CA1105096A - Method and apparatus for electrical perforation of webs - Google Patents

Abstract

METHOD AND APPARATUS FOR ELECTRICAL
PERFORATION OF WEBS.

Abstract of the Disclosure A web is electrically perforated at high hole density by applying alternating-current voltage across an electrode pair facing the web while pressurized gas is supplied to the electrode gap. Voltage amplitude level and gas flow level are selected to provide for the striking of multiple arcs per half-cycle, thereby enabling reduction in spacing between adjacent perforations in the web. Perforation practice involving preselection of cigarette filter tipping paper based on chemical composition of the paper is also disclosed as is apparatus for use in implementing the described practices, wherein electrode support members have gas flow conduits formed integrally therewith and wherein webs are taken up by a capstan drive unit resiliently biased into engage-ment with a web take-up roll.

Description

~ ~ 5 ~ ~ ~

This invention relates generally to the electrical perforation of web matPrial and more particularly to apparatus ~or perforating cigarette filter tipping paper.
In the manufacture of tobacco products, effoxts at diluti.on, i.e., controlled introduction of air in the course of smoki.ng, have looked to perforating the tobaceo wrapper and to perforating the filter or plug tipping paper. Since the ~ilter tipping paper is of substantially less area than the tobacco wrapper and portions of such area are not usefully per~oratable by reason uf being en-gaged by the smoker or a~hesive-coated, the perforated area per unit length of use~uL area need be increased to attain desired di.lution levels. O~fsetting considerations apply her~9 however, since as hole area per perforatlon is :increased, subdued hole visibility lessens and smoke issuance through the tipplng paper during non-pufing periods increases. An adequate compromise is to increase the derlsity o~ hol~s of individual size sufficiently small to render the ~ipping pap~r apparently unperforated and ~ub-s~antially ~on-smoke issuing. :
: For relatively hi8h dilution levels of interest, and consider~
ing tipping paper area usable ~or perforatlng, appllcants concluded tha~ holes in ~iæe ranging rom ten to one hundred microns in dla-meter and spaced mutually by less than ons millimet r are required.
On examln~ilter tipping papers perforated by commercial suppliers at sufeici.ent porosity (iOe., in per~orated area), applicants found ~he produc~ either insufficient in hole density or not available : in produc~ion volume. While applicants have no kn~ledge as to the details of methods by which sue~ examined filter tipping papers:~

were made, i~ w~s evident from hole charac~er.i~tics and density ~.

; :

that the papers were electrically perforated. In respect of one sample paper found to have sufficient hole density, subsequently received production samples thereof evidenced substantially less hole density, leading to the conclusion that the sample paper was laboratory-tailored an~ not reproducible in production volume.
Concerning electrical pexforation of webs, a vast number of prior efforts are known.
It is an object of the present invention to provide apparatus for high density electrical perforating ~f webs.
A more particular object of the invention is to provide perforated cigarette filter tippiny paper for use in d.iluted cigarettes.
In attaining the above and other objects, applicants have developed an electrical perforating system wherein a web is displaced relative to electrodes defining a spark gap therebetween with pressurized gas and voltage applied to the electrodes to provide web perforations per pair of opposed electrodes in excess of the customary single perforation per 20 half-cycle or like amplitude excursion of the voltage source.
By such practice, applicants provide filter tipping paper evidencing holes of size ranging from ten to one hundred microns in diameter and in density exceeding one thousand per square inch. In porosity, such paper is rom ten to twenty-five hlmdred Coresta permeability units, enabling its use in cigarettes having dilution levels of five to seventy percent.
In accordance with one broad aspect, the invention relates to a method of perforating a planar web comprising the steps of: laj supporting a pair of electrodes in spaced opposing relation to de~ine a gap therebetween for passage of , .

