DEVICE AND METHOD FOR EXPANSION FORMING

Abstract: A device and method for expansion forming of an article used in conjunction with an isostatic press comprising a pressure cell equipped with an elastomeric membrane (3). The expansion forming tool (4) comprises at least one pressure intensifier (8) and at least one moulding cavity (6). A blank (5) is arranged in the moulding cavity (6) and an elastomeric forming body (7) is arranged inside the blank (5). The pressure intensifier is arranged such that it is acted upon by the elastomeric membrane (3) and pressure exerted on the elastomeric forming body (7) inside the blank (5) inside the moulding cavity (6) so that the blank (5) is expanded into an article shaped according to the shape of the moulding cavity (6).

Description: Device and method for expansion forming, TECHNICAL AREA The present invention relates to a device and method for forming hollow articles . More precisely the present invention refers to expansion forming hollow and complex shaped metal articles by means of high pressure. BACKGROUND ART In the production of hollow articles there are well known methods to force a hollow or tubular blank against the walls of a surrounding cavity using a fluid pressure, for example a pressurised fluid such as hydraulic oil. US 5644829 "Method for expansion forming of tubing" discloses a method for expanding a tube-shaped blank against the walls of a surrounding cavity. The method includes the use of two axial cylinders to seal the ends of the tube. The tube is then filled with a fluid which is then pressurised. The pressure from the pressurised fluid forces the tube against the walls of the surrounding cavity. US 5419171 "Isostatic bulge forming" discloses a method for bulge-forming a tube-shaped blank using a pressure fluid in an isostatic press. It also discloses how a bulge or protrusion may be formed in a wall of the tube-shaped blank, a bulge that could be subsequently knocked out, thus forming an opening in the wall of the blank. In an article entitled "HYDROFORMING APPLICATION IN THE AUTOMOTIVE INDUSTRY" published in the periodical "Advanced Technologies and Processes", the author discusses hydroforming . In particular hydroforming is discussed in relation to methods for forming hollow components such as parts of exhaust systems for automobiles. In the process described, a mould is held in a conventional two directional press. The mould also comprises two axial cylinders which seal the open ends of a pipe-shaped blank. Steps in the process are disclosed including the following : A pipe blank is loaded into the cavity of the tool, the press closes the tool, axial cylinders seal the pipe ends and fill-in a liquid which is pressurised. In the article a multiple fitting tool for multi-cavity production is also shown. This comprises a multi-cavity mould held in a horizontal moulding press with internal pressure supplied to the cavities by a pressure intensifier via a pressurised liquid. The presses used in the hydroforming processes described supply internal pressure against the blank via a pressurised liquid. A brochure issued by "KRUPP HOESCH AUTOMOTIVE GmbH" describes how a hydroforming process may be used to manufacture certain products such as chassis sub-frames and floor pans for the automotive industry. The process is described as comprising a press in which is mounted a mould or a tool. A tube-shaped blank or a specially contoured blank is placed in the mould. The mould is closed and in the case of the tube shaped blank, the ends are sealed with axial punches. By means of internal pressure supplied by a hydraulic fluid, as well as axial pressure in the tube example, the article is moulded against the walls of the mould cavity. These two processes both have the disadvantage that the amount of pressure available that can be exerted on a blank by a conventional two-directional press is limited. The limits are in terms of both direction of application, usually limited to one axis, and limited in magnitude of pressure which can be applied. This means that pressure may be uniformly applied via a hydraulic fluid to the blank in the interior of the cavity, but the principal closing pressure applied to the exterior of the mould will be in one axial direction. This in turn means the internal pressure available in the mould is limited by the strength of the mould, and that the moulding tools must be large enough and strong enough to withstand any tensile stresses produced by the internal pressure. The contact between the pressure fluid and the blank results in a requirement for seals within the mould that resist liquids under high pressure, and requires extra stages in the moulding cycle to clean the blank before and after the process. The use of a pressure fluid inside a blank makes the use of trimming edges or piercing forms inside a mould cavity difficult or impractical, so that trimming or piercing operations require additional process steps. High pressures are required for at least two reasons. First, to reduce the degree of springback which occurs. When subjected to low pressures, metal blanks typically deform elastically, which means that after the pressure is removed, the metal returns in part to its original shape. Permanent changes in the shape of metal blanks are the result of plastic deformation, which occurs at pressures greater