HK1231811B - Die casting mold evacuation valve assembly - Google Patents

Description

Die casting mold evacuation valve assembly

Technical Field

The invention relates to a die casting die evacuation valve assembly.

Background Art

In order to reliably prevent air pockets or gas pockets in the finished casting, the mold or its cavity is automatically evacuated during casting. To this end, not only must the air present in the casting mass and the mold cavity be able to escape, but it must also be ensured that gases present in the casting mass can also escape.

The problem of evacuating a die casting mould involves preventing the molten casting mass from escaping into the environment under all circumstances. In order to prevent this from happening, a valve assembly is used, which is provided with an evacuation pipe connected to the mould cavity of the die casting mould, an evacuation valve arranged in the evacuation pipe and an actuator for closing the evacuation valve. Known valve assemblies are designed for pilot control and are made of casting material. The latter has an actuator, which includes a force sensor actuated by the casting material and a member for transmitting the force of the closing member from the force sensor to the evacuation valve. A valve assembly of this type is known from EP 0 612 573 A2. Although such valve assemblies are very reliable in operation and thus achieve a very fast closing response, it is still desirable to enable the valve assembly to be more specifically adapted to the requirements of locations subjected to high stresses and strains.

Summary of the Invention

It is an object of the present invention to provide a valve assembly for evacuating a die casting mould, as described in this application, which valve assembly, although wear-resistant, is nevertheless relatively cost-effective to produce.

This object is achieved by the valve assembly described in this application.

Since the housing front is now provided with a first sleeve surrounding the head portion of the valve piston and/or a further sleeve surrounding at least the head portion of the force sensor, where each sleeve is stiffer than the housing front, the valve housing is particularly reinforced at locations subject to high stresses and strains.

Further preferred embodiments of the valve assembly are described in other features of this application.

Therefore, in another preferred embodiment, each sleeve is inserted into a hole drilled in the front of the housing. Each hole has a diameter that decreases toward the rear of the housing. The transition from the larger diameter to the smaller diameter forms a step that contacts each sleeve, and each sleeve has a heel corresponding to the step. This configuration ensures that each sleeve is accurately positioned in the axial direction. Furthermore, it ensures that each sleeve does not shift when subjected to the very high forces generated during the die-casting process.

In another further preferred embodiment, the head portion of the force sensor is configured as a cylinder, with the end face of the head portion transforming a sharp edge into a shell surface area. This configuration is intended to ensure that no metal debris becomes trapped between the head portion of the force sensor and the inner surface of each sleeve.

Preferably, the valve piston of the evacuation valve also has a cylindrical head portion at the front end, the end face of which transitions to a sharp edge in its shell area. This is also intended to ensure that no metal fragments become trapped between the head portion of the valve piston and the inner surface of the corresponding sleeve.

In yet another further preferred embodiment, the sleeve or sleeves and the evacuation duct are arranged in the housing front. This configuration has the advantage of allowing cost-effective production.

In yet another further preferred embodiment, the actuator has a follower arranged between the laminated spring assembly and the clamp of the force sensor.Providing the follower as part of the actuator has proven successful, particularly where high stresses and strains are involved.

By providing the actuator with a moving pressure plate, which is fitted with a laminated spring assembly to return the actuator and/or discharge the riser, this component is also made particularly adaptable to high demands on wear and tear.

In yet another preferred embodiment, the actuator comprises a pneumatic closing member comprising a working piston housed in a bore bored in the valve housing, an inlet nozzle in contact with the working piston in the closed position via its end face, the inlet nozzle comprising a central air outlet and a sealing member surrounding the central air outlet. This arrangement ensures that, in the front closed position, the working piston can come into contact with the inlet nozzle in the front closed position even if, for example, the actuator is housed slightly tilted in the bore.

In a further preferred embodiment, in addition to the cylindrical head portion, the force sensor has at least two further cylindrical sections, each of which is guided in the sleeve. This configuration has the advantage that the force sensor is very accurately guided in the sleeve via its head portion. Furthermore, these guide sections, which are subject to high stresses and strains, can fit perfectly within the sleeve.

Preferably, the end face of the valve piston and/or the force sensor protrudes by 0.01-1 mm from the front end face of the corresponding sleeve facing the valve housing when retracted. Such a configuration has proven successful, in particular with regard to the exit of the riser from the force sensor or the valve piston, respectively, especially since, in such a configuration, casting material cannot enter the sleeve assigned to the force sensor or the valve piston, respectively.

