Method and apparatus using electrostatic charges to stabilize the upper sheets of a stack of paper

In a paper processing machine, the method and apparatus of this invention apply an opposite electrostatic charge to stacks and reams of paper to eliminate the repelling force from like charges on both sides of the individual sheets of the stack or ream to thereby temporarily bond them together. With the sheets thus temporarily bonded, the stacks and reams of paper can be easily transported between work stations without undesirable sliding of the sheets.

BACKGROUND OF THE INVENTION 
The present invention relates to moving and transporting stacks of paper 
and similar materials, and more particularly to the deliberate use of 
electrostatic charges to pin sheets of paper together in a stack which can 
then be easily transported and stored. 
Electrostatic forces on webs or sheets of paper often interfere with the 
operation of paper converting machinery. These charges are of opposing 
polarity so that one side of the continuous web is of positive polarity 
while the other side is of negative polarity. This causes the web to be 
attracted to machinery components or to adjacent webs (in case of multiple 
web processing with webs being unwound from several mill rolls, or when a 
single web is slit into ribbons which are then superimposed in the 
processing machine for further processing). Once sheeted, the webs are 
then stacked onto skids up to about six or seven feet high. The skid loads 
are then transported by fork-lifts to a temporary storage area or moved 
directly to the next processing machine such as a destacker and 
cartonizing machine manufactured and sold by Involvo, 33 Brook Street, 
West Hartford, Conn. 
In order to prevent machine jam-ups due to sheets clinging together from 
the time they are cut from the endless web until they are stacked at the 
delivery end of the sheeter, great care is taken to avoid or eliminate 
electro-static charges between the paper layers. This facilitates the flow 
of the cut sheets into the delivery stack at the sheeter. As the endless 
web (ribbons) advances from the unwind roll through pull roller systems 
before and after the printing stations and through the printing stations, 
it comes in contact with metal rollers or metal cylinders on one side and 
rubber or plastic covered rollers or cylinders on the other. Typical 
examples are: (1) driven steel pull rollers on one side of the web and 
rubber or plastic covered nipper wheels (or nipper rollers) on the other; 
(2) rubber covered impression cylinder on one side of the web and a metal 
gravure cylinder on the other (in gravure presses); and (3) rubber covered 
blanket cylinder on one side and steel impression cylinder on the other 
side (in web offset presses). While passing through the nips of such 
cylinders or rollers of different materials on each side of the web, the 
electrostatic charges on the paper surfaces are altered and may add to or 
subtract from the already existing electrostatic charge on that side of 
the paper and thereby cause an imbalance that interferes with the high 
speed operation of the paper processing machine. 
The prior art teaches the use of static eliminators to neutralize the 
electrostatic charges on both sides of the webs to reduce or eliminate 
jam-ups and other interference with the free flow of the still endless 
paper webs (or ribbons). The simplest method of prior art is the use of 
metal tinsel connected to ground and with the free ends of the tinsel 
touching the moving web. 
Other prior art methods for eliminating undesired electrostatic charges 
from moving webs of paper and similar materials include the use of the 
nuclear static eliminators, air ionizing devices or static eliminator 
rods. These devices are commercially available and will effectively 
neutralize the static charges on a moving web or sheet of paper and thus 
eliminate the undesired clinging together of webs or sheets and their 
undesired attraction to machinery components or to each other. Eliminating 
these electrostatic charges causes the sheets to repel each other in a 
manner similar to that of magnetic poles having like charges. In addition 
to this, a boundary layer of air remains between several of the uppermost 
sheets on the stack, such that these sheets slide easily around when the 
stack is moved such as during transport by fork-lifts. The boundary layer 
of air is squeezed out between the lower layers when the weight of the 
sheets accumulated on top exceeds the repelling force from the like 
charges between the layers of paper plus the force required to move the 
boundary layer of air. Normal friction then prevents the lower sheets in 
the stack from sliding, while the upper sheets still are free to slide 
around. 
