Technology for monitoring a thermoforming process

Abstract

A method for monitoring a thermoforming process is provided. Furthermore, a thermoforming tool and a thermoforming machine are provided which implement such a method. The method for monitoring a thermoforming process comprises recording at least two measured values ​​at at least two mold cavities of the thermoforming tool, wherein at least one measured value is recorded at each of the at least two mold cavities, which value indicates a stretching force or stretching energy to be applied during pre-stretching of the material layer into the respective mold cavity. The method further comprises comparing the measured values ​​recorded at the at least two mold cavities in order to obtain a comparison result, wherein the comparison result is an indicator of the homogeneity of the thermoforming process.

Description

Technical FieldThe invention relates to the field of thermoforming. More particularly, the invention relates to a technique for monitoring a thermoforming process, and to a thermoforming tool and thermoforming machine implementing such a technique.Prior ArtThe use of thermoforming tools and thermoforming methods for forming packaging articles into two-dimensional (planar) material layers (sheets or blanks) made of plastic material is known. Polypropylene (PP), polyethylene terephthalate (PET) or polystyrene (PS) or other thermoplastic plastics materials can be used as the plastics material in this case. The shaped packaging articles are used above all for portioned packaging of foods and can be designed as containers, cups, trays or capsules depending on the use.Thermoforming tools for producing cup-shaped packaging articles are known, for example, from EP 1 541 320 A1, EP 1 163 996 B1 and DE 10 2016 103 237 A1. Thermoforming tools of this type comprise a first mold part (e.g. upper mold part) and a second mold part (e.g. lower mold part), which are arranged coaxially with respect to one another and are movable along a common axis. The upper mold part has at least one hold-down device and at least one pre-stretcher mounted displaceably therein. The lower mold part has at least one mold insert with at least one mold cavity and at least one mold base. The at least one mold cavity together with the at least one mold base forms a mold space which simulates the shape of the cup to be molded.In the production of cup-shaped packaging articles, the plastic material present in the form of a two-dimensional layer (this can be present, for example, in the form of a rollable sheet / film or a blank / sheet blank) is first heated to a desired forming temperature. Subsequently, the two-dimensional material layer heated to forming temperature is supplied to the thermoforming tool (intermittently). In this case, the two mold parts are moved apart (open state of the thermoforming mold) in order to be able to position the material layer between the two mold parts. The thermoforming tool is then closed by moving the two mold parts towards each other. With the at least one prestretcher, the material layer brought to forming temperature is mechanically stretched or prestretched into the at least one mold cavity, whereby a preformed blank is formed. The mechanical stretching of the film with the aid of the at least one prestretcher can also be omitted or can be effected with the aid of compressed air (prebubbling) or vacuum (pre-suction), if the deep-drawing depth and thus the forming or stretching ratio is small on account of the geometry of the article to be formed (for example during the forming of a lid).A process parameter that significantly influences the thermoforming process is the temperature of the two-dimensional layer to be formed. In order to be able to obtain packaging articles of desired quality (for example desired transparency, desired shape sharpness), it is necessary to bring the layer to be formed to a desired forming temperature. The desired forming temperature can depend here on the material composition and the physical properties (for example crystallinity) of the thermoplastic material used and must be adjusted accordingly for each material layer or determined by test tests. It has been found that, in particular in the case of partially crystalline plastic material layers, such as for example in the case of polypropylene layers, minor changes in the forming temperature (for example a change in the forming temperature of 2-3 K) can already lead to considerable changes in the shape sharpness of the articles produced.In multiple thermoforming tools having a plurality of mold cavities for simultaneously producing a plurality of packaging articles per thermoforming cycle, which are currently preferably used in industrial production of packaging articles, a challenge further consists in heating the material layer in such a way that it has substantially the same desired forming temperature over the entire forming surface. This is because it can only be ensured in this way that all articles produced in a thermoforming cycle have the same quality.To achieve the most homogeneous possible temperature distribution over the forming surface of the material layer, heating beam elements, preferably infrared heating beam elements, are used in practice, which are arranged in the form of a matrix in a heating device (heating station) arranged in front of the thermoforming tool and, in addition, can be individually regulated. The arrangement and size of the used radiant heating elements is predetermined by the thermomachine manufacturer.The homogeneity of the heating of a material layer over its forming surface depends above all on the arrangement and ready-to-use of the heating jet elements in the heating station. If, for example, a heat radiation element fails, this can lead to local temperature fluctuations in the material layer, which are often not detected equally and can lead to rejects during article production.However, the use of a feed device which grips and clamps the material layer in the edge region in order to feed the material layer to the heating station and subsequently to a forming station comprising the thermoforming tool (intermittently) can also influence the homogeneity of the temperature profile over the forming surface. It has been found that, as a result of the influence of the feed device, the temperature of the material layer is lower in the edge region, i.e. transversely to the feed direction, where the material layer is gripped by the feed device. Accordingly, the temperature of the radiant heating elements provided for heating the material layer in the edge region must be set higher than the radiant heating elements provided for heating the material layer in the central region of the material layer in order to be able to compensate for this temperature gradient.A further problem is the determination of the actual temperature (forming temperature) of the heated material layer. In practice, this is effected contactless via infrared sensors or thermal cameras. These measure primarily the surface temperature of the material layer, but not the temperature distribution over the material layer thickness. The temperature distribution over the material layer thickness, however, depends not only on the heating power of the heating jet elements, but also on the dwell time of the material layer in the heating station. Excessively short residence times in conjunction with an excessively low heating power may lead to the temperature distribution over the thickness of the material layer not being homogeneous, but rather having a negative temperature gradient proceeding from the material layer surface. This means that although the material layer on the surface can have a desired forming temperature and this is also measured by the infrared sensors, the material layer in the interior has significantly lower temperatures (i.e. is significantly colder). However, it is not so important for the quality of the thermoforming process (i.e. transparency, sharpness of the articles, etc.) whether the surface of the material layer has the desired forming temperature, but whether the material layer has the desired forming temperature as a whole, i.e. also over the layer thickness.In practice, the thermoforming process parameters, in particular the forming temperature, are set manually as a function of the material layer used when starting the thermoforming process (heating up the material layer and the thermoforming tool). Manually set means that test articles are produced and evaluated for their quality (sharpness of shape, transparency, etc.). The process parameters, in particular the temperature of the radiant heating elements, are then manually set or readjusted by an operator (for example, via an operator interface (HMI)) until the thermoforming tool produces articles of desired quality. Thereafter, regular production of the packaging articles can begin. Manually adjusting thermoforming process parameters based on inspection of produced articles is time consuming and requires skilled operators. Furthermore, disturbances or undesired changes in the process parameters during the thermoforming process (for example a temperature change due to failure of a heating element) can be detected only on the produced article and thus relatively late.The object of the present invention is to eliminate these disadvantages. In particular, it is an object of the present invention to provide an automated technique for monitoring a thermoforming process. The technique should also be capable of monitoring the homogeneity of the thermoforming process in a multiple thermoforming tool, in particular the homogeneity of the temperature distribution in the material layer to be formed. It is also desirable that the monitoring technique be carried out in real time and initiate appropriate countermeasures in the event of deviations.Short tear-offTo achieve at least one of the above objects, according to a first aspect of the invention, there is provided a method of monitoring a thermoforming process adapted to form a plurality of articles in a sheet of material. The method comprises the following steps: acquiring at least two measured values at at least two mold cavities, wherein at least one measured value is acquired at each of the at least two mold cavities, said measured value indicating a stretching force or stretching energy to be applied during the prestretching of the material layer into the respective mold cavity; and comparing the measured values acquired at the at least two mold cavities in order to obtain a comparison result, wherein the comparison result is an indicator for the homogeneity of the thermoforming process.According to the present invention, the thermoforming process is designed for simultaneously producing a plurality of articles in a layer of material. By the simultaneous production of a plurality of articles is meant the simultaneous production of at least two articles, preferably at least three, more preferably at least four articles or more. This can be achieved by using a multiple thermoforming tool which, as will be described further below, has at least two, preferably at least three, more preferably at least four, mold cavities for simultaneously forming the at least two, preferably at least three, more preferably at least four articles. The shaped articles can be packaging articles, in particular packaging articles for portioned reception of foods, such as