Title:
METHOD AND APPARATUS FOR ELECTRICALLY PERFORATING SHEET OR WEB MATERIAL
United States Patent 3622751
Abstract:
A method and apparatus for perforating a web or sheet of material by passing a plurality of sparks through the web, with the first spark forming the perforation and the remaining sparks enlarging the perforation, and controlling the number of sparks to control the size of the perforation.
US Patent References:
Method and apparatus for controlling the porosity of electrically perforated sheet material
Menke - October 1950 - 2528158
Article perforation and food preservation
Shawfrank - February 1962 - 3020377
Apparatus for perforating and bonding moving sheets of material by electrical discharges
Bancroft et al. - May 1968 - 3385951
Multiple spark printer
Schmidt - December 1968 - 3419752
Apparatus for perforating sheet material
Parmela et al. - January 1965 - 3167641
Method and apparatus for controlling the porosity of electrically perforated sheet material
Menke - October 1950 - 2528158
Article perforation and food preservation
Shawfrank - February 1962 - 3020377
Apparatus for perforating and bonding moving sheets of material by electrical discharges
Bancroft et al. - May 1968 - 3385951
Multiple spark printer
Schmidt - December 1968 - 3419752
Apparatus for perforating sheet material
Parmela et al. - January 1965 - 3167641
Inventors:
Larive, Rene (Grand'Mere, Quebec, CA)
Knight, Richard L. C. (Grand'Mere, Quebec, CA)
Knight, Richard L. C. (Grand'Mere, Quebec, CA)
Application Number:
04/832592
Publication Date:
11/23/1971
Filing Date:
06/12/1969
View Patent Images:
Export Citation:
Assignee:
Consolidated, Paper
Limited (Nassau, BA)
Limited (Nassau, BA)
Primary Class:
Other Classes:
83/365
International Classes:
B23H1/02; B23H9/14; B26F1/28; B23H9/00; B26F1/00; H05B7/18
Field of Search:
219/383-384,121EB 83/16,171,365 156/272
US Patent References:
Primary Examiner:
Mayewsky, Volodymyr Y.
Claims:
We claim
1. A method of perforating a web comprising the steps of:
2. A method as claimed in claim 1, including controlling the number of perforations produced in relation to the relative rate of travel of the web with respect to the electrodes.
3. A method as defined in claim 1, including the step of providing a source of alternating voltage, causing said pairs of electrodes to discharge on every half-cycle of said alternating voltage, and adjustably controlling the number of said half-cycles thereby to control the number of said further electrical discharges.
4. A method as claimed in claim 3, including providing a series of control pulses of predetermined width, each control pulse corresponding to a half-cycle of said alternating voltage.
5. A method as claimed in claim 4, wherein the frequency of the control pulses is a function of web speed.
6. An apparatus for perforating a web comprising means for relatively moving the web in a path of travel between a gap defined by a plurality of pairs of electrodes, at least one electrode of each said pair of electrodes being a point electrode, a source of high-frequency electrical pulses electrically connected to said electrodes and having sufficient potential to cause electrical discharges to pass between each pair of electrodes and through the web whereby the first discharge between each pair of electrodes forms a perforation in the web and each succeeding discharge across each pair of electrodes enlarges the perforation formed, electronic control means for adjustably controlling the number of electrical discharges enlarging the perforation to control the size of the perforation formed.
7. An apparatus as claimed in claim 6, wherein said source is an alternating voltage source.
8. An apparatus as claimed in claim 7, wherein the adjustable control means includes electronic gate means controlling the passage of a predetermined number of half-cycles of alternating voltage from said alternating voltage source.
9. An apparatus as claimed in claim 8, wherein said electronic gate means controlling the passage of the half-cycles includes two oppositely conducting controlled rectifiers conducting respectively on the positive and negative peaks of each voltage cycle.
10. An apparatus as claimed in claim 9, wherein said electronic control means further includes means for generating a series of control pulses, said series of control pulses being applied to said control rectifiers to govern the conducting period for said control rectifiers.
11. An apparatus as claimed in claim 15, including means controlling the frequency of the control pulses as a function of web speed.
