Method and devices for monitoring a thermoforming process

Abstract

The invention relates to a method for monitoring a thermoforming process. The invention also relates to a thermoforming tool and a thermoforming machine which implement such a method. The method for monitoring a thermoforming process comprises detecting at least two measurement values at at least two mould cavities of the thermoforming tool, wherein at each of the at least two mould cavities at least one measurement value is detected which indicates a stretching force or stretching energy to be applied to the respective mould cavity during prestretching of the material layer. The method also comprises comparing the measurement values detected at the at least two mould cavities in order to obtain a comparison result, wherein the comparison result is an indicator of the homogeneity of the thermoforming process.

Description

[0001] Methods and devices for monitoring a thermoforming process

[0002] Technical area

[0003] The invention relates to the field of thermoforming. In particular, the invention relates to a technique for monitoring a thermoforming process, as well as a thermoforming tool and a thermoforming machine implementing such a technique. State of the art

[0004] The use of thermoforming tools and thermoforming processes for forming packaging items in two-dimensional (flat) layers (sheets or blanks) of plastic is well known. The plastic materials used can be polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), or other thermoplastic materials. The formed packaging items are primarily used for portioned food packaging and can be designed as containers, cups, trays, or capsules, depending on their application.

[0005] 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. Such thermoforming tools comprise a first mold part (e.g. upper mold part) and a second mold part (e.g. lower mold part), which are arranged coaxially 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 displaceably mounted 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 molding space which replicates the shape of the cup to be formed.

[0006] In the production of cup-shaped packaging articles, the plastic material in the form of a two-dimensional layer (this can be in the form of a rollable sheet / film or a blank / board blank, for example) is first heated to a desired forming temperature. The two-dimensional material layer, heated to forming temperature, is then fed (intermittently) to the thermoforming tool. The two mold parts are moved apart (open state of the thermoforming tool) in order to be able to position the material layer between the two tool parts. The thermoforming tool is then closed by moving the two mold parts towards each other. Using the at least one pre-stretcher, the material layer brought to forming temperature is mechanically stretched or pre-stretched into the at least one mold cavity, thereby creating a pre-formed molded article.The mechanical stretching of the film with the aid of at least one pre-stretcher can also be omitted or carried out with the aid of compressed air (pre-blowing) or vacuum (pre-suction) if the deep-drawing depth and thus the forming or stretching ratio is small due to the geometry of the article to be formed (for example when forming a lid).

[0007] One process parameter that significantly influences the thermoforming process is the temperature of the two-dimensional layer to be formed. In order to obtain packaging items of the desired quality (e.g. desired transparency, desired forming sharpness), it is necessary to bring the layer to a desired forming temperature. The desired forming temperature can depend on the material composition and the physical properties (e.g. crystallinity) of the thermoplastic material used and must be adjusted accordingly for each material layer or determined through test experiments. It has been shown that, particularly with semi-crystalline plastic material layers, such as polypropylene layers, small changes in the forming temperature (e.g. a change in the forming temperature of 2-3 K) can lead to significant changes in the forming sharpness of the produced articles.

[0008] In the case of multi-cavity thermoforming tools with a multitude of mold cavities for the simultaneous production of a large number of packaging items per thermoforming cycle, which are currently preferred in the industrial production of packaging items, a further challenge is to heat the material layer in such a way that it has essentially the same desired forming temperature across the entire forming surface. This is the only way to ensure that all items produced in a thermoforming cycle have the same quality.

[0009] To achieve the most homogeneous temperature distribution possible across the forming surface of the material layer, radiant heating elements, preferably infrared radiant heating elements, are used in practice. These elements are arranged in a matrix in a heating device (heating station) located in front of the thermoforming tool and are also individually controllable. The arrangement and size of the radiant heating elements used are specified by the thermoforming machine manufacturer.

[0010] The homogeneity of heating a material layer across its forming surface depends primarily on the arrangement and operational readiness of the radiant heating elements in the heating station. For example, if a radiant heating element fails, this can lead to local temperature fluctuations in the material layer, which are often not immediately detected and can result in rejects during product production.

[0011] However, the use of a feeding device that grips and clamps the material layer in the edge area in order to feed the material layer to the heating station and then (intermittently) to a forming station comprising the thermoforming tool can also influence the homogeneity of the temperature profile across the forming surface. It has been shown that due to the influence of the feeding device, the temperature of the material layer in the edge area, i.e. transverse to the feed direction, where the material layer is gripped by the feeding device, is lower. Accordingly, the temperature of the radiant heating elements intended for heating the material layer in the edge area must be set higher than the radiant heating elements intended for heating the material layer in the middle area of ​​the material layer in order to compensate for this temperature gradient.

[0012] A further problem lies in determining the actual temperature (forming temperature) of the heated material layer. In practice, this is done contactlessly using infrared sensors or thermal imaging cameras. These primarily measure the surface temperature of the material layer, not the temperature distribution across the material layer thickness. However, the temperature distribution across the material layer thickness depends not only on the heating power of the radiant heating elements, but also on the residence time of the material layer in the heating station. Residence times that are too short in combination with too low a heating power can result in the temperature distribution across the thickness of the material layer being inhomogeneous, but exhibiting a negative temperature gradient starting from the material layer surface.This means that while the material layer on the surface may have a desired forming temperature, which is also measured by the infrared sensors, the material layer inside has significantly lower temperatures (i.e., is significantly colder). However, the decisive factor for the quality of the thermoforming process (i.e., transparency, sharpness of the formed parts, etc.) is not so much whether the surface of the material layer has the desired forming temperature, but whether the material layer as a whole, i.e., across the entire layer thickness, has the desired forming temperature.

[0013] In practice, the thermoforming process parameters, in particular the forming temperature, are set manually depending on the material layer used when starting up the thermoforming process (heating up the material layer and the thermoforming tool). Manually set means that test articles are produced and inspected for their quality (shape definition, transparency, etc.). The process parameters, in particular the temperature of the radiant heaters, are then manually set or adjusted by an operator (e.g. via a human-machine interface (HMI)) until the thermoforming tool produces articles of the desired quality. After that, regular production of the packaging articles can begin. The manual setting of thermoforming process parameters based on the inspection of produced articles is time-consuming and requires experienced operators. Furthermore, malfunctions or errors can occur.Undesirable changes in the process parameters during the thermoforming process (e.g. a temperature change due to the failure of a heating element) are only detected on the produced article and thus relatively late.

[0014] The object of the present invention is to eliminate these disadvantages. In particular, the object of the present invention is to provide an automated technology for monitoring a thermoforming process. The technology should also be capable of monitoring the homogeneity of the thermoforming process in a multi-cavity thermoforming tool, in particular the homogeneity of the temperature distribution in the material layer to be formed. Furthermore, it is desirable for the monitoring technology to operate in real time and initiate appropriate countermeasures in the event of deviations.

[0015] Brief outline

[0016] To achieve at least one of the above-mentioned objects, according to a first aspect of the invention, a method is provided for monitoring a thermoforming process designed to form a plurality of articles in a material layer. The method comprises the following steps: recording at least two measured values ​​at at least two mold cavities, wherein at each of the at least two mold cavities at least one measured value is recorded, which indicates a stretching force or stretching energy to be applied during pre-stretching of the material layer into the respective mold cavity; and comparing the measured values ​​recorded at the at least two mold cavities to obtain a comparison result, wherein the comparison result is an indicator of the homogeneity of the thermoforming process.

