GB2084626A - Machine for producing pleated capacitors - Google Patents

Abstract

A pleated capacitor manufacturing machine in which double sided metallised polymer film 14 is pulled from a roll 16 at a defined tension and the strip is turned through a right angle over a curved bar 20 before passing through pleating teeth 22 and through a set of pinch rollers 24 to form the pleated capacitive portion of the capacitor. Leads are subsequently attached to form the pleated capacitor. Preferably, the curved bar 20 is of square section and air bearings 32 cause the film to "float" above the surface thereof. Alternatively, a barrel roller or a segmented roller, Figs. 4 and 5 (both not shown), may be used. <IMAGE>

Description

SPECIFICATION Improvements in or relating to machines for producing pleated capacitors The present invention relates to machines for producing pleated and to capacitors formed by the machine.

The pleating process as described in the invention is an improvement over winding in that winding is an intermittent process. There is a limit to the speed with which a capacitor can be wound because controlled acceleration into the winding cycle and deceleration at the end of the winding cycle is required to prevent film breakage. The production of discrete components at the beginning of the manufacturing process results in subsequent stages e.g.

lead attachment and encapsulation also having to be done in an intermittent fashion. Thus the whole process of capacitor manufacture is forced to be inter mittentwith a limited maximum speed. The equipment required to subsequently handle the individual items produced at the beginning of the production line is necessarily complex and expensive. Greater speeds can only be achieved by parallel operation of the slower stages of the production process which again adds to complexity and expense.

The production of capacitor bodies by folding or pleating of the dielectric and electrodes is known.

Any early patent -- British Patent 759,614 tells how this can be done. However, with the materials available at that time (separate tissues or polymer film together with tin foil electrodes), pleating or folding processes were difficult to control and work in a reliable manner. With the advent of vacuum metallised film for use as a combined electrode/dielectric system the problems of handling film were reduced and a number of developments on the earlier patent have been put forward as a means of making pleated or folded capacitors using such metallised material.

These methods are not however found to be practical because of major shortcomings in the methods used to achieve this pleating. The two areas where shortcomings are most apparent are: (1 ) Wrinkling and creasing of the film in the folding process.

(2) Damage to the very thin (30-50 nm typically) metallisation layer on the polymer dielectric due to scratching during the process.

It is an object of the present invention to achieve a method of manufacturing capacitors which is quicker and simpler than the winding technology currently employed.

The immediate application of this patent is in the production of low value metallised film capacitors.

The invention has possible applications however in other electronic components where multilayer structures are requiredm Examples are other types of capacitor or windings for chokes and transformers.

Possible areas of non electronic interest are (1) filters, e.g. for dust extraction or liquid/solid separation in chemical engineering or (2) large area membranes e.g. for kidney dialysis or desalination.

The inventive features of the patent are: (1 ) the use of a curved bar of the correct geometry combined with the change in direction of travel of the film as a means of initiating pleating across a web of material.

(2) The use of air bearings to prevent damage to the metallisation on the film.

(3)Arrangementof"upgoing"and"downgoing" pleats on opposite faces of the capacitor so that side contacts can be made.

(4) The use of half height end pleats so that two contacts can be made to one face or strips can be stacked together sideways while maintaining correct electrical functioning, thus forming a unit of higher capacitance.

The present invention therefore provides a machine for making pleated capacitors including means for rotatably mounting a roll of metallised film, drive means for said film to draw the film over a curved surface, thereby changing the direction of travel of said film through substantially a right angle, through a plurality of pleating teeth for forming the pleats in said film and through a set of pinch rollers to squeeze the pleated film into a compact package.

Preferably the curved surface is formed by a curved bar with a substantially sharp corner, said bar being provided with air bearing means to support said film away from the substantially sharp corner.

Alternatively the curved surface may be formed by a series of rollers.

The metallised film is preferably provided across its entire width with non metallised strips to enable the finished pleated capacitor to be cut to provide non metallised edges.

In a first embodiment the metallised film is provided with a non-metallised edge to inhibit short circuits in the finished capacitor.

