A METHOD OF CUTTING A POLYMER FILM AND A DEVICE FOR WEB-CUTTING

Abstract: This method of cutting a polymer film (F) is based upon relative linear or primary movement between the film (F) and a cutting edge (22) in a film-cutting position and uses no secondary cutting motion; films having a thickness in the range of from 10 micrometers to 500 micrometers are used and the cutting edge (22) is a substantially continuous edge (22) at the periphery (21) of a substantially circular steel sheet disc (20) having a thickness in the range of from 10 micrometers to 500 micrometers and a diameter in the range of from 10 millimeters to 100 millimeters; the disc (20) is maintained for a first and preferably predetermined length of cutting operation in a first position where a predetermined first incremental portion of the cutting edge (22) is in film-cutting position; then, disc (20) is indexed, i.e. step-­ switched, for removing the first incremental portion of cutting edge (22) from the film-cutting position and for moving a subsequent incremental portion (23) of cutting edge (22) into film-cutting position; then the subsequent portion (23) is held in position for another length of cutting operation; this indexing step is repeated until a major part, at least, of the substantially continuous edge (22) of steel sheet disc (20) has been indexed. The subject web-cutting device (2) has an indexing blade in the form of a substantially circular steel sheet disc (20) having (a) a substantially uniform thickness in the range of from 10 micrometers to 500 micrometers, (b) a diameter in the range of from 10 millimeters to 100 millimeters, and (c) a substantially continuous cutting edge (22) extending around the periphery (21) of disc (20).

Description: This invention relates to a method of slit-cutting polymer films and to a device for carrying out the method. Various machines used for continuous production or processing of polymer films, e.g. winders of the type disclosed in U.S. Patents Nos. 1,687,928, 2,915,255, 3,494,566 and in my U.S. Patent No. 4,191,341, may require a continuous cutting operation to be performed at the moving polymer web, generally at a marginal area thereof and in longitudinal direction (parallel to machine direction), e.g. for continuously opening a blown polymer film hose at its sides so as to produce two separate polymer webs that can be wound up separately. Further, a wider polymer web may require division into a number of parallel strips, or a web may require longitudinal side portions to be cut away, e.g. after coating, etc. Any such operation requires prolonged cutting of polymer films, generally at relatively high speeds, and dulling of the cutting edge must be prevented or controlled if undesired tearing or rupturing of the polymer film is to be prevented. Broadly, four types of mechanical cutting modes can be distinguished in polymer film cutting: (a) press cutting, i.e. when the cutting edge is pressed onto the polymer film which in turn is supported by a surface or anvil; (b) shear cutting, i.e. when two cutting edges interact upon the polymer film in the manner of shear blades; (c) rotational cutting, i.e. when a circular knife is rotated at high rotating velocities in the general manner of a circular saw while simultaneously moving relative to the polymer film; (d) slit cutting, i.e. when a sharp edge of a blade is contacted with an unsupported polymer film. Most devices used for continuous longitudinal cutting of polymer films are those developed in the paper industry, i.e. press cutting or shear cutting devices comprising rotatable circular knives which, in press cutting, are pressed onto a counter-roller having an extremely hard surface or, in shear cutting, cooperate with a second rotatable circular knife to form a shear edge; in either case, the circular knives used must be of a rugged construction, i.e. have a substantial thicknes of several millimeters to support the stresses of coacting with the support roller or the second knife. In general, previous devices for longitudinal continuous cutting of polymer films have performed satisfactorily with many conventional polymer films; however, there is a growing tendency to include various additives in polymer films to improve or modify certain properties and some typical additives are very abrasive. As a consequence, rapid and, sometimes, uncontrolled dulling of the knives becomes a problem of increased importance. The possibilities to compensate the problems of knife-dulling by improving the performance of conventional press-cutting or shear cutting knives when cutting polymer films are limited, however, for reasons of cost of materials and maintenance. Devices for slit-cutting of fast moving (e.g. 10-150 m/min) polymer films having a thickness of from 10 to 500 micrometers and using conventional razor blades have been available on commercial machines. Such previously used cutters will be explained below in connection with Figure 1c. However, when cutting polymer films that contain abrasive additives, the cutting life of such throw-away razor blades is limited and controlled placement of fresh edge portions of such razor blades into cutting position presents problems in any prolonged (e.g. 10 hours or more) continuous operation. Reversing a blade or exchanging one blade for another requires an interruption of the operation. Thus, the invention aims at preserving the advantages of the extremely sharp edge provided by such throw-away blades while