US2550366A - Method and apparatus for electroperforating sheet material - Google Patents

Description

April 24, 1951 J. w. MEAKER ET AL METHOD AND APPARATUS FOR ELECTROPERFORATING SHEET MATERIAL 2 Sheets$heet 1 Filed Oct. 25, 1946 INVENTORJ Jaw/v H WA/QVP J W MEAKER ETAL METHOD AND APPARATUS FOR ELECTROPERFORATING SHEET MATERIAL April 24, 1951 Filed Oct. 25, 1946 Patented Apr. 24, 1951 2,550,366 METHOD AND APPARATUS FOR ELECTRO- PERFO'RATING SHEET John W. Meakcr, New York,

H. McShan,

signor to said Meaker Application October 25, 1946, Serial No.

2 Claims. 1

Thi invention relates to a method and an apparatus for perforating sheet material by an electric discharge and more particularly to a method and an apparatus for the electro-perforation of sheet material having relatively high dielectric properties.

An object of this invention is to provide a method and an apparatus to facilitate the electroperforation of sheet materials having relatively high dielectric strength such as plastics, Koroseal, leather, leatherette, cellulose products, rubberized or plastic coated fabrics and the like. The invention is particularly adapted for the perforation of relatively thin sheets of such materials as ordinarily used for protective coverings of various sorts. The process and apparatus described and claimed herein is especially suitable for electro-perforating thin sheets of such material without damaging the material mechanically or affecting its appearance.

Reference is made to Patent No. issued October 30, 1945, to John W. Meaker and Edward H. Yonkers, Jr., in which the difficulties experienced in the electro-perforation of sheet material having high dielectric strength, unless the material is pretreated to reduce its dielectric strength are described in detail.

A preferred embodiment of the invention is illustrated in the accompanying drawings wherein:

Fig. l is a perspective view of an apparatus suitable for carrying out the invention having the electricai connections thereto illustrated diagrammatically;

Fig. 2 is a perspective view in section of a portion of a sheet of material prior to treatment;

Fig. 3 is a similar view of the material after it has been abraded;

Fig. 4 is a similar view of the material after it has been abraded and then perforated by an electric discharge;

Fig. 5 is a perspective view illustrating a modified apparatus for electrically perforating the ma terial; and

Fig. 6 is a side View abrading the material.

Referring to the drawings in detail, a thin sheet of material It, which is to be treated, is fed from a roll I l and passes between a pair of pressure rollers l2 and I3. The pressure rollelS l2 and 13 are preferably hard surfaced rolls made of steel or the like, and the upper roll I2 has a sheet of abrasive carrying material l4 fixed to its surface. However, the abrasive carrying material may, if desired, be placed on both of the T0115.

of a modified apparatus for MATERIAL N. Y., and Clarence Newark, N. J., said McShan as- The lower roll I3 is journalled at each end in supports l5. The upper roll I2 is journalled at each end in bearing blocks 16 which are slidably mounted in the supports l5. The pressure exerted between the rolls may beadjusted by pressure adjusting screws ll which extend through the upper portion of the support 15 and bear against the bearing blocks 16. If desired, compression springs may be installed between the rolls and the adjusting screws. It has been found that a pressure of from 30 to lbs. between the rolls will produce satisfactory indentation of the surface of a sheet of Koroseal six thousandths of an inch .006") thick for subsequent electro-perforation. However, it may be found terial to be indented and the thickness thereof. Thi treatment produces a series 0f indentations Illa in the sheet 10 which vary somewhat in depth and which are distributed at random over the surface of the material as illustrated in Fig. 3. The indentations have a lower dielectric strength than the untreated fabric with the deepest abrasions having the lowest dielectric strength. Thus, materials when indented in this manner may be more readily penetrated by an electric discharge.

After the sheet material has been indented, it is passed between a set of spaced electrodes I8 and IS. The spaced electrodes l8 and I!) are positioned. relative to the pressure rolls I2 and i3 and extend across the width of the sheet material to be perforated. The upper electrode 18 may consist of a series of individual electrodes connected electrically to each other and pivotally carried by a support 25. The upper electrode may extend across the entire width of the material or a number of rows of such electrodes may be provided to cover the entire width of the material by placing the electrodes in the various rows in staggered relation to each other. The lower electrode [9 may be a rod or series of rods positioned beneath the upper electrodes.

The electrodes I 8 and 19 are connected to opposite sides of the secondary of a transformer the primary of which is connected to a source of electric energy S. Thus, the transformer T supplies to the electrodes a high tension, alternating current having such characteristics that an electric discharge will pas between the electrodes and through the indented material as it passes between the electrodes thereby electroperforating the material. After the sheet has been perforated, it is taken up in a roll 2| that may be driven by any suitable means.

As previously mentioned, the discharge between the electrodes will occur at the point in the material where the resistance to the passage of such discharge is lowest due to the indentation. Since the indentations are distributed at random over the surface of the material, as illustrated in Fig. 4, the perforations illb are also distributed at random over the surface of the material so that there is no definite pattern or arrangement of the perforations.

