High internal pressure forming process

Details High internal pressure forming process High internal pressure forming process

2003-01-17

2004-12-28

Freay, Charles G. (Department: 3746)

Metal deforming

By application of fluent medium, or energy field

With actuated tool engaging work

C072S061000, C072S062000, C417S401000

Reexamination Certificate

active

06834522

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a device for forming a workpiece exhibiting at least one hollow space in its interior by means of high internal pressure created by a medium capable of flow in the sealed hollow interior of the workpiece, said device containing a forming tool with a cavity accommodating the workpiece and pressure generating equipment for generating high internal pressure. The invention also relates to a process for forming the said workpiece using a device according to the invention.
In so called high internal pressure forming processes, in the following referred to as HIPF processes, a hollow body is stretched or formed by means of high internal pressure. For that purpose the openings in the hollow body are sealed by sealing stems in order to build up and maintain a positive pressure in the said space by means of a working or pressure medium which is introduced past the sealing stem into the hollow interior of the workpiece. The positive pressure leads to the workpiece being stretched or formed, whereby the hollow body takes on the shape of the tool cavity in which the hollow body was laid.
For some time now the HIPF process has been employed in combination with shaping and cutting, stamping or perforating. That means that during the HIPF process openings have been made or created in the walls by means of the high internal pressure.
The pressures that have to be built up depend, amongst other things, on the material to be formed, the shape and the wall thickness of the workpiece and on the geometry and degree of deformation. Further, pressing out openings by means of the HIPF process makes special requirements on the internal pressure.
Up to now, in order to create the high internal pressure, so called uniformly working pressure converters with hydraulic drive for liquid media have been employed. Uniformly working pressure converters are in their function two longitudinally coupled cylinders with working surfaces A
1
, A
2
of different sizes, practically designed as equipment with differential pistons by means of which the pressure p
1
built up on the primary side is transformed to a higher final pressure—the so called secondary pressure p
2
, whereby the differential pistons traverse a single distance in order to generate the pressure. The pressure increase factor is obtained—neglecting frictional losses—by the ratio of the working surface areas:
p
2
/p
1
=A
1
/A
2
As the pressure converter on the secondary side operates with a relatively large piston area, the primary side drive is normally made using liquid media.
Normally, the pressure generating equipment contains a pre-fill pump which feeds the high internal pressure system with pressure medium at the start of the forming process and at the same time creates an elevated base pressure. Subsequently, the final or forming pressure is created using pressure converters. As the volume flow of the almost incompressible liquid medium is small because of the previously generated base pressure, the forming pressure created by the differential piston is reached right away, whereby the single distance traversed by the differential piston is as a rule less than the maximum possible single piston displacement.
The maximum volume flow of pressure medium that can be created is limited by the single piston displacement i.e. after the differential piston has accomplished the single piston displacement, it is not possible to achieve a further increase in pressure. This problem arises e.g. in cases in which the pressure converter has to generate relatively high volume flow, such as for example when there is a leak on the high pressure side or for very large stretching of the section in the forming process. It can happen, therefore, that the pressure converter is not able to generate the necessary maximum pressure as this has not yet been reached after a single stroke of the piston.
Conventional HIPF devices fitted with pressure converters are relatively inflexible as the volume flow that a pressure converter is able to generate to reach the high internal pressure is limited by the single piston stroke. Further, the installation and operation of conventional pressure converters is relatively complicated, therefore e.g. temporary installation of an HIPF device for trials or for small series production is hardly economical using pressure converters that operate in the conventional manner.
SUMMARY OF THE INVENTION
The object of the present invention is to propose an HIPF device with a pressure generating device which permits flexible adaptation with respect to the maximum pressures, and is flexible from the standpoint of installation and operation while at the same time being cost favourable.
That objective is achieved by way of the invention in that the pressure generating equipment contains a displacement pump, preferably a reciprocating pump, with pressure converter (pressure intensifier) from a longitudinally coupled displacement body with different working surface areas A
1
, A
2
, and the displacement body operates with a stroke frequency f, and a pressure p
1
created on the primary side can be converted in a higher secondary pressure p
2
by means of strokes of the pressure converter until an end pressure is reached on the secondary side, whereby the relationship:
p
2
/p
1
=A
1
/A
2
defines the increase in pressure.
Further, the invention also relates to a device for forming a workpiece exhibiting at least one hollow space in its interior by means of a high internal pressure created by a medium capable of flow in the sealed hollow interior of the workpiece, said device containing a forming tool with a cavity accommodating the workpiece, whereby the forming tool contains means for producing recesses, in particular holes in the workpiece during the high internal pressure forming process, and the means comprise a stem fitting in space in the wall of the forming tool, whereby the recesses are created by withdrawing the stem from the workpiece, and the stem movement is controlled by the counterpressure on the stem and the stem counterpressure is generated via pressure generating equipment. The said pressure generating equipment contains a displacement pump with pressure converter comprising a longitudinally coupled displacement body with different working surface areas A
1
, A
2
, and the displacement body operates with a stroke frequency f, and by means of strokes of the pressure converter a pressure p
1
created on the primary side can be converted until a higher secondary pressure p
2
is reached on the secondary side, whereby the relationship:
p
2
/p
1
=A
1
/A
2
defines the increase in pressure.
The pressure converter of the displacement pump working with a stroke frequency f during the forming process in order to reach a final pressure on the secondary side is, consequently, in contrast to uniform working pressure converters, an oscillating pressure converter which traverses the given piston distance more than once. Therefore, in contrast to uniform working pressure converters, an unlimited volume flow of pressure-building medium can be produced continuously, as this is not dependent on a single stroke of the piston.
The workpieces may be e.g. single or multi-chamber hollow sections with openings at the ends. Further, the workpieces may also be hollow bodies having the shape of a container. The workpieces are preferably of metal, in particular aluminium or an aluminium alloy.
The pressure generating equipment for generating the high internal pressure and/or the counterpressure on the stem preferably contains a multi-stroke high pressure piston pump also known as high pressure piston pump. On the primary side the high pressure piston pump is preferably operated pneumatically i.e. with a gas such as compressed air in the range 1-15 bar, whereby the compressed air is brought to a higher pressure by means of a separate facility such as a compressor and fed to the displacement pump. The high pressure piston pump may also be operated by hydraulic means on the primary side i.

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