minimizing or avoiding the problems caused by knife-dulling in prolonged continuous operation (e.g. 100 hours or more), notably when cutting polymer films that contain highly abrasive components, such as antiblocking agents. It has been found according to the present invention that this aim can be achieved by a method as defined in claim 1. Preferred embodiments of the inventive method have the features of claims (2-3) the device for carrying out the method according to the invention is defined in claim 4 and preferred embodiments of the device have the features of claims (5-10). Blades or knives in the form of steel sheet discs having the features (a), (b) and (c) of claim 5 are believed to be novel per se and will be termed "indexing blade" herein; the indexing drive preferably includes a conventional step-motor. Normally the "size" of the indexing steps will determine the lengths of the incremental edge portions of the disc moved successively into cutting position after each edge portion has remained therein for a length of cutting operation. With a given diameter of the disc as defined herein, the step size can be defined in terms of angular degrees of a circle that encompasses 360°. Theoretically, when a polymer film held in a plane, such as a web, is relatively moved against a cutting edge held normally to the plane, the actual cutting position of first interaction between polymer film and cutting edge is a line on top of the cutting edge, the length of that line being defined by the film thickness, as the cutting edge is assumed to have virtually no "thickness". Thus, the minimum length of the incremental cutting edge portions required in steps (B) and (C) of the inventive method is the thickness or gauge of the polymer film (10 to 500 µm). In practice, a moving web of polymer film may deviate somewhat from its theoretical plane of travel so that the location of the film-cutting position (or first point of contact between polymer film and cutting edge) may deviate somewhat from its theoretical position; the length of each incremental portion of the cutting edge will, typically, be in the range of from about 0.5 to 5 mm, preferably about 1 to 4 mm. As the peripheral length of a circular disc having a diameter between 10 mm and 100 mm will be in the range of from 31 to 314 mm, it is apparent that discs of such diameters will provide up to several hundred of incremental edge portions for use in the cutting position. For many purposes and with disc diameters in the preferred range of from 20 to 60 mm, each indexing step will involve changing of the angular position of the disc (viewed normally to the disc plane and with 360° for full turn) by shifting the angular position of the disc in steps of from about 1° to about 10°; typically, the disc thus provides from about 30 to about 300 discrete portions of the cutting edge that can be used in succession in the cutting position until the blade is exhausted. The term "length of cutting operation" could be quantified in terms of the geometrical length of the polymer film that has been cut; in practice, the length of the cutting time period is more convenient, notably as the speed of the web is frequently defined by a producing or processing plant where continuous cutting is required. For example, on such a time basis ("length of cutting operation" expressed in terms of "period of cutting time") an incremental cutting edge portion of an indexing blade of the device according to the invention having a Rockwell hardness C of at least 50 will have a cutting life in continuous operation in the order of, for example, from 100 to 2000 minutes with typical web speeds (10 to 150 meters/minute, e.g. 20 to 80 meters/minute) at film gauges in the 50 to 500 µm range and with various polymers containing abrasive additives. Thus, a typical indexing blade of the device according to the invention will have a cutting life in the range of days to weeks and some simple tests will be sufficient to establish optimized use periods for the incremental portions and the indexing blade. Preferably, the length of the cutting operations is monitored, e.g. on a time basis or on the basis of the cut web length, for generating signals that can be used to automatically control the indexing "frequency", i.e. the operational distance between the subsequent changes of the angular blade position. For example, the above values of cutting life periods of 100 to 2000 minutes per incremental edge portion would indicate a typical indexing frequency range of 14 indexing steps per 24 hours to 5 indexing steps per week of continuous operation. While these examples of suitable indexing frequencies are given for illustration, the virtual infinity of variations in the polymer material-plus-additives systems may make it advisable to optimize the indexing frequency. In practice, indexing periods of below 50 minutes (between two subsequent shifts) will be the exception, while periods well above 1000 minutes have been found to be operable in many instances. Visual inspection of the cut edges of the film will show when a cutting edge portion is becoming blunt by the appearance of undulations, stretch-orientations and irregular ruptures. So, when optimizing operation with the indexing blade according to the invention, one would watch the time period between putting the first incremental edge portion into cutting position and the first appearance of undulations and/or stretch-orientations. This period might typically be in the