A METHOD FOR SEALING A TRAY BY MEANS OF A LID

Abstract: A method for sealing a tray (1) by means of a lid (3), the lid consisting of a paperboard material, and the tray of a plastic material or of a plastic-laminated paperboard and being particularly intended for storing and heating food. The lid (3) is provided with an opening (6) and is adhesively secured to a plastic film (7). The lid is then, together with its plastic film, welded to the tray. The adhesive securing is so dimensioned that the adhesion between the lid and the plastic film will be weaker than the weld seal between the film and the tray, such that the lid - without being heated and without causing any loosening of the plastic film from the tray - can be removed from the plastic film after this has been welded to the tray. The side of the plastic film that faces the tray is coated with a layer of so-called peel-off-type which enables a removal of the film after this has been welded to the tray.

Description: A method for sealing a tray by means of a lid FIELD OF THE INVENTION The present invention relates to a method for sealing a tray by means of a lid, the lid consisting of a paperboard material, and the tray consisting of a plastic material or of a plastic-laminated paperboard and being particularly intended for storing and heating food. BACKGROUND ART Such a method is known from SE-B-464.629. This known method involves a fastening of the lid to the tray by interposing therebetween a plastic film which is, in one single opera¬ tion, welded to the lid and to the tray as well. That method has the following disadvantages. A rather complicated heating tool is required at every place where food is to be packaged into the tray. It must be able to accomplish, in one single operation, a feeding of the plastic film and simultaneously welding it against the tray and against the lid as well. That method will also result in a so strong bond between the lid and the plastic film that they can be removed from the tray only together as one unit. Therefore, it will not be possible for the consumer to separate the paperboar lid from the plastic film, which is of course a serious problem when taking into account environmental aspects in waste disposal. In many cases where the food has to be warmed in an oven i is prescribed that the tray must be sealed also during the warming. With the known method, this would mean that the entire contents of the tray can not be visually observed by the consumer, since the paperboard member will obstruct any visual observation of the contents. There will also be a risque that the paperboard portion of the lid - in case it is left on the tray during the heating thereof - can be discoloured or emit unpleasant sulphite odour. In order to eliminate the lastmentioned problem the known method would require that a paperboard material of a very high and ex¬ pensive quality always had to be used. SUMMARY OF THE INVENTION The object of the present invention is, therefore, to pro¬ vide a method of the aforementioned kind, where all said disadvantages have been eliminated. This object will be achieved by a method according to the invention and having the characteristics set forth in Claim 1. Further developments of the invention are set forth in the subclaims. BRIEF DESCRIPTION OF THE DRAWING The drawing provides a schematical perspective view of a heating tool and a tray which is being sealed by the me¬ thod according to the invention. PREFERRED EMBODIMENT In the drawing, a tray 1 of plastic is illustrated, for in¬ stance fabricated of a polyester plastic material or of a polyester-laminated paperboard. The opening edge of the tray is bent outwardly about 90 to form a peripheral flan¬ ge 2 extending around the opening edge. The tray is adapted to be moisture resistantly sealed against the flange 2 by a closure in the form of a lid 3 fabricated of a paperboard material. The sealing of the tray 1 is carried out by means of a heating tool consisting of a seat portion 4 in which the tray 1 is to be placed, and of a welding head 5 being mo¬ vable relative to the seat portion 4 in order to weld the lid 3 against the tray 1 under pressure. The tray is sealed in the following way. The lid 3 is fabricated of untreated paperboard, and is provided with a through window opening 6 in order to enable a visual observation of the interior of the tray 1. A transparent plastic film 7, suitably of a polyester mate¬ rial, for instance a film sold under the trade name Mylar (registered trademark) OL25, is used as a window for the lid 3. This type of plastic film has one side thereof, i.e. the tray side in this case, coated with a so called peel- off-layer, enabling the film, after it has been welded a- gainst any plastic surface, to be pulled away therefrom. The lid 3 is adhesively secured to the plastic film 7 with the non-coated side of the film facing the tray side of the lid. The adhesive used here is heat resistant, for instance in the form of a water based dispersion, and is suitably applied between the lid 3 and the film 7 in a string or at discrete points, as is indicated in dotted line 8. This use of an adhesive which is heat resistant requires no heat, but the lid 3 and the film 7 have only to be pressed together in order to secure the lid to the film. The film 7 extends over substantially the entire surface of the lid 3, such that the window 6 will be covered thereby. Two of the corners of the lid 3 have been cut away, so that the film 7 will thereby protrude outwards at 7a and 7b, res¬ pectively, in order to form tabs to facilitate any desired removal of the film when this has been welded, as will be described below. The water based dispersion may for instance be a Gludan (registered trademark) adhesive. This adhesive is suffici¬ ently heat resistant to withstand the subsequent welding, and has also been approved for contact with foodstuffs. The lid 3 with the plastic film 7 adhered thereto is then delivered to a food packer who will fill the tray 1 with the desired food contents. The tray 1 is then placed into the seat 4. The lid 3 is then placed with its plastic film 7 facing the flange 2 of the tray. The welding head 5 will then, under heat, press the lid 3 against the flange 2 of the tray such that the film 7 will be welded to the flange 2. The tray 1 with its contents is now ready to be distri¬ buted to the consumer. If the consumer is desirous of having a maximum visual ob¬ servation of the interior of the tray, for instance if he wants to choose carefully among several various purchased food meals, he can readily loosen the lid 3 and carefully pull this away without loosening the film 7 from the tray. Alternatively, if he chooses to leave the lid 3* on the tray, he can easier open the tray - after a heating thereof in an oven -if he first loosens the lid 3 a little, and then pulls one of the corner tabs 7a or 7b, respectively, so that the film 7 can be pulled away from the tray. The adhesive securing of the lid 3 to the plastic film 7 by means of the adhesive substance 8 is dimensioned such that the adhesion between the lid 3 and the film 7 will be weaker than the heat seal that will be obtained between the film and the tray 1 as a result of the welding of the film against the tray. Thus, the lid 3 can - without ha¬ ving to be heated and without causing any loosening of the plastic film from the tray - be completely or partially pulled away from the film after the film has been welded to the tray. For the manufacture of the lid 3 such paperboard material is used that will give the tray 1 a sufficient strength and rigidhess during any transport and treatment of the tray before it will reach the consumer. The paperboard member of the lid 3 may suitably be provided with any desired text, for instance indicating the kind of contents of the tray or having instructions how to heat the contents.

