GB2049535A

GB2049535A - Sheet forming

Info

Publication number: GB2049535A
Application number: GB7918330A
Authority: GB
Original assignee: Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1979-05-25
Filing date: 1979-05-25
Publication date: 1980-12-31
Also published as: JPS55159935A; PT71274A

Abstract

In the manufacture of an article having a base and an integral peripheral wall, a clamped heat-softened thermoplastic sheet (41) is prestretched by a flexible diaphragm (31) into the cavity (2) of a female mould (1) in which the forming of the article is completed by positive pneumatic pressure and/or vacuum applied to the prestretched sheet. The diaphragm may be expanded by gas or liquid admitted at (22), air under pressure being admitted at (15) to effect the final forming. <IMAGE>

Description

SPECIFICATION A method and a device for the manufacture of articles from a thermoplastic sheet The invention relates to a method and to a device for the manufacture of an article having a base (a bottom or a top) and a peripheral wall integral therewith from a heat-softened thermoplastic sheet.

In a known method, known as "plug assist forming", a heat-softened thermoplastic sheet is mechanically prestretched by a plug into a cavity prior to fully shaping it into the finished article by either vacuum or positive pneumatic pressure. The aim of prestretching is to improve material distribution in the article. This method improves material distribution when used with a female mould in which the depth-diameter ratio equals or is greater than 1:2.

For shallow forming, such as the forming of trays, the advantages of this method are less significant.

Often an excessively thick base is an inevitable consequence of obtaining adequate strength at the base of the peripheral wall and along the edge between the bottom and the peripheral wall. A heavy base spoils the shape, is wasteful on material and may reduce manufacturing speed as a result of increased cooling time.

The aim of the invention is to devise a method of manufacture of articles from a thermoplastic sheet which has advantages over the described known method, and also a device suitable for carrying out the method.

According to a first aspect thereof the invention provides a method for the manufacture of an article having a base and a peripheral wall integral therewith from a thermoplastic sheet, in which a clamped heat-softened thermoplastic sheet is prestretched by a flexible diaphragm into the cavity of a female mould in which the forming of the article is completed by positive pneumatic pressure and/or vacuum applied to the prestretched sheet.

This method, which might be called "diaphragm assisted forming" must be distinguished from the so-called "diaphragm forming" in which the article is formed from a sheet solely by the pressure applied by a flexible diaphragm. Known "diaphragm forming" has the disadvantages of excessive straining of the diaphragm, which is the sole means of shaping, and of being suitable for the manufacture of only a limited number of shapes.

The invention also provides, according to a second aspect, a device for the manufacture of an article having a base and a peripheral wall integral therewith from a heat-softened thermoplastic sheet, the device comprising a resilient diaphragm, means for carrying the diaphragm, means for clamping the sheet, a female mould providing a cavity, means for pneumatically or hydraulically expanding the diaphragm into the cavity to prestretch the sheet therein, and means for providing positive pneumatic pressure and/or vacuum to the prestretched sheet for final forming of the article in the cavity.

In a preferred embodiment of the device the means for carrying the diaphragm are displaceable towards and away from or into and out of the cavity, and the diaphragm is of non-uniform thickness.

U.K. Patent Specification 1 213 059 describes a method of "diaphragm forming" wherein fluid is supplied to the interface between the diaphragm and the sheet to be formed, so as, by reducing the frictional resistance between the diaphragm and the sheet, to allow relative lateral movement between them.

Applicants have found however, that in order for the distribution of the sheet within the female mould to be accurately controlled it is preferred that substantially no relative movement should occur between the sheet and the diaphragm. Applicants accordingly propose that the diaphragm material and/or its surface characteristics should be chosen to provide a high coefficient of friction with the sheet material, and the thickness of the diaphragm should be made non-uniform in such a manner that the thickness of material over the area of the finished article is as required.In general, the greater the thickness of the diaphragm in a particular locality, the greater will be the thickness imparted to the adjacent sheet material because the degree of stretching of the latter will be correspondingly small; correspondingly, the smaller the diaphragm thickness, the more the diaphragm, and hence the sheet material, will stretch, and the smaller will be the sheet material thickness.