order of several hundred minutes and a portion, say 50 to 75% or higher, say up to 90 %, of that period might be selected for the time length of each interval between subsequent indexing motions. Starting operation with a fresh, i.e. sharp, indexing blade according to the invention, such blade will be "exhausted" or "blunt" upon completion of a full indexing cycle and will be replaced by another fresh indexing blade. To warn operating personnel of the approaching end of an indexing cycle of a blade, various optical or acoustic signals can be used; preferably, the indexing drive, e.g. step-motor, is geared to produce or trigger such signal. In general, replacement of an exhausted indexing blade should be simple and, preferably, entail no substantial effort for demounting and remounting of the blades. To that end, a blade support member for easy blade exchange may be provided on the indexing drive, e.g. a magnetic plate and positioning means on the support member and/or the blade; preferably, the blade is provided with at least one perforation for cooperating with at least one corresponding protuberance, e.g. a pin or the like, on the blade support. As the indexing blade must be refrained from rotating, such positioning means can serve as a lock for preventing blade rotation. In general, the continuous cutting edge of indexing blades of the device according to the invention should be substantially as sharp as the cutting edge of conventional razor blades of comparable thickness. As the production of razor blades is a highly developed and mature art, it is believed that the term "provided with a razor-type edge" provides for a clear definition in the subject context and providing steel sheet in the required thickness range with a razor edge is known per se. Consequently, novel indexing blades of the device according to the invention can be manufactured by conventional grinding and honing techniques but starting from circular or polygonal pieces of steel sheet meeting the required thickness and shape parameters, and further providing a finished hardness of at least about 50 RHC, e.g. 55 to 58 RHC. It has been found, according to the invention, that the novel indexing blades used in the method disclosed herein provide for suprising advantages in view of cost and operation. While not wishing to be bound by any theory, it is believed that these advantages are due, at least in part, to (a) a cutting mode based entirely on primary (linear) motion, thus avoiding spreading or the cut edges of the polymer film in directions normal to the plane of the film as would be the case if the blade were rotated; obviously, the indexing motion has no cutting effect of its own; (b) a blade thickness in the same range of magnitude as the thickness of the polymer film; this seems to minimize spreading of the cut edges of the polymer film in directions parallel to the plane of the film; (c) blade flexibility combined with substantial stability of shape. In connection with stability of shape it should be noted that blade thickness and blade diameter preferably are correlated to avoid blade fluttering when used with a polymer film of a given thickness; for that reason, a blade thickness range of from 20 to 500 µm, more preferably of from 30 to 300 µm, and particularly of from 50 to 200 µm, is preferably combined with a diameter range of from about 20 to 60 mm. For many purposes, a diameter:thickness ratio of the indexing blades in the range of from about 100:1 to 3000:1 is suitable. Disc diameters below about 20 mm have the disadvantage of providing relatively few incremental cutting positions and diameters below 10 mm are not suitable for that purpose. On the other hand, at diameters of above 60 mm, an increased fluttering tendency may occur; this may be compensated by increasing the thickness within the limits given. Generally, the disc thickness-primarily geared to minimize film spreading upon and immediately after cutting-may have an impact upon blade fluttering in the sense that lower blade thicknesses tend to increase the flutting tendency. For that reason, a blade thickness in the lowest part (10 to 30 µm) of the range given is not preferred and a minimum blade thickness of at least 50µm is a more preferred lower limit. At the uppermost part (300 to 500 µm) of the blade thickness range fluttering is avoided but the blade may be too thick so that blade thicknesses in this uppermost region are not generally preferred and a preferred upper limit of blade thickness is 300 µm and even a blade thickness of below 200 µm will be suitable for most purposes of the invention, notably in the preferred diameter range. Blade fluttering may, of course, depend upon the speed of the motion of the film. For many purposes, e.g. for tube slitting copening of extruded polymer hose at one or both sides of the flat hose), margin cutting or web division in longitudinal or machine direction, typical web speeds are in the range given above (10 to 150 m/min). The term "polymer film" is used herein to encompass webs or web portions of polymer films and comparable organic materials; generally, this implies a generally "flat" structure as is typical for moving webs of films in the plastics industry; this includes laminates in the thickness range given. Representative but non-limiting examples of polymer films or webs for use in the inventive method include single-layer webs and multi-layer webs provided that the total web thickness does not substantially exceed the 500 µm