EP1675695B1

EP1675695B1 - Method and apparatus for supplying fluid

Info

Publication number: EP1675695B1
Application number: EP03759131A
Authority: EP
Inventors: Jan Ericson; Kenneth Lindgren; Magnus HAGSTRÖM
Original assignee: Hydroforming Design Light AB
Current assignee: Hydroforming Design Light AB
Priority date: 2003-10-24
Filing date: 2003-10-24
Publication date: 2009-04-22
Anticipated expiration: 2023-10-24
Also published as: WO2005039799A1; DE60327376D1; AU2003274862A1; EP1675695A1; ATE429296T1

Abstract

In a hydroforming process low pressure fluid (LPF) is supplied through inlet means (3) to a working chamber (2) and the pressure of said fluid is then considerably raised for deforming the workpiece against a die (22). Vacuum (V) is applied to the working chamber through separate outlet means (5) as low pressure fluid is fed thereto. This application of vacuum is then terminated prior to raising the fluid pressure to the forming pressure (HPF), and the outlet means is simultaneously blocked. Through the application of vacuum the pre-filling of the working chamber with low pressure fluid will be will speeded-up to a considerable degree, while the fluid supply may still be performed in a closed fluid system. Since the pre-filling of the working chamber with low pressure fluid conventionally constitutes a considerable part of the complete hydroforming cycle, such a speeded-up fluid supply will have a significant effect on the productivity of the forming technique.

Description

TECHNICAL FIELD

The present invention relates generally to hydroforming processes, and more specifically relates to the handling of fluid for a hydroforming process.

BACKGROUND OF THE INVENTION

The hydroforming technique in general is based on the utilization of hydraulic fluid under very high pressure to form metal workpieces. A common problem experienced in all hydroforming processes and specifically in the internal hydroforming of tubular workpieces, is to provide an efficient handling of the fluid that is employed to form a workpiece in a hydroforming tool. Such handling shall secure a rapid supply of hydroforming fluid to the workpiece in order to obtain minimum cycle times for each forming operation. In addition, it is important that air is effectively removed from the workpiece in association with the supply of hydroforming fluid thereto. Air remaining against the workpiece may otherwise, in combination with the very high fluid pressures developed during hydroforming, cause hazardous explosive reactions or serious forming problems, such as forming defects in the workpiece, and may also slow up the filling procedure.
By the above mentioned internal hydroforming, a commonly practiced filling procedure is such that the workpiece is pre-filled with hydroforming fluid at a pressure that is very much lower than the forming pressure. Said fluid is supplied to the workpiece through supply channels in workpiece end plugs. Other prior art filling solutions apply a slightly different approach by suggesting that the workpiece is lowered into a "fluid pool", whereupon the end plugs are secured to the workpiece ends to enclose the fluid therein. The filling rates obtained by means of said prior art procedures do still not meet highly set productivity demands. A further drawback of said prior art procedures is the inevitable spillage of hydroforming fluid, both during supply and discharge of the fluid and especially due to fluid remaining inside a tubular workpiece being removed from the hydroforming equipment. The hydroforming processes are therefore considered to be undesirably messy and require additional cleaning work, both with regard to the actual formed workpiece and to the entire hydroforming equipment. An additional problem of the "open" fluid handling is that the fluid is in most cases quite contaminated and must be cleaned or even regenerated before it is used in a new forming process.
The above mentioned problem of air becoming trapped in the workpiece is particularly common by the supply of hydroforming fluid to a tubular workpiece, especially in cases where the workpiece has bends etc. In the prior art attempts have been made to solve this problem by allowing trapped air to escape around the perimeter of end plugs that have not been finally sealed to the workpiece at this stage. Alternatively, it has also been common to performed venting of trapped air through specific vent openings. However, the prior art filling procedures do not at all times secure successful venting of all trapped air, especially not in larger workpieces of complex shape.
US 3,837,200 describes a press apparatus for making sheet metal pulleys. The press apparatus makes it possible to form groove walls of the sheet metal pulley from a hat- or cup-shaped blank in a continuous manner including several pressing steps. The press apparatus comprises a number of movable dies forming the groove walls of the pulley by means of a number of pressing operations. The same fluid tubes are used to feed and discharge a fluid to different parts of the interior of the workpiece in connection with some of the pressing operations. The pressure of the fluid in the interior of the workpiece is raised to a level such that a part of the workpiece bulges out in an empty space between two dies. The fluid is only used to temporarily form the blank such that it is possible to press it to a desired shape by a conventional pressing operation.
WO 03/013757 describes a method and an apparatus for superplastic forming. Only few metals and metal alloys have such properties that they can be used for superplastic forming. During superplastic forming, a gaseous medium is applied with an overpressure to one side of a workpiece in a vessel. In this case, the gaseous medium is an inert gas. The air in the vessel is evacuated by a vacuum pump before the inert gas is applied to the vessel. The workpiece is heated in the vessel by a laser to a superplastic temperature range. The gas pressure on one side of the workpiece and its increased temperature cause the workpiece to expand slowly in a direction towards a mould. The forming process usually takes 30 to 120 minutes.