~ .

said webi ~b~ determining the hreakdown voltage level of said gapt with said web therein; (c) generating voltage having a succession of a-c half cycles each commencing and ceasing at times spaced by a preselected time period; (d) conveying said web linewise through said gap at such lineal speed that a prescribed lineal extent of said web traverses said gap during each said time period; and (e) while issuing pressurized gas onto the surfaces of said electrodes adjacent said gap and said web therein and applying said a-c half cycle voltage to said electrodes, establishing an amplitude level for said voltage ln excess of twice said gap breakdown voltage level and establishing a flow level for said pressurized gas to produce per~orations in said web spaced apart longitudinally of said web by a distance less than said prescribed lineal extent.
In accordance with another aspect, the invention relates to apparatus for perforating a planar web comprising:
(a) electrode means including first and second electrodes;
(b) first and second plate members supporting said first and second electrodes in spaced opposing relation to define a gap therebetween for passage of said web; (c) means for issuing plural separate streams o~ pressurized gas onto surface of éach of said electrodes adjacent said gap and at an acute angle wi.th respect to the plane of said web therein; (d) means or generating voltage having amplitude excursions with the passage of time and or applying such generated voltage to said electrode means; and (e) means for conveying said web linewise through said ~ap.
In accordance with a further aspect, the invention relates to apparatus for perforating a ~eb comprising: -~(a) electrode means includins first and second electrodes;

-2a- -~S6~6 (b) first and second plate members supporting said first and second electrodes in spaced opposing relation to define a gap therebetween for passage of said web; ~c~ means for issuing plural streams of pressurized gas onto each of said electrodes, such pressurized gas issuing means defining diametrically opposed issuance ports adjacent each of said electrodes;
(d) means for generating voltage having amplitude excursions with the passage of time and for applying such generated voltage to said electrode means; and (e) means ~or conveying said web linewise through said gap.
The foregoing and other objects and ~eatures of the invention will be further evident rom the following detailed description of 3~ ~

2b-- . , practices and appara~us and from ~he dra~ings~ wherein like re-ference numerals identify like parts throughout. In the drawings:
Fig. 1 is a plan VL~W of web perforating apparatus.
; Fig~ 2 is a front elevation partly in section taken alon~
line II-II of Fig. 1.
Fig. 3 i~ an electri~al schematic dia~ram o the circuit component~ ~or the preferred embodiment of the invent ion .
~ ig, ~ is a mechanical schematic drawing of web rewind appara~us~
Reerring to Figs. 1 and 2~ web 10 may be a continuou~ sh~et of cigarette ~ilter tipping paper and i8 advanced rightwardly in ; the drawings ~rom a web supply roll (not shown). Considering the far righthand portion o~ Fig. l; two cross-hatched areas A
and B are show~ on ~eb 10. Such areas are approximately five millLmeters in width W, are spaced apart by ~pacing S of approxi-mately twenty~four millimeters and are designated as the portions of web 10 which are per~ora~ed at high hole densi~y.
As is the custom in cigaret~e making, opposed tobacco rod sections and an intervening double ilter link are brought ~o and-~0 to~end abutting relation and ~ilter tipping paper applied theretofor ~oining the two tobacco rod ~ections and the intervening doubl.e ~iLter link. Subsequently a Cllt is made ~y~metrically, such as between ::ross-hatched areas A and B in Fig~ 1 severing .

