Improvements in or relating to thermoformed articles

Abstract: The invention provides a method of making an article, e.g. a tray, from sheet material (7) such as thermoplastics, whereby sharp edges are avoided, comprising the step of deforming the sheet to form the article (3) with a marginal flange (5) having a down-turned lip (6), and cutting the article from the sheet to form a cut edge (12) at the extremity of the down-turned lip, and characterised by the steps of forming the down-turned lip (6) with an inwardly projecting recess or depression (10) extending lengthwise of the lip (6), and crimping the down-turned lip to form a folded or rounded corner (11) in the down-turned lip and to bring the cut edge (12) into the vicinity of the folded or rounded corner to form a finished edge (11, 12). From another aspect the invention is a forming tool for finishing or rolling the edge (12) of an article (3) of the kind referred to above characterised by means (2) for engaging the lip (6) and cutting the article (3) from the sheet (7) and by means (1, 4) for engaging and crimping the down-turned lip (6) to form a folded or rounded corner (11) in the lip and to bring the cut edge (12) into the vicinity of the folded or rounded corner to form a finished edge (11, 12).

Description: The invention relates to thermoformed articles and more particularly, but not exclusively, to disposable thermoformed plastics trays of the kind used in the packaging and display of foodstuffs. Normally such thermoformed trays are made in successive thermoforming operations on a sheet or strip of plastics material, typically transparent polyvinylchloride, after which the trays are separated from the plastics sheet or strip by means of a cutter which may include a blade acting against an anvil or may comprise a punch and die set. This operation of cutting the thermoformed articles from the plastics sheet or strip tends to leave the articles with a sharp marginal edge which can be undesirable for a number of uses and for example the edges can be sharp enough to cause minor injury and can be sharp enough to puncture plastics film which is often placed over such trays to protect their contents. It is known to provide the marginal edges of thermoformed plastics containers with a rounded edge and for example this is done in the case of thermoformed plastics drinking cups in a rolling operation which is only applicable to cylindrical thermoformed articles. US-A-3357053 of W.B. LYON et al discloses apparatus for forming a thin-walled article or container with a rolled of curled lip in single operation. It is however also known to round-off the edges of thermoformed plastics trays, which are usually square or rectangular in shape and formed with a marginal flange, by means of an additional process which folds the cut edges under the top marginal flange of the tray. This is achieved with the aid of a forming machine with formers which can be advanced towards the marginal flange of the tray from all sides to engage the cut edges and fold them under the marginal flange. This process is time consuming and involves expensive machinery and accordingly is not used as widely as might otherwise be the case. An object of the invention is to provide a simple process and means for finishing or rolling the edges of thermoformed articles such as disposable trays such that they do not present unacceptably sharp edges. According to the invention there is provided a method of making an article from sheet material comprising the step of deforming the sheet to form the article with an outwardly extending marginal flange having a down-turned lip and cutting the article from the sheet to form a cut edge at the extremity of the downturned lip, characterised by the steps of forming the down-turned lip with an inwardly projecting continuous recess or depression extending lengthwise of the lip and having an upper edge and being terminated by the cut edge and squeezing together the flange and the cut edge permanently to deform the recess or depression and bring the cut edge and upper edge together. By this process the edge is shrouded by the folded or rounded corner so that the adverse effects of the edge are at least mitigated. The invention is particularly applicable to articles made by thermoforming plastics sheet but might also find application in articles made from metal sheet, such for example, as in the making of aluminium food containers. Where thermoplastic sheet material is concerned, it is preferred that the step of permanently deforming the recess or depression is carried out without additional heating of the thermoplastics sheet material. The invention is diagrammatically illustrated by way of example in the accompanying drawings in which:- Figure 1 is a cross-sectional side elevation of punch and die apparatus; Figure 2 is an enlarged scrap view of part of Figure 1, and Figure 3 is an enlarged scrap view corresponding to Figure 2 and showing the punch and die in a different position. Referring to the drawings there is shown a thermoformed plastics tray 3, of the kind intended for the packaging and display of foodstuffs, which is generally rectangular in shape and which is formed about its periphery with an outwardly extending marginal flange 5 having a down- turned lip 6. As shown in Figures 1 and 2 of the drawings the thermoformed tray 3 is still attached to a flat strip or sheet 7 of plastics material from which the tray was formed and the tray 3 is arranged in the cavity 8 of a punch and die set 4 and 2 respectively which is to be used to cut the tray 3 from the sheet 7. To carry out the cutting operation the punch 4 is moved in the direction of the arrow 9 of Figure 1 so that the cutting edges of the punch and die engage the junction of the lip 6 with the sheet 7 whereupon further movement of the punch causes the tray to be severed from the sheet. In accordance with the invention the lip 6 of the tray is formed with an undercut portion 10 which takes the form of a concavity, that is to say an inwardly projecting continuous recess or depression, extending lengthwise of the lip 6, and which has an upper edge 11. When the punch 4 is moved to shear the tray 3 from the sheet 7 the recess or depression is terminated by a cut-edge 12. It is arranged that the punch continues to move after shearing has taken place to crimp the undercut portion 10 against the marginal flange 5 as shown in Figure 3 of the drawings so that the down-turned lip 6 is terminated by a folded or rolled corner which tends to shroud the cut or sheared edge 12 of the tray. Thus the flange 5 and the cut edge 12 are squeezed together between the punch 4 and an anvil 1 so as permanently to deform or crimp the recess or depression 10 and bring the cut edge 12 into the vicinity of the upper edge 11 of the recess or depression 10. If desired the top edge of the punch may be profiled to tend to force the cut edge 12 inwardly and away from the folded or rolled corner 11. The die 2 is provided with an ejector to remove the tray 3 after cutting and crimping and the ejector is formed by the anvil 1 against which the punch 4 crimps the marginal flange. It will be appreciated that the invention could be carried out in other ways and with articles other than trays. Thus the cutting of the article from the sheet could be achieved with the aid of a so-called steel rule blade which acts against a cutting shoe or anvil 16. The invention thus provides a simple and effective method and means for finishing the cut edges of trays or other articles formed from sheet material, particularly articles thermoformed from plastics material, to avoid or mitigate against the sharp edges exhibited by such articles produced by conventional means. It is conceivable that the method of the invention could be used with sheet materials other than thermoplastics, e.g. thin aluminium and steel sheet.

Claims: A method of making an article from sheet material (7) comprising the step of deforming the sheet to form the article (3) with an outwardly extending marginal flange (5) having a down-turned lip (6), and cutting the article from the sheet to form a cut edge (12) at the extremity of the down-turned lip, characterised by the steps of forming the down-turned lip (6) with an inwardly projecting continuous recess or depression (10) extending lengthwise of the lip (6) and having an upper edge (11) and being terminated by the cut edge (12), and squeezing together the flange (5) and the cut edge (12) permanently to deform the recess or depression (10) and bring the cut edge (12) and upper edge (11) together. A method according to claim 1, wherein the sheet material (7) is thermoplastic and comprising the step of thermoforming the article (3) in the sheet, characterised in that the step of permanently deforming the recess or depression (10) is carried out without additional heating of the thermoplastics sheet material (7).

Inventors: DICKENS JOHN, DOCKERTY JAMES NELSON, DICKENS, JOHN, DOCKERTY, JAMES NELSON
Applicants: ANSON PACKAGING LTD, ANSON PACKAGING LTD.
Application year: 1990
Main classification: B29C57
IPC codes: B29C51, B29C57, B65D1