Cutting device, particularly for slitting plastic material webs

Abstract: A cutting device is provided for slitting a web of plastics material which is continuously moving between first (3) and second (4) transfer rolls rotating about horizontal axes while being kept under mechanical tension between said rolls, the device including a knife (6) with a circular cutting edge (6B) composed of a thin disc with a bevelled edge (6D,6E) which is slowly driven in rotation about an axis substantially parallel to the axis of said transfer rolls (3,4) and which, in the cutting position, intersects from below, and over a circular segment, the oblique plane inclined at substantially 45° to the horizontal formed by said web between said rolls, the direction of rotation of said disc (6) being such that the tangential movement of the cutting edge (6D,6E) thereof in the region that intersects said web is in the opposite direction to the movement of said web.

Description: BACKGROUND OF THE INVENTION The present invention relates to a cutting device, which is particularly intended for slitting or edge trimming a web or film of plastics material that is continuously moving between first and second transfer rolls rotating about horizontal axes and which is mechanically kept under tension between the two said rolls, the device comprising a knife with a circular cutting edge mounted on a support arm (see for example FR-A-2152857). Such a device typically finds application in the continuous production of thin webs or films of plastics material, particularly where there is a requirement to eliminate the rough or irregular edges that are produced on such webs or films as a result of transverse stretching. Up until now, a razor blade secured to a holder was employed for such edge cutting. One disadvantage of using a razor blade was that it was used over a part of its length, leading to fast and irregular wear. The razor blade insert had to be frequently replaced with a subsequent detrimental effect on productivity. One object of the present invention is consequently to provide a cutting device in which the cutting edge of the knife has a high resistance to wear and which enables a considerable length of web or film to be cut before there is a need to replace or resharpen the knife. A further object of the invention provides a cutting device which regularly distributes wear over a considerable length of the cutting edge of the knife, thus ensuring that the knife has a long working life while still ensuring that a cut which is as good, true and regular as that provided by a razor blade is obtained. A further object of the invention consists in providing a cutting device which yields a cut having improved straightness such that it is no longer possible to visually distinguish the successive layers of web or film on a wound roll and which enables a roll of such material to be accurately cut into narrower elements. It is still a further object of the invention to provide a cutting device for a web or film of plastics material which provides more reliable elimination of possible electrostatic charges which can be present on the web or film of plastics material. A further object of the invention is to provide a cutting device for webs or films of plastics material which is less sensitive to clogging and wear as a result of dust and debris originating from such cutting. A further object of the invention is to provide a cutting device in which the debris and waste material from cutting can easily be collected. It is a further object of the invention to also provide an installation for the slitting of plastics material webs and films which allows an accurate and clean cut of the film edges to be achieved subsequent to transverse stretching of the film, such edges often exhibiting, at the point where the transverse and longitudinal stretching has been applied, a somewhat serrated edge and a thickness which is different from that of the rest of the film or web. SUMMARY OF THE INVENTION These and other objects are realized in a cutting device for slitting or edge trimming a web of plastics material which is continuously moving between a first and a second transfer roll rotating about horizontal axes and which is mechanically kept under tension between said first and second transfer rolls, comprising a knife consisting of a thin disc having a sharpened circular cutting edge that is rotated slowly about an axis that is substantially parallel to the axes of said first and second transfer rolls, the direction of rotation of said disc being such that the tangential displacement of the cutting edge thereof in the region that intersects said web is in a direction which is opposite to the direction of movement of said web, characterized in that said knife, when in the cutting position, intersects from below, over a circular-shaped sector, an oblique plane inclined substantially at 45° to the horizontal formed by the web between said first and second transfer rolls, and in that the cutting edge of said knife which passes through said web rotates in the proximity of said first transfer roll during