WO1989011962A1

WO1989011962A1 - Plastics moulding apparatus

Info

Publication number: WO1989011962A1
Application number: PCT/GB1989/000608
Authority: WO
Inventors: Peter Reginald Clarke
Original assignee: Basildon Moulding Company Limited
Priority date: 1988-06-04
Filing date: 1989-06-02
Publication date: 1989-12-14
Also published as: GB2219238A; AU3692589A; GB8813240D0

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 finds particular application in the moulding of articles of polyethylene terephthalate (herein referred to as PET) though it 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. This property and the fact that it is a safe material in contact with food, make it an excellent material for the manufacture of containers, such as dishes, for heating food in a conventional or in a microwave oven.

Oven dishes have been manufactured from PET by passing a continuous sheet of amorphous material sequentially through a heating station and a forming station, then severing the formed dishes from the web. However, once crystalline, the material is difficult to cut and causes serious wear to the cutting blades. For this reason, it is preferred to form the articles from individual blanks so that the need for later trimming can be avoided.

In moulding a PET dish 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. With a view to mitigating the foregoing problem, the present invention provides an apparatus for moulding plastics articles from preformed blanks, including a loading station containing a supply of blanks, a heating station at which blanks are preheated, a forming station containing a mould in which the blanks are formed into the shape of the finished articles by pressure, vacuum or plug assisted moulding and a plurality of clamping frames for gripping the edge region of blanks and transporting the individual blanks between the stations, characterised in that each clamping frame comprises a pair of clamping members of which one has a first end surface against which the blank is clamped by the other clamping member, an opposite end surface which mates with the mould in the forming station, and an edge surface extending between the two end surfaces which forms part of the mould cavity.

In the present invention, the final- shape of the articles is defined in part by the mould in the forming station and in part by the shape of one of the clamping members of the clamping frames which carries the blanks between stations. The clamping frames thus serve a dual function. Another way of viewing the invention is that the mould which defines the shape of the finished article is in two parts, one part remaining in the forming station and the other moving with the blank from loading station until it is discharged as a finished article. The resultant effect is that the individual blanks can be reliably moved between stations and no trimming is required once the article has left the forming station.

There is known from US-3,810,731 a vacuum forming machine having a plurality of processing stations connected by an endless track on which forming frames carry plastic sheet material from station to station. These frames play no part in shaping the articles. 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 forming station 16. A further station 18 is provided 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 clamping frames 32 as there are stations. The column rotates to index the frames 32 from one station to the next, each frame spending an equal length of time in each station.

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

Each pair of clamping members comprises a lower member 32a and an upper member 32b, the clamping force on the rim of the blank being exerted by the weight of the upper member 32b. Both the members are annular and grip only the rim of the blank. The lower member 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 members to permit the formed dish to be removed and a new blank to be inserted. To this end, the upper member 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 members 32b is raised from the lower member 32a but remains parallel to it. When the air cylinder 40 slowly releases the upper clamping member 32b, the latter is lowered by its own weight onto the fresh blank.

The clamping members 32a, 32b are both heated by circulating oil and as they have separate oil supplies the temperatures of the two members 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 mould 48 mates accurately with the lower clamping member 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.

It will be noted that the clamping member 32a has three accurately machined surface. The end surface 54 acts in conjunction with the upper clamping members 32b to grip the edge of the blank. The opposite end surface 56 mates with the mould 48 and the edge surface 52 which extends between the end surfaces 54 and 56 forms part of the mould cavity and partially defines the shapes of the finished article. 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 frames 32 thereby permitting the manufacture of dishes at a higher rate. In this case, the blank placed on a frame 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. An apparatus for moulding plastics articles from preformed blanks, including a loading station (12) containing a supply of blanks, a heating station (14) at which blanks are preheated, a forming station (16) containing a mould (48) in which the blanks are formed into the shape of the finished articles by pressure, vacuum or plug assisted moulding and a plurality of clamping frames (32) for gripping the edge region of blanks and transporting the individual blanks between the stations, characterised in that each clamping frame (32) comprises a pair of clamping members (32a,32b) of which one (32a) has a first end surface (54) against which the blank is clamped by the other clamping member, an opposite end surface (56) which mates with the mould (48) in the forming station (16) , and an edge surface (52) extending between the two end surfaces (54,56) which forms part of the mould cavity.

2. Apparatus as claimed in claim 1, wherein the clamping members (32a,32b) are heated.

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

4. Apparatus as claimed in any preceding claim, wherein the other clamping member (32a) is mounted on two arms so that it may be raised in the loading station while remaining parallel to said one clamping member (32a) .

5. Apparatus as claimed in any preceding claim, wherein the stations (12, 14, 16) are arranged circumferential about a central axis (10) about which the individual clamping frames (32) means can be rotate .

6. Apparatus as claimed in any preceding claim, wherein an annealing station (18) is provided after the moulding station (16) , to permit the re-crystallisation of the plastics material to take place under controlled conditions.

7. Apparatus as claimed in any preceding claim, wherein the heating station comprises means (42,44) 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.