DE102018131126A1 - Process monitoring in the production of injection molded parts - Google Patents

Abstract

The following describes a measuring system for monitoring a casting process. According to one embodiment, the measuring system has an ultrasound transducer, which is designed to couple ultrasound pulses into a casting tool. The measurement system further comprises a sensor circuit coupled to the ultrasound transducer and configured to trigger the ultrasound transducer with a drive signal having a sequence of pulses for generating the ultrasound pulses, and one of the ultrasound transducer (or a further ultrasound transducer) received signal, which represents reflected in the casting tool ultrasonic pulses to process. The received signal has a sequence of pulses corresponding to the drive signal, each comprising a main pulse and ringing. The processing of the received signal includes, inter alia-for each pulse of the received signal-generating a first measured value representing the amplitude of the main pulse and a second measured value representing the amplitude of the ringing.

Description

TECHNICAL AREA

The present description relates to the monitoring of an injection molding process by means of an ultrasonic sensor.

BACKGROUND

Injection molding is one of the most important manufacturing processes in plastics processing. Increasingly high quality demands on the manufactured components with regard to mechanical and / or optical properties, the minimization of waste (keyword "zero defect production") as well as the independent optimization (ie the shortening of production times or the automated compensation of variable external influences) are increasingly playing a role to meet required quality requirements.

A so-called shaping unit consists in principle of an injection molding machine and the injection mold. While the injection molding tool has at least one negative mold - the so-called cavity - of the molded part to be produced, the injection molding machine processes the usually granular plastic base material into a flowable melt and injects this material under high pressures into the mold, where it cools down in the cavity (or in reaction is brought). In many cases, the finished molded part can be removed from the mold (demoulded). The injection molding tool and the injection molding machine are often a purely structural unit without any connection by means of sensors.

If it is not possible to dispense with monitoring the process by means of sensors - be it for reasons of the required / prescribed quality assurance or because sensor data are needed to control the injection molding process - in-mold pressure sensors and molded part temperature sensors have become established on the market.

For an accurate measurement, however, it is usually necessary that the sensor head touches the plastic molded part, for which a hole in the cavity is necessary. In order to guarantee a faultless measurement and tightness of the cavity at high pressures, strict tolerance requirements are imposed on the bores in the tool, which results in corresponding costs. The resulting impression of the sensor head which is visible on the finished molded part is a reason for exclusion for many applications in which a measurement would make sense, such as optical components (for example, lenses, headlamp housing, etc.).

In addition, sensor data from in-mold pressure sensors are typically used to monitor the process, which in the best case does not necessarily correlate with the product characteristics of the finished molded part. As a rule, the standard molded part temperature sensors are not suitable for quantitatively determining the cooling rate (cooling rate) which is important for the product properties. The high pressures in injection molding require massive sheathing of the actual sensor element (e.g., thermocouples), resulting in high second-order response times. The sensor head is also in contact with the (tempered) tool, which can also falsify the result of the temperature measurement.

One promising option for simultaneously monitoring process and product properties during injection molding is the use of pulsed ultrasonic sensor systems. In a variety of publications (eg DE 197 37 276 C2 . DE 198 34 797 C2 . EP 2 657 801 A2 , DE 20 2012 008 359 Ul, US 5,951,163 A . WO 03/089214 A2 For example, direct or indirect measurement of sound velocity and sound attenuation in a reflecting assembly (ie, ultrasonic transmitter = ultrasonic receiver) or a transmission assembly is described. The direct coupling of viscoelastic or thermodynamic properties with the sound damping or speed of sound theoretically allows a large number of conclusions regarding the process and material condition during injection molding. As problematic, the strong attenuation of the ultrasound can be seen in plastic melts; the damping can make the determination of acoustic characteristics difficult or impossible, for example, with larger wall thickness and in the presence of fillers (eg glass fibers, talc, etc.) in the plastic melt. Therefore, this type of ultrasonic measurement in injection molding has also found no relevant distribution in industrial practice.

In the publications US 7,017,412 B2 It is described to circumvent the above-mentioned problems of pulsed ultrasound systems by a continuous ultrasound system. However, the results here are always strongly dependent on the local conditions (eg distance to the cavity, cavity thickness, coupling efficiency, individual sensor characteristics, etc.), and for each installation situation and tool temperature a resonance frequency must be determined on the system, which makes such a system less efficient industrial practice. In the publication US 5,951,163 For the first time, the possibility of using simple reflectance measurements for flow front detection is qualitatively reported for the first time, with a quantitative evaluation methodology as well as proposals for system calibration There is a lack of sensor aging effects and a decrease in coupling efficiency, which is indispensable for long-term measurements in industrial practice.

