AT515780B1 - Method for monitoring the movement of a piston-cylinder unit - Google Patents

Abstract

In a method of monitoring the movement of a piston-cylinder unit (1) of a cascade control (11) for cascade-like opening and closing of hot ducts of a molding machine, one of a pressure transducer (9) is expected to be due to reaching a piston stopper (K) Pressure rise (P +) measured pressure (P) in the piston-cylinder unit (1) with a predetermined, at least once set constant pressure threshold (Psoll) compared. A signal (S), preferably a warning signal (SW), is output when the measured pressure (P), preferably in a time period (tr) in which the pressure rise (P +) is expected, is below the predetermined constant pressure threshold (Psoll ) lies. If the pressure threshold (Psoll) is reached outside the predetermined period (tr) in which the pressure rise (P +) is expected, a warning signal (SW) can also be issued.

Description

Description: The invention relates to a method for monitoring the movement of a piston-cylinder unit of a cascade control for the cascade-like opening and closing of hot channels of a shaping machine. In addition, the invention relates to a shaping machine, in particular injection molding machine for carrying out such a method, with at least one piston-cylinder unit of a cascade control for the cascade-like opening and closing of hot channels of a molding machine, wherein the piston-cylinder unit, a pressure source, a from the pressure source Pressure settable fluid, a fluid line, a valve, a cylinder, a guided in the cylinder and can be acted upon by the fluid piston and a pressure transducer, with the pressure of the fluid in the fluid line is measurable, auf¬weist, and a switching device for switching the with the switching device signal-connected piston-cylinder unit.

In the molding machine industry, there are various ways to use piston-cylinder units. Most such piston-cylinder units are used in Hydrauliki¬ drive units. In particular, such hydraulic drive units are used in injection molding machines or molding machines in the region of the core pull, the Kask control or the ejector. An example of such a hydraulic drive unit for an injection molding machine can be found in DE 10 2011 012 714 A1.

Very often so-called hot runners of injection molds are opened and closed via hydraulic cylinders. An example of such hot runners, which are actuated in cascade fashion via needle closure nozzles, for example, is known from WO 00/30824 A2hervor. Hot runner seals associated with an evaluation unit are disclosed in DE 41 10 445 C2. In this case, position monitoring elements in the form of cams are provided, which cooperate with limit switches.

Known hydraulic cylinders for hot runners are in most cases hydraulic cylinders with a very short stroke (a few millimeters). Due to the already very limited installation space in injection molds and often very high number of built-in hot runners, these can usually only poorly - and according to DE 41 10 445 C2 only with additional space and design effort - be equipped with Endschalterüber¬wachungen. WO 2014/085321 A1 also discloses a hot runner system with elaborate displacement sensors. In addition, a reliable adjustment of these limit switches or displacement sensors would prove to be very difficult with the short strokes.

This means that in the injection molding process when opening and closing the hot runners in most cases simply trusts that the switching of a hydraulic valve follows an immediate movement of the corresponding hot runner cylinder without being able to observe or control the actual movement, since these cylinders are fully in "Inside" of the tool are installed. Most often one notices a malfunction of a single Heißkanaleserst the component quality of the molded part, which is usually very difficult to figure out the causing hot runner.

Another frequent weak point of these hot runner circuits is also the importance of the speed and reproducibility of the switching operations. For example, in the case of a filling process of a few seconds, it is absolutely crucial that several hot runners open and close in an absolutely reproducible manner within a few 1 / 10th of a second at the same time (or depending on worm position or filling pressure). In the event of a fault (poor component quality), it is hardly possible to prove whether the cause is due to an excessively or non-reproducibly switching hot runner.

Again and again, there are discussions and lengthy measurements at customers to clarify whether an insufficient component quality or weight fluctuations caused by Fehl¬funktion or even by switching too slowly of hot runners.

The measurement with flow meters in each case a supply line of a cascade cylinder is a known, but very expensive design to recognize a correct switching operation.