than those that produce elastic deformation. Secondly, most of the metals used exhibit beneficially increased strength characteristics after they have been deformed plastically. This effect is sometimes described as cold working. Thus using high or extremely high pressures can produce a finished shape which is more accurate, which is more close to the required shape, and a stronger product. In the use of a high pressure press such as an isostatic press of the Quintus type, there exists a difficulty in providing moulds and moulding devices that are sufficiently strong. Moulds and moulding devices are often required to withstand very high pressures of up to 10,000 bar during each moulding cycle in a isostatic press. Such moulding devices are often very large, very heavy and very expensive. DESCRIPTION OF THE INVENTION, ADVANTAGES The object of the invention is to provide a device and method for the expansion forming of an article according to the claims. A further object of the invention is to provide a device and method where the edges of the article may be trimmed by means of the expansion forming method. A still further object of the invention is a device and method with which an opening may be pierced in an article during the expansion forming. A yet still further object is, amongst others, that the expansion forming device be inexpensive to produce and economic to use. These and other objects are achieved according to the invention by an expansion forming tool used with an isostatic press equipped with a membrane as described in the claims. The expansion forming tool is designed to produce extremely high pressures inside a hollow or tubular blank and shape the blank into an article by expansion against the walls of a moulding cavity. The expansion forming tool is designed so that the membrane of the isostatic press acts not only on the expansion forming tool, but on one or more built-in pressure intensifiers . The pressure intensifiers increase the pressure received from the membrane and transfers it to a metal blank inside a moulding cavity. This is carried out in the following way. Pressure from the membrane influences the pressure intensifier piston and is increased in proportion to the relative diameters of the pressure intensifier piston. The pressure fluid in the channel transfers the increased pressure via a tube-shaped membrane and exerts it against an elastomeric forming body, typically made of rubber. The elastomeric forming body is positioned inside the metal blank which is to be formed into the required hollow shape. Under the pressure applied by the elastomeric forming body the blank is expanded and forced against the walls of the moulding cavity in the expansion forming tool. Thus the blank takes the shape of the cavity walls. The moulding cavity walls can also be arranged with trimming edges and/or piercing forms. In this way, excess parts of the blank may be trimmed off during the expansion forming process by the pressure applied by the elastomeric forming body, forcing the blank against the trimming edges. Similarly one or more openings may be pierced through the walls of the blank by the pressure applied by the elastomeric forming body, forcing the blank against a suitably shaped piercing form. Although the pressures on the blank may be extremely high the expansion forming tool is arranged to use compression forces only. This means that the expansion forming tool may be relatively small and light in construction in relation to the pressures employed. The pressure produced by the pressure intensifier depends on a ratio of diameters of a piston in the pressure intensifier. The relative diameters may be varied and selected to suit the characteristics of the blank to be formed. The expansion forming tool is not limited to a single moulding cavity, and may comprise of several moulding cavities and pressure intensifiers . A tubular shaped blank is used and shaped by expansion with the present invention into a hollow form. It is also within the scope of the invention to use a blank which only partially encircles the elastomeric forming body. The advantages of the invention over the known prior art are many. The expansion forming tool is small, light and relatively inexpensive to produce, which means in turn that relatively low production volumes become more economic . The forming pressure inside the blank may be greater than the closing pressure of the press. There is no contact between pressure fluids and the metal blanks, thereby avoiding sealing, cleaning and contamination problems. A blank may be trimmed and/or pierced during the expansion forming process by trimming edges and/or piercing forms included in the walls of the moulding cavity for that purpose. Expansion forming thus reduces the number of process steps necessary for shaping an article. The availability of extremely high moulding pressures compared to devices of the prior art means that more detailed shapes can be achieved in a single moulding step. Greater plastic deformation of a blank is available, which results in more cold working and thus products with greater strength.. It also means that high strength alloys and thicker gauge metals can be shaped economically. Higher strength achieved in the forming of the product also offers opportunity to reduce the weight of material used in the article which is important in applications such as parts for automobiles. