Abstract:
An electrostatic charging system holds a film web to a roll without significant slippage, thus allowing for proper tension control of the web even with extreme differences in tension upstream and downstream of the motorized roll. The system can electrostatically transfer a web using motorized drum rolls with an internally integrated web severing knife.

Description:
BACKGROUND 
   The present inventions relate to improving the quality of products produced by plastic resin extrusion lines. 
   When film is extruded, it typically is in the form of a flat continuous web as in cast film extrusion or a tubular form as in blown film extrusion. In blown film, the inflated tubular, bubble-type form passes through stabilizers of various designs and into a flattening device. This device, known as a collapsing frame, flattens the tube into a two sided, connected film with no air inside. In a flat state, webs are conveyed by various combinations of non-motorized and motorized rollers to a winder or to in-line downstream conversion, equipment such as printing presses, laminators, or bag machines. In a winder, a web may be cut into individual webs before the winding process, and the resulting rolls are converted at a later time in what is commonly termed an out-of-line converting process. 
   As film webs are conveyed to a winder or in-line conversion equipment, motorized rollers can be used at various points to maintain control of web tension. Except for the initial motorized roll immediately downstream of the extrusion process (commonly called the primary nip roll) where constant motorized roll speed is maintained without feedback, tension is typically measured in some way and provided as feedback to a motor controller. This motor controller is programmed to respond to maintain tension at a constant preset setting. 
   Typically, tension is measured either statically by measuring the applied force due to tension on load cell devices attached to idler rolls, or somewhat dynamically through what is commonly referred to as a dancer. A dancer is a series of idler rolls that move against springs, counterweights, air cylinders, or other such force applying devices in such a way as to allow the film&#39;s path length to change in response to tension variations and thus provide indication of film tension. In other cases, tension is measured indirectly by measuring the torque applied by the motorized roll and comparing the measurement to a no-load torque pre-measured in the absence of the web, taking into consideration roll geometry, and converting this to applied web tension. 
   An issue with maintaining proper tension control is web slippage when passing over motorized rolls. In many cases, additional rolls are pressed against the motorized roll to form a nip point that the web passes. The nip point acts to help hold the film against the motorized roll to prevent slippage. In other cases, large wrap angles around the motorized roll or more than one roll are used to provide a large enough surface area for friction to act and prevent slippage. Combinations of nipping and large wrap angle may not always prevent slippage and thus can lead to tension control problems as the web becomes uncontrollable. 
   One such area is the motorized roll used in the winding process known commonly as the lay-on or winding drum. The winding drum is the final motorized roll the web passes over before being wound on the finished roll. Good tension control should be maintained by the winding drum or undesirable defects in the roll can result. In some winding machines, the finished roll shaft is also motorized to aid in maintaining proper tension control of the web as the finished roll builds in diameter to a final roll diameter over time such as disclosed in U.S. Pat. No. 5,275,348. 
   In practice it is desirable to maintain constant tension before reaching the winder drum for such purposes as slitting the web into multiple webs, slitting trim from the edges or middle for maintaining high quality roll ends, and minimizing wrinkling and deformation of the web before winding on the finished roll. It is further desirable to control the tension on the finished roll to tensions that are different from upstream tension especially when making very large rolls. Excessive winding tension can create roll quality issues or even crush the central winding core. However, higher tensions are beneficial to the upstream cutting and trimming processes. 
   A limitation of some devices, such as the winding devices described in U.S. Pat. No. 5,275,348, is that they require essentially zero winding drum for proper tension control to be achieved. In practice, devices such as these exhibit slippage when significant differences exist in tension upstream and downstream of the motorized drum roll and thus limit the tension difference achievable. In practice, depending on web and drum materials used, small differences of as little as 0.1 or 0.2 pounds per linear inch of web width across the face of the motorized roll are enough to cause slippage of the web. Typical web tensions within these extrusion processes range from 0.25 to 2.0 pounds per linear inch. 
   A situation for maximum tension differential exists when winders, which are required to continuously handle a web without interruption, are transitioning from winding one roll to the next. Extrusion processes are run continuously, so the web is severed at the conclusion of building a finished roll to final diameter and the loose incoming end is taken up onto a new winding core to start building a new roll without stopping or slowing the upstream process. The severing process causes tension in the web local to the severing device to suddenly drop to zero, creating instantaneous slippage on the motorized drum roll since now tension differences are maximized. 
   U.S. Pat. No. 5,848,761 shows one example of a device in which a severing knife is contained within a motorized winding drum. This arrangement makes the slippage problem especially troublesome. In this case, a vacuum chamber within the drum roll is energized to hold the web in place and prevent slippage until a new winding core acquires a loose incoming cut edge of the web and reestablishes web tension. In practice, some slippage does still occur and the complexity of such systems is very costly. 
