Method for monitoring an injection molding process, and plastic injection molding machine

Abstract

The invention relates to a method for monitoring an injection molding process, wherein a plastic material is injected into a cavity (16) of an injection mold (15), and the plastic material is held in the cavity (16) during a cooling phase (53, 54). The injection mold (15) comprises a mold core (18) which protrudes into the cavity (16) and which is provided with an evaporation chamber (28). During the cooling phase (53, 54), a liquid is conveyed into the evaporation chamber (28) so that the liquid is evaporated in the evaporation chamber (28), and an audio signal (38) of noises generated in the injection mold (15) is recorded, wherein the end (62) of the evaporation process is inferred from a decrease in the level of the audio signal (38). The invention also relates to a plastic injection molding machine.

Description

Method for monitoring an injection molding process, plastic injection molding machine

[0001] The invention relates to a method for monitoring an injection molding process and a plastic injection molding machine.

[0002] In plastic injection molding machines, a plastic material in a liquid or pasty state is injected into a cavity of an injection mold. The plastic material cools and solidifies. This creates an injection-molded part that has a shape corresponding to the cavity. The injection mold can be opened to remove the injection-molded part from the mold.

[0003] In certain applications, the injection mold comprises a mold core that extends into the cavity of the injection mold. An evaporation chamber can be arranged within the mold core, in which a liquid is evaporated as the plastic material cools to extract heat from the plastic material in the vicinity of the mold core (WO 2019 / 158521 A1).

[0004] The quality of the injection-molded part depends on the cooling process in the mold core being carried out according to specifications. Both excessive and insufficient cooling can compromise the quality of the injection-molded part. Quality control is not easy because the quality of the injection-molded part depends on the internal structure of the material, and this often cannot be determined non-destructively.

[0005] The invention is based on the object of providing a method for monitoring an injection molding process and a plastic injection molding machine with which these disadvantages can be reduced. The problem is solved by the features of the independent claims. Advantageous embodiments are specified in the subclaims.

[0006] In the method according to the invention for monitoring an injection molding process, a plastic material is injected into a cavity of an injection mold. During a cooling phase, the plastic material is held in the cavity. The injection mold comprises a mold core which projects into the cavity and is provided with an evaporation chamber. During the cooling phase, a liquid is conveyed into the evaporation chamber so that the liquid evaporates in the evaporation chamber. During the cooling phase, an acoustic signal of noises generated in the injection mold is recorded.

[0007] The invention is based on the discovery that the evaporation process, which takes place in the evaporation chamber during the cooling phase, is associated with noise. A sound signal recorded during the cooling phase includes the noise generated by the evaporation process and thus allows conclusions to be drawn about the progress of the cooling process.

[0008] The cooling phase describes a temporal segment of an injection molding machine's working cycle. At the beginning of a working cycle, the injection mold is closed so that the cavity forms a closed hollow space. In an injection phase, plastic material in a liquid or pasty state is injected into the cavity so that the cavity is filled with the plastic material. The injection phase can last a few seconds, for example, between 0.2 s and 5 s. After the end of the injection phase, pressure is continued to be exerted from the feed unit onto the plastic material in the cavity during a post-pressing phase. The post-pressing phase ends when quantities of the plastic material adjacent to the feed unit have hardened, so that the pressure is no longer on quantities of plastic material removed from the feed unit. The length of the holding phase can, for example, be between 2 s and 50 s. The time remaining for the plastic material to harden is referred to as the residual cooling time. The length of the residual cooling time can, for example, be between 2 s and 50 s. When the plastic material has hardened sufficiently, the injection mold can be opened and the molded part removed. This completes one working cycle of the injection molding machine and the next working cycle can begin. The injection molding machine can be set up to repeat this sequence cyclically.

[0009] During the cooling phase, the temperature of the plastic material in the cavity drops. The cooling phase begins after the end of the injection phase and ends when the plastic material has reached its lowest temperature within the cavity. The cooling phase ends at the latest when the injection mold is opened to eject the finished injection-molded part. The feeding of liquid to the evaporation chamber can extend over a conveying phase. The conveying phase can correspond to part of the cooling phase. The end of the conveying phase can be before the end of the cooling phase. During the conveying phase, liquid can be fed to the evaporation chamber continuously or intermittently.

