CUTTING OF PLASTIC SHEET

Abstract: An apparatus for slitting foil (14) of thermoplastics material comprises means to move the foil (14) through a slitting station (1), the slitting station (1) having a first slitting blade (2) having a sharp front edge (4) located to extend transversely through the path of the moving foil (14), and that part (5) of the blade (2) behind the front edge (4) having each side (6) shaped to be non-intrusive into the slit and melted thermoplastics material foil (14), means (8, 9) to raise and maintain a temperature of the slitting blade (2) to a temperature equal to or above a melting temperature of the thermoplastics material foil (14) selected to be slit by the slitting blade (2), and a cooling blade (3) located behind and aligned with the slitting blade (2) and having two sides each adapted to be aligned with the corresponding side of the slit of thermoplastics material foil (14) and provide by both contact and radiant heating effects, a substantial cooling effect.

Description: CUTTING OF PLASTIC SHEET This invention relates to effecting cutting or slitting of thermoplastics sheet or foil. The problem to which this invention is directed relates to a difficulty associated with providing a continuous cutting or slitting of a sheet or sheets of thermoplastics material where such sheet or foil is required to be separate for whatever purpose. Such purpose might occur, for instance, where the thermoplastics material in sheet form is welded to other sheets and is formed in a plurality of adjacent conduits having integrated water resistant outlet channels integrated therein. There are efficiencies in forming such channels as a multiple number of channels in adjacent relationship but it then becomes necessary to separate by slitting such conduits one from the other. There are, accordingly, three sheets of the appropriate thermoplastics material which are welded together and which have to be slit. It would appear at first sight that all that would be necessary would be to insert a sharp blade at a slitting station and this would be sufficient to effect the slitting as the foil is moved through the slitting station. Unfortunately, the problem is somewhat more difficult. A first problem is that any sharpened blade very quickly becomes blunt. Self-evidently, this is solved by replacing the blade or providing a mechanism by which the blade is replaced on a regular basis but there are costs involved to provide such blades on such a continuing replacement basis and there is furthermore a necessity that the mechanism should be reliable to effect such replacement. There is, however, a further difficulty in the mere use of a blade. Because the sheet plastics material can be formed by an extruding process in which the polymer chains may only be approximately aligned in a longitudinal direction, it is found that when a blade is caused to cut on a continuing basis through such sheet, there can be a side tracking effect which can cause substantial misalignment of the slitting location and also can cause undue strain on other parts of an apparatus causing the plastic to pass the slitting station. Both the life of the cutting blade and the tracking effect can be overcome by providing that the blade, while still sharp, is nonetheless caused to be heated and kept at a temperature so that the cutting effect is combined with a melting effect as the tape engages the blade. This implicitly implies that the blade is made from a thermally conducting material and is heated to such a temperature that such elevated temperature can be rapidly transferred to the approaching plastics material. One further problem arises, however, in that with a melting of the plastics material, it is found that this causes a substantial build up of material which while partially melted or adhering to parts of the blade, nonetheless accumulate immediately subsequent to the blade and can reach retained quantities that it is necessary, from time to time, to manually clean such materials away from the location. Apart from the inconvenience from having to constantly monitor such build up, there is the decided difficulty that if it is overlooked at any time such a build up can interfere eventually with the cutting process and can eventually cause a blockage resisting further passage of foil through a slitting station. According to a further form of this invention then, it is proposed that there be located adjacent but behind the first slitting blade, a cooling means whereby to cause resolidification of the plastics material very soon after the slitting action. The result of this action is to cause such a previously melted plastics material to stay adhered in the close vicinity of the slit edges so that it is then carried away by the continuing passage of the plastics material. Accordingly, by merely causing a cooling effect behind the first heated slitting blade, the debris can be caused to remain attached to the passing plastics material and it, therefore, becomes self cleansing. For a better understanding of this invention it will now be described in relation to a preferred embodiment which shall now be described with the assistance of drawings in which :- FIG. 1 is a side elevation of a slitting station constructed in accordance with the embodiment, and, FIG. 2 is a plan view from a cross section along the lines 2-2 in FIG. 1 , showing, furthermore, the approaching thermoplastics foil. Referring in detail to the drawings, there is shown a slitting station 1 which includes a slitting blade 2 and a cooling blade 3. The slitting blade 2 has a sharpened front end 4 and a rearwardly extending body part 5 which is defined by parallel sides 6 and 7. The slitting blade is comprised of steel and is connected between two electrodes 8 and 9 by which a substantial electric current is caused to flow through the generally uniform cross sectional area defined by the blade shape as shown and in this manner is caused to be heated to an appropriate heat. Such a temperature can be varied in accord with the type and grade of plastic that is being slit but typically for a polythene plastics material having a melt temperature of approximately 120 degrees centigrade would mean that a temperature on a surface of the blade of 150 degrees centigrade is appropriate. Located immediately behind the slitting blade 2 and separated only by a small gap 10, there is provided a cooling blade 3 which is made from copper and which is kept cool by a plurality of water carrying conduits 11 carrying water that is kept to relatively cold temperature so that the blade is kept to a temperature of approximately 10 degrees centigrade. The cooling blade 3 has sides 12 and 13 which are aligned with the sides 6 and 7 of the slitting blade 2 so that any material after slitting will tend to maintain some peripheral contact so that by a combination of radiant loss and conducting loss, the melted plastics will be rapidly cooled and, therefore, caused to maintain adherence to the passing foil material. It has been found to be of advantage that the foil material 14 which, in this embodiment is comprised of three welded together sheets of polythene plastics material, is supported during the slitting operation on the outer peripheral surface of a drum 15 which includes a slot corresponding in position to the lower end of the blade 2. Such an arrangement assists in keeping the thermoplastics foil in a fixed position relative to the slitting action of the blade 2. This then describes the preferred arrangement. By using heating to effect at least substantial melting and then quickly resolidifying the melted material, has two very significant advantageous results. The first of these is to remove or at least significantly reduce the extent to which an offsetting bias can be otherwise caused by using a cutting blade through extruded polythene sheet type materials or other extruded thermoplastics materials. Secondly, while the life of the blade 2 and particularly the effectiveness of a sharp cutting edge is very extensively extended, the previous disadvantage of debris build up has been avoided by simply hardening the debris very quickly and then causing this to be kept with the moving plastic foil and therefore, being effectively disposed of on a continuing basis. The results, in any event, have disclosed that this has shown itself to be a very effective technique for the purposes described.