When the skid loads of paper are moved, the uppermost sheets have a 
tendency to slide off, fall down to the floor where they are spoiled. This 
is particularly severe when handling sheets having the so-called 
"Kromekote" surfaces which are very smooth (very low coefficient of 
friction) and contain chemicals having a very high dielectric constant 
(such as the titanates). "Kromekote" sheets (coated on both sides) and 
having a thickness of 0.008 to 0.010 inches and a sheet size of about 
23.times.35 inches sell for approximately one US dollar ($1.00) per sheet 
wholesale so that the loss of a few sheets from each stack at the paper 
processing plant could be substantial. 
Additionally, when the skid loads of paper stacks are automatically 
destacked on the INVOLVO destacker, the uppermost sheets of the individual 
reams being destacked again slide around and cause undesirable trouble 
until the reams are cartonized. This occurs because the weight has been 
removed and the charges of like polarity again want to levitate the upper 
sheets. The faster the machine runs, the more disturbance there is. This 
then limits the production speed of the machine to well below the rated 
mechanical speed. 
SUMMARY OF THE INVENTION 
The present invention is designed to overcome the above noted limitations 
that are attendant in the "prior art" and toward this end it contemplates 
the provision of a novel method and apparatus for deliberately inducing 
electrostatic charges of opposite polarities into stacks of sheets of 
paper in order to hold them together during transport. 
An object of this invention is the deliberate application of an 
electrostatic charge having a polarity opposite to the charge existing in 
the pile (or stack) to neutralize the repelling force between the 
uppermost layers, allowing their weight to push out the entrapped layers 
of air and thereby to cause intimate contact between the sheets of paper 
so that the normal coefficient of friction is restored and the undesired 
slippage eliminated. 
Another object is to provide a method and apparatus which induces 
electrostatic charges of opposite polarity in individual stacks of paper 
being destacked from skid loads of paper for cartonizing. 
It is a further object to provide such a method which can be easily 
practiced using automated machinery. 
Still another object is to provide such an apparatus which may be readily 
and economically fabricated and will enjoy a long life and operation. 
It has now been found that the foregoing and related objects can be readily 
attained in an apparatus using electrostatic charges to temporarily hold 
sheets of paper in assembly which assembles a skid load stack of sheets of 
paper with each sheet having an electrostatic charge introduced thereon 
which tends to repel the sheet from adjacent sheets in the skid load 
stack. In addition, as the skid load stack is assembled, a layer of air is 
introduced between adjacent sheets whereby the electrostatic forces and 
layers of air cause adjacent sheets to slide relative to one another if 
the skid load stack is moved. By introducing an opposite electrostatic 
charge into at least an upper portion of the skid load stack, the opposite 
electrostatic charge having an opposite polarity to the electrostatic 
charge introduced during the assembly step, the sheets in the upper 
portion of the skid load stack are caused to attract to one another 
thereby decreasing the tendency of the sheets in the upper portion to 
slide relative to one another if the skid load stack is moved. 
During further processing, predetermined numbers of sheets from the skid 
load are separated to form individual stacks of sheets. An electrostatic 
charge of opposite polarity to the existing electrostatic charge in the 
lower portion sheets is introduced thereby causing the sheets in the 
individual stacks to attract to one another thereby decreasing the 
tendency of the sheets to repel one another. The individual stacks can 
then be moved for further processing. 
Desirably, to introduce the opposite electrostatic charge, an electrostatic 
charging device is provided adjacent a path along which the individual 
stacks of sheets pass. The electrostatic charging device can be moved 
relative to the individual stacks by a computer controller to optimize the 
distance between the electrostatic charging device and the top of the 
individual stacks. 
The invention will be fully understood when reference is made to the 
following detailed description taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning first to FIG. 1, therein illustrated is the delivery end of a paper 
converting machine generally indicated by the numeral 10. As a printed 
perforated, punched or otherwise processed endless web or ribbon of paper 
12 moves toward the delivery end of the paper converting machine 10, it 
usually passes through a rotary sheeter section generally indicated by the 
numeral 14 where sheets are cut from the endless web 12 and moved through 
high speed and low speed conveyor belt sections, respectively indicated by 
the numerals 16 and 18, into a pile delivery or other stack forming device 
20 for further in-line or off-line processing. 
