1. Field of the Invention
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.
2. Description of the Related 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 pressurized fluid such as hydraulic oil. U.S. Pat. No. 5,644,829, entitled xe2x80x9cMethod for Expansion Forming of Tubing,xe2x80x9d 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 pressurized. The pressure from the pressurized fluid forces the tube against the walls of the surrounding cavity. U.S. Pat. No. 5,419,171, entitled xe2x80x9cIsostatic Bulge Forming,xe2x80x9d 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 xe2x80x9cHYDROFORMING APPLICATION IN THE AUTOMOTIVE INDUSTRYxe2x80x9d published in the periodical xe2x80x9cAdvanced Technologies and Processesxe2x80x9d, 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 mold is held in a conventional two directional press. The mold 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 pressurized. In the article a multiple fitting tool for multi-cavity production is also shown. This comprises a multi-cavity mold held in a horizontal molding press with internal pressure supplied to the cavities by a pressure intensifier via a pressurized liquid. The presses used in the hydroforming processes described supply internal pressure against the blank via a pressurized liquid.
A brochure issued by xe2x80x9cKRUPP HOESCH AUTOMOTIVE GmbHxe2x80x9d 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 mold or a tool. A tube-shaped blank or a specially contoured blank is placed in the mold. The mold 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 molded against the walls of the mold cavity.
Conventional processes such as those described above 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 mold will be in one axial direction. This in turn means the internal pressure available in the mold is limited by the strength of the mold, and that the molding 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 mold that resist liquids under high pressure, and requires extra stages in the molding 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 mold cavity difficult or impractical, so that trimming or piercing operations require additional process steps.
High pressures are required for at least two reasons. The first is to reduce the degree of spring back, 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 closer to the required shape, and is a stronger product.
In the use of a high-pressure press such as an isostatic press of the Quintus(copyright) type, manufactured by Flow Pressure Systems in Sweden, there exists a difficulty in providing molds and molding devices that are sufficiently strong.
Molds and molding devices are often required to withstand very high pressures of up to 10,000 bar during each molding cycle in an isostatic press. Such molding devices are often very large, heavy and expensive.
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 molding cavity.
The expansion forming tool is designed so that the membrane of the isostatic press acts not only on the expansion forming tool, but also 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 molding 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 molding cavity in the expansion forming tool. Thus the blank takes the shape of the cavity walls.
The molding 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 pressure 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 molding cavity, and may comprise several molding 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 molding cavity for that purpose. Expansion forming in accordance with the present invention thus reduces the number of process steps necessary for shaping an article.
The availability of extremely high molding pressures compared to devices of the prior art means that more detailed shapes can be achieved in a single molding 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 the opportunity to reduce the weight of material used in the article, which is important in applications such as parts for automobiles.