GB2192359A - Moulding plastics materials - Google Patents

Abstract

The method comprises injection moulding a blank (10) of non-crystalline plastics material, heating the blank, introducing the blank into a thermo-forming station (28) in which the blank is deformed under pressure into the shape of a mould cavity (22), and applying heat for sufficient time following the deformation to crystallise the plastics material at least partially. <IMAGE>

Description

SPECIFICATION Moulding plastics materials The present invention relates to the moulding of plastics articles and more particularly to the moulding of articles made from crystalline or partly crystalline plastics materials such as polyestertetrafilate, commonly referred to as PET.

Crystalline PET has recently been used in the making of baking dishes to replace the previously used aluminium foil. These dishes are capable of being heated in a conventional oven and have the advantage over aluminium dishes of being well suited for use in microwave ovens.

It should perhaps be mentioned that the need for the total or partial crystallisation of the plastics material is to strengthen the material so as to be better suited to its intended use. The crystallised material is distinguishable visually from the uncrystallised material in that the latter is clear while the former is opaque.

The process of injection moulding does not lend itself well to the making of articles from a partly cryalline plastics material since it is essential for the mould cavity to be heated and furthermore the cycle time would in practice be too long for injection moulding to be commercially viable. The method therefore employed in the making of such crystalline plastics dishes has been to extrude a sheet of PET and coil it. At some later stage, the sheet material is uncoiled and after preheating is passed through a thermoforming station where the sheet is deformed under pressure to conform to the surface of the mould cavity.

The sheet is heated further in the mould for it to be at least partly crystallised. The pressure formed dish is then separated from the web by some suitable cutting means and after passing through a further heating station to continue the crystallisation process the formed dishes are stacked and the remaining web is withdrawn for later reprocessing.

The method described above suffers from several disadvantages which will now be discussed briefly.

One problem is that of wastage of the plastics material which results from the limited extent of deformation that can be achieved during the thermo-forming process. Because the dish is stretched from an initially flat sheet, a substantial thickness of sheet must be employed to avoid excessive thinning in the corners of the dish and this causes other parts of the articles to be unnecessarily thick. The remaining web is also wasteful of plastics material because after crystallisation it cannot merely be recycled by being re-extruded but must instead be totally reprocessed.

A further problem is that the separation of the dishes from the web is difficult to achieve efficiently because of the abrasive properties of the material. This causes undue wear on the cutting edges and leaves poor edges in the finished article.

Additionally, the need to minimise the web wastage also places limitations on the design of the article and generally these are found to be rectangular in outline, which is not of necessity the most aesthetically desirable shape.

The invention seeks to provide a method of forming articles from crystalline plastics material which does not suffer from the foregoing disadvantages.

In accordance with the present invention, a method of forming a plastics article comprises injection moulding a blank of non-crystalline plastics material, heating the blank, introducing the blank into a thermo-forming station wherein the blank is deformed under pressure into the shape of a mould cavity, and applying heat for sufficient time following the deformation to crystallise the plastics material at least partially.

The important difference between the #inven- tion and the prior resides in the fact that the plastics material to be thermoformed is not produced by extrusion of a flat sheet but by injection moulding. Because each blank is individually moulded the problems associated with severing a formed article from a continuous web do not arise nor does there arise any problem in relation to the wastage caused by the material in the web itself.

Unlike an extruded sheet, the injection moulded blank need not be of uniform thickness and can have thickened portions in the regions which are to be stretched during the thermo-forming process while the regions which do not undergo deformation in the thermo-forming can be made relatively thin. In this way the amount of material contained in each article can be reduced without weakening the article at its most vulnerable points such as the corners. The blank may furthermore be formed with reinforcements so that large areas in the finished article may be rendered more rigid and less prone to bending or twisting.

The injection moulding of the blank does not present problems as severe as the moulding a finished article because the distances over which the plastics material must flow in the mould are shorter and the thickness of the mould cavity is greater. It is also not required at this stage to crystallise the palstics material. Because of these various factors, it is possible to employ an unheated mould for the making of the blank. Furthermore, as no attempt is being made to heat and crystallise the blank during the blank forming process the cycle rate of an injection moulding machine producing the blanks can be high.

The apparatus for thermo-forming the blanks preferably comprises a stacker arranged above a conveyor having a plurality of individual mould cavities each for holding one blank, a preheating station, for example an infrared heater, arranged to pre-soften the non-crystalline plastics material, a thermo-forming station in which the edges of the blank are gripped against the mould cavity and the centre of the blank is deformed under pressure to conform to the shape of the mould cavity and a heating station for heating the deformed blank to continue the crystallisation of the plastics material.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a blank formed by injection moulding for use in the making of a crystallised PET oven dish, and Figure 2 shows schematically an apparatus for forming oven dishes from the blank shown in Figure 1.