Furthermore, from DE 10 2004 002 029 A1 a waveform monitoring device is known. In this case, a measured value waveform is generated via a determination device on the basis of print data and it is determined whether the print data exceeds a reference pressure waveform beyond a predetermined range. In other words, it is constantly checked whether a measured pressure curve deviates from a desired (desired) pressure profile. This is complicated since a reference cycle is always necessary. In addition, this embodiment is very expensive in terms of performance, since a control loop must be constantly run through. So there must be a constant comparison.

In a very similar complex manner, also in WO 2010/138302 A1, a continuous comparison of measured values and desired values takes place during a complete switching operation of a piston-cylinder unit.

The object of the present invention is therefore to provide a comparison with the prior art improved method or an improved shaping machine.In particular, the known disadvantages are to be solved.

This is achieved by a method having the features of claim 1. Accordingly, it is envisaged that a pressure increase in the piston-cylinder unit to be expected by a pressure transducer when a piston stop is reached is compared to a predetermined, constant pressure threshold value, a signal being output if the measured pressure is below the predetermined, constant pressure This provides a possibility for verifying or recording the switching operation of the piston-cylinder units (and thus the functionality of the hot runners) during an injection molding process. In other words, the invention is based on the fact that a moving oil column causes a pressure peak at abrupt stop, which is significantly higher than the originally available supply pressure. Typically, straight cascade cylinders are dimensioned to achieve high acceleration of the relatively low-moving masses (valve pins) and, therefore, very short switching times can be realized. The movement itself is suddenly stopped by the mechanical stop of the lock needle or the achievement of the hydraulic cylinder stroke, causing the above-mentioned significant pressure spike. This pressure peak need only be interpreted more appropriately, and thus not only can it be ascertained that the corresponding cylinder has actually carried out a movement, but it is even possible to determine the duration of the movement.

Basically, it is sufficient for the operability of the present invention, gen¬rell to check whether just the measured pressure reaches the Druckschwellwert the pressure increase. With this variant, a pure pressure increase monitoring is possible. This already provides meaningful results for a likely malfunction.

In addition, a time monitoring of a detected pressure change, in particular a pressure increase, is possible. In fact, it could well be that, for example, a hot runner needle is mechanically damaged or any particle is clamped in such a way that, although a movement and thus at the stop a usual - ie lying in the range of Druckschwellwerts - pressure increase takes place, but the required stroke was not reached , This could be detected via the time deviation by comparing the time of the pressure threshold value to a predetermined time period in which the pressure rise is expected, the signal being output if the time of the pressure threshold value is outside the predetermined time period that is, before or after this period.

Particularly preferred are thus both variants, so both the Druckanstiegsüberwa¬chung as well as the timing monitoring provided in combination with each other.

Preferred embodiments are specified in the subclaims.

An increase in pressure can well be well beyond the pressure threshold, without a signal is output, since the necessary for the fundamental function check pressure peak has been reached. Above all, if the pressure increase does not only deviate from the pressure threshold value but also ends below this pressure threshold value, a deviation which hinders the functional capability is actually given.

The output signal may be identical at a deviating from the pressure threshold pressure increase and also at the time deviating from the time. That is, it does not matter to the system whether there is only a pressure increase deviation, only a temporal deviation, or both. For example, the output signal may be merely indicia, thereby providing an on-screen operator with knowledge of the functionality and actual travel times of individual piston-cylinder units. Already this can be very helpful for a user in the spray process optimization. However, in order to give an operator an additional, even clearer indication of the presence of an unwanted deviation, it is preferably provided that the signal is output as a warning signal. Particularly preferably, it can be provided here that the operator himself can set the limits for each piston-cylinder unit and for a corresponding warning signal. As a result, the operator can introduce his knowledge about each piston-cylinder unit or each cascade, since often only this one The operator knows if a particular cascade is more critical to the process than another.