[0010] The liquid can be supplied to the evaporation chamber under high pressure. For example, the pressure of the liquid can be at least 5 bar, preferably at least 10 bar, more preferably at least 20 bar higher than the pressure in the evaporation chamber. In this way, it can be prevented that the liquid evaporates before entering the evaporation chamber. This information refers to the pressure in a liquid line via which the liquid is supplied to the evaporation chamber. In particular the specification refers to a section of the liquid line which is adjacent to an outlet opening through which the liquid exits into the evaporation chamber. The section of the liquid line can be formed by a capillary tube whose diameter can be between 0.5 mm and 2 mm, for example, and preferably between 0.8 mm and 1.2 mm. A section of the liquid line which is further away from the outlet opening can have a diameter which is significantly larger than the diameter of the capillary tube. The injection moulding machine can comprise a high-pressure pump with which the liquid is conveyed along the conveying line.

[0011] The evaporation process causes the temperature in the area surrounding the mold core to drop until the evaporation temperature of the liquid is reached. If liquid is added beyond this point, the liquid no longer evaporates and cooling to a value below the evaporation temperature takes place. The conveying phase can continue until the supplied liquid no longer evaporates in the evaporation chamber. In other words, the conveying phase can include the point in time at which the evaporation process ends.

[0012] The time at which the evaporation process ends is of particular interest in the context of the method according to the invention. The start of the conveying phase is generally known, since the liquid feed is initiated by a control signal. If the time at which the evaporation process ends is also known, the duration of the evaporation process can be determined, which can be of interest, for example, for determining suitable parameters for carrying out the method or for quality control purposes. The method can be carried out in such a way that the sound signal is recorded while the evaporation process ends . In other words, the recorded sound signal can cover a period of time in which the end of the evaporation process occurs .

[0013] The sound recording can include additional phases of the injection molding machine's operating cycle. For example, the sound signal can be recorded during the entire conveying phase. It is also possible for the sound signal to be recorded during the entire injection molding machine's operating cycle or for the sound signal to be recorded continuously across multiple injection molding machine operating cycles.

[0014] The injection molding machine can include a data memory in which the sound signal is saved. The data memory can, for example, be part of a control unit of the injection molding machine or part of an evaluation unit of the sound sensor. The storage can take place together with a time signal, so that it is possible to subsequently assign the sound signal or sections of the sound signal to an injection molding process. During an operating cycle of an injection molding machine, a sequence of noises is generated. In addition to the noise of the evaporation process, this includes, for example, a noise when the injection mold is closing, a noise caused by the injection process, a noise when the injection mold is opening, and a noise when the produced injection-molded part is ejected from the injection mold. During an injection molding process, these noises normally follow one another in time.In particular, the evaporation process takes place during a period in which the mold is neither opened nor closed, nor is a finished injection-molded part being ejected, nor is an injection process taking place. By correctly chronologically assigning the time, it is possible to determine which part of a longer sound recording belongs to the evaporation process.

[0015] The sound signal can be recorded using a sound sensor. The sound sensor can be arranged adjacent to the injection mold, so that a noise generated by the evaporation process stands out clearly in the sound signal compared to ambient noise. The injection molding machine can include a control unit configured to control the sound sensor so that a sound signal is recorded during desired phases of an injection molding process.

[0016] It is not necessary for the control unit to be a structural unit. For example, it is also possible for the injection molding process to be controlled by a first control module and the cooling device to be controlled by a second control module. It can be a master-slave configuration, with the first control module acting as the master and the second control module acting as the slave. The sound sensor can be controlled by one of these control modules. It is also possible for the sound sensor to be provided with its own control module.

[0017] The evaporation process is accompanied by a characteristic noise which fades away when the evaporation process ends. The end of the evaporation process therefore normally corresponds to a drop in the level of the sound signal. The control unit can be designed to conclude that the evaporation process has ended from a drop in the level of the sound signal. For example, the control unit can assume that the evaporation process has ended when the level of the sound signal has dropped from a maximum value reached during the evaporation process to a predetermined threshold value. The threshold value can, for example, be 40% below the maximum value.

