GB2139934A

GB2139934A - Pressing contoured shapes

Info

Publication number: GB2139934A
Application number: GB08313417A
Authority: GB
Inventors: Anthony Green
Original assignee: TKR INTERNATIONAL Ltd
Current assignee: TKR INTERNATIONAL Ltd
Priority date: 1983-05-16
Filing date: 1983-05-16
Publication date: 1984-11-21
Also published as: GB8313417D0; GB2139934B

Abstract

Sheet material (6) such as sheet metal and, especially, sheets of composite material such as aromatic polymer composites, for instance carbon fibre reinforced polyetheretherketone, is formed into contoured shapes having high configurational accuracy and finish on both surfaces by pressing such sheet material between a rigid contoured tool (1) and a complementarily contoured semirigid diaphragm (7) thrust against the sheet material (6) and the tool (1) by fluid pressure in a manner to cause the sheet material to conform to the tool (1) configuration and to experience substantially isostatic pressure. <IMAGE>

Description

SPECIFICATION Pressing contoured shapes This invention concerns the pressing of contoured shapes in sheet metal and composite sheet material.

There are many available techniques for pressing contoured shapes from sheet metal some of which are applicable to the pressing or moulding of contoured shapes from sheets of composite material such as fibre-reinforced plastics. However the pressing of saddle-section shapes, especially in workpieces of extended length, is difficult to accomplish particularly when configurational accuracy and/or a high surface finish is required for both surfaces of the product. The reason for this is that techniques employing matched male and female press tools involve drawing of the sheet material - which may be undesirable especially in the case of relatively thick composite material - and also involve the application of a variable, non-isostatic, pressure on the thickness of the sheet material in the closed condition of the tools.This is particularly disadvantageous in the case of composite materials that require to be subjected to substantially isostatic pressure for uniform consolidation and optimum realisation of their properties in the product. Accord inglytechniques involving the use of relatively flexible diaphragms engaging one surface of the workpiece and subject to fluid pressure to force the workpiece against a tool have been developed.

However such techniques in general result in a product in which only the surface engaged by the tool has the desired finish and configurational accuracy.

An object of the present invention is to provide a process and equipment for pressing contoured shapes from sheet metal and composite sheet material, that avoids these and other disadvantages of the known techniques.

In its broadest aspect the present invention provides a process for the forming of sheet material into contoured shapes, comprising introducing such sheet material into the space between a rigid contoured tool and a complementarily contoured semirigid diaphragm and applying fluid pressure to said diaphragm to thrust the same against the sheet material and the rigid tool in a manner to cause the sheet material to conform to the tool configuration and to experience substantially isostatic pressure.

The physical properties of the diaphragm to be used in this process depend to an extent upon the properties of the sheet material to be formed by the process. Thus the diaphragm must be sufficiently shape-holding to ensure that the sheet material will adapt and conform to the configuration of its contoured surface, and yet be sufficiently flexible to enable the development of substantially isostatic pressure on the sheet material by the application of fluid pressure to the diaphragm. In general, however, the diaphragm will be formed of a sheet metal selected for its ability to be formed into the required configuration to complement the rigid tool by the property of superplasticity.

Moreover because the forming process of the invention will normally be carried out at elevated temperature, the material from which the diaphragm is formed must be capable of maintaining its configuration at the forming temperature concerned and, ideally, it should have a coefficient of thermal expansion closely matching that of the rigid tool so that the complementary configurations of the tool and diaphragm are maintained overtheworking temperature range.

The process of the invention may be applied, for example, to the forming of sheet metal such as aluminium and other light alloy sheet, titanium and titanium alloy sheet and stainless steel sheet.

The process of the invention is also especially applicable to the forming of contoured shapes from composite sheet material and especially from the aromatic polymer composites (APC) such as the carbon fibre reinforced polyetheretherketone (PEEK) resin sheet material as manufactured by ICI under its Trade Mark VICTREX. The satisfactory consolidation of materials of this character requires the application of temperatures ranging up to about 400"C and the application of substantially isostatic pressure to avoid displacement of the carbon fibre reinforcement within the resin matrix during consolidation.

APC materials based on PEEK resins are thermoplastic in character and the process of the invention is applicable to other composite materials of similar thermoplastic character, and also to composite materials having a thermosetting resin matrix such as composites based on epoxy resins.

