EP1318906B1 - Controller for a hydraulic press and method for the operation thereof - Google Patents

Description

• The invention relates to a hydraulic press referred to in the preamble of claim 1, to a method for their operation according to the preamble of claim 8 and to a use according to claim 11.
• Such hydraulic presses are used when it comes to molding or forming workpieces. Also for cutting operations hydraulic presses are used. The required force of the hydraulic press depends on the workpiece. In the ceramic industry, presses are used whose pressing force is 20,000 kN or more. In view of an economical production while the cycle time for a pressing operation should be as short as possible. Clock rates of 20 strokes per minute are indicative. By pressing force and cycle time, the energy to be expended is determined in hydraulic presses so the performance of pumps and these pumps driving electric motors. In hydraulic presses according to the prior art and memory are applied, so accumulator or flywheels.
• A hydraulic press of this kind is from the DE-A1-43 20 213 known. Here is in the feed circuit of the hydraulic pressure cylinder, a pressure medium reservoir is present, which is loaded during the return stroke of the press and is used in the advancement of the pressing tool for driving. Energy can thus be saved with the main drive.
• Out JP-A-63 256 300 is known a press that is operated with a multi-stage pressure converter. After a first pressing operation with low pressure, the hydraulic oil is drained into the tank. Then a second pressing operation with high pressure takes place. An energy recovery is therefore not possible here.
• Out US-A-5,852,933 and DE-A1-44 36 666 a hydraulic drive system for a press is known. It contains a low pressure and a high pressure circuit. In this there are three hydrostatic machines, two of which are mechanically coupled. To allow satisfactory operation, these machines must be adjustable in their displacement or delivery volume, which is associated with considerable costs. The system described here can only be used if the press has differential cylinders or synchronous cylinders.
• It is also known ( DE-A1-43 08 344 ) to apply the principle of secondary control when controlling the drive of a hydraulic press. The various movements of the press ram are combined in such a way that the pressure network operates in a closed circuit, wherein the maximum system pressure is determined by the accumulator.
• When regulating a hydraulic press plays according to DE-A1-43 08 344 also the fact a role that the hydraulic oil is quite compressible. This affects in a press cycle in both the compression and the decompression and is a source of losses. Largely disregarded in the prior art is the fact that the mechanical parts of the press absorb energy by elastic deformation of their components. When closing the press this energy must be expended. During the opening process, this energy is not recovered.
• The invention has for its object to provide a hydraulic press, the hydraulic control is constructed so that in the sum of the energy consumption is reduced, without causing an increased expenditure on equipment is needed. The control should also be applicable to a press with Plungerzylindem.
• The above object is achieved by the features of claims 1 and 8. Advantageous developments emerge from the dependent claims.
• An embodiment of the invention will be explained in more detail with reference to the drawing.
• Fig. 1 ein hydraulisches Schema einer Pressensteuerung,
• Fig. 2 bis 6 dieses Schema mit der Darstellung einzelner Schritte innerhalb eines Taktzyklus und
• Fig. 7 ein Schema einer Ausführungsvariante der Pressensteuerung.
Show it:
• Fig. 1 a hydraulic diagram of a press control,
• Fig. 2 to 6 this scheme with the representation of individual steps within a clock cycle and
• Fig. 7 a diagram of an embodiment of the press control.
• In the Fig. 1 1 means a press cylinder, which is assigned a reservoir 2 for the hydraulic medium. Reference numeral 3 denotes a valve group including a series of valves which will be mentioned later. Over a Cylinder line 4, the hydraulic medium between the press cylinder 1 and the valve group 3 is promoted.
• To the valve group 3, a storage line 5 is connected. In this storage line 5 into a hydraulic pump 6, which is driven by an electric motor, which is not shown here, promotes hydraulic medium. With the extending within the valve group 3 storage line 5 is a pressure fluid accumulator 7 in conjunction. This also means that the hydraulic pump 6, the hydraulic medium in the accumulator 7 is able to promote. In the line section between the hydraulic pump 6 and the storage line 5, a non-illustrated check valve may be arranged to relieve the hydraulic pump 6 from the pressure prevailing in the pressure accumulator 7 pressure when the hydraulic pump 6 is not running.
• From the valve group 3, a tank line 8 leads to the reservoir 2. According to the invention, a pressure converter 9 is connected to the valve group 3, which can act according to the general inventive concept on the one hand as a pressure intensifier, on the other hand as a pressure reducer. For this purpose, the pressure converter 9 has a piston 9K which is displaceable within a cylinder 9Z and which separates a low pressure chamber 9.1 with a large effective cross section from a high pressure chamber 9.2 with a small effective cross section. In order to achieve the smaller effective cross section, there is a piston rod 9S connected to the piston 9K in the high-pressure space 9.2. The effective ratio in terms of pressure and flow rate is determined by the cross sections of the two pressure chambers 9.1 and 9.2. For the low pressure chamber 9.1, the cross section is determined by the inner diameter of the cylinder 9Z according to $${\mathrm{A}}_{\mathrm{9.1}}\mathrm{=}\mathrm{¼}\mathrm{*}{{\mathrm{d}}_{\mathrm{9}\mathrm{Z}}}^{\mathrm{2}}\mathrm{*}\mathrm{\pi }$$

