GB2219271A

GB2219271A - Plastics dishes

Info

Publication number: GB2219271A
Application number: GB8813242A
Authority: GB
Inventors: Peter Reginald Clarke
Original assignee: Clarke R & Co Ltd
Current assignee: Clarke R & Co Ltd
Priority date: 1988-06-04
Filing date: 1988-06-04
Publication date: 1989-12-06
Also published as: GB8813242D0

Abstract

A rimmed dish formed of moulded PET plastics material has the upper surface of the rim of the dish textured to provide a key capable of ensuring an improved bond with a sealing film adhered to the rim. The texturing preferably comprises a decorative regular raised pattern. During moulding the upper surface of the rim is less crystallised than the remainder of the thickness of the rim by clamping the periphery of a dish blank between two plates 32a, 32b, the upper plate 32b having a textured pattern and being less heated than the lower plate 32a. The body part of the dish is moulded into shape in a heated lower mould part under vacuum or by a pressure plug (Figure 4). Different parts of the mould may be kept at different temperatures so that the degree of crystallinity varies from one region of the dish to another, eg. to maintain flexibility at the corners of the dish. <IMAGE>

Description

Plastics Dishes The invention relates to moulded plastics dishes and in particular to dishes moulded from preformed blanks of polyethylene terephthalate, usually referred to as PET.

PET has an amorphous form and a crystalline form which exhibit different mechanical properties, the amorphous form being less strong and less brittle. To change the crystallinity, the material must be raised above a threshold temperature and then allowed to cool down gradually. Once in the crystalline form the material cannot be softened by heating and makes an excellent container for food to be heated in a conventional or a microwave oven.

It is known for frozen foods to be packaged in dishes which may be placed in a conventional oven or in a microwave oven. Aluminium dishes have been used in the past but PET dishes present a better alternative and have also been used. When used for frozen foods, it is desirable to seal off the dish by adhering a film to the rim of the dish so as to seal the contents of the dish hermetically. It is however difficult to make a reliable seal with the rim of the PET and the present invention seeks to mitigate this problem.

According to the present invention, there is provided a dish formed of moulded PET plastics material wherein the surface of upper rim of the dish is textured to provide a key capable of ensuring an improved bond with a sealing film adhered to the rim.

In the prior art no attention was paid to the surface texture of the mould forming the dishes in the region of the upper surface of the rim and the rim of the dish was flat and smooth. Other parts of the mould were roughened to avoid air being trapped but not the rim.

The present invention is predicated on the realisation that the surface texture is of importance in ensuring a proper seal.

The texturing of the surface of the rim may be formed by random etching or erosion of the mould surface but it is preferred that a regular pattern be formed on the rim surface. In this way, the texturing which performs an important physical function in improving the bonding of the sealing film also improves the aesthetic appearance of the dish.

It has also been found that the crystallinity of the plastics material affects the bond with the sealing film and it is preferable for the rim of the dish not to be recrystrallised. On the other hand, this weakens the rim and creates the need for a compromise between the strength of the rim and the ability to bond a sealing film to it.

In a preferred embodiment of the invention, a temperature gradient is maintained across the thickness of the film during the moulding process so that the upper surface of the rim is less crystallised than the remainder of the thickness of the rim. This removes the need for compromise in that the strength of the rim may be achieved without impairing the bond with the sealing film.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is schematic plan view of a moulding apparatus for forming a dish of the invention, Figure 2 is schematic section though the loading station, Figure 3 is a schematic section through a heating station, and Figure 4 is a schematic section through a moulding station.

The drawings show an apparatus for converting preformed blanks into dishes for containing food. The blanks are preformed by injection moulding and are not of even thickness, being thickened in regions which are to be stretched most during the forming of the blanks into food dishes. The perimeter of each blank is shaped to conform to that of the finished dish and its rim, which does not undergo deformation, is the part that is gripped to enable the blank to be carried between the various stations of the apparatus.

The apparatus as shown in Figure 1 in plan, comprises four stations which are arranged around a central vertical axis 10. The stations comprise a loading and unloading station 12, a heating station 14 and a moulding station 16. A further station 18 is formed for annealing but this is merely an empty station, which may not be required, and is provided to allow the formed article to complete its heat treatment under controlled conditions.