The inventors have set themselves the task to improve the signal processing / evaluation in pulsed ultrasonic reflection measurements in injection molds, so that they can be used for a larger number of plastics (with or without fillers) and molding wall thicknesses for process control and product monitoring and further sensor aging effects and changing the coupling efficiency of the sensor to the tool can be compensated by suitable signal normalization.

SUMMARY

The above object is achieved by the measuring system according to claim 1 or the method according to claim 6. Various embodiments and further developments are the subject of the dependent claims

The following describes a measuring system for monitoring a casting process. According to one embodiment, the measuring system has an ultrasound transducer, which is designed to couple ultrasound pulses into a casting tool. The measurement system further comprises a sensor circuit coupled to the ultrasound transducer and configured to trigger the ultrasound transducer with a drive signal having a sequence of pulses for generating the ultrasound pulses, and one of the ultrasound transducer (or a further ultrasound transducer) received signal, which represents reflected in the casting tool ultrasonic pulses to process. The received signal has a sequence of pulses corresponding to the drive signal, each comprising a main pulse and ringing. The processing of the received signal includes, inter alia-for each pulse of the received signal-generating a first measured value representing the amplitude of the main pulse and a second measured value representing the amplitude of the ringing.

Another embodiment relates to a method comprising: injecting a sequence of ultrasonic pulses into a casting tool; receiving a corresponding sequence of reflected ultrasound pulses and generating a received signal having a corresponding sequence of pulses each having a main pulse and a ringing. For each pulse of the received signal, a first measured value and a second measured value are generated, which respectively represent the amplitude of the main pulse or the amplitude of the ringing.

list of figures

• 1 enthält Zeitdiagramme zur Illustration von Ultraschallpulssignalen und deren Hüllkurve.
• 2A ist ein Zeitdiagramm zur Illustration eines von einem Ultraschallempfänger empfangenen Ultraschallpulses, der an der Wand einer Kavität eines Spritzgusswerkzeugs reflektiert wurde.
• 2B illustriert die Entstehung von Interferenzen durch Mehrfachreflexion in der Wandschicht des abkühlenden Kunststoffformteils.
• 3 ist ein Blockdiagramm zur Illustration eines Beispiels einer Schaltung zur Signalverarbeitung und Signalauswertung eines reflektierten Ultraschallpulses.
• 4 enthält Zeitdiagramme zur Illustration der resultierenden Messsignale im Verlauf eines Spritzgießvorganges.
• 5 illustriert den messbaren Einfluss der Temperatur des Spritzgießwerkzeugs auf das Wachstum der Randschicht des abkühlenden Kunststoffformteils.

Embodiments will be explained in more detail with reference to figures. The illustrations are not necessarily to scale, and the embodiments are not limited to only the aspects illustrated. Rather, it is important to represent the underlying principles of the embodiments. In the pictures shows:

• 1 contains timing diagrams to illustrate ultrasound pulse signals and their envelope.
• 2A FIG. 4 is a timing diagram illustrating an ultrasonic pulse received from an ultrasound receiver that has been reflected on the wall of a cavity of an injection molding tool. FIG.
• 2 B illustrates the generation of interference by multiple reflection in the wall layer of the cooling plastic molding.
• 3 is a block diagram illustrating an example of a circuit for signal processing and signal evaluation of a reflected ultrasonic pulse.
• 4 contains timing diagrams to illustrate the resulting measurement signals in the course of an injection molding process.
• 5 illustrates the measurable influence of the temperature of the injection mold on the growth of the surface layer of the cooling plastic molding.

DETAILED DESCRIPTION

Before referring to the figures in more detail, some general aspects of the embodiments described here will be explained first. According to the embodiments described herein, short ultrasonic pulses are repeatedly generated by means of an ultrasonic transducer and emitted in the direction of the cavity of the injection molding tool. The ultrasound transducer can be arranged in the injection mold or on its surface. The injection molding tool is usually made of metal, eg steel. Before the plastic melt is injected, there is air in the cavity. Due to the very high density differences between metal and air, the sound energy is almost completely reflected at the metal / air boundary layer (ie at the inner wall of the air-filled cavity). The reflected signal can be received by the same ultrasonic transducer and converted into electrical signal. This signal can now be received by a signal processing unit further processed and evaluated. According to the examples described here, for example, a measured value is determined which represents the amplitude of the reflected signal (a so-called pulse or burst). For an air-filled cavity (before injection of the plastic melt), this measured value can be used as reference value; Further measurements during a spiking process are normalized (scaled) with this reference value. This eliminates the aging effects of the ultrasonic transducer and the coupling media, temperature differences in the tool steel, and other potential effects, allowing comparison of readings over long periods of time.

