A METHOD AND A DEVICE FOR WELDING TOGETHER PLASTIC OR PLASTIC COVERED SURFACES

Abstract: A method and a device for welding together plastic or plastic covered surfaces (6, 11) by the aid of laser beam heating (13), especially for joining the lateral seam of a container or a blank for a container. Between the plane areas (11, 6) to be melted together a reflector means (12) is provided with surfaces having a reflecting angle orthogonal to the faces to be melted together (6, 11). The laser beam (13) is emitted while the surface (6, 11) to be melted together are directed past said reflector means (12) in its longitudinal direction. The faces (6, 11) are pressed and melted together after having left the area at said reflector means (12). Said reflector means (12) is, e.g. a reflecting prism.

Description: The invention relates to a method and a device for welding together plastic or plastic covered surfaces by the aid of laser beam heating. Containers intended for liquid comestibles, e.g. dairy produce, beverages, or juice, are conventionally manufactured from thermoplastic covered cardboard, but they may also be manufactured entirely from cardboard. Such containers are manufactured of a plane blank provided with folding indications in the shape of embossed lines, and said blank is commonly folded into a rectangular tube shaped body which is closed in a suitable manner at its top and bottom. In assembling the blank to a tube shaped body by use of only one starting blank a flap is joined with a side edge area on a side surface, and it is of great importance to achieve a completely tight seal. The conventional manner of carrying out such joining or sealing is by the aid of glue or by heating to make the plastic surfaces melt together. If a seam without any leakage is to be granted it is, however, difficult to carry out this technique rapidly and efficiently on a production line. A difficulty arising in this connection is that both surfaces to be mutually contacted should be quite uniformly heated in order to obtain a uniform joining. This also being a case of opposed surfaces it may be necessary to keep said surfaces at a relatively large mutual distance to be able to exert the necessary influence of heat to achieve the desired heating. There will then be a hazard of cooling said surfaces in an uncontrolled manner before they are joined. It is an object of the present invention to provide a method and a device permitting two plastic or plastic covered surfaces to be welded together without encountering the above mentioned disadvantages and by which entirely constant heating of both opposite surfaces is achieved. This object is achieved by use of a heating device in the shape of a laser, and the invention is characterized by the features appearing from the claims. In European Patent Application 0147333 use of laser beams for melting or plasticising two seam surfaces to be welded is disclosed. A device is disclosed by the aid of which a laser beam is split permitting simultaneous heating to two surfaces areas by the same beam while being conveyed along a line path. In another embodiment two surfaces to be welded together are provided at a mutual angle, and the laser beam is made to radiate into said angle heating and plasticising said surfaces so that they can melt together. This last mentioned method which is especially suited for surfaces containing a metal film in their composition will cause reflections between said lateral surfaces extending in a mutually inclined manner resulting in unequal heating and uncertainty as regards the final tightness of the seam. With the present invention this problem is avoided by use of the reflecting surfaces located between the surfaces to be welded together and those surfaces are pressed together while continuing to be conveyed along said line. In the same manner as in the above mentioned European application an unfocused laser beam can be used. Further features of the intention will appear from the following description of an embodiment diagrammatically shown in the drawing and in a side elevation illustrating the principle of laser beam heating via a reflector means of two surface to be melted together. The drawing shows an end elevation of a blank for a thermoplastic covered container that is to be joined along one side face. The cardboard container may be covered by a thermoplastic, e.g. polyethylene, on its outside faces and may, if desired, also comprise a metal film, e.g. an aluninium film. In a first assembling step of the container the blank, designated 1 in the figure, is folded to make two and two side faces mate. The desired rectangular container has four side faces 2, 3, 4, and 5. Additionally, said blank is provided with a side flap 6 which is to form the seam-connection with the end portion of side face 5. The side wall portions of the container are separated by grooved or embossed lines 7, 8, 9, and 10. At the phase of manufacture shown in the drawing said cardboard blank is folded at embossed lines 7 and 9, so that faces 2 and 3 mate, whereas faces 4 and 5 enclose an acute angle. The container blank may either have the form of separate blanks for each container, supplied one after the other, or in the shape of a web intended for being cut transversally, before bottom and top portions are joined. Such a process of manufacture of containers is commonly known technology. In order to achieve welding of the side edge areas said container blank is conveyed in the position as shown in the figure in a lateral direction, i.e. transversal to the paper plane, over a reflector means 12, the container blank 1 being provided in such a relation to reflector means 12 that said means 12 will limit the two surfaces to be welded together, i.e. the interior face area 11 of surface 5 and seam flap 6 both having a length indicated by x in the figure. To heat said faces a laser beam 13 is emitted into reflector means 12 which in the shown embodiment is a reflector prism that is symmetrically arranged, so that the laser beam is reflected equally to both sides towards the faces to be melted together. This occurs continuously at the same time as blank 1 is conveyed on in a lateral direction. By the aid of a correct section of the intensity of said laser beam in relation to the length of reflector prism 12 plasticisation or melting of said two side faces is achieved, which faces can be pressed together in a known manner after having left the area of said reflector prism to form a homogeneous and good melted connection or weld. Because of the high intensity of said laser beam heating will be very quick so that the process of melting together can be carried out on a working path feeding blanks 1 at comparatively high speed. It should be mentioned that reflector means 12 does not have to be a prism, but can be designed in a desired manner the only consideration being that said laser beam must be equally divided between both faces to be plasticized. Necessary adjustments will be obvious to those skilled in the art.

Claims: A method for welding together plastic or plastic covered surfaces (6, 11) by the aid of a laser beam heating along a line seam, reflecting the laser beam with a pair of reflecting surfaces respectively equally to said surfaces, conveying the surfaces along the line of the seam past the reflecting surfaces and subsequently pressing together the melted surfaces together after having left the reflector surfaces (6, 11), characterised in that the reflecting surfaces are located between said surfaces (6, 11) and the melted surfaces are pressed together while continuing to be conveyed along said line. A method as claimed in Claim 1 wherein the surfaces (6, 11) are conveyed while inclined to each other at an acute angle (9), the reflector (12) being arranged to direct the laser beam to only a portion (x) of said surfaces. A device for welding together plastic or plastic covered surfaces (6, 11) by the aid of a laser beam heating along a line seam, comprising a laser beam emitter (13), a reflector (12) with a pair of reflecting surfaces for reflecting the beam from the laser equally to the respective surfaces, a conveyor for conveying the surfaces past said reflector means and means for pressing together said melted surfaces characterised in that the reflecting surfaces are located between said surfaces (6, 11) and said means for pressing together is arranged to act on said surfaces while being conveyed by said conveyor. A device as claimed in Claim 3 wherein said conveyor is arranged to convey the surfaces (6, 11) inclined towards each other at an acute angle (9) and said reflector is arranged to direct the laser beam to only a portion (x) of said surfaces (6, 11). A device as claimed in Claim 3 or Claim 4 wherein said reflector (12) is a prism.

Inventors: DYRE-HANSEN, HANS, THORSTENSEN, BERNT
Applicants: ELOPAK A/S
Application year: 1987
Main classification: B29C65
IPC codes: B29C65, B23K26