Specifically, it is preferably provided for the method that it is durchgeführtwird with a Schaltvor¬richtung for switching the signal-technically connected to the switching device piston-cylinder unit of a molding machine, in particular injection molding machine, wherein the piston-cylinder unit, a pressure source, a pressurizable fluid from the pressure source, a fluid conduit, a valve, a cylinder, a piston guided in the cylinder and acted upon by the fluid, and the pressure transducer, with which the pressure of the fluid in the fluid conduit is measurable, characterized by the steps of: outputting an opening signal of the switching device to the valve, whereby the piston is moved by means of the pressurized fluid, measuring the pressure of the fluid in the fluid line through the pressure transducer after outputting the opening signal, at least once setting the predetermined, constant pressure threshold, optionally at least one time F establishing a predetermined period of time following the outputting of the opening signal, comparing the pressure measured by the pressure transducer with the predetermined, constant pressure threshold value, preferably in the predetermined time period, and outputting a signal, preferably a warning signal, when the pressure measured by the pressure sensor, preferably in the predetermined period is below the pressure threshold. The pressure transducer may also be referred to as a pressure sensor or pressure measuring device. It is preferably provided for the valve that it is designed as at least one two-way valve.

In order to achieve as constant a pressure as possible in front of this valve, a pressure reducing valve arranged in the fluid line upstream of the valve is preferably provided, by means of which the fluid can be placed under a constant pressure, preferably below a pressure of 30 to 50 bar, before the opening signal is issued. Specifically, the level of Versorgungs¬ pressure should be about 45 bar.

With the output of the opening signal is preferably provided that the pressure of the fluid by at least 25%, preferably at least 50%, breaks down. Thereafter, it is preferably provided that the pressure in the fluid after the pressure drop increases to a substantially constant value that is between 10% and 30% lower than the pressure of the fluid before the opening signal is issued. Subsequently, it is preferably provided that just before or with the reaching of the piston stop the pressure of the fluid by at least 10%, preferably by at least 40%, and preferably to a pressure of about 50 bar, increases. The piston stop may correspond to the piston end position in the cylinder. Especially in cascades, the abrupt stop of movement is caused by the contact of the "needle" (corresponds to the piston rod end) with the sealing point just when driving in Kask¬den.

Furthermore, it is preferably provided that the predetermined period is between 50 ms and 5 seconds, preferably between 100 ms and 500 ms, after the output of the opening signal. For a core pull, this range is 2 to 3 seconds. In a cascade control in the preferred 100 ms to 500 ms.

In general, the times given here and in particular the percentages of the pressure changes can vary greatly, since they depend on very many details of the respective application and the respective design of the piston-cylinder units.

The object of the present invention is also achieved by a molding machine having the features of claim 9. Accordingly, a comparison device and an output unit is provided, wherein in the comparison device a predetermined, constant pressure threshold value is stored and the comparison device, a pressure value signal of the pressure sensor can be supplied, wherein the pressure value signal with the predetermined, constant pressure threshold is comparable and wherein the output unit, a signal can be output, if the pressure value signal supplied to the comparison device by the pressure sensor is below the stored and predetermined, constant pressure threshold value. As a result, a relatively simple tool can be made available to a customer with a simple tool, with which the function of each desired piston-cylinder unit, even during the process, can be monitored virtually constantly.

As already stated, the signal can not only be easily output, but it can preferably be provided that the output unit has a warning device, wherein the signal from the warning device as, preferably acoustic or visual, warning signal can be output. With regard to the forming machine, it is also provided according to the invention that the piston-cylinder unit is part of a cascade control. In particular, the cascade control may comprise a plurality of parallel or serially connected piston-cylinder units.

Further details and advantages of the present invention will be explained in more detail below with reference to the Figu¬renbeschreibung with reference to Ausführungsbei¬spiele shown in the drawings. 1 shows schematically a single cascade with pressure transducer, FIG. 2 shows a single cascade with two pressure transducers, FIG. 3 shows a multiple cascade with one pressure transducer each, FIG. 4 shows a multiple cascade with one Figure 5 shows a single core pull with two pressure transducers. Figure 6 is a pressure course diagram. Figure 7 shows schematically a forming machine with clamping unit and injection unit.

