Film perforating apparatus

Abstract

 The invention is concerned with an apparatus and a process for perforating synthetic plastics film, for example polyethylene or polypropylene, with an electrical spark discharge set up between a set of first electrodes which may be pointed rods and second electrodes which also may be complementary pointed rods or a single electrode which may be an electrically conducting guide plate, vane or roller. The film is drawn between the first and second electrodes. Each first electrode is supplied with high voltage sufficient to cause the spark discharge from a separate charged capacitor whereby each first electrode is electrically isolated from the others and thus a discharge from one first electrode does not prevent the formation of discharges from the other electrodes. The charging and discharge of the capacitors is controlled by a series of coupled switching units.

Description

[0001] This invention is concerned with film perforating apparatus.

[0002] In the manufacture of wrapping and packaging films from synthetic plastics materials, such as polyethylene and polypropylene, it is known for some packaging purposes to provide small holes or pores in the film at regular intervals to permit the controlled passage of vapours and/or gases. The pores may vary in size and density per unit area depending upon the intended use.

[0003] In one known method of forming pores in synthetic plastics film, the film is drawn between opposing electrodes across which a high electrical potential is applied of sufficient magnitude to initiate spark discharges between the electrodes through the film. Each discharge perforates the film forming a pore having a beaded edge derived from molten plastics material. However, when a series of high potential electrodes is used and each electrode is energised from a single source, on the formation of the first spark discharge between a pair of electrodes all the current passes through that discharge and so precludes spark discharges between other electrodes. As a consequence, the film is not perforated with the desired pattern of pores. It is known to limit such a current surge in electrode circuits by providing current limiting resistors so that discharges can take place between other electrodes.

[0004] The present invention is concerned with an improved film perforating apparatus for perforating synthetic plastics film with electrical spark discharges in which each electrode has an independent source of high potential.

[0005] According to the present invention an apparatus for perforating a synthetic plastics film by means of electrical spark discharges comprises a set of first electrodes directed towards but spaced from a second electrode or electrodes so defining a gap through which the film may be drawn between the first electrodes and the second electrode or electrodes, a capacitor associated with each first electrode, a unidirectional high voltage source and a switching means for electrically connecting the high voltage source across each capacitor for the purpose of charging each capacitor and then disconnecting each capacitor from the high voltage source and electrically connecting each charged capacitor between its associated first electrode and the second electrode or electrodes whereby each capacitor is discharged by an electric spark discharge passing through the film.

[0006] In a preferred form of the invention each first electrode is a rod directed towards the second electrode or electrodes. The rod preferably has a sharp point. The second electrode may consist of an electrically conducting film guide plate or roller, or a bar or a vane with which the film makes no. contact. Preferably, however, the second electrode consists of a series of rod electrodes, preferably pointed, directed towards and corresponding with the first electrodes.

[0007] The invention also includes a process for the perforation of synthetic plastics film comprising passing the film through a gap between a set of first electrodes spaced from a second electrode or electrodes and applying a high voltage across the first electrodes and second electrode or electrodes such that electrical discharges pass between the first electrodes and second electrodes through the film causing the film to be perforated wherein each first electrode is supplied with high voltage from a separate charged capacitor.

[0008] The invention also includes a plastics film when perforated by an apparatus in accordance with the invention.

[0009] A specific apparatus constructed in accordance with the invention will now be described with reference to the accompanying drawings in which,

Figure 1 is a partly diagrammatic front elevation,

Figure 2 is an end elevation,

Figure 3 is an enlarged view in plan of a fragment of perforated film and,

Figure 4 is a more enlarged view of a part of Figure 3.

[0010] Referring to Figures 1 and 2, a film 1 of polypropylene is drawn through a gap between a set of pointed rod electrodes 2 directed towards a second set of earthed pointed rod electrodes 3 which correspond mirror-like with the electrodes 2, connected to a stud 4 on a four stud switching unit 5 (one unit 5 for each electrode 2) which comprises a connector arm 6 mounted upon a common rotatable electrically insulated rod 7. The connector arm 6 in its vertical position, as shown in Figures 1 and 2, electrically connects stud 4 to a diametrically opposite stud 8 in each of the units 5. On rotation of the rod 7 through 90°, the connector arm 6 on each unit 5 electrically connects diametrically opposite studs 9, 10 as shown in dotted outline in Figure 2.