Claims: C l a i s 1. A method for sealing a tray by means of a lid, the lid consisting of a paperboard material, and the tray consis- ting of a plastic material or of a plastic-laminated paper¬ board and being particularly intended for storing and heat¬ ing food, c h a r a c t e r i z e d in that a) the lid (3) is provided with at least one window ope- ning (6) in order to enable a visual observation of any contents of- the tray (1) , b) a transparent plastic film (7) is adhesively secured to the tray side of the lid by means of a heat resistant adhesive substance (8) such that the film will cover the window opening (6) , c) the tray (1) is filled with said contents, d) the lid (3) and the plastic film (7) adhered thereto is placed over the opening edge (2) of the tray with the plastic film (7) facing the tray (1) , whereafter the plas- tic film is heat sealed to the opening edge (2) of the tray by welding, and that e) said adhesive securing is dimensioned such that the adhesion between the lid (3) and the plastic film (7) will be weaker than the weld seal between the film and the tray (1) , such that the lid - without having to be heated and without causing any loosening of the plastic film from the tray - can be completely or partially removed from the plastic film after this has been welded to the tray. 2. A method according to Claim 1, c h a r a c t e r i ¬ z e d in that the tray side of the transparent plastic film (7) is suitably of a so-called peel-off-type. 3. A method according to any preceding Claim, c h a - r a c t e r i z e d in that the plastic film (7) is pro¬ vided with portions (7a, 7b) protruding outwards from the lid (3) in order to form tabs for said removal of the plastic film from the tray.

Inventors: NOREN TONY, NOREN, TONY
Applicants: NOREN TONY, NOREN, TONY
Application year: 1993
Main classification: B65B7
IPC codes: B29C65, B65D77