In the centre of the four stations, there is formed a vertical column 30 which supports as many blank carriers 32 as there are stations. The column rotates to index the carriers from one station to the next, each carrier spending an equal length of time in each station.

Referring to Figure 2, which shows the loading station 12, each carrier comprises a pair of clamping plates 32a and 32b which are heated by independent supplies of hot oil fed to the plates by means of pipes 34 leading from the central column 10.

Each pair of clamping plates comprises a lower plate 32a and an upper plate 32b, the clamping force on the rim of the blank being exerted by the weight of the upper plate 32b. Both the plates are annular and grip only the rim of the blank. The lower plate 32a also forms part of the mould into which the blank is to be deformed by the application of temperature and pressure.

The surface of the upper plate 32b in contact with the blank has a decorative pattern, such as a basket weave and this pattern is impressed on the softened plastics material of the blank. The textured surface is more aesthetically appealing and enables an improved bond to be achieved with an applied sealing film.

In the loading station, it is necessary to separate the clamping plates 32a, 32b to permit the formed dish to be removed and a new blank to be inserted. To this end, the upper plate 32b is mounted on two pairs of pivoted parallel arms 36 which define a parallelogram. One of the pairs of arms 36 extends beyond its pivot point and is connected by means of a suitable linkage 38 to an air cylinder 40. When the air cylinder 40 is operated to pull down on the end of the arms 36, the upper clamping plates 32b is raised from the lower plate 32a but remains parallel to it. When the air cylinder 40 slowly releases the upper clamping plate, the latter is lowered by its own weight onto the new blank.

The clamping plates 32a and 32b are both heated by circulating oil and as they have separate oil supplies the temperatures of the two plates may be different from one another. It is preferred that the upper clamping should be cooler as the plastics material of the blank then remains less crystalline on its surface to which a sealing film is to be bonded.

Referring now to Figure 3, while passing through the heating station 14, the blank 46 is held only by its rim and its centre is totally exposed both from above and from below. This enables the blank to be heated in the heating station by two radiant heaters 42 and 44, for example infra-red heaters. Because the blank 46 is heated from both sides simultaneously it can be brought rapidly to the temperature at which it can be moulded.

This reduces the cycle time of the apparatus and therefore the manufacturing cost of the finished article.

In the moulding station (Figure 4), a lower mould part 48 mates accurately with the lower clamping plate to define the annular cavity having the desired shape of the dish. The mould is heated by means of cartridge heaters 50 to raise the temperature of the blank still further to the level at which recrystallisation is initiated. The lower mould part 48 is also formed with small perforations to permit air to be sucked out from under the blank, the vacuum serving to draw the softened blank into contact with the walls of the mould.

Alternatively, the blank may be urged from above by high pressure or forced mechanically by a male plug member into the shape of the cavity.

The temperature of the mould is accurately maintained during the moulding process and different parts of the mould may be kept at different temperatures so that the degree of crystallinity in the finished dish may vary from one region to the other. This may be required as rigidity of the material may cause it to be brittle and risk of damage to the dish during transportation may be reduced by maintaining flexibility in certain regions, for example the corners.

After the forming of the blank into a dish, recrystallisation of the PET material commences and continues after removal of the dish from the mould. The annealing station allows the dish to cool down under controlled temperature conditions. If the dish is sufficiently rigid to handle almost immediately after leaving the moulding station and the annealing station may be omitted.

The dish is now brought around again to the loading station from which it can now be removed, to be replaced at the same time by a fresh blank.

The apparatus has been shows with only one loading and unloading station but it is of course possible for two or more stations of each kind to be placed around the circumference of a circle and to provide a larger number of carriers thereby permitting the manufacture of dishes at a higher rate. In this case, the blank placed on a carrier at one loading station will be removed as a finished dish at another loading station.

It has been assumed that the blanks are loaded manually and the finished dishes removed in the same manner. Of course, it is alternatively possible for a separate loading jig to perform these transfers automatically.

Claims

1. A dish formed of moulded PET plastics material wherein the surface of upper rim of the dish is textured to provide a key capable of ensuring an improved bond with a sealing film adhered to the rim.

2. A dish as claimed in claim 1, wherein the texturing comprises a decorative regular raised pattern.

3. A dish as claimed in claim 1 or 2, wherein a temperature gradient is maintained across the thickness of the rim during the moulding process so that the upper surface of the rim is less crystallised than the remainder of the thickness of the rim.