upper thickness limit. Webs in the form of tubular extrudates preferably are cut, after local spreading of mutually superimposed web layers if required, in single-layer mode; generally, the single-layer mode is preferred even though the "single layer" may be a laminate. The lower limit of the film thickness range (10 µm) is due mainly to practical reasons, such as lack of cohesiveness and self-supporting strength of extremely thin films. "Polymer" includes homopolymers, copolymers, polymer mixtures and polymer compositions containing non-polymeric constituents, e.g. additives, dyes, plasticizers, etc. Illustrative examples of suitable polymers are polyolefins (e.g. polyethylene, polypropylene) including copolymers of such olefins (e.g. copolymers of ethylene and acrylic acid or vinyl chloride) and the so-called ionomers; polyhaloalkylenes, polyesters, polyamides; polyacrylates, polymethacrylates, polystyrene and styrene-based copolymers, polyvinylidene chloride, polyvinylidene fluorides. When using films of polyvinyl chloride (PVC) or similar materials that have a variable degree of plastification in a relatively "hard" form, the optimum upper limit of film thickness may be substantially below 500 µm. For example, films of hard PVC (shore A hardness of 90 or more) can be cut best when having a thickness of about 50 µm. In general, polymer films suitable for use in the inventive method have a shore hardness (A, C or D) of up to about 90 or less and a ball-pressure hardness (German Industrial Standards DIN, in kg/cm²) of up to about 1000 or less. Most thermoplastic polymers are suitable but films of regenerated cellulose, of chemically modified cellulose and of partially cross-linked polymers and the like are suitable as well as long as the films made thereof have a sufficient flexibility for processing as webs and have a hardness in the range just cited. Additives including abrasive types such as anti-blocking agents, can be incorporated into the films; in fact, problems of continuously cutting such films with conventional cutting devices operating in the press-cutting, shear-cutting or rotation-cutting mode can be avoided entirely according to the invention by simply adapting the indexing frequency so that web ruptures, irregular edges and the like disadvantages of blade dulling do not occur. Even if the films contain substantial amounts of abrasive anti-blocking agents, a typical indexing blade according to the invention will permit continuous cutting for periods of days to weeks. The invention will be explained further by reference to the annexed drawings, wherein: Figure 1a and 1b are diagrammatic illustrations of film cutting blades according to the art; Figure 1c is a diagrammatic illustration of a previously used razor blade cutter shown for comparative purposes; Figure 2 is a diagrammatic side-view of a preferred embodiment of the invention having a circular indexing blade; Figure 3 is a diagrammatic top-view of the device shown in Figure 2; Figure 4 is a diagrammatic view of a polygonal indexing blade of the device according to the invention; Figure 5 is a diagrammatic top-view of an inventive device comprising a film-guiding means, and Figure 6 is a semi-diagrammatic side-view of an inventive device in operative position on a machine used in the production of films by blow extrusion. The prior art cutting device 10 of Figure 1a comprises a circular knife 101 (shown in front view, upper portion broken away) rotatingly supported by a shaft (not shown) and in pressing engagement with an extremely hard rotating anvil or counter roller 102 (only a fragment being shown in section). This is an example of the press-cutting mode where the cutting edge angle a of circular knife 101 typically is well above 10°. A substantial thickness is required, of course, for knife 101. The plane of the film that is cut is indicated as F in all Figures, that plane being assumed to extend normal to the plane of drawing, at least at the cutting point. If the anvil 102 is omitted in the device of Figure 1 a and if the knife 101 is connected with a drive to rotate at, say, 1000 to 5000 rotations per minute, this would illustrate the rotation cutting mode. A conventional shear-type cutter 11 is illustrated in Figure 1b comprising an upper rotating circular knife 111 (fragment shown) that cooperates with a lower rotating circular knife 112 (fragment shown) to form an endless shearing edge. This is an example of the shear-cutting mode and, again, the knife edge angle a would be substantially greater than 10°. Figure 1c illustrates, for purposes of comparison, a cutting device 12 using a conventional razor blade 121. Such blades are known to have many uses other than for shaving and various devices for cutting with such blades are conventional; thus, Figure 1c is intended to show the result of using such blades for continuous cutting of polymer films. To this end, razor blade 121 can be arranged on a magnetic support 123 that holds blade 121 in cutting position and provides for easy replacement of used blades. A film-guiding means including, if desired, a spread 141 and a guide member 142 cooperates with blade 121. Operation of device 12 in Figure 1c illustrates the slit cutting mode; physical contact between blade 121 and guide member 142 should be avoided as blade 121 has the thickness of a conventional razor blade, i.e. in the range of from about 40 to 100 µm, and is much too flexible for co-acting effectively with an anvil, counter-knife or