During the injection molding process plastic melt is injected into the cavity of the tool under pressure, whereby the plastic successively displaces the air from the mold and fills the cavity. As soon as the plastic melt reaches the position at which the ultrasound pulse is reflected during this process, a reduction of the aforementioned measured value, which represents the amplitude of the reflected signal, can be observed since a part of the sound energy is now transmitted through the boundary layer metal / plastic ( ie by the now covered with plastic inner wall of the cavity) is transmitted into the plastic melt inside. In practice, about 1-5% of the coupled sound energy can be transmitted into the plastic melt.

The above-mentioned normalization to the measured amplitude of the reflected signal in the case of an air-filled cavity essentially corresponds to the calculation of the reflection coefficient R on the inner wall of the cavity, which indicates how much of the injected acoustic energy will be reflected at an interface. Because this reflection coefficient R In the case of a metal / air boundary layer, practically 1 (100%), it can be easily determined by expressing the measured values of the amplitudes of the reflected signal as a multiple of the mentioned reference value, which represents the amplitude of the reflected signal in air-filled cavity. A normalized measurement of 1 thus corresponds to a reflection factor of 100% and indicates an air-filled cavity. A normalized measured value of less than 1 (eg between 0.95 and 1) indicates a plastic-filled cavity. Using the reflection coefficient R and its change can be made conclusions about the density of the plastic during the injection molding process. Ultrasonic transducers may be located at multiple locations of an injection molding tool, permitting measurements at different positions of the cavity. In such applications, the ultrasonic measurements can be used for example for detecting the flow front of the plastic melt.

In addition to the detection of the flow front of the plastic melt in the cavity, the time of complete filling of the cavity with plastic melt can also be determined by suitable signal processing. If the cavity is completely filled with plastic melt, a pressure increase in the plastic melt leads to a density increase and thus to a further reduction of the reflection coefficient. The time of complete filling of the cavity is an important process parameter in injection molding. At this time, usually switched by speed-controlled injection of the plastic melt to the so-called "reprinting". Typically, a (e.g., constant) pressure is applied to the plastic melt in the cavity to reduce volume contraction due to cooling of the plastic. The ultrasound measurements can be used to control the switchover time. Too early switching can result in incompletely filled cavities and too late switching overfilling and thus burr formation on the finished molded part at the mold parting plane or in the worst case for overmoulding the tool, whereby the tool halves open at the parting planes during the process; This can lead to serious damage to the tool.

With the aid of the concept described here, the optimum switching time can be detected with the aid of the ultrasonic measuring system, and e.g. be signaled by emitting a trigger signal to the injection molding machine. According to one embodiment, the mentioned switching time can be detected by detecting that the normalized measured value for the signal amplitude of the reflected signal falls below a threshold value.

The measured values for the amplitude of the reflected signals (ultrasound pulses) determined by the measuring system can also be used to assess the homogeneity of the plastic melt with regard to undesired gas inclusions or unmelted plastic granules. These two defects are mainly caused by moist plastic base material (granules) or by errors in the plasticization of the base material and lead to serious optical and / or mechanical errors in the produced plastic molded part. Gas inclusions result in an increase in the reflection coefficient (due to a decrease in density in comparison to the plastic melt) and lead to an increase in the amplitude in the reflected signal. Conversely, unmelted granules lead to a reduction in the reflection coefficient (due to an increase in density compared to the plastic melt).

Another detectable quantity is the time of the mold part being shut down from the cavity wall. The increasing cooling of the molding allows from a certain time no further transport of plastic melt in the auskühlende molding by the applied emphasis. As a result, the volume contraction can not be compensated by the cooling and the molding shrinks and dissolves from the inner wall of the cavity. This point in time is of importance because the cooling effect on the molding drastically deteriorates as a result of the forming air layer between the cavity wall and the molding, and further residence in the cavity may unnecessarily prolong the cycle time. The time of the downshift is detectable by a sudden increase in the measured value representing the amplitude of the reflected signal.

Additional information that can be determined using the concept described herein is the thickness and growth rate of the cooling edge layer of the molding in the cavity. In the interior of the plastic, in the vicinity of the cavity wall, a further reflection of the ultrasonic signal occurs at the boundary layer between solidified plastic and plastic melt. This further reflection can be constructively or deconstructively superimposed on a part of the main reflection (which takes place on the cavity wall), whereby interferences can be observed when evaluating the measured values for the signal amplitudes. These interferences provide information about the growth rate of the already hardened surface layer of the molding in the cavity. A change in the growth rate in the molded part can give indications of a malfunction of the tool cooling (see also 5 ). Due to the concept of monitoring a casting process described here, deviations from a desired profile during edge layer growth of the solidifying melt can be detected. Such deviations may, as described, indicate inhomogeneities (eg bubbles, incompletely molten plastic granulate, etc.) in the melt or incorrect cooling of the casting tool.