containers, cups, trays or capsules.By material layer is meant a two-dimensional (sheet-like) material layer made of a thermoplastic material and suitable for thermoforming articles. Polystyrene (PS), polypropylene (PP), polyethylene terephthalate (PET) or another thermoplastic material can be used as thermoplastic material. Depending on the requirements of the articles to be produced, the material layer can be present in the form of a rollable material sheet or in the form of a panel-shaped material blank.The stretching force to be applied is the force which has to be applied / applied during thermoforming for stretching or prestretching the heated material layer into the respective mold cavity (of the multiple thermoforming tool). Accordingly, the stretching energy is the energy which is to be applied / applied during the prestretching of the material layer over a predetermined prestretching path (deep-drawing path). It can be obtained by integrating the stretching force over the preliminary stretching path (deep-drawing path).It has been found that the stretching force or stretching energy to be applied depends primarily on the temperature (forming temperature) of the material layer. The stretching force or stretching energy to be applied is less dependent on the surface temperature of the material layer than rather on the temperature which the material layer has over the entire thickness of the material layer (i.e. also in the interior of the material layer). Higher temperatures of the material layer result in lower stretching forces or stretching energies to be applied, while lower temperatures result in higher stretching forces or stretching energies.It has also been found that changes in the temperature (forming temperature) of the material layer to be formed by a few Kelvin lead to considerable, readily measurable changes in the stretching force (changes in the range of 100 N with temperature changes of only 2 K). This correlation between the stretching force or stretching energy to be applied on the one hand and the actual temperature (forming temperature) of the material layer on the other hand can thus be used to infer the actual temperature (forming temperature) of the material layer via the detected stretching force (stretching energy) or via a measured value which points to the stretching force or stretching energy. Thus, the measuring principle according to the invention based on the detection of at least one measured value indicative of the stretching force or stretching energy is substantially more reliable than temperature measurements with the aid of infrared sensors or thermal imaging cameras, which are only surface-sensitive.It is understood that the stretching force or stretching energy to be used in stretching or prestretching the material layer can also depend on the speed and / or geometry of the prestretcher used for stretching the material layer. These tool-specific parameters are known, however, and their contribution / influence on the stretching force or stretching energy can easily be distinguished from the contribution / influence of the temperature (forming temperature) of the material layer on the stretching force or stretching energy. On the other hand, these tool-specific influences are not important in the method according to the invention, since the at least one measured value (measurement parameter) indicative of the stretching force or stretching energy is recorded at at least two mold cavities of the multiple thermoforming tool independently of one another and the recorded measured values are then (directly) compared with one another or with a predefined reference value (desired measured value) in order to obtain the comparison result. Since in the multiple thermoforming tool the pre-extenders are actuated identically for each mold cavity and have the same geometry, their contribution / influence on the at least one measured value recorded at each of the at least two mold cavities is the same. When comparing (relative comparison) the knife values recorded at the at least two mold cavities with one another, the influence of the geometry and speed of the pre-stretchers on the recorded measurement values is thus not important, since this influence is the same for all measurement values.If deviations occur in the measured values recorded at the at least two mold cavities and indicative of the stretching force or stretching energy, these deviations are associated with changes in the actual forming temperature of the material layer at the respective mold cavities. The method according to the invention can thus be used to monitor the homogeneity of the temperature distribution in the material layer.In order to be able to monitor the homogeneity of the temperature distribution over the entire forming surface of the material layer, the at least two measured values can be recorded at at least two mold cavities which are arranged in a thermoforming tool at a distance from one another in a direction transverse to a feed direction of the material layer and / or in a direction parallel to the feed direction of the material layer. The feed direction means the direction in which the material layer is fed to the thermoforming tool. It corresponds to the longitudinal direction of the thermoforming tool, while the transverse direction corresponds to the width direction of the thermoforming tool.Preferably, at least one measured value can be detected at each of at least two mold cavities which are arranged diagonally to one another (i.e. diagonally to the feed direction) in the thermoforming tool. Thus, with little measurement effort, the homogeneity of the temperature distribution both in the feed direction / longitudinal direction and in the transverse direction / width direction of the material layer / of the thermoforming tool can be monitored reliably.In a particularly simple implementation of the method, the at least one measured value indicative of the stretching force can be recorded at each of at least two mould cavities of the thermoforming tool arranged diagonally spaced apart from one another, wherein a first mould cavity is arranged centrally in the thermoforming tool and a second mould cavity is arranged diagonally further outwards (for example in a corner region) in the thermoforming tool. Proceeding from this implementation, in a development, the at least one measured value indicative of the stretching force can be recorded at each of at least three mould cavities arranged diagonally with respect to one another, wherein a first mould cavity can be arranged centrally in the thermoforming tool, a second mould cavity can be arranged diagonally further outwards (for example in a first corner region) in the thermoforming tool and a third mould cavity can likewise be arranged diagonally further outwards (for example in a second corner region which lies diagonally opposite the first corner region) in the thermoforming tool. By acquiring and comparing measured values at the mold cavities in the middle and in the edge region, in particular at the corner regions of the thermoforming tool, a reliable statement can be made about the homogeneity of the thermoforming process, in particular about the homogeneity of the temperature distribution over the entire forming surface of the material layer (i.e. both in the feed direction / longitudinal direction and in the transverse direction / width direction).Deviations (inhomogeneities) in the temperature distribution of the material layer can be caused above all by the feed device which is provided for feeding the material layers and grips and clamps them in the edge regions transversely to the feed direction. Due to the feed device, the temperature in the edge regions opposite transversely to the feed device can be somewhat lower than in the central region of the heated material layer. In order to detect such an non-uniformity of the temperature distribution transversely to the feed direction, it may be sufficient to detect the at least one measured value at at least two mold cavities, preferably at least three mold cavities, wherein a first mold cavity is arranged centrally in the thermoforming tool in the transverse direction and a second mold cavity is arranged further outwards (for example in a first corner region) in the thermoforming tool in the transverse direction. If the at least one measured value is additionally also recorded at a third mold cavity, the third mold cavity can be arranged in the thermoforming tool lying further outwards in the transverse direction (for example in a second corner region which lies opposite the first corner region in the transverse direction).It is understood that the method described herein is not intended to be limited to acquiring and comparing measurement values indicative of the stretching force or stretching energy at at least two or three mold cavities. Depending on the number of mold cavities in the thermoforming tool, the method described here can also be extended to four or more mold cavities in order to obtain a reliable statement about the homogeneity of the temperature over the forming surface. In particular, by acquiring and comparing measured values at a plurality of mold cavities (at 4 or more mold cavities), the resolution of the method can be further improved and locally limited temperature deviations / temperature inhomogeneities (caused, for example, by the failure of a heating beam element) can be better monitored. On the other hand, it is not necessary to record and compare the measured values for each mold cavity of the thermoforming mold, which are indicative of the stretching force or stretching energy, provided that the mold cavities for which the measured values are recorded are arranged distributed over the mold.The at least one measurement value, which points to the stretching force or stretching energy at the respective mold cavity, can be a measurement value which is proportional to the stretching force or stretching energy which is applied during the stretching of the material layer into the respective mold cavity. An implementation of a measurement principle for acquiring / proportional measurement values indicative of the stretching force or stretching energy is described in more detail further below in connection with a thermoforming tool according to the invention. Due to the proportionality between the at least one recorded measured value and the stretching force or stretching energy to be applied, it is possible to infer the stretching force or stretching energy and thus also the actual temperature of the material layer in the region of the respective mold cavity, which is correlated with the stretching force or stretching energy. Deviating from this, however, it is also conceivable to measure and compare the stretching force or stretching energy directly at the at least two mold cavities in order to obtain a comparison result which points to a deviation / non-homogeneity in the thermoforming process, in particular to a deviation / non-homogeneity of the temperature distribution in the heated material layer.The step of comparing the measured values recorded at the at least two mold cavities can comprise comparing the measured values recorded at the at least two mold cavities with one another. The step of comparing can further comprise determining a deviation of the at least two detected measured values from one another in order to obtain a comparison result. The comparison result can comprise