Methods and devices for monitoring a thermoforming process

Abstract

Method for monitoring a thermoforming process designed to form a large number of articles in a single 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 when pre-stretching the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c); and Comparing the measured values ​​recorded at the at least two forming cavities (160, 160a, 160b, 160c) to obtain a comparative result, the comparative result being an indicator of the homogeneity of the thermoforming process.

Description

Technical field

[0001] 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 which implement such a technique. State of the art

[0002] The use of thermoforming tools and processes for forming packaging articles in two-dimensional (flat) layers (sheets or blanks) of plastic material is well-known. Polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), or other thermoplastic materials can be used as the plastic material. The formed packaging articles are primarily used for portioning food and can be designed as containers, cups, trays, or capsules, depending on the application.

[0003] Thermoforming tools for the production of 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 with each other and are movable along a common axis. The upper mold part has at least one hold-down and at least one pre-stretcher slidably 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 that replicates the shape of the cup to be formed.

[0004] In the production of cup-shaped packaging articles, the plastic material, which is in the form of a two-dimensional layer (this can be, for example, a rollable sheet / film or a blank / sheet), is first heated to the desired forming temperature. The heated two-dimensional material layer is then fed (intermittently) into the thermoforming tool. During this process, the two tool parts are moved apart (open state of the thermoforming tool) to position the material layer between them. The thermoforming tool is then closed by moving the two tool parts towards each other. Using at least one pre-stretcher, the material layer, heated to forming temperature, is mechanically stretched or pre-stretched into the at least one mold cavity, thus creating a pre-formed part.The mechanical stretching of the film using at least one pre-stretcher can also be omitted or carried out using 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).

[0005] A key process parameter influencing thermoforming is the temperature of the two-dimensional layer being formed. To obtain packaging articles of the desired quality (e.g., desired transparency, desired shape definition), it is necessary to bring the layer to a specific forming temperature. This desired forming temperature can depend on the material composition and physical properties (e.g., crystallinity) of the thermoplastic material used and must be set accordingly for each layer or determined through testing. Experience has shown that, particularly with semi-crystalline plastic material layers, such as polypropylene, even small changes in the forming temperature (e.g., a change of 2-3 K) can lead to significant changes in the shape definition of the produced articles.

[0006] In multi-cavity thermoforming tools, which are now predominantly used in the industrial production of packaging items and are designed for the simultaneous production of numerous packaging products per thermoforming cycle, a further challenge lies in heating the material layer in such a way that it maintains essentially the same desired forming temperature across the entire forming surface. Only in this way can it be ensured that all items produced in a single thermoforming cycle exhibit the same quality.

[0007] 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 are arranged in a matrix within a heating unit (heating station) positioned 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.

[0008] The homogeneity of the heating of a material layer across its forming surface depends primarily on the arrangement and operational readiness of the heating elements in the heating station. If, for example, a heating element fails, this can lead to local temperature fluctuations within the material layer, which are often not immediately detected and can result in rejects during production.

[0009] However, the use of a feeding device that grips and clamps the material layer at the edge in order to feed it to the heating station and subsequently (intermittently) to a forming station encompassing 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 at the edge, i.e., perpendicular to the feeding direction where the material layer is gripped by the feeding device, is lower. Accordingly, the temperature of the heating elements intended for heating the material layer at the edge must be set higher than that of the heating elements intended for heating the material layer in the center of the material layer in order to compensate for this temperature gradient.

[0010] Another 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, but not the temperature distribution across the thickness of the material. However, the temperature distribution across the material thickness depends not only on the heating power of the radiant heating elements, but also on the dwell time of the material layer in the heating station. Insufficient dwell times combined with insufficient heating power can result in a non-homogeneous temperature distribution across the thickness of the material layer, exhibiting a negative temperature gradient starting from the surface.This means that while the surface layer of material may have a target forming temperature, which is also measured by the infrared sensors, the material layer inside is significantly colder. However, what is crucial 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 rather whether the entire material layer, including across its thickness, has the desired forming temperature.

[0011] In practice, the thermoforming process parameters, especially the forming temperature, are manually set depending on the material layer used when starting the thermoforming process (heating the material layer and the thermoforming tool). Manually set means that test articles are produced and inspected for their quality (form sharpness, transparency, etc.). Subsequently, the process parameters, especially the temperature of the heating elements, are manually adjusted or readjusted by an operator (for example, via a human-machine interface (HMI)) until the thermoforming tool produces articles of the desired quality. Regular production of the packaging articles can then begin. Manually setting thermoforming process parameters based on the inspection of produced articles is time-consuming and requires experienced operators. Furthermore, malfunctions or other issues can arise.Undesirable changes in process parameters during the thermoforming process (for example, a temperature change due to the failure of a heating element) can only be detected on the produced item and therefore relatively late.