12. A circuit for use in a system to perforate a web comprising rate-of-movement-sensing means for generating a control pulse in relation to the rate of movement of said web;
13. In a circuit as set forth in claim 12, in which said current rectifier means comprises a pair of controlled rectifiers in back-to-back configuration for alternately generating a static discharge during each half-cycle of alternating current when made conducting by the control pulse.
1. A method of perforating a web comprising the steps of:
2. A method as claimed in claim 1, including controlling the number of perforations produced in relation to the relative rate of travel of the web with respect to the electrodes.
3. A method as defined in claim 1, including the step of providing a source of alternating voltage, causing said pairs of electrodes to discharge on every half-cycle of said alternating voltage, and adjustably controlling the number of said half-cycles thereby to control the number of said further electrical discharges.
4. A method as claimed in claim 3, including providing a series of control pulses of predetermined width, each control pulse corresponding to a half-cycle of said alternating voltage.
5. A method as claimed in claim 4, wherein the frequency of the control pulses is a function of web speed.
6. An apparatus for perforating a web comprising means for relatively moving the web in a path of travel between a gap defined by a plurality of pairs of electrodes, at least one electrode of each said pair of electrodes being a point electrode, a source of high-frequency electrical pulses electrically connected to said electrodes and having sufficient potential to cause electrical discharges to pass between each pair of electrodes and through the web whereby the first discharge between each pair of electrodes forms a perforation in the web and each succeeding discharge across each pair of electrodes enlarges the perforation formed, electronic control means for adjustably controlling the number of electrical discharges enlarging the perforation to control the size of the perforation formed.
7. An apparatus as claimed in claim 6, wherein said source is an alternating voltage source.
8. An apparatus as claimed in claim 7, wherein the adjustable control means includes electronic gate means controlling the passage of a predetermined number of half-cycles of alternating voltage from said alternating voltage source.
9. An apparatus as claimed in claim 8, wherein said electronic gate means controlling the passage of the half-cycles includes two oppositely conducting controlled rectifiers conducting respectively on the positive and negative peaks of each voltage cycle.
10. An apparatus as claimed in claim 9, wherein said electronic control means further includes means for generating a series of control pulses, said series of control pulses being applied to said control rectifiers to govern the conducting period for said control rectifiers.
11. An apparatus as claimed in claim 15, including means controlling the frequency of the control pulses as a function of web speed.
12. A circuit for use in a system to perforate a web comprising rate-of-movement-sensing means for generating a control pulse in relation to the rate of movement of said web;
13. In a circuit as set forth in claim 12, in which said current rectifier means comprises a pair of controlled rectifiers in back-to-back configuration for alternately generating a static discharge during each half-cycle of alternating current when made conducting by the control pulse.
Description:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to an improved method and apparatus for electrically perforating a sheet or web or material.
More particularly the invention relates to an improved method and apparatus for electrically perforating a moving web or sheet whereby the size of the perforations can be readily controlled.
2. Description of the Prior Art
Electrical perforating of webs or sheets by sparks is well known. However, the perforations are generally of one size only. The size of the perforations could be varied by increasing the size of the single spark used to form the perforations. However, it has been found extremely difficult in practice to do this.
Mechanical perforating of webs is also well known whereby the web is passed between mating rollers with one roller having pins cooperating with grooves in the other roller to punch perforations in the web. This apparatus has disadvantages in that the size of perforations cannot be varied unless the rollers are replaced. The pins become worn and dull rapidly thus requiring frequent replacement and servicing of the rolls. The pins also tend to bend and thus, do not perforate properly. The perforating operation frequently raises the surface of the web adjacent the perforation formed, thus forming a rough finish to the web surface. Additionally, when the pins are worn slightly, the perforation is not always completely formed, resulting in the formation of a flap which can close the perforation.
It is the purpose of the present invention to overcome the prior art disadvantages and provide a method and apparatus for perforating webs or sheets which can be easily controlled to form perforations of varying size, which does not mar the surface appearance of the perforated webs, which cleanly forms the perforations, and which is simple and reliable in operation.