In carrying out the process of the invention, the sheet material to be formed is heated to the required forming temperature (if it is not to be cold-formed) either in situ adjacent to the rigid tool or externally in a pre-heating chamber such as an infra-red oven. If external pre-heating is employed the pre-heated sheet, at the forming temperature, is transferred into the space between the tool and the diaphragm.

The sheet material is next pressed into conformity with the configurations of the rigid tool and diaphragm respectively, by causing the diaphragm to move towards the tool by the application of fluid pressure to the diaphragm. When the tool and diaphragm are fully "closed' upon the sheet material, the complementary configuration of the tool and diaphragm enables the fluid pressure applied to the diaphragm to develop substantially isostatic pressure throughout the sheet material thereby to cause this to conform uniformly with the complementary configurations of the tool and diaphragm.

If the sheet material is sheet metal the fluid pressure on the diaphragm may be immediately relieved and the shaped sheet metal removed while still at or near the forming temperature. However, if the sheet material is a composite of thermoplastic character the process will involve cooling of the material, while still under cnsolidation pressure, to a suitable temperature to render the product shapeholding before the pressure in relieved and the product removed.

If the sheet material is a composite of thermosetting character the consolidation pressure will be maintained until the sheet material has cured.

The invention also extends to apparatus for per forming the process. Thus in another aspect the invention provides apparatus for forming sheet material to produce contoured shapes therefrom, characterised by a rigid tool and a semirigid diaphragm having a configuration complementary to the tool and adapted to be pressed theretowards by fluid pressure in a manner to apply substantially isostatic pressure to sheet material interposed between the tool and the diaphragm.

As noted, the diaphragm is conveniently formed of a material that exhibits superplasticity at elevated temperature: suitable materials for forming the diaphragm are, for example, superplastic zinc; titanium and certain alloys thereof; superplastically formable stainless steel such as "Avesta 3RE60"; and certain high performance nickel alloys such as "Nimonic 718". A diaphragm is conveniently formed to the required configuration complementary to the rigid tool by using the latter as a matrix against which the diaphragm sheet, raised to the appropriate elevated temperature (for instance a 1 000 C in the case of stainless steel) by the application of gas pressure.To provide an allowance for the thickness of the sheet material to be formed by the equipment a suitable sheet having a thickness corresponding with that of the intended product may be fitted to the rigid tool during the process of forming the diaphragm for use therewith.

The diaphragm may conveniently constitute one wall of a bag-like member adapted to be interposed between the rigid tool and a reacting or backing member proximate thereto, the bag being capable of being inflated by introduction of gas so as to apply the required fluid pressure to the diaphragmforming wall of the bag during operation of the equipment in performance of the process of the invention.

Atypical embodiment of apparatus in accordance with the invention is illustrated in the single Figure of the accompanying drawing that is an exploded, semi-diagrammatic, perspective view of the principal components of the apparatus.

The apparatus shown in the drawing comprises an essentially rigid male tool 1 adapted to be mounted directly or indirectly on a lower press platen 2. A reacting or backing member 3 of channel-like configuration providing clearance over the tool 1 is carried by the upper platen (not shown) of a press so asto be movable into position overthetool 1 and held, by the press, to retain the tool 1 and member 3 in fixed relative positions during a forming operation.

The apparatus further comprises a bag-like component 4, initiallyformed by welding togetherthe edges of a pair of superposed metal sheets of, e.g., superplasticallyformable stainless steel while providing one or more inlet connections 5 to the interior space between the sheets.

The component 4 is formed to the arched shape illustrated by heating it to a suitably high temperature, for instance about 1 000 C, placing it over the tool 1 and then lowering the backing member 3 into place over the tool 1 while introducing gas through the inlet(s) 5 sufficiently to inflate the component and enable it to be approximately formed to a configuration matching the space between the tool 1 and the inner surface of the backing member 3 as the press closes. With the press fully closed, the gas pressure within the component 4 is then increased to an extent sufficient to effect superplastic forming of the wall of the component 4 adjacent to the tool 1 into conformity with the surface configuration of the tool 1.