  

  and for the high-pressure chamber 9.2 by the difference between the inner diameter of the cylinder 9Z and 9S piston rod according to $${\mathrm{A}}_{\mathrm{9.2}}\mathrm{=}\mathrm{¼}\mathrm{*}{\left({\mathrm{d}}_{\mathrm{9}\mathrm{Z}}\mathrm{-}{\mathrm{d}}_{\mathrm{9}\mathrm{S}}\right)}^{\mathrm{2}}\mathrm{*}\mathrm{\pi }\mathrm{,}$$

  

  A _9.1 is the hydraulically effective cross section of the Nederdruckraums 9.1, A _9.2 that of the high-room space 9.2, d _9Z, the inner diameter of the cylinder 9Z and d _9S, the diameter of the piston rod 9S.
• The pressure ratio of the pressure converter 9 and accordingly also the ratio of the volume flows is thus determined by A _9.1 : A _9.2 . The ratio A _9.1 : A _9.2 is for example 2: 1. The position of the piston 9K is detected by means of a displacement transducer 9W.
• The low-pressure chamber 9.1 communicates with a pressure converter Nederdruckleitung 10.1 of the valve group 3 in conjunction. At this pressure converter Nederdruckleitung 10.1 are three switching valves, namely a Vorpreßventil 11, the second port is connected to the cylinder line 4, a Nederdruckkammer outlet valve 12 whose second port is connected via the tank line 8 to the reservoir 2, and a low-pressure chamber inlet valve 13, whose second terminal is connected to the storage line 5 and thus also to the accumulator 7.
• The high-pressure chamber 9.2 communicates with a pressure converter high-pressure line 10.2 of the valve group 3 in conjunction. At this pressure converter high-pressure line 10.2 are also valves, namely a Hauptpreß valve 14 whose second port is connected to the cylinder line 4, and a check valve 15 whose second port is connected to the storage line 5 and thus with the accumulator 7. A pressure relief valve 16 is located between the cylinder line 4 and the tank line 8. At the pressure converter high-pressure line 10.2 is also a third valve, namely a 3-way valve 17 connected to an upstream check valve 18, wherein the 3-way valve 17 on the other hand with the storage line 5 and thus also with the accumulator 7 and with its further connection to the tank line 8 and thus connected to the reservoir 2. The line section between the check valve 18 and the 3-way valve 17 is referred to as Preßleitung and is provided with the reference numeral 19. The check valve 18 is functionally a return check valve. About the operation of the various valves 11, 12, 13, 14, 15, 16 and 17 is then based on the FIGS. 2 to 6 reported in detail. The valves are electrically controllable and are controlled by a control unit 20. The naturally existing connection lines from the control unit 20 to the valves 11, 12, 13, 14, 15, 16 and 17 are not shown in the figures for reasons of clarity.
• Shown are in the hydraulic scheme, only the invention essential elements, in addition to a press safety lowering and retraction control 21, which is necessary for the safe operation of the press cylinder 1, in view of the invention but without relevance. Also necessary is a pressure sensor 22 which detects the pressure in the cylinder line 4.
• Not shown are for reasons of clarity, the electrical connections between the controller 20, transducer 9W, pressure transducer 22, press safety lowering and retraction control 21 and other safety-related elements on the press.
• Based on Fig. 2 a first phase of the press operation is described below, namely the structure of the form. The press cylinder 1 is filled in a conventional manner from the reservoir 2 with hydraulic medium, which is indicated by an arrow. As a result, the upper die is lowered and thus closed the mold. At the same time, the piston 9K is in an upper position near its upper end position A.
• Now, the 3-way valve 17 is driven so that it releases the flow from the terminal of the storage line 5 to the connection of the Preßleitung 19. The control of the 3-way valve 17 is in the Fig. 2 marked by the fact that its electrically driven drive is filled in black. Through this opening of the 3-way valve 17 can now hydraulic medium from the accumulator 7 via said 3-way valve 17 through the Preßleitung 19, through which because of the pressure of the hydraulic medium forcibly opening check valve 18 and by the pressure converter high-pressure line 10.2 in the High-pressure chamber 9.2 of the pressure converter 9 flow, which in the Fig. 2 indicated by arrows. At the same time the Vorpreßventil 11 is driven, which in turn is marked by