the like counter-members used in press-cutting and shear-cutting. Generally, film guide means are preferred for slit cutting operation, notably when using this cutting mode for one-sided or two-sided splitting of tubular films produced by blow-extrusion methods of the type disclosed, for example, in U.S. Patent No. 2,668,323 to Johnson. Returning to razor blade 121 of the device shown in Fig. 1c it is apparent that, as such blade has two parallel cutting edges, the practically feasible way of exchanging a blunted cutting edge of blade 121 is to reverse blade 121. Thereafter, a fresh blade is needed. In theory, each cutting edge of blade 121 might be used in incremental portions by manual displacement but with little or no positional control; in practice, this is impossible, however. The device 2 shown diagrammatically in a side-view in Figure 2 comprises an indexing blade in the form of a circular steel sheet disc 20 having a diameter of 45 mm and provided at its periphery 21 with a continuous or endless cutting edge 22. An enlarged portion of the peripheral part of disc 20 is shown in section in the circle connected with Figure 2: steel sheet disc 20 having a substantially uniform thickness of about 200 µm and a Rockwell hardness C in the range of from 50 to 58 presents a razor-sharp edge formed by two converging edge surfaces 22, 221 obtained, e.g. by grinding and honing. Surfaces 22, 221 are shown to be "planar", i.e. presenting a linear taper, but could be slightly curved, i.e. form a cutting edge with a concave taper or a convex taper as can be obtained by grinding and honing techniques conventionally used in production of razor blades. The angle a enclosed by surfaces 22, 221 in a linear taper will generally be below 10°, e.g. 8 to 9°. Typically, the radial length of surfaces 22, 221 will be about 4 to 6 times greater than the thickness of disc 20, regardless of the type of taper. For indexing or step-switching, steel sheet disc 20 is rigidly connected with a step-switching drive 25 (indicated in Figure 2 diagrammatically as a circle and also referred to as "actuator" herein) preferably, a stepping motor. Such motors, generally for electrical operation, are conventional in the step-switching art and provide for a predetermined angular displacement of an axis in response to a signal. Actuator 25 is, in turn, rigidly connected with a mounting plate 27 or equivalent mounting means for holding the indexing blade 20 in a web cutting position. The web plane is indicated by line F and is assumed to be normal to the plane of drawing moving continuously in a "downward" direction, i.e. downwards, from the upper side of Figure 2, and the indexing blade is kept stationary, both in planar and in axial direction once the position of mounting plate 27 is fixed, e.g. after moving into a desired position by sliding displacement on two rods (not shown) mounted on the frame of a web-processing machine (not shown) and securing in that position. The web-processing machine might be a group of web-moving rollers, connected with a blownhose extruder, a web-winding apparatus, a coating machine or the like requiring continuous longitudinal slitting or trimming of a polymer web. Three mutually adjacent incremental portions of cutting edge 21 are indicated between broken lines of Figure 2 and designated by "A" and reference numerals 23, 24. The radial lengths of the incremental portions are exaggerated in Figure 2 for clarity and would, in practice, cover only about 3 to 6° of the total 360° periphery. Assuming that cutting of the downwardly moving web F is started when portion A of indexing blade 20 is in cutting position as depicted in Figure 2 and further assuming a typical speed of movement of web F of about 30 meters per minute: now, portion A will be held in cutting position as long as that portion remains sufficiently sharp for smooth cutting of web F. Depending mainly upon the abrasive effect of web F (e.g. its anti-blocking constituent and proportion thereof), it may typically take about 500 minutes of cutting time (i.e. 15000 meters of cutting length) until incremental edge portion A begins to loose its original sharpness by continued abrasion. Accordingly, a predetermined and safe (for continued smooth cutting) length of cutting operation would be about 250 minutes of cutting time or 7500 meters of cutting length with an abrasive film. This length may be determined by previous runs (operating instructions) or by a simple test run when a hitherto untried web material is to be cut. The predetermined value for a safe length (time-wise or length-wise) of cutting operation is used as a first or "step-trigger" indexing parameter, i.e. to trigger actuator 25. An example for a suitable triggering arrangement will be given below. When actuator 25 is triggered, it will move an adjacent and fresh incremental portion of cutting edge 21 into cutting position. Assuming that the sense of operation of actuator 25 is anti-clockwise, the subsequent incremental portion indexed into the original position of A is cutting edge portion 23 which now remains in that position for the above explained safe cutting time or length of 250 minutes or 7500 meters and will be indexed out of cutting position by actuator 25 thereafter. Thus, a continuous cutting operating can be maintained until the "last" fresh incremental portion 24 is indexed into cutting position A. As will be understood from the explanation, a second indexing parameter is required that in effect