SUMMARY OF INVENTION
The invention is particularly directed toward a method and apparatus for electrically perforating a sheet or web where the sheet or web is relatively moved between a pair of opposed electrodes and two or more electrical discharges of a current are passed between the electrodes through the sheet or web, the first discharge perforating the sheet and each succeeding discharge enlarging the perforation, and controlling the number of discharges to control the size of the perforation.
The apparatus and method are particularly suitable for perforating layers or laminations of plastic and paper webs or sheets, used in manufacturing bags. The perforations make the bags porous or air permeable and the porosity can be readily controlled merely by controlling the size of the perforations without having to change the number of perforations made or speed of operation of the perforating apparatus. Paper or plastic webs alone could be perforated if desired.
Polyethylene and other similar plastic materials, when produced as thin sheets or webs, are substantially impervious to the passage of moisture. This characteristic of polyethylene has led to it use as a coating material for kraft paper, for example, and more particularly, for coating kraft paper used in manufacturing multiwalled cement bags where it is important to keep the cement in a dry condition.
Prior to the use of the polyethylene-coated material for cement bags, the kraft paper in its uncoated condition had a porosity of about 400 ml./min. over a sample area of 5 cm. 2 with a pressure drop of 150 mm. of water (as measured with a Bendtsen porosity tester) which was adequate to permit the escape of trapped air through the bag wall as the bag was being filled with cement. With the advent of polyethylene-coated kraft paper for cement bags, it was found that although the polyethylene provided a substantial "moisture barrier" to keep the cement dry, it was so impervious that it prevented the passage of air therethrough, unlike uncoated bags; and accordingly, resulted in bursting the bags during filling or alternatively, required slow filling of the bags.
In order to overcome the undesirable characteristics of polyethylene-coated kraft paper, the coated paper was minutely, mechanically perforated in a manner to permit the escape of air while still forming a suitable moisture barrier.
However, mechanical perforating has the disadvantages previously referred to. The present apparatus has been found to suitably perforate the bag material with minute perforations to provide the necessary porosity while still providing a material which acts as a suitable moisture barrier. BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in detail having reference to the accompany drawings in which: FIG. 1 is a diagrammatic circuit view showing a preferred embodiment of the perforating apparatus; and
FIG. 2A to FIG. 2F illustrate the waveforms occurring in various portions of the apparatus
DESCRIPTION OF PREFERRED EMBODIMENT
The apparatus for perforating a web or sheet W of material includes 1 or more sets 3 of spaced-apart electrodes 1, 2. As shown in FIG. 1, the electrodes 1, 2 of each set 3 are spaced apart across the width of the web W to provide a gap through which the web W is moved. The electrodes 1, 2 are point electrodes although, preferably, one of the electrodes will be a line electrode. The electrodes 1, 2 of each set 3 are connected in series to the secondary 5 of a high-voltage transformer 4. The high-voltage transformer 4 can comprise an ignition coil similar to that used in automobiles.
A high-frequency alternating voltage is applied to the primary 6 of the transformer 4 from a voltage source 7. The voltage source 7 can comprise motor-generator set. The frequency of the motor-generator set normally can range from 400 Hz. to 10 kHz., although a wider range can be used when perforating webs running at very low or high speeds.
A control circuit 8 is provided to control the application of the high-frequency alternating voltage, the waveform of which is shown at A, to the primary 6 of transformer 4 and thus, the number of sparks passing between each pair of electrodes 1, 2. The control circuit 8 preferably includes a pair of oppositely conducting silicon-controlled rectifiers 9, 10 arranged in back-to-back configuration. Each SCR 9, 10 conducts on alternative half-cycles of the alternating voltage as shown by waveforms B and C. Each time one of SCR's conducts, a spark is passed between each pair of electrodes 1, 2. To control the number of sparks discharged, the length of time that the SCR's 9, 10 are capable of conducting is controlled. This is done by applying square wave control pulse of constant width, as shown by D, to the gates of the SCR's, to make them conducting. The width of the control pulse controls the number of sparks discharged between the electrodes. The control pulse is produced by a conventional pulse-shaping circuit 12 and fed to a primary 13 of a transformer 14, and through secondaries 15, 16 to the gate of each respective SCR 9, 10 connected thereto.
