on the surface of the lower sheet. 
         [0009]    One aspect of the present disclosure comprises an arrangement that prevents the above described scraping by holding a sheet exiting the digital printer above the stack on the receiving conveyor until the sheet can be released to fall vertically onto the next lower sheet on the stack. This may be accomplished, for example, by providing a selectively extendable structure, which may comprise a shelf and/or one or more selectively extendable rollers, for example, for supporting a printed sheet exiting the digital printer and holding the printed sheet above the stack and/or receiving conveyor until its trailing end exits the nip of the stacker and then allowing the printed sheet to drop onto the stack. 
         [0010]    It may also be advantageous to use such a shelf or selectively extendable rollers with rotary die cut machines and/or other feeding devices other than digital printers. Thus, another aspect of the present disclosure involves using the shelf or selectively extendable rollers as a pre-accumulator for certain stackers. Stackers often include movable plates for squaring or tamping a stack as it forms. These tamping structures may perform their tamping function on a stack at a location below the accumulator shelf In such a case, sheets are tamped as they fall onto the receiving conveyor or immediately after they are dropped by the accumulator shelves. However, in order to provide a more compact hopper and/or better square a stack as it forms, it may be desirable to providing the tamping structures at the level of the accumulator so that stacks can be tamped in the accumulator section before they are released onto the receiving conveyor. 
         [0011]    In such a case, the selectively extendable structure, such as rollers, may act as a pre-accumulator or an accumulator for the accumulator to catch a few (e.g., two or three) sheets while the sheets on the accumulator are tamped or squared and before they are dropped onto the receiving conveyor. The tamping structures are maintained at the level of the stack top as the stack forms on the receiving conveyor. When the accumulator shelf is extended to catch falling sheets (when the finished stack is being moved away from the stacking device) the tamping pauses temporarily and sheets accumulate on the accumulator shelf When the receiving conveyor is back in position, the selectively extendable rollers are extended to receive a small number of sheets while the sheets on the accumulator are tamped and squared, the squared sheets on the accumulator are dropped onto the receiving conveyor, the rollers are retracted to drop sheets onto the top of the stack, and the tamping structures proceed to tamp the sheets falling on the top of the stack. 
         [0012]    One aspect of the disclosure comprises a buffer apparatus configured to hold a stack of sheets and that includes an input section, and the sheets arrive at the input section in a travel direction from a source. The apparatus also includes a top, a bottom, a first side guide having a first face facing transverse to the travel direction, a second side guide having a second face facing transverse to the travel direction, and a backstop facing the input of the buffer apparatus and spaced from the input of the buffer apparatus in the travel direction. The first side guide, second side guide and backstop define a hopper for holding a plurality of the sheets of material. The apparatus includes at least one first retractable support that is shiftable from a first position in which a first portion of the at least one first retractable support projects a first distance into the hopper and a second position in which the first portion of the at least one first retractable support projects a second distance into the hopper or does not project into the hopper, and the second distance is less than the first distance. The apparatus includes at least one second retractable support shiftable from a first position in which a first portion of the at least one second retractable support projects a third distance into the hopper and a second position in which the first portion of the at least one second retractable support projects a fourth distance into the hopper or does not project into the hopper, and the fourth distance is less than the third distance. The at least one second retractable support is located between the input and the at least one first retractable support. 
         [0013]    Another aspect of the disclosure includes a method of buffering a stream of sheets by forming a stack. The method includes moving a first sheet in a travel direction into a hopper such that the first sheet impacts against a backstop, catching the first sheet on a first set of retractable supports, and retracting the first set of retractable supports and allowing the first sheet to fall onto a second set of retractable supports. The second set of retractable supports is located between the first set of retractable supports and a bottom of the hopper. The method also includes retracting the second set of retractable supports and allowing the first sheet to fall out of the bottom of the hopper. 