the assembled unit into ~wo independent cigare~tesO While such clgare~te making practlce orms no part of ~he subject inven~ion~
it serveJ to lndics~e that" in the ul~ima~ely manufac~ured ~: cigarette, ~he areàs of peroratiorls in filter tipping paper made in acc~rdance with ~he sub3ect invention occupy a pOsiti : 3-substantially distant from the end o the filter engaged by the ~moker .
In electrically per~orating cross-hatched area~ A and B o elongate web 10, ~vur groups of electrodes are employed3 de signated RL (rear let~, RR (rear right), ~L (fro~t left) and FR
(front right). Th~ right and left pair~ of elertrode groups are identical in components but are displaced mutually laterally so that each group perforates one-haIf of e~ch o~ ~he cross-hatched areas A and B. This shortens the unsupported pa~h length involved 10 in the travel of elongate web 10 through the perforating system.
Each electrode group includes ~ive electrodes, identlfied by the reference numeral~ -1 through ~5~ usecl jolntly wi~h the electrode group designati~n. Thé electrodes in each group are spaced uni-formly longi~udinally of web 10 and are further disposed in unique locatio~s laterally of the web. Considering electrode group RR, electrodes ~R-l and RR-2 ar~ spaced laterally of web 10 by distance Dl, an amoun~ approximately one-tenth o~ the ln~ended , width of the cros~-hatched area A~ Li~e lateral spacing ~xists be~ween the remainder of th~ electrodes of group RR ~nd the same applies to ~he electrodes o group FR wi~h respec~ to area A and to ~he elec~rodes o.~ ~roup~ E`L and RL with re~pect to area B.
Laterally adjacent electrodes in groups RR and RL~ i.e., ~R~l and : ~L 51 are ~paced laterall~ by distance D2 also equal ~o appro~i-mately on~-tenth o the width o cross~hatched area A and ~he same applies~or ~lectrodes FL 1 and FR--5 w;~h respec~ to are~ B.
Accordinglys each electrod~ of the ~eries of ~he twenty electrodes confronting ~eb lO :in the~ course of its conveyance through the apparatus o~ Figso l and 2 ~s in acing relation to a unique lateral : -subzone o areas A and B. Und~r certain conditions, it is en-visioned that both right and left groups would not be needed and therefore a single electrode group would be used to perora~e a ~ull band wldth.
~ lectrodes RL~l through RL-5 and FL-l through FL-5 are supported in electrically insulative plate 12, being removably secured ~o the upper surface of the plate by fittings 14. The electrodes extend both upwardly of fittings L4, to be engaged by electrical conductors (not shown) and dow~wardly of plate 12 to terminate in needle-shaped ends adjacent the upper surface of web 10. A further electrically insulative plate 16 is disposed adjacent the undersur~ace o~ web 10 an~ supports electrodes in gap-de~ining al.ignment with the electrodes supported by plate 12.
In the sectionally shown lef~ward portiGn of Fig. 2, such lower electrodes RL-10 through RL-50, are in alignment respectively with elec~rodes RL-l ~hrough RL~S. Electrodes RR~l through RR-5 and PR~l through FR-S are supported . in plate member 18 adjacent the upper surace of web 10 and a ~urther plate mamber 2V supports an electrode in gap-defining alignment with each o these electrodes.
As shown in Fig. ~, plate ~0 has ~rontal electrodes FR-10 ~hrough FR-50, resp~ctively in alignment with electrodes FR-l through FR-5.
Each o~ the plate members 12~ 16, 18 and 20 includes an iden~ical air conduit network for issuing pressuriæed air or other gas onto web 10 and ~he electrode ends adiacen~ web 10. Consider-ing plate member lZ in this respec~, an inle~ conduit 22 e~tend~
from ~itting 24 to service longltudinal onduit 26 and ~rans~erse conduit 27 which:in turn service~ longitudinal conduit 28. Con duit 28 communicates with transv~rse conduit 29 ~hich in turn services longi~udinal condui~ 30. Branch conduits for issuing onto electrodes are associated with each of longitudinal conduits 26 and 30. For example, branch conduit 32a extends from longi-tudinal conduit 30 downwardly to an issue port 34 (Fig. 2) for issuing pressurized air applied to fitting 24 onto the surface o~ elec~rode RL-l. Branch condui~ 32b ~xtends to an issue port (not shown) opposite port 34 whereby further pressuri~ed air issues onto electrode RL-l. Fittings 36, 38 and 40 are provided for plate members 16, 18 and 20 respectively~ Completing the des-cription of parts shown in Figs 4 l and 2, roller ~2 i~ rotativelysupported on shaft 44 contiguous with web lO to support the web in its course be~ween it~ courses respectively between plate membe~s 12 and 16 and plate members 18 and 20. Exi~ roller 46 (Flg. 2~ supports web 10 as it e~it~ the space beL~een plate mem-bers 18 and 20. A housing ~not shown) is pivotally supported ~or moveEnent into substantially enclosing relation with the apparatus o~ Fi~s. 1 and 2 and is connected to a vacuum source for wi~h-dr~wal and disposal of ozone and other gases generated during operation of the apparatus. The housing has side openings ~or antry and exit of the web.
Re~erring to Fig. 3, the electrode-energizing syst0m o the invention is preferably alternating-current ~a-c) based and in-cludes~motor 48 driving g~nerator 50 ~hrough mecha~ical linkage 52~ The ~enerator output is applied over lines 54, 56 and 58 in para~llel to au~otransormers ~0, 6~, 64 and 66 which ~ay be con-ventional variacs, furnish:ing variably selectable amplitude vol-tage, respectively, to t~e primary winding~ o~ transormers 68, 70, 7~ and 74. Secondary windings o~ the transformers are - .. . .. . . ..

