GB2205063A - Moulded plastics articles - Google Patents

Abstract

The invention relates to a food container in the form of a dish made of partly crystalline P.E.T., in which the P.E.T. has different degrees of crystallisation in different regions, so that parts of the containers are selectively hardened while others remain relatively flexible and less frangible upon impact. The mould (30) for moulding the article includes heaters (38) and thermal barriers (36) or cooling passages (40) so that the article is heated to different temperatures and the degree of crystallinity is varied from one region in the container to the other so that some parts remain amorphous and flexible while other parts are hardened by crystallisation.

Description

MOULDED PLASTICS ARTICLES The invention relates to moulded plastics articles and more especially to articles, such as food containers, made of partly crystalline poly(ethylen terephthalate), herein referred to as P.E.T.

The conventional manner of constructing such a food container is to form it from a blank, usually cut from an extruded flat sheet. The term "forming" is used here to include any process by which the sheet is stretched or deformed into the shape of a mould, for example by air pressure, vacuum or mechanical pressure.

P. E. T. has been used in the packaging of food because of its strength and its chemically inert character. It also has the property that it changes in crystal structure when heated above a predetermined temperature and upon partial crystallisation its physical properties are changed significantly. In particular, the strength is improved and this changed is accompanied by a change in the appearance of the material and more importantly an increase in brittleness. The increase in strength is naturally desirable but the brittleness can cause problems by making the finished article more frangible upon impact.

According to a first aspect of the present invention, there is provided an article made of partly crystalline P.E.T, having different degrees of crystallisation in different regions, whereby parts of the article are selectively hardened while others remain relatively flexible and less frangible upon impact.

Conventionally, an article is heated uniformly after it has been formed or during its formation and undergoes crystallisation uniformly. The present invention, by contrast, recognizes that in certain articles the physical properties suited to some regions are not desirable for others. In the case where a blank is formed into a dish, for example, strength is required in the side walls of the dished compartment and in its base, but the corners are easily damaged by impact and therefore it is preferable to retain in the corner regions the flexibility, resilience and impact resistance of the amorphous material.

Also in the case of a food container where a foil is used to seal the container, it has been found that the foil can be made to adhere well to the amorphous plastics material but tends to form a poorer bond with the partly crystallised material.

If the article is a container formed with a carrying handle bent out of the plane of the blank after the article is moulded, the hinge section between the handle and the rim of the container is another region which should preferably remain amorphous in order not to break during use.

Because the properties of the P.E.T. are controlled by the degree of crystallisation and this is in turn temperature dependent, the invention permits all parts of an article to have the physical properties best suited to their function by controlling the temperature to which the various regions of the article are heated during the manufacturing process.

In a second aspect of the invention, there is provided a mould for forming an article from a blank which includes means for locally heating different regions of the mould so as to maintain a temperature gradient within the mould and heat the moulded article unevenly, only selected parts of the article which are to be hardened by partial crystallisation being heated above the temperature necessary to initiate crystallisation.

The mould may conveniently include so-called cartridge heaters which consist of electrically heated rod like elements which are inserted in suitably positioned bores in the mould.

The temperature difference required to vary the physical properties of P.E.T. significantly is only small, typically some 20or. A temperature of 1350C is required to initiate crystallisation while a temperature of 1150C is sufficient to permit the article to be moulded and to remain amorphous. Maintaining such a temperature gradient can readily be effected by including thermal barriers in the mould and carefully positioning the heating elements, without resorting to forced cooling of any region. It is however also possible to circulate a coolant through the mould so that both heat sources and heat sinks should be present to create the required gradient.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a perspective view of a food container, and Figure 2 is a section through a mould for forming the container in Figure 1.

The container 10 shown in Figure 1 has a single dished compartment 12 and two carrying handles 14. The container is made by vacuum moulding a blank which may be a flat blank cut from an extruded sheet or is preferably an injection moulded blank in which the thickness is not uniform to permit local strengthening of the container without increasing the entire thickness of the blank.