In accordance with a third aspect of the invention there is provided a method forthe manufacture of a article having a base and a peripheral wall integral therewith from a thermoplastic sheet, in which a clamped heat-softened thermoplastic sheet is stretched into a mould cavity by a flexible diaphragm of non-uniform thickness, with substantially no relative movement occuring between the diaphragm and the sheet at the interface between them.

Such a method may include the further step of completing the formation of the article by positive pneumatic pressure and/or vaccum applied to the sheet, but this further step is not essential. The method may therefore be of the kind referred to above as "diaphragm assisted forming", or of the kind referred to as "diaphragm forming".

The invention also provides, according to a fourth aspect, a device for performing the method of the third aspect, the device comprising a resilient diaphragm of non-uniform thickness, means for carrying the diaphragm, means for clamping the sheet, a female mould providing a cavity, and means for pneumatically or hydraulically expending the diaphragm into the cavity to prestretch the sheet therein.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a device according to the invention in vertical section at the beginning of moulding when the diaphragm is deflated; Figure 2 shows the device when the diaphragm is fully expanded by inflation; Figure 3 shows the device at the completion of the forming cycle; Figure 4 is a plan view of the diaphragm used in Figures 1-3; Figure 5 is a section along line A-A in Figure 4; and Figure 6 is a section along line B-B in Figure 4.

Figures 1 to 3 show a device comprising a female mould 1, which defines a cavity 2 and is provided with a base insert 3, which serves also as an ejector, and with a raised land 4 having a top end face. The illustrated device is designed for the manufacture of relatively shallow plastics trays which are substantially rectangular in plan, and the cavity 2 is shaped correspondingly. In the descriptive part of this specification the term "substantially rectangular" includes "rectangular with rounded corners", such as shown in Figure 4.

The device also comprises a chamber 11 having a top 12 and a peripheral wall 13 having a bottom end face, the shape and size of which correspond to those of the end face of the land 4. The top 12 is provided with a central opening 14 and a further opening 15, which will be referred to later.

The device further comprises a carrier 21 having a hollow shaft 22 provided with a carrier plate 23. The shaft 22 is fixed within the opening 14 non-movably and air-tightly. The plate 23 is substantially rectangular in plan and is near its periphery provided with a plurality of holes.

The carrier 21 carries a diaphragm 31 which is substantially in the shape of a tray and is substantially rectangular in plan, as shown in Figure 4. It has a peripheral wall 32 and a bottom 33 having a central area 34 and a peripheral area 35. An inner flange 36 extends from the peripheral wall 32 and is near its periphery provided with a plurality of holes.

The peripheral wall 32 is relatively thick, while the central area 34 is relatively thin, the peripheral area 35 being thickest at the peripheral wall 32 and becoming gradually thinner towards the central area 34 into which it eventually merges. The thickness of the peripheral area 35 need not be uniform along the whole periphery, it may be of different thickness e.g.

in the corner regions. The diaphragm 31 is made from a resilient material, such as silicon rubber, the resilient properties of which do not appreciably deteriorate when exposed to heat.

The diaphragm 31 is attached to the carrier plate 23 by means of screws 24, which pass through the holes in the plate 23 and through bushes 25 in the holes in the flange 36 and are screwed into threaded holes in a clamp ring 26 which is also substantially rectangular in plan.

In operation a heat-softened sheet 41 of thermoplastic material, e.g. polypropylene, is positioned on the land 4 and the chamber 11 is pressed against the sheet 41 so that the latter is clamped between the land 4 and the peripheral wall 13. In that position the diaphragm is either in light contact with or at a small distance above the sheet 41. Then air is forced under pressure through the hollow shaft 22 to the diaphragm 31 which is thereby inflated so that it expands into the cavity 2 and thereby deforms the sheet 41 correspondingly, as shown in Figure 2. At that stage air is forced under pressure into the chamber 11 through the opening 15 while the diaphragm 31 is deflated through the hollow shaft 22 and returns by its own resilience and by the air pressure in the chamber 11 into its initial position.