In a second embodiment the metallised film is located on the rotatable mounting and guided through the pleating teeth in such a manner that a half pleat is formed on one face of the finished capacitorthusenabling both terminal leadsto be attached to that face.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a first type of pleated capacitor produced according to the present invention, Figure 2 shows a second type of pleated capacitor produced according to the present invention, Figure 3A shows in elevation a pleating machine according to the present invention, Figure 3B shows the machine of Figure 3A in plan view, Figure 4 shows a barrel roller for use on the machine of Figure 3, Figure 5 shows an alternative curved segmented roller for use on the machine of Figure 3.

This invention is concerned with a pleating machine whereby doubly metallised film is rapidly and continuously pleated in a longitudinal direction to produce a capacitor strip which is cut to length on The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

emergence from the pleating machine, the cut length determining the capacitance. Certain features of the capacitor thus produced as a result of the pleating process are also included. The characteristics of the pleating machine are such that: 1. The film will flow through the machine at a rapid rate in an amenable fashion so that regular uniform pleats are obtained.

2. Creasing and damage to the film is avoided.

3. The film metallisation is essentially undamaged by the process. This particularly is important in longitudinal pleating because, as will be seen later, longitudinal scratches, e.g. at the apex of a pleat, would result a capacitor with only part of its electrode area connected.

So that the operation of the machine shown in Figures 3A and 3B can be better understood the structure of the complete capacitor body is described below. With reference to Figure 1 the pleated strip as it emerges from the pleating machine is shown. It should be emphasised that as the film is metallised on both sides the essential capacitor structure of electrode/dielectric/electrode is present in the unpleated structure. The pleating operation merely serves to package the inconvenient sheet structure into a shape more acceptable as an electronic component. The strip is cut into short segments, and short circuiting at the cut edges may be prevented by fusing, sawing, solvent action or other methods already reported in British Patents 1,229,565:1,242,467; and 1,268,962.Alternatively, as shown in Figure 1, metallisation free stripes 10 can be provided across the metallised film and cuts performed through these areas. This approach of course, requires attention to the metallisation stage of the process to produce the necessary metallisation free transverse bands. By the application of pressure or pressure and heat during the final pleating stage a compact thin pleated continuous strip structure is produced. Two further important features of the pleated strip are: 1) The metallisation 12 does not coverthe full depth of the outer pleats but stops, for instance, half-way down the outer pleat. This feature is easily achieved by longitudinal metallisation free bands at the edges of the original double metallised web.The advantage of this feature is that the possibility of short circuiting around the edge X to the metallisation on the other side of the film is prevented.

2) The pleating is so arranged that at one side of the pleated body there is a "downgoing" pleat (the near side in Figure 1 ) and atthe other side an "upgoing pleat" (the far side in Figure 1). Contacts can then be applied at points A A' (major face contacts), the contacts being to opposite electrodes. It should be noted that in the interior of the pleated body adjacent pleats have electrodes of like polarity pressed into contact with each other. Because of this large scale short circuiting of like polarity electrodes current from a contact such as A does not have a long distance to travel to any specific area of electrode but "hops" across the pleat edges (B B') to reach the specific area.

As an alternative to contacts A A1 contacts may be applied at B B'. This may be done in conjunction with metal spraying (Schooping process) of these edges as is done with conventional woundn staggered film type capacitors. Examination of Figure 1 will show that opposite electrodes are again contacted by this procedure.

Afurther possibility for applying contacts is shown in Figure 2. Here one of the outer pleats is only half the height of the strip. Contacts at C and C' make connection to electrodes of opposite polarity. This form of connection may be attractive where it is required to take both lead wires from the same major face of the capacitor e.g. for a dual-in-line package type of design for the finished component.