[0018] As long as the liquid in the evaporation chamber is evaporating, the cooling effect depends directly on the evaporation temperature. If liquid is fed to the evaporation chamber beyond this point, further cooling of the plastic material would result from heating of the liquid. This would result in the temperature of the plastic material falling below the evaporation temperature. Such excessive cooling is undesirable because it can have a negative effect on the structure of the plastic material and thus on the quality of the injection-molded part. The process can therefore be carried out in such a way that the end of the evaporation process triggers the termination of the liquid supply to the evaporation chamber, thus ending the conveying phase.The period between the end of the evaporation process and the end of the conveying phase can be shorter than 5 s, preferably shorter than 2 s, more preferably shorter than 1 s.

[0019] The method can be carried out in such a way that the recorded sound signal is made available for manual evaluation via an evaluation unit of the sound sensor. An operator can use the recorded data to determine how long the evaporation process takes for a specific type of injection molding operation. The injection molding machine can then be configured so that the conveying phase has a suitable length for this type of injection molding operation. In this case, it is not necessary for the sound signal to be processed in the control unit of the injection molding machine during operation.

[0020] In one embodiment, the sound signal is fed back to a control unit of the injection molding machine, wherein the control unit is designed to influence the ongoing injection molding process based on the sound signal. The feed back of the sound signal to the control unit can be a Be part of a closed control loop. In one embodiment, the delivery phase is terminated within a closed control loop, within which the sound signal is used as a controlled variable. The delivery phase can be terminated, for example, by switching off the high-pressure pump used to supply the liquid to the evaporation chamber.

[0021] At the beginning of the conveying phase, atmospheric pressure may be present in the evaporation chamber, meaning that the evaporation process takes place at a temperature of the order of 100 °C. Alternatively, the evaporation temperature can be influenced by adjusting the pressure in the evaporation chamber. The process can be carried out in such a way that, at the beginning of the conveying phase, the pressure in the evaporation chamber differs from atmospheric pressure.

[0022] The pressure in the evaporation chamber at the start of the delivery phase can be higher than atmospheric pressure. This leads to a reduction in the cooling effect compared to a condition in which atmospheric pressure is present in the evaporation chamber. This is due on the one hand to the fact that the evaporation temperature increases with increasing pressure. On the other hand, the pressure difference compared to the pressure in the liquid line decreases, which has the effect that the amount of liquid entering the evaporation chamber is reduced. A pressure increase in the evaporation chamber resulting from the evaporation process can be reduced via a discharge line of the evaporation chamber, for example by arranging an overflow valve in the discharge line, which opens when the pressure at the overflow valve exceeds a predetermined threshold value.

[0023] Alternatively, at the beginning of the conveying phase, the pressure in the evaporation chamber may be lower than atmospheric pressure. The injection molding machine may include a vacuum pump to create such a pressure in the evaporation chamber. Negative pressure in the evaporation chamber increases the cooling effect.

[0024] The invention also relates to a plastic injection molding machine with an injection mold and with a feed unit for injecting a plastic material into a cavity of the injection mold. The injection mold comprises a mold core projecting into the cavity and provided with an evaporation chamber and a high-pressure pump arranged upstream of the evaporation chamber for conveying a liquid into the evaporation chamber. The injection molding machine further comprises a sound sensor which is designed to record a sound signal from noises generated in the injection mold while the high-pressure pump conveys liquid into the evaporation chamber.

[0025] The plastic injection molding machine can comprise a control unit. The control unit can be designed to control the interaction of the components during a working cycle of the plastic injection molding machine. For this purpose, the control unit can provide control signals that trigger one or more of the following steps. The steps can be carried out in the order specified below or in a different order. The control unit can control an actuator with which the injection mold is brought into a closed state. The control unit can control the feed unit so that, in an injection phase, a plastic material is injected into the cavity of the injection mold. The control unit can control the feed unit so that, in a post-pressure phase following the injection phase, pressure is exerted on the plastic material in the cavity. The control unit can control the high-pressure pump so that liquid is continuously pumped into the evaporation chamber during a pumping phase. The control unit can control the high-pressure pump so that the pumping process is ended. The control unit can control the actuator of the injection mold so that the injection mold is opened. The control unit can control a sound sensor to start or stop a sound recording. The control unit can control the high-pressure pump depending on a recorded sound signal.