[0012] 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 multi-cavity thermoforming tool, especially the homogeneity of the temperature distribution in the material layer being formed. Furthermore, it is desirable that the monitoring technique operates in real time and initiates appropriate countermeasures in the event of deviations.

[0013] The problem is solved according to the invention by means of a method for monitoring a thermoforming process with the features of main claim 1, as well as by means of a thermoforming tool with the features of dependent claim 12 and a thermoforming machine with the features of dependent claim 20. Advantageous embodiments of the present invention are the subject of dependent claims 2 to 11 and 13 to 19. Brief overview

[0014] To solve at least one of the aforementioned problems, a method for monitoring a thermoforming process designed for forming a plurality of articles in a single layer of material is provided according to a first aspect of the invention. 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, indicating a stretching force or stretching energy to be applied when pre-stretching the layer of material into the respective mold cavity; and comparing the measured values ​​acquired at the at least two mold cavities to obtain a comparative result, wherein the comparative result is an indicator of the homogeneity of the thermoforming process.

[0015] According to the present invention, the thermoforming process is designed for the simultaneous production of a plurality of articles in a single layer of material. The simultaneous production of a plurality of articles means the simultaneous production of at least two articles, preferably at least three, and more preferably at least four or more. This can be achieved by using a multi-cavity thermoforming tool which, as described below, has at least two, preferably at least three, and more preferably at least four mold cavities for the simultaneous forming of the at least two, preferably at least three, and more preferably at least four articles. The formed articles can be packaging articles, in particular packaging articles for portioned food products, such as containers, cups, trays, or capsules.

[0016] A material layer is a two-dimensional (planar) sheet of material made from a thermoplastic material suitable for thermoforming articles. Suitable thermoplastic materials include polystyrene (PS), polypropylene (PP), polyethylene terephthalate (PET), or other thermoplastic polymers. Depending on the requirements of the articles to be produced, the material layer can be supplied as a rollable sheet or as a flat blank.

[0017] The required stretching force 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). Similarly, the stretching energy is the energy that must be applied to pre-stretch the material layer over a predetermined pre-stretching path (deep drawing path). It can be obtained by integrating the stretching force over the pre-stretching path (deep drawing path).

[0018] It has been shown that the required stretching force or energy depends primarily on the temperature (forming temperature) of the material layer. The required stretching force or energy depends less on the surface temperature of the material layer than on the temperature throughout its entire thickness (including the interior of the material layer). Higher material layer temperatures result in lower required stretching forces or energies, while lower temperatures result in higher required stretching forces or energies.

[0019] Furthermore, it has been shown that changes in the temperature (forming temperature) of the material layer being formed, even by a few Kelvin, lead to considerable, readily measurable changes in the stretching force (changes in the range of 100 N for temperature changes of only 2 K). This correlation between the required stretching force or stretching energy on the one hand and the actual temperature (forming temperature) of the material layer on the other can thus be used to deduce the actual temperature (forming temperature) of the material layer from the measured stretching force (stretching energy) or from a measurement that indicates the stretching force or stretching energy.Thus, the measuring principle according to the invention, based on the acquisition of at least one measured value indicating the stretching force or stretching energy, is significantly more reliable than temperature measurements using infrared sensors or thermal imaging cameras, which are only surface-sensitive.

[0020] It is understood that the stretching force or energy required for stretching or pre-stretching the material layer can also depend on the speed and / or geometry of the pre-stretcher used for stretching the material layer. However, these tool-specific parameters are known, and their contribution / influence on the stretching force or energy can easily be distinguished from the contribution / influence of the temperature (forming temperature) of the material layer on the stretching force or energy.On the other hand, these tool-specific influences are not significant in the inventive method, since the at least one measured value (measurement parameter) indicating the stretching force or stretching energy is acquired independently at at least two mold cavities of the multi-cavity thermoforming tool, and the acquired measured values ​​are then compared (directly) with each other or with a predetermined reference value (target measured value) to obtain the comparison result. Since the pre-stretchers in the multi-cavity thermoforming tool are actuated identically for each mold cavity and have the same geometry, their contribution / influence on the at least one measured value acquired at each of the at least two mold cavities is the same.When comparing (relatively comparing) the measurement values ​​recorded at the at least two mold cavities, the influence of the geometry and speed of the pre-stretchers on the recorded measurements is therefore negligible, since this influence is the same for all measurements.