operation of said device. By virtue of the fact that the knife is rotating, wear on the actual cutting edge thereof is regularly distributed over the whole of its periphery, and the fact of penetrating the film from below over a circular segment up to a position close to the first or feed transfer roll ensures an excellent cut is obtained while keeping the knife protected from offcuts and debris from cutting. According to one feature of the invention, the knife is detachably mounted on a shaft journaled at the end region of a fixed support arm, said shaft carrying a driven pulley which is driven in rotation by a continuous belt using a drive pulley linked to a drive motor means. According to another feature, the support arm has a concave hollowed upper surface which substantially follows, in the cutting position, the contour of the first transfer roll the cutting edge of said knife approches to a distance on the order of one millimeter to said first transfer roll. The cutting edge of the knife is advantageously ground to an angle of about 45° by providing on each side thereof a bevelled annular face obtained by a tangential grinding or machining operation. The invention also relates to installations for cutting or slitting weblike or film material, notably a continuously moving plastics material film, that incorporate a cutting device of the type specified herein. Other objects, features and advantages of the invention will become clear from the description that follows of some embodiments of the invention provided by way of non-limiting example and with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a side view of an embodiment of the cutting device according to the invention; FIGURE 2 is a fragmentary view taken in cross-section along line II-II of figure 1 of a first embodiment of the knife; FIGURE 3 is a fragmentary view in cross-section taken along line II-II of figure 1 of a second embodiment of the knife. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the embodiment illustrated in figure 1, the cutting device according to the invention, generally indicated by reference numeral 1, is designed to slit a film or web of plastics material 2 travelling in the cutting region between first and second transfer rolls 3 and 4, the film or web moving from the upper roll 3 to the lower roll 4 as indicated by arrows f. The film or web 2 may, for certain plastics materials, have a relatively large thickness of several tens of millimeters, while for other materials it may have a very small thickness of the order of only several micrometers. The web or film (hereinafter referred to as the "film") 2 is moving at high speed, which can reach several meters per second, between the first and second transfer rolls 3 and 4 in an oblique plane 5 inclined at substantially 45° with respect to the horizontal. The cutting device 1 consists of a knife 6 having a circular cutting edge 6b taking the form of a thin disc which is sharpened at about 45° on its peripheral edge and is made of a suitable material such as a suitably tempered hard steel. The knife 6 is mounted on axial shaft 7 which is arranged horizontally and parallel to the plane 5 of the film 2, rotating inside the end portion 8 of a holder arm 9 that is integral with a fixed frame structure which is not shown. Shaft 7 has a rotatably driven, rigidly fixed toothed pulley 10, preferably mounted on the side of the support arm end 8 opposite to the knife 6 (see figure 3). This driven toothed pulley 10 receives an endless toothed belt 11. The belt 11 is driven by a drive pulley 12 which is mounted on a shaft 13 the axis of which is parallel to the axis of shaft 7 and is driven rotatably by a speed reduction unit 14. In accordance with a first embodiment (figure 2), the arm 9 can consist of two parallely aligned metal plates 8a, 8b separated by cross members which are not illustrated and which maintain the two plates spaced by a distance i substantially equal to the thickness of the disc of knife 6. Shaft 7 is thus journaled at each side of the knife disc in a respective journal bearing 16, 17 provided in each one of the plates 8a, 8b. The endless belt 11 is preferably toothed but can be a thin flat belt which is retained in a flat groove as illustrated in figure 2, or alternatively a highly elastic round section belt 11b can be used, as illustrated in figure 3. The systems using belts and pulleys of figures 2 and 3 are completely interchangeable. In order to lock toothed belt 11 in position, the end 8 of the arm includes, a locking detent means 21 housed within the loop of the belt 11 and having a projecting tooth. The locking detent means 11 is secured to an eccentric component (not shown) by any suitable means such as by screws having a central axis 20. The projecting tooth, after displacement of the eccentric component, engages between two adjacent teeth of the belt 11 to lock the belt axially and prevent shaft 7 from turning. The direction of rotation of the speed reduction motor unit 14 is selected so that knife 6 rotates in the direction of arrow F, so that the tangential displacement of cutting edge 6b of