[0017] According to the present invention, the thermoforming process is designed for the simultaneous production of a plurality of articles in one layer of material. The simultaneous production of a plurality of articles means 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 described below, has at least two, preferably at least three, more preferably at least four mold cavities for the simultaneous formation of the at least two, preferably at least three, more preferably at least four articles. The molded articles can be packaging articles, in particular packaging articles for the portioned storage of food, such as containers, cups, bowls or capsules.

[0018] A material layer is a two-dimensional (sheet-like) layer of material made of a thermoplastic material suitable for thermoforming articles. The thermoplastic material can be polystyrene (PS), polypropylene (PP), polyethylene terephthalate (PET), or another thermoplastic material. Depending on the requirements of the articles to be produced, the material layer can be in the form of a rollable sheet of material or in the form of a plate-like cut material.

[0019] The stretching force to be applied is the force that must be applied during thermoforming to stretch or pre-stretch the heated material layer into the respective mold cavity (of the multi-cavity thermoforming tool). Accordingly, the stretching energy is the energy that must be applied when pre-stretching the material layer over a given pre-stretching path (deep-drawing path). It can be obtained by integrating the stretching force over the pre-stretching path (deep-drawing path).

[0020] It has been shown that the applied stretching force or stretching energy depends primarily on the temperature (forming temperature) of the material layer. The applied stretching force or stretching energy depends less on the surface temperature of the material layer than on the temperature of the material layer across its entire thickness (including the interior of the material layer). Higher temperatures of the material layer result in lower applied stretching forces or stretching energies, while lower temperatures result in higher stretching forces or stretching energies.

[0021] It has also been shown that changes in the temperature (forming temperature) of the material layer to be formed by a few Kelvin lead to significant, easily measurable changes in the stretching force (changes in the range of 100 N for temperature changes of only 2 K). This correlation between the applied stretching force or stretching energy 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 from the recorded stretching force (stretching energy) or from a measured value that indicates the stretching force or stretching energy.Thus, the measuring principle according to the invention based on the detection of at least one measured value indicating the stretching force or stretching energy is considerably more reliable than temperature measurements using infrared sensors or thermal imaging cameras, which are only surface-sensitive.

[0022] It is understood that the stretching force or stretching energy required for stretching or pre-stretching the material layer can also depend on the speed and / or geometry of the pre-stretcher used to stretch the material layer. However, these tool-specific parameters are known, 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 significant in the method according to the invention, since the at least one measured value (measurement parameter) indicating the stretching force or stretching energy is recorded independently of one another at at least two mold cavities of the multiple thermoforming tool, and the recorded measured values ​​are then compared (directly) with one another or with a predetermined reference value (target measured value) to obtain the comparison result. Since in the multiple thermoforming tool, the pre-stretchers 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 measured values ​​recorded at the at least two mold cavities with each other, the influence of the geometry and speed of the pre-stretchers on the measured values ​​is thus not significant, since this influence is the same for all measured values. If deviations occur in the measured values ​​recorded at the at least two mold cavities and indicating the stretching force or stretching energy, these deviations are related to changes in the actual forming temperature of the material layer at the respective mold cavities. Thus, the method according to the invention can be used to monitor the homogeneity of the temperature distribution in the material layer.

[0023] In order to monitor the homogeneity of the temperature distribution across the entire forming surface of the material layer, the at least two measured values ​​can be recorded in at least two mold cavities 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 refers to 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.

[0024] Preferably, at least one measured value can be recorded in each case at at least two mold cavities arranged diagonally to each other (i.e., diagonally to the feed direction) in the thermoforming tool. Thus, the homogeneity of the temperature distribution can be reliably monitored both in the feed direction / longitudinal direction and in the transverse direction / width direction of the material layer / thermoforming tool with minimal measurement effort.

[0025] 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 mold cavities of the thermoforming tool arranged diagonally spaced from one another, wherein a first mold cavity is arranged centrally in the thermoforming tool and a second mold cavity is arranged diagonally further outward (for example in a corner region) in the thermoforming tool.Based on this implementation, in a further development, the at least one measured value indicative of the stretching force can be recorded at each of at least three mold cavities arranged diagonally to one another, wherein a first mold cavity can be arranged centrally in the thermoforming tool, a second mold cavity can be arranged diagonally further outwards (for example in a first corner region) in the thermoforming tool and a third mold cavity can be arranged diagonally further outwards (for example in a second corner region which is diagonally opposite the first corner region) in the thermoforming tool.By recording and comparing measured values ​​at the mold cavities in the center and in the edge area, especially at the corner areas 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).

[0026] Deviations (inhomogeneities) in the temperature distribution of the material layer can be caused primarily by the feeding device, which is provided for feeding the material layers and grips and clamps them in the edge regions transverse to the feed direction. Due to the feeding device, the temperature in the edge regions opposite transverse to the feeding device can be somewhat lower than in the central region of the heated material layer. In order to detect such an inhomogeneity in the temperature distribution transverse to the feed direction, it may be sufficient to record the at least one measured value in at least two mold cavities, preferably in 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 in the transverse direction (for example in a first corner region) in the thermoforming tool.If the at least one measured value is additionally recorded at a third mold cavity, the third mold cavity can be arranged further outward in the transverse direction (for example in a second corner region which is opposite the first corner region in the transverse direction) in the thermoforming tool.

[0027] It is understood that the method described here is not intended to be limited to the recording and comparison of measured 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 reliable information about the temperature homogeneity across the forming surface. In particular, by recording and comparing measured values ​​at a large number of mold cavities (at four 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 radiant heating element) can be better monitored.On the other hand, it is not necessary to record and compare the measured values ​​indicating the stretching force or stretching energy for each mold cavity of the thermoforming tool, provided that the mold cavities for which the measured values ​​are recorded are distributed over the mold.

[0028] The at least one measured value which indicates the stretching force or stretching energy at the respective mold cavity can be a measured value which is proportional to the stretching force or stretching energy which is applied when the material layer is stretched into the respective mold cavity. An implementation of a measuring principle for recording measured values ​​which are indicative of / proportional to the stretching force or stretching energy is described in more detail 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 draw conclusions about the stretching force or stretching energy and thus also about the actual temperature of the material layer in the region of the respective mold cavity which correlates with the stretching force or stretching energy. As an alternative to this, however, it is also conceivable to record the stretching force orTo directly measure and compare the stretching energy at the at least two mold cavities in order to obtain a comparison result which indicates a deviation / inhomogeneity in the thermoforming process, in particular a deviation / inhomogeneity of the temperature distribution in the heated material layer.