The timing diagrams in 1 serve to illustrate an ultrasonic pulse and an example of the measurement of the signal amplitude. The diagram (a) in 1 shows an ultrasonic pulse, in fact the waveform (ie the instantaneous value of the signal depending on the time) of an electrical signal At) which is converted and radiated by an ultrasonic transducer into an acoustic signal or which is received by the ultrasonic transducer and generated by converting the received (reflected) acoustic signal. A pulsed ultrasonic signal usually consists of a sequence of pulses. Due to the limited bandwidth of real ultrasonic transducers, a pulse usually has the waveform of a burst signal (Burst for short), ie a pulse includes a few oscillations with a rapidly increasing and then falling amplitude, the frequency of the oscillation such as Resonant frequency of the transducer corresponds. In 1 is the maximum amplitude (approximately in the middle of a pulse) with A _p designated.

The mentioned reflection coefficient R can (neglecting the attenuation of the sound wave) as the quotient A _R / A _{T of} the amplitude A _T an emitted ultrasonic pulse and the amplitude A _R be defined of the corresponding reflected ultrasonic pulse. Considering the attenuation, R is equal to (A _R / A _T ) * e ^{-2L * α} (α> 0), where L is the distance between ultrasonic transducers 8th and the reflecting cavity wall, and α represents a damping coefficient (see also Figs 2 B) , Because like in 1 represented the amplitude of an ultrasound pulse At) is not constant, instead of the amplitude and the maximum amplitude A _p be used for the determination of the reflection coefficient. One way of metrological determination of the maximum amplitude A _p a burst signal is to first rectify the signal and then low pass it. The rectified signal | A (t) | is in diagram (b) the 1 and the filtered rectified signal in diagram (c) of FIG 1 , This filtered rectified signal also represents the envelope of the ultrasound pulse and is referred to herein as env {| A (t) |}. The amplitude of the envelope corresponds to the maximum amplitude A _p of the ultrasound pulse At) , ie A _P = max {env {| A (t) |}}. The amplitude of the envelope can therefore be used as a measured value for the amplitude of the ultrasound pulse At) used (and consequently also for the determination of the reflection coefficient).

Alternatively, the hatched area under the envelope env {| A (t) |} can also be used as a measure of the amplitude. As the pulse duration t _p is substantially constant and given by the properties (including bandwidth) of the transducer, the area is approximately proportional to the maximum amplitude A _p , This area can also be used as a measure of the signal energy (strictly speaking, the square root of the signal energy) of the ultrasound pulse At) be interpreted. A measured value for the amplitude of the ultrasound pulse At) can therefore be determined by the envelope signal env {| A (t) |} over the pulse duration t _p is integrated. Instead of rectification (diagram (b) off 1 ) alternative could be the signal At) also be squared. In this case, the obtained measured value would be proportional to the amplitude square A _P ² the envelope (ie the signal energy), which nonetheless a suitable measured value for the amplitude of the ultrasound pulse At) is.

2A shows a more realistic waveform of an electrical signal generated by an ultrasound transducer u (t) (Eg, an electrical voltage), which represents the waveform of a received ultrasonic pulse, which was previously reflected on the cavity wall of an injection molding tool. How to get in 2A can see points the signal u (t ) (reflected ultrasound pulse) has two distinct portions, a first portion called the "main pulse" and a second portion called "ringing". The main pulse has a pulse length t _H , eg the duration t _H a few (eg the first three) halfwaves of the signal u (t) include. The ringing has its origin in the finite bandwidth of the ultrasonic transducer and the duration of ringing is with t _N designated. In the area of this reverberation phase, multiple reflections in the solidifying surface layer of the molded part cause interference (see also FIG 2 B) ; These multiple reflections have a different cause than the ringing. The end of the second section is not clearly visible on the signal, as the ringing decays slowly. However, for example, one may define the end of the second section as the time at which the amplitude of the oscillation has fallen below some threshold (eg, 10% of the maximum amplitude). For example, the time duration t _N a certain number, eg ten) of half waves after the main pulse. The times t _H and t _N are for a particular measurement setup (pulse generator, ultrasonic transducer, geometry of the tool and the cavity, etc.) approximately constant sizes and are therefore known system parameters.

During the injection molding process, ultrasonic pulses are continuously generated, reflected at the cavity wall (main pulse) and reflected back to the transducer. Some process parameters in injection molding can be identified by the change in the amplitude of the main pulse. Other interesting parameters can be derived from the change in the amplitude of the ringing. For example, the following process parameters can be detected by evaluating the main pulse: time of flow front detection, time of complete filling of the cavity with plastic melt, checking / detection of the time of beginning of the holding pressure phase, duration and end of the holding pressure phase, time of the mold part being lowered off the cavity wall, Assessment of the homogeneity and density of the melt. The evaluation of the ringing allows the monitoring of the surface layer of the cooling plastic melt (edge layer growth rate or edge layer thickness).