The handles 14 initially lie in the plane of the blank and are bent up during use of the container. Clearly therefore, the hinge of the handles needs to remain flexible if it is not to snap during use. For this to be the case, the hinge region 18 should remain amorphous.

The corners 20 also a further region where flexibility is an advantage as brittle containers tend to be damaged there when dropped.

When food is placed in the container, it is sealed by means of a foil adhered to its rim 22. It has been found that the foil adheres better to amorphous P.E.T.

than it does to the crystallised material, for which reason it is preferable not to harden the rim 22.

As for the remainder of the container, it is preferred that these parts should be hardened by crystallising the P.E.T. as this adds to the strength of the container and to its rigidity.

From the above description, it will be clear that the hardness requirements of the container differ from one region to the other and conventionally the container was hardened uniformly. A compromise had to be reached to strengthen the container sufficiently without make it too frangible in the corners.

In the preferred embodiment of the invention, the P.E.T.

container is hardened during the forming process by using a mould cavity 30 in which steps are taken to maintain a temperature gradient.

The mould cavity 30 of Figure 2 is for vacuum moulding of an injection moulded blank. The blank is injection moulded as this makes more efficient use of the plastics material and can permit the thickness in each region to be varied to provide reinforcement where necessary.

The blank which is initially flat is placed in a recess 32 in the mould cavity 30 and clamped against the recess by means of a movable collar 34. The mould cavity 30 includes passages which are not shown for evacuating the space beneath the blank to cause the heating blank to be formed against the inner surface of the mould cavity.

The mould cavity 30 has cartridge heaters 38 which are fitted within long receiving bores in the mould. In the mould cavity, the formed container is heated above the temperature where crystallisation takes place and once hardened, the container is ejected from the mould cavity.

As so far described, the mould cavity is generally conventional. To maintain parts of the container in the amorphous state, the mould cavity 30 additionally contains means for creating a temperature gradient either by forced cooling or by careful use of thermal insulators.

It has been mentioned previously that the corners 20 should remain amorphous. This is achieved using the mould of Figure 2 by a thermal barrier 36. The barrier 36 is disposed between the corners and the adjacent cartridge heaters to impede heat flow to the corners.

The barrier 36 may be an air gap or a thermal insulator.

Because the heat flow is impeded, during use the corners reach a lower temperature that the rest of the container and remain amorphous while other parts are crystallised.

To keep the rim 20 in an amorphous state, the collar 34 is shown including passages 40 for circulation of a coolant, such as water or cold air. This active cooling can of course also be used in place of a thermal barrier to achieve the desired cooling of the corners 20.

Claims (8)

1. An article made of partly crystalline P.E.T, having different degrees of crystallisation in different regions, whereby parts of the article are selectively hardened while others remain relatively flexible and less frangible upon impact.

2. An article as claimed in claim 1, wherein the article is a container with a dished compartment and wherein the corners of the dished compartment are less crystalline than the walls and the base of the compartment.

3. An article as claimed in claim 2, wherein the container comprises a rim to which a foil is to be adhered when the container is full, and wherein the rim is less crystalline that the walls of the dished compartment.

4. An article as claimed in claim 3, wherein the container includes handles bent out of the plane of the rim and wherein the hinge region between the handles and the rim is less crystalline than the walls of the dished compartment.

5. A mould for forming an article from a blank which includes means for locally heating different regions of the mould so as to maintain a temperature gradient within the mould and thereby heat the moulded article unevenly, only selected parts of the article which are to be hardened by partial crystallisation being heated above the temperature necessary to initiate crystallisation.

6. A mould as claimed in claim 5, which includes thermal barriers to impede heat flow to selected regions of the surface of the mould cavity.

7. A mould as claimed in claim 5 or 6, which includes means for locally cooling selected regions of the surface of the mould cavity.

8. A mould cavity constructed substantially as herein described with reference to and as illustrated in the accompanying drawings.