connected t:hrough resistors 76, 78, 80 and 82 to the electrod of ~igs. 1 and 2. Each transformer i~ arranged to serviee a distinct eleetrode group. Accordingly, re~istor 7~ is con-nected to electrode RL-l, resistor 78 to ele.ctrode RR-l, re-sistor 80 to electrode FL-l. and resistor 82 to electrode FR-l~
The electrodes in each group are connected correspondingly as indicated for lectrode group RL in Fig. 3. For ~his group, eleetrodes RL-10 and RL 20 are intereonnected by conductor 84, electrodes RL-30 and RL 40 are interconnec~ed by conductor 86, electrodes RL-2 and RL-3 are interconnerted by conductor 88 and electrodes RL-4 and RL-5 a~e interconnected by conductor 90.
Electrode RL-50 is connected to ground. In the illustrated cir-: cuit arra~gement, the gaps between aligned electrodes of each group are series-connected across each trans~ormer secondary.
The windings o~ all autotransformers and trans~ormers are ground-ed as i~dicated.
Turning now to Fig. 4 to complete khe description o~ appara-tu~ employed in practicing the invention, web 10 is conveyed from rol.ler 46 (discusged above in connertion wi~h Fig. 2) down wardly around ro~ler 92 and upwardly there~rom. In lts upward travel from ro].ler 9~, web 10 :is loaded sl.ightLy by roller 9~.
This roller is rotatively supported in lever 96 which i~ itsel~
rotatively supported about pin ~8. Wei~hted arm 100 i~ rigidly arranged wi~h lever 96 ~o rotate therewith. On breakage of web 10 or predetermi.ned le~senlng of longitudinal web tension, lever 96 rotatas courlterclockwise by gravity displacing arm 100 into engagement with opera~:or lû2 o~ switch 104, thereby opening ~he switeh and inte:xrupting web transpor~ and supply o~ voltage to ~7 -.