In another further preferred embodiment of the valve assembly, a sleeve for guiding the valve piston is provided with a radial bore, through which the interior of the sleeve is connected to the outlet pipe, and the valve piston includes two guide sections, one of which is arranged upstream of the radial bore and the other is arranged downstream of the radial bore, wherein the guide section arranged upstream of the radial bore has an axial port. This configuration enables any gas to be discharged through the valve piston.

Preferably, the working stroke of the force sensor is limited to less than 2 mm so that the kinetic energy transferred by the molten casting material to the force sensor or actuator as a whole remains within an acceptable range.

In yet another further preferred embodiment of the valve assembly, a working stroke of the valve piston corresponds to at least three times the working stroke of the force sensor, and the idle movement of the valve element exceeds the working stroke of the force sensor. Such a configuration enables the working stroke of the valve element to be larger and independent of the working stroke of the force sensor.

In an alternative further embodiment, the actuator has a follower as the force transmission member, which is constructed integrally with the force sensor. This is a particularly cost-effective alternative to the valve assembly and is particularly suitable when the casting material impacts the force sensor with a low kinetic energy.

In another further preferred embodiment, both the valve piston and the working piston are spring-biased in the direction of the advanced opening position, which enables the force acting on the follower in the direction of the advanced opening position to be more uniform.

In summary, in a particularly preferred further embodiment of a valve assembly, at least one pilot-actuated closing piston is provided, by means of which the force transmission component and at least the valve piston connected thereto can be displaced into the closed position. Such a configuration has the advantage that the closing of the evacuation valve can also be freely timed at any point in time by pilot actuation. Closing the evacuation valve by pilot actuation is useful, for example, for testing purposes, particularly at the start of a casting process, i.e., until the temperatures of the various parts within the valve assembly have become uniform.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in detail with reference to the accompanying drawings, in which:

FIG1 is a section through a valve assembly according to a first example version of the invention;

FIG2 is a section through a valve assembly according to a second example version of the invention;

FIG. 3 is a section through a valve assembly according to a third example version of the invention.

DETAILED DESCRIPTION

1 , a section through a valve assembly 1 for evacuating a die casting mould is shown. As the basic principle of such a valve assembly is already known from EP 0 612 573 A1 , the following will focus only on the particularly prominent elements or components designed according to the invention.

The valve assembly 1 has a valve housing 2, which comprises a housing front part 2a and a housing rear part 2b. The two housing parts 2a, 2b are connected together by fasteners (not shown), such as bolts. The valve housing 2 is provided with an evacuation duct, which consists of an inlet duct 3 located at the bottom of the housing front part 2a and an outlet duct 5 leading upwards out of the housing front part 2a. The inlet duct 3 is connected to the mold cavity of the die-casting mold to be evacuated, while the outlet duct 5 is usually connected to an exhaust device. An evacuation valve 7 is arranged in the valve housing 2, by means of which the outlet duct 5 can be closed from the inlet duct 3. The inlet duct 3 is open in the direction of the front face 4 of the valve housing 2, to which a so-called compensator is applied. How the compensator seals the valve housing 2 to the front is not shown.

The evacuation valve 7 includes a valve piston 8 that can be displaced between a front open position and a rear closed position by an actuator 14. The actuator 14 generally comprises a force sensor 15, a force-transmitting member in the form of a follower 20, a fixed pressure plate 22, a movable pressure plate 24, a laminated spring assembly 23 disposed between the two pressure plates 22, 24, and a pneumatic closing member 36. Two tappets (not shown) for tensioning the laminated spring assembly 23 penetrate the follower 20. Initially, the tappets protrude from the front face 4 of the valve housing 2, and then retract, acting against the compensator, thereby biasing the laminated spring assembly 23.

Furthermore, the actuator 14 may also include one or more pneumatically operated, pre-actuated closing pistons 21 (termed vacustops), which will be described in further detail below. The single closing piston 21 shown in FIG. 1 is of course merely suggestive. The closing piston 21 is pre-actuated, preferably pneumatically, and serves to close the valve piston 8 at the appropriate time at the start of the casting cycle. An external signal is used to trigger the closing action.

The two housing parts 2a, 2b are made of a high-temperature material, preferably steel, particularly preferably hot-worked steel such as 1.2343 ESU. To adapt the valve housing 2 to locations exposed to high stresses and strains (i.e., in the area of the valve piston 8 of the pump-down valve 7 and the force sensor 15 of the actuator 14), two sleeves 26, 31 are inserted into the housing front part 2a for specific requirements. Each sleeve 26, 31 respectively encloses the head portion 9 of the valve piston 8 and the section 16 of the force sensor 15. Both the first sleeve 26 and the further sleeve 31 are preferably made of wear-resistant tool steel, such as cold-worked K340 steel. In any case, both sleeves 26, 31 are made of a material that is substantially harder and more resistant to wear, stress, and strain than the two housing parts 2a, 2b.