[0029] The step of comparing the measured values ​​acquired at the at least two mold cavities can comprise comparing the measured values ​​acquired at the at least two mold cavities with one another. The comparing step can further comprise determining a deviation of the at least two acquired measured values ​​from one another in order to obtain a comparison result. The comparison result can comprise the determined deviation. In other words, by comparing the measured values ​​acquired at the at least two mold cavities with one another, it can be determined how much the measured values ​​at the at least two mold cavities differ from one another.This direct comparison method has the advantage that measurement-specific influences, such as the geometry of the pre-stretchers or the stretching speed, have no influence on the comparison result, since such measurement-specific influences are the same across all mold cavities and thus cancel each other out when comparing the measured values. A deviation (difference) determined in the comparison step between the measured values ​​recorded at different mold cavities thus represents a relative measure of the extent to which the thermoforming process taking place at the respective mold cavities, in particular the forming temperature of the material layer at the respective mold cavities, differs from each other.

[0030] 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 predetermined reference value. The comparing step can further 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 target measured value that indicates a target forming temperature.Such a reference value can be calculated / specified taking into account the stretching speed and / or the geometry of the thermoforming tool (for example the geometry of the pre-stretcher used to stretch the material layer into the respective mold cavity) in such a way that it indicates a desired forming temperature (target forming temperature) of the material layer.

[0031] A deviation of the measured values ​​recorded at different mold cavities from the reference value determined in the comparison step thus represents 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).

[0032] Using the comparison methods described above, based on comparing the recorded measured values ​​with each other or based on comparing the recorded measured values ​​with a specified reference value (target measured value), it is easy to determine whether the forming temperature of the material layer is the same at the respective cavities within a multiple thermoforming tool and thus whether the thermoforming process in the mold cavities of the thermoforming tool proceeds in the same way. Due to the direct relationship between the recorded measured values, the stretching force or stretching energy, and the forming temperature of the material layer, it is not necessary to convert the recorded measured values ​​into corresponding stretching force values ​​or stretching energy values ​​and forming temperature values, which can save computing capacity.

[0033] On the other hand, due to the direct relationship between the measured values ​​and the applied stretching force, as well as the stretching force and the forming temperature of the material layer, corresponding stretching force values ​​or stretching energy values, and from these corresponding thermoforming process values, in particular the forming temperature values ​​of the material layer at the respective cavities, can be calculated and compared with each other. This allows the user to better understand how much the thermoforming process, in particular the forming temperature of the material layer at the respective mold cavities of the thermoforming tool, deviates from one another or from a target value (target temperature).

[0034] In order to be able to objectively compare the measured values ​​recorded at the at least two mold cavities with each other or with a target value, the recording step can comprise a simultaneous recording of the at least one measured value or a measured value curve at each of the at least two mold cavities during the stretching / pre-stretching of the material layer into the respective mold cavities. In order to be able to record a measured value curve at each of the at least two mold cavities, a plurality of measured values ​​can be recorded during the stretching / pre-stretching of the material layer. However, it is also conceivable that the measured value curve is recorded during an entire thermoforming cycle (i.e., from the closing to the reopening of the mold).

[0035] The simultaneously recorded measured values ​​or measured value curves can be compared with each other. For example, it is conceivable that recorded measured value maxima are compared with each other. Alternatively, measured value increases (gradients in the measured value curve) can be compared with each other, provided the measured value curve is recorded during the pre-stretching of the material layer. Alternatively, it is also conceivable that recorded measured value curves at the at least two mold cavities are integrated over a predetermined pre-stretching distance (pre-stretching time) and the integrated values ​​are compared with each other to achieve a comparison result. The integration of recorded measured value curves has the advantage that statistically caused measurement errors, in particular outliers, are eliminated, thus making the obtained comparison result even more accurate.The method may further comprise the step of generating and providing at least one feedback signal if the comparison result includes a deviation between the at least two measured values ​​acquired at different mold cavities or a deviation of the measured values ​​acquired at the at least two mold cavities from a predetermined reference value. If the deviation determined in the comparison result exceeds a predetermined threshold value (tolerance value), this may be an indicator (indication) of an inhomogeneous thermoforming process, in particular an indication of an uneven / inhomogeneous temperature distribution across 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, for example, comprise 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 actively controlled. In particular, with the aid of the output control signal, the heating device provided for the thermoforming process can be automatically readjusted, thus eliminating inequalities in the temperature distribution across the forming surface of the material layer.

[0036] To achieve at least one of the above-mentioned objects, according to a second aspect of the invention, a thermoforming tool is provided for forming a plurality of articles in a material layer fed to the thermoforming tool, the thermoforming tool comprising: at least two mold cavities for forming at least two articles; at least two pre-stretchers corresponding to the at least two mold cavities, each pre-stretcher being designed to pre-stretch (or pre-stretch) the material layer into the mold cavity corresponding thereto.to stretch); and a measuring device which is designed to record at least one measured value on at least two mold cavities, which indicates a stretching force or stretching energy to be applied during pre-stretching (stretching) of the material layer into the respective mold cavity, and to provide the recorded measured values ​​to an evaluation device (for comparing the recorded measured values ​​and generating a comparison result). The thermoforming tool according to the present invention is a multiple thermoforming tool which is designed for the simultaneous forming of a plurality of articles (at least two, three or more articles) in one 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-stretchers, which are arranged, for example, in a second thermoforming tool part corresponding to the first thermoforming tool part. The first thermoforming tool part can be a lower thermoforming tool part, while the second thermoforming tool part can 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.

[0037] The measuring device can comprise at least two sensors, wherein a first sensor can be provided for detecting at least one measured value that indicates the stretching force or stretching energy that is to be applied when pre-stretching the material layer into a first mold cavity by a corresponding first pre-stretcher. A second sensor of the measuring device can be provided for detecting at least one measured value that indicates the stretching force or stretching energy that is to be applied when pre-stretching the material layer into a second mold cavity by a corresponding second pre-stretcher.The first pre-stretcher can be arranged at a distance from the second mold cavity or the second pre-stretcher 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). In this way, the at least two sensors can record and compare measured values ​​at different mold cavities in the transverse / width direction, the longitudinal direction, or the diagonal direction, and thus 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 fed to the thermoforming tool, as described above in connection with the method.It is understood that the measuring device does not have to be limited to the above-mentioned first and second sensors. Rather, the measuring device can comprise further sensors for recording measured values ​​indicative of the stretching force or stretching energy at further mold cavities of the thermoforming tool. For example, in one development, the measuring device can comprise a third sensor for recording a measured value at a third mold cavity or third pre-stretcher, which is arranged in the transverse direction / width direction of the thermoforming tool and / or in the longitudinal direction of the thermoforming tool at a distance both from the first mold cavity or the first pre-stretcher and from the second mold cavity or the second pre-stretcher. In yet another development, the measuring device can comprise a fourth sensor for recording a measured value at a fourth mold cavity ora fourth pre-stretcher, which is arranged at a distance from the first, second, and third mold cavities / pre-stretchers in the transverse / width direction and / or in the longitudinal direction of the thermoforming tool. By recording and comparing measured values ​​indicative of the stretching force or stretching energy at three, four, or more mold cavities / pre-stretchers, the homogeneity of the thermoforming process in the multiple thermoforming tool, in particular the homogeneity of the temperature distribution across the forming surface of the material layer fed to the thermoforming tool, can be monitored even better.