SUMMARY OF THE INVENTION

The invention provides a solution overcoming the above discussed problems experienced with the prior known techniques.
It is a basic object of the invention to provide an improved method of supplying fluid to a workpiece for performing a hydroforming operation, by means of which the productivity of the forming operation may be increased.
It is a further basic object of the invention to provide an improved fluid supply system for performing the method of the invention.
It is a further object of the invention to provide an improved method as well as fluid system for further increasing productivity by also optimizing discharge of fluid after a completed hydroforming operation.
It is a further object of the invention to provide an improved method as well as fluid system by means of which the supply of fluid to a hydroforming process is rendered even more efficient by removing air from the workpiece during fluid supply.
Briefly, the invention provides a method and a system for the efficient supply of fluid to a workpiece to be formed in a hydroforming process. In such a process low pressure fluid is supplied through inlet means to a working chamber and the pressure of said fluid is then considerably raised for deforming the workpiece against a die. According to the invention vacuum is applied to the working chamber through separate outlet means as low pressure fluid is fed thereto. This application of vacuum is then terminated prior to raising the fluid pressure to the forming pressure, and the outlet means is simultaneously blocked. Through the application of vacuum the pre-filling of the working chamber with low pressure fluid will be will speeded-up to a considerable degree. Since the pre-filling of the working chamber with low pressure fluid conventionally constitutes a considerable part of the complete hydroforming cycle, such a speeded-up fluid supply will have a significant effect on the productivity of the forming technique.
In further developed embodiments of the invention vacuum is also applied to the working chamber subsequent to the completion of the actual forming step. This application of vacuum will speed-up also the fluid discharge from the working chamber, with the resulting further gain in productivity. In alternative embodiments this post-forming vacuum application for discharge purposes may be performed through either the inlet or the outlet means. This will likewise permit handling of the fluid in a closed system throughout the entire hydroforming operation, including fluid supply, forming and fluid discharge, to thereby minimize contamination of the fluid.
In other embodiments of the invention the post-forming vacuum application is replaced by or combined with an application of pressurized air through one of the inlet or the outlet means or alternatively through the inlet or outlet means that is not used for the vacuum application. These measures will speed-up fluid discharge even further and will not least also provide for an automatic clearing of the portion of the workpiece that has been in contact with the hydroforming fluid.
In yet another embodiment of the invention the fluid supply is directed generally towards a bottom surface of a fluid working chamber whereas the vacuum is applied to an upper portion of the working chamber. This will promote a relatively calm filling and simultaneously an efficient removal of air from the fluid working chamber.
These and further objects of the invention are met by the invention as defined in the appended patent claims.
Advantages offered by the present invention, in addition to those described above, will be readily appreciated upon reading the below detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention along with further object, features and advantages thereof are described further in detail below in connection with the attached drawings, of which:

Fig. 1: is a very schematic illustration of a first basic embodiment of a fluid system according to the invention;
Fig. 2A: illustrates the fluid system of fig. 1 in a filling mode;
Fig. 2B: illustrates the fluid system of fig. 1 in a forming mode;
Fig. 2C: illustrates a further developed method of the invention applied to the fluid system of fig. 1;
Fig. 2D: illustrates the fluid system of fig. 1 at the end of a full hydroforming cycle;
Fig. 3: is an illustration corresponding to fig. 2C of a modified second embodiment of the inventive fluid system in the emptying and clearing mode; and
Fig. 4: is a very schematical partial view of a practical embodiment of end closure devices designed for optimum supply of fluid to a working chamber.