For a web W travelling between the sets of electrodes 1, 2 at constant speed, the pulse-shaping circuit can be set up to provide a control pulse of predetermined constant width and at a predetermined frequency which will, for a fixed frequency from the voltage source 7, cause a predetermined number of sparks to pass between the electrodes. The first spark perforates the web, and because the time for the predetermined number of sparks to occur is relatively much less than the speed of the web, each succeeding spark of the predetermined number will pass through the perforation to enlarge it. Thus, the size of each perforation can be easily controlled through the pulse-shaping circuit.
While the size of the perforations can be simply controlled by controlling the width of the control pulse as described, the size can also be controlled by means of the voltage source (in changing its frequency or voltage), either alone or in conjunction with the use of the control circuit.
If desired, the perforating operation can be directly controlled by the speed of travel of the web. As shown in FIG. 1, an actuating circuit 17 is connected to operate the pulse-shaping circuit 12. The actuating circuit can comprise a toothed steel wheel 18 connected to a rotating shaft of the web-feeding apparatus. A magnetic pickup 19 is located adjacent the periphery of the wheel 18 and every time a tooth 20 of the wheel 18 passes the pickup 19 during rotation of the wheel, a pulse E is generated to control the operation of the shaping circuit 12. The frequency of the control pulse is thus controlled by the web speed. This arrangement is useful where the speed of the web varies during its passage between the electrodes. The arrangement permits the web to be uniformly perforated despite variation in its speed.
Other actuating means to control the operation of the pulse-shaping circuit can be used. For example, for relatively slow web speeds, an optical sensing means can be used with the toothed wheel instead of the magnetic pickup to provide pulses controlling the pulse-shaping circuit.
The operation of the apparatus having reference to both FIG. 1 and FIGS. 2A- 2F shows diagrammatically the time relationship of the waveforms illustrated in FIG. 1. The output pulse E of the magnetic pickup 19 is shown in FIG. 2A. The output pulse E is fed to the pulse-shaping circuit 12 which provides control pulses D of constant predetermined width WP as shown in FIG. 2B. These pulses D are the input to the gates of the SCR's 9, 10. FIG. 2C shows the alternating voltage output A from the voltage source 7 having voltage half-cycles A 1 and A 2 . Upon the application of the input pulses D to the SCR's, the SCR's are made conducting on their respective forward biased half-cycles and each conducts or passes a number voltage half-cycles A 1 and A 2 , respectively, depending on the width of control pulse D as shown in FIGS. 2D and 2E to provide outputs B and C, respectively. Each of the half-cycles A 1 , A 2 are transformer coupled to the electrodes to produce a spark F for each set as shown in FIG. 2F.
The number of sparks F in each group or burst can be easily controlled by varying the control pulse width WP. The first spark F 1 passing between the electrodes 1, 2 forms the perforation in the moving web W, and each succeeding spark F 2 --F 6 of the group, taking the path of least resistance, passes through the perforation in the web to enlarge it. The burst of sparks occurs within a relatively very short time and the web has moved a short distance within this allowing all the sparks F 1 -F 6 to pass through the perforation.
If a large series of sparks are supplied in a single burst, the approximate first half of the series of sparks can form and enlarge a first perforation. This occurs as a result of the web moving relative to the electrodes. The sparks between the electrodes take a path of least resistance and as the first perforation moves away from the electrodes, the sparks follow the perforation until the distance of the perforation from the electrodes becomes so great that the electrical resistance of the unperforated web located between the electrodes is less than the airpath through the perforation which is moving away from the electrodes. Thus, the remainder of the sparks of the burst form a second perforation and enlarge it as well.
The apparatus, by way of example, can comprise a 3-kHz. 300-v. motor-generator set which is thus capable of producing 6,000 sparks/sec. if desired. With a web travelling at 300 feet/min. and a desired perforation spacing of one-half inch, the maximum time available for each burst of sparks is 4.1 milliseconds, and thus a burst will contain approximately 25 individual sparks. For any desired porosity, the number of sparks per hole depends on the speed of operations. Using a 24-v. ignition coil as the transformer 4, the 300-v. motor-generator set can provide a voltage of approximately 25 kv. at the secondary 5 of the transformer which is sufficient to produce a spark 1 inch long. By using a series of electrodes with small gaps, several sparks can be produced from a single transformer. For example, 12 electrode sets may be used to produce 12 rows of holes off the same transformer, A plurality of transformers and electrode sets can be arranged to extend across the width of the web or sheet. The holes produced are minute and barely visible to the naked eye. For example, to obtain a Bendtsen porosity of 120 ml./min./in. 2 in a web running at up to 400 ft./min. with one-half-in. hole spacing, the holes produced are 0.0055 inch in diameter.