The abrasive carrying material [4 may be sand-paper, garnet paper, emery cloth or the like, and may be either fine, coarse, or medium grade. The grade of the abrasive material will depend upon the resistance of the material being treated to such indentation, the thickness of the material and the size of the perforation or porosity desired. Normally, a fine grade of abrasive material may be used on thin material such as the Korcseal sheet mentioned. The main requirement in this respect is to have an abrasive material with a grain of sufiicient size to penetrate or indent the material substantially, and thicker materials normally require an abrasive material with a larger size grain. It is also desirable that the recesses between the grains of the abrasive material be approximately equal to or greater than the thickness of the material being treated.

As a coarse grade of abrasive material will produce a smaller number of indentations than a finer grade for any given area, there will be fewer perforations per square inch produced in the subsequent treatment of the material as the grain size of the abrasive material increases. However, an adequate number of perforations will be obtained even under such conditions as the number of indentations on the material will normally exceed the number of the electric discharges which are passed through any given area. Where there is an excess of indentations, the electric discharge will pass through the deepest of the indentations, or the one having the lowest dielectric strength of those between the electrodes at a given instant. For a given number of holes per square inch, the dielectric strength of the material should not exceed a maximum value determined by the equation:

where K equals dielectric strength along the surface of the material (approximately that of air). D equals the distance between indentations to be electro-perforated and an adjacent hole perforated. t equals thickness of material, and k equals dielectric strength of an indentation. If tk is greater than K (2D+t) then the electric discharge will occur through a previously perforated hole, rather than through a new indentation. Therefore, the value of It must be reduced sufficiently by weakening the material as described to obtain a given number of holes per given area. This is accomplished by employing an abrasive of suihcient size to meet such requirements.

While emery cloth or the like has been specifically mentioned above, various types of abrasive materials may be used satisfactorily. For example, a solid Carborundum wheel may replace the steel roller covered with sandpaper or a steel roller having a roughened surface may be used. This apparatus and method may also be used in perforating materials which do not have particularly high dielectric strength. For example, in perforating cellophane for use as a container for material such as tea, from which an infusion is to be made, it has been found particularly advantageous to emboss the cellophane sheet by means of a fine mesh metal screen carried on one of the rollers. This indents the cellophane sheet as it passes between the rollers and embosses it with a regular pattern. When the cellophane is then subjected to an electric discharge, the discharge will pass through the indentations eliminating the tendency of the holes to run together and thereby weaken the material when such indentations are not present.

The porosity or the number of perforations per square inch, may also be controlled by the speed at which the material travels and the frequency, voltage and current of the electric discharge, all of which will be governed by the characteristics of the material being treated and the results which it is desired to obtain. Inasmuch as the regulation of these factors is well understood by those skilled in the art, they will not be described in detail here.

By this means, the size and distribution of the perforations may be controlled so that perforations may be obtained that will permit the passage of air through the material for ventilation or other purposes, but which will not materially decrease the materials resistance to water penetration or destroy the water repellant characteristics of the material, if any. Also, in the case of the cellophane tea bag, larger perforations may be obtained, for instance, by increasing the current, that will readily permit water penetration if that is desired.

With material of extremely high dielectric strength, it may be found desirable to moisten the surface of the material just prior to its electro-perforation in order to reduce the dielectric strength of the material. The moistening of the material assists the passage of the electric discharge through the material, and it also has a tendency to prevent carbonization which appears as unsightly black rings at the perforations in some materials.

In the modified form of electric discharge apparatus illustrated in Fig. 5, a piece of copper screening 22 has been substituted for the pivoted electrodes l8. This copper screen 22 extends across the width of the material to be treatedand is carried by a support 23. The screen electrode 22 extends downwardly at an angle and contacts the surface of the material to be treated above the lower electrode 19. This arrangement provides an electrode which is flexible and which is of simple construction. A flexible metal plate may also be used as an electrode with very satisfactory results.

In the modified form of abrading device shown in Fig. 6, there are two sets of oppositely disposed pressure rolls 24, and an endless belt 25 of abrasive carrying material is placed around each set of the pressure rolls. The sheet material l0 passes between the opposed surfaces of the abrading material and frictionally engages therewith so that abrasive material is moved by the sheet at the same speed as the sheet material. This arrangement provides an indenting surface which will not require replacement or renewal as frequently as the single rolls. If desired, a single endless belt may be used on one set of the rolls to indent only one surface of the material.

While a preferred embodiment of the invention has been described and illustrated herein, it will be understood that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

We claim:

1. A method of electro-perfor-ating sheet material, comprising the steps of applying an abrasive carrying material to a surface of said sheet material under sufficient pressure to produce a series of indentations distributed at random over the surface of the material, and then passing said sheet material between a pair of spaced electrodes extending substantially across the width of said indented surface and perforating said material at random by discharging an electric current between said electrodes and through said material.

2. An apparatus for electi e-perforating sheet material of a relatively high dielectric strength including a pair of oppositely disposed rolls, said rolls being positioned to engage with the opposite surfaces of a moving web of sheet material, an abrasive material carried by at least one of the rolls and contacting under pressure with one sur- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 692,834 Davis Feb. 11, 1902 1,410,899 Duncan Mar. 28, 1922 2,141,869 Kom'g Dec. 27, 1938 2,388,069 Meaker et a1 Oct. 30, 1945

Images