~ 3~ ~

the electrode groups. Beyond roller 94, web 10 traverses roller 106 and is fed to take-up or rewind roll l.0~.
The drive arrangement for web-take-up includes a driven roller 110 keyed to sha~t 112 which is rotatively supported at the rightward end of take-up control arm 114. Shaft 112 pr~fer-ably ~a~ a gear belt pulley ke~ed to its periphery :~or engaging gear belt 116 whereby roller 110 is driven counter~lockwise upon advance of chain 116 in the direction lndicated by the arrow~ in Fig. 4. At its letward end, control arm 114 supports gear belt pulley 118 for rotation relative thereto, gaar belt pulley 118 being keyed to trans~er shat 120. Shaft 120 includes a gear belt pulley keyed peripherally thereto engaged by a second gear belt 122. Gear bel~ 122 is driven in the direction indicated by the arrows by tachometer-driven motor 124, the motor output shaft 126 being keyed to a peripheral ~ear belt pulley engaging gear belt 122. The field winding of motor 24 is connected to excitation source ~ through variable re~istor 128 whereby the speed of conveyance of web 10 may be preselected. Once selec~ed, ~uch w~b spead is maintained constant by the tachometer control 20 in mo~or 124.
Housing 130 supports control arm 11.4 for rotation about the leftward control arm end. The angular position o~ arm 114 is controlled by positio~ing o~ lever 132, integral wlth arm 114 and rotatably secured to the output actuator 134 of air cylinder 13S. The cylinder is urnished with pressurized air through in~-let~138 whereby arm 114 is rotated to pLace roll~r 110 in driving engagemen~ with web 10 after ~he wrap is wound on roll 108. In operatlon, motor 124 awaits it~ energization, by suitable time ~8 delay circuitry, until suffioi~nt air pressure is developed in cylin~er 136 to insure that roller 110 is in driving engagement with web 10. In the course of web take-up, cylinder 136 serves to apply bias to control arm 114 to maintain driving engagement o~ roller 110 with roll 108.
By way of in~roduetion to one aspect of pra~tice hereunder, consider an a-c source operating to provide one ~houæand cycles per second (cps). The period per full cycle is 0.001 seconds or one millisecond. The half-p~riod, i.e., the duration of each half-cycle, is 0.5 ms. With such vol~age applied across opposed gap-deinillg electrodes, assume a web traversing the gap at a lineal speed of nine hundred feet per mi.nute (fpm)7 The lineal extent of web passing by the elec~rodes in the half-cycle period may be calculated to be 2.29 mm. A~ such voltage source ~re-quency and web speed one obtains a minimum hole spacing of ~.29 mm per electrode. In prior art practices, idenki~ied in applicantsl aforesaid s~atement under 37 C.F,R. 1.97 and 1.98, one preselects source frequency and web speed to achieve desired uniorm spacing and adjus~s sourc.e amplitude to attain desixed hole siza. Pro-vision may be made for modifyitlg on~ or more o~ these parametersto compensate for change in another o~ the parameters to maintain uni~ormi~y of hole spacing or constancy in porosi.ty.
In practice discussed below, adjacent hole spacing of 0.79 mm is:achieved hereby under ~he aforesaid conditions of web speed (900 fpm) and eouxce requency (1000 cps)~ an improvement in hole : ~ density o about 2.9 over th~t obtained by prior known methods.
: Such improvement of almost a factor of three provides for : porosi~y levels in fi.lter tipping paper suf~içient for the high ~g _ dilution level cigarettes of current interest.

In approaching the subject problem, conditions were sought under under which arcs rould be struck with higher time density than the single arc per hal-cycle or other voltage excursion of thP known prior art methods. In one aspect o the subject study, the practice o spark blasting was considered, i.e., applying pressurized gas through the spark gap ~o enhance arc-quenching by the removal of ionized media in the gap. Additively ~o this practice, voltage over-driving of electrod~s was under-taken, i.e., applying voltage to electrodes in amplitude levelsubstantlally in excess of the amplitude level reql~ired to inltiate an arc in a given gap length through a web of given dielectric const~nt. ~s results reported below indicate, proper coordination o these two parameters, gas flow and voltage ampli-~ude level for a given gap length and dielectric breakdown ~trength produces the phenomenon of multiple arc-striking per half-cycle or equivalent voltage excursion. Conversely, reductions in level o~ either of such parameters ~rom the coordination thereof producing such multiple arc-striking bring~ on an asymptomatic approach ~o the customary single-arcirlg condition.
; With a ga5 10w of 0.40 cubic ~eet per minute (c~m) per each electrode, a-c voltaga ~V~C) across ~he electrodes is varied in the table below with indica~ed results. Source requency is set at 1000 cps and paper speed at 400 fpm. The electrodes have a eonical arc tip wî~h interior a~gle o 60 and the gap there-~etween is 0.030 inch~ The web is commerciall~available Ecusta cork 52 mm ~ipping paper, 36 gram~m basis weight~