the fact that its electrically driven drive is filled in black. Thus, hydraulic medium can now flow from the low-pressure chamber 9.1 via the pressure converter low-pressure line 10.1, through the pilot-pressure valve 11 and the cylinder line 4 into the press cylinder 1. Because of the area ratio A _9.2 to A _9.1, the pressure converter 9 now acts as a pressure reducer, wherein the amount of hydraulic _{medium is} increased according to the area ratio A _9.2 to A _9.1 . If the area ratio A _9.2 to A _{9.1 is} 1: 2, for example, then the pressure converter 9 of FIG Pressure reduced in the ratio 1: 2, but the amount of hydraulic medium increased in the ratio 1: 2. By the flow of the hydraulic medium, the piston 9K is moved in the direction B.
• It should also be noted that the 3-way valve 17 is a proportionally controllable valve, so that the drive of the 3-way valve 17, for example, a proportional solenoid, so that the pressure in the Preßleitung 9 and in the pressure converter high-pressure line 10.2 and thus also the pressure in the pressure converter low pressure line 10.1. in the cylinder line 4 and in the press cylinder 1 can be controlled or regulated.
• If the desired pre-pressure is reached, which is detected by the pressure transducer 22, transmitted by the latter to the control unit 20 and thus detected by the control unit 20, then the control unit 20 causes the 3-way valve 17 and the pre-pressure valve 11 to be closed.
• Subsequently, the pressure relief valve 16 is now activated and thus opened. This results in a pressure reduction in the press cylinder 1 and in the cylinder line 4, which is detected by the pressure transducer 22. Hydraulic medium thus flows from the press cylinder 1 and the cylinder line 4 via the pressure relief valve 16 and through the tank line 8 to the reservoir 2. Determines the pressure transducer 22, that the press cylinder 1 and the cylinder line 4 are depressurized, the pressure relief valve 16 is closed again.
• It may be advantageous to connect another phase of the construction of a form. This is done in the manner described above, but now with a higher form, which is achieved by appropriately modified control of the 3-way valve 17. This phase can take place while the upper tool, not shown, is located on the pressed material, also not shown. But it may also be advantageous to slightly raise the upper tool.
• After the pre-pressure or pre-pressure build-up phase, the piston 9K is located within the cylinder 9Z in a position near the lower limit position B, which is detected by the position transducer 9W. This position is required to subsequently produce the required Hauptpreßdruck can.
• Now follows the next phase of the press operation, the structure of the main pressure. This is based on the Fig. 3 and 4 described below. In the Fig. 3 is the first step of this phase shown. In this figure, in turn, the controlled valves are represented by black marking of the electric drives and the flow of the hydraulic medium is indicated by arrows next to the lines. How about the Fig. 3 can be seen, now the check valve 15 and the Hauptpreß valve 14 are driven. Lock valve 15 and Hauptpreß valve 14 are then open. These two valves 14, 15 are advantageously electrically controllable open-close valves. Also Vorpreßventil 11, low-pressure chamber inlet valve 13, low-pressure chamber outlet valve 12 and pressure relief valve 16 are advantageously of this type.
• By driving check valve 15 and Hauptpreß valve 14, the flow of the hydraulic medium from the accumulator 7 via the storage line 5, through the check valve 15, the Hauptpreß valve 14 and through the cylinder line 4 to the press cylinder 1 allows. In the press cylinder 1 thus a pressure is built up, which is preselected, but at most equal to the pressure in the accumulator 7.