[0038] Regardless 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 further be designed (operated in such a way) that they record the measured values ​​simultaneously. In this way, the measured values ​​recorded by the at least two (three, four or more) sensors can be better compared (with each other). Furthermore, the at least two (three, four or more) sensors of the measuring device can be designed (operated in such a way) that they each record a plurality of measured values ​​during the pre-stretching of the material layer by the respective pre-stretchers into the respective mold cavities. In this way, a measured value curve can be recorded by the respective sensors which indicates the curve of the stretching force during the pre-stretching process into the respective mold cavities.

[0039] The stretching force acts on the respective pre-stretcher, which pre-stretches the material layer into the respective mold cavity. Cost-effective and reliable acquisition of measured values ​​indicative of the stretching force or stretching energy at at least two spaced-apart mold cavities can be achieved by utilizing the first sensor to measure an elongation or deflection caused by the stretching force of a first bending element coupled to the first pre-stretcher, and the second sensor to measure an elongation or deflection caused by the stretching force of a second bending element coupled to the second pre-stretcher. The first bending element and the second bending element thus cooperate with the respective first and second sensors of the measuring device.

[0040] In a further development, the measuring device can comprise additional sensors designed to detect the elongation or deflection of additional bending elements coupled to additional pre-stretchers arranged in the thermoforming tool at a distance from the first and second pre-stretchers in the transverse direction and / or longitudinal direction. For example, the measuring device can comprise a third sensor designed to detect the elongation or deflection of a third bending element coupled to a third pre-stretcher arranged in the thermoforming tool at a distance from the first pre-stretcher and the second pre-stretcher in the transverse direction and / or longitudinal direction.In a further exemplary implementation, the measuring device may comprise a fourth sensor configured to detect the elongation or deflection of a fourth bending element coupled to a fourth pre-stretcher 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 longitudinal direction.

[0041] To reliably measure the strain or deflection, the first sensor can be arranged on a top and / or bottom side of the first bending element, while the second sensor (third sensor, fourth sensor) can be arranged on a top and / or bottom side of the second bending element (third bending element, fourth bending element). Strain gauges can be used as sensors to measure the strain or deformation of the respective bending elements. Instead of strain gauges, other sensors designed to reliably detect strain, deformation, or deflection of the respective bending elements can also be used.

[0042] According to one implementation, the first bending element and the second bending element (as well as the additional bending elements, if present) can be integrated into a pre-stretching plate, which is provided for the simultaneous actuation of the at least two pre-stretching elements. The bending elements can thus be part of the pre-stretching plate. This allows for a particularly space-saving and simple coupling of the bending elements to the respective pre-stretching elements.

[0043] Each of the bending elements has a known elastic deformability, so that the elongation or deformation detected by the sensors at the respective bending elements is proportional to the stretching force acting on the respective pre-stretchers. Short-term force peaks can, under certain circumstances, lead to plastic deformation of the bending elements, which can result in the loss of proportionality between the elongation or deflection (strength of elongation or deflection) and the stretching force (strength of stretching force). The bending elements can then no longer be used for measurement. To prevent undesired deformation of the bending elements, the thermoforming tool can further comprise a protective device designed to limit the elongation or deflection of the respective bending elements (to an elastic deformation range).

[0044] The measuring device can further comprise an evaluation device. The evaluation device can be configured to compare the measured values ​​acquired for the at least two mold cavities in order to obtain and provide a comparison result. In particular, the measuring device can be configured to compare the measured values ​​acquired at the at least two mold cavities with one another and to determine a deviation between the at least two acquired measured values ​​and to provide this as a comparison result, as described above in connection with the method according to the invention.

[0045] 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 it as a comparison result, as has been described above in connection with the method according to the invention.

[0046] The evaluation device can further be designed 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 if the determined deviation in the comparison result exceeds a predetermined threshold value (tolerance value). In such a case, the evaluation device can generate and output a feedback signal. The generated feedback signal can, for example, comprise 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 described above in connection with the method according to the invention.

[0047] To achieve at least one of the above-mentioned objects, according to a third aspect of the invention, a computer program is provided that comprises instructions that 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 computing unit can be implemented as a software and / or hardware module; in particular, the computing unit can be part of the evaluation device described above.

[0048] To achieve at least one of the above-mentioned objects, a thermoforming machine is provided according to a fourth aspect of the invention. The thermoforming machine comprises the above-described thermoforming tool according to the second aspect 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 using the thermoforming tool.

[0049] The heating device can be controlled using the feedback signal provided by the evaluation device of the thermoforming tool.

[0050] Short description of the drawings

[0051] Further details and advantages of the invention will be explained in the following

[0052] Explained by drawings. They show:

[0053] Figures la / lb show views of an exemplary thermoforming tool as seen in the prior art.

[0054] technology known;

[0055] Figure 2 is a diagram showing a correlation between the temperature of a material layer and the stretching force to be applied;

[0056] Figure 3 is a flow chart illustrating a method according to the invention for monitoring a thermoforming process; Figure 4 is a diagram illustrating the time course of the stretching force during the

[0057] Pre-stretching of a material layer in the thermoforming tool;

[0058] Figure 5 schematic representations of mold cavities of a thermoforming tool and a material layer with different temperature distributions;

[0059] Figure 6 schematic representations of different temperature distributions in the thickness direction of the material layer;

[0060] Figure 7 is a schematic representation of a thermoforming tool according to the invention, which is designed to implement the method described in connection with Figure 2;

[0061] Figures 8a / 8b show three-dimensional views of a thermoforming tool according to the invention with a measuring device for recording measured values ​​indicative of stretching forces; and

[0062] Figures 9a / 9b are schematic views of tool components of the thermoforming tool shown in Figures 8a and 8b.

[0063] Detailed description

[0064] In conjunction with Figures 1a and 1b, a thermoforming tool 1000 is first described, as is known from the prior art, in which the inventive technology described here for monitoring a thermoforming process can be implemented. Figure 1 shows a sectional view, while Figure 1b shows an isometric representation of the thermoforming tool 1000.

[0065] The thermoforming tool 1000 is a multiple thermoforming tool that has at least two mold cavities 160 and at least two pre-stretchers 232 cooperating with the mold cavities 160. Figures 1a and 1b show, by way of example, ten mold cavities 160 and ten pre-stretchers 232 arranged in a 2x5 matrix 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 can also have more than 10 mold cavities 160 / pre-stretcher 232 or fewer 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 one another in the axial direction (in the height direction) and movable relative to one another.The first thermoforming tool part 100 is designed as a lower thermoforming tool part 100 in the thermoforming tool 1000 shown in Figures 1a and 1b; correspondingly, the second thermoforming tool part 200 is designed as an upper thermoforming tool part 200.

[0066] 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, each mold base 130 being axially displaceably received at the axial lower end of a respective mold insert 140. Mold base 130 and mold insert 140 together form a mold cavity 160, into which a material layer fed to the thermoforming tool 1000 can be molded to form an article, in particular a packaging article. The article can be a cup, container, bowl, or capsule; The geometry of the molded article is determined 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.

[0067] The mold base 130 of the thermoforming tool 1000 assigned to each mold insert 140 is coupled to a common ejection bar 136 via a corresponding ejection rod 134. The ejection bar 136 is operable and can, in particular, be axially raised (displaced upward), thereby raising the mold bases 130 in the respective mold inserts 140. This allows the articles formed in the mold cavities 160 to be ejected.