~ 5 ~ ~ ~

VAC HOLES Rl R2 R3 ___ _ _ _ 5000 38 1.0 1.0 1,0 6000 45 1~06 1.18 l.ll 7000 49 1.12 1.~9 1.15 7500 51 1.15 1.34 L.17 85Q0 5~ ~.19 1.53 1.29 For the source fr~quency and web speed conditions sta~ed, thP
singlP-arc ~ituation can be calculated to yieLd one hole per mm or 39 holes per 40 mm length. As indicated in the table, the single-arc si.tuation applies a~ 5000 volts~ which is an ampli-tude level abou~ twice the gap breakdown level (approximately 2500 volts). Rl defines the ratio ~on-time~ of time, per half-cycle, during which ~he amplitude level o~ appl.ied voltage VAC
exceeds the gap breakdown level, ~o the corresponding tlme or 5000 volts V~C. R2 defines the ratio o~ observed holes to the number of holes obtaining for the single-arc-strike situa~ion.
R3 ls the ratio of R2:to Rl. For 6000 volts, the number o~ holes observed ln a~40 mm web length increases to 45, an improvemen~ of 1.18~ As wlll be ob~erved, increasing VAC levels give rise ~o increased hole densities, an increase exceeding one and one-half beln~ reached at the V~C level o~ 8sao volts~ The values for R2 and R3 are further illustrative of the phenomenon whereby the change in numb~r of holes increases by a ~igure o~ merit in excess of the improvem~nt which might be expected simply from increased on t~ime. Thus, at 8500 volts VACl on-time increase~ by 1.19 (R2 : while hol density is improved by a ~a~tor Qf 1.53 (Rl), the further improvemènt beyond on-time being indicated ~R3 - 1.243.

A practical limi~ to maximum hole density has been observed which is dependent on th~ type of paper being perforated. Thu~, or a given paper, it has been found that a point is reached where greater hole density cannot be achieved de3pite ~ur~her increase of the control features of voltage amplitude and gas ~low. While the arc ~requency does increase, the arcs are ex-pended through previously perforated holes rather t~an through the unper~orated paper direo~ly be~ween an electrode pair. In thi~ case, ~he dlelectric breakdown s~rength of air through the path including a previous hole i5 less than the dielectric break-down strength of the ~traight pa~h including the unperfora~ed paper between the electrodes. This situation, i.e., non-perforating arcing, i8 ma:intairled until the strai~ht path becomes the path o~ leas~ resistance.
An und~rstanding of certain paper parameters has been achieved which helps to det~rmine ~he optimum paper type for a given u~age. The di~lectric breakdown strength is proportional to ~he basic weight o~ a particular type paper as more material (thickness) will incr~ase the dii-ference be~ween the straight pa~h plus paper and the longer path through alr and a previously perorat~d hole. Preferred thic~ness for 11ter tipping paper is ~rom 0.0005 inch to 0~01 lnch.
The chemical composition of the paper has also been found to bear upon maxim~m attainable hole density as well as contribute to material bulld-up on the electrodes during ope~tion. This ~: build-up, dependin~ upon its amount and composition, can change : ~ the elec~rical properties of the a~o as well a~ ac~ually p~ysical-ly tear or brea~ the web being perforated. Preerred paper .

- ~ . .. - .. . ... . . . ..