• In the Fig. 4 the second step of the phase of building up the main pressure is shown. Now, the low-pressure chamber inlet valve 13 and the Hauptpreß valve 14 are driven, that is opened, which is marked as in the previous figures, characterized in that the electrical drives of the valves 13, 14 are shown in black. The thus adjusting flow of the hydraulic medium is in turn marked with arrows next to the lines. Now, hydraulic medium flows from the accumulator 7 through the storage line 5, the Nederdruckkammer open inlet valve 13 and through the pressure converter Nederdruckleitung 10.1 in the low pressure chamber 9.1 of the pressure converter 9. The pressure prevailing in the pressure accumulator 7 pressure thereby arises even in the low pressure chamber 9.1. Due to the area ratio A _9.2 to A _9.1 arises at the same time in the high pressure chamber 9.2, a higher pressure, which is twice as large as the pressure in the accumulator 7 at an already mentioned area ratio A _9.2 to A _9.1 of 1: 2 because now but also the Hauptpreß Valve 14 is opened, an equally high pressure builds up in the press cylinder 1. At the conclusion of this phase of the press operation so the pressure in the press cylinder 1 under the given conditions is twice as high as the pressure in the pressure fluid accumulator. 7
• The structure of this pressure in the press cylinder 1 is followed by the pressure transducer 22. Once the desired pressure is reached, the low pressure chamber inlet valve 13 and the Hauptpreß valve 14 are closed again. It is understood that this pressure build-up is connected to a flow of hydraulic medium from the accumulator 7 in the low-pressure chamber 9.1 and the high-pressure chamber 9.2 via the cylinder line 4 to the press cylinder 1, whereby the piston 9K is moved in the direction A. Because of the area ratio A _9.2 to A _9.1 is the amount of hydraulic medium flowing from the high-pressure chamber 9.2, under the given conditions of area ratio A _9.2 to A _9.1 of 1: 2 only half as large as the amount of hydraulic medium from the accumulator 7th her in the low pressure chamber 9.1 flows.
• The press now reaches its maximum pressure and performs the pressing. Under the effect of this pressure, the stresses in the components of the press are at the maximum values. Since the components deform elastically, so energy is stored in these components. Another energy potential represents the compressible hydraulic fluid volume in the press cylinder 1, the press line 4, the pressure converter high-pressure line 10.2 and in the high-pressure chamber 9.2 of the pressure converter 9.
• Thereafter, a phase of discharge with voltage reduction and decompression takes place. This phase takes place in three steps, of which the first two in the Fig. 5 and 6 are shown. The first step is in the Fig. 5 shown. Now the Hauptpreß valve 14 and the check valve 15 are open, which is analogous to the previous figures with black marking of the drives of the valves 14, 15 is shown. Now, the hydraulic fluid can flow from the press cylinder 1 to the accumulator 7, taking the path through the cylinder line 4, the Hauptpreß-valve 14, the check valve 15 and the storage line 5. The flow is due to the fact that, as previously mentioned, the pressure in the press cylinder 1 is greater than in the pressure fluid reservoir 7. The first step lasts until the pressures in the press cylinder 1 and in the pressure fluid reservoir 7 are the same. However, this also means that a very considerable part of the energy stored in the components of the press is recovered by increasing the pressure in the accumulator 7. This is a decisive advantage of the control device according to the invention and the method for its operation.
• The second step of the discharge phase will be based on the Fig. 6 described, in turn, the drives of the controlled valves are shown filled in black and the flow of hydraulic medium is marked with arrows on the lines. This second step is the preparation of the next press cycle. For this, the pressure converter 9 must assume a certain position in the direction of the end position B. The remaining volume in the low-pressure chamber 9.1 of the pressure converter is then so large that the pre-pressures for the next power stroke can be realized with this volume. With the transducer 9W can be checked whether this is the case. If this is not the case, the pressure prevailing in the press cylinder 1, in the cylinder line 4 and in the pressure converter high pressure line 10.2 residual pressure by opening the Hauptpreß valve 14 and the Niederdruckkaanmer outlet valve 12 is used to the piston 9K of the pressure converter 9 in the to bring desired position. This desired position is in the Fig. 6 shown. In this case, the high-pressure chamber is 9.2 again filled with pressurized hydraulic fluid, so that no hydraulic fluid from the pressure accumulator 7 must be removed for the filling. That means a further energy saving. The displaced during the movement of the piston 9K from the low pressure chamber 9.1 hydraulic fluid passes through the Nederdruckkammer outlet valve 12 through the tank line 8 into the reservoir 2. Has the piston 9K reaches the desired position, which is determined as mentioned by the transducer 9W, so Low-pressure chamber outlet valve 12 and Hauptpreß valve 14 closed again.