Each sleeve 26, 31 is inserted into a respective borehole 28, 33 machined into the housing front portion 2a. To ensure precise axial positioning of the sleeves 26, 31, both boreholes 28, 33 and sleeves 26, 31 are provided with a heel 27, 32 or step 29, 34, respectively. Due to the step 29, 34 of each borehole 28, 33, the diameter of the corresponding borehole 28, 33 increases toward the front portion 4. In other words, the diameter of each borehole 28, 33 decreases toward the housing rear portion 2b. The step 29, 34 is formed at the transition from larger to smaller diameter, where each sleeve 26, 31 contacts the heel 27, 32 corresponding to the step 29, 34. With this configuration, each sleeve 26, 31 must be pressed into the corresponding borehole 28, 33 from the front portion 4. When pressed as described above, the heel 27, 32 of each sleeve 26, 31 contacts the step 29, 34 of the corresponding bore 28, 33, respectively, thereby accurately positioning each sleeve 26, 31 axially. This ensures that even if a force is applied to the front end directed toward the front face 4, each sleeve 26, 31 cannot be pushed further inward in the valve housing 2, toward the housing rear portion 2b. This is important because, during die casting, the casting material (metal) from the inlet pipe 3 strikes the front end of each sleeve 26, 31 with high kinetic energy.

The sleeve 26 for guiding the valve piston 8 is provided with a radial bore 30, through which the interior of the sleeve 26 is connected to the outlet pipe 5. The force sensor 15 is provided with a collar 19, with which the follower 20 comes into positive contact during the forward-directed movement. An annular recess is machined into the rear side of the housing front part 2a to receive the collar 19 of the force sensor 15 in the forwardly displaced position shown. The rear side of the sleeve 31 extends axially upward to the recess in the housing front part 2a.

The front of the valve piston 8 is provided with a front guide section 11 and a rear guide section 12 for guiding in the sleeve 26. The front guide section 11 is provided with axial ports 11a. When the evacuation valve 7 is open, any gas involved is free to flow from the inlet pipe 3 to the outlet pipe 5 through these ports 11a.

The valve piston 8 of the evacuation valve 7, when in the open state, has a head portion 9 with a cylindrical outer surface 10. The outer diameter of the head portion 9 is adapted exactly to the inner diameter of the corresponding sleeve 26. As will be explained in more detail below, the front face of this head portion 9 transforms into its outer surface 10 with a sharp edge.

The force sensor 15 of the actuator 14 is composed of three cylindrical sections 16, 18, by means of which the force sensor 15 is guided in the sleeve 31. The front section 16, which points toward the inlet pipe 3, forms the head of the force sensor 15 and is provided with a cylindrical outer surface 17. The outer diameter of the front section 16 is precisely adapted to the corresponding inner diameter of the sleeve 31. Likewise, the front face of this head transforms into a sharp edge into the outer surface 17 of the front section 16.

In this example, the term "sharp edge" should be understood as meaning that there are no visible chamfers or rounded edges between the front face of valve piston 8 and the outer surface 10 of its front section, and between the front face of force sensor 15 and the outer surface 17 of its front section, respectively. Obviously, "sharp edge" does not mean a completely sharp edge, but rather may have a small radius or radius, for example, a few tenths of a millimeter. The distinction between a sharp-edged transition and a chamfer arises from the fact that, in the closed state of valve piston 8 and force sensor 15, respectively, the molten casting material cannot form an annular conical shell around the front end of head portion 9 or force sensor 15, respectively, and around the inside of each sleeve 26, 31. Such annular shells tend to solidify rapidly due to their small size. Furthermore, chamfers pose the risk that the resulting annular shell, its components, or particles could become lodged between each head portion 9, 16 and the inner surface of each sleeve 26, 31 (particularly during forward displacement of valve piston 8 or force sensor 15, respectively). This, of course, results in increased wear and tear. This can be avoided by the above-mentioned sharp-edged configuration. In particular, in combination with the hardened sleeves 26, 31 that close the head parts 9, 16, a sharp-edged configuration of each head part 9, 16 can be achieved, especially since the dimensions of the parts 9, 26; 16, 31 involved are very accurate and can fit one another.