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in more detail in connection with the enclosed drawings. Figure 1 shows schematically a device for expansion forming according to the invention. Figure 2 shows a piercing form in an expansion forming device according to the invention. Figure 3 shows an article with an opening pierced in it according to the invention. Figure 4 shows schematically a development of an expansion forming device according to the invention. DESCRIPTION OF THE EMBODIMENTS Figure 1 shows schematically a isostatic press which comprises a pressure cell. The isostatic press comprises an upper, moveable part 1 and a lower, static part 2. In the upper, moveable part 1 of the isostatic press a membrane 3 is arranged with means available to supply pressurised fluid to it. An expansion forming tool 4 is situated in the lower, static part 2 of the isostatic press. The expansion forming tool 4 comprises a moulding cavity 6 and may comprise more than one moulding cavity. The expansion forming tool 4 is separable into, for example, two parts about a plane surface corresponding to the centre line A through the moulding cavity 6 as shown in Figure 1. An expansion forming method is as follows. A tubular or hollow blank 5 is placed in a lower part of the moulding cavity 6 of the expansion forming tool 4. An elastomeric forming body 7 is placed inside the blank 5. The upper part of the moulding cavity 6 is placed on top of the lower part so enclosing blank 5 with the elastomeric forming body 7 inside it. The press is closed, bringing the membrane 3 in the upper part of the isostatic press in contact with the expansion forming tool 4. Pressure is developed in the upper part 1 of the press, applying a pressure against the membrane 3 and thus over the expansion forming tool 4. The expansion forming tool 4 comprises one or more pressure intensifiers 8 each of which contains a piston 9. The piston 9 is acted upon by the membrane 3 and exerts a pressure on a fluid 10 in a cylinder 11 inside the pressure intensifier 8. The pressure in the fluid is transferred via a channel 12 via a tube-shaped membrane 13 to the elastomeric forming body 7 inside the blank 5 in the moulding cavity 6. The elastomeric forming body 7 expands and exerts a pressure against the blank 5 forcing it against the walls of the moulding cavity 6. The blank 5 is thereby expanded into an article shaped according to the shape of the walls of the moulding cavity 6. The moulding cavity may also be shaped such that excess parts of the blank are trimmed off by trimming edges 16 shown in Figures 1 and 4. In a further development of the embodiment shown in Figure 2 one or more piercing forms 17 are included. During the expansion forming method, the blank 5 is forced against the piercing form 17 and a passageway or opening 18 is pierced through the wall of blank 5 as shown in Figure 3. Upon the completion of the expansion forming method pressure is removed. The upper part 1 of the press is raised, and the expansion forming tool 4 separated by, for example removing an upper part containing an upper part of the moulding cavity 6. The blank 5, which has now become a shaped article, is removed from the moulding cavity 6. Trimmed off or scrap material is removed from the trimming edge 16. Scrap material from a piercing operation is removed from the piercing form 17. The trimming edge 16 shown in Figure 1 is in the form of a circumferential groove in the walls of the moulding cavity 6. It is within the scope of the invention that the expansion forming tool may also comprise trimming edges and/or piercing forms that protrude from the surrounding walls of the cavity and into the cavity. Such trimming edges and/or piercing forms include both static devices fixed in the cavity walls and moveable devices that may be moved out from the walls of the cavity into walls of the blank using pressure intensifiers or other hydraulic or mechanical means. The pressure increase achieved by the pressure intensifier is determined by the selection of a diameter of the upper part 15 of the piston 9 in the pressure intensifier 8 relative to a diameter of the lower part 14 of the piston 9. The elastomeric forming body 7 may be shaped so as to fit closely with blanks of different shapes which are used to produce finished articles of different shapes. For example, Figure 4 shows a blank 5a that has a relatively thick wall thickness which may be used together with an elastomeric forming body 7a which has a corresponding change in cross section. To ensure a close fit between the elastomeric forming body and blanks with different shapes and/or blanks with an undercut, the elastomeric forming body 7, 7a may also consist of more than one piece. An alternative method of placing the elastomeric forming body 7 inside the blank 5 is as follows . An elastomeric forming body 7 for an elongated tube-shaped blank 5 may be greater in diameter than the diameter of the tube-shaped blank 5 before expansion forming. In this case, the elastic forming body 7 may be stretched longitudinally and the tube shaped blank 5 slipped over the stretched elastomeric body 7. The tube shaped blank 5 together with the elastomeric forming body 7 already inserted may then be inserted in the moulding cavity 6 of the expansion forming tool 4.