         [0014]    A further aspect of the disclosure comprises a method that includes providing a buffer apparatus configured to hold a stack of sheets. The buffer apparatus has an input section, a top, a bottom, a first side guide having a first face facing transverse to the travel direction, a second side guide having a second face facing transverse to the travel direction, and a backstop facing the input of the buffer apparatus that is spaced from the input of the buffer apparatus in the travel direction. The first side guide, second side guide and backstop define a hopper for holding a plurality of the sheets of material. The apparatus includes at least one first retractable support shiftable from a first position in which a first portion of the at least one first retractable support projects a first distance into the hopper and a second position in which the first portion of the at least one first retractable support projects a second distance into the hopper or does not project into the hopper, and the second distance is less than the first distance. The apparatus includes at least one second retractable support shiftable from a first position in which a first portion of the at least one second retractable support projects a third distance into the hopper and a second position in which the first portion of the at least one second retractable support projects a fourth distance into the hopper or does not project into the hopper, and the fourth distance is less than the third distance. The at least one second retractable support is located between the input and the at least one first retractable support. The method includes placing the at least one first retractable support in the first position of the at least one retractable support, receiving at least one sheet moving in the travel into the hopper and catching the at least one sheet on the extended at least one first retractable support, placing the at least one second retractable support in the first position of the at least one second retractable support, shifting the at least one first retractable support to the second position of the at least one first retractable support and allowing the at least one sheet to fall onto the at least one second retractable support, and shifting the at least one second retractable support to the second position of the at least one retractable support and allowing the at least one sheet to fall from the bottom of the buffer apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    These and other aspects and advantages of the present disclosure will be better understood after a reading of the following detailed description together with the attached drawings in which: 
           [0016]      FIG. 1  is a perspective view of a buffer apparatus according to an embodiment of the disclosure. 
           [0017]      FIG. 2  is a front elevational view of the buffer apparatus of  FIG. 1 . 
           [0018]      FIG. 3  is a rear elevational views of the buffer apparatus of  FIG. 1  in a first configuration. 
           [0019]      FIGS. 4  is a rear elevational view of the buffer apparatus of  FIG. 1  in a second configuration. 
           [0020]      FIGS. 5  is an end elevational detail view of the buffer apparatus of  FIG. 1  with a first set of sheets stacked on the accumulator and a sheet on the pre-accumulator. 
           [0021]      FIG. 6  is top plan view of the buffer apparatus of  FIG. 1 . 
           [0022]      FIG. 7  is a sectional side elevational view taken along line VII-VII in  FIG. 6 . 
           [0023]      FIG. 8  is a side elevational view of a conventional stacking apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Referring now to the drawings, wherein the showings are for the purpose of illustrating presently preferred embodiments of the disclosure only and not for the purpose of limiting same,  FIG. 1  shows a buffer apparatus  100  for use in a stacking system which buffer apparatus  100  includes a frame  102  having a front  104 , a rear  106 , a left side  108  and a right side  110 . As used herein, the front side  104  is the side from which sheets will enter the buffer apparatus  100  during use, and the terms “left side” and “right side” refer to those respective sides of the buffer apparatus  100  as viewed by a person facing the front  104  of the stacking apparatus. Moreover, the phrases “x-axis” or “x-direction” may be used to refer to the direction from the left side  108  of the frame  102  to the right side  110  of the frame  102 . In addition, the phrases “y-axis” or “y-direction” may be used to refer to the direction from the front  104  of the frame  102  to the rear  106  of the frame  102 . This direction may also be referred to as the “travel direction,” that is, the direction in which sheets travel as they enter the buffer apparatus  100 . Finally, the vertical direction may be referred to as the “z-axis” or “z-direction.” 
         [0025]    A knurled roller  112  is supported at the front  104  of the frame  102 , and first and second nip rollers  114 ,  116  are mounted above the knurled roller  112  and spaced from the knurled roller  112  by a gap  117  (see, e.g.,  FIG. 2 ). The gap  117  is selected based on the thickness of the material to be handled by the buffer apparatus  100  such that sheets that arrive at the gap  117  are nipped by the first and second nip rollers  114 ,  116  and the knurled roller  112  and drawn by the rotation of the knurled roller  112  into the interior of the frame  102 . In the disclosed embodiment, the first nip roller  114  is mounted to the frame  102  such that it is fixed in the x-direction, and the second nip roller  116  is mounted so that it is adjustable in the x-direction and such that the distance between the first nip roller  114  and the second nip roller  116  is adjustable. Making the second nip roller  116  adjustable in this manner allows the nip rollers  114 ,  116  to be used to handle sheets of different widths and to ensure that the first and second nip rollers  114 ,  116  engage only the edges of the sheets leaving the digital printer. The positioning of nip rollers is less important in conventional stacking operations. However, the nip rollers could potentially damage printing on the top surface of printed sheets exiting a digital printer, and the adjustability of the second nip roller  116  helps avoid such damage. Of course, if desired, the first nip roller  114  could also be adjustable in the x-direction, or, if the incoming sheets are leaving the conveyor section of a stacking apparatus, they will be moving with enough speed that the first and second nip rollers  114 ,  116  and the knurled roller  112  could be omitted entirely. 
