GB2219239A - Plastics moulding apparatus - Google Patents

Abstract

Apparatus for pressure, vacuum or plug assisted moulding of a PET article which comprises a mould formed in two parts, a first part being an annular clamping member (32a, 32b) for gripping only the edge of the PET blank (46) and the second being a cover (48) movable relative to the clamping member and capable of mating with the annular clamping member to complete the mould cavity, and heating means (42 and 44) for heating the blank from both sides prior to the mating of the two parts of the mould cavity. <IMAGE>

Description

Plastics Moulding Apparatus The invention relates to the moulding of plastics articles and in particular to moulding articles 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.

Once in the crystalline form, the material can only be softened by heating to considerably higher temperatures and thus makes an excellent container for food to be heated in a conventional or a microwave oven.

In moulding a PET article from a preformed blank, the blank must first be heated to soften the plastics material and then stretched by the application of pressure, vacuum or mechanical pressure to conform to a mould of the desired shape. Because the softened PET plastics material is not easy to transport, it is usual in an automated apparatus to heat the PET material while it is disposed within the mould and this results in prolonged cycle times.

With a view to mitigating the above disadvantage, the present invention provides apparatus for pressure, vacuum or plug assisted moulding of a PET article, comprising a mould formed in two parts, a first part being an annular clamping member for gripping only the edge of the PET blank and the second being a cover movable relative to the clamping member and capable of mating with the annular clamping member to complete the mould cavity, and heating means for heating the blank from both sides prior to the mating of the two parts of the mould cavity.

Because of the split mould cavity, both sides of the blank are exposed while the blank is retained by the clamping member thereby permitting more rapid heating of the blank. However, the blank does not need to be removed from its clamping means for transfer to the mould cavity since the clamping means forms part of the mould cavity. After heating, it is only necessary to close the mould cavity and urge the softened plastics material into contact with the mould cavity by the use of vacuum, pressure, a plug or a combination of these.

Preferably, the heating means are radiant heaters, such as infra-red heaters.

As the part of the blank gripped by the clamping means is not exposed to the heat energy, it is preferred that the clamping member be heated.

The heating of the plastics material by the heating means is necessary only to soften the plastics material to permit it to be moulded. A further rise in temperature is required to permit recrystallisation of the material to occur and this is effected by heating of the mould cavity. Conveniently, cartridge heaters may be used for this purpose.

In the formation of an oven dish by the apparatus of the invention, the rim of the dish is captured in the clamping means and is held clamped while the centre of the blank is softened and deformed. The rim usually is required to receive a sealing film to permit the food content of the dish to remain fresh and it has been found that the level of crystallinity of the plastics material is important in determining the ease with which a sealing film can be adhered to the rim of the dish.

As the rim is retained between clamping means, the clamping surfaces can be maintained at different temper ature so that, if desired, the upper surface of the rim may have a different degree of crystallinity from the remainder of the rim and of the dish.

A temperature gradient may be created within the mould, if variable crystallinity is required in the finished article, by using heaters at different temperature in different parts of the moulds or by selectively shielding parts of the mould.

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 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 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.

In the loading station 12, it is necessary to separate the clamping plates 32a and 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 32b, 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 32b 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, 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 46 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 32a to define the 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 maintains the necessary temperature conditions to assure the required level of recrystallisation. If the dish is sufficiently rigid to handle almost immediately after leaving the moulding station then 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 shown with only one loading and unloading station 12 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.

It will be clear that while the invention has been described by reference to food dishes, the apparatus can be used for other forms of article made from PET.

Claims (8)

1. Apparatus for pressure, vacuum or plug assisted moulding of a PET article, comprising a mould formed in two parts, a first part being an annular clamping member for gripping only the edge of the PET blank and the second being a cover movable relative to the clamping member and capable of mating with the annular clamping member to complete the mould cavity, and heating means for heating the blank from both sides prior to the mating of the two parts of the mould cavity.

2. Apparatus as claimed in claim 1, wherein the heating means are radiant heaters.

3. Apparatus as claimed in claim 1 or 2, wherein the the clamping member be heated.

4. Apparatus as claimed in any preceding claim, wherein heating means are provided in the mould cavity to raise the temperature of the plastics material to the level at which recrystallisation occurs.

5. Apparatus as claimed in claim 4, wherein the heating means in the mould cavity comprises cartridge heaters.

6. Apparatus as claimed in claim 4 or 5, a temperature gradient is created within the mould, by using heaters at different temperature in different parts of the moulds or by selectively shielding parts of the mould.

7. Apparatus as claimed in any preceding claim, wherein means are provided for heating the clamping member and the cover member, the temperatures of the clamping member and the cover member being separately controllable.

8. Apparatus for pressure, vacuum or plug assisted moulding of a PET article constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.