The pneumatic closing member 36 of the actuator 14 has a working piston 37 housed in a borehole 35 in the valve housing 2 and biased toward the housing front 2a by means of a pressure spring 38. The actuator 14 is thus held in a forward, open position by means of the pressure spring 38. Furthermore, the closing member 36 includes an inlet nozzle 39 provided with a central air outlet (not shown). A sealing member 40 is arranged at the end face of the inlet nozzle 39 around the air outlet, with which the working piston 37 comes into contact with its end face in the closed position. The sealing member 40 ensures that the working piston 37 forms a sealing contact with the inlet nozzle 39 in the forward position, particularly if, for example, the borehole 35 becomes symmetrical due to wear and tear, resulting in the end face of the working piston 37 becoming slightly skewed.

In particular, the pneumatic closing member 36 serves to hold the valve piston 8 in the closed position, as achieved by the momentum of the force sensor 15 .

As mentioned at the outset, since the function of the valve assembly is known from patent EP 0 612 573 A1, its operation will be discussed only briefly below. With the evacuation valve 7 open, any gas in the die casting mold cavity can be evacuated through the valve assembly's outlet conduit 5. This gas flows through the inlet conduit 3 and the open evacuation valve 7 into the sleeve 26, where it passes through radial bores 30 and into the outlet conduit 5, which is connected to an exhaust device (not shown). As soon as the die casting mold cavity is filled with casting material, the molten casting material escapes through the evacuation conduit, via the inlet conduit 3, into the valve assembly, and upwards to the force sensor 15. At this point, the casting material, due to its kinetic energy, advances at high speed, causing the force sensor 15 to suddenly retract in the direction of the rear portion 2b of the housing. Since the clamp 19 of the force sensor 15 comes into contact with the follower 20, the follower 20 is also retracted. In doing so, the follower 20 simultaneously drives the valve piston 8 of the evacuation valve 7 and the working piston 37 of the pneumatic valve 36 , thereby closing the evacuation valve 7 so that no casting material can enter the outlet pipe 5 or escape from the valve assembly 1 .

The operating stroke of the force sensor 15 is limited to a portion of the respective operating strokes of the valve piston 8 and the actuator 14 of the evacuation valve 7. Preferably, the operating stroke of the force sensor 15 is limited to at most one-third of the operating stroke of the valve piston 8, which in turn means that the operating stroke of the valve piston 8 corresponds to at least three times the operating stroke of the force sensor 15. Once the force sensor 15 comes into contact with the housing rear portion 2b via its rear side, the actuator 14 is free to retract until the corresponding elements come into contact with the respective stops. The kinetic energy transferred from the force sensor 15 to the various elements of the actuator 14 is sufficient to retract their elements (i.e., the follower 20, together with the valve piston 8 and the working piston 37) to their rearward stops, thereby transferring energy that is, of course, sufficient to overcome the force of the compression spring 38. However, the retraction of the actuator 14 is supported by the working piston 37, which has been lifted from the inlet nozzle 39, and the compressed air flow from the inlet nozzle 39 into the bore 41, which retracts the working piston 37. However, the primary task of the closing member 36 is to maintain the actuator 14 in the rearward closed position. For this purpose, the closing member 36 is sized so that the pressure on the end of the working piston 37 is sufficient to hold the working piston 37 and thereby the entire actuator 14 in the rear closed position against the force of the compression spring 38 .

When the casting process is complete, the compensator (not shown) is removed from the front face 4, releasing the tappet (not shown), allowing the laminated spring assembly 23 to relax freely. The laminated spring assembly 23 then pushes the movable pressure plate 24 against the follower 20, which in turn pushes the valve piston 8, force sensor 15, and working piston 37 forward in the direction of the front face 4. During this process, the valve piston 8 and force sensor 15 push the solidified casting mass (riser) in the inlet duct 3 within the valve housing 2 and forward out of the valve housing 2. During this process, the laminated spring assembly 23 serves to retract the actuator 14 and eject the solidified casting mass.

Since the valve piston 8 or force sensor 15 is in the forward closing member, solidified casting mass, which may be located in the corresponding sleeve, is completely removed. The sharp-edged design of the valve piston 8 and force sensor 15 facilitates the removal of this solidified casting mass. Since both the valve piston 8 and the sleeve 26 are made of very hard material, and the outer diameter of the valve piston 8 and the outer diameter of its head portion 9 are perfectly matched to the inner diameter of the corresponding sleeve 26 (i.e., only a few hundredths of a millimeter smaller), and the end face of the head portion 9 also transforms into a sharp edge as the shell surface 10, even small and minimal metal chips can be removed from the sleeve 26. The same applies to the force sensor 15 with its associated sleeve 31.