DETAILED DESCRIPTION OF EMBODIMENTS

An exemplary illustrative embodiment of the fluid system of the invention is illustrated in figs. 1 and 2A-D. The fluid system is here illustrated as applied to hydroforming equipment employed for the internal hydroforming of tubular workpieces 1 and comprising a forming tool or die 22 consisting of an upper and a lower tool half 23 and 24, respectively. The tool halves 23, 24 each have a recess 23A, 24A formed in facing surfaces thereof, for receiving the workpiece 1 during the forming operation. During forming the two tool halves 23, 24 are firmly clamped against each other by means of a press, not illustrated, or special clamps as disclosed in our earlier International Patent Application no. WO 01/36123 A1 . In the clamped condition the recesses 23A, 24A of the tool halves 23, 24 form a channel receiving the workpiece 1 and having a shape corresponding to the desired final outer shape of the formed workpiece 1A (see figs. 2B-2D).
When the workpiece 1 has been received in the tool 22 recess 23A, 24A, end closure means 7 and 8 are extended by means of operating means 9 and 10, respectively, until they engage the respective one of opposite ends of the workpiece 1. In fig. 1 solid lines illustrate the end closure means in their retracted positions whereas dash-dot lines illustrate them in their extended positions engaging the workpiece 1 ends. Hydroforming fluid HF is basically supplied to the hydroforming equipment from a main fluid tank 12 that is provided with filter equipment 12A and/or 12B on the pressure and/or return side. In view of the inventive closed fluid supply and discharge system, as described below, fluid contamination will be modest and the required filtering capacity will normally be equally low. From the main tank fluid HF is here pumped to a fluid supply tank 16 by means of a fluid pump 14. Through a fill valve 4, a pressure intensifier 11, fluid inlet means 3 with a fluid supply channel 3A and the respective connecting fluid lines 13, 15, 17 hydroforming fluid HF is supplied to the workpiece 1 in a manner that will be described more closely below, with reference to figs. 2A-2D. Return of fluid from the workpiece 1 to the main tank 12 is similarly performed through fluid outlet means 5 with a fluid discharge channel 5A, a discharge valve 6, a fluid discharge tank 20 and the respective connecting fluid lines 21, 18, as will likewise be described more closely below.
In a first process step the workpiece 1 ends are closed and sealed in the above described manner by means of the end closure means 7 and 8, respectively. Then, a subatmospheric pressure or vacuum V is applied to the fluid outlet means 5 and its schematically indicated fluid channel 5A, to a fluid working chamber 2 formed by the interior of the workpiece 1 and to the fluid inlet means 3 and its schematically indicated fluid channel 3A, as is illustrated in Fig. 2A. Specifically, this is done by first activating a vacuum generator 30, such as a vacuum pump, that is connected to the discharge tank 20 and by opening the discharge valve 6. Fluid LPF at a first low pressure level, preferably at or close to atmospheric pressure, is then supplied from the fluid supply tank 16 to the working chamber 2 by opening the fill valve 4. With the simultaneous application of vacuum V the complete filling of the fluid working space 2 with low pressure fluid LPF is performed very quickly. Once filling is completed the vacuum generator 30 is stopped and the fill 4 and discharge 6 valves are closed, preferably controlled by appropriate sensors, not shown, automatically detecting the complete filling of the fluid working space 2. Such a sensor may for instance be a level sensor detecting a rise in the fluid level of the fluid discharge tank 20 or other appropriate known sensor.
The application of vacuum V to speed-up the filling of the fluid working chamber 2 will also have the additional favorable effect of reducing the risk that air remains trapped in the working chamber 2. Basically, this removal of air is related to the strength of the vacuum applied to the working chamber 2, so that air is more effectively removed the stronger the applied vacuum is.