The webs being perforated are preferably polyethylene (or similar plastic)-coated Kraft paper although other materials having similar dielectric qualities can likewise be perforated. Additionally, although the particular problem of maintaining a "moisture barrier" occurs in the cement bags, perforations of this character can also be used, for example, on glue flaps of various web or sheet materials in order to permit the moisture to evaporate and enable the glue to dry properly. Depending on the voltage source and type of web, several layers of plastic or plastic-coated papers can be perforated. For example, using a 300-v. motor-generator set, plastic material up to a thickness of 1 mil or plastic-coated paper with a plastic film less than 1 mil thick can be perforated.
Although a 24-v. ignition coil is suitable for use as transformer 4, other types of high-voltage coils could be used, such as for example, a 6-or 12-v. ignition coil.
1. Field of Invention
This invention relates to an improved method and apparatus for electrically perforating a sheet or web or material.
More particularly the invention relates to an improved method and apparatus for electrically perforating a moving web or sheet whereby the size of the perforations can be readily controlled.
2. Description of the Prior Art
Electrical perforating of webs or sheets by sparks is well known. However, the perforations are generally of one size only. The size of the perforations could be varied by increasing the size of the single spark used to form the perforations. However, it has been found extremely difficult in practice to do this.
Mechanical perforating of webs is also well known whereby the web is passed between mating rollers with one roller having pins cooperating with grooves in the other roller to punch perforations in the web. This apparatus has disadvantages in that the size of perforations cannot be varied unless the rollers are replaced. The pins become worn and dull rapidly thus requiring frequent replacement and servicing of the rolls. The pins also tend to bend and thus, do not perforate properly. The perforating operation frequently raises the surface of the web adjacent the perforation formed, thus forming a rough finish to the web surface. Additionally, when the pins are worn slightly, the perforation is not always completely formed, resulting in the formation of a flap which can close the perforation.
It is the purpose of the present invention to overcome the prior art disadvantages and provide a method and apparatus for perforating webs or sheets which can be easily controlled to form perforations of varying size, which does not mar the surface appearance of the perforated webs, which cleanly forms the perforations, and which is simple and reliable in operation.
SUMMARY OF INVENTION
The invention is particularly directed toward a method and apparatus for electrically perforating a sheet or web where the sheet or web is relatively moved between a pair of opposed electrodes and two or more electrical discharges of a current are passed between the electrodes through the sheet or web, the first discharge perforating the sheet and each succeeding discharge enlarging the perforation, and controlling the number of discharges to control the size of the perforation.
The apparatus and method are particularly suitable for perforating layers or laminations of plastic and paper webs or sheets, used in manufacturing bags. The perforations make the bags porous or air permeable and the porosity can be readily controlled merely by controlling the size of the perforations without having to change the number of perforations made or speed of operation of the perforating apparatus. Paper or plastic webs alone could be perforated if desired.
Polyethylene and other similar plastic materials, when produced as thin sheets or webs, are substantially impervious to the passage of moisture. This characteristic of polyethylene has led to it use as a coating material for kraft paper, for example, and more particularly, for coating kraft paper used in manufacturing multiwalled cement bags where it is important to keep the cement in a dry condition.
Prior to the use of the polyethylene-coated material for cement bags, the kraft paper in its uncoated condition had a porosity of about 400 ml./min. over a sample area of 5 cm. 2 with a pressure drop of 150 mm. of water (as measured with a Bendtsen porosity tester) which was adequate to permit the escape of trapped air through the bag wall as the bag was being filled with cement. With the advent of polyethylene-coated kraft paper for cement bags, it was found that although the polyethylene provided a substantial "moisture barrier" to keep the cement dry, it was so impervious that it prevented the passage of air therethrough, unlike uncoated bags; and accordingly, resulted in bursting the bags during filling or alternatively, required slow filling of the bags.