Alternatively or additionally, the force sensor 15 can be sized and adapted to the sleeve 31 such that the end face of the force sensor 15 in the retracted state protrudes by 0.01-1 mm beyond the corresponding end face of the sleeve 31 pointing toward the front face 4 of the valve housing 2, effectively preventing any casting mass from entering the sleeve 31. The same applies to the valve piston 8, by sizing and adapting the valve piston 8 to the sleeve 26 such that its head portion 9 in the retracted state protrudes by approximately 0.01-1 mm beyond the corresponding end face of the sleeve 26 pointing toward the front face 4 of the valve housing 2.

With reference now to FIG. 2 , a cross-section through a valve assembly of a second example version is shown, in which in particular only the differences compared to the above-described example version are described in detail below. Thus, a significant difference compared to the above-described example version is that the force sensor 15 a and the follower 20 a do not form separate components, but are instead made integrally. Furthermore, the follower 20 a does not comprise a rearward protrusion, but rather a pressure spring 43 is supported on the rear side of the follower 20 a, which acts as a return spring for returning the actuator 14 to the forward open position. Furthermore, a closing piston 21 a is visible, by means of which the follower 20 a can be retracted. Retracting the follower 20 a also causes the evacuation valve 7 connected to the follower 20 a to be closed. Preferably, two closing pistons 21 a are provided, for example arranged in a horizontal line or diagonally to each other, where only one closing piston 21 a is visible in the figure. The closing piston 21a is pre-actuated, preferably for pneumatically actuating the valve piston 8 at the start of the casting cycle at the appropriate time, preventing the casting mass from entering the outlet pipe 5. The pneumatic passages or pipes on the end surface facing the front face 4 that enable pneumatic actuation of the closing piston 21a are not shown in this figure. As mentioned above, one or more closing pistons 21a are used to close the valve piston 8 at the appropriate time at the start of the casting cycle. However, in any case, the valve piston 8 must be closed before the casting material enters the inlet pipe 3. Once the temperature within the valve assembly has stabilized after several casting cycles, the valve piston 8 of the evacuation valve 7 can be closed by the casting mass impact force sensor 15a. Experience has shown that closing the evacuation valve 7 by actuation of the casting mass can be achieved after approximately 10-50 casting cycles. While the two housing parts 2a, 2b are fastened together by bolts 42, it is clear that multiple such bolts 42 may be used.

Referring now to FIG3 , a cross-section of a valve assembly of a third example version is shown, wherein here again particular attention is paid to the differences compared to the first example version. One notable difference from the example version shown in FIG1 is that both the valve piston 8b and the working piston 37b are biased in the direction of the forward open position by pressure springs 44, 45, respectively. In addition, in this case, at least one closing piston 21b is provided, by means of which the follower 20b can be retracted. The rear portion of the follower 20a is formed by a conical protrusion. The stationary pressure plate 22a is provided with an annular recess, the rear end of which guides the force sensor 15a. Since the valve piston 8b and the working piston 37b are biased in the same direction by the pressure springs 44, 45, respectively, the forward impact of the follower 20 is made more uniform.

It will be appreciated that certain features of the different exemplary variants shown in Figures 1 to 3 can be combined as necessary. In this connection, it should be mentioned that, for example, also in the exemplary version shown in Figure 1 , both the valve piston 8b and the working piston 37b can be biased by a compression spring in the direction of the advanced open position.

The clear advantages of the valve assembly shown can be summarized as follows:

The use of a hardened sleeve enables the valve assembly to be configured to be particularly wear-resistant in locations subject to high stresses and strains, and yet cost-effective in production.

- The valve assembly allows safe and reliable evacuation until the mold cavity is completely filled.

- The valve assembly has a simple construction and is designed to have a long service life.

- Components exposed to wear, such as the two sleeves, the two pressure plates as well as the valve piston and the force sensor, can be quickly and easily replaced with new ones when necessary.

By providing one or more pre-actuated closing pistons, it is now possible to ensure that the valve piston is closed at the start of the casting cycle and at the right time, so that no casting material can enter the outlet duct.