• Subsequently, the residual pressure in the press cylinder 1 and in the cylinder line 4 is still completely degraded in the third step, which takes place in that now the pressure relief valve 16 is opened. In this case, flows under the action of the residual pressure hydraulic fluid from the press cylinder 1 through the cylinder line 4, the pressure relief valve 16 and the tank line 8 in the reservoir 2. The flow ceases as soon as the residual pressure in the press cylinder 1 is completely degraded. Then, the pressure relief valve 16 is closed again.
• At the same time, however, the pressure in the high pressure chamber 9.2 and in the pressure converter high pressure line 10.2 is maintained. This pressure can be used at the next press cycle, which in turn results in energy savings, since the pressure does not have to be rebuilt.
• In the Fig. 7 a variant of the press control according to the invention is shown. Compared to the example of Fig. 1 the only change is that the pressure converter 9 'has a different design than the pressure converter 9 after the Fig. 1 to 6 , The pressure converter 9 'essentially consists of a first pump 23 whose shaft 24 is rigidly coupled to a second pump 25, so that the shaft 24 is common to both pumps 23, 25. The first pump 23 is connected on the one hand to the pressure converter low pressure line 10.1, this side of the pump 23 acts as a low pressure chamber 9.1, on the other hand with a tank 26. The second pump 25 is connected on the one hand to the pressure converter high pressure line 10.2, this side of the pump The two pumps 23, 25 are not driven by a motor, but act through the rigid connection each as a unit of pump and hydraulic motor. As a pressure converter, this combination of the two pumps 23, 25 is effective in that the specific delivery volume, ie the volume per revolution, is different, which in the Fig. 7 symbolically represented by the different size of the pumps 23, 25. For example, this ratio is 2: 1. This is also due to the fact that the promotion of the hydraulic medium by the two pumps 23, 25 in these effective areas correspond to the areas A _9.1 and A _9.2 according to the first embodiment. Accordingly, the pressure converter 9 'behaves exactly as the pressure converter 9 during the in the Fig. 2 to 6 illustrated and described with reference to these figures different phases of the press operation. During the aforementioned first phase of the press operation, for example, the pressure converter 9 'acts as a pressure reducer, wherein the second pump 25 operates as a hydraulic motor and drives the first pump 23. When acting as a pressure booster, the first pump 23 acts as a hydraulic motor, which drives the second pump 25. The individual phases and their steps of a press cycle correspond to those described above.
• It is also advantageous that a transducer 9W is not required and the pressure converter 9 'must take a certain position in preparation for the next press cycle, which simplifies the control process.
• Despite the very simple structure of the control device according to the invention can be recovered with this energy of individual pressing steps. Thus, as described above, even in the press, in the pressed material and in the compressible hydraulic oil elastic stored energy recovered. The control device comes without expensive components such as adjustable pumps.
• Through experiments it has been found that a significant energy saving over the prior art can be achieved by the control device according to the invention. The energy savings can reach around 40%.
• The invention can be used in principle in hydraulic presses of various types for different applications with great advantage. The press can be equipped with differential cylinders, DC cylinders or plunger cylinders. It is particularly advantageous if the control device according to the invention is used in presses for the shaping of ceramic parts such as tiles.
• Based on the structure described above and the simultaneously described mode of action, it follows that both the structure of the device and the mode of operation, ie the control method, are the subject of the invention.
LIST OF REFERENCE NUMBERS