the deviation determined. In other words, by comparing the measured values recorded at the at least two mold cavities with one another, it can be determined how strongly the measured values differ / deviate from one another at the at least two mold cavities. This direct comparison method has the advantage that measurement-specific influences, such as the geometry of the pre-extenders or the stretching speed, do not have any influence on the comparison result, since such measurement-specific influences are the same at all mold cavities and thus cancel one another out when comparing the measurement values with one another. A deviation (difference) determined in the comparison step between the measured values recorded at different mold cavities thus represents a relative measure of how much the thermoforming process taking place at the respective mold cavities, in particular the forming temperature of the material layer at the respective mold cavities, deviates / differs from one another.Additionally or alternatively, the step of comparing the measured values recorded at the at least two mold cavities can comprise comparing the measured values recorded at the at least two mold cavities with a predefined reference value. The step of comparing can furthermore comprise determining a deviation of the measured values recorded at the at least two mold cavities from the reference value in order to obtain a comparison result. In other words, each of the measured values recorded at the at least two mold cavities can be compared with the reference value and a possible deviation therefrom can be determined. The reference value can be a desired measured value, which indicates a desired forming temperature. Such a reference value can be calculated / predefined taking into account the stretching speed and / or the geometry of the thermoforming tool (for example the geometry of the pre-stretcher used for stretching the material layer into the respective mold cavity) in such a way that it points to a desired forming temperature (target forming temperature) of the material layer. A deviation of the measured values recorded at different mold cavities from the reference value determined in the comparison step thus constitutes an absolute measure of how much the thermoforming process taking place at the respective mold cavities, in particular the forming temperature of the material layer at the respective mold cavities, deviates from a target forming temperature (and thus from a target thermoforming process).Using the above-described comparison methods based on the comparison of the recorded measured values with one another or based on the comparison of the recorded measured values with a predefined reference value (desired measured value), it can be easily determined whether the forming temperature of the material layer is the same at the respective cavities within a multiple thermoforming tool and thus the thermoforming process takes place the same in the mold cavities of the thermoforming tool. Due to the direct relationship between the acquired measured values, the stretching force or stretching energy and the forming temperature of the material layer, it is not necessary to convert the acquired measured values into corresponding stretching force values or stretching energy values and forming temperature values, whereby computing capacities can be saved.On the other hand, on the basis of the direct relationship between the measured values and the stretching force to be applied and the stretching force and the forming temperature of the material layer, corresponding stretching force values or stretching energy values and thermoforming process values corresponding therefrom, in particular forming temperature values of the material layer at the respective cavities, can be calculated from the detected measured values and compared with one another. As a result, it can be illustrated even better to a user how strongly the thermoforming process, in particular the forming temperature of the material layer at the respective forming cavities of the thermoforming tool, deviate from one another or deviate from a setpoint value (setpoint temperature).In order to be able to compare the measured values recorded at the at least two mold cavities objectively with one another or with respect to a setpoint value, the step of recording can comprise a simultaneous recording of the at least one measured value or of a measured value profile at each of the at least two mold cavities during the stretching / prestretching of the material layer into the respective mold cavities. In order to be able to record a measurement value profile at each of the at least two mold cavities, a multiplicity of measurement values can be recorded during the stretching / prestretching of the material layer. However, it is also conceivable that the measured value profile is recorded during an entire thermoforming cycle (i.e. from closing to re-opening the forming tool).The simultaneously recorded measured values or measured value profiles can be compared with one another. For example, it is conceivable that recorded measurement value maxima are compared with one another. Alternatively, increases in the measured value (slopes in the measured value profile) can be compared with one another, provided that the measured value profile is recorded during the pre-stretching of the material layer. Alternatively, it is also conceivable that recorded measured value profiles are integrated at the at least two mold cavities over a predefined prestretch travel (prestretch time) and the integration values are compared with one another in order to achieve a comparison result. The integration of recorded measured value profiles has the advantage that statistically induced measurement errors, in particular outliers, are eliminated and the comparison result obtained is thus even more accurate.The method can further comprise the step of generating and providing at least one feedback signal if the comparison result comprises a deviation between the at least two measured values recorded at different mold cavities or a deviation of the measured values recorded at the at least two mold cavities from a predefined reference value. If the deviation determined in the comparison result exceeds a predefined threshold value (tolerance value), this can be an indicator (indication) for an inhomogeneous thermoforming process, in particular an indication of an uneven / inhomogeneous temperature distribution over the forming surface of the heated material layer. In such a case, a feedback signal may be generated and output. The generated feedback signal can comprise, for example, a warning signal for a user; additionally or alternatively, the generated feedback signal can comprise a control or regulating signal, in particular a control or regulating signal for a heating device for heating the material layer, which is generated as a function of the comparison result. Thus, the method according to the invention not only allows the thermoforming process to be monitored in real time, but also to be actively controlled. In particular, the heating device provided for the thermoforming process can be automatically readjusted with the aid of the control signal that is output, and thus irregularities in the temperature distribution over the forming surface of the material layer can be eliminated.In order to achieve at least one of the above-mentioned objects, according to a second aspect of the invention, a thermoforming tool for forming a plurality of articles in a material layer supplied to the thermoforming tool is provided, wherein the thermoforming tool comprises: at least two mold cavities for forming at least two articles; at least two pre-extenders corresponding to the at least two mold cavities, wherein each pre-extender is configured to pre-extend (or extend) the material layer into the mold cavity corresponding thereto; and a measuring device which is designed to record at least one measured value at each of at least two mold cavities, which measured value is indicative of a stretching force or stretching energy to be applied in the respective mold cavity during pre-stretching (stretching) of the material layer, and to provide the recorded measured values to an evaluation device (for comparing the recorded measured values and for generating a comparison result).The thermoforming tool according to the present invention is a multiple thermoforming tool designed for simultaneously forming a plurality of articles (of at least two, three or more articles) in a thermoforming cycle. For this purpose, the thermoforming tool has at least two (at least three or more) mold cavities which are formed, for example, in a mold insert of a first thermoforming tool part. Accordingly, the thermoforming tool also has at least two (at least three or more) pre-extenders, which are arranged, for example, in a second thermoforming tool part corresponding to the first thermoforming tool part. The first thermoforming tool part may be a lower thermoforming tool part, while the second thermoforming tool part may be an upper thermoforming tool part. In an alternative embodiment, the first thermoforming tool part can also be an upper thermoforming tool part, while the second thermoforming tool part can be a lower thermoforming tool part.The measuring device can comprise at least two sensors, wherein a first sensor can be provided for capturing at least one measured value, which is indicative of the stretching force or stretching energy to be applied by a corresponding first pre-stretcher during pre-stretching of the material layer into a first mold cavity. A second sensor of the measuring device can be provided for capturing at least one measured value, which is indicative of the stretching force or stretching energy to be applied by a corresponding second pre-stretcher during pre-stretching of the material layer into a second mold cavity. The first mold cavity or the first pre-stretcher can be arranged here in a direction transverse to the feed direction of the material layer to the thermoforming tool (corresponding to the width direction of the thermoforming tool) and / or in a direction parallel to the feed direction of the material layer to the thermoforming tool (corresponding to the longitudinal direction of the thermoforming tool) at a distance from the second mold cavity or the second pre-stretcher. In this way, the at least two sensors can be used to record and compare measured values at different mold cavities in the transverse direction / width direction, longitudinal direction or diagonal direction and thus to monitor the homogeneity of the thermoforming process at different mold cavities in the thermoforming tool, in particular the homogeneity of the temperature distribution of the material layer supplied to the thermoforming tool, as was described in connection with the method further above.It is understood that the measuring device does not have to remain limited to the above-mentioned first and second sensors. Rather, the measuring device can comprise further sensors for detecting measured values at further mold cavities of the thermoforming tool, which values are indicative of the stretching force or stretching energy. For example, in a development, the measuring device can comprise a third sensor for detecting a measured value at a third mold cavity or third pre-stretcher, which is / is arranged spaced apart both from the first mold