There is a further advantage to this particular embodiment of a pleated capacitor. The capacitance values obtainable with a single strip are limited by the length of strip cut off to make the capacitor (for a component intended for printed circuit board or similar use this could be no more than say 1 cm) and also by the width of the web available for pleating (perhaps 1-2 m). Capacitance values are limited by these considerations to 20.1 1LF. However, if two or more pleated strips are taken through pinch rollers together a thicker strip is produced of higher capacitance.If metallisation free transverse bands are used to maintain isolation at the cut edges of capacitors derived from such a strip it would of course be necessary to maintain the strips in registry so that when they were rolled together the metallisation free bands would be aligned. It is also necessaryto have the correct geometry for the outer pleats of each strip for correct electrical functioning. Referring to Figure 2 if two such strips are brought into sideways contact by rolling together, proper electrical continuity is achieved between the strips. Incorrect geometries of the outer pleats result in short circuiting of the capacitor or only part of the capacitor being connected. A thicker strip produced by this process may be contacted in mannersAA1, B B' or C C' in the same way as a single strip.

The pleating machine according to the present invention is shown in Figure 3A and B. Double metallised film 14 is held on a roller 16 which is equipped with a light friction brake 18 so that a gentle tension on the film pulls it from the roll in a continuous jerk free fashion. The film then travels vertically to a curved square section bar 20. The film is drawn over this bar to a horizontal direction. The actual orientations are unimportant but the relationship is. It is practically desirable for success that a change in direction of 90" is achieved over a well defined sharp edge. The film is thereafter drawn through a set of teeth 22 which impart pleats to the film and finally to a set of pinch rollers 24 which compress the pleated strip 26 and provide the tension which draws the film through the machine. The function and importance of each part of the machine will now be discussed in more detail.

Firstly consider the transition from vertical to horizontal orientation of the film over a curved bar. In any practical, continuous, longitudinal pleating process the film must come initially from a roll at full width and at the end of the process must come essentially to a point where the actual pleated compact 26 is formed (in this case at the pinch rollers). It is essential that the transition between the two states be achieved in a smooth continuous manner if wrinkling and damage to the film is to be avoided. If a vertical to horizontal transition is not used and, for instance, film is drawn from the roll and taken through pleating teeth to the pinch roller in just one dimension the low friction of the metallised film allows it to slide sideways across the roll as it is drawn off causing the film to wrinkle and bunch before it comes to the pleating teeth.This uncontrolled change in geometry of the incoming film cannot be accommodated by the teeth and thus further wrinkling and damage to the film ensues. Neither is it possible to successfully take film from a vertical to horizontal direction over an edge of the incorrect geometry. Consider for example the use of a straight edge instead of a curved edge in Figure 3. If the edge is smooth and applies no frictional forces to the film, the film can again slip sideway and tend to bunch in the centre of the bar. The correct geometry of pleating can only be maintained if the film travels from the roll to the bar 20 in a parallel fashion and initiates pleating at the bar, across the full width of the film, in a uniform manner. This can only be achieved with a straight bar if sufficient sideways frictional force on the film keeps it spread out along the full width of the bar.The maintenance of such force requires pressure on the surface of the film which damages the metallisation to an unacceptable extent. Only a bar with the correct radius of curvature is successful, this radius being the distance from the bar to the junction of the pinch rollers as shown in Figure 3. With this correct curvature there are no tangential forces acting on the film as it moves over the bar, the film is maintained at full width over the bar, without bunching or creasing by the radial tension in the film acting from the pinch rollers.

The bar 20 must be of such a section that the edge over which the vertical to horizontal transition of the film occurs is sharp. A square section bar is chosen for reasons which will become apparent later. It may be thought that it would be less damaging to the film to travel over a roller. To maintain the correct curvature either a barrel roller 28, (Figure 4) or a segmented roller 30 (Figure 5) would have to be used as an alternative to the square section bar. The barrel roller is difficult to use as instead of the pleats coming together at the pinch rollers in a fanwise manner in two dimensions the pleat wou Id be brought over the surface of a cone whose height is r and base radius also r. This geometry results in poor conformance to a parallel sided uniform strip which is required at the pinch roller exit.Segmented rollers also give a poor result as small creases tend to form at the edges of each roller segment. These creases are incorporated as creases in the finished compact.