2 B illustrates the generation of the mentioned interferences in the ringing phase of an ultrasound pulse. 2 B shows a sketch of the ultrasound transducer 8th , a tool 7 with a cavity 9 , which is filled with plastic melt P. When cooling / solidification of the plastic forms in the molding one of the wall 9 ' the cavity 9 outgoing growing boundary layer P 'out. That is, the thickness d of the peripheral layer increases during the cooling time. As in 2 B A part of the ultrasonic power is sketched on the wall 9 ' reflected. The unreflected part penetrates into the boundary layer P ' and at the other end of the boundary layer (at the transition solid / liquid) is reflected back. The part of the sound reflected in the boundary layer has a longer path (twice the thickness d) than the sound at the cavity wall 9 ' is reflected, which is a transit time difference of .delta.t of 2d / c _p corresponds to ( c _p denotes the speed of sound in the boundary layer P ' ). The runtime difference .delta.t is thus proportional to the thickness d of the surface layer P ' and becomes larger during cooling (since the surface layer P ' gets thicker). In contrast, the runtime 2L / c _s remains in the steel wall 71 of the tool 7 constant (cs denotes the speed of sound in the wall of the tool, L the wall thickness). In practice, the transit time difference .delta.t shorter than the duration of an ultrasound pulse, so that of the transducer 8th Pulse received a superposition of the two reflections (on the cavity wall 9 ' and in the outer layer P ' ), this superposition occurring essentially in the ringing phase, which occurs in 2 B as interference area I is designated. The change of interference and its meaning will be discussed later with reference to 4 and 5 discussed.

3 is a block diagram showing an example of a possible implementation of a sensor circuit for the ultrasonic measurements described above in an injection mold. It is understood that there are many other ways to implement the same or an equivalent function in terms of circuitry. In view of the above and the following explanations, a person skilled in the art will readily be able to implement the illustrated or other suitable implementations.

According to the in 3 Example shown is an ultrasonic transducer 8th in an injection mold 7 or arranged on the outside of the surface. In 3 Also shown is an ultrasonic pulse coming from the transducer 8th through the injection mold 7 to one in the tool 7 arranged cavity 9 is running, (at least) on the wall 9 ' the cavity 9 is reflected back to the transducer 8th runs and is received by this again. During the injection molding process plastic melt P is pressed into the mold. 3 shows the flow front of the plastic melt P in the cavity 9 and the flow direction.

The ultrasound transducer 8th serves as both transmitter and receiver. The with the transducer 8th coupled sensor circuit 22 can therefore be considered as an ultrasonic transceiver (transceiver). The sensor circuit 22 thus has a transmit path and a receive path. The transmission path includes an oscillator 1 which generates a signal sp (t) with pulses of defined pulse duration and adjustable pulse repetition frequency PWF. For example, the oscillator 1 an astable multivibrator (relaxation oscillator). Other possibilities of signal generation can also be used. The pulse repetition frequency PWF is via one at the input 2 received signal adjustable. The oscillator 1 can via a trigger signal s _ON (t) which can come from the control of the injection molding machine can be activated or deactivated. There are many ways to implement this activation or deactivation. In the example shown, this is from the injection molding machine at the entrance 4 received trigger signal s _ON (t) above a certain logic level (eg high level) that the oscillator should be active. These are the oscillator signal s _P (t) and the trigger signal s _ON (t) an AND gate 3 supplied, which for the oscillator signal s _P (t) is transparent as long as the trigger signal s _ON (t) has a high level. Changes the level of the trigger signal s _ON (t) to a low level, becomes the oscillator signal sp (t) through the AND gate 3 blanked and the signal level at the output of the AND gate 3 is almost zero (low level). As a trigger signal s _ON (t) For example, a signal can be used that indicates whether the tool 7 the injection molding device is closed.

The output signal s _P '(t) of the AND gate 3 (corresponds to the oscillator signal s _P (t) if the oscillator is active) is a trigger input of a high voltage pulse generator 5 fed. The high voltage pulse generator 5 is configured to generate a high voltage pulse (ie a burst signal) of defined pulse duration (eg, a few microseconds) in response to a pulse received at the trigger input. As long as the oscillator 1 is active (ie the AND gate 3 the oscillator signal sp (t) does not blank), the high voltage pulse generator generates 5 So high voltage pulses with the pulse repetition frequency PWF ,