cavity or from the first pre-stretcher and from the second mold cavity or from the second pre-stretcher in the transverse direction / width direction of the thermoforming tool and / or in the longitudinal direction of the thermoforming tool. In yet another development, the measuring device can comprise a fourth sensor for detecting a measured value at a fourth mold cavity or a fourth pre-stretcher, which is / are arranged spaced apart from the first, second and third mold cavity / pre-stretcher in the transverse direction / width direction and / or in the longitudinal direction of the thermoforming tool. By acquiring and comparing measured values indicative of the stretching force or stretching energy at three, four or more mold cavities / prestretchers, the homogeneity of the thermoforming process in the multiple thermoforming tool, in particular the homogeneity of the temperature distribution over the forming surface of the material layer supplied to the thermoforming tool, can be monitored even better.Independently of the specific arrangement of the sensors in the thermoforming tool, the at least two (three, four or more) sensors of the measuring device can furthermore be configured (operated) to record the measured values simultaneously. In this way, the measured values detected by the at least two (three, four or more) sensors can be better (compared with one another). Furthermore, the at least two (three, four or more) sensors of the measuring device can be designed (operated) to record a plurality of measurement values in each case during the pre-stretching of the material layer by the respective pre-stretchers into the respective mold cavities. In this way, a measured value profile can be detected by the respective sensors, which is indicative of the profile of the stretching force during the pre-stretching process into the respective mold cavities.The stretching force acts on the respective pre-stretcher, which pre-stretches the material layer into the respective mold cavity. A cost-effective and reliable detection of measured values indicative of the stretching force or stretching energy at at least two mould cavities arranged at a distance from one another can be achieved in that the first sensor can be designed to measure an elongation or deflection of a first bending element, which is coupled to the first pre-stretcher, caused by the stretching force, and the second sensor can be designed to measure an elongation or deflection of a second bending element, which is coupled to the second pre-stretcher, caused by the stretching force. The first bending element and the second bending element thus cooperate with the respective first and second sensors of the measuring device.In a development, the measuring device can comprise further sensors which can be designed to detect the elongation or deflection of further bending elements which are coupled to further pre-extenders which are arranged in the thermoforming tool in each case at a distance from the first and second pre-extenders in the transverse direction and / or the longitudinal direction. For example, the measuring device can comprise a third sensor which is designed to detect the elongation or deflection of a third bending element which is coupled to a third pre-stretcher which is arranged in the thermoforming tool at a distance from the first pre-stretcher and the second pre-stretcher in the transverse direction and / or the longitudinal direction. In a further exemplary implementation, the measuring device can comprise a fourth sensor which is designed to record the elongation or deflection of a fourth bending element which is coupled to a fourth pre-stretcher which is arranged in the thermoforming tool at a distance from the first pre-stretcher, the second pre-stretcher and the third pre-stretcher in the transverse direction and / or the longitudinal direction.In order to be able to measure the strain or deflection reliably, the first sensor can be arranged on an upper side and / or on an underside of the first bending element, and likewise the second sensor (third sensor, fourth sensor) can be arranged on an upper side and / or underside of the second bending element (third bending element, fourth bending element). Strain gauges can be used as sensors in each case, which measure the strain or deformation of the respective bending elements. Instead of strain gauges, other sensors can also be used which are designed to reliably detect an expansion or deformation or deflection of the respective bending elements.According to one implementation, the first bending element and the second bending element (as well as the further bending elements, if present) can be integrated in a pre-stretcher plate which is provided for the simultaneous actuation of the at least two pre-stretchers. The bending elements can thus be part of the pre-stretcher plate. A particularly space-saving and simple coupling of the bending elements to the respective pre-extenders can thus be achieved.Each of the bending elements has a known elastic deformability, so that the strain or deformation at the respective bending elements detected with the aid of the sensors is proportional to the stretching force acting at the respective pre-stretchers. Briefly acting force peaks may possibly lead to the plastic deformation of the bending elements, as a result of which the proportionality between the elongation or deflection (strength of the elongation or deflection) and the stretching force (strength of the stretching force) can be lost. The bending elements can then no longer be used for measurement. In order to prevent undesired deformation of the bending elements, the thermoforming tool can further comprise a protection device which is configured to limit the stretching or bending of the respective bending elements (to an elastic deformation region).The measuring device can also comprise an evaluation device. The evaluation device can be configured to compare the measured values recorded for the at least two mold cavities in order to obtain and provide a comparison result. In particular, the measuring device can be designed to compare the measured values recorded at the at least two mold cavities with one another, and to determine a deviation of the at least two recorded measured values from one another and to provide them as a comparison result, as has been described further above in connection with the method according to the invention.Additionally or alternatively, the measuring device can be designed to compare the measured values recorded at the at least two mold cavities with a predetermined reference value (target measured value) and to determine a deviation of the at least two recorded measured values from the reference measured value and to provide them as a comparison result, as has been described further above in connection with the method according to the invention.The evaluation device can furthermore be configured to generate and provide a feedback signal if the comparison result indicates an inhomogeneous thermoforming process, in particular an inhomogeneous temperature profile in the material layer in the thermoforming tool. This can be the case when the determined deviation in the comparison result exceeds a predefined threshold value (tolerance value). In such a case, the evaluation device can generate and output a feedback signal. The generated feedback signal can comprise, for example, a warning signal for a user; additionally or alternatively, the generated feedback signal can comprise a control or regulating signal, in particular a control or regulating signal for a heating device for heating the material layer, as has been described further above in connection with the method according to the invention.To achieve at least one of the above-mentioned objects, according to a third aspect of the invention, a computer program is provided which comprises instructions which cause the method according to the first aspect to be executed when the computer program is executed in a computing unit (with a processor). The arithmetic unit can be implemented as a software and / or hardware module; in particular, the arithmetic unit can be part of the evaluation device described above.In order to achieve at least one of the above objects, according to a fourth aspect of the invention, a thermoforming machine is provided. The thermoforming machine comprises the thermoforming tool according to the second aspect described above for forming a plurality of articles in a material layer; and a heating device for heating the material layer before the material layer is formed by means of the thermoforming tool.The heating device can be regulated with the aid of the feedback signal provided by the evaluation device of the thermoforming tool.Brief Description of the DrawingsFurther details and advantages of the invention are explained with reference to the following drawings. The following are shown: FIGS. 1 a / 1 b are views of an exemplary thermoforming tool as known from the prior art; FIG. 2 is a graph showing a correlation between the temperature of a material layer and the stretching force to be applied; FIG. 3 is a flow chart illustrating a method of monitoring a thermoforming process according to the invention; FIG. 4 is a diagram showing the time profile of the stretching force during the prestretching of a material layer in the thermoforming tool; FIG. 5 shows schematic representations of mold cavities of a thermoforming tool and of a material layer having different temperature distributions; FIG. 6 shows schematic representations of different temperature distributions in the thickness direction of the material layer; FIG. 7 shows a schematic illustration of a thermoforming tool according to the invention, which is designed to implement the method described in connection with FIG. 2 ; FIGS. 8 a / 8 b are three-dimensional views of a thermoforming tool according to the invention with a measuring device for acquiring measured values indicative of stretching forces; and FIGS. 9 a / 9 b show schematic views of tool components of the thermoforming tool shown in FIGS. 8 aand 8 b.Detailed DescriptionIn conjunction with FIGS. 1 aand 1 b, a thermoforming tool 1000 is first described, as is known from the prior art, and in which the technique according to the invention described here for monitoring a thermoforming process can be implemented. FIG. 1 shows a sectional view, while FIG. 1 bshows an isometric representation of the thermoforming tool 1000.The thermoforming tool 1000 is a multiple thermoforming tool that has at least two mold cavities 160 and at least two pre-extenders 232 cooperating with the mold cavities 160. In FIGS. 1 aand 1 b, ten mold cavities 160 and ten pre-extenders 232 are indicated by way of example, which are arranged in a 2x5matrix arrangement in the thermoforming tool 1000. However, the invention does not depend on the specific number of mold cavities 160 and pre-stretchers 232. The thermoforming tool 1000 may also have more than 10 mold cavities 160 / pre-stretcher 232 or less than 10 mold cavities 160 / pre-stretcher 232, but at least two mold cavities 160 / pre-stretcher 232.The thermoforming tool 1000 comprises a first thermoforming tool part 100 and a second thermoforming tool part 200 which are arranged opposite each other in the axial direction (in the height direction) and movable relative to each other. The first thermoforming tool part 100 is formed as a lower thermoforming