In order to overcome the undesirable characteristics of polyethylene-coated kraft paper, the coated paper was minutely, mechanically perforated in a manner to permit the escape of air while still forming a suitable moisture barrier.
However, mechanical perforating has the disadvantages previously referred to. The present apparatus has been found to suitably perforate the bag material with minute perforations to provide the necessary porosity while still providing a material which acts as a suitable moisture barrier. BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in detail having reference to the accompany drawings in which: FIG. 1 is a diagrammatic circuit view showing a preferred embodiment of the perforating apparatus; and
FIG. 2A to FIG. 2F illustrate the waveforms occurring in various portions of the apparatus
DESCRIPTION OF PREFERRED EMBODIMENT
The apparatus for perforating a web or sheet W of material includes 1 or more sets 3 of spaced-apart electrodes 1, 2. As shown in FIG. 1, the electrodes 1, 2 of each set 3 are spaced apart across the width of the web W to provide a gap through which the web W is moved. The electrodes 1, 2 are point electrodes although, preferably, one of the electrodes will be a line electrode. The electrodes 1, 2 of each set 3 are connected in series to the secondary 5 of a high-voltage transformer 4. The high-voltage transformer 4 can comprise an ignition coil similar to that used in automobiles.
A high-frequency alternating voltage is applied to the primary 6 of the transformer 4 from a voltage source 7. The voltage source 7 can comprise motor-generator set. The frequency of the motor-generator set normally can range from 400 Hz. to 10 kHz., although a wider range can be used when perforating webs running at very low or high speeds.
A control circuit 8 is provided to control the application of the high-frequency alternating voltage, the waveform of which is shown at A, to the primary 6 of transformer 4 and thus, the number of sparks passing between each pair of electrodes 1, 2. The control circuit 8 preferably includes a pair of oppositely conducting silicon-controlled rectifiers 9, 10 arranged in back-to-back configuration. Each SCR 9, 10 conducts on alternative half-cycles of the alternating voltage as shown by waveforms B and C. Each time one of SCR's conducts, a spark is passed between each pair of electrodes 1, 2. To control the number of sparks discharged, the length of time that the SCR's 9, 10 are capable of conducting is controlled. This is done by applying square wave control pulse of constant width, as shown by D, to the gates of the SCR's, to make them conducting. The width of the control pulse controls the number of sparks discharged between the electrodes. The control pulse is produced by a conventional pulse-shaping circuit 12 and fed to a primary 13 of a transformer 14, and through secondaries 15, 16 to the gate of each respective SCR 9, 10 connected thereto.
For a web W travelling between the sets of electrodes 1, 2 at constant speed, the pulse-shaping circuit can be set up to provide a control pulse of predetermined constant width and at a predetermined frequency which will, for a fixed frequency from the voltage source 7, cause a predetermined number of sparks to pass between the electrodes. The first spark perforates the web, and because the time for the predetermined number of sparks to occur is relatively much less than the speed of the web, each succeeding spark of the predetermined number will pass through the perforation to enlarge it. Thus, the size of each perforation can be easily controlled through the pulse-shaping circuit.
While the size of the perforations can be simply controlled by controlling the width of the control pulse as described, the size can also be controlled by means of the voltage source (in changing its frequency or voltage), either alone or in conjunction with the use of the control circuit.
If desired, the perforating operation can be directly controlled by the speed of travel of the web. As shown in FIG. 1, an actuating circuit 17 is connected to operate the pulse-shaping circuit 12. The actuating circuit can comprise a toothed steel wheel 18 connected to a rotating shaft of the web-feeding apparatus. A magnetic pickup 19 is located adjacent the periphery of the wheel 18 and every time a tooth 20 of the wheel 18 passes the pickup 19 during rotation of the wheel, a pulse E is generated to control the operation of the shaping circuit 12. The frequency of the control pulse is thus controlled by the web speed. This arrangement is useful where the speed of the web varies during its passage between the electrodes. The arrangement permits the web to be uniformly perforated despite variation in its speed.