[0026] The cooling device may comprise a liquid reservoir from which the liquid is supplied to an outlet opening leading into the evaporation chamber. The liquid may be supplied through a liquid line extending between the liquid reservoir and the outlet opening. The injection molding machine may comprise a high-pressure pump with which the liquid is conveyed to the outlet opening.

[0027] The mold core can extend from a proximal end adjoining the body of the injection mold to a distal end. The distal end can form a free end in the cavity. The advantages of the invention are particularly evident when the mold core is of slim design. A distal section of the mold core adjacent to the distal end preferably has a largest diameter of no more than 20 mm, preferably no more than 10 mm, more preferably no more than 7 mm. The largest diameter is generally not less than 3 mm. The section adjacent to the distal end can extend over at least 30%, preferably at least 50%, more preferably at least 70% of the length that the mold core projects into the cavity.

[0028] The evaporation chamber can be arranged adjacent to the distal end of the mold core. A first channel can be formed within the mold core, through which the liquid is supplied to the evaporation chamber. The first channel forms a section of the liquid line between the liquid supply and the outlet opening. A second channel can be formed within the mold core, through which the gas-liquid mixture is discharged from the evaporation chamber. The cross-sectional area of ​​the second channel can be larger than the cross-sectional area of ​​the first channel, preferably at least by a factor of two, more preferably at least by a factor of five, further preferably at least by a factor of ten. In one embodiment, the first channel extends inside the tube and the second channel inside the annular channel. The reverse design is also possible.

[0029] The disclosure includes further developments of the method with features that are described in connection with the plastic injection molding machine according to the invention. The disclosure includes further developments of the plastic injection molding machine that are described in connection with the method according to the invention.

[0030] The invention is described below by way of example with reference to the accompanying drawings using advantageous embodiments. They show: Fig. 1: a schematic representation of an injection molding machine according to the invention; Fig. 2: the mold core of the injection molding machine from Fig. 1 in an enlarged view; Fig. 3: a schematically illustrated sound signal during a working cycle of the injection molding machine; Fig. 4: a block diagram of an inventive injection molding machine; Fig. 5, 6: the representation according to Fig. 3, 4 in an alternative embodiment of the invention; Fig. 7: the representation according to Fig. 4 in an alternative embodiment of the invention.

[0031] An injection molding machine shown in Fig. 1 comprises a frame 14 which supports an injection mold 15 consisting of two mold halves 11, 12. In Fig. 1, the injection mold 15 is shown in the open state. To close the injection mold 15, the two mold halves 11, 12 are moved towards one another so that a completely enclosed cavity 16 is formed inside the injection mold 15. A mold core 18 connected to the first mold half 11 projects into the cavity 16. The injection molding machine comprises actuators 58 (Fig. 4) with which one or both mold halves 11, 12 can be moved in a suitable manner relative to the frame 14.

[0032] After the injection mold 15 is closed, a piston screw 13 of a feed unit 17 is started to inject a plastic material in a liquid state into the interior of the injection mold 15 in an injection phase, so that the cavity 16 is completely filled with the plastic material. After the end of the injection phase, the cooling phase begins, in which the temperature of the plastic material drops by releasing heat to the environment. At the beginning of the cooling phase, the piston screw maintains the pressure acting on the plastic material in a post-pressing phase until the plastic material has hardened in an area 40 between the feed unit 17 and the cavity 16. The plastic material then hardens completely in the cavity 16 without any external pressure being exerted. After the plastic material has completely hardened, the injection mold 15 is opened. An injection molded part whose shape corresponds to the cavity 16 is removed from the injection mold 15. The injection molded part has a recess that corresponds to the mold core 18.

[0033] During a section of the cooling phase, heat is specifically extracted from the plastic material in the vicinity of the mold core 18. For this purpose, a blind bore is formed inside the mold core 18, which extends almost to the distal end of the mold core 18, see Fig. 2. The end of the bore is formed by an evaporation chamber 28 arranged inside the mold core 18. A capillary tube 30 extends in the center of the bore, at the distal end of which an outlet opening 43 is formed, which opens into the evaporation chamber 28. A proximal end of the capillary tube 30 is connected to a supply line 32. An annular return channel is formed between the capillary tube 30 and the wall 29 of the mold core 18, which is connected to a return line 33.