tool part 100 in the thermoforming tool 1000 shown in FIGS. 1 aand 1 b; correspondingly, the second thermoforming tool part 200 is formed as an upper thermoforming tool part 200.The lower thermoforming tool part 100 comprises a lower tool carrier 110 for receiving a cooling block 120. The cooling block 120 has at least two cavities for receiving at least two mold inserts 140. Furthermore, the lower thermoforming tool part 100 has at least two mold bases 130, wherein in each case one mold base 130 is accommodated in an axially displaceable manner at the axial lower end of a respective mold insert 140. Together, the mold base 130 and the mold insert 140 each form a mold cavity 160 into which a material layer supplied to the thermoforming tool 1000 can be shaped in order to form an article, in particular a packaging article. The article may be a cup, container, shell or capsule; the geometry of the shaped article is dictated by the mold cavity 160, in particular by the inner wall / contact surface 162 of the mold insert 160 and the contact surface 132 of the mold bottom 130.The mold base 130 of the thermoforming mold 1000 assigned to each mold insert 140 is coupled to a common ejection rail 136 via a corresponding ejection rod 134. The ejector bar 136 is actuatable and can in particular be axially raised (displaced upwards), whereby the mold bases 130 in the respective mold inserts 140 are raised. Thereby, the articles molded in the mold cavities 160 can be ejected.The upper thermoforming tool part 200 has a block-shaped upper tool carrier 210, in which at least two recesses 240 are formed. In each of the at least two recesses 240, a hold-down device 220 is provided for holding down the material layer during a thermoforming process. Furthermore, the upper thermoforming tool part 200 has a pre-stretcher device 230. The pre-stretcher device 230 comprises at least two pre-stretchers 232, wherein each pre-stretcher 232 is arranged in a corresponding hold-down recess so as to be axially displaceable and coaxial to a corresponding mold cavity 160 of the first thermoforming tool part 100. Each of the at least two pre-extenders 232 is coupled at its end axially facing away from the mold cavity 160 to a pre-extender rod 234. Each pre-stretcher bar 234 is in turn coupled to a common pre-stretcher plate 236. The pre-stretcher plate 236 is actuatable in the axial direction, so that the at least two pre-stretchers 232 can be actuated simultaneously. In particular, the at least two pre-extenders 232 can be simultaneously disengaged and moved into the respective mold cavity 160 (i.e. moved downward) with the aid of the actuatable pre-extender plate 236; likewise, the at least two pre-extenders 232 can be simultaneously re-engaged and moved back into their initial position (i.e. moved upward). In FIG. 1 a, the pre-extenders 232 are in their initial position, in which the pre-extenders 232 are accommodated in the corresponding hold-down recess. By disengaging the respective pre-extenders 232 into the mold cavities 160, a material layer arranged between the lower thermoforming tool part 100 and the upper thermoforming tool part 200 can be pre-stretched into the respective mold cavity.A standardized thermoforming process is described further below in connection with the thermoforming tool 1000 shown in FIGS. 1 aand 1 b. In the open state of the thermoforming tool 1000 (i.e. when the two thermoforming tool parts 100, 200 are moved apart→shown in FIG. 1 ), a heated material layer can be arranged between the first thermoforming tool part 100 and the second thermoforming tool part 200. Subsequently, the thermoforming tool 1000 is closed, for example by at least one of the two thermoforming tool parts 100, 200 being moved against the other thermoforming tool part 100, 200. With the aid of the prestretchers 232, the material layer held down (clamped) between the two thermoforming tool parts 100, 200 with the aid of the hold-downs 220 is stretched or prestretched into the respective mold cavities 160, as a result of which preformed moldings which do not yet have the final article shape are produced.For complete molding, compressed air is introduced into the respective mold cavities 160 via channels (these are not shown in FIGS. 1 aand 1 b), whereby the pre-molded molded molded articles are pressed against the contact surfaces 132 and 162 of the mold base 130 and of the mold insert 140 and are thus molded into the final articles. By contacting with the contact surfaces 132, 162, the shaped material layer is cooled, whereby a stable article is produced.Subsequently, the thermoforming tool 1000 can be evacuated (i.e. the molding air built up in the respective mold cavities 160 can be reduced again) and the thermoforming tool 100 can be opened, i.e. the two thermoforming tool parts 100, 200 can be moved apart. By raising the mold bottoms 130 by means of the ejection bar 136, the molded articles can be ejected.In order to be able to produce articles of desired quality at the at least two mold cavities 160, it is important that the same thermoforming process takes place at each of the at least two mold cavities 160. A process parameter that significantly influences the thermoforming process and thus the quality of the shaped articles is the temperature / forming temperature to which the material layer is heated for the thermoforming process. The forming temperature can be subject to fluctuations in material layers with a large forming surface, which are provided for multiple thermoforming tools having a plurality of forming cavities; such temperature fluctuations can be caused, for example, by an unexpected failure of heating jet elements of a heating device, which are provided for heating the material layer, or by a feed device, which is provided for feeding the material layer to the heating device and to the thermoforming tool. In order to be able to detect and counter-control fluctuations in the process parameters, in particular in the forming temperature of the material layer, it is therefore desirable to continuously monitor the process parameters in the thermoforming tool 1000 and, if possible, to monitor them in a mold cavity-resolved manner.Test experiments have shown that, in a thermoforming tool 1000, as described in connection with FIGS. 1 aand 1 b, the stretching force which must be applied by the respective pre-stretchers 232 during pre-stretching of the heated material layer into the respective mold cavities 160 correlates with the temperature / forming temperature of the material layer. This correlation between the stretching force to be applied and the temperature / forming temperature of the heated material layer is shown in the diagram in FIG. 2.The diagram in FIG. 2 shows the profile of the stretching force as a function of the temperature / forming temperature of a material layer made of polypropylene which has been thermoformed with the aid of a thermoforming tool as described above in connection with FIGS. 1 aand 1 b. In the diagram, the stretching force recorded during thermoforming of the material layer is plotted as a function of a continuous thermoforming machine running time, wherein the thermoforming machine running time is plotted on the X axis and the recorded stretching force is plotted on the Y axis. The polypropylene layer was fed intermittently to the thermoforming tool in order to continuously form articles, wherein for each thermoforming cycle (machine cycle, wherein several cycles per minute were carried out) the maximum stretching force to be applied during the prestretching of the material layer was recorded and was depicted in the diagram as a function of the continuous thermoforming machine running time.In these test thermoforming tests, the temperature / forming temperature of the material layer was varied several times. In a heating phase in the thermoforming time interval from t 0 to t 1 the material layer was first continuously heated to a reference temperature T ref with the aid of a heating device and subsequently kept at this value until the time t 2. During this heating phase between t 0 and t 1 and the subsequent first temperature holding phase between t 1 and t 2 a plurality of test articles were molded, wherein the maximum stretching force applied by the pre-stretcher for pre-stretching the material layer was recorded during each molding cycle and entered in the diagram. As can be seen from the diagram, the detected stretching force decreases continuously with increasing forming temperature of the material layer and remains virtually unchanged in the first temperature holding phase between t 1 and t 2.At the time t 2 the temperature of the material layer was further increased by a predetermined amount with respect to the reference temperature T ref with the aid of the heating device (in the present case an increase by 2 Kelvin) and was kept constant until the time t 3. During this second temperature holding phase between t 2 and t 3 a plurality of test articles were again formed and the maximum stretching force applied by the prestretcher for prestretching the material layer was detected during each forming cycle. As can be seen from the diagram, the detected stretching force decreases in a stepwise manner as a result of the temperature increase at the time t 2 and then remains substantially constant at a lower force level (approximately 100 N less compared to the force level in the time range t 1 to t 2).At the times t 3 and t 4 the temperature of the material layer was increased again by the same amount (2 Kelvin each) with the aid of the heating device and the maximum stretching force during the prestretching of the material layer was recorded. The detected maximum stretching force decreases by a further 100 N at the points in time t 3 and t 4 as a result of the temperature increase carried out and then remains at a lower force level in each case.As is clearly evident from the diagram, the detected stretching force correlates with the temperature / forming temperature of the supplied material layer. In particular, changes in the temperature of the material layer can be easily resolved by detecting the stretching force. This is because, as can be seen from the diagram, even small temperature changes in the material layer lead to considerable changes in the stretching force (100 N for temperature changes of 2 Kelvin). Similar relationships also arise if instead of a polypropylene layer a material layer is used which consists of another thermoplastic material, such as PET or polystyrene (PS).Based on these test experiments and their results, the inventors have recognized that the stretching force to be applied for a mold cavity correlates with the actual forming temperature of the material layer in the region of the mold cavity and that a precise statement about the actual temperature of the material layer in the region of each mold cavity can be made by recording the stretching force or the stretching energy or a measured value indicative of the stretching force or stretching energy. In particular, the inventors have recognized that the stretching force to be applied to each mold cavity during the prestretching or stretching of the material layer is not determined as much by the surface temperature of the material layer, but rather by the actual temperature or temperature profile over the material thickness of the material layer at the mold cavity. Thus, the stretching force to be applied is a measure