Claims (16)

1. A valve assembly (1) for evacuating a die-casting mold, comprising a valve housing (2) including a front portion (2a) and a rear portion (2b), wherein an evacuation conduit (3, 5) capable of being closed by means of an evacuation valve (7) is machined in the front portion (2a), and a valve piston (8) having an actuator (14) for driving the evacuation valve (7), the actuator (14) including a force sensor (15) actuated by the casting material and for transmitting the closing motion of the force sensor (15) to the valve piston (8). The force transmission member (20) of the valve piston (8) of the vacuum valve (7), and the actuator (14) with a stacked spring assembly (23) for returning and/or for discharging the riser, is characterized in that the front portion (2a) of the housing is provided with a sleeve (26) surrounding at least the head portion (9) of the valve piston (8) and/or another sleeve (31) surrounding at least the head portion (16) of the force sensor (15), each sleeve (26, 31) being stiffer than the front portion (2a) of the housing. 2. The valve assembly (1) according to claim 1, characterized in that each sleeve (26, 31) is inserted into a hole (28, 33) drilled in the front part (2a) of the housing, each hole (28, 33) having a diameter decreasing in the direction of the rear part (2b) of the housing, a step (29, 34) is formed at the transition from the larger diameter to the smaller diameter for contacting each sleeve (26, 31), and each sleeve (26, 31) has a heel (27, 32) corresponding to the step (29, 34). 3. The valve assembly (1) according to claim 1 or 2, characterized in that the head portion (16) of the force sensor (15) is configured as a cylinder, and the end face of the head portion (16) of the force sensor (15) transforms the sharp edge into a shell surface area (17). 4. The valve assembly (1) according to claim 1 or 2, characterized in that the valve piston (8) of the evacuation valve (7) includes a cylindrical head portion (9) at the front end, and the end face of the head portion (9) of the valve piston (8) transforms the sharp edge into its shell surface area (10). 5. The valve assembly (1) according to claim 1 or 2, characterized in that each sleeve (26, 31) and the evacuation pipe (3, 5) are arranged in the front part (2a) of the housing. 6. The valve assembly (1) according to claim 1 or 2, characterized in that the actuator (14) as the force transmission member (20) has a follower disposed between the stacked spring assembly (23) and the clamp (19) of the force sensor (15). 7. The valve assembly (1) according to claim 1 or 2, characterized in that the actuator (14) is provided with a movable pressure plate (24) on which the stacked spring assembly (23) is mounted for returning the actuator (14) to and/or discharging the riser. 8. The valve assembly (1) according to claim 1 or 2, characterized in that the actuator (14) has a pneumatic closing member (36) including a working piston (37) received in a hole (35) drilled in the valve housing (2), an inlet nozzle (39) contacting the working piston (37) in the closed position through its end face, the inlet nozzle (39) including a central air outlet and a sealing member (40) around the air outlet. 9. The valve assembly (1) according to claim 1 or 2, characterized in that, in addition to the cylindrical head portion (16) of the force sensor (15), the force sensor (15) has at least two additional cylindrical segments (18), both of which are guided in the sleeve (31). 10. The valve assembly (1) according to claim 1 or 2, characterized in that the end face of the valve piston (8) and/or the end face of the force sensor (15) protrudes 0.01-1 mm from the end face of the corresponding sleeve (26, 31) facing the front (4) end face of the valve housing (2) when retracted. 11. The valve assembly (1) according to claim 1 or 2, characterized in that the sleeve (26) for guiding the valve piston (8) is provided with a radial bore (30), the interior of the sleeve (26) is connected to the outlet pipe (5) through the radial bore (30), and the valve piston (8) includes two guide sections (11, 12), one guide section (11) being arranged upstream of the radial bore (30) and the other guide section (12) being arranged downstream of the radial bore (30), wherein the guide section (11) arranged upstream of the radial bore (30) has an axial port (11a). 12. The valve assembly (1) according to claim 1 or 2, wherein the working stroke of the force sensor (15) is limited to less than 2 mm. 13. The valve assembly (1) according to claim 12, wherein the working stroke of the valve piston (8) corresponds to at least three times the working stroke of the force sensor (15). 14. The valve assembly (1) according to claim 1, wherein the actuator (14) has a follower (20a) integrally constructed with the force sensor as the force transmission member. 15. The valve assembly (1) according to claim 8, wherein both the valve piston and the working piston are spring biased by pressure springs (44, 45) in the direction of the forward opening position. 16. The valve assembly (1) according to claim 1 or 2, characterized in that at least one actuated closing piston (21) is provided, wherein the force transmission member (20) and at least the valve piston (8) connected to the force transmission member (20) are displaced to a closed position by means of the closing piston (21).