determines the cutting life of the indexing blade, i.e. the number of indexing steps per full periphery of 360°. This second parameter, in effect, determines the peripheral length of each incremental cutting edge portion, and while this length is dependent both upon the diameter of indexing blade 20 as well as upon the angular displacement of actuator 25 per switching step, it will be termed "angular" indexing parameter. An actual peripheral length of each incremental cutting edge portion of about 1 mm will be sufficient for many cutting purposes and this length may be double if required for safety of continuous cutting, e.g. to compensate for minor deviations of the web from its theoretical plane of movement. Accordingly, the 45 mm diameter of indexing blade 20 having a peripheral length of about 140 mm may provide for 140 or 35 incremental portions corresponding with angular indexing parameters of 2.5° or 10°. Accordingly, the actuator 25, or its variable setting, will have to provide for indexing blade 20 by 2.5 or 10 degrees per step in this example. As conventional indexing actuators such as stepping motors provide for control, no further explanation is believed to be required here. By the same token, generation of a signal that indicates complete or substantially complete indexing of blade 20 can be achieved by conventional means, e.g. standard design of stepping motors or stepping motor control. For example, when the "last" incremental edge portion 24 is indexed into position A, a contact in the actuator that is activated once per full turn, may close a circuit that powers an optical or acoustical warning device such as a bell; for additional safety, a timer triggered in the same manner may interrupt operation of the machine that produces or moves web F. For safely guiding web F into the cutting position A of Figure 2, it may be advantageous to provide for a web-guide that supports web F, e.g. in the position marked S. Depending upon the conformation of F, the support at S may have a plane or a curved surface. A physical contact between the web-guide at S and indexing blade 20 must be prevented, however. A top-view of device 2 of Figure 2 is shown in Figure 3 to illustrate that a generally normal position of indexing blade 20 relative to web F is preferred. It should be emphasized, however, that only that portion of web F at the cutting position A need be so oriented. As apparent from Figure 3, a protecting shield 36 may be used for operating safety. Further, the indexing actuator or stepping motor 25 is shown to consist of a drive 39 and a reduction gear 38; further, blade 20 is connected with gear 38 by a support plate 31 that may have one or more positioning pins (not shown) matching with corresponding perforations (not shown) of indexing blade 20. For a convenient exchange of a used indexing blade by a fresh blade, support plate 31 is a magnetic plate. While indexing blade 20 of Figures 2 and 3 is shown to be circular in accordance with a preferred embodiment, Figure 4 illustrates a "substantially circular" indexing blade 40 in a polygonal (regular polygon) shape; preferably, the continuous cutting edge 42 at periphery 41 of blade 40 is subdivided to present at least twelve, and preferably more than twelve, linear segments, for example twentyfour or thirtysix segments. In general, one segment should be provided for each indexing step. Figure 5 indicates, in a diagrammatic top-view, two different positions of indexing blade 50 relative to two polymer film webs F¹, F², each of which is guided in a typical conformation. Web F¹ shows a side or edge portion of a normally compressed tubular film of the type produced by extrusion and subsequent inflation ("blow-extrusion") of the type mentioned above. Web F² is moved in planar conformation normal to blade 50. In order to maintain web F¹ in a substantially normal position relative to indexing blade 50 in the area of the cutting position, a film or web guide 58 is held in a stationary position, e.g. by being secured to the same mounting means (not shown) that holds actuator 55 and blade 50. Guide 58 has a recess 581 to receive blade 50 without contacting same, and an outlet 582 for blowing air into tubular web F² so as to facilitate spreading thereof. This is particularly advantageous when cutting up tubular films of very thin or rupture-sensitive polymer films. In practice, tubular films in an originally compressed or folded state will be cut up in two portions, e.g. at each folding edge, so that a pair of cutting devices will be used. A similar guide 58 (minus air outlet 582) can be used to guidingly support a web F², moved in a generally planar configuration, at or near positions S indicated in Figures 2 and 3. Indexing blade 50 and actuator 55 of Figure 5 correspond with blade 20 and actuator 25 of Figures 2 and 3 and an actuator control 56 is shown to supply a triggering signal or impulse to actuator 55 in accordance with the first or step-triggering indexing parameter explained above. Actuator control 56 may be a timer device connected, if desired, with the drive (not shown) of the web producing or web processing plant. Alternatively, or complementary, the actuator control 56 may be connected with a conventional device 561, 562 for metering the length of a moving web so as to adapt the indexing frequency to a change of the speed of web movement. A cutting device 6 according to the invention is shown in Figure 6 in a semi-diagrammatic side-view, partially