for the heating through of the material layer, in particular a measure for the homogeneity of the heating of the material layer in the thickness direction.Furthermore, the inventors have recognized that the correlation between the stretching force or stretching energy and the temperature / forming temperature can be used to record the thermoforming process, in particular the temperature / forming temperature of the material layer in a multiple thermoforming tool depending on the mold cavity and (compare it with one another). In this way, it becomes possible to monitor the thermoforming process in a multiple thermoforming tool depending on the mold cavity and to detect possible deviations / inhomogeneities in the thermoforming process at the respective mold cavities. In particular, it is possible to record the actual temperature / forming temperature of the material layer supplied to the thermoforming tool as a function of the mold cavity; in particular, deviations / inhomogeneities in the forming temperature of the material layer can be recorded at the respective mold cavities, and the homogeneity of the temperature distribution of the material layer in the transverse and / or longitudinal direction can thus be determined.In conjunction with FIGS. 3 to 6, a method according to the invention for monitoring thermoforming processes in a thermoforming tool which is designed for forming a plurality of articles in a material layer will now be further described. The method is based on the correlation described further above in connection with FIG. 2 between the actual temperature / forming temperature of the material layer and the stretching force or stretching energy to be applied for prestretching or stretching the material layer.FIG. 3 shows a flow chart which illustrates the steps of the method according to the invention.In a first step S 10 of the method, at least two measured values are recorded at at least two mold cavities 160 a, 160 b, 160 c(see also FIGS. 5 and 6 ), wherein at least one measured value is recorded at each of the at least two mold cavities 160 a, 160 b, 160 c, said measured value indicating a stretching force or stretching energy to be applied during the prestretching of the material layer into the respective mold cavity 160 a, 160 b, 160 c.The at least one measured value recorded at each of the at least two mold cavities 160 a, 160 b, 160 cmay be an individual measured value, for example a maximum value, which is indicative of a maximum stretching force to be applied during the prestretching of the material layer into the respective mold cavity (for example by a respective prestretcher of the thermoforming tool). Since the thermoforming process is a dynamic process in which tool components are in motion during each thermoforming cycle, such as the movement of the pre-extenders during the pre-stretching of the material layer into the respective mold cavities, the acquisition of individual measured values can be subject to fluctuations. It can therefore be advantageous not only to record a measured value, such as a maximum value, which points to the maximum stretching force to be applied, but to record a plurality of measured values at the respective at least two mold cavities 160 a, 160 b, 160 cduring the pre-stretching process or during the entire thermoforming cycle. As a result, a measured value profile can be obtained for each molding cycle or prestretching process, which is indicative of the stretching force profile at the respective mold cavities.In the diagram in FIG. 4, such a measurement value profile is shown, wherein the recorded measurement values were converted to corresponding stretching force values and plotted as a function of the forming cycle time (see X axis) (see values on the Y axis). It can be clearly seen that the stretching force depends on the prestretch travel or the prestretch time and, as the prestretch travel (prestretch time) increases, the detected stretching force continuously increases up to a maximum value (time range 610 in FIG. 4 ). The measurement value profile or stretching force profile in the measurement region 610 is strongly temperature-dependent (temperature-sensitive) and can be used in the method according to the invention. In particular, the slope or the integration of the measured value profile in the prestretching time range 610 can be used, which is a measure of the stretching energy to be applied. For the sake of completeness, it should also be mentioned that the stretching force falls again in the time range 620 adjoining the time range 610, since the maximum prestretch travel has been reached and molding air has been introduced into the respective mold cavity for the further shaping of the material layer (range 620 in FIG. 4 ).As described above, in the first step S 10, the at least one measurement value (single measurement value or a plurality of measurement values for acquiring a measurement value profile during the pre-stretching process) is acquired at at least two mold cavities 160 a, 160 b, 160 c. For example, the at least one measured value indicative of a stretching force or stretching energy can be recorded at at least three mold cavities 160 a, 160 b, 160 c, as is illustrated in FIG. 5. The three mold cavities 160 a, 160 b, 160 c, at each of which at least one measured value indicative of a stretching force or stretching energy is detected, are marked by an "X" in FIG. 5.FIG. 5 shows, in the left and right illustration, in each case a lower thermoforming tool part 100 (indicated by a dashed rectangle in FIG. 5 ) which has a multiplicity of mold cavities 160 which are arranged in a 5x3 matrix mold (see dashed circles). Furthermore, both illustrations show a material layer 10 which is fed intermittently to the thermoforming tool in the feed direction 22 (vertical direction in FIG. 5 ) and is arranged above the lower thermoforming tool part 100. In the left illustration, the material layer has a temperature gradient in the direction transverse to the feed direction 22 (i.e. in the transverse direction 24), while in the right illustration the temperature is homogeneous over the entire forming surface and does not have any temperature variation or temperature gradients.By acquiring at least one measured value indicative of a stretching force or stretching energy at each of the three mold cavities 160 a, 160, 160 carranged at a distance from one another in a direction diagonal to the feed direction 22, the temperature gradient of the material layer 10 indicated in the left illustration can be easily detected. This is because, due to the respectively different temperature of the material layer 10 in the region of the mold cavities 160 a, 160 band 160 c(see grey scale 30 in FIG. 5 ), during the prestretching of the material layer 10 into the respective mold cavities 160, 160 b, 160 c, respectively different measured values or measured value profiles are measured, which indicate the stretching force or stretching energy. The situation is different in the right-hand illustration, where the material layer 10 has been heated homogeneously, with the result that the same measurement values are obtained in each case at the respective mold cavities 160 a, 160 b, 160 c. For better comparison of the measured values, the measured values at the respective mold cavities 160 a, 160 b, 160 care preferably recorded simultaneously.In the exemplary illustration in FIG. 5, the measured values are recorded at three mold cavities 160 a, 160 b, 160 carranged in the diagonal direction. Thus, a statement can be made about the temperature of the material layer 10 in the center and in the respective outer corner regions, whereby a reliable statement about the homogeneity of the temperature profile over the entire material layer 10 is obtained. It is understood that depending on the size of the mold, in particular on the number of mold cavities present in the mold, the measured value can also be recorded at more than two or three mold cavities in order to be able to easily resolve local temperature changes or deviations from a setpoint temperature. As a rule, however, it is sufficient to record the measured values at a few mold cavities at suitable positions; it is not necessary to record the temperature at each mold cavity in order to obtain a valid statement about the homogeneity of the temperature / forming temperature over the entire forming surface of the material layer 10.In addition, the actual temperature / forming temperature can also be detected via the material layer thickness using the measuring principle described here, as is indicated further above in connection with FIG. 2 and is described further below in connection with FIG. 6.FIG. 6 shows the thermoforming tool from FIG. 5 comprising the lower thermoforming tool part 100 with the mold cavities 160; the material layer 10 has a homogeneous surface temperature, which corresponds, for example, to a setpoint temperature / setpoint forming temperature, as is shown on the left in FIG. 6. Nevertheless, the measured values measured at the mold cavities 160 a, 160 band 160 cmay indicate stretching forces or stretching energies, which deviate from a target stretching force or target stretching energy corresponding to the target temperature of the material layer 10.To illustrate this situation, reference is made to the cross-sectional illustration of the material layer 10 along the line A-A in the middle of FIG. 6 and to the cross-sectional illustrations BT 1, BT 2, BT 3 shown on the right, with different temperature profiles in the cross-sectional region B of the material layer 10. Whereas in the upper illustration BT 1 the temperature at the surface 10 aand in the interior of the material layer 10 (i.e. along the thickness d) substantially corresponds to the desired forming temperature and is thus homogeneous, the illustrations BT 2 and BT 3 each have a negative temperature gradient in the thickness direction. In BT 2 the temperature decreases sharply towards the middle of the material layer, while in BT 3 the temperature of the material layer decreases more slowly starting from the surface 10a, but continuously towards the rear surface of the material layer 10. The measured value or measured value profile recorded at the respective mold cavities 160 a, 160 b, 160 c, which points to the stretching force or stretching energy to be applied during prestretching of the material layer 10, will only correspond to the setpoint value or setpoint value profile for the homogeneous temperature profile in the illustration BT 1 while a strong deviation of the recorded measured value from the setpoint value is to be expected for the inhomogeneous temperature profile in the illustration BT 2 and an average deviation of the recorded measured value from the setpoint value is to be expected for the inhomogeneous temperature profile in the illustration BT 3. Thus, the acquired measured values at the at least two mold cavities 160 a, 160 b, 160 cmay also be used to make a statement as to whether the temperature / forming temperature of the material layer 10 has a desired (homogeneous) temperature profile in the thickness direction of the material layer.Returning to FIG. 3, after the at least two measured values have been recorded at at least two mold cavities 160 a, 160 b, 160 cin a first method step, the measured values recorded at the at least two mold cavities 160 a, 160 b, 160 care compared in a subsequent step S 20 in order to obtain a