GB2219238A

GB2219238A - Plastics moulding apparatus

Info

Publication number: GB2219238A
Application number: GB8813240A
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: GB8813240D0; AU3692589A; WO1989011962A1

Abstract

The invention relates to an apparatus for moulding plastics articles which comprises clamping frames (32) for gripping a preformed blank only by its edge region and transporting the blank first through a heating station (14) where the blank is heated and then to a moulding station (16) where the softened blank is made to conform to the shape of a mould. The clamping frames move with the blank from one station to the next and mate with the mould (48) which defines the shape of the finished article.

Description

Plastics Moulding Apparatus The invention relates to the moulding of plastics articles and in particular to moulding articles from preformed plastics blanks. The invention finds particular application in the moulding of articles of polyethylene terephthalate (herein referred to as PET) but can be used for other plastics materials.

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. If a conventional automated apparatus with multiple station is used for the manufacture a problem is experienced in that the softened blank is not easy to transport from the heating station to the moulding station and the present invention seeks to mitigate this problem.

According to the present invention, there is provided apparatus for moulding plastics articles, comprising clamping means for gripping a preformed blank only by its edge region and transporting the blank first through a heating station where the blank is heated and then to a moulding station where the softened blank is made to conform to the shape of a mould, the clamping means being movable with the blank from one station to the next and forming at least part of the mould which defines the shape of the finished article.

As the clamping means form part of the mould, it is preferred that they should be heated.

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 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 temperature 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.

Preferably, the clamping means consist of two separable rings, each separately heated, for example by a hot circulating fluid, the upper ring being mounted on two arms so that it may be raised as part of a parallelogram while remaining parallel to the lower ring.

The stations of the apparatus may conveniently comprise a a loading station, a heating station and a moulding station arranged circumferential about a central axis about which the individual clamping means can be rotated.

If desired, an annealing station may be provided after the moulding station, to permit the re-crystallisation of the plastics material to take place under controlled conditions.

The heating station may conveniently comprise means for heating the blank by radiant energy from above and below. The radiant energy may advantageously be in the infra-red range but other forms of heating may be employed.

The moulding station of the preferred embodiment of the invention comprises a heated part mould which mates with the lower of the clamping rings to define the shape of the mould cavity. The deformation of the softened blank into the heated mould may be effected by vacuum, pressure and/or a plug.

Cartridge heaters arranged in bores in the mould may serve to maintain the desired temperature during the moulding operation and 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 and in regions where a thickened wall section is required in the finished 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, 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.

In the loading station, it is necessary to separate the clamping plates 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 slower releases the upper clamping plate 32b, the latter is lowered by its own weight onto the fresh blank.

The clamping plates 32a, 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 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 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 forming 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 12 from which it can be removed, and 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 32 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 or an other plastics material.

Claims

1. Apparatus for moulding plastics articles, comprising clamping means for gripping a preformed blank only by its edge region and transporting the blank first through a heating station where the blank is heated and then to a moulding station where the softened blank is made to conform to the shape of a mould, the clamping means being movable with the blank from one station to the next and forming at least part of the mould which defines the shape of the finished article.

2. Apparatus as claimed in claim 1, wherein the clamping means are heated.

3. Apparatus as claimed in claim 2, wherein the parts of the clamping means contacting the opposite surfaces of the rim are heated to different temperatures.

4. Apparatus as claimed in any preceding claim, wherein the clamping means comprise two separable rings, the upper ring being mounted on two arms so that it may be raised as part of a parallelogram while remaining parallel to the lower ring.

5. Apparatus as claimed in any preceding claim, wherein the stations comprise a loading station, a heating station and a moulding station arranged circumferential about a central axis about which the individual clamping means can be rotated.

6. Apparatus as claimed in claim 5, wherein an annealing station is provided after the moulding station, to permit the re-crystallisation of the plastics material to take place under controlled conditions.

7. Apparatus as claimed in claim 5 or 6, wherein the heating station comprises means for heating the blank by radiant energy from above and below.

8. Apparatus as claimed in claim 7, wherein the radiant energy is infra-red radiation.

9. Apparatus for moulding plastics articles constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.