         [0026]    The frame  102  includes first and second guide rails  118  mounted near the top of the frame  102  which guide rails  118  extend in the x-direction. A carriage  120  is mounted on the first and second guide rails  118  for rolling or sliding movement in the x-direction. A first side plate assembly  122  is mounted at the right side  110  of the frame  102 , and a second side plate assembly  124  is mounted on the carriage  120  so that it is level with the first plate assembly  122  in the z direction and so that the second side plate assembly  124  is movable toward and away from the first side plate assembly  122  in the x-direction. 
         [0027]    Referring now to  FIG. 3 , the first side plate assembly  122  includes a first vertical side plate  126 , and a first accumulator shelf  128  is mounted on the side of the first side plate  126  and connected to suitable actuators so that it can be controllably moved toward the side of the frame  102 , by extending it through a slot in the first side plate  126 , for example. A plurality of first rollers  130 , sometimes referred to as “pop-out” rollers, are mounted on the right side of the first side plate  126  plate and connected to actuators that cause the first rollers  130  to extend through the first side plate  126 . The first rollers  130  could alternately comprise non-rotatable rods, or a shelf without exceeding the scope of this disclosure. The first rollers  130  may be controlled to move simultaneously or individually or as parts of individually controllable sets of rollers. 
         [0028]    The second side plate assembly  124  includes a vertical second side plate  132 , and a second accumulator shelf  134  is mounted on the side of the side plate  132  and connected to suitable actuators so that it can be controllably moved toward the first side plate assembly  122 , by extending it through a slot in the second side plate  132 , for example. A plurality of second rollers  136  are mounted on the side of the side plate  132  and connected to actuators that cause the second rollers  136  to extend through the second side plate  132  toward the rollers  130  on the first side plate assembly  122 . The second rollers  136  may also be controlled to move simultaneously or individually or in sets but in any case are preferably aligned with the rollers on the first side plate assembly  122  and configured to extend and retract at the same time as the corresponding first rollers on the first side plate assembly  122 . 
         [0029]    A backstop  138 , which may be adjustable in the x direction and the y direction, is provided on the frame  102  for limiting movement of sheets in the y direction. A plurality of back tamping plates  140  are provided generally in the plane of the backstop. These back tamping plates  140  are movable back and forth in the y direction to tamp and align the sheets of a stack in the y direction. 
         [0030]    A set of first side tamping plates  142  is located in the first side plate assembly  122 , and a set of second side tamping plates  144  is located in the second side plate assembly  124 . (Alternately, tamping can be performed in the x-direction from one side only.) The first and second side tamping plates  142 ,  144  are movable toward and away from each other in the x direction to align sheets in the x direction and are located above the first and second actuator shelves  128 ,  134  in the z direction and below the first and second rollers  130 ,  136  in the z direction. The tamping operation of the back tamping plates  140  and the first and second sets of side tamping plates  142 ,  144  squares the stack of sheets, and the operation of these tamping plates is discussed in more detail below. 
         [0031]    The first accumulator shelf  128 , second accumulator shelf  134 , first rollers  130  and second rollers  136  are controlled by actuators, such as pneumatic cylinders, electric motors, etc. to move between extended and retracted positions relative to the first side plate  126  and the second side plate  132 . The actuators are under the control of a conventional controller, such as a programmable logic controller (PLC), microprocessor, etc. which may be the same controller that controls the overall operation of the stacking apparatus  10 . The general concept of controlling the movement of such actuators is well known. The particular arrangement of and manner of controlling the actuators to move the first accumulator shelf  128 , second accumulator shelf  134 , first rollers  130  and second rollers  136  is part of the disclosed method, and two methods of using the system described above are discussed below. 