[0011] A capacitor 11, earthed by one terminal to a common earth 12 is connected by the other terminal to stud 8 on each unit 5 and each stud 8 is connected to corresponding stud 9. The remaining stud 10 in each unit 5 is connected by a common line 13 to a high potential DC generator 14 which, itself, is earthed by its second terminal.

[0012] In operation, the film 1 is drawn at a constant speed between the electrodes 2, 3. At the same time, the rod 7 rotates at a predetermined rate (by means not shown) which, in turn, rotates each arm 6 in each unit 5 in unison. When the arm 6 in each unit 5 connects stud 10 to stud 9, current flows to the capacitors 11 charging them with respect to earth to a high potential up to the level of the generator 14. On further rotation of the rod 7 the arm 6 in each unit 5 breaks contact with the studs 9, 10 and connects the studs 4, 8 when the charge on each capacitor 11 passes through to its associated electrode 2 and a spark discharge shown at 15 in Figure 2 occurs between each electrode 2 and its corresponding earthed electrode 3 through the film 1. Each discharge 15 perforates the film forming a pore 16 (Figure 3) which is edged with a bead 17 (Figure 4) of molten plastic material which solidifies on cessation of the discharge. The bead 17 reinforces the edge of the pore 16 and inhibits tearing.

[0013] The size of the pore 16 depends upon the current flowing and duration of the discharge which in turn depends upon the electrical capacity of the capacitors 11. As this can be determined with some precision, close control upon hole size can be obtained independently of the speed of travel of the film 1, unlike the electric spark perforation apparatus of the prior art in which the discharge ceases less precisely as a pore moves away from the electrode area and the spark gap becomes too long for the potential difference to maintain the discharge.

[0014] The distance apart of the pores 16 in the film 1 is determined in the transverse direction of the film 1 by the spacing of the electrodes 2 and in the longitudinal direction by the speed of travel of the film 1 and the speed of rotation of the rod 7.

[0015] Since the time of discharge is very short the pores 16 are formed over a very short arc of travel of the film 1 during which time the discharge follows the perforation initially formed in the film 1.

[0016] The pores 16 may range in size from about 6 microns to several hundred microns depending upon the capacity of the capacitors 11 and the magnitude of the potential from the generator 14 and the thickness of the film 1. In one example, the generator 14 was rated at 23 kilovolts and the capacitors 11 had a capacity of 3C picofarads. Polypropylene film 1 of thickness 25 microns was used and pores 16 formed in the film 1 had a diameter of about 50 microns.

[0017] In modifications of the apparatus shown in Figures 1 and 2, the second set of electrodes 3 are replaced by an earthed vane or plate set on edge or by an earthed roller over which the film 1 travels. In another form of the apparatus, the stud 4 and electrode 2 may be removed and the arm 6 may have pointed ends and serve both as a connector for the charging of the capacitor 11 when in the horizontal position and as an electrode when in the vertical position. It is understood that a film produced by the apparatus or process of the present invention is included in the scope of the invention.

Claims

1. Apparatus for perforating a synthetic plastics film by means of electical spark discharges comprising a set of first electrodes directed towards but spaced from a second electrode or electrodes so defining a gap through which the film may be drawn between the first electrodes and the second electrode or electrodes, a capacitor associated with each first electrode, a unidirectional high voltage source and a switching means for electrically connecting the high voltage source across each capacitor for the purpose of charging each capacitor and then disconnecting each capacitor from the high voltage source and electrically connecting each charged capacitor between its associated first electrode and the second electrode or electrodes whereby each capacitor is discharged by an electric discharge passing through the film.

2. Apparatus as claimed in claim 1 in which each first electrode is a rod directed towards the second electrode or electrodes.

3. Apparatus as claimed in claim 1 or claim 2 in which the second electrodes are rods directed towards and corresponding with the first electrodes.

4. Apparatus as claimed in claim 2 or claim 3 in which the rod electrodes are pointed.

5. Apparatus as claimed in claim 1 or claim 2 in which the second electrode is an electrically conducting guide plate or roller or bar or a vane.

6. A process for the perforation of synthetic plastics film comprising passing the film through a gap between a set of first electrodes spaced from a second electrode or electrodes and applying a high voltage across the first electrodes and second electrode or electrodes such that electrical discharges pass between the first electrodes and second electrodes through the film causing the film to be perforated wherein each first electrode is supplied with high voltage from a separate charged capacitor.

Drawing