[0021] 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.

[0022] To monitor the homogeneity of the temperature distribution across the entire forming surface of the material layer, at least two measured values ​​can be acquired at at least two forming cavities. These cavities are spaced apart from each other in a thermoforming tool, one in a direction transverse to the feed direction of the material layer and / or the other in a direction parallel to the feed direction. The feed direction refers to the direction in which the material layer is fed to the thermoforming tool. This corresponds to the longitudinal direction of the thermoforming tool, while the transverse direction corresponds to the width direction of the thermoforming tool.

[0023] Preferably, at least one measurement can be taken at at least two mold cavities that are arranged diagonally to each other (i.e., diagonally to the feed direction) in the thermoforming tool. This allows the homogeneity of the temperature distribution to be reliably monitored with minimal measurement effort, both in the feed direction / longitudinal direction and in the transverse / broad direction of the material position / thermoforming tool.

[0024] In a particularly simple implementation of the method, the at least one measured value indicating the stretching force can be recorded at each of at least two diagonally spaced forming cavities of the thermoforming tool, wherein a first forming cavity is located centrally in the thermoforming tool and a second forming cavity is located diagonally further outwards (for example in a corner area) in the thermoforming tool.Based on this implementation, in a further development, at least one measured value indicating the stretching force can be recorded at each of at least three diagonally arranged mold cavities, wherein a first mold cavity can be located centrally in the thermoforming tool, a second mold cavity can be located diagonally further outwards (for example in a first corner area) in the thermoforming tool and a third mold cavity can also be located diagonally further outwards (for example in a second corner area that is diagonally opposite the first corner area) in the thermoforming tool.By recording and comparing measured values ​​at the mold cavities in the center and at the edges, especially at the corners 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).

[0025] Deviations (inhomogeneities) in the temperature distribution of the material layer can be caused primarily by the feeding device, which grips and clamps the material layers at their edges, perpendicular to the feeding direction. Due to the feeding device, the temperature in the edge regions opposite the feeding device may be somewhat lower than in the central area of ​​the heated material layer. To detect such an inhomogeneity in the temperature distribution perpendicular to the feeding direction, it may be sufficient to take at least one measurement at at least two mold cavities, preferably at at least three mold cavities, wherein a first mold cavity is located centrally in the thermoforming tool in the transverse direction and a second mold cavity is located further outwards in the transverse direction (for example, in a first corner region) in the thermoforming tool.If at least one measurement is additionally recorded at a third mold cavity, the third mold cavity can be located further out in the transverse direction (for example, in a second corner area that is opposite the first corner area in the transverse direction) in the thermoforming tool.

[0026] It is understood that the method described here is not limited to acquiring and comparing measured values ​​indicative of 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 temperature homogeneity across the forming surface. In particular, by acquiring and comparing measured values ​​at a large number of mold cavities (at four or more), the resolution of the method can be further improved, and localized temperature deviations / temperature inhomogeneities (caused, for example, by the failure of a 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 across the mold.

[0027] The at least one measured value indicating the stretching force or stretching energy at the respective mold cavity can be a measured value proportional to the stretching force or stretching energy applied when stretching the material layer into the respective mold cavity. An implementation of a measuring principle for acquiring measured values ​​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 acquired measured value and the stretching force or stretching energy to be applied, it is possible to deduce the stretching force or stretching energy and thus also the actual temperature of the material layer in the area of ​​the respective mold cavity, which correlates with the stretching force or stretching energy. However, it is also conceivable to determine the stretching force or stretching energy in a different way.To directly measure and compare the stretching energy at the at least two mold cavities in order to obtain a comparative result that indicates a deviation / inhomogeneity in the thermoforming process, in particular a deviation / inhomogeneity of the temperature distribution in the heated material layer.

[0028] The step of comparing the measured values ​​acquired at the at least two mold cavities can include comparing the measured values ​​acquired at the at least two mold cavities with each other. The comparison step can further include determining the deviation between the at least two acquired measured values ​​in order to obtain a comparison result. The comparison result can include the determined deviation. In other words, by comparing the measured values ​​acquired at the at least two mold cavities with each other, it can be determined how much the measured values ​​at the at least two mold cavities differ from each other.This direct comparison method has the advantage that measurement-specific influences, such as the geometry of the pre-stretcher or the stretching speed, do not affect the comparison result, since such measurement-specific influences are the same at 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 therefore 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, differs from each other.