         [0032]    First, the use of the buffer apparatus  100  for stacking relatively long sheets printed by a digital printer is discussed. Sheets, including corrugated sheets, printed by a digital printer are often printed in their length direction—that is, the sheet is arranged so that its longest dimension is aligned with the y direction of the buffer apparatus  100 . As such, the leading end of a sheet may tend to droop and drag along the top surface of a lower sheet in a stack on the receiving conveyor  26  or the accumulator shelf  128  and damage the print on that sheet. This problem is avoided with the present buffer apparatus  100 . 
         [0033]    The buffer apparatus  100  is positioned at the output end of a digital printer (not illustrated), and the spacing between the first nip roller  114  and the second nip roller  116  is set so that these nip rollers nip the sheet exiting the digital printer close to the side edges of the sheet, i.e., at locations on the sheet that have not been printed. The gap  117  between the knurled roller  112  and the first and second nip rollers  114 ,  116  is also set so that the rollers can accept and control the movement of the sheet without damaging it. 
         [0034]    Before a first sheet is received from the digital printer, the controller causes the first rollers  130  and the second rollers  136  to extend toward each other and through the first side plate  126  and the second side plate  132 , respectively. The gap between the first rollers  130  and the second rollers  136  is less than the width of the sheet leaving the printer, and thus the sheet contacts the first and second rollers  130 ,  136  after it passes the nip rollers  114 ,  116  and is pushed along the tops of the first and second rollers  130 ,  136  toward the rear  106  of the frame  102 . At approximately the same time as the trailing edge of the sheet leaves the gap  117  between the knurled roller  112  and the first and second nip rollers  114 ,  116 , the controller causes the first rollers  130  and the second rollers  136  to retract, and this releases the sheet from the rollers and allows it to drop onto the surface of the receiving conveyor  26  or accumulator shelf  128  or onto the top of a previously deposited sheet and/or partial stack of sheets that is present on the receiving conveyor  26  or the accumulator shelf  128 . The controller cycles the actuators controlling the first and second rollers  130 ,  136  quickly so that the next printed sheet, which begins to emerge from the gap  117  shortly after the earlier sheet leaves the gap  117 , can be received on the rollers  130 ,  136 , and the foregoing process repeats. In this manner, printed sheets are dropped substantially vertically onto the receiving conveyor  26  or onto the accumulator shelf  128  or any partial stack of sheets on the receiving conveyor  26  or the accumulator shelf  128 , and the problem of damaging print on sheets already in the stack is avoided. 
         [0035]    All of the first rollers  130  and all of the second rollers  136  can be controlled to move simultaneously. Alternately, the rollers  130 ,  136  may be individually controlled and/or controlled in groups. For example, if it is desired to cause the front end of the sheet to drop onto the receiving conveyor  26  before the rear end of the sheet drops, the rollers closest to the rear  106  of the frame  102  may be caused to retract shortly before the rollers closest to the front  104  of the frame  102  retract. In order to simplify control, the rollers may be retracted in sets. For example, the four rollers closest to the rear  106  of the frame may first be simultaneously retracted, the four centrally located rollers may next be retracted, and the four rollers closest to the front  104  of the frame  102  may be retracted after that. The rollers can then be extended in the opposite order from which they were retracted or all may be re-extended at the same time to receive the next incoming sheet. 
         [0036]    The lift table  28  on which the receiving conveyor  26  is mounted is controlled to maintain the top of the stack being formed at a desired height or within a range of heights. Specifically, it may be desirable to maintain the top of the stack above the level of the first and second accumulator shelves  128 ,  134  so that the side tamping plates  142 ,  144  can tamp the stack as it forms and, together with the back tamping plates  140 , keep the stack square. 
         [0037]    The foregoing receiving and releasing process continues until the stack on the receiving conveyor  26  reaches a desired height. At that time, the lift table  28  lowers the receiving conveyor  26  so that the top of the stack drops below the level of the first and second accumulator shelves  128 ,  134 , the controller extends the first and second accumulator shelves  128 ,  134 , and the receiving conveyor  26  moves the finished stack away from the frame  102  of the buffer apparatus  100 . The first rollers  130  and the second rollers  136  continue to operate to support sheets exiting the digital printer and to drop them individually onto the stack forming on the first and second accumulator shelves  128 ,  134 . When the receiving conveyor  26  is back in position and ready to receive more sheets, the controller retracts the accumulator shelves  128 ,  134  and drops the accumulated sheets onto the receiving conveyor and the process of receiving sheets on the first and second rollers  130 ,  136  continues as before until the new stack on the receiving conveyor has reached a desired height. 