The air forced through the opening 15 completes the moulding of the sheet 41 in the cavity 2 of the mould 1, as shown in Figure 3, and is then released through the opening 15 or some other opening (not shown).

The described device and method may be modified in a number of ways. So for instance the same device may be used for vacuum moulding, in which case the opening 15 communicates the chamber 11 with atmosphere and the mould is a vacuum mould of a type know per se.

Also hydraulic liquid may be used instead of air or other gas to expand the diaphragm 31.

The carrier 21 may be arranged to be displaceable in which case its shaft 22 is adapted to move axially, and if necessary also air-tightly, through the central opening 14. This movement will be relatively small, preferably not more than 2 cm.

Afemale mould 1 having a single cavity 2 of a simple shape has been described. The invention is, however, suitable for use with a female mould having either a single cavity 2 of a more complex shape or having two or more cavities which may be all of the same shape or which may be of different shape, either a simple or a complex one.

It is important to ensure consistency of the maximum expansion of the diaphragm 31 to obtain articles having the same material distribution. This may be achieved by controlling either the pressure or the volume of the fluid inflating the diaphragm 31.

The latter may be achieved e.g. by delivering the fluid by a cylinder-and-piston means in which the pistion sweeps always the same volume.

The material of the diaphragm should preferably be chosen having the material of the plastics sheet in mind. The aim of the choice is to select material so as to obtain such friction between the diaphragm and the hot plastics sheet that no relative movement, or hardly any relative movement, would occur between the diaphragm and the sheet, so that the latter will stretch in a manner dictated by the diaphragm.

The shape and the non-uniform thickness of the diaphragm are chosen to expand the diaphragm in a predetermined manner which would directly influence distribution of material in the finished product.

Claims

1. A method for the manufacture of an article having a base and peripheral wall integral therewith from a thermoplastic sheet, in which a clamped heat-softened thermoplastic sheet is prestretched by a flexible diaphragm into the cavity of a female mould in which the forming of the article is completed by positive pneumatic pressure and/or vacuum applied to the prestretched sheet.

2. A device for the manufacture of an article having a base and a peripheral wall integral therewith from a heat-softened thermoplastic sheet, the device comprising a resilient diaphragm, means for carrying the diaphragm, means for clamping the sheet, a female mould providing a cavity, means for pneumatically or hydraulically expanding the diaphragm into the cavity to prestretch the sheet therein, and means for providing positive pneumatic pressure and/or vacuum to the prestretched sheet for final forming of the article in the cavity.

3. A device according to Claim 2 wherein the means for carrying the diaphragm are displaceable towards and away from or into and out of the mould cavity.

4. A device according to Claim 2 or 3 wherein the diaphragm is of non-uniform thickness.

5. A device according to Claim 2, 3 or 4 wherein the diaphragm is so made and its material so selected that substantially no relative movement occurs between the diaphragm and the sheet at the interface between them.

6. A method for the manufacture of an article having a base and a peripheral wall integral therewith from a thermoplastic sheet, in which a clamped heat-softened thermoplastic sheet is stretched into a mould cavity by a flexible diaphragm of non-uniform thickness, with substantially no relative movement occurring between the diaphragm and the sheet at the interface between them.

7. A method according to Claim 6 wherein the formation of the article is completed by positive pneumatic pressure andior vacuum applied to the sheet.

8. A device for carrying out a method according to Claim 6 or 7, the device comprising a resilient diaphragm of non-uniform thickness, means for carrying the diaphragm, means for clamping the sheet, a female mould providing a cavity, and means for pneumatically or hydraulically expanding the diaphragm into the cavity to prestretch the sheet therein.

9. A method according to Claim 1 substantially as herein described with reference to the accompanying drawings.

10. A device according to Claim 2 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown, in the accompanying drawings.

11. A method according to Claim 6 substantially as herein described with reference to the accompanying drawings.

12. A device according to Claim 8 substantially as herein described with reference to and shown in the accompanying drawings.

13. An article made by, or with the use of, a method according to any one of Claims 1, 6, 7, 9 or 11, or by, or with the use of, a device according to any one of Claims 2 to 5,8,10 or 12.