[0068] 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-stretching device 230. The pre-stretching device 230 comprises at least two pre-stretchers 232, wherein each pre-stretcher 232 is arranged axially displaceably and coaxially with a corresponding mold cavity 160 of the first thermoforming tool part 100 in a corresponding hold-down recess. Each of the at least two pre-stretchers 232 is coupled to a pre-stretcher rod 234 at its end axially remote from the mold cavity 160. Each pre-stretcher rod 234 is in turn coupled to a common pre-stretcher plate 236.The pre-stretcher plate 236 is operable in the axial direction, so that the at least two pre-stretchers 232 can be operated simultaneously. In particular, with the aid of the operable pre-stretcher plate 236, the at least two pre-stretchers 232 can be simultaneously disengaged and moved into the respective mold cavity 160 (i.e., moved downwards); likewise, the at least two pre-stretchers 232 can be simultaneously reengaged and moved back to their starting position (i.e., moved upwards). In Figure 1a, the pre-stretchers 232 are in their starting position, in which the pre-stretchers 232 are received in the corresponding hold-down recess. By disengaging the respective pre-stretchers 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.

[0069] In the following, a standardized thermoforming process is further described in connection with the thermoforming tool 1000 shown in Figures 1a and 1b. In the open state of the thermoforming tool 1000 (i.e., when the two thermoforming tool parts 100, 200 are moved apart (not shown in Figure 1) a heated material layer can be arranged between the first thermoforming tool part 100 and the second thermoforming tool part 200. The thermoforming tool 1000 is then closed, for example by moving at least one of the two thermoforming tool parts 100, 200 against the other thermoforming tool part 100, 200. With the aid of the pre-stretchers 232, the material layer held down (clamped) between the two thermoforming tool parts 100, 200 with the aid of the hold-down devices 220 is stretched or pre-stretched into the respective mold cavities 160, whereby pre-formed moldings are produced which do not yet have the final article shape.

[0070] For complete molding, compressed air is introduced into the respective mold cavities 160 via channels (these are not shown in Figures 1a and 1b), whereby the pre-formed blanks are pressed against the contact surfaces 132 and 162 of the mold base 130 and the mold insert 140 and thus formed into the final articles. By contacting the contact surfaces 132, 162, the formed material layer is cooled, resulting in a stable article. The thermoforming tool 1000 can then be vented (i.e. the molding air built up in the respective mold cavities 160 can be released again) and the thermoforming tool 100 can be opened, i.e. the two thermoforming tool parts 100, 200 can be moved apart. The molded articles can be ejected by lifting the mold bases 130 with the aid of the ejector bar 136.

[0071] In order to produce articles of the desired quality in the at least two mold cavities 160, it is important that the same thermoforming process runs in each of the at least two mold cavities 160. One process parameter that significantly influences the thermoforming process and thus the quality of the molded 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 with a plurality of mold cavities. Such temperature fluctuations can be caused, for example, by an unexpected failure of radiant heating elements of a heating device provided for heating the material layer, or by a feeding device provided for feeding the material layer to the heating device and the thermoforming tool.In order to be able to detect fluctuations in the process parameters, especially in the forming temperature of the material layer, at an early stage and to counteract them, it is therefore desirable to monitor the process parameters in the thermoforming tool 1000 continuously and, if possible, with mold cavity resolution.

[0072] Tests have shown that in a thermoforming tool 1000, as described in connection with Figures 1a and 1b, the stretching force that must be applied by the respective pre-stretchers 232 when pre-stretching 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 illustrated in the diagram in Figure 2.

[0073] The diagram in Figure 2 shows the stretching force curve as a function of the temperature / forming temperature of a polypropylene material layer that was thermoformed using a thermoforming tool as described above in connection with Figures 1a and 1b. The diagram plots the stretching force recorded during thermoforming of the material layer as a function of continuous thermoforming machine running time, with the thermoforming machine running time on the X-axis and the recorded stretching force on the Y-axis. The polypropylene layer was intermittently fed to the thermoforming tool in order to continuously form articles. For each thermoforming cycle (machine cycle, with several cycles being carried out per minute), the maximum stretching force to be applied during pre-stretching of the material layer was recorded and plotted in the diagram as a function of continuous thermoforming machine running time.

[0074] In these test thermoforming experiments, the temperature / forming temperature of the material layer was varied several times. In a warm-up phase in the thermoforming time interval from t0 to ti, the material layer was first continuously heated to a reference temperature Tref using a heater and then held at this value until time t2. During this warm-up phase between t0 and ti and the subsequent first temperature hold phase between ti and t2, a large number of test articles were formed, with the maximum stretching force applied by the pre-stretcher to pre-stretch the material layer being recorded and plotted in the diagram during each forming cycle. As can be seen from the diagram, the recorded stretching force decreases continuously with increasing forming temperature of the material layer and remains virtually unchanged during the first temperature hold phase between ti and t2.

[0075] At time t2, the temperature of the material layer was reduced by a predetermined amount with respect to the reference temperature T re f was further increased (in this case, an increase of 2 Kelvin) and kept constant until time t3. During this second temperature-holding phase between t2 and t3, a large number of test articles were again molded, and the maximum stretching force applied by the pre-stretcher to pre-stretch the material layer during each molding cycle was recorded. As can be seen from the diagram, the recorded stretching force decreases gradually as a result of the temperature increase at time t2 and then remains essentially constant at a lower force level (approximately 100 N less compared to the force level in the time range t1 to t2).

[0076] At times t3 and t4, the temperature of the material layer was increased again by the same amount (by 2 Kelvin each time) using the heating device, and the maximum stretching force during pre-stretching of the material layer was recorded. The recorded maximum stretching force decreases by a further 100 N at times t3 and t4 as a result of the temperature increase and then remains at a lower force level in each case. As is clearly evident from the diagram, the recorded stretching force correlates with the temperature / forming temperature of the fed material layer. In particular, changes in the temperature of the material layer can be well resolved by recording the stretching force. As can be seen from the diagram, even small changes in the temperature of the material layer lead to significant 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 that consists of another thermoplastic material, such as PET or polystyrene (PS).

[0077] Based on these tests and their results, the inventors have recognized that the stretching force to be applied to a mold cavity correlates with the actual forming temperature of the material layer in the area of ​​the mold cavity, and that by recording the stretching force or the stretching energy, or a measured value indicative of the stretching force or stretching energy, a precise statement can be made about the actual temperature of the material layer in the area of ​​each mold cavity. In particular, the inventors have recognized that the stretching force to be applied to each mold cavity during pre-stretching or stretching of the material layer is determined not so much by the surface temperature of the material layer, but rather by the actual temperature or temperature profile across the material thickness of the material layer in the mold cavity.Thus, the stretching force to be applied is a measure of the heating of the material layer, in particular a measure of the homogeneity of the heating of the material layer in the thickness direction.