g~ ~

components, apart from the predominant struc~ural fiber, e.g.
cellulosic fiher, have been found to be non-acidic components, such as CaC03, MgC03 and TiO2o Filter tipping pap~r rendered chemically non-acidic is accordingly preferably preselec~ed in practice under the inventivn. Acidic product~ such as iron clay (Kaolinite~ tend to bond more easily with the metal elec~
trodes (i.e.S tungsten or other suitable metal~ and lead to harder build-up produc~s which are no~ removed readily by the moving paper and tend to finally break the web. The basic or neutral paper components, a~ opposed to acidic, such as CaC0 ~gC03 and TiO2 do not bond as readily to such metal electrodes~
Thus, deposits which may occur on the electrodes are more readily removed from the electrodes by the moving web and therefore web breakage is minimiæ2d.
In brief summary of the foregoing, in one aspect of practice herein, ~election o~ voltage level and gas flow is made relative to the voltag~ excursion tlme and web speed such ~hat per~oration spacing is Less ~han the lineal e~tent o~ web tra versing an electrode gap during such voltage excur~loll time. A
longitudinally and la~erally spaced grouping of n electrode pairs, n being ~'ive ~or each o~ the Fig~ l electrode groups, is preferably employed. As shown for rearward and fron~al elec~rodes, one rearward group is laterally successive to ~he other rearward group and one fron~al group i~ lateraLly successive to the o~her ronta1 group.
~ In anoth~r aspect of practice herein, a practice ~or per-forating filter tipping paper involves a preselection o~ paper bas~d on chemical composition, a selection found to bear upon hole densit:y.

~ ~ 5~t~ ~

In other aspects of the disclosure, apparatus desirably încorporates, in uni~ary structure, electrode support capability and pressurized gas conduits issuing onto supported elertrode~, wherein plate members have conduits a~d issue ports therefor structurally ln~egral ~herewi~h. In providing for enhanced re-sidence of a web in desired position throughout extended convey-ance thereof, one elec~rode pair or group may be arranged ~ucce~sive to another pair or group in tha direction o~ linewise conveyance of the web with a web support contiguously engaging the web and loca~ed successive to the one pair and precedent to ~he o~her. Apparatus ~or use in perforation practice has a wab take-up arrangement of resiliently biased capstan type wherein a pivotally movable control arm includes a drive link-age for a driven roller engaging the web has taken up and ai.r-pres~ure bia~ maîntains engagement of the take-up roll and driven roller during pivotal movement of the control arm occasion-ed by web take-up.
While a complete theoreti~al explanation of the phenornana reached by practice herein in its first-noted aspect i~ not pre-~ntly known, the attainment of holes iTI exce~s of that obtainingin prior knowrl practlces is established as be:Lng attributable to multiple arc str Lking a~ shown by e~ample and observed by oscillo~copic s~udies. An approach to the single-arcing situation has been observed ~o occur concomitantly wi~h change in the con-trol parameters of gas ~low and w ltage amplitude level, As noted, a-c volt~ge is preferably employed in practicing : the invention whe.reby ~wo voltage amplitude excursions per cycle are defined by the positlve and negative half-cycle~, the ormer :
. ~ . , ,:. . . . . . . '1 :
.. ..
.

S~ 6 co~mencing a~ 0 and ceasin~ at 180 and the la~er commencing at 180 and endirlg at 360. Ei~her half-cycle may be inverted to provide unidirectional half-cycles. Practice hereund~r likewise contelnplates pulsed direct current voltage which al~o provides voltage amplitude excursions suitable for use in effecting multiple arc-striking.
In respect of issue port 34 (Fig. ~), the results tabulated above were reached with 1/32 inch noæzles for each of the opposed pair of the noæzles for each oppo~ed electrode being pressurized with aix at 20 psi. Other nozzles employed were 1/6~ and 1/1~
inch, providing gas flow levels from 0.2 to 3.0cfm per electrode.
In ~he Fig~ 3 preerred electrical circuit arrangement, ; the secondary windings o each of transformers 68~74 preerably apply 12,000 volts across the series-circuit o~ fiv~. electrode pairs. On arcing, the voltage across each electrode gap i5 in the order o~ 400 vol~s, providing an overvoltage of 10,000 volts.
Wi~h gap breakdown voltage being 1,50~ ~o 2,500 vol~s for typical ilter ~ipping papers, the overvoltage multiple is seen ~o be from four ~o about seven during arcing. In an al~ernate circuit arrangement, a transformer is provided for each eLectrode pair, furnishing 6,000 volts ac~ the electrode pair ~nd hence an overvoltage multiple o:E rom about two to four~

'