The from the high voltage pulse generator 5 generated sequence of high voltage pulses is via a transmitting-receiving switch 6 the ultrasound transducer 8th supplied, which is adapted to emit corresponding ultrasonic acoustic pulses, ie in the tool 7 to emit into it. The ultrasound transducer 8th converts the electrical (burst) signal into an audible signal. The transceiver 6 is designed to prevent a coupling of high voltage signal components in the reception path. Such transceivers are known per se and are therefore not explained in detail here. The one from the transducer 8th radiated acoustic ultrasonic pulse passes through the material (eg steel) of the tool 7 towards the cavity 9 , will be there at the cavity wall 9 reflects and runs back to the ultrasound transducer 8th , which converts the reflected acoustic ultrasound pulse back into an electrical signal. The received electrical signal is transmitted via the transceiver 6 fed to an analog filter, for example a bandpass filter 10 to suppress unwanted frequency components and amplified (see amplifier 11 ). In the example shown, the bandpass filtered and amplified signal is with u (t) designated; an exemplary waveform of the signal u (t) was eg in 2A already shown. In this regard, reference is also made to the above explanations. Also, the received (filtered and amplified) signal u (t) includes a sequence of pulses at the pulse repetition rate PWF , where in the 2A and 2 B only one pulse was displayed.

As explained above, there are various possibilities for determining a measured value which is the amplitude of a pulse in the received signal u (t) represents. In the present example, the area under the envelope of the pulse is determined by integration, the main pulse and the ringing (cf. 2A) be evaluated separately. As mentioned, the main pulse is the first received echo of the emitted ultrasound pulse which due to its relatively high amplitude has no measurable interference phenomena with the (very weak) signal of the boundary layer solidified plastic / melt from the interior of the molding.

For the determination of the envelope, the received signal u (t) is first rectified (see 3 , Rectifier 12 ) and then low-pass filtered (see 3 , Low pass 13 ). The effect of rectification and low pass filtering was discussed above with reference to FIG 1 explained and is therefore not repeated here. The resulting envelope signal env {| u (t) |} becomes a first integrator 14 and a second integrator 15 fed, which are reset regularly at the beginning of a pulse. For example, the oscillator signal sp (t) (alternatively also s _P '(t) ) a reset input of the integrators 14 . 15 be fed. The time windows of integration are determined by the pulse generators 16 and 17 given, for example, each may include a monostable multivibrator and a delay element. The pulse generators 16 and 17 are trained to respond upon receipt of a pulse of the oscillator signal s _P '(z) to generate a pulse of defined length and defined delay (relative to the oscillator signal). These pulses indicate the integration time window and can, for example, enable inputs of the integrators 14 . 15 be fed.

The length and timing (relative to the oscillator signal) of the pulse generators 16 and 17 generated pulses define the integration time constraints for the integrators 14 and 15 , For the first integrator 14 is the length of the time window t _H and for the second integrator 15 is the length of the time window t _N , These time windows are also in 2A represented and correspond to the length of the main pulse and the Nachschwingens. The length and timing of the time window t _H and t _N depend on the duration of the acoustic signal and on the frequency and bandwidth of the ultrasonic transducer 8th and are thus known system parameters. At the end of the integration time window t _H and t _N becomes the output signal of the associated integrator 14 or. 15 using the sample and hold circuits 18 or. 19 sampled. The resulting discrete-time output signals of the sample and hold circuits 18 or. 19 are denoted u _H [k] and u _N [k] respectively, where k is a time index. The sampling rate corresponds to the pulse repetition frequency PWF. The time discrete output signals u _H [k] and u _N [k] can be at the outputs 20 or. 21 output and, for example, digitized and further processed by a processor (not shown). Alternatively, an analog (quasi-continuous) signal processing is possible.

According to one embodiment, the sensor circuit 22 designed as an assembly, with a connection for the ultrasonic transducer 8th , an entrance 4 for the trigger signal s _ON (t) from the injection molding machine, an entrance 2 for selecting the pulse repetition frequency and two outputs 20 and 21 for the output signals u _H [k] or. u _N [k] , which respectively represent the area under the envelope of the main pulse and the Nachschwingens. These surfaces can - as initially with reference to 1 explained - be used as a measured value for the (maximum) amplitude of the main pulse or Nachschwingens.

The timing diagrams off 4 show an example of a typical waveform of the (discrete-time, at high pulse repetition frequency quasi-continuous) output signals u _H [k] and u _N [k] representing the course of the (maximum) amplitude of the main pulses and the associated ringing phases. As mentioned at the beginning, normalization to 100% ensures that aging effects and varying coupling efficiency of the ultrasound transducer do not play any significant role. In the 4 represented signals U _H [k] and U _N [k] are therefore based on the measured values for an empty (air-filled) cavity (eg U _H (t _k ) = u _H [k] / u _H [0] and U _N (t _k ) = u _N [k] / u _N [ 0]). The time k = 0 (corresponds to t ₀ = 0s) represents the start of a measurement with a closed, still filled with air cavity before the injection of plastic melt and the time t _k corresponds to the multiple of the reciprocal of the pulse repetition frequency (t _k = k · PWF ^-1 ).