The rotary sheeter section 14 includes a rotary cutting knife device 22 
upstream from a pair of nipper rollers 24, the upper of which is mounted 
for swinging movement as indicated by arrow 26. At the instant the rotary 
cutting knife 22 severs a sheet 28 from the leading end of the continuous 
web 12, web tension downstream of the continuous web 12 is lost and the 
nipper rollers 24 just upstream of the rotary cutter knife device 22 push 
the leading end of the web 12 beyond the rotary cutter knife device 22 and 
into the high speed belt section 16 with its upper and lower belts 30 and 
32. The upper and lower belts 30 and 32 are staggered across the web 12 so 
that they can be adjusted to make contact with the leading end of the web 
12 and pull it forward. The surface speed of these belts is considerably 
higher than the speed of the moving web 12 for several reasons. First, the 
belts 30 and 32 pull the web 12 taught to allow cutting under tension. 
Secondly, they quickly move the cut sheet 28 away from the endless web 12 
as soon as the rotary cutter knife device 22 has severed it from the 
continuous web 12. Finally, they create a space 34 between successive cut 
sheets so as to facilitate overlapping (shingling) of cut sheets as 
indicated by numeral 36 in the slow speed belt section 18 further 
downstream. Since the high speed belts 30 and 32 have rubber or plastic 
surfaces which rub on the slower speed leading end of the continuous web 
12 until the sheets have been cut and accelerated to the higher belt 
speed, electrostatic charges of the same negative polarity 38 are being 
generated on both sides of the just cut sheet 28. 
As the spaced apart sheets 28 coming from the high speed belt section 16 
move into the low speed belt section 18, the low speed belts rub on both 
sides of the sheets to cause them to slow down, and the sheets partially 
slide over each other as shown by numeral 36. This increases the 
electrostatic forces 38 of negative polarity on both sides of the sheets. 
Since the charge on the lower side of the upper sheet is of the same 
polarity as the charge on the top side of the next lower sheet, the sheets 
are being repelled by the electrostatic forces and a thin layer of air is 
permitted to stay between the sheets. No effort is made to remove the 
electrostatic forces of same polarity because they facilitate the sliding 
of the sheets over each other as they form a pile or a stack in the 
stacking device 20. The stack can form a skid load 40 up to about six feet 
(6') high. Often air blasts from a compressor 44 are deliberately 
introduced to enhance the floating of the sheets into the pile or stack. 
When the pile or stack builds up sufficiently, the weight of the upper 
sheets in the pile or stack overcomes the repelling force from the 
electrostatic charges of equal polarity and will push out some of the air 
entrapped between the lower sheets; however, the electrostatic forces 
remain in the accumulated pile or stack and cause problems when the skid 
load 40 is moved to an off-line, automatic destacking and cartonizing 
machine (FIG. 3) where reams of sheets are destacked from the skid load 40 
and cartonized. 
Depending upon moisture content, surface coatings and basis weight of the 
paper as well as the intensity of the electrostatic forces between the 
sheets, the weight of the upper 1/8" to 2" thick layers of sheets is often 
insufficient to overcome the repelling electrostatic forces existing 
between the upper sheets so that the boundary layers of air between the 
upper sheets are not pushed out. When the skid load 40 of paper is removed 
from the stacking device 20 to storage or further processing in the 
destacker/cartonizing machine, the upper sheets float around, get 
displaced, or even fall off. This results in costly damage and 
inefficiency. 
To neutralize the repelling force between the uppermost sheets of the skid 
load 40 before it is moved, an electrostatic emitting device 46, with its 
power pack 48, as shown in FIG. 2 is used to neutralize the repelling 
forces by deliberately applying an electrostatic charge 49 having a 
polarity opposite to the charge existing in the skid load 40. The weight 
of the uppermost sheets then pushes out the entrapped layers of air 
whereby intimate contact between the sheets is achieved allowing the 
normal friction therebetween to prevent undesired slippage. 