1

press cylinder

2

reservoir

3

valve group

4

cylinder line

5

storage line

6

hydraulic pump

7

Accumulator

8th

tank line

9

Pressure converter (first embodiment)

9 '

Pressure converter (second embodiment)

9.1

Low-pressure chamber

9.2

High-pressure chamber

9Z

cylinder

9K

piston

9S

piston rod

9W

transducer

10.1

Pressure converter low-pressure line

10.2

Pressure converter high-pressure line

11

prepressing

12

Low-pressure chamber outlet valve

13

Low-pressure chamber inlet valve

14

Main pressing valve

15

check valve

16

Pressure relief valve

17

3-way valve

18

check valve

19

pressing line

20

control unit

21

Press safety lowering and retraction control

22

Pressure transducer

23

first pump

24

wave

25

second pump

26

tank

Claims (11)

1. A hydraulic press with a controller, having a pressing cylinder (1), a reservoir (2), a valve group (3), a pressure medium reservoir (7) and a hydraulic pump (6), the pressing cylinder (1), the reservoir (2), the valve group (3), the pressure medium reservoir (7) and the hydraulic pump (6) being connected together by means of a cylinder line (4), a reservoir line (5) and a tank line (8), a pressure converter (9; 9') being assigned to the valve group (3) and which can be operated as a pressure amplifier and as a pressure reducer, characterised in that the valve group (3) can be controlled to establish a primary pressure in the pressing cylinder (1) such that the pressure medium flows out of the pressure medium reservoir (7) to the pressure converter (9; 9') and can be controlled to discharge the press by reducing the stress and by decompression such that the pressure medium flows from the pressing cylinder (1) to the pressure medium reservoir (7).
2. The hydraulic press according to Claim 1, characterised in that the pressure converter (9) comprises a piston (9K) that can be moved within a cylinder (9Z) and a piston rod (9S) connected rigidly to the piston (9K), the pressure converter (9) having a low-pressure chamber (9.1) and a high-pressure chamber (9.2) which are separated from one another by the piston (9K), and that the low-pressure chamber (9.1) has a larger cross-section A_9.1 than the high-pressure chamber (9.2) which has a cross-section A_9.2.
3. The hydraulic press according to Claim 1, characterised in that the pressure converter (9') comprises a first pump (23) with a greater specific delivery volume and a second pump (25) with a smaller specific delivery volume which are connected rigidly by means of a shaft (24), the one side of the first pump (23) acting as a low-pressure chamber (9.1) and the one side of the second pump (25) acting as a high-pressure chamber (9.2).
4. The hydraulic press according to Claim 2 or 3, characterised in that the low-pressure chamber (9.1) is connected to the valve group (3) by means of a pressure converter low-pressure line (10.1), and that this pressure converter low-pressure line (10.1) is connected