the knife takes place in the opposite direction to that of the movement of film 2. During operation, the linear velocity of rotation of the knife's circular cutting edge is small compared to the velocity of movement of the film. The bearings of shaft 7, which can be radially-prestressed roller bearings, preferably deep-race ball bearings, exhibit minimal play while still allowing the knife to rotate with very limited opposition due to the low speed of rotation. This also ensures minimal increase in play with the passage of time. If necessary, an adequate and sealed lubrication system, grease for example, can be used for the single bearing (see figure 3), or for the double bearings (figure 2). Arm 9 has a generally tapered shape so that the knife 6 can be positioned when in the cutting position between the two rolls 3 and 4, the cutting edge 6b being extremely close (the spacing being shown by e in figure 1) to the first or feed roll 3. The distance e can, for example, be of the order of a millimeter. In order to render such close approach possible, the upper contour of arm 9 has a concave shape 15. During its movement, the film 2 encounters in plane 5, the cutting edge 6b of knife 6 which is inclined with respect to this plane 5 (leading to reduction in the cutting angle) and which slits the film. As can be clearly seen in figure 1, the knife 6 is arranged in such a manner that its cutting edge 6b intersects the plane 5 of film 2 along the line of cutting by penetrating, from below, over a circular sector 6a, through the film 2. The line joining the axes of the driven and drive pulleys 10 and 12 is slightly inclined (by 10 to 30°) with respect to the horizontal to take advantage of the available 45° angle area under the plane 5 of the film for housing the end 8 of arm 9. It is important for the film 2 to be kept moving under mechanical tension between the two rolls 3 and 4 so that the film does not lose contact with the knife. The radial and axial guiding provided by the bearing or bearings of shaft 7 ensure that the cutting edge 6b is accurately positioned. Where it is necessary to provide two clean and completely rectilinear edges on the film being produced, two cutting devices such as 1 are obviously provided each of them being located close to an edge of the film. A similar device can be used at each place where slitting of the film is required. As shown in figure 2, the cutting edge 6b of knife 6 has an angle x that is close to 45°. Obviously, other cutting angles are possible in order to take account, if necessary, of the nature of the material to be cut. Additionally, several manners of sharpening the cutting edge are possible. In accordance with the embodiment of figure 2, a single annular bevelled face 6c making an angle x with the plane face 18 can be provided by tangential grinding of the knife 6. In the embodiment shown in figure 3, two annular bevelled faces 6d and 6e making a respective angle of y and z with the cutting plane 19 of knife 6 can be used, the plane 19 being the plane through which the whole circular cutting edge 6b of the knife passes when there is no warp present. Otherwise stated, the cutting plane 19 which is shown extended by dash-dot lines in figure 3, is a plane that is parallel to the two parallel sides 18 and 18b of the knife 6 and which passes through the cutting or slicing edge 6b (in fact the circular edge) of knife 6. The angles y and z are usually substantially identical and equal to half the angle x, in other words 22.5°, which reconstructs the 45° sharpening of razor blades. The angles y and z can obviously be different as a function of the material that is to be cut or slit, and the sum of the angles can differ from 45° for the same reasons. In accordance with one embodiment shown in figure 1, the end 8 of arm 9 carries a brush or scraper means 26 to permanently clean or wipe the cutting edge 6b and the annular tapered sides 6c or 6d and 6e to remove debris adhering thereto and originating from the slitting operation. Such debris 27 falls onto a plate 28 which protects roll 4 and is evacuated without any risk of interfering with slitting or bring carried along with the moving web of film 2. A self-sharpening means (such as grindstones or the like) can also be provided on arm 9 for sharpening cutting edge 6b either continuously or at regular intervals, during production or during the time film 2 is not moving. The cutting device according to the invention has proved to be capable of achieving remarkable service, effectively slitting in excess of a thousand kilometers of film, before resharpening is required. Resharpening can be readily done in the workshop after removing knife 6 or the entire arm 9. In the embodiment shown in figure 3, which has undergone comprehensive trials, shaft 7 is permanently mounted in the end portion 8 of arm 9 using a rolling bearing 22 that is very rigid in the radial and axial directions (for example a deep race ball bearing or a roller bearing) with sealed lubrication. In order mount shaft 7 in a cantilevered manner into bearing 22 and on one side of arm 9, the circular knife 6 is