Fig. 2B illustrates the next step of the hydroforming process, namely the actual forming process that is basically performed in a well known manner. Initially, as described above, with the fluid working chamber 2 filled with low pressure fluid LPF and with the workpiece 1 ends closed and sealed, the fill valve 4 and the discharge 6 valve are both closed. Then, the pressure intensifier 11 is operated by supplying operating fluid OP thereto. The design or operation of the pressure intensifier 11 will not be described in detail herein, but reference may be made our earlier International Patent Application no. WO 01/42662 A1 that in detail discloses the design and operation of a pressure intensifier. The pressure intensifier 11 raises the pressure of the fluid to a second forming level HPF of often several thousand bar. In the fluid working chamber 2 said raised pressure deforms the walls of the workpiece 1 into contact with the walls of the tool 22 recess 23A, 24A thereby producing the shape of the hydroformed product 1A. The formed product 1A maintains the original tubular shape in the end portions so that the end closure means 7, 8 maintain their closing and sealing positions therein throughout and after forming.
When forming is completed, the pressure intensifier 11 is stopped and the high pressure is relieved, in the schematically illustrated example by applying operating fluid OF in a reverse direction to the intensifier 11. In a further development of the basic method of the invention, illustrated in Fig. 2C, the vacuum generator 30 is then activated again for supporting the discharge of fluid LPF from the working chamber 2. Specifically, the fill 4 and discharge 6 valves are opened to once more apply the vacuum V to the fluid working chamber, thereby speeding-up also the discharge of fluid HF therefrom. When all fluid HF has been discharged from the product 1A and from the input means 3 and output means 5 the fill 4 and discharge 6 valves are closed again. By maintaining the end closure means 7, 8 in their positions engaging the ends of the formed product 1A also during fluid discharge the entire filling, forming and discharge process is performed with a closed fluid system. This promotes cleanliness in and around the hydroforming equipment and of the hydroforming fluid HF.
With all fluid LPF discharged from the product 1A and from the input means 3 and output means 5 the end closure means 7 and 8 are retracted from the tubular ends of the product 1A, as is illustrated in fig. 2D, and finally the formed product 1A is removed from the hydroforming tool or die 22 that has been opened after completion of the forming process. To prepare for a new forming cycle the fill 4 and discharge 6 valves are closed again and fluid is pumped from the fluid discharge tank 20 to the main tank 12 and from the main tank 12 to the fluid supply tank 16.
The manner of or means for controlling the various valves and pumps have not been specifically indicated or described herein, and it should be obvious that any conventional, manual or pilot operated controls could be used therefore. Thus, the invention also covers any manual, semi-automated or fully automated control of the entire process.
The described vacuum supported filling of the workpiece 1 or fluid working chamber2 with low pressure hydroforming fluid LPF will not only considerably speed-up the filling procedure to increase productivity, but will also significantly reduce the risk of air being trapped in the working chamber during high pressure forming. In the further developed embodiment of the inventive method the additional vacuum supported discharge of the fluid after forming will even further increase productivity by speeding-up also said fluid discharge. By also enabling a closed fluid system operation the invention promotes a clean process without fluid spillage and without any substantial fluid contamination.
A modified second embodiment of the inventive fluid system is schematically illustrated in Fig. 3 that shows the system in a situation corresponding to that of Fig. 2C. The modifications to the system do in effect mainly concern the actual emptying or discharge phase of the process and consists of the addition of a source 19 of pressurized air PA that is connected to the fluid inlet means 3 through an additional air valve 25, the fluid supply tank 16 and the fill valve 4. In this embodiment the fluid discharging method differs from the one described in connection with Fig. 2C in that after the high pressure forming is completed and the high pressure has been relieved, the pressurized air source 19 is activated and the air valve 25 is opened essentially at the same time as the vacuum generator 30 is activated and the fill 4 and discharge 6 valves are opened. Through the application of pressurized air PA to the fluid present predominantly in the working chamber 2, the discharge of the fluid LPF will be extremely efficient and quick and above all the interior of the formed product 1A will be almost completely cleaned from fluid. The latter fact will be of great importance for products that otherwise require separate cleaning measures.