sectioned. Indexing blade 60 is a steel sheet disc having a uniform thickness of 100 to 300 µm and a diameter of 30 to 60 mm. A continuous cutting edge 62 is provided at periphery 61 of blade 60 and a securing member 63 holds blade 60 in rigid connection with actuator 65 which is mounted on suppport 691 of slide-carriage 69. Carriage 69 is slideably mounted on a guide bar 67 of a web processing machine (not shown); rod 671 connected with carriage 69 is used to slightly pull the spreader device 68 towards the inner surface of one edge F³ of a tubular film moving in downward direction. It is to be understood that rod 671 carries a second device 6 (not shown) in opposite position at the other edge ( not shown ) of the tubular film extending from F³ and beyond the right side of Figure 6. Spreader 68 is provided with an air-outlet 64 supplied with compressed air via line 66 and bores (broken lines) within carriage 69. A free-wheeling circular film guide 682 having a peripheral recess 681 for receiving an edge portion of indexing blade 60 but without contacting the latter in the same general manner as explained in connection with Figure 5 is provided so that edge F³ of the tubular film will be guided into cutting position A. Again, as explained above, indexing blade 60 is not moved except when indexed for removing an incremental portion of cutting edge 62 from cutting position A and for introducing a fresh subsequent incremental cutting edge portion into that position. Again, each incremental portion of cutting edge 62 will have a peripheral length in the range of typically 1 to 10° providing for 36 to 360 incremental edge portions for indexing into, and out of, cutting position A. With a typical residence time of each incremental portion of about 250 minutes in cutting position, the total cutting time of indexing blade 60 will be in the range of from 9000 to 90,000 minutes; as each indexing motion of blade 60 is substantially momentary and, typically, last for a second only, the aggregated total time of indexing motion during complete indexing of blade 60 will amount from 36 seconds to 6 minutes and thus has no effect upon cutting. Accordingly, there is no appreciable difference if indexing is clockwise or counter-clockwise. In general, indexing blades of the device according to the invention can be obtained from sheets of tool-grade steel, e.g. steel sheets of the type conventionally used in the manufacture of razor blades. Typical examples are ferrous alloys containing carbon and chromium as the essential alloying elements. For example, a steel containing about 0.4 %, by weight, of carbon and 13.5 %, by weight, of chromium is illustrative but numerous other types of cutting-grade steel are known and can be used for the indexing blades disclosed herein. Examples will be given to illustrate, but not to limit, the inventive method. Examples I-IV A polymer film producing plant was modified as follows: Two indexing cutters 6 as illustrated in Fig. 6 were slidingly arranged on the frame-supported slide bar 67 of the withdrawing roller group of a conventional and commercially available blow extruder (Type A 90-32, manufactured by AFEX AG of Uznach, Switzerland). The plant was set to produce a primary web in the form of a folded and compressed tubular film having a width of 1000 mm and at a web speed at slide bar 67 of 30 meters/minute for subsequent cutting-up at both lateral folding edges so as to produce two films, each having a width of 1000 mm. The two cutters 6 were positioned on bar 67 so that each guide wheel 681 of guide 68 was in contact with the inner surface of one of the two folding edges. The actuators 65 were commercially available standard stepping motors ("Saia-stepping motors", supplied by Saia AG of Murten, Switzerland) comprising an electric motor, a gear and a dial for setting axial displacement per switch; a setting for 9° displacement was selected for both stepping motors. The electrical input to the stepping motors was controlled by the main power switch of the blow extruder so that the actuators 65 were operative only as long as the extruder was in operation. The actuator control for each stepping motor was a commercially available standard timing switch (also supplied by Saia AG, Switzerland) with a dial to set a time interval between subsequent switching impulses. Setting of this dial was selected for the "safe length" time periods given in Table I below. Each cutter 6 was connected at 671 with a weight-loaded (500 g) wire so that each guide wheel 681 was lightly pressed against the inner side of the corresponding folding edge of the tubular web. Compressed air was supplied via a flexible conduit connected with each cutter 6 at 66 to provide a continuous air stream of 2 to 5 liters/minute at the outlet end of nozzle 64. Each indexing blade 60 had a diameter of 45 mm, a thickness of 200 µm and a Rockwell C-hardness of 56. Edge 62 was obtained by honing to razor blade sharpness. The calculated length of each incremental edge portion was 3.53 mm. A standard counting device was connected with one stepping motor to activate a buzzer after 40 switches of that stepping motor. A continuous winder as disclosed in U.S. Patent 4,191,341, Figure 7, was used to receive the two webs resulting from cutting up of the blow-extruded tubular film. The web-cutting quality was judged by visual inspection of the side faces of the coils obtained on the winder. The cutting quality was judged "good" when and as long as the coil side