Claims: CLAIMS : 1. Apparatus for slitting a sheet or sheets of thermoplastics material comprising a slitting blade, means to raise and maintain a temperature of the slitting blade to a temperature equal to or above a melting temperature of the thermoplastics material selected to be slit by the slitting blade, and cooling means downstream of the slitting blade adapted to effect a rapid cooling of any melted thermoplastics material. 2. Apparatus as in claim 1 in which the cooling means include a cooling blade located behind the slitting blade, and means to effect a cooling of the said cooling blade. 3. Apparatus for slitting foil of thermoplastics material comprising means to move the foil through a slitting station, the slitting station having a first slitting blade having a sharp front edge located to extend transversely through the path of the moving foil, and that part of the blade behind the front edge having each side shaped to be non intrusive into the slit and melted thermoplastics material, and a cooling blade located behind and aligned with the said slitting blade and having two sides each adapted to be aligned with the corresponding side of the slit of thermoplastics material and provide by both contact and radiant heating effects, a substantial cooling effect. 4. Apparatus for slitting foil of thermoplastics material as in the last preceding claim in which the slitting blade is comprised of an electrically conducting metal, and the heating means comprise a supply of electrical current adapted to effect the passing through the said metal of electric current. 5. Apparatus as in either of the last two preceding claims in which the cooling blade is comprised of a material being a thermal conductor, and the cooling means include a supply of cooling fluid passing through conduit within the thermal conducting material of the said cooling blade. 6. Apparatus as in the last preceding claim in which the cooling fluid is water. 7. Apparatus as in any one of the preceding claims in which the slitting blade has sides downstream of the sharpened front edge which are parallel one to the other, and the cooling blade has sides which are aligned with a respective side of each of the sides of the said slitting blade. 8. Apparatus as in any one of the preceding claims in which there is a slitting station and in which there is a drum on which the foil is held as it passes through the slitting station, and the slitting blade has an outermost end engaging into a slot within the peripheral surface of the drum. 9. A method of effecting a slitting of a thermoplastics foil comprising joining of the thermoplastics foil through a slitting station in which there is a slitting blade having a sharp front edge and which has been kept at a temperature and is of such material that a slitting effect is being achieved with the substantial melting of the plastics material, and effecting retention of the thus melted plastics with the passing plastics material by effecting a rapid cooling of such melted plastics after such slitting. 10. A method as in the last preceding claim further characterised in that the thermoplastics material is polythene, the slitting blade is held to a temperature of approximately 150 degrees centigrade and the cooling is effected by a cooling blade which is kept to a temperature of approximately 10 degrees centigrade. 11. Apparatus for effecting slitting of thermoplastics material substantially as described in the specification with reference to and as illustrated by the accompanying drawings. 12. A method of slitting thermoplastics foil substantially as described in the specification with reference to and as illustrated by the accompanying drawings.

Inventors: SNELLING PETER JOHN, SNELLING, PETER, JOHN
Applicants: VINIDEX TUBEMAKERS PTY LTD, VINIDEX TUBEMAKERS PTY. LIMITED
Application year: 1988
Main classification: B26D7
IPC codes: B26D7, B29C67