In Figure 1, there is shown an oval blank 10 which is of non-uniform cross section. The regions 12 of the blanks 10 which are to be stretched during the later thermo-forming process are of greater thickness than the remainder of the blank. If desired, reinforcement ribs for the finished article may also be moulded in the blank 10 at this stage. The thickened regions of the blank may protrude from only one of from both surfaces of the blank.

The mould cavity for making the blanks 10 may be a conventional mould and no steps need be taken to heat the mould during or after the moulding process. The outline shape of the blank may be varied at will and the design of the thickened regions is selected in dependence upon the shape that is later to be produced. The thickened regions act as spurs to assist in the distribution of the injected plastics material and it will be clear to the person skilled in the art that the moulding of such a blank does not present any serious problems affecting the design of the mould cavity nor reducing the cycle time.

After moulding of the blanks, these are stacked for later input into the apparatus of Figure 2, which converts the blanks into oven dishes. The apparatus comprises a conveyor 20 which consists of pressure forming mould cavities 22 joined end to end and driven in an endless loop by a chain which is not shown.

The conveyor passes beneath several stations each of which is conventional and is therefore only shown schematically.

In a loading station 24, there is stacked the preformed injection moulded blanks 10 shown in Figure 1 and one such blank in picked up by the mould cavity 22 from the stack as it passes through loading station 24.

The cavity 22 now passes through a preheating station 26 which softens the plastics material of the blank 10 in preparation for thermo-forming in the next station deisgnated 28 in Figure 2. The length of the station 26 determines the duration of the pre-heating and several blanks may be present in the pre-heat ing station at the same time.

The thermo-forming station 28 is essentially a heated press in which the softened plastics blank is stretched to conform to the shape of the mould cavity 22. As the press descends, the edges of the blank are first gripped between the press and the mould cavity 22.

Further pressure deforms the blank into the shape of the cavity by stretching the thick ened regions of the blank. The press is heated in order to crystallise the plastics material.

After deformation and partial crytallisation in the station 28, the cavity 22 passes through a further heating station 30 where the temperature of the plastics material is controlled to continue or complete the crystallisation pro cess. The heating station 30 may in fact com prise several heating stages at different tem peratures and once again several cavities may be present in the heating station 30 at the same time.

The heating stations can be of generally conventional design and can employ any suit able form of radiation. In particular, the heat ers may comprises electrical heating elements or infra-red radiation sources.

After their formation, the oven dishes either fall out or are pushed out of the mould cavities 22 onto a stack 32 allowing the cavities to return empty to the loading station 24.

The thermo-forming process is therefore an essentially continuous one and the heating of the blank is not merely carried out in the thermo-forming press but is spread over sev eral stations. The time spent by any cavity in the thermo-forming press press 28 is only that necessary to carry out the deformation of the blank and therefore the cycle time of the apparatus of Figure 2 is short and the number of dishes that can be formed in a given time is commercially acceptable.

Though the cavities 22 have been assumed to be include the entire female mould cavity for each blank this is not necessarily the case as the female mould cavity may be formed in two parts of which one moves with the con veyor and the other forms part of the thermdforming station. For example, the moving cavities 22 may just overlap the edges of the blanks and have a central aperture in which a female mould is received in the thermo-form ing station 28. This construction reduces the cost of the moving cavities and is also desir able in that only the tools in the press have to be modified if one wishes to alter the design of the finished article.

The invention can be seen to avoid the problems associates with cutting the dishes from a web and can furthermore reduce the material wastage. The invention further affords more freedom in the design of the dishes and allows the forming of dishes with inner partitions if desired.

Claims (5)

1. A method of forming a plastics article comprises injection moulding a blank of noncrystalline plastics material, heating the blank, introducing the blank into a thermo-forming station wherein the blank is deformed under pressure into the shape of a mould cavity, and applying heat for sufficient time following the deformation to crystallise the plastics material at least partially.

2. A method as claimed in claim 1, wherein the blank is moulded with a non-rectangular outline.

3. A method as claimed in claim 1 or 2, wherein the blank is moulded with uneven thickness, being thickened in the regions which are subsequently stretched during the thermo-forming process.

4. Apparatus for carrying out the method of any preceding claim, comprising a loading station arranged above a conveyor having a plurality of individual mould cavities each for holding one blank, a preheating station arranged to pre-soften the non-crystalline plastics material, a thermo-forming station in which the edges of the blank are gripped against the mould cavity and the blank is deformed under pressure to conform to the shape of the mould cavity and a heating station for heating the deformed blank to continue the crystallisation of the plastics material.

5. A method of forming a plastics article, substantially as herein described with reference to and as illustrated in the accompanying drawings.