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of perforating a planar web comprising the steps of:
(a) supporting a pair of electrodes in spaced opposing relation to define a gap therebetween for passage of said web;
(b) determining the breakdown voltage level of said gap, with said web therein;
(c) generating voltage having a succession of a-c half cycles each commencing and ceasing at times spaced by a preselected time period;
(d) conveying said web linewise through said gap at such lineal speed that a prescribed lineal extent of said web traverses said gap during each said time period; and (e) while issuing pressurized gas onto the surfaces of said electrodes adjacent said gap and said web therein and applying said a-c half cycle voltage to said electrodes, establishing an amplitude level for said voltage in excess of twice said gap breakdown voltage level and establishing a flow level for said pressurized gas to produce perforations in said web spaced apart longitudinally of said web by a distance less than said prescribed lineal extent.

2. The method claimed in claim 1. wherein said web is of thickness of from 0.0005 inch to 0.010 inch, said voltage amplitude level exceeding 5 kv and said flow level being from 0.20 cfm to 3.0 cfm per electrode.

3. Apparatus for perforating a planar web comprising:
(a) electrode means including first and second electrodes;
(b) first and second plate members supporting said first and second electrodes in spaced opposing relation to define a gap therebetween for passage of said web;

(c) means for issuing plural separate streams of pressurized gas onto surface of each of said electrodes adjacent said gap and at an acute angle with respect to the plane of said web therein;
(d) means for generating voltage having amplitude excursions with the passage of time and for applying such generated voltage to said electrode means; and (e) means for conveying said web linewise through said gap.

4. The apparatus claimed in claim 3 wherein said gas issuing means defines diametrically opposed issuance points adjacent to each of said electrodes.

5. The apparatus claimed in claim 4 wherein said electrode means comprises a group of n pairs of such opposed electrodes for perforating a continuous lateral expanse of width W of an elongate web material, the electrode pairs of said group being supported in facing relation to said expanse with a lateral spacing of W divided by n between laterally adjacent electrode pairs.

6. The apparatus claimed in claim 5 wherein said electrode means further comprises a second group of n pairs of such opposed electrodes supported in facing relation to said expanse and longitudinally separated from said first-mentioned group with a lateral spacing of W divided by n between laterally adjacent electrode pairs of said second group, said second group being supported laterally successively to said first-mentioned group such that each such group is in facing relation to a unique lateral extent of said expanse.

7. The apparatus claimed in claim 4 wherein said voltage generating means generates alternating-current voltage.

8. The apparatus claimed in claim 5 wherein said electrode means supports said electrode pairs successively longitudinally with respect to said web and wherein said voltage applying means applies said voltage across the first electrode in such electrode pair succession adjacent one side of said web and the last electrode in said electrode pair succession adjacent the other side of said web.

9. The method claimed in claim 1 wherein said voltage is alternating-current voltage.

10. The method claimed in claim 1 wherein said step (e) is practiced in part by issuing said pressurized gas from separate issuance locations whereby oppositely directed pressurized gas streams are issued into said gap.

11. The apparatus claimed in claim 4 wherein said means (c) includes plural separately located ports for issuance of laterally oppositely directed gas streams into said gap.

12. The apparatus claimed in claim 4 including first and second plate members supporting said pair of electrodes, said pressurized gas issuing means including first and second conduits extending respectively interiorly of said first and second plate members separately to first and second issue ports respectively adjacent said pair of electrodes.

13. The apparatus claimed in claim 12 wherein said first and second conduits and said first and second issue ports are structurally integral respectively with said first and second plate members.

14. The apparatus claimed in claim 12 including web conveying means comprising a web take-up roll and drive means for said roll comprising a roller, motive means for driving said roller, control arm means for supporting said roller at a given location thereon for driving engagement of said roller with said take-up roll, means for positively supporting said control arm means about a further location thereof distal from said given location and means for biasing said control arm to maintain said driving engagement of said roller with said take-up roll during pivotal movement of said control arm means in the course of the take-up of said web by said take-up roll.