   Prior methods exist for acquiring a loose incoming cut edge of a web. Sticky substances such as glue or tape are popular but messy and typically create undesirable impressions in the wound web. Other techniques as shown in U.S. Pat. No. 4,852,820 employ an electrostatic charging device between a motorized winding drum and a finished roll that is about to be cut free. This eliminates the problems associated with using glue or tape. The incoming winding core is held generally opposite the electrostatic charging device after the motorized winding drum with the web passing in between. Just before severing the web, an electrostatic charge is applied to the web opposite the incoming winding core. This causes an electrostatic force that acts to push the web toward the incoming winding core. This force of attraction is due to electrostatically formed ions preferably placed on the opposite side of the web. These ions are drawn to the ground potential of the incoming conductive and grounded winding shaft which holds the incoming typically not conductive, winding core. These ions act to carry the web with them and try to attach the web to the winding core. The web is then severed nearby the charging device and the completed finished roll. The loose incoming web end is electrostatically attracted to the incoming winding shaft where it attaches to the new core and begins to build a new roll. 
   One drawback of motorized drum rolls as shown in U.S. Pat. No. 5,848,761 containing the severing knife within itself is that the elect positioned between the severing knife and the motorized winding drum because these latter two are formed in one unit. This makes it impossible to use the electrostatic transfer method described in U.S. Pat. No. 4,852,820. 
   SUMMARY 
   The systems described here relate to electrostatic charging systems that hold a film web to a motorized roll. This is done without significant slippage, thus allowing for proper tension control of the web even with extreme differences in tension upstream and downstream of the motorized roll. In one embodiment, a wrap angle of at least approximately 10 degrees is used to provide a surface area in contact with the motorized roll over which electrostatic forces act to hold the web without slippage. A nip roll is not required, but if present, should not be placed in such a way that the electrostatic charge is bled off the web prematurely, although in most cases it is desirable to remove excess electrostatic charge before reaching the finished roll since this can cause significant undesirable problems in finished rolls. Removal of undesirable electrostatic charge can be accomplished in many ways, such as by direct contact of intervening conductive rolls or by well known non-contacting electrostatic elimination devices, but at a location that does not significantly affect tension controlling properties of the device. 
   These systems can avoid a need for complex vacuum holding systems and nip rolls, especially as it pertains to winding with motorized drum rolls with integral web severing knives. The systems can also provide improved tension control without unwanted electrostatic buildup within finished rolls because there is no web slippage even under extremes of differential tension present on opposite sides of motorized rolls within an extrusion process such as those that occur during web transfer or within machines that highly stretch the web. An additional feature of these systems is that they can provide a torque boost to the motor of a motorized drum roll equivalent to a downstream tension force effect on the web upon severing of that web on or nearby the motorized drum roll to substantially maintain a constant upstream web tension whether electrostatically or otherwise maintained. 
   The motorized roll itself can be electrically charged or un-grounded to neutralize its attractive effect on electrostatic charges present, especially those on the opposite side of the web, which produce attractive forces that carry the web and hold it to the motorized roll. This neutralizing effect can be virtually instantaneous. 
   One possible benefit is allowing for electrostatically transferring a web using motorized drum rolls with an internally integrated web severing knife. In this case, a non-conductive coating, such as polyurethane or any other such suitable substance is applied to the motorized drum roll containing an internal severing knife to form a motorized transfer drum. A static charge is optionally applied between the surface of the motorized transfer drum and the web, then also to the surface of the web opposite the motorized transfer drum such that electrostatic charge is present on both sides of the web. In this case, both layers of electrostatic charges will be attracted to the motorized transfer roll as long as it is at ground potential and the web will be attached to the motorized transfer drum by the charges located opposite the drum. A grounded conductive shaft holding an incoming winding core is subsequently brought substantially parallel and in close proximity which acts to attract the web due to charges on the opposite side of the web which act to substantially cancel the attractive force of the still grounded motorized transfer drum. The web is then severed and more or less simultaneously, the motorized transfer drum is electrically charged to the same polarity of the charged ions adjacent the web or un-grounded to neutralize its effect on the ions and thus on the web. This allows the web to be drawn toward the incoming grounded shaft and attach to the winding core as subsequent electrostatic transfer occurs. 
   While certain benefits are described, a given system need not have all of these benefits. 
   Other features and advantages will become apparent from the following detailed description, drawings, and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of an electrostatic web transfer device for transferring a web in a winding process employing a motorized drum roll containing an integrated severing knife. 
       FIG. 2  is a schematic side view of an electrostatic web transfer device for transferring a web in a winding process employing a motorized drum roll containing an integrated severing knife. 
       FIG. 3  is a schematic side view of an electrostatic web holding system used for motorized drum rolls containing an integrated severing knife to transfer webs using non-electrostatic attraction to incoming cores and shafts. 