Reference is first made to FIG. 7, which schematically shows a forming machine 3, in particular an injection molding machine. On the one hand, this shaping machine 3 has an injection unit 24 in the form of an injection unit and, on the other hand, a closing unit 23. This clamping unit has at least one mold mounting plate 25 movable by a drive device 32 and a fixed mold mounting plate 26. On each of these two mold clamping plates 25 and 26, a mold half 27 is clamped. These two mold halves 27 form a cavity 28 in the closed state. The injection unit 24 injects melt into this cavity 28 via the injection channel 29. In this Fig. 7 is schematically illustrated that the injection channel 29 before reaching the cavity 28 in several branches - so-called cascades 11 - branches. Each individual branch or hot runner can be closed with a piston-cylinder unit 1 not shown in detail here. However, such piston-cylinder units 1 according to the invention can also be used in the area of an ejector, not shown here, in a core pull or similar components likewise not shown become.

In Fig. 1 is a schematic diagram in conjunction with the logical components for performing the method is shown schematically. First of all, reference is made to the optical pressure supply shown on the left side. The accumulator 16 and the pump 17 together form a pressure source 4, from which a fluid F in the fluid line 5 is pressurized. Die¬ses fluid F may for example be a gas, such as air, whereby the piston-cylinder unit 1 is pneumatically driven. Preferably, however, this fluid F is a liquid, preferably a hydraulic oil, whereby the piston-cylinder unit 1 is hydraulically driven. After the battery 16 and the pump 17 is followed by a check valve 18.Über the Zuschaltventil 19, the battery 16 can be switched on. Downstream of the first pressure transducer 30 follows the pressure reducing valve 10. By this pressure reducing valve 10, the pressure P in the fluid line 5 to a constant pressure P, preferably set to 45 bar. This is followed by a further pressure transducer 22. For the basic function of the present invention, no pressure transducer 22 must be provided, but it is helpful in setting or checking the correct setting of the pressure-reducing valve 10 and thus the supply pressure. This is followed by the dash-dotted line hydraulic block. In this hydraulic block, a check valve 20 is initially provided, after which the pressure transducer 9 follows. Subsequently, downstream of a valve 6 is arranged, which is formed in die¬sem case as a two-way valve with a spool. This control slide is an opening signal Ozu performed by the switching device 2 shown schematically. As a result, the valve 6 is opened and fluid F enters the displacement of the cylinder 7, whereby the piston 8 is moved in this case to the right. With this opening signal O occurs a sudden pressure drop, which is measured by the pressure transducer 9. This pressure transducer 9 sends a corresponding pressure value signal Pist to the comparison device 12. In this comparison device 12, a pressure threshold value PSOii is also stored. From the valve 6 so long fluid F is supplied to the displacement of the cylinder 7 until the piston 8 reaches the Kol¬benanschlag K. Upon reaching this piston stop K, a pressure increase P + is expected. This pressure rise P + is forwarded as a pressure value signal Pist from the pressure transducer 9 to the comparison device 12. If the expected pressure increase P + deviates from the stored or predetermined pressure threshold value Pson, a corresponding signal S is output by the output unit 13. This signal S can be forwarded to an operating unit 15 and displayed there as a concrete pressure value. But it is also possible that the operating unit 15 has a warning device 14 or a warning device 14bildet and outputs the signal S as a warning signal Sw or displays. As a result, a user is reminded acoustically and / or visually insistently on a malfunction. However, the comparison device 12 can not only monitor whether there is a pressure deviation, but also when a pressure increase P + occurs. For this purpose, a time interval for the desired achievement of the pressure threshold value Psom is specified or stored in the comparison device. If the time t at which the pressure rise P + occurs or the pressure threshold value PsoN is reached is outside of this predetermined period tr, a signal S is output in the same manner as described above. Both the retraction and the extension of the piston 8 are controlled or measured via the pressure transducer 9.