Since at the time t ₀ Air is located in the cavity, becomes virtually 100% of the sound energy at the cavity wall 9 ' (see. 3 ) reflected. The normalized signal U _H (t) Therefore, it can also be interpreted as the reflection coefficient R. During the injection molding process, the flow front of the plastic melt reaches the "field of view" of the ultrasonic transducer 8th , take the reflection coefficient R and thus also the (maximum) amplitudes of the reflected main pulses (as well as the amplitudes of the associated ringing) significantly. The flow front of the plastic melt P , (see. 3 ) can be detected, for example, that the signal U _H (t) monitored during the injection molding process and that time t _FF is detected, at which the normalized signal U _H (t) a predefined threshold S _FF below. The threshold S _FF may be in the range of 98-99%, for example. In this way, a fast and accurate detection of the flow front is possible. Additionally or alternatively, the signal U _N (t) be used for the flow front detection. Changes in the measured value U _H (t) indicate changes in the reflection coefficient R and thus indirectly also on density fluctuations in the melt, which can be caused for example by the inhomogeneities mentioned. For process monitoring, for example, an ultrasonic transducer could be placed on the casting tool (eg, just past the injection point) so that the entire injected melt must pass the measuring field ("field of view") of the sensor. Short, erratic changes in the measured value U _H (t) (ie the reflection coefficient) may indicate inhomogeneities.

Once the cavity 9 completely with plastic melt P (see. 3 ) is filled, there is a significant compression of the plastic material in the cavity, and consequently take the reflection coefficient R and thus also the (maximum) amplitudes of the reflected main pulses (as well as the amplitudes of the associated ringing) continue to significantly decrease. The corresponding time tu the complete filling of the cavity 9 can also be detected by means of a threshold value Su, which is smaller than the threshold value S _FF (eg. lies S _FF in the range of 95-97%). The time tu is thus detected by the normalized signal U _H (t) falls below the threshold Su. This point in time tu can be used to initiate the repressurization phase, in which the pressure is switched from a constant press-in speed of the plastic melt to, for example, constant pressure. In this Repressing phase begins to cool the molding, and it forms a thicker edge layer in which the plastic melt is solidified. In addition or alternative, the signal U _N (t) be used for the detection of the switching time tu.

With the application of the emphasis, the growth of the surface layer begins in the molded part, starting from the surface of the molded part into the interior. Because of this (growing) boundary layer P ' , there is a reflection that coincides with the reflection on the cavity wall 9 ' superimposed (see 2 B) ; This superposition (interference) makes itself above all in the (maximum) amplitude of the Nachschwingens - thus in the signal U _N (t) - noticeable. This superposition (interference) results in a kind of beat signal which can be found in the signal after the time tu U _N (t) can recognize.

The effect of the emphasis on the molded part can be on the signal U _H (t) , which represents the amplitudes of the main pulses, recognize. Shortly after the switching time tu, the injection molding machine regulates the pressure to the (usually constant) set reprint. The time of reaching this pressure level is in the signal U _H (t) visible; the amplitude increases again slightly. The corresponding time is in 4 With t _{N, start} designated. The duration of the repressurization phase is usually set by the user on the injection molding machine. In fact, it can only act as long as plastic melt can be conveyed through the liquid core of the molded part into the molded part. The actual time t _{N, end} where the reprint no longer affects the course of the signal U _H (t) be read, since the pressure drop leads to a change in density and thus sudden change in the maximum amplitudes of the main pulses. The actual pressure duration acting on the molded part results from the difference t _N = t _{N, end} -t _{N, start} .

At the signal U _N (t) , which represents the maximum amplitudes of the ringing phases, the above-mentioned interference can be seen, which is reflected in the signal U _N (t) manifests as oscillation, which with the beginning of the holding pressure phase (at about the time t _{N, start} ) begins. As mentioned, this oscillation is a result of the overlapping reflection on the cavity wall 9 ' with the reflection at the growing edge layer P 'of the molding (see 2 B) , In this phase, in the signal U _N (t) (eg by means of digital signal processing) the times t _{MAX, 1} . t _MAX2 the maxima (additionally or alternatively also the minima) of the oscillation are detected.

The times t _{MAX, 1} . t _MAX2 or the period of the oscillation Δt _MAX can be monitored (and, if necessary, stored and documented) during the injection molding process to monitor the consistency of growth of the surface layer over multiple production cycles. The period of the oscillation Δt _MAX can serve as a measure of the growth rate of the boundary layer. From this measured value, taking into account the speed of sound in the plastic and the frequency of the ultrasound, the actual growth rate can be approximately calculated.