[0034] The supply line 32 and the return line 33 are connected to a central module 19, which together with the evaporation chamber 28 and other components forms a cooling device of the injection molding machine. The central module 19 comprises a high-pressure pump 42, which is connected to the other end of the supply line 32. The high-pressure pump 42 is designed, in a delivery phase, to deliver water towards the capillary tube 30 under a high pressure, which may, for example, be 20 bar higher than atmospheric pressure. The water emerges as a fine jet from the outlet opening 43 of the capillary tube 30 and is distributed in the evaporation chamber. 28 . The temperature in the evaporation chamber 28 and in the distal region of the annular return channel is so high that the liquid evaporates. The evaporation process removes heat from the plastic material in the vicinity of the mold core 18.

[0035] In this exemplary embodiment, an evaporation process at atmospheric pressure would cool the plastic material too much. A pressure control unit 57 is therefore used to apply a pressure in the evaporation chamber 28 that is higher than atmospheric pressure. This has a twofold effect. Firstly, as the pressure increases, the evaporation temperature increases, whereby the temperature difference compared to the plastic material decreases. Secondly, the amount of liquid emerging from the capillary tube 30 is reduced. Both of these effects result in less heat being extracted from the plastic material.

[0036] In order to be able to apply the desired pressure in the evaporation chamber 28, a pressure source 27 is connected to the return line 33 via an adjustable pressure reducer 26 and a first valve combination 25. The first valve combination 25 comprises a check valve and a switchable shut-off valve which, when open, provides a free passage between the return line 33 and the check valve and which, when closed, closes the passage. In the pressure source 27, which can be, for example, a pressure accumulator or a pressurized line, there is a pressure PI which is reduced to a lower pressure P2 by the adjustable pressure reducer 26. The pressure P2 specified by the pressure reducer 26 spreads, when the valve combination 25 is open, via the return line 33 to the evaporation chamber 28, provided that a pressure is present in the return line 33 and the evaporation chamber 28 which is lower than the pressure P2.Conversely, the pressure in the return line is. 33 is higher than the pressure P2 specified by the pressure reducer 26 , the check valve of the valve combination 25 closes and no pressure equalization takes place.

[0037] At the start of a working cycle of the injection molding machine, atmospheric pressure prevails in the evaporation chamber 28. After the first valve combination 25 opens, the pressure P2 spreads into the evaporation chamber 28. The cooling phase is accompanied by active cooling, with water being pumped into the evaporation chamber by the high-pressure pump 42. The pressure P2 is set such that heat is removed from the plastic material in the vicinity of the mold core 18 without the structure of the plastic material being damaged by excessive cooling.

[0038] During the conveying process, the pressure in the evaporation chamber rises because the pressure at which the liquid is conveyed into the evaporation chamber is higher than the pressure P2 present in the evaporation chamber. Furthermore, the evaporation process is accompanied by an expansion which also causes a pressure increase in the evaporation chamber 28. The increased pressure spreads through the return line 33 and causes the check valve of the first valve combination 25 to close. A second valve combination 24 is arranged in the return line 33 and comprises an overflow valve and a switchable shut-off valve connected in parallel. The shut-off valve is closed. The overflow valve has an opening pressure which is somewhat higher than the pressure P2 set by the pressure reducer 26, so that the overflow valve opens quickly when the pressure in the evaporation chamber 28 increases.Therefore, even during the delivery phase, the pressure in the evaporation chamber 28 does not increase significantly.

[0039] After the end of the delivery phase, the second valve combination 24 is opened and the first valve combination 25 is closed, so that pressure equalization to atmospheric pressure takes place via the return line 33. The water-gas mixture is returned from the evaporation chamber 28 to the central module 19. Any water contained in the mixture is condensed in the central module 19 and can be reused in a subsequent delivery phase.

[0040] The supply line 32 arranged between the central module 19 and the evaporation chamber 18 comprises a branch to which a compressed air line 22 is connected. After the pressure in the evaporation chamber 28 has equalized to atmospheric pressure, a burst of compressed air can be directed from the central module 19 into the compressed air line 22, which propagates through the capillary tube 33 to the evaporation chamber 28, so that any remaining liquid residues are blown out through the return line 33. In this way, defined starting conditions for the subsequent working cycle of the injection molding machine can be created, if desired.