In contrast, a drive unit with a single cascade and two pressure transducers 9 is shown in FIG. This variant is more accurate, since the pressure loss via the valve 6 is not measured by the arrangement of the two pressure transducers 9 directly in the supply lines to the consumer (ie between the valve 6 and the cylinder 7) and therefore the retraction and extension can be checked and measured even more precisely , Generally, the valve 6 may be formed as a 4/2 or 4/3 way valve. In the case of FIG. 2, the left-side pressure sensor 9 is provided for closing and the right-side pressure sensor 9 for opening. The advantage of these two pressure sensors 9 is, inter alia, that no line resistance is given. In addition, a more accurate signal from the cylinder 7 can be determined. These two pressure sensors 9 are particularly useful for very sensitive cascades. In this embodiment variant, the difference of the pressure value signal Pist from the two pressure sensors 9 is used or taken into account for monitoring the movement of the piston-cylinder unit 1.

Fig. 3 shows a drive unit with a multiple cascade, wherein the basic formation of a single cascade is again as in Fig. 1. In this variant according to Figure 3, a separate pressure transducer 9 is arranged in front of each valve 6, each ent Responsive pressure value signals Pist to the comparison device 12 forwards.

In the figures 2 to 5, the logic components comparison device 12, output unit 13 and switching device 12 and warning device 14 or control unit 15 are generally not shown separately again. Analogously, however, they are provided as in FIG. 1. These logical components or components may be-at least partially-integrated into a single structural unit (eg in the form of a control or regulation unit).

Fig. 4 shows a cost effective variant, with which several cascades can be controlled with a pressure transducer 9, especially if they are operated sequentially. Even in parallel operation, detection or monitoring can take place, namely, when all cascades have switched through. Here it is crucial that the individual check valves 20 with respect to FIG. 3 are omitted and replaced by a central check valve 20 in the supply line before dividing into the individual cascade sections. This has only the disadvantage that thereby parallel switching operations influence each other. An inventive monitoring of the movement of Kolbenzylindereinhei¬ten 1 is also given in this variant. In other words, in the case of this variant, it is known that an error exists at the pressure value increase P + deviating from the pressure threshold value Pson, but it is still not known exactly where this is present.

Compared to FIGS. 1 to 4, FIG. 5 shows no cascade control 11 but a single-core cable. In this diagram, it can be seen that only one pump 17 is set as the pressure source 4. The fluid F opens at the end of the hydraulic line via a tank line in the tank 31. In between, as in the previous variants in the supply line a Druckminderven¬til 10 is provided, after which again a pressure transducer 22 is arranged. Subsequently, a multi-way valve with a central position is arranged as the valve 6. This is followed by a Zwillingstperrven¬til 21, which has unlockable check valves. Only then are two pressure transducers 9 arranged directly in the supply lines to the piston-cylinder unit 1, which - similarly as inFig. 2 - ensure even more accurate monitoring. This circuit diagram arrangement is equally suitable for an ejector.

Fig. 6 shows a pressure waveform diagram as recorded by the pressure transducers 22 and 9, respectively. Corresponding signals are also taken into account in the comparison device 12. In the case of a switching operation, the pressure profile of the pressure value signal Pist shown in FIG. 6 along the time axis is shown by way of example. At the beginning, ie before the switching of the valve 6, there is a constant pressure P, the exact value of which depends on the setting of the pressure-reducing valve 10. In this diagram, the switching position of the switching device 2 is additionally shown schematically. At the beginning, no signal is output from the switching device 2. However, as soon as the opening signal O is output by the switching device 2, the valve 6 opens correspondingly, as a result of which fluid F enters the stroke space of the cylinder 7. Due to this switching process, there is a significant short drop in pressure, in particular of the pressure value signal Pist measured by the pressure transducer 9. When the traversing movement of the piston 8 to the right is triggered, the pressure Pistwieder increases significantly. The pressure Pist measured by the pressure transducer 9 then settles to a relatively constant pressure curve during the subsequent uniform displacement movement. This pressure profile is lower than the level of the supply pressure P (45 bar). But as soon as the end position or the piston stop K is reached, there is a sudden pressure increase P +, which is caused by the "braking" of the oil column. It can be clearly recognized that this pressure peak is above the level of the pressure supply. The evaluation of these signals detected by the pressure transducers 9 and 22 is now carried out via the comparator device 12. If the time t from the switching of the valve 6 (usually a 24 volt signal) to the shown pressure rise P + (for example supply pressure plus 10%) is measured via this comparison device 12, an exact statement about the duration of the switching process is obtained.