faces had a smooth and uniform appearance. The cutting quality was judged "poor" when the coil sides showed stratification due to irregularities at the film edges. Films of polymers known to have low or high intrinsic abrasive effects on cutting devices and with or without abrasive additives were produced in the plant under continuous operating conditons (three shifts per day) with continuous operation during 3 to 6 days. When starting production with a given polymer composition, the actuator control was deactivated (Zero-setting) for observation of the time-dependence of the cutting quality, i.e. without indexing. The first appearance of irregularities at the coil sides indicated a "critical length" of the cutting operation per edge increment; 50 to 80% of that critical length (time-wise) was taken as the "preliminary safe length" and the actuator control was set at that value. When continued operation showed any indications of poor cutting quality, the "preliminary safe length" was further shortened. When a run was completed without indication of poor cutting quality, the last "preliminary safe length" was taken as "safe length". The results are summarized in Table I together with the polymer systems used and show that the indexing blades performed well even with very abrasive systems requiring a blade exchange only after more than 150 hours of continuous cutting. In view of the low costs of such thin indexing blades and the simplicity of the blade-exchange operation, this provides for a marked improvement, both as regards cost and maintenance, over the best prior art shear or press cutting devices, notably when used for highly abrasive systems. Blade exchange operation with the inventive device was timed to take from 7 to 10 seconds; a conventional shear cutter used for comparative purposes with the abrasive polymer system of Example IV requires knife-reconditioning after about one week of continuous operation; demounting and remounting of one shear cutter takes about 10 minutes while knife-reconditioning may take several hours.

Claims: 1. A method of slit-cutting a fast-moving polymer film having a thickness in the range of from 10 to 500 micrometers by means of an easily replaceable knife provided with a razor-type cutting edge and held in a cutting position where the film is not in contact with a support, characterized in that (i) the cutting edge is a substantially continuous edge (22) at the periphery (21) of a circular or polygonal steel sheet disc (20) having a thickness in the range of from 10 micrometers to 500 micrometers and a diameter in the range of from 10 millimeters to 100 millimeters; (ii) maintaining disc (20) for a first length of cutting operation in a first position where a predetermined first incremental portion of the cutting edge (22) is in film-cutting position (A); (iii) automatically indexing disc (20) for removing the first incremental portion of cutting edge (22) from film-cutting position (A) and for moving a subsequent incremental portion (23) of cutting edge (22) into film-cutting position (A) and maintaining the subsequent portion (23) in said position for another length of cutting operation, and . (iv) repeating step (iii) until a predominant portion, at least, of the continuous edge (22) of disc (20) has been indexed, but discontinuing indexing before an incremental portion previously maintained in said cutting position (A) is again moved into said cutting position (A). 2. The method of claim 1, characterized by measuring the length of the cutting operation and by generating indexing control signals in proportion with said length. 3. The method of claim 1-2, characterized by providing a warning signal before disc (20) has completed a full turn. 4. A device for carrying out the method of claim 1, characterized by an indexing blade consisting essentially of a circular or polygonal steel sheet disc (20, 60) having (a) a substantially uniform thickness in the range of from 10 micrometers to 500 micrometers, (b) a diameter in the range of from 10 millimeters to 100 millimeters and (c) a substantially continuous razor-type cutting edge (22) extending around the periphery (21) of steel sheet disc (20, 60); and by an indexing drive (25, 65) in operative connection with the disc (20, 60) for automatically indexing said disc by not more than one full turn. 5. The device of claim 4, characterized in that disc (20) has a thickness in the range of from 20 to 300 microns, a diameter in the range of from 20 to 60 millimeters and a Rok-well hardness of at least about 50 RHC. 6. The device of claims 4 or 5, characterized by a holder (27, 69) for keeping disc (20, 60) in web-cutting position (A). 7. The device of any of claims 4-6, characterized in that the indexing drive (25) includes a stepping motor (29) and that a support (31) is provided for removably holding disc (20) in operative connection with the drive (25). 8. The device of any of claims 4-7, characterized by metering means (56, 561, 562) for generating signals indicative of the length of the cutting operation. 9. The device of any of claims 4-8, characterized in that a web-guide (68) is provided for directing the web (F³) into film-cutting position (A). 10. The device of claim 9, characterized in that the web-guide (68) includes means (64, 68) for spreading mutually adjacent layers of a tubular polymer film.

Inventors: LOOSER, GOTTLIEB
Applicants: LOOSER, GOTTLIEB
Application year: 1981
Main classification: B26D1
IPC codes: B26D1, B26D7