Accepting that a sharp edged bar of the correct curvature is the preferable geometry capable of success the problem remains of preventing damage to the delinate metallisation on the film as it passes over the bar. This is achieved by the use of correctly placed air bearings 30 (Figure 3A). These air bearings consist of two rows of nozzles from which compressed air is ejected. This air causes the film to "float" a very short distance above the surface of the bar. As the film is drawn over the edge of the bar a cushion of air is trapped against the horizontal and vertical faces of the bar and the film is drawn over the sharp edge without actually contacting it.This may be demonstrated by turning the air supply to the nozzles off; the film still runs over the bar smoothly but microscopic examination of the metallisation shows itto be longitudinally scratched by its journey over the bar. It can now be seen why a square section bar is preferred since two flat faces at right angles are required to establish the conditions for an air cushion. Other shapes, e.g. a right triangle, meeting these conditions could also obviously be used but a square section was chosen for reasons of manufacturing convenience. Angles slightly greater or smaller than 90" may be used but practical success is achieved by using substantially 90".

The pleating teeth 22 may in a similar fashion also be provided with air bearings (not shown) to prevent contact of the film with the teeth. The problem of contact with the teeth is not in practice found to be so serious as may be expected however. Once the machine is running correctly and with the correct alignment of the various parts, pleats will run from the curved bar 20 to the pinch roller 24 without need for constant guidance by teeth. It is a measure of the success of this pleating system that pleats form themselves naturally without being forced to conform into unnatural geometries by teeth etc. This should be compared with other patents concerning pleated capacitors e.g. Offenlegungsch rift 261 33o0 where teeth are required to provide continuous guiding forces to the pleats and thereby unavoidably damage the film.

It might be thought therefore, that the teeth could be dispensed with entirely but this is not so for two reasons. Firstly because of slight misalignments in the system, inhomogeneities in the film and imper fectwinding on the original roll, the film will need a little guiding from time to time to keep it in the correct pleat profile. It should be noted (see inset Figure 3A) that the teeth are widely set apart (a gap of 1-2 mm is provided) and that the film flows through them generally without contact. The second reason the teeth are required is for start up of the machine.

The top set of teeth are removable and to start the machine, film is pulled from the roll by hand taken over the curved bar, laid on the bottom set of teeth, and roughly bunched at the end to push it into the pinch rollers (these are spring loaded and may be opened to admit the bunched film).

Drive to the pinch rollers is then applied and as the film is drawn through, the top set of teeth are slowly lowered into position. This progressively introduces the correct pleat profile into the film which thereafter flows smoothly through the system. Using this procedu re the first few metres of film are of course scrap but this is of little consequence as only one start-up per reel of film is required.

Claims (7)

1. A pleated capacitor manufacturing machine including means for rotatably mounting a roll of metallised film, drive means for said film to draw the film over a curved surface thereby changing the direction of travel of said film through substantially a right angle and through a plurality of pleating teeth for forming the pleats in said film and through a set of pinch rollers to squeeze the pleated film into a compact package.

2. A pleated capacitor manufacturing machine as claimed in claim 1 in which the curved surface is formed by a curved bar with a substantially sharp corner, said bar being provided with air bearing means to support said film away from the substantially sharp corner.

3. A pleated capacitor manufacturing machine as claimed in claim 1 in which the curved surface is formed by a series of rollers.

4. A pleated capacitor manufacturing machine as claimed in claim 1 or claim 2 in which the metallised film is provided across its entire width with nonmetallised strips to enable the finished pleated capacitor to be cut to provide non-metallised edges.

5. A pleated capacitor manufacturing machine as claimed in any one of claims 1 to 4 in which the metallised film is provided with a non-metallised edge to inhibit short circuits in the finished capacitor.

6. A pleated capacitor manufacturing machine as claimed in claim 1 in which the metallised film is located on the rotatable mounting and guided through the pleating teeth in such a manner that a half pleat is formed on one face of the finished capacitor thus enabling both terminal leads to be attached to that face.

7. A pleated capacitor manufacturing machine substantially as described with reference to the accompanying drawings.