If the apparatus is intended only for the forming of relatively thin sheet material and the surface configuration of the tool 1 does not involve re-entrants or small radii, direct forming ofthe diaphragmconstituting wall of the component 4 against the tool 1, in the manner described above, will be satisfactory. However if the tool configuration is more complex and/orthe equipment isto be used for forming a workpiece 6 of relative thick sheet material, the forming of the diaphragm-constituting wall 7 of the component 4 may be accomplished with the interposition of a dummy workpiece 6 between the tool 1 and the component 4.The dummy workpiece may be preformed to the configuration of the ultimate product or, if of a material suitably deformable at the temperature involved, may be a simple sheet that will accommodate to the configuration of the tool 1 during the diaphragm-forming operation.

In use of the illustrated apparatus in performing the process of the invention, a workpiece 6 is placed on the tool 1, overlaid by the component 4 and the backing member 3 thereupon closed on the tool 1.

Gas pressure is then applied to the inlet(s) 5 so as to apply fluid pressure to the interior of the component 4 and thus against the diaphragm-constituting wall 7 of the component 4 to cause this wall to apply substantially isostatic pressure to the workpiece 6 and force the latter into conformity with the complementary configurations of the tool 1 and wall 7.

If the forming operation is to be conducted at elevated temperature, the workpiece may be preheated to forming temperature before being placed on the tool 1 or it may be heated in place by heating the tool and/or by irradiation while on the tool and before superposition of the component 4 and backing member 3. Heat may be applied to the tool by the provision of heating elements therein or by conduction from a heated press platen 2, as convenient.

If the nature of the workpiece 6 requires itto be cooled in contact with the tool, the latter may be provided with means for accomplishing cooling: e.g.

with passages for the flow of coolant liquid.

We have found that in pressing composite sheet material consisting of carbon fibres embedded in a matrix of PEEK, preheating of that material in an infra-red oven to a temperature approaching 4000C followed by immediate forming, in the manner described, with the tool 1 at a temperature of the order of 1 50 C achieves satisfactory consolidation of such sheet material and the production of a contoured product with high grade finish on both surfaces and uniformity of consolidation demonstrated by uniformity of physical properties of the product throughout its extent.

We have also found that superplastic zinc may be hot-formed at temperatures about 250 C; aluminium may be hot-formed at about 400 C; titanium at temperatures of about 5000C and titanium alloys at temperatures ranging from about 6500C up to about 900 C; and stainless steel at temperatures of about 900 C. The use of the higher forming temperatures requires the use of a diaphragm of stainless steel or high performance nickel alloy.

Claims

1. A process for the forming of sheet material into contoured shapes, comprising introducing such sheet material into the space between a rigid contoured tool and a complementarily contoured semirigid diaphragm and applying fluid pressure to said diaphragm to thrust the same against the sheet material and the rigid tool in a manner to cause the sheet material to conform to the tool configuration and experience substantially isostatic pressure. Ire.

2. A process according to claim 1, characterised by the use of a diaphragm of sheet metal having said contoured configuration developed by superplastic forming.

3. A process according to claim 2, wherein said diaphragm is superplastically formed to said contoured configuration by use of said rigid tool as a matrix.

4. A process according to claim 1, 2 or 3, wherein said diaphragm has a coefficient of thermal expansion closely matching that of the rigid tool.

5. A process according to any preceding claim, wherein the sheet material is an aromatic polymer composite.

6. A process according to claim 5, wherein the sheet material is a composite of carbon fibre in a matrix of polyetheretherketone resin.

7. Apparatus for forming sheet material to produce contoured shapes therefrom, characterised by a rigid tool and a semirigid diaphragm having a configuration complementary to the tool and adapted to be pressed theretowards by fluid pressure in a manner to apply substantially isostatic pressure to sheet material interposed between the tool and the diaphragm.

8. Apparatus according to claim 7, wherein diaphragm is formed of a material that exhibits su perplasticity.

9. Apparatus according to claim 8, wherein said diaphragm is superplastically formed to the said complementary configuration from a material selected from superplastic zinc, aluminium, titanium and its alloys, stainless steel and nickel alloys.

10. Apparatus according to claim 7 or 8, wherein said diaphragm constitutes one wall of an expandible bag-like member adapted to be contained by a backing member proximate the rigid tool and to be inflated by fluid under pressure.

11. Apparatus according to claim 7,8 or 9, wherein the rigid tool and the diaphragm have closely matched coefficients of thermal expansion.

12. Apparatus for forming sheet material, substantially as described with reference to the accompanying drawing.

13. A process for forming sheet material, substantially as described.

14. Every novel feature and every novel combination of features disclosed herein.