       FIG. 4  is a schematic side view of an electrostatic, tension controlling, motorized roll showing an optional electrostatic elimination process. 
   

   DETAILED DESCRIPTION 
     FIGS. 1-3  illustrate a web transfer device that can replace a known vacuum based web transfer system for winding continuous webs. A web  10  is conveyed onto a motorized drum, in this embodiment a transfer drum (conveying roll)  20 , which is substantially earth grounded, for subsequent winding onto a finished roll  30 . Finished roll  30  can be in contact, or free from contact, with transfer drum  20 . Drum  20  is motorized and as shown is driven counter-clockwise by motor  50 , but can also turn clockwise with web  10  passing in the opposite direction around transfer drum  20  to cause web  10  to wind on finished roll  30  with the opposite side of web  10  to the outside of finished roll  30 . 
   Referring particularly to  FIG. 2 , a charging device  40  energizes an electrostatic bar  42 , which generates ions  44  that are attracted to earth ground presently found on transfer drum  20 . Web  10  is pressed against transfer drum  20  due to electrostatically attractive forces between ions  44  and presently grounded transfer drum  20 , thus preventing significant slippage of web  10  on transfer drum  20 . The electrostatic bar can be operated while the web is being conveyed over the drum, and can be used while the web is being wound, and not just to move a web during transfer of the web to another drum. The electrostatic bar provides tension without a nip roll and without an S-wrap series of rollers (although in some embodiments these parts could be used also). The ions can be provided continuously, rather than just during a process of transferring a web from a finish roll to a new core, and can be provided substantially all the time, i.e., during normal operation. 
   Motor  50 , controlled by motor drive  52 , is responsive to signals from upstream web tension sensor  54  measured by any of several typically known means to maintain the upstream web tension at desired levels. Advantageously, drive  52  may apply a calculated torque dependent on desired web tension, a known radius of transfer drum  20 , and pre-measured frictional losses, through motor  50  to transfer drum  20  to maintain the desired tension in web  10  without resorting to direct measurement of web tension. Motor drives, such as model ACS-600 or 800 from ABB Corporation, are based on technology know as direct torque control which can be used to facilitate the torque measuring and control functions. 
   An optional electrostatic neutralizer  70 , which can be any suitable device such as a conductive roll or (if there is a lack of physical space) an electrostatic eliminator bar may be used for removing charged ions from the surface of web  10  and transfer drum  20 . If used, electrostatic neutralizer  70  is positioned to allow for sufficient wrap angle of at least 10° around transfer drum  20  to allow for positive tension control before neutralization of ions  44 . It is desirable to neutralize ions  44  before being wound on finished roll  30  because excessive charge can cause the web to have undesirable clinging effects to other surfaces and can cause sudden electrostatic discharge to other nearby objects or to personnel working in the area. Electrostatic neutralizer  70 , if present, may be advantageously turned off just before transferring web  10  at the completion of winding finished roll  30  so as not to interfere with the transfer process. 
   Referring only to  FIGS. 1 and 2 , grounded transfer drum  20  is coated with substantially non-conductive covering  22  to prevent dissipation of any applied electrostatic charges. Just before completion of winding finished roll  30 , an incoming core  60  is brought adjacent to grounded transfer drum  20  by incoming winding shaft  62 , which is grounded to prepare for the transfer of web  10 . A second electrostatic bar  46  energizes to generate ions  48 . Ions  48  are positioned between web  10  and the surface of transfer drum  20 . Ions  44  and  48  are attracted to grounded transfer drum  20  and to grounded incoming winding shaft  62  which carries incoming core  60  but are not dissipated due to the presence of non-conductive covering  22  and incoming core  60 . Severing knife  24  within drum  20  cuts web  10 . Transfer drum  20  is then purposefully ungrounded to release the attractive forces on ions  44  and  48  which sandwich web  10  between at point A. Optionally, charging device  40  may energize transfer drum  20  to enhance the release of attractive forces on ions  44  and  48  which sandwich web  10  such as at point A to enhance the forces which act to attach web  10  to core  60  due to attractive forces felt between ions  44  and  48  and still grounded incoming winding shaft  62 . 