In the illustrated embodiment the pressurized air PA is applied through the fluid inlet means 3 whereas the vacuum V for the forced fluid discharge is applied through the fluid outlet means 5. It should be emphasized that according to the invention the direction of the fluid discharge could be reversed. Specifically, during the fluid discharge phase appropriate valve means, not illustrated, could be employed to redirect the vacuum application to the fluid inlet means 3, through the fluid supply tank 16, and to apply the pressurized air (PA) to the fluid outlet means 5, through the fluid discharge tank (20).
Furthermore, the invention is not restricted to the use of pressurized air PA in combination with vacuum V for the discharge process, but also covers an embodiment where pressurized air PA alone is employed during the fluid discharge phase and the vacuum is not applied at all for the forced fluid discharge but only for the fluid supply phase.
In Fig. 4 is finally disclosed a modified embodiment according to the invention. In Fig. 4 the modification is illustrated as applied to end closure means 107, 108 similar to those disclosed in connection with the previously described embodiments. The embodiment illustrated in Fig. 4 is based on the insight that when performing the forced fluid supply according to the invention, the removal of trapped air from the fluid working chamber 2 is dramatically improved if the infeed of low pressure fluid LPF is directed to a lower part of the fluid working chamber 2 and the application of the vacuum V and therefore the trapped air removal is performed at an upper part of the fluid working chamber.
This is done by providing a generally fan-shaped infeed slit 107A in the first end closure means 107, opening in the lower area of the end closure means close to the outer circumference thereof and thereby near a lower wall of the working chamber 2, here the lower portion of the inner wall of the workpiece 1. Said infeed slit 107A is part of the fluid supply channel 3A of the input means 3, is connected thereto in or near the center of the end closure means and is directed obliquely downwardly against the lower wall of the working chamber 2. Furthermore, a generally fan-shaped output slit 108A is provided in the second end closure means 108, opening in the upper area of the end closure means close to the outer circumference thereof and thereby close to an upper wall of the working chamber 2, here the upper portion of the inner wall of the workpiece 1. Preferably the output slit 108A is part of the fluid discharge channel 5A of the output means 5, is connected thereto in or near the center of the end closure means and is directed obliquely upwardly against the upper wall of the working chamber 2.
In this configuration low pressure fluid LPF is sucked rather calmly into the working chamber 2, at a lower part of its circumference, without any significant turbulence that might otherwise tend to cause air to be suspended in the fluid. By applying the vacuum at the uppermost area of the working chamber 2, at a position distant from the infeed slit 107A, air TA trapped in the chamber, schematically illustrated in Fig. 4, will be effectively removed from the working chamber 2. For the effective discharge of fluid from the working chamber 2 after the forming operation a second output slit 108B may be provided in the end closure means 108, opening close to the lower wall of the working chamber 2. A further valve means, not illustrated, should then be provided in the second output slit 108B, said valve being closed during the fluid supply phase to prevent vacuum from being applied therethrough during said phase.
The invention has been described above with specific reference to the illustrated embodiments thereof that refer to the internal hydroforming of generally tubular workpieces where the fluid working chamber is equal to the interior of the workpiece. However, it shall be understood that the invention is not restricted to these exemplifying embodiments or to such an application. The basic principles of the invention may likewise be applied to other embodiments for use in other hydroforming processes for forming generally planar metal plates, such as flexforming or hydrostatic forming. Generally, with regard to the inventive method, said other hydroforming processes differ from the internal hydroforming process in that the fluid working chamber is not directly (flexforming) or only partly (hydrostatic forming) formed by the workpiece, in this case the metal plate. Therefore, modifications and variations of the invention that may be required in such applications fall within the scope of the invention as defined by the appended claims.