Claims: CLAIMS 1. A device for expansion forming of an article, used in conjunction with an isostatic press comprising a pressure cell equipped with an elastomeric membrane (3), characterised by an expansion forming tool (4) which comprises at least one pressure intensifier (8) and at least one moulding cavity (6), in which moulding cavity (6) is arranged a blank (5, 5a) and an elastomeric forming body (7, 7a) is arranged inside the blank (5, 5a), wherein the pressure intensifier (8) is arranged such that it is acted upon by said elastomeric membrane (3), the pressure intensifier (8) is arranged to exert pressure on the elastomeric forming body (7, 7a) such that the elastomeric forming body (7, 7a) forces said blank (5, 5a) against the walls of said moulding cavity (6) with the result that the blank (5, 5a) is expanded into an article shaped according to the shape of the moulding cavity (6) . 2. A device according to claim 1, characterised in that the pressure intensifier (8) comprises a piston (9) arranged in a cylinder (11), which cylinder (11) contains a pressure fluid (10) connected via a channel (12) to a tube-shaped membrane (13), which tube-shaped membrane is arranged in contact with an elastomeric forming body (7, 7a) arranged inside said blank (5, 5a) inside the moulding cavity (6). 3. A device according to claims 1 and 2, characterised in that a diameter of an upper part (14) of a piston (9) in the pressure intensifier (8) is greater than a diameter of a lower part (15) of the piston (9) . 4. A device according to claim 1, characterised in that said moulding cavity (6) comprises one or more trimming edges (16) to trim off excess parts of the blank (5, 5a) , and one or more piercing forms (17) in the walls of the moulding cavity (6) corresponding to one or more openings (18) to be pierced in the walls of said blank (5, 5a) . 5. A method for expansion forming an article in an isostatic press comprising a pressure cell equipped with an elastomeric membrane (3) and an expansion forming tool (4), characterised by the steps of: -placing a blank (5, 5a) inside a moulding cavity (6) of an expansion forming tool (4) inside said isostatic press -placing an elastomeric forming body (7, 7a) inside the blank (5, 5a), -closing said isostatic press, -developing a pressure in said isostatic press and influencing said elastomeric membrane (3) which is in contact with said expansion forming tool (4), -influencing by means of said elastomeric membrane (3) at least one pressure intensifier (8) arranged in said expansion forming tool (4) causing a pressure in the pressure intensifier (8), -exerting a pressure by means of the pressure intensifier (8) against the elastomeric forming body (7, 7a) inside the blank (5, 5a), -forcing the blank (5, 5a) against the walls of the moulding cavity (6) and forming the blank (5, 5a) by expansion into an article shaped according to the shape of the moulding cavity (6) 6. A method for expansion forming an article according to claim 4, in which the step of placing the elastomeric forming body (7, 7a) inside the blank (5, 5a) is characterised by a further step of: -stretching the elastomeric forming body (7, 7a) longitudinally -slipping the blank (5, 5a) over the elastomeric forming body (7, 7a) . 7. A method for expansion forming an article according to claim 5, characterised in that the moulding cavity (6) comprises a shape that defines one or more trimming edges (16) to trim off excess parts of the blank (5, 5a), and one or more piercing forms (17) in the walls of the moulding cavity (6) corresponding to one or more openings (18) to be pierced in the walls of the blank (5, 5a), which method comprises the further steps of: -trimming the blank (5, 5a) at a position of the one or more trimming edges (16) and piercing one or more openings (18) in the blank (5, 5a) at a position of the one or more piercing forms (17 ) .

Inventors: HELLGREN KEIJO, SVENSSON LENNART, HELLGREN, KEIJO, SVENSSON, LENNART
Applicants: ABB AB, HELLGREN KEIJO, SVENSSON LENNART, HELLGREN, KEIJO, SVENSSON, LENNART
Application year: 1999
Main classification: B21D26
IPC codes: B21D26