[0078] Furthermore, the inventors have recognized that the correlation between 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 to compare them (with each other). In this way, it is 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 fed to the thermoforming tool depending on the mold cavity; in particular, deviations / inhomogeneities in the forming temperature of the material layer at the respective mold cavities can be recorded, and thus the homogeneity of the temperature distribution of the material layer in the transverse and / or longitudinal direction can be determined.

[0079] In conjunction with Figures 3 to 6, a method according to the invention for monitoring thermoforming processes in a thermoforming tool designed to form a plurality of articles in a material layer will now be further described. The method is based on the correlation described above in connection with Figure 2 between the actual temperature / forming temperature of the material layer and the stretching force or stretching energy to be applied for pre-stretching or stretching the material layer.

[0080] Figure 3 shows a flow chart illustrating the steps of the method according to the invention.

[0081] In a first step S10 of the method, at least two measured values ​​are recorded at at least two mold cavities 160a, 160b, 160c (see also Figures 5 and 6), wherein at least one measured value is recorded at each of the at least two mold cavities 160a, 160b, 160c, which indicates a stretching force or stretching energy to be applied when pre-stretching the material layer into the respective mold cavity 160a, 160b, 160c.

[0082] The at least one measured value recorded at each of the at least two mold cavities 160a, 160b, 160c can be an individual measured value, for example, a maximum value, which indicates a maximum stretching force to be applied when pre-stretching the material layer into the respective mold cavity (for example, by a respective pre-stretcher 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-stretchers when pre-stretching the material layer into the respective mold cavities, the recording of individual measured values ​​can be subject to fluctuations.It can therefore be advantageous not only to record one measured value, such as a maximum value which indicates the maximum stretching force to be applied, but also to record a large number of measured values ​​during the pre-stretching process or during the entire thermoforming cycle at the respective at least two mold cavities 160a, 160b, 160c. In this way, a measured value curve can be obtained for each molding cycle or pre-stretching process which indicates the stretching force curve at the respective mold cavities. The diagram in Figure 4 shows such a measured value curve, wherein the recorded measured values ​​were converted to corresponding stretching force values ​​and plotted as a function of the molding cycle time (see X-axis) (see values ​​on the Y-axis). It can be clearly seen that the stretching force depends on the pre-stretching path orthe pre-stretching time and with increasing pre-stretching distance (pre-stretching time) the recorded stretching force increases continuously up to a maximum value (time range 610 in Figure 4). The measured value curve or stretching force curve in the measuring range 610 is strongly temperature-dependent (temperature-sensitive) and can be used in the method according to the invention. In particular, the gradient or the integration of the measured value curve in the pre-stretching 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 drops again in the time range 620 following the time range 610, since the maximum pre-stretching distance has been reached and molding air has been introduced into the respective mold cavity for further shaping of the material layer (range 620 in Figure 4).

[0083] As described above, in the first step S10, the at least one measured value (single measured value or a plurality of measured values ​​for recording a measured value curve during the pre-stretching process) is recorded at at least two mold cavities 160a, 160b, 160c. 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 160a, 160b, 160c, as illustrated in Figure 5. The three mold cavities 160a, 160b, 160c, at each of which at least one measured value indicative of a stretching force or stretching energy is recorded, are marked by an "X" in Figure 5.

[0084] Figure 5 shows, in the left and right views, a lower thermoforming tool part 100 (indicated by a dashed rectangle in Figure 5) having a plurality of mold cavities 160 arranged in a 5x3 matrix shape (see dashed circles). Furthermore, both views show a material layer 10 that is intermittently fed to the thermoforming tool in the feed direction 22 (vertical direction in Figure 5) and is arranged above the lower thermoforming tool part 100. In the left view, 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 view, the temperature is homogeneous across the entire forming surface and exhibits no temperature variation or temperature gradients.

[0085] By recording at least one measured value indicating a stretching force or stretching energy at each of the three mold cavities 160a, 160, 160c, which are arranged spaced apart from one another in a direction diagonal to the feed direction 22, the temperature gradient of the material layer 10 indicated in the left-hand illustration can be easily detected. This is because, due to the respective different temperatures of the material layer 10 in the region of the mold cavities 160a, 160b, and 160c (see gray scale 30 in Figure 5), different measured values ​​or measured value curves indicating the stretching force or stretching energy are measured during the pre-stretching of the material layer 10 into the respective mold cavities 160, 160b, 160c. The situation is different in the right-hand illustration, where the material layer 10 was heated homogeneously, so that the same measured values ​​are obtained at the respective mold cavities 160a, 160b, 160c.For better comparability of the measured values, the measured values ​​at the respective mold cavities 160a, 160b, 160c are preferably recorded simultaneously.

[0086] In the exemplary illustration in Figure 5, the measured values ​​are recorded at three mold cavities 160a, 160b, 160c, which are arranged diagonally. This allows a statement to be made about the temperature of the material layer 10 in the center as well as in the respective outer corner areas, thereby obtaining a reliable statement about the homogeneity of the temperature profile across the entire material layer 10. It is understood that, depending on the size of the mold, in particular 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 accurately resolve local temperature changes or deviations from a target 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.

[0087] In addition, the measuring principle described here can also be used to record the actual temperature / forming temperature across the material layer thickness, as indicated above in connection with Figure 2 and described further below in connection with Figure 6. Figure 6 shows the thermoforming tool of Figure 5, comprising the lower thermoforming tool part 100 with the mold cavities 160; the material layer 10 has a homogeneous surface temperature which, for example, corresponds to a target temperature / target forming temperature, as shown on the left in Figure 6. Nevertheless, the measured values ​​measured at the mold cavities 160a, 160b and 160c can indicate stretching forces or stretching energies that deviate from a target stretching force or target stretching energy corresponding to the target temperature of the material layer 10.

[0088] To illustrate this situation, reference is made to the cross-sectional view of the material layer 10 along the line AA in the middle of Figure 6 as well as to the cross-sectional views BTi, BT2, BT3 shown on the right with different temperature profiles in the cross-sectional area B of the material layer 10. While in the upper view BTI the temperature at the surface 10a and in the interior of the material layer 10 (i.e. along the thickness d) essentially corresponds to the target forming temperature and is thus homogeneous, the views BTz and BT3 each show a negative temperature gradient in the thickness direction. In BT2 the temperature decreases sharply towards the middle of the material layer, while in BT3 the temperature of the material layer decreases more slowly, starting from the surface 10a, towards the rear surface of the material layer 10. The measured value recorded at the respective mold cavities 160a, 160b, 160c orThe measured value curve, which indicates the stretching force or stretching energy to be applied during pre-stretching of the material layer 10, will only correspond to the target value or target value curve for the homogeneous temperature curve in the representation BTi, while a strong deviation of the recorded measured value from the target value is to be expected for the inhomogeneous temperature curve in the representation BT2, and a moderate deviation of the recorded measured value from the target value is to be expected for the inhomogeneous temperature curve in the representation BT3. Thus, the recorded measured values ​​at the at least two mold cavities 160a, 160b, 160c can also be used to make a statement about whether the temperature / forming temperature of the material layer 10 exhibits a desired (homogeneous) temperature curve in the thickness direction of the material layer.