The electrostatic emitting device 46 is a charging bar made by SIMCO, 2257 
N. Penn Road, Hatfield, Pa. 19440 and described in U.S. Pat. No. 
3,735,198, which is hereby incorporated by reference. It provides up to 
25,000 volts, 8 mA. The depth of penetration varies depending upon the 
strength of the applied charge, the moisture content of the paper 
(conductivity), and the distance between the electrostatic emitting device 
46 and ground (earth). After the application of the charge, the skid load 
40 of paper can then be moved for further processing without the upper 
sheets sliding around or falling off. Since the deliberate application of 
electrostatic force penetrates only a short distance into the top of the 
pile, it does not neutralize the electrostatic force of opposite polarity 
further down in the pile. 
Turning now to FIG. 3, therein is illustrated a destacking and cartonizing 
installation generally indicated by numeral 50 and modified in accordance 
with the present invention. The installation 50 is manufactured and sold 
by Involvo, 33 Brook Street, West Hartford, Conn. The skid load of paper 
40 is placed onto an infeed conveyor 52 of the installation 50 and is 
automatically, advanced to the destacker 54. At the destacker 54 as shown 
in FIG. 4, the total height of the skid load 40 is automatically sensed by 
a proximity switch 56 and the ream thicknesses of the proper sheet count 
are then computer calculated by a computer controller 58 in a well known 
manner. The computer controller 58 is a conventional microprocessor of the 
type generally found as original equipment in the Involvo destacking and 
cartonizing machine 50 which is programmed to perform the functions of the 
present invention. The computer controller 58 causes a lifting platform 60 
holding the skid load 40 to be elevated as indicated by arrows 62 allowing 
reams 64 of the proper count to be automatically destacked and moved onto 
a conveyor belt 66 by a pusher 67. As the ream 64 is pushed off the skid 
load 40, the electrostatic charges 69 of equal polarity (still existing 
between the sheets of paper in the lower portion of the skid load 40 being 
destacked) reduce the friction between sheets allowing them to slide 
around under the slightest external force. To eliminate this problem, an 
electrostatic emitting device 68 is mounted at the beginning of the 
conveyor 66 emitting electrostatic charges 71 of positive polarity and 
moves automatically up or down as indicated by arrow 73 in accordance with 
a computer generated signal from the controller 58 corresponding to the 
height of the ream of paper 64 that is being passed underneath the 
electrostatic emitting device 68. 
The electrostatic emitting device 68 is identical to the electrostatic 
emitting device 46 shown in FIG. 2 and has emitter points spaced 1/2 to 2 
inches apart and positioned above the conveyor belt 66 close to the 
nearest edge of the skid load 40 in such a way that the ream 64 being 
destacked passes below the electrostatic emitting device 68. The tips of 
the emitter points are desirably no more than three inches above the top 
surface of the ream. The distance between the emitter tips and the top of 
the ream 64 is adjusted automatically as indicated by arrow 73 by the 
controller 58 to account for the thickness of the ream power pack 72 so as 
to keep this distance at a minimum for maximum efficiency. The actual 
movement indicated by arrow 73 can be accomplished by a servomotor or 
pneumatic cylinder with a position controller (not shown). Both, the 
voltage applied to the electrostatic emitting device 68 and the gap 
distance between the ream of paper 64 and the bottom of the electrostatic 
emitting device 68 can be adjusted by the machine operator to control the 
static charges that hold the ream together. A metal support plate 70, over 
which the conveyor belt 66 moves, is connected to ground potential (earth) 
to maximize static charging by the electrostatic emitting device 68. A 
power pack 72 is connected to the existing house power line 74 (usually 
120 or 220 Volts AC) and provides an adjustable (up to 25,000 Volts, 8 mA) 
voltage of positive polarity to the emitter points on the electrostatic 
emitting device 68. For optimum results, the air gap between top of ream 
64 and bottom of the electrostatic emitting device 68 is between 1/2" and 
1"; although acceptable results can also be achieved with somewhat greater 
air gaps provided the ream 64 being destacked is thinner and/or the 
operating voltage of the electrostatic emitting device 68 is increased. 