   - to a pre-press valve (11) the second connection of which lies on the cylinder line (4),

   - to a low-pressure chamber inlet valve (13) the second connection of which lies on the reservoir line (5), and

   - to a low-pressure chamber outlet valve (12) the second connection of which lies on the tank line (8),
   and that the high pressure chamber (9.2) is connected by means of a pressure converter high-pressure line (10.2) to the valve group (3), and that this pressure converter high-pressure line (10.2) is connected

   - to a main press valve (14) the second connection of which lies on the cylinder line (4),

   - to a closing valve (15) the second connection of which lies on the reservoir line (5), and

   - by means of a check valve (18) and a press line (19) to a 3-way valve (17) the second connection of which lies on the reservoir line (5), and the third connection of which lies on the tank line (8).
5. The hydraulic press according to Claim 4, characterised in that the 3-way valve (17) is proportionally controllable.
6. The hydraulic press according to Claim 4 or 5, characterised in that a pressure release valve (16) is disposed between the cylinder line (4) and the tank line (8).
7. The hydraulic press according to Claim 6, characterised in that the pre-press valve, the low-pressure chamber inlet valve (13), the low-pressure chamber outlet valve (12), the main press valve (14), the closing valve (15) and the pressure release valve (16) are electrically controllable OPEN-CLOSED valves.
8. A method of controlling a hydraulic press having a pressing cylinder (1), a reservoir (2), a valve group (3), a pressure medium reservoir (7) and a hydraulic pump (6), the pressing cylinder (1), the reservoir (2), the valve group (3), the pressure medium reservoir (7) and the hydraulic pump (6) being connected together by means of a cylinder line (4), a reservoir line (5) and a tank line (8) with which a pressure converter (9; 9') assigned to the valve group (3) can be operated as a pressure amplifier and as a pressure reducer, characterised in that the valve group (3) is controlled to establish a preliminary pressure in the pressing cylinder (1) such that the pressure medium flows out of the pressure medium reservoir (7) to the pressure converter (9; 9') and is controlled to discharge the press by reducing the stress and by decompression such that the pressure medium flows from the pressing cylinder (1) to the pressure medium reservoir (7).
9. The method according to Claim 8, characterised in that valves disposed in the valve group (3) according to Claims 3 and 5 are operated such that

   - in a first procedural step, by controlling the 3-way valve (17) and the pre-press valve (11) the pressure converter (9; 9') acts as a pressure reducer and a preliminary pressure is established in the pressing cylinder (1),

   - in a further procedural step, by controlling the closing valve (15) and the main press valve (14) a pressure is established in the pressing cylinder (1) which can be pre-selected and corresponds as a maximum to the pressure in the pressure medium reservoir (7),

   - in a subsequent further procedural step, by controlling the main press valve (14) and the low-pressure chamber inlet valve (13) the pressure converter (9; 9') acts as a pressure amplifier and a pressure is established in the pressing cylinder (1) which is higher than the pressure in the pressure medium reservoir (7),

   - in a subsequent further procedural step, by controlling the main press valve (14) and the closing valve (15) the pressure prevailing in the pressing cylinder (1) is reduced until it is at the same level as the pressure in the pressure medium reservoir (7),

   - if appropriate, in a subsequent further procedural step, by controlling the main press valve (14) and the low-pressure chamber outlet valve (12) the piston (9K) of the pressure converter (9) is brought into a position desired for a next pressing cycle, and

   - finally, by controlling the pressure release valve (16) the residual pressure in the pressing cylinder (1) is reduced.
10. The method according to Claim 9, characterised in that after completion of the first procedural step, this first procedural step is repeated, by means of modified control of the 3-way valve a higher preliminary pressure being established.
11. The use of a hydraulic press according to Claims 1 to 7 and according to the method according to Claims 8 to 10 for the forming of ceramic parts as tiles.

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