[0089] Back to Figure 3. After the at least two measured values ​​have been recorded at at least two mold cavities 160a, 160b, 160c in a first method step, the measured values ​​recorded at the at least two mold cavities 160a, 160b, 160c are compared in a subsequent step S20 to obtain a comparison value. The comparing step can comprise comparing the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c with one another and / or comparing the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c with a reference measured value. The reference measured value can be a target measured value that would be obtained during pre-stretching of the material layer 10 at a desired forming temperature.

[0090] Furthermore, the step of comparing may comprise determining a deviation between the measured values ​​acquired for the at least two mold cavities 160a, 160b, 160c and / or determining a deviation of the measured values ​​acquired for the at least two mold cavities 160a, 160b, 160c from the reference measured value.

[0091] The determined deviation between the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c is a measure (indicator) of the extent to which the forming temperature of the material layer deviates between the at least two mold cavities 160a, 160b, 160c. As a rule, it is sufficient to compare the measured values ​​at at least two spaced-apart mold cavities 160a, 160b, 160c and determine their deviation in order to monitor the homogeneity of the temperature distribution / forming temperature distribution across the forming surface of the material layer 10a. In the case of large thermoforming tools with many mold cavities, it may be advantageous to compare the measured values ​​of at least three, at least four or more mold cavities 160a, 160b, 160c arranged 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 10a.

[0092] The determined deviation of the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c from the reference measured value is also a measure (indicator) of how much the actual temperature of the material layer (not only on the surface, but also inside the material layer) deviates from a target forming temperature.

[0093] If, in the first step S10, an individual measured value is recorded for each of the at least two mold cavities 160a, 160b, 160c, such as a maximum value (measured value maximum), these values ​​are compared with one another and / or with a corresponding reference measured value (target measured value); deviations detectable during the comparison can be provided as a comparison result. If, however, in the first step S10, a measurement curve is recorded for each of the at least two mold cavities 160a, 160b, 160c by recording a plurality of measured values ​​during the pre-stretching process, the recorded measurement curves can be compared with one another and / or with a target measured value curve. For example, gradients of the recorded measured value curves that indicate an increase in the stretching force to be applied during the pre-stretching process (see Figure 4, gradient of the measured value curves in the measuring range 610) can be compared with one another.Alternatively, it is also conceivable that the recorded measured value curves are integrated over a certain pre-stretching path (pre-stretching time in Fig. 4) and the integration values, which indicate the stretching energy to be applied, are compared with each other.

[0094] The comparison result provided by the comparison (i.e., the determined deviation(s)) can be compared with a predefined threshold value (tolerance value), which represents a tolerance level that is non-critical for the thermoforming process. If the determined deviation(s) exceed the threshold value, this is a measure (indicator) that the thermoforming process, in particular the forming temperature of the material layer 10 across the mold cavities of the multiple thermoforming tool, is not sufficiently homogeneous. Furthermore, it can be determined whether the temperature of the material layer generally corresponds to the reference value and thus whether the correct forming temperature is present.

[0095] In a subsequent third step S30, 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 if the determined deviation(s) exceed the tolerance value, as described above.

[0096] In connection with Figure 7, a thermoforming tool 1000a according to the invention is described, which is designed to implement the method described above.

[0097] The thermoforming tool 1000a is only schematically indicated in Figure ?. 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 the simultaneous forming of at least two articles in one forming cycle. The upper thermoforming tool part 200 comprises at least two pre-stretchers 232 corresponding to the at least two mold cavities 160. Each pre-stretcher 232 of the at least two pre-stretchers 232 is designed to pre-stretch a material layer (not shown in Figure 7) into the corresponding mold cavity 160.

[0098] For the sake of clarity, only the components of the thermoforming tool 1000a that are most essential to the invention are shown and described in Figure 7. It is understood that the at least two pre-stretchers 232 can be coupled via respective pre-stretcher rods to a pre-stretcher bar for joint actuation, as described in connection with the thermoforming tool 1000 in Figure 1. Likewise, 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 bar.

[0099] The thermoforming tool 1000a further comprises a measuring device 300. The measuring device 300 is designed to record at least one measured value at at least two mold cavities 160, which indicates a stretching force or stretching energy to be applied during pre-stretching 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 / width direction (see Figure 7) and / or in the longitudinal direction of the thermoforming tool 1000a. A specific implementation of measuring sensors 320 is described in more detail below in connection with Figures 8, 9a, and 9b.Regardless of the specific implementation, each of the at least two measuring sensors 320 of the measuring device 300 is designed to detect at least one measured value that indicates a stretching force or stretching energy that is to be applied by the respective pre-stretcher 232 when pre-stretching the material layer into the respective mold cavity 160.

[0100] 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.

[0101] The evaluation device 400 is configured to compare the measured values ​​acquired at the at least two mold cavities 160 or pre-stretchers 232 in order to obtain and provide a comparison result. In particular, the measuring device 400 can be configured to compare the measured values ​​acquired at the at least two mold cavities 160 and to determine a deviation of the at least two acquired measured values ​​from one another or from a reference measured value (target measured value) and to provide this deviation as a comparison result, as described above in connection with the method (step S20).

[0102] The evaluation device 400 can further 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 1000a. This can be the case if the determined deviation in the comparison result exceeds a predetermined 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, for example, comprise 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 adjusted.

[0103] While the at least two sensors 320 of the measuring device 300 are installed in the thermoforming tool (for example, coupled to the respective pre-stretchers 232), the evaluation device 400 can be arranged outside the thermoforming tool 1000a and can be wirelessly or wired connected to the sensors 320 installed in the thermoforming tool 1000a via a communication interface 440. The communication interface 440 can be coupled to the communication interface 340 of the measuring device 300 and can read the sensor measurement values ​​provided by the respective sensors 320 in real time during the thermoforming process.

[0104] 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.

[0105] A concrete implementation of a measuring device 500 in connection with a thermoforming tool 1000 according to the invention is further described in connection with Figures 8a and 8b and Figures 9a and 9b. Figure 8a shows an isometric representation of the thermoforming tool 1000, while Figure 8b shows the pre-stretching device 230 of the thermoforming tool 1000, comprising a plurality of pre-stretchers 232 (10 in the specific tool) which are coupled to a common, actuatable pre-stretcher plate 236 via respective pre-stretcher rods 234. The thermoforming tool 1000 of Figures 8a and 8b essentially corresponds to the thermoforming tool 1000 of Figure 1. Only the differences from the thermoforming tool 1000 of Figure 1 are described below. With regard to the components that are structurally and functionally identical, reference is made to the description in connection with Figure 1 above.

[0106] In contrast to the thermoforming tool of Figures 1a / 1b, 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 bending elements 310 are in turn coupled to respective pre-stretchers 232.

[0107] The bending elements can be implemented directly in the pre-stretcher plate 236 using slot structures 312 formed in the pre-stretcher plate 236, as shown in Figure 2b. This implementation 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 shown in Figures 8a and 8b. In addition or alternatively to the sensor arrangement shown in Figures 8a and 8b, the sensors 320 can also be arranged on the underside of the respective bending elements 310 facing away from the upper side.

[0108] Each bending element 310 has a certain elasticity and can be (slightly) elastically deformed or bent when a force, in particular a stretching force, is applied to the pre-stretcher 232 coupled to the bending element 320. This principle is further described in connection with Figures 9a and 9b.