comparison value. The step of comparing can in this case comprise comparing the measured values recorded for the at least two mold cavities 160 a, 160 b, 160 cto one another and / or comparing the measured values recorded for the at least two mold cavities 160 a, 160 b, 160 cwith a reference measured value. The reference measured value can be a desired measured value which would be obtained during the prestretching of the material layer 10 with a desired forming temperature.Furthermore, the step of comparing can comprise ascertaining a deviation between the measured values recorded for the at least two mold cavities 160 a, 160 b, 160 cof one another and / or ascertaining a deviation of the measured values recorded for the at least two mold cavities 160 a, 160 b, 160 cof the reference measured value.The determined deviation between the measured values recorded for the at least two mold cavities 160 a, 160 b, 160 cis a measure (indicator) for how much the forming temperature of the material layer between the at least two mold cavities 160 a, 160 b, 160 cdiffers. As a rule, it is sufficient to compare the measured values at at least two mould cavities 160 a, 160 b, 160 c, which are spaced apart from one another, and to determine their deviation in order to monitor the homogeneity of the temperature distribution / forming temperature distribution over the forming surface of the material layer 10 a. In the case of large thermoforming tools having many mold cavities, it may be advantageous to compare the measured values at at least three, at least four or more mold cavities 160 a, 160 b, 160 carranged at a distance from one another and to determine their deviation from one another in order to monitor the homogeneity of the temperature distribution / forming temperature distribution over the forming surface of the material layer 10 a.The determined deviation of the measured values recorded for the at least two mold cavities 160 a, 160 b, 160 cfrom the reference measured value is furthermore a measure (indicator) for how much the actual temperature of the material layer (not only on the surface but also in the interior of the material layer) deviates from a setpoint forming temperature.If an individual measured value is detected in each case in the first step S 10 for the at least two mold cavities 160 a, 160 b, 160 c, such as a maximum value (measured value maximum), these values are compared with one another and / or with a corresponding reference measured value (desired measured value); deviations which can be determined during the comparison can be provided as a comparison result. If, on the other hand, in the first step S 10, a measurement profile is recorded for each of the at least two mold cavities 160 a, 160 b, 160 cby recording a plurality of measurement values during the pre-stretching process, the recorded measurement profiles can be compared with one another and / or with a setpoint measurement profile. For example, slopes of the recorded measured value profiles, which correspond to an increase in the stretching force to be applied during the pre-stretching operation (see FIG. 4, slope of the measured value profiles in the measurement range 610), can be compared with one another. Alternatively, it is also conceivable that the recorded measured value profiles are integrated over a specific prestretch travel (prestretch time in FIG. 4 ) and the integration values, which indicate the stretching energy to be applied, are compared with one another.The comparison result provided by the comparison (i.e. the determined deviation(s)) can(can) be compared with a predefined threshold value (tolerance value), which represents a tolerance measure not critical for the thermoforming process. If the determined deviation(s) exceeds the threshold value, this is a measure (indicator) for the fact that the thermoforming process, in particular the forming temperature of the material layer 10, is not sufficiently homogeneous across the mold cavities of the multiple thermoforming tool. In addition, it can be detected whether the temperature of the material layer corresponds in principle to the reference value and thus the correct forming temperature is present.In a subsequent third step S 30, a feedback signal can be generated and provided if the comparison result indicates an inhomogeneous thermoforming process, in particular an inhomogeneous temperature distribution of the material layer. This can be the case when the determined deviation(s) exceeds the tolerance value, as described above.In conjunction with FIG. 7, a thermoforming tool 1000 aaccording to the invention is described, which is designed to implement the method described above.The thermoforming tool 1000 ais only schematically indicated in FIG. 7. It comprises a lower thermoforming tool part 100 and an upper thermoforming tool part 200. The lower thermoforming tool part 100 comprises at least two mold cavities 160 for simultaneously forming at least two articles in a molding cycle. The upper thermoforming tool part 200 comprises at least two pre-extenders 232 corresponding to the at least two mold cavities 160. Each pre-stretcher 232 of the at least two pre-stretchers 232 is configured to pre-stretch a material layer (not shown in FIG. 7 ) into the corresponding mold cavity 160.For clarity, only the components of thermoforming tool 1000 awhich are most important for the invention are shown and described in FIG. 7. It is understood that the at least two pre-stretchers 232 may be coupled via respective pre-stretcher rods to a pre-stretcher bar for common actuation, as described in connection with the thermoforming tool 1000 in FIG. 1. In the same way, the at least two mold cavities 160 can each have a mold base at their axial lower end, which are coupled to respective ejector rods and a common ejector strip.The thermoforming tool 1000 afurther comprises a measuring device 300. The measuring device 300 is designed to record at least one measured value at each of at least two mold cavities 160 which is indicative of a stretching force or stretching energy to be applied during the prestretching of the material layer into the respective mold cavity 160. For this purpose, the measuring device 300 has at least two sensors 320, which are arranged in the thermoforming tool at a distance from one another in the transverse direction / width direction (see FIG. 7 ) and / or in the longitudinal direction of the thermoforming tool 1000 a. A specific implementation of measurement sensors 320 is described in more detail below in connection with FIGS. 8, 9 aand 9 b. Independently of the specific implementation, each of the at least two measurement sensors 320 of the measuring device 300 is designed to record at least one measurement value which is indicative of a stretching force or stretching energy which is to be applied by the respective pre-stretcher 232 during the pre-stretching of the material layer into the respective mold cavity 160.The measured values recorded by the at least two sensors 320 can be provided to an evaluation device 400 via a (wireless or wired) communication interface 340 of the measuring device 300.The evaluation device 400 is designed to compare the measured values recorded at the at least two mold cavities 160 or pre-extenders 232 in order to obtain and provide a comparison result. In particular, the measuring device 400 can be configured to compare the measured values recorded at the at least two mold cavities 160 and to determine a deviation of the at least two recorded measured values from one another or from a reference measured value (desired measured value) and to provide them as a comparison result, as described further above in connection with the method (step S 20).The evaluation device 400 can furthermore be configured to generate and provide a feedback signal 500 if the comparison result indicates an inhomogeneous thermoforming process, in particular an inhomogeneous temperature profile of the material layer in the thermoforming tool 1000 a. This can be the case when the determined deviation in the comparison result exceeds a predefined threshold value (tolerance value). In such a case, the evaluation device can generate and output the feedback signal 500. The generated feedback signal 500 can comprise, for example, a warning signal for a user; additionally or alternatively, the generated feedback signal 500 can comprise a control or regulating signal, in particular a control or regulating signal for a heating device for heating the material layer. Thus, a heating device can be automatically readjusted.While the at least two sensors 320 of the measuring device 300 are installed in the thermoforming tool (for example are coupled to the respective pre-extenders 232), the evaluation device 400 can be arranged outside the thermoforming tool 1000 aand can be connected wirelessly or by wire to the sensors 320 installed in the thermoforming tool 1000 avia a communication interface 440. The communication interface 440 can be coupled to the communication interface 340 of the measuring device 300 and read out the sensor measurement values provided by the respective sensors 320 in real time during the thermoforming process.The evaluation device 400 can be implemented as a combined software and hardware module and comprise at least one processor 420 for executing the above-mentioned functionalities.In conjunction with FIGS. 8 aand 8 band FIGS. 9 aand 9 b, a specific implementation of a measuring device 500 in conjunction with a thermoforming tool 1000 according to the invention is further described. FIG. 8 ashows an isometric view of the thermoforming tool 1000, while FIG. 8 bshows the pre-stretcher device 230 of the thermoforming tool 1000, comprising a plurality of pre-stretchers 232 ( 10 in the specific tool) which are coupled via respective pre-stretcher rods 234 to a common, actuatable pre-stretcher plate 236.The thermoforming tool 1000 of FIGS. 8 aand 8 b substantially corresponds to the thermoforming tool 1000 of FIG. 1 Only the differences from the thermoforming tool 1000 of FIG. 1 will be described below. With regard to the components which are structurally identical and functionally identical, reference is made to the description in connection with FIG. 1 further above.In contrast to the thermoforming tool of FIGS. 1 a / 1 b, the thermoforming tool 1000 additionally comprises a measuring device 300. The measuring device 300 has at least three sensors 320, which are coupled to respective bending elements 310. The flexures 310, in turn, are coupled to respective pre-extenders 232.The bending elements can be realized directly in the pre-stretcher plate 236 with the aid of slot structures 312 formed in the pre-stretcher plate 236, as shown in FIG. 2 b. This realization of the bending elements 310 is simple and space-saving and enables easy positioning and wiring of the sensors 320. For example, the respective sensors 320 can be arranged on the upper side of the respective bending elements 310 facing away from the respective pre-stretcher 232, as illustrated in FIGS. 8 aand 8 b. In addition or as an alternative to the sensor arrangement shown in FIGS. 8 aand 8 b, the sensors 320 can also be arranged on the underside of the respective bending elements 310 facing away from the upper side.Each bending element 310 has a specific elasticity and can be elastically deformed or bent (slightly) upon the application of a force, in particular a stretching force, to the pre-stretcher 