Operation of the respective piston-cylinder unit 1 (hot runner cylinder). After a first evaluation, corresponding frame regions can be set or stored in the form of a pressure threshold value Pson and a period of time tr for reaching this pressure threshold value PSOii. In subsequent renewed switching operations, these form a Vergleich¬basis for monitoring the movement of the piston-cylinder units 1. With this Überwa¬chungsverfahren several types of malfunction can be detected. For example, can be detected, whether a valve 6 is defective. If so, there is no oil flow and thus no pressure increase P +. Furthermore, it is possible that the hot runner needle is inserted. Then, although it comes to a very small flow of oil to compress the oil or fluid F in the supply line, but there is no sudden stop and thus no noticeable pressure increase P +. In both cases, it would be recognized that the piston 8 does not move in the cylinder 7 and thus there is a malfunction.

With regard to the different embodiment variants, it should again be pointed out that the preferred variant provides only one pressure transducer 9, via which both directions of movement of the piston 8 can be evaluated. A check valve 20 in the supply line to the valve 6 prevents the occurring pressure peaks from influencing other adjacent hot runners (see FIGS. 1 and 3). According to FIGS. 2 and 5, two pressure transducers 9 are arranged after the valves 6. By measuring directly after the valve 6 in the Zulei¬tung the respective direction of movement can be evaluated more accurately and safely. The most cost-effective embodiment is shown in Fig. 4, according to which a pressure transducer 9 is provided after the pressure reducing valve 10 centrally for a plurality of hot runners. In this case, the individual check valves 20 can be dispensed with. With this one pressure sensor 9, it is possible to evaluate the switching of several hot runners simultaneously. Also individual hot runners can be evaluated, but only if they are operated exclusively sequentially.

Of course, all ideas and variants mentioned herein are not only implementable in hot runners but in each piston-cylinder unit 1, which is operated in ähnli¬cher manner, ie with high acceleration and a stop by a mechanical piston stop 6. This The invention thus makes use of the fact that the consumer pressure (pressure peak) at the mechanical stop (piston stop K) rises significantly above the supply pressure (pressure P after pressure reduction valve 10), since the movement stops so abruptly due to the high dynamics and low moving masses.

Claims (13)