Claims

A method of handling fluid (HF) for a hydroforming process, whereby fluid at a first pressure level (LPF) is supplied to a fluid working chamber (2) and the pressure of said fluid is then considerably raised to a second level (HPF) in the fluid working chamber for deforming a workpiece (1) against a die (22), said fluid being supplied through fluid inlet means (3) associated with the fluid working chamber, characterized in that:
a) in association with the supply of fluid at the first pressure level (LPF) through the inlet means (3), a subatmospheric pressure (V) is applied to the fluid working chamber (2) through separate fluid outlet means (5); and

b) prior to raising the fluid pressure to the second level (HPF), the application of the subatmospheric pressure is terminated and fluid passage through the separate fluid outlet means (5) is blocked.
A method according to claim 1, wherein fluid (LPF) is supplied from a fluid reservoir (12, 16) communicating with the fluid working chamber (2) through a fill valve (4) associated with the fluid inlet means (3), characterized by:
a) first activating a vacuum generating means (30) communicating with the fluid working chamber (2) through a discharge valve (6) associated with the fluid outlet means (5);

b) then opening the fill valve (4) to allow vacuum-supported fluid feed through the fluid inlet means (3) to fill the fluid working chamber; and

c) blocking the fluid outlet means (5) by closing the discharge valve (6) and deactivating the vacuum generating means (30) and then raising the fluid pressure to the second level (HPF) through the fluid inlet means (3).
A method according to claims 1 or 2 for an internal type of hydroforming process where low pressure fluid (LPF) is supplied to a generally tubular workpiece (1) the ends of which are closeable and sealable by means of end closures means (7, 8; 107, 108) to form the working chamber (2) in the interior of the workpiece, whereby fluid is supplied through a fluid supply channel (3A; 3A, 107A) extending through a first end closure means (7; 107), characterized by applying the subatmospheric pressure (V) through a discharge channel (5A; 5A, 108A) extending through a second end closure means (8; 108) to thereby remove air from the fluid working chamber (2) inside the workpiece (1).
A method according to claim 3, characterized in that fluid (LPF) supplied to the fluid working chamber (2) is directed towards a lower portion thereof and in that subatmospheric pressure (V) is applied to the working chamber at an upper portion thereof.
A method according to any of claims 1-4, characterized in that after the raising of the fluid pressure to the second level (HPF) and a subsequent relieving of said raised fluid pressure and thereby after the completed forming of the workpiece (1), subatmospheric pressure (V) is once more applied to the fluid working chamber (2) through one of the inlet and outlet means (3 and 5, respectively) to thereby rapidly discharge fluid (LPF) from the fluid working chamber (2).
A method according to any of claims 1-4, characterized in that after the raising of the fluid pressure to the second level (HPF) and a subsequent relieving of said raised fluid pressure and thereby after the completed forming of the workpiece (1), pressurized air (PA) is applied to the fluid working chamber (2) through one of the inlet and outlet means (3 and 5, respectively) to thereby rapidly discharge fluid (LPF) from the fluid working chamber (2) and to effectively clear the fluid working chamber of fluid (LPF).
A method according to claim 6, characterized in that the pressurized air (PA) is applied during or subsequent to a renewed application of subatmospheric pressure (V) to the fluid working chamber (2) through the other one of the inlet and outlet means (3 and 5, respectively).
A method according to claim 3 and any of claims 4-7, characterized in that the first and second end closure means (7, 8; 107, 108) are both maintained in closing and sealing engagement with the respective workpiece (1) ends until the post-forming application of the subatmospheric pressure (V) and/or the pressurized air (PA) has been terminated, to thereby provide a closed leakage-free fluid supply and discharge.
A fluid system for a hydroforming process, having at least one fluid reservoir (11, 16) for supplying fluid (HF) at a first pressure level (LPF) to a fluid working chamber (2), a pressure intensifier (11) for considerably raising the fluid pressure to a second level (HPF) in the fluid working chamber for deforming a workpiece (1) against a die (22) and fluid inlet means (3) associated with the fluid working chamber, characterized by:
a) a source (30) of subatmospheric pressure (V) connected to the fluid working chamber (2) through separate fluid outlet means (5); and by

b) fill and discharge valves (4 and 6, respectively) controllable between open and closed positions for allowing the application of subatmospheric pressure (V) to the fluid working chamber and for blocking fluid passage through the fluid outlet means (5), respectively.
The fluid system of claim 9, characterized by a source (19) of pressurized air (PA) communicating with either of the fluid inlet (3) or fluid outlet (5) means through an air valve (25).
The fluid system of claims 9 or 10 for an internal type hydroforming process where low pressure fluid (LPF) is supplied to a generally tubular workpiece (1) the ends of which are closeable and sealable by means of first and second end closure means (7, 8; 107, 108) to form the working chamber (2) in the interior of the workpiece, whereby fluid is supplied through a supply channel (3A; 3A, 103A) extending through a first end closure means (7; 107), characterized by a fluid outlet channel (5A; 5A, 105A) extending through an opposite second end closure means (8), said fluid outlet channel communicating with the source (30) of subatmospheric pressure (V) and with a fluid reservoir (12, 20).
The fluid system of claim 11, characterized in that the first end closure means (107) has an infeed slit (107A) extending obliquely outwardly from a generally central main infeed channel (3A) and opening in a lower area of the end closure means close to the outer circumference thereof to thereby direct the supplied fluid (LPF) towards a lower wall of the working chamber (2) and in that the second end closure means (108) has an output slit (108A) extending obliquely outwardly from a generally central main discharge channel (5A) and opening in an upper area of the end closure means close to the outer circumference thereof to thereby discharge fluid from an upper portion of the working chamber (2).