The cooling of the molding results in a volume contraction, which in turn causes the molding to shrink and detach from the interior wall of the cavity ("shrinkage"). Point of time t _AB to which the molding separates from the tool inner wall is in both signals U _H (t) and U _N (t) clearly. Due to the resulting gap between cavity wall 9 ' (please refer 3 ) and molded part, the reflection factor increases again to approximately 100% and the signals rise again to their initial values.

The exemplary signal curves of the normalized output signals U _H (t) and U _N (t) make it clear that the concepts described here can be used both for monitoring and for controlling an injection molding process. In the examples described here, an ultrasonic transducer is used, which works both as a transmitter and as a receiver. Instead, two separate transducers can be used as transmitter and receiver. In this case the transceiver is not needed. Finally, it should be noted that the concepts described here not only for the monitoring and control of a (plastic) injection molding (injection moudling), but also used analogously for the monitoring and control of a (metal) die casting process (the casting) or other casting processes can be.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19737276 C2 [0007]
• DE 19834797 C2 [0007]
• EP 2657801 A2 [0007]
• US 5951163 A [0007]
• WO 03/089214 A2 [0007]
• US 7017412 B2 [0008]
• US 5951163 [0008]

Claims (10)

A measuring system for monitoring a casting process, comprising: an ultrasonic transducer (8) adapted to couple ultrasonic pulses into a casting tool (7); a sensor circuit (22) coupled to the ultrasound transducer (8) and adapted to generate the ultrasound pulses to drive the ultrasound transducer (8) with a drive signal having a sequence of pulses and one of the ultrasound Transducer (8) or a further ultrasonic transducer provided receive signal (u (t)), which in the casting tool (7) reflected ultrasonic pulses to process, wherein the received signal (u (t)) a corresponding to the drive signal sequence of pulses each having a main pulse and a ringing, and wherein the processing of the received signal (u (t)) comprises: generating - for each pulse of the received signal (u (t)) - a first measured value (U _H (t )), which represents the amplitude of the main pulse, and a second measured value (U _N (t)), which represents the amplitude of the ringing. The measuring system according to Claim 1 , wherein the first measured value (U _H (t)) in each case based on the maximum amplitude of the main pulse and the second measured value (U _N (t)) in each case based on the maximum amplitude of the ringing. The measuring system according to Claim 1 or 2 , wherein the first measured value (U _H (t)) is based in each case on the area under the envelope of the main pulse and the second measured value (U _N (t)) in each case on the area under the envelope of the ringing. The measuring system according to one of Claims 1 to 3 wherein generating the first measured value (U _H (t)) comprises integrating the envelope of the respective pulse of the received signal (u (t)) in a first time window (t _H ), and generating the second measured value (U _N (t )) comprises the integration of the envelope of the respective pulse of the received signal (u (t)) in a second time window (t _N ). The measuring system according to Claim 4 wherein the first time window (t _H ) starts a defined first delay time after the corresponding pulse in the drive signal, and wherein the second time window (t _N ) starts a defined second delay time after the corresponding pulse in the drive signal. A method comprising: injecting a sequence of ultrasonic pulses into a casting tool (7); Receiving a sequence of reflected ultrasonic pulses and producing a received signal (u (t)) having a corresponding sequence of pulses each having a main pulse and a ringing; Generating - for each pulse of the received signal (u (t)) - a first measured value (U _H (t)) representing the amplitude of the main pulse; and generating - for each pulse of the received signal (u (t)) - a second measured value (U _N (t)) representing the amplitude of the ringing. The method according to Claim 6 , wherein the first measured values (U _H (t)) and the second measured values (U _N (t)) normalized in such a way that they assume a predefined value for an empty casting tool (7). The method according to Claim 6 or 7 , wherein based on the change of the first and / or second measured values (U _H (t), U _H (t)) during a casting process, one or more process parameters are determined, which includes at least one of the following: the time (t _FF ) of Detection of the flow front of the melt injected into the casting tool (7), the time (tu) of the complete filling of a cavity of the casting tool (7) with melt, time and duration of a holding pressure phase, growth rate of the edge layer of the melt in the casting mold, time of the mold part to go down the wall of the cavity. The method according to Claim 6 or 7 wherein, based on the change of the second measured values during a casting process, in which melt is introduced into a cavity of the tool (7), unwanted deviations in the edge layer growth in the melt from a desired course are detected, while the melt cools and solidifies. The method according to one of Claims 6 to 9 in which, based on the first measured value (U _H (t)) and its change, density fluctuations of the melt injected into the casting tool (7) are detected, in particular to detect inhomogeneities in the melt.

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