         [0038]    The buffer apparatus  100  described above can also beneficially be used to stack sheets that are relatively short, that is, sheets short enough that they do not droop and drag along the top surface of lower sheets in a stack. Specifically, as discussed in greater detail below, the side tamping plates  142 ,  144 , can beneficially be used to tamp and square sheets of a stack on the receiving conveyor  26  as well as to tamp and square sheets that are located on the accumulator shelves  128 ,  134 . Tamping and squaring the sheets being accumulated may contribute to the formation of a neater stack that requires less pressure to square. That is, each sheet is tamped as it drops onto a stack, either on the accumulator shelves or on the receiving conveyor, and this reduces the need to tamp and square multiple sheets at once as might have formerly occurred when a partial stack of sheets was dropped from the accumulator shelves onto the receiving conveyor. If, for example,  10  to  20  sheets accumulate on the accumulator shelves without being squared, these  10  to  20  sheets would have to be squared simultaneously by conventional tamping plates. However, the weight of the upper portion of the stack pressing on the lower sheets of the stack while the sheets are slid in the x-y plane and squared could potentially damage the printing on the lower sheets of the stack. Squaring the sheets as they are accumulated on the accumulator shelves avoids the problem without the need for additional sets of tamping plates. 
         [0039]    In operation, the second nip roller  116  and the backstop  138  are positioned to accommodate the dimensions of the shorter sheets being processed. Based on the position of the backstop  138 , the controller may control fewer than all of the first and second rollers  130 ,  136 ; that is, the rollers  130 ,  136  that will not contact the shorter sheets being processed in this embodiment may be left idle. 
         [0040]    Short sheets pass through the gap  117 , impact the backstop  138  and drop onto the receiving conveyor  26  or onto a partial stack forming on the receiving conveyor  26 . Because the sheets are short (in the y direction) they do not droop, and they drop substantially vertically onto the stack forming on the receiving conveyor. The first and second rollers  130 ,  136  are therefore not used to receive and release individual sheets as discussed in the embodiment handling long sheets described above. Instead, the top of the stack is maintained at a level somewhat above the level of the first and second accumulator shelves in the z direction, and the stack is squared by the operation of the back tamping plates  140  and the first and second side tamping plates  142 ,  144 . When the stack reaches the desired height, the first rollers  130  and the second rollers  136  can be used as follows to facilitate the removal of the stack on the receiving conveyor  26 . 
         [0041]    When the stack reaches a desired height, the first rollers  130  and the second rollers  136  are extended to receive incoming sheets while the lift table  28  drops the stack away from the side tamping plates  142 ,  144  and back tamping plate  140 . When the top of the stack has dropped below the level of the first and second accumulator shelves  128 ,  134 , the first and second accumulator shelves  128 ,  134  are extended, and the first rollers  130  and the second rollers  136  are retracted to drop the small number of accumulated sheets onto the accumulator shelves  128 ,  134 . The first rollers  130  and the second rollers  136  thus function as a “pre-accumulator” or accumulator for the accumulator shelves  128 ,  134  in a manner that enables the use of tamping plates as a level higher than the level of the accumulator shelves  128 ,  134  and helps provide time for the top of the stack to clear the accumulator before the accumulator shelves  128 ,  134  are extended. The rollers  130 ,  136  are then retracted, and sheets are deposited directly on the accumulator shelves  128 ,  134  until the receiving conveyor  26  returns to a position for receiving sheets directly from the printer. 
         [0042]    The rollers  130 ,  136  can be used in a similar manner as pre-accumulators when stacks of long sheets are transferred by the receiving conveyor  26 . 
         [0043]    The present invention has been described above in terms of several presently preferred embodiments. Additions and changes to these embodiments will become apparent to persons of ordinary skill in the art upon a reading of the foregoing description. It is intended that all such modifications and additions comprise a part of the present invention to the extent they fall within the scope of the several claims appended hereto.