[0041] A check valve 21 is arranged in the supply line 32 adjacent to the mold core 18. The check valve 21 becomes effective when, at the beginning of a working cycle of the injection molding machine, the pressure equalization between the pressure P2 of the pressure reducer and the evaporation chamber 28 is carried out. The pressure P2, which is higher than atmospheric pressure, only spreads as far as the check valve 21. In this way, the liquid can more easily penetrate to the evaporation chamber 28 in the subsequent conveying phase.

[0042] The injection molding machine comprises a control unit 41 which controls the interaction of the components. At the beginning of a working cycle of the injection molding machine, the injection mold 15 is closed and the first valve combination 25 is opened so that the pressure in the evaporation chamber 28 increases from atmospheric pressure to the pressure P2 set by the pressure reducer 26. The feed unit 17 is activated to fill the cavity 16 to fill the injection mold 15 with plastic material. The pressure exerted by the feed unit 17 is maintained during the subsequent pressing phase. The high-pressure pump 42 is activated so that during the cooling phase water is pumped into the evaporation chamber 28 and evaporates there. The injection mold 15 is opened so that the injection-molded part can be removed. At the same time, the first valve combination 25 is closed and the second valve combination 24 is opened in order to equalize the pressure in the evaporation chamber 28 to atmospheric pressure via the central module 19. A blast of compressed air emitted by the central module 19 creates defined starting conditions for the subsequent working cycle of the injection molding machine.

[0043] The evaporation process creates a characteristic noise that propagates across the wall of the mold core 18 and the structure of the injection mold 15, so that a sound signal is emitted from the injection mold 15 into the environment. A sound sensor 35 is arranged adjacent to the injection mold 15 and records the noise generated during the injection molding process.

[0044] In Fig. 3, a sound signal 38 recorded during an injection molding process with the sound sensor 35 is shown schematically. The level A of the sound signal 38 is plotted against time T. In a first phase 51 of the injection molding process, the injection mold 15 is closed, which causes a short noise with a high level A. In a second phase 52 (injection phase), the feed unit 17 is put into operation in order to supply plastic material to the cavity 16 of the injection molding mold 15, which is accompanied by a longer noise of medium level A. During a third phase 53 (feeding phase), water is fed to the evaporation chamber 28, where it evaporates and causes a longer lasting noise at a higher level. The evaporation noise ends as soon as the temperature in the area around the mold core 18 is no longer higher than the evaporation temperature of the water. The energy required for evaporation is then no longer available in the area around the mold core 18. In a fourth phase 54, in which no particular noise is generated, the plastic material continues to cool without the cooling being supported by an evaporation process. The third phase 53 and the fourth phase 54 together form the cooling phase of the injection molding process.Further noises arise in a fifth phase 55 through the opening of the injection mold 15 and in a sixth phase 56 through the ejection of the finished injection molded part.

[0045] If water were supplied to the evaporation chamber 28 beyond the end of the evaporation process, the plastic material in the vicinity of the mold core 18 would be cooled to a temperature below the evaporation temperature, which is undesirable. Therefore, the injection molding machine according to the invention features a control circuit with which the conveying phase 53 is terminated as a function of the sound signal 38.

[0046] The interaction of the components of the injection molding machine during the injection molding process is subject to the control of the control unit 41, see Fig. 4. After the desired pressure in the evaporation chamber 28 has been set via the pressure control unit 57, the actuator 58 is controlled in the first phase 51 in order to close the injection mold 15. In the injection phase 52, the feed unit 17 is activated in order to fill the cavity 16 of the injection mold 15 with plastic material. In the delivery phase 53, the high-pressure pump 42 receives a Control signal according to which water is pumped into the evaporation chamber 28. During the pumping phase 53, the sound signal 38 recorded by the sound sensor 35 is evaluated in the control unit 41. After the level A of the sound signal 38 has reached a maximum value 59, the control unit 41 waits until the level A of the sound signal 38 has dropped to an intermediate value 60 which is 40% lower than the maximum value 59. It is assumed that when this condition occurs, the end 62 of the evaporation process is reached. The control unit 41 reacts by switching off the high pressure pump 42 with a further control signal. After the remaining cooling time in the fourth phase 54 has elapsed, the control unit 41 emits further control signals in order to reopen the injection mold 15 and eject the finished injection molded part.