[0109] Figures 9a and 9b each show, by way of example, a bending element 310 and the pre-stretcher 232 coupled to the bending element 310 via the pre-stretcher rod 234. In Figure 9a, no force acts on the pre-stretcher 232, so that the bending element 310 is not deformed. In Figure 9b, a force acts on the pre-stretcher 232. The force acting on the pre-stretcher 232 is indicated in Figure 9b by an arrow, wherein the direction of the arrow indicates the direction of force in which a stretching force acts on the pre-stretcher 232 when the material layer is pre-stretched or stretched with the aid of the pre-stretcher 232. Due to the stretching force acting on the pre-stretcher 232, the bending element 310 is elastically deformed or bent / deflected in proportion to the amount of force. This bending / deflection can be detected by means of the sensors 320 on the respective bending element 310, whereby the measured value is proportional to the bending / deflection of the bending element 310.The bending / deflection is in turn proportional to the stretching force applied to the pre-stretcher, which in turn is proportional to the temperature / forming temperature of the material layer 10, as described above. Thus, the measured values ​​recorded by the sensors 320 at 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.

[0110] For example, strain gauges can be used as sensors 310, which change their electrical resistance depending on the deformation (bending / deflection) of the respective flexural elements 310. Strain gauges have proven to be reliable and sufficiently precise for measuring the deformation or strain of the flexural elements 310; however, 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 flexural elements 310 is conceivable.

[0111] The electrical resistance of the respective strain gauges 320 on the respective bending elements 310 can be (continuously) recorded. The resistance values ​​recorded on the respective bending elements 310 can be directly compared with each other. Alternatively, the recorded resistance values ​​can be converted into corresponding stretching force values ​​and compared with each other or with a target measurement value. The deviation(s) between the resistance values ​​determined during the comparison are an indicator of whether the thermoforming process is proceeding uniformly (and thus homogeneously) at the respective mold cavities or not.

[0112] The technology described here enables reliable and cost-effective monitoring of a thermoforming process in a multiple-cavity thermoforming tool. In particular, the homogeneity of the temperature / forming temperature of the material layer fed to the multiple-cavity thermoforming tool can be reliably monitored (and in real time) across the entire forming surface of the material layer as well as in the thickness direction of the material layer. The technology according to the invention can also be used for the automatic readjustment of a heating device of the thermoforming machine when deviations in the temperature distribution of the material layer are detected.

Claims

Patent claims 1. A method for monitoring a thermoforming process designed to form a plurality of articles in a layer of material (10), the method comprising: Recording at least two measured values at at least two mold cavities (160, 160a, 160b, 160c), wherein at least one measured value is recorded at each of the at least two mold cavities (160, 160a, 160b, 160c), which indicates a stretching force or stretching energy to be applied during pre-stretching of the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c); and Comparing the measured values recorded at the at least two mold cavities (160, 160a, 160b, 160c) to obtain a comparison result, wherein the comparison result is an indicator of the homogeneity of the thermoforming process.

2. The method according to claim 1, wherein the at least two measured values are recorded at at least two mold cavities (160, 160a, 160b, 160c) which are arranged spaced apart from one another in a thermoforming tool (1000, 1000a) 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).

3. The method according to claim 1 or 2, wherein the measured value detected at each of the at least two mold cavities (160, 160a, 160b, 160c) is a measured value which is proportional to the stretching force or stretching energy which is applied during pre-stretching of the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c).

4. 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 measured values from one another and / or a deviation of the at least two recorded measured values from a predetermined reference value.

5. The method according to any one of claims 1 to 4, wherein the step of detecting comprises simultaneously detecting at least one measured value or a measured value curve at each of the at least two mold cavities (160, 160a, 160b, 160c) during the pre-stretching of the material layer (10).

6. The method according to any one of claims 1 to 5, wherein the step of comparing comprises comparing measured values acquired at the at least two mold cavities (160, 160a, 160b, 160c) or comparing measured value curves acquired at the at least two mold cavities (160, 160a, 160b, 160c).

7. The method according to claim 6, wherein the comparison result is obtained by comparing measured value increases or measured value maxima of the measured value curves recorded at the at least two mold cavities (160, 160a, 160b, 160c) during pre-stretching of the material layer (10).

8. The method according to claim 6, wherein the comparison result is obtained by integrating the recorded measured value curves at the at least two mold cavities (160, 160a, 160b, 160c) and comparing the integration values.

9. The method according to any one of claims 1 to 8, wherein the measured value recorded at each of the at least two mold cavities (160, 160a, 160b, 160c) correlates with the temperature distribution in the material layer (10) at the respective mold cavity (160, 160a, 160b, 160c) and wherein the comparison result is used as an indicator of the homogeneity of the temperature distribution of the material layer (10).

10. The method according to any one of claims 1 to 9, further comprising generating and providing at least one feedback signal (500) if the comparison result indicates an inhomogeneous thermoforming process, in particular an inhomogeneous temperature distribution of the material layer (10).

11. The method according to any 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.

12. A thermoforming tool (1000, 1000a) for forming a plurality of articles in a material layer (10) fed to the thermoforming tool (1000, 1000a), the thermoforming tool (1000, 1000a) comprising: at least two mold cavities (160, 160a, 160b, 160c) for forming at least two articles; at least two pre-stretchers (232) corresponding to the at least two mold cavities (160, 160a, 160b, 160c), each pre-stretcher (232) of the at least two pre-stretchers (232) being designed to pre-stretch the material layer (10) into the corresponding mold cavity (160, 160a, 160b, 160c);a measuring device (300) which is designed to record at least one measured value on at least two mold cavities (160, 160a, 160b, 160c) in each case, which value indicates a stretching force or stretching energy to be applied when pre-stretching 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); 13. 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) for first mold cavity (160, 160a, 160b, 160c) or 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).

14. The thermoforming tool (1000, 1000a) of claim 13, wherein the first sensor (320) is configured to measure an elongation or deflection of a first flexural element (310) coupled to the first pre-stretcher (232) caused by the stretching force, and the second sensor (320) is configured to measure an elongation or deflection of a second flexural element (310) coupled to the second pre-stretcher (232) caused by the stretching force.

15. 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).

16. 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) which is provided for the simultaneous actuation of the at least two pre-stretchers (232).

17. Thermoforming tool (1000, 1000a) according to one of claims 14 to 16, further comprising a protective device which is designed to limit the extension or deflection of the first bending element (310) and / or the second bending element (310).

18. Thermoforming tool (1000, 1000a) according to one of claims 12 to 17, further comprising the evaluation device (400) which is designed to compare the measured values recorded for the at least two mold cavities (160, 160a, 160b, 160c) in order to obtain and provide a comparison result.

19. Thermoforming tool (1000, 1000a) according to claim 18, wherein the evaluation device (400) is further designed 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).

20. A thermoforming machine comprising: the thermoforming tool (1000, 1000a) according to any one of claims 12 to 19 for forming a plurality of articles in a material layer (10); and a heating device for heating the material layer (10) before the material layer (10) is formed by means of the thermoforming tool (1000, 1000a).

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