   More or less simultaneously, and when the trailing end of now severed web  10  begins to slip off transfer drum  20  onto finished roll  30  and until firm attachment of web  10  to incoming core  60 , a boost in torque equivalent to the calculated loss in downstream tension due to severing of web  10  leading to finished roll  30  optionally may be applied to transfer drum  20  by motor  50  and motor drive  52  to maintain substantially uniform tension  54  upstream of transfer drum  20  during web  10  transfer. This torque boost is generally not required in applications where finished roll  30  is not also powered by a suitable motor to apply tension on its own accord to web  10  as such process is well known in the industry. Incoming grounded winding shaft  62  continues to apply attractive forces to ions  44  and  48 . Ions  44  tend to directly attach to incoming core  60 , whereas ions  48  carry with them and attach web  10  to incoming core  60  where web  10  begins to wind as new finished roll  31 . Electrostatic bar  46  is then turned off in preparation for the next transfer cycle. Electrostatic bar  42  may also be turned off if not required to maintain positive tension control over web  10 . Electrostatic neutralizer  70 , if present, can be turned on until the next transfer cycle occurs. The now complete finished roll  30  is removed and the process is allowed to repeat itself continuously. Any applied torque boost to transfer drum  20  is removed upon proper tensioning of new finished roll  31 . 
   Referring to the embodiment of  FIG. 3 , a tacky substance  64  such as glue, tape or any other substance suitable for adhering web  10  to incoming core  60  can be pre-applied to incoming core  60 . Just before completion of winding finished roll  30 , incoming core  60  with tacky substance  64  is brought adjacent to transfer drum  20  by incoming winding shaft  62  in preparation for the transfer of web  10 . Ions  44  continue to press web  10  against transfer drum  20  due to electrostatically attractive forces between ions  44  and presently grounded transfer drum  20 . Covering  22  can either be conductive or non-conductive since ions  44  adjacent to web  10  are insulated from conductive transfer drum  20  by web  10  to prevent dissipation of any applied electrostatic charges in the area of web  10 . Severing knife  24  acts to cut web  10 . More or less simultaneously, and when the trailing end of now severed web  10  begins to slip off transfer drum  20  onto finished roll  30 , and until firm attachment of web  10  to incoming core  60 , a boost in torque equivalent to the calculated loss in downstream tension due to severing of web  10  leading to finished roll  30  optionally may be applied to transfer drum  20  by motor  50  and motor drive  52  to maintain substantially uniform tension  54  upstream of transfer drum  20  during web  10  transfer. This torque boost is generally not required in applications where finished roll  30  is not also powered by a suitable motor to apply tension on its own accord to web  10  as such process is well known in the industry. 
   Incoming winding shaft  62  moves incoming core  60  with tacky substance  64  into contact with web  10  which until contact with tacky substance  64  remains attached to still grounded transfer drum  20  due to electrostatically attractive forces between ions  44  and presently grounded transfer drum  20 . Web  10  is forcibly attached onto incoming core  60  by tacky substance  64  which overcomes the electrostatically attractive forces between ions  44  and presently grounded transfer drum  20  and causes web  10  to begin to wind as a new finished roll  31 . The now complete finished roll  30  is removed and the process is allowed to repeat itself continuously. 
   Referring again to  FIGS. 1-3 , after transfer when web  10  is attached to incoming core  60 , electrostatic bar  42  can optionally be de-energized if it is not required to prevent slippage and maintain proper tension control of web  10  as it passes over transfer drum  20 . In these cases optional electrostatic neutralizer  70  may not be needed since electrostatic ions are only applied to web  10  during transfer which occurs typically over a very small time frame compared to the time required to wind up a complete finished roll  30 . 
   Referring now to  FIG. 4 , web  10  is conveyed onto roller  18  which is conductive and substantially earth grounded. Charging device  41  energizes electrostatic bar  43  which generates ions  45  that are attracted to earth ground of roller  18 . Web  10  is pressed against grounded roller  18  due to electrostatically attractive forces between ions  45  and roller  18 , thus preventing significant slippage of web  10  on roller  18 . Motor  51 , controlled by motor drive  53 , is responsive to upstream web tension sensor  55  that measures tension to maintain the upstream web tension at desired levels. The web conveying system shown in  FIG. 4  can be used as part of a transfer system to a finish roll for winding the web, or can be used to convey a web to other equipment. 
   Advantageously and optionally, torque equivalent to the calculated desired difference in upstream and downstream tension plus pre-measured torque required to just overcome frictional losses may be applied to roller  18  by motor  51  and motor drive  53  to maintain substantially uniform tension both upstream and downstream of roller  18 . 
   Optional electrostatic neutralizer  71 , which can be any suitable device such as an electrostatic eliminator bar, or due to their common use in areas other than between winding transfer drums such as transfer drum  20  of  FIGS. 1 ,  2 , and  3 , preferably a conductive roll, may be used for removing charged ions from the surface of web  10 . If used, electrostatic neutralizer  71  is positioned downstream to allow for sufficient wrap angle of at least 10 degrees around roller  10  to allow for positive tension control before ion  45  neutralization. 
   While certain structural embodiments have been described, it should be understood that various modifications can be made to the above-described embodiments without departing from the spirit and scope of the invention as defined in the appended claims. For example, further rolls (passive or motorized) and other equipment for sensing and/or conveying can be provided before or after the web is conveyed by roller  18  or drum  20 .