[0029] Additionally or alternatively, the step of comparing the measured values ​​acquired at the at least two mold cavities can include comparing these values ​​with a predetermined reference value. Furthermore, the comparison step can include determining any deviation of the measured values ​​acquired 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 ​​acquired at the at least two mold cavities can be compared with the reference value, and any deviation from it can be determined. The reference value can be a target measured value indicating a target forming temperature.Such a reference value can be calculated / specified, taking into account the stretching rate 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. A deviation of the measured values ​​recorded at different mold cavities from the reference value, determined in a 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).

[0030] Using the comparison methods described above, based on comparing the recorded measurements with each other or with a predefined reference value (target measurement), it is easy to determine whether the forming temperature of the material layer is the same in the respective cavities of a multi-cavity thermoforming tool, and thus whether the thermoforming process is uniform across all cavities. Due to the direct correlation between the recorded measurements, the stretching force or energy, and the forming temperature of the material layer, it is not necessary to convert the recorded measurements into corresponding stretching force or energy values ​​and forming temperature values, thereby saving computing resources.

[0031] On the other hand, due to the direct correlation between the measured values ​​and the required stretching force, and between the stretching force and the forming temperature of the material layer, corresponding stretching force values ​​or stretching energy values, and consequently corresponding thermoforming process values, especially forming temperature values ​​of the material layer at the respective cavities, can be calculated and compared from the recorded measured values. This allows a user to better understand how much the thermoforming process, and in particular the forming temperature of the material layer at the respective forming cavities of the thermoforming tool, deviates from each other or from a target value (target temperature).

[0032] To objectively compare the measured values ​​acquired at the at least two mold cavities with each other or with a target value, the acquisition step can include simultaneously acquiring at least one measured value or a measured value profile at each of the at least two mold cavities during the stretching / pre-stretching of the material layer into the respective mold cavities. To acquire a measured value profile at each of the at least two mold cavities, a multitude of measured values ​​can be acquired during the stretching / pre-stretching of the material layer. It is also conceivable, however, that the measured value profile is acquired during an entire thermoforming cycle (i.e., from closing to reopening the mold).

[0033] The simultaneously recorded measurements or measurement profiles can be compared. For example, it is conceivable to compare recorded measurement maxima. Alternatively, increases in the measurement profile (slopes in the measurement profile) can be compared, provided the measurement profile is recorded during the pre-stretching of the material layer. Another alternative is to integrate recorded measurement profiles at the at least two mold cavities over a predetermined pre-stretching distance (pre-stretching time) and compare the integrated values ​​to obtain a comparative result. Integrating recorded measurement profiles has the advantage that statistically induced measurement errors, especially outliers, are eliminated, thus making the resulting comparative result even more accurate.

[0034] The method may further include the step of generating and providing at least one feedback signal if the comparison result reveals 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 (tolerance value), this may indicate 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, include a warning signal for a user; additionally or alternatively, the generated feedback signal can include a control signal, in particular a control signal for a heating device for heating the material layer, which is generated depending on the comparison result. Thus, the method according to the invention allows not only 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 output control signal, thereby eliminating inequalities in the temperature distribution over the forming surface of the material layer.

[0035] To solve at least one of the aforementioned problems, 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 forming cavities for forming at least two articles; at least two pre-stretchers corresponding to the at least two forming cavities, each pre-stretcher being configured to pre-stretch (or extend) the material layer into the corresponding forming cavity.to stretch); and a measuring device designed to record at least one measured value at at least two mold cavities, indicating a stretching force or stretching energy to be applied when pre-stretching (stretching) 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).

[0036] The thermoforming tool according to the present invention is a multi-cavity thermoforming tool designed for the simultaneous forming of a plurality of articles (at least two, three, or more articles) in a single thermoforming cycle. For this purpose, the thermoforming tool has at least two (at least three or more) forming cavities, which are, for example, formed in a mold insert of a first thermoforming tool part. Correspondingly, the thermoforming tool also has at least two (at least three or more) pre-stretchers, which are, for example, arranged 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 version, 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 indicating the stretching force or stretching energy 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 indicating the stretching force or stretching energy to be applied when pre-stretching the material layer into a second mold cavity by a corresponding second pre-stretcher. The first mold cavity orThe first pre-stretcher can be arranged 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 length direction of the thermoforming tool), spaced apart from the second mold cavity or the second pre-stretcher. In this way, the at least two sensors can acquire and compare measured values ​​at different mold cavities in the transverse / width direction, length direction, or diagonal direction. This allows the homogeneity of the thermoforming process at different mold cavities in the thermoforming tool, and in particular the homogeneity of the temperature distribution of the material layer fed to the thermoforming tool, to be monitored, as described above in connection with the method.