232 coupled to the bending element 320. This principle will be further described in connection with Figs. 9a and 9b.FIGS. 9 aand 9 b show, by way of example, in each case one bending element 310 and the pre-stretcher 232 coupled to the bending element 310 via the pre-stretcher rod 234. In FIG. 9 a, no force acts on the pre-stretcher 232 so that the bending element 310 is not deformed. In FIG. 9 b, a force acts on the pre-stretcher 232. The force acting on the prestretcher 232 is indicated in FIG. 9 b by an arrow, wherein the arrow direction indicates the force direction in which a stretching force acts on the prestretcher 232 when the material layer is prestretched or stretched with the aid of the prestretcher 232. Due to the tensile force acting on the pre-stretcher 232, the bending element 310 is elastically deformed or bent / bent in proportion to the amount of force.This bending / bending can be detected by means of the sensors 320 on the respective bending element 310, wherein the measured value is proportional to the bending / bending of the bending element 310. The bending / bending is in turn proportional to the stretching force applied to the prestretcher, which is in turn proportional to the temperature / forming temperature of the material layer 10, as described above. Thus, the measured values recorded by the sensors 320 on the respective bending elements 310 are proportional to the stretching force applied to the respective pre-stretcher, and thus also proportional to the actual forming temperature of the material layer.As sensors 310, for example, strain gauges can be used which change their electrical resistance depending on the deformation (bending / bending) of the respective bending elements 310. Strain gauges have proven to be reliable and sufficiently precise for measuring the deformation or strain of the bending elements 310; nevertheless, the use of other sensors (displacement sensors, piezoelectric sensors, inductive or capacitive sensors or optical sensors) for measuring the strain, bending or deflection of the bending elements 310 is conceivable.The electrical resistance of the respective strain gauges 320 at the respective flexures 310 may be detected (continuously). The resistance values detected at the respective flexures 310 may be directly compared to each other. Alternatively, the detected resistance values can be converted into corresponding stretching force values and compared with one another or with a desired measurement value. The deviation(s) between the resistance values determined during the comparison are an indicator of whether or not the thermoforming process takes place identically (and thus homogeneously) at the respective mold cavities.The technique described here enables reliable and cost-effective monitoring of a thermoforming process in a multiple thermoforming tool. In particular, the homogeneity of the temperature / forming temperature of the material layer supplied to the multiple thermoforming tool can be monitored reliably (and in real time) over the entire forming surface of the material layer and in the thickness direction of the material layer. The technique according to the invention can also be used for automatic readjustment of a heating device of the thermoforming machine if deviations in the temperature distribution of the material layer are detected.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedEP 1 541 320 A1

[0003] EP 1 163 996 B1

[0003] DE 10 2016 103 237 A1

[0003]

Claims

A method of monitoring a thermoforming process configured to form a plurality of articles in a material sheet (10), the method comprising: acquiring at least two measurement values at at least two mold cavities (160, 160a, 160b, 160c), wherein at each of the at least two mold cavities (160, 160a, 160b, 160c) at least one measurement value is acquired indicative of a stretching force or stretching energy to be applied during pre-stretching of the material sheet (10) into the respective mold cavity (160, 160a, 160b, 160c); and comparing the measurement values acquired at the at least two mold cavities (160, 160a, 160b, 160c) to obtain a comparison result, wherein the comparison result is an indicator of homogeneity of the thermoforming process.Method according to Claim 1, wherein the at least two measured values are recorded at at least two mould cavities (160, 160a, 160b, 160c) which are arranged in a thermoforming tool (1000, 1000a) at a distance from one another in a direction transverse to the feed direction of the material layer (10) and / or in a direction parallel to the feed direction of the material layer (10).The method of claim 1 or 2, wherein the measurement value acquired at each of the at least two mold cavities (160, 160a, 160b, 160c) is a measurement value proportional to the stretching force or stretching energy applied during pre-stretching of the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c).Method according to one of Claims 1 to 3, wherein the comparison result is obtained by determining a deviation of the at least two recorded measurement values from one another and / or a deviation of the at least two recorded measurement values from a predefined reference value.Method according to one of claims 1 to 4, wherein the step of acquiring comprises simultaneously acquiring at least one measured value or one measured value profile at each of the at least two mold cavities (160, 160a, 160b, 160c) during the pre-stretching of the material layer (10).Method according to one of Claims 1 to 5, wherein the step of comparing comprises comparing measured values recorded at the at least two mould cavities (160, 160a, 160b, 160c) or comparing measured value profiles recorded at the at least two mould cavities (160, 160a, 160b, 160c).Method according to Claim 6, wherein the comparison result is obtained by comparing increases in measurement values or measurement value maxima of the measurement value profiles recorded at the at least two mould cavities (160, 160a, 160b, 160c) during the prestretching of the material layer (10).Method according to Claim 6, wherein the comparison result is obtained by integrating the recorded measured value profiles at the at least two mould cavities (160, 160a, 160b, 160c) and comparing the integration values.Method according to one of claims 1 to 8, wherein the measured value recorded at each of the at least two mould cavities (160, 160a, 160b, 160c) correlates with the temperature distribution in the material layer (10) at the respective mould cavity (160, 160a, 160b, 160c) and wherein the comparison result is used as an indicator for the homogeneity of the temperature distribution of the material layer (10).Method according to one of Claims 1 to 9, further comprising generating and providing at least one feedback signal (500) if the comparison result points to an inhomogeneous thermoforming process, in particular to an inhomogeneous temperature distribution of the material layer (10).Method according to one of Claims 1 to 10, wherein the feedback signal (500) comprises a warning signal for a user and / or a control or regulating signal, in particular a control or regulating signal for a heating device for heating the material layer (10), which is generated as a function of the comparison result.Thermoforming tool (1000, 1000a) for forming a plurality of articles in a material layer (10) supplied to the thermoforming tool (1000, 1000a), the thermoforming tool (1000, 1000a) comprising: at least two forming cavities (160, 160a, 160b, 160c) for forming at least two articles; at least two pre-extenders (232) corresponding to the at least two forming cavities (160, 160a, 160b, 160c), wherein each pre-extender (232) of the at least two pre-extenders (232) is configured to pre-extend the material layer (10) into the corresponding forming cavity (160, 160a, 160b, 160c); a measuring device (300) which is designed to record at least one measured value at each of at least two mold cavities (160, 160a, 160b, 160c), which measured value points to a stretching force or stretching energy to be applied during the prestretching of the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c), and to provide the recorded measured values to an evaluation device (400).Thermoforming tool (1000, 1000a) according to claim 12, wherein the measuring device (300) comprises at least two sensors (320), wherein a first sensor (320) is provided for detecting at least one measured value indicative of the stretching force or stretching energy, which is to be applied by a first pre-stretcher (232) for pre-stretching the material layer into a first mold cavity (160, 160a, 160b, 160c), and a second sensor (320) is provided for detecting at least one measured value indicative of the stretching force or stretching energy, which is to be applied by a second pre-stretcher (232) for pre-stretching the material layer into a second mold cavity (160, 160a, 160b, 160c), wherein the second mold cavity (160, 160a, 160b, 160c) or the second pre-stretcher (232) is arranged spaced apart from the first mold cavity (160, 160a, 160b, 160c) or from the first pre-stretcher (232) in a direction transverse to the feed direction of the material layer (10) and / or in a direction parallel to the feed direction of the material layer (10).The thermoforming tool (1000, 1000a) of claim 13, wherein the first sensor (320) is configured to measure an extension or deflection of a first flexure (310) coupled to the first pre-stretcher (232) caused by the stretching force, and the second sensor (320) is configured to measure an extension or deflection of a second flexure (310) coupled to the second pre-stretcher (232) caused by the stretching force.Thermoforming tool (1000, 1000a) according to claim 14, wherein the first sensor (320) is arranged on an upper side and / or on an underside of the first bending element (310), and wherein the second sensor (320) is arranged on an upper side and / or on an underside of the second bending element (310).Thermoforming tool (1000, 1000a) according to claim 14 or claim 15, wherein the first bending element (310) and the second bending element (310) are part of a pre-stretcher plate (236) provided for simultaneously actuating the at least two pre-stretchers (232).Thermoforming tool (1000, 1000a) according to any of claims 14 to 16, further comprising a protection device configured to limit the stretching or bending of the first bending element (310) and / or the second bending element (310).Thermoforming tool (1000, 1000a) according to any of claims 12 to 17, further comprising the evaluation device (400) configured to compare the measured values acquired for the at least two mold cavities (160, 160a, 160b, 160c) to obtain and provide a comparison result.Thermoforming tool (1000, 1000a) according to claim 18, wherein the evaluation device (400) is further configured to generate and provide a feedback signal (500) if the comparison result indicates an inhomogeneous thermoforming process, in particular an inhomogeneous temperature profile of the material layer (10) in the thermoforming tool (1000, 1000a).A thermoforming machine comprising: the thermoforming tool (1000, 1000a) according to any of claims 12 to 19 for forming a plurality of articles in a material sheet (10); and a heating device for heating the material sheet (10) before the material sheet (10) is formed using the thermoforming tool (1000, 1000a).

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