A method for monitoring the movement of a piston-cylinder unit (1) of a Kask¬densteuerung (11) for cascade-like opening and closing of hot runners of a shaping machine, characterized in that one of a pressure transducer (9) in a due to reaching a Piston stop (K) expected pressure rise (P +) measured pressure (P) in the piston-cylinder unit (1) with a vorgege¬benen, constant pressure threshold value (Pson) is compared, wherein a signal (S) ausgege¬ ben, when the measured pressure (P) is below the predetermined, constant Druck¬schwellwerts (Psoii). 2. The method according to claim 1, characterized in that the time (t) of Errei¬chens the pressure threshold (Pson) is compared with a predetermined period (tr) in which the pressure increase (P +) is expected, the signal (S ) is output when the time (t) of reaching the pressure threshold (Pson) is outside the predetermined period (tr), that is, before or after the period (tr). 3. The method according to claim 1 or 2, characterized in that the signal (S) is output as a warning signal (Sw). 4. The method according to any one of claims 1 to 3, characterized in that the Verfah¬ren with a switching device (2) for switching with the switching device (2) signal technically connected piston-cylinder unit (1) of a shaping machine (3 ), in particular injection molding machine, wherein the piston-cylinder unit (1) comprises a pressure source (4), a fluid (F) pressurizable from the pressure source (P), a fluid conduit (5) Valve (6), a cylinder (7), a guided in the cylinder (7) and the fluid (F) acted upon piston (8) and the pressure transducer (9), with which the pressure (P) of the fluid (F) in the fluid line (5) is measurable, characterized by the steps of: - outputting an opening signal (0) from the switching device (2) to the valve (6) whereby the piston (8) is pressurized (P) by the fluid (F) - Measuring the pressure (P) of the fluid (F) in the fluid line (5) by the pressure transducer (9) after the Ausg just the opening signal (O), - at least once setting the predetermined, constant pressure threshold (Psoll) i - comparing the pressure (P) measured by the pressure transducer (9) with the predetermined, constant pressure threshold (Pson), preferably in a predetermined time ¬raum (tr), and - output of a signal (S), preferably a warning signal (Sw), when the pressure transducer (9), preferably in the predetermined period (tr), measured pressure (P) below the pressure threshold (Pson). A method according to claim 4, characterized in that the valve (6) is at least a two-way valve. 6. The method according to claim 4 or 5, characterized by a in the fluid line (5) in front of the valve (6) arranged pressure reducing valve (10) through which the fluid (F) before outputting the opening signal (O) under a constant pressure ( P), preferably under a pressure (P) of 30 to 50 bar, can be set. 7. The method according to any one of claims 4 to 6, characterized in that just before or with the achievement of the piston stop (K) of the pressure (P) of the fluid (F) by at least 10%, preferably by at least 40% and preferably to a pressure (P) of over 50 bar, increased. 8. The method according to any one of claims 4 to 7, characterized in that the predefined period (tr) is between 50 ms and 5 seconds, preferably between 100 ms and 500 ms, after the output of the opening signal (O). A molding machine (3), in particular an injection molding machine for carrying out a method according to any one of claims 1 to 8, comprising - at least one piston-cylinder unit (1) of a cascade control (11) for opening and closing hot passages of a molding machine, the Piston-cylinder unit (1) a pressure source (4), one of the pressure source (4) under pressure (P) settable fluid (F), a fluid line (5), a valve (6), a cylinder (7), ei ¬nen in the cylinder (7) guided and acted upon by the fluid (F) piston (8) and a Druckaufnehmer (9) with which the pressure (P) of the fluid (F) in the fluid line (5) can be measured, has, - A switching device (2) for switching the switching device (2) signaltech¬nisch associated piston-cylinder unit (1), characterized by - a comparison device (12), wherein in the comparison device (12) a vorgebe¬ner, constant Pressure threshold (Pson) is deposited and the Vergleichv orrichtung (12) a pressure value signal (Pist) of the pressure transducer (9) can be fed, wherein the Druckwert¬ signal (Pist) with the predetermined, constant pressure threshold (PSOii) is comparable, and - an output unit (13), wherein of the output unit ( 13), a signal (S) can be output if the pressure value signal (Pist) supplied to the comparison device (12) by the pressure sensor (9) lies below the stored and predetermined, constant pressure threshold value (Psoil). 10. Forming machine according to claim 9, characterized in that the output unit (13) has a warning device (14), wherein the signal (S) from the warning device (14) as, preferably acoustic or pictorial, warning signal (Sw) is dispensable. Forming machine according to claim 9 or 10, characterized in that the piston-cylinder unit (1) is part of a cascade control (11). 12. Forming machine according to claim 11, characterized in that the Kask¬densteuerung (11) has a plurality of parallel or serially connected piston-cylinder units (1). 13. Forming machine according to one of claims 9 to 12, characterized in that from the comparison device (12) the time (t) of the pressure transducer (9) measured, the pressure threshold (Pson) corresponding pressure value signal (Pist) with ei¬nem predetermined and stored period (tr), in which a pressure increase (P +) due to the achievement of a piston stop (K) is expected, is comparable and from the output unit (13) a signal (S) can be output, if the time (t) of reaching the pressure threshold (Pson) is outside the stored and predetermined period (tr), ie before or after the period (tr). For this 7 sheets drawings