supported laterally on an annular flange bearing 23 at the point of exit of shaft 7 and an integral annular securing ring 25 having at least one screw 24 is used to secure the bearing onto shaft 7. In order to remove knife 6 for the purpose of replacing or resharpening, locking means 21 is first pressed against the toothed belt 11 in order to lock the belt in position and prevent shaft 7 from rotating. After this, screw or screws 24 are loosened after which the securing ring 25, together with knife 6 which is integral therewith, are removed from shaft 7. Accurate refitting of knife 6 is achieved by slipping it along shaft 7 until the side 18 of knife 6 comes into abutment with the annular flange bearing 23. This is followed by tightening of screw or screws 24 of securing ring 25 against shaft 7 which has remained in its rotating position on its rigid bearing 22. It will be noticed, as has already been indicated, that the cutting life without the need for resharpening is considerably improved when compared to the service life of a conventional industrial razor blade. In the embodiment shown in figure 2, replacement of the knife 6, notably in order to resharpen it, can be readily achieved after removing the support arm 9. Removal of the knife 6 from its axial shaft 7 is carried out after removing the driven pulley 10, and after additionally removing one of the plates 8a or 8b. For reasons of productivity of the plastic film winding plant that is associated with the cutting device according to the invention, the support arm 9 can be very quickly removed after shutting off the power to the speed reduction motor unit 14, in order to withdraw the whole cutting device and replace it by another new or overhauled cutting device. The cutting device that has been removed is then subject to a full workshop service both as regards to the knife 6 (which is removed or resharpened), and the belt, the bearings and the speed reduction motor unit 14, all the parts being completely cleaned, checked and lubricated if necessary. Use of the cutting device according to the invention provides slit film edges which are particularly tear-free and crack-free. When the film 2 slows down or stops moving, rotation of knife 6 ensures that the film is cut over the whole region that is in contact with the knife's cutting edge, unlike with conventional razor blades where the cut can become irregular.

Claims: Cutting device for slitting or edge trimming a web (2) of plastics material which is continuously moving between a first (3) and a second (4) transfer roll rotating about horizontal axes and which is mechanically kept under tension between said first and second transfer rolls, said cutting comprising a knife (6) with a circular cutting edge (6b) mounted on a support arm (9) and consisting of a thin disc having a ground circular edge (6d, 6e) and being driven slowly in rotation about an axis that is substantially parallel to the axes of said first and second transfer rolls, the direction of rotation of said disc being such that the tangential displacement of the cutting edge (6d, 6e) thereof in the region that intersects said web (2) is in a direction which is opposite to the direction of movement of said web, characterized in that said knife (6), when in the cutting position, intersects from below, over a circular-shaped sector (6a), an oblique plane (5) inclined substantially at 45° to the horizontal formed by the web (2) between said first and second transfer rolls, and in that the cutting edge of said knife (6) which passes through said web (2) rotates in the proximity (distance e ) of said first transfer roll (3) during operation of said device. Device according to claim 1, characterized in that said knife (6) is detachably mounted on a shaft (7) journaled at the end region of a fixed support arm (9), said shaft (7) carrying a pulley (10) which is driven in rotation by a continuous belt (11) using a drive pulley (12) linked to a drive motor means. Device according to claims 1 and 2, characterized in that said support arm (9) has a concave hollowed upper surface (15) which substantially follows, in said cutting position, the contour of said first transfer roll (3) whereby the cutting edge (6b) of said knife (6) approches to a distance on the order of one millimeter to said first transfer roll (3). Device according to claims 1 to 3, wherein the cutting edge (6b) of said knife (6) is ground to an angle of about 45°. Device according to claim 4, wherein said knife cutting edge (6b) is ground to about a 45° angle by providing on each side thereof a bevelled annular face (6d, 6e) obtained by a tangential grinding or machining operation. An installation for cutting or slitting weblike or film material (2), notably continuously moving plastics material films, characterized in that said installation includes at least one cutting device according to claims 1 to 5.

Inventors: DE CAUWER JACQUES GILBERT, NICOLAI LUC, PETITJEAN GILLES, DE CAUWER, JACQUES GILBERT, NICOLAI, LUC, PETITJEAN, GILLES
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1992
Main classification: B26D1
IPC codes: B26D1