Other actuating means to control the operation of the pulse-shaping circuit can be used. For example, for relatively slow web speeds, an optical sensing means can be used with the toothed wheel instead of the magnetic pickup to provide pulses controlling the pulse-shaping circuit.
The operation of the apparatus having reference to both FIG. 1 and FIGS. 2A- 2F shows diagrammatically the time relationship of the waveforms illustrated in FIG. 1. The output pulse E of the magnetic pickup 19 is shown in FIG. 2A. The output pulse E is fed to the pulse-shaping circuit 12 which provides control pulses D of constant predetermined width WP as shown in FIG. 2B. These pulses D are the input to the gates of the SCR's 9, 10. FIG. 2C shows the alternating voltage output A from the voltage source 7 having voltage half-cycles A 1 and A 2 . Upon the application of the input pulses D to the SCR's, the SCR's are made conducting on their respective forward biased half-cycles and each conducts or passes a number voltage half-cycles A 1 and A 2 , respectively, depending on the width of control pulse D as shown in FIGS. 2D and 2E to provide outputs B and C, respectively. Each of the half-cycles A 1 , A 2 are transformer coupled to the electrodes to produce a spark F for each set as shown in FIG. 2F.
The number of sparks F in each group or burst can be easily controlled by varying the control pulse width WP. The first spark F 1 passing between the electrodes 1, 2 forms the perforation in the moving web W, and each succeeding spark F 2 --F 6 of the group, taking the path of least resistance, passes through the perforation in the web to enlarge it. The burst of sparks occurs within a relatively very short time and the web has moved a short distance within this allowing all the sparks F 1 -F 6 to pass through the perforation.
If a large series of sparks are supplied in a single burst, the approximate first half of the series of sparks can form and enlarge a first perforation. This occurs as a result of the web moving relative to the electrodes. The sparks between the electrodes take a path of least resistance and as the first perforation moves away from the electrodes, the sparks follow the perforation until the distance of the perforation from the electrodes becomes so great that the electrical resistance of the unperforated web located between the electrodes is less than the airpath through the perforation which is moving away from the electrodes. Thus, the remainder of the sparks of the burst form a second perforation and enlarge it as well.
The apparatus, by way of example, can comprise a 3-kHz. 300-v. motor-generator set which is thus capable of producing 6,000 sparks/sec. if desired. With a web travelling at 300 feet/min. and a desired perforation spacing of one-half inch, the maximum time available for each burst of sparks is 4.1 milliseconds, and thus a burst will contain approximately 25 individual sparks. For any desired porosity, the number of sparks per hole depends on the speed of operations. Using a 24-v. ignition coil as the transformer 4, the 300-v. motor-generator set can provide a voltage of approximately 25 kv. at the secondary 5 of the transformer which is sufficient to produce a spark 1 inch long. By using a series of electrodes with small gaps, several sparks can be produced from a single transformer. For example, 12 electrode sets may be used to produce 12 rows of holes off the same transformer, A plurality of transformers and electrode sets can be arranged to extend across the width of the web or sheet. The holes produced are minute and barely visible to the naked eye. For example, to obtain a Bendtsen porosity of 120 ml./min./in. 2 in a web running at up to 400 ft./min. with one-half-in. hole spacing, the holes produced are 0.0055 inch in diameter.
The webs being perforated are preferably polyethylene (or similar plastic)-coated Kraft paper although other materials having similar dielectric qualities can likewise be perforated. Additionally, although the particular problem of maintaining a "moisture barrier" occurs in the cement bags, perforations of this character can also be used, for example, on glue flaps of various web or sheet materials in order to permit the moisture to evaporate and enable the glue to dry properly. Depending on the voltage source and type of web, several layers of plastic or plastic-coated papers can be perforated. For example, using a 300-v. motor-generator set, plastic material up to a thickness of 1 mil or plastic-coated paper with a plastic film less than 1 mil thick can be perforated.
Although a 24-v. ignition coil is suitable for use as transformer 4, other types of high-voltage coils could be used, such as for example, a 6-or 12-v. ignition coil.