[0047] The sound signal 38 is stored in a data memory 61 and made available in a form that allows it to be assigned to a specific injection molding process. This also allows for subsequent verification of whether the duration of the evaporation process was in accordance with the specifications.

[0048] 5 and 6 show an alternative embodiment of the invention. The return line 33 coming from the evaporation chamber 28 opens into the environment, so that atmospheric pressure is present in the evaporation chamber 28. The sound signal 38 is not recorded during the entire injection molding process 51-56, but only in the third phase 53, in which the evaporation noise occurs. The control unit 41 activates the high-pressure pump 42 and the sound sensor 35 simultaneously. As soon as the level A of the sound signal 38 has dropped to the intermediate value 60, the control unit 41 switches both the high-pressure pump 42 and the sound sensor 35 off again. The sound signal 38 stored in the data memory 61 is limited to the part of the injection molding process 51-56 that is important for quality documentation.

[0049] In Fig. 7 an embodiment is shown in which the sound sensor 35 detects the sound signal 38 during the entire injection molding process. The sound signal 38 is stored in an evaluation unit 63 of the sound sensor 35. In a configuration phase, several injection molding processes are carried out and the associated sound signals 38 are recorded. An operator calls up the sound signals 38 from the evaluation unit 63 and evaluates them manually in order to determine a duration of the conveying phase 53 that is suitable for the respective injection molding process. The control unit 41 of the injection molding machine is then configured so that the conveying phase 53 has a fixed, predetermined length that results from the operator's findings.

Claims

Patent claims 1. Method for monitoring an injection molding process in which a plastic material is injected into a cavity (16) of an injection mold (15) and in which the plastic material is kept in the cavity during a cooling phase (53, 54) (16), in which the injection mold (15) has a mold core (18) projecting into the cavity (16) and provided with an evaporation chamber (28), wherein during the cooling phase (53, 54) a liquid is conveyed into the evaporation chamber (28) so that the liquid evaporates in the evaporation chamber (28), and wherein during the cooling phase (53, 54) a sound signal (38) of noises generated in the injection mold (15) is recorded, wherein a drop in the level of the sound signal (38) is closed to the end (62) of the evaporation process.

2. The method according to claim 1, wherein liquid is supplied to the evaporation chamber (28) during a conveying phase (53) and wherein the conveying phase (53) ends before the cooling phase (53, 54).

3. The method according to claim 2, wherein the conveying phase (53) is continued until the supplied liquid no longer evaporates in the evaporation chamber (28).

4. Method according to one of claims 1 to 3, wherein the recorded sound signal (38) comprises a period of time in which the end (62) of the evaporation process falls.

5. The method according to claim 4, wherein the end (62) of the evaporation process is inferred from a drop in the level of the sound signal (38).

6. Method according to one of claims 2 to 5, wherein the end (62) of the evaporation process triggers the termination of the conveying phase (53).

7. Method according to one of claims 2 to 6, wherein the sound signal (38) is made available for manual evaluation via an evaluation unit (36) of the sound sensor (35).

8. Method according to one of claims 2 to 7, wherein the conveying phase (53) is terminated within a closed control loop within which the sound signal (38) is used as a controlled variable.

9. Method according to one of claims 1 to 8, wherein at the beginning of the conveying phase (53) a pressure different from atmospheric pressure is present in the evaporation chamber (28).

10. The method according to claim 9, wherein the pressure in the evaporation chamber (28) at the beginning of the conveying phase (53) is higher than atmospheric pressure.

11. Plastic injection molding machine with an injection mold (15), a feed unit (17) for injecting a plastic material into a cavity (16) of the injection mold (15) , wherein the injection mold (15) has a cavity (16) and provided with an evaporation chamber (28), with a high-pressure pump (42) for conveying a liquid into the evaporation chamber (28), with a sound sensor (35) which is designed to record a sound signal (38) of noises generated in the injection mold (15), with a control unit (41) which is designed to control the sound sensor (35) in such a way that a sound signal (38) is recorded, while the high-pressure pump (42) conveys liquid into the evaporation chamber, and wherein the control unit (41) is designed to conclude the end (62) of the evaporation process from a drop in the level of the sound signal (38).

Images