Typically, one can work with a voltage of 16,000 Volts when the gap is 
about 1" and the ream thickness is about 6", or when the gap is about 4" 
and the ream thickness is only 4". A 6" thick ream would require 
approximately 20,000 Volts when the gap is increased to about 4" and 
approximately 24,000 Volts when the air gap is increased to about 7". The 
computer controller can be used to automatically adjust both the gap and 
the voltage to achieve optimum results. 
Directing this electrostatic force 71 of positive polarity toward the top 
of the ream of paper 64 being destacked and against the grounded plate 70 
below the conveyor belt 66 balances the negative electrostatic charge 
existing between the layers of paper (described above) and thereby 
eliminates the pre-existing repelling force, and the positive charge 71 
being applied from the electrostatic emitting device 68 through the ream 
of paper 64 toward the grounding plate actually causes the individual 
layers of paper to attract each other and to temporarily bond the sheets 
together. This electrostatic bonding force will diminish with time until 
the deliberately induced excessive charges are again at their natural 
neutral level. The time to again reach this natural level varies from 
somewhat less than one hour to several days but is of more than sufficient 
duration to hold the reams or stacks of paper together during the 
processing and packaging operations. 
Referring again to FIG. 3, after the conveyor 66, the reams of paper 64 
then flow into a squaring station 76, on through inspection stations at 
numerals 78 through 88 and into a cartonizing machine 90 where a bottom 
carton is formed around the ream and so is a top lid. Once the skid load 
40 is emptied, an empty skid removal device 92 removes the empty skid from 
the destacker 54. 
Typically, reams of 500 sheets, each up to 0.012" thick (total of 6" thick 
pack) can retain the electrostatic holding effect long enough to process 
the ream through the entire machine until the finished and sealed cartons 
are palletized. The holding charge does diminish with time so that the 
sheets will no longer cling together when the printer receives the cartons 
of paper for further processing. 
Although the above specifically describes the intentional use of 
electrostatic charges to eliminate undesired sliding around of the 
uppermost sheets of paper on skid loads being destacked into reams for 
cartonizing, the method of this invention can be also used to eliminate 
problems with the upper sheets of paper sliding around on thinner stacks 
or packs of paper that are automatically moved from the batch counter 
delivery end of a sheeter for other types of further processing. Typical 
applications are on machines that automatically produce and package 
products such as loose leaf filler sheets, spiral bound notebooks, steno 
pads, memo pads, and the like. In each such case, the paper is unwound 
from one or more mill rolls, printed, perforated or otherwise improved and 
then cut into sheets which are collected in batches of predetermined count 
and then finished in-line. The in-line finishing operations may consist of 
punching file holes, applying front and back cover sheets, spiral binding 
or padding, wrapping and/or cartonizing. As the stacks or packs are being 
formed and batched, the same electrostatic charges of negative polarity 
described above are on both sides of each sheet in the stack or pack. As 
the collated sets, packs, stacks, or reams are automatically pushed from 
the collecting tray toward the finishing end of the processing line, the 
individual sheets want to slide around because the entrapped air and the 
electrostatic forces of equal polarity do not allow intimate contact and 
friction between those layers of paper. Deliberately forcing an 
electrostatic charge of positive polarity by the method of this invention 
into the negatively charged sheets of paper will temporarily hold the 
individual packages of sheets together during the finishing operations and 
permit increased production speeds by eliminating disturbances from the 
undesired sliding around of sheets. 
Thus, it can be seen from the foregoing specification and attached drawings 
that the method and apparatus of the present invention provides a unique 
means for causing intimate contact between adjacent sheets of paper in a 
stack thereby eliminating undesirable slippage. 
The preferred embodiment described above admirably achieves the objects of 
the invention; however, it will be appreciated that the departures can be 
made by those skilled in the art without departing from the spirit and 
scope of the invention which is limited only by the following claims.