[0038] It is understood that the measuring device need not be limited to the first and second sensors mentioned above. Rather, the measuring device can include further sensors for acquiring measured values ​​indicative of the stretching force or stretching energy at additional mold cavities of the thermoforming tool. For example, in a further embodiment, the measuring device can include a third sensor for acquiring a measured value at a third mold cavity or third pre-stretcher, which is arranged at a distance in the transverse / width direction and / or longitudinal direction of the thermoforming tool from both the first mold cavity or first pre-stretcher and the second mold cavity or second pre-stretcher. In yet another embodiment, the measuring device can include a fourth sensor for acquiring a measured value at a fourth mold cavity orThe system includes a fourth pre-stretcher, which is arranged at intervals in the transverse / lateral direction and / or longitudinal direction of the thermoforming tool from the first, second, and third mold cavities / pre-stretchers, respectively. 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 multi-cavity thermoforming tool, and in particular the homogeneity of the temperature distribution across the forming surface of the material layer fed into the thermoforming tool, can be monitored even more effectively.

[0039] 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 configured (operated) to acquire the measured values ​​simultaneously. This allows for a better comparison of the measured values ​​acquired by the at least two (three, four, or more) sensors. Furthermore, the at least two (three, four, or more) sensors of the measuring device can be configured (operated) to acquire a multitude of measured values ​​during the pre-stretching of the material layer by the respective pre-stretchers into the respective mold cavities. In this way, the respective sensors can record a measurement profile that indicates the progression of the stretching force during the pre-stretching process into the respective mold cavities.

[0040] 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 having the first sensor designed to measure the elongation or deflection caused by the stretching force of a first bending element coupled to the first pre-stretcher, and the second sensor designed to measure the elongation or deflection caused by the stretching force of a second bending element coupled to the second pre-stretcher. The first and second bending elements thus cooperate with the respective first and second sensors of the measuring device.

[0041] In a further development, the measuring device can include additional sensors designed to detect the strain or deflection of further bending elements coupled to additional pre-stretchers, each arranged transversely and / or longitudinally spaced from the first and second pre-stretchers in the thermoforming tool. For example, the measuring device can include a third sensor designed to detect the strain or deflection of a third bending element coupled to a third pre-stretcher arranged transversely and / or longitudinally spaced from the first and second pre-stretchers in the thermoforming tool.In another exemplary implementation, the measuring device can include a fourth sensor designed to detect the strain or deflection of a fourth bending element coupled to a fourth pre-stretcher arranged in the thermoforming tool spaced transversely and / or longitudinally from the first pre-stretcher, the second pre-stretcher and the third pre-stretcher.

[0042] To reliably measure strain or deflection, the first sensor can be positioned on the top and / or bottom of the first bending element, and similarly, the second (third, fourth) sensor can be positioned on the top and / or bottom of the second (third, fourth) bending element. Strain gauges can be used as sensors to measure the strain or deformation of the respective bending elements. Alternatively, other sensors designed to reliably detect strain, deformation, or deflection of the respective bending elements can be used.

[0043] According to one implementation, the first bending element and the second bending element (as well as any further bending elements, if present) can be integrated into a pre-stretcher plate, which is designed for the simultaneous actuation of at least two pre-stretchers. The bending elements can thus be part of the pre-stretcher plate. This allows for a particularly space-saving and simple coupling of the bending elements with the respective pre-stretchers.

[0044] Each bending element has a known elastic deformability, so the strain 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 potentially lead to plastic deformation of the bending elements, which can disrupt the proportionality between the strain or deflection (magnitude of strain or deflection) and the stretching force (magnitude of stretching force). The bending elements can then no longer be used for measurement. To prevent unwanted deformation of the bending elements, the thermoforming tool can also include a protective device designed to limit the strain or deflection of the respective bending elements (to an elastic deformation range).

[0045] 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 any 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.

[0046] Additionally or alternatively, the measuring device can be configured to compare the measured values ​​recorded at the at least two mold cavities with a predetermined reference value (target measured value) and to determine any deviation of the at least two recorded measured values ​​from the reference measured value and to provide this as a comparison result, as described above in connection with the method according to the invention.

[0047] The evaluation unit can further 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 within the thermoforming tool. This can occur if the determined deviation in the comparison result exceeds a predefined threshold (tolerance value). In such a case, the evaluation unit can generate and output a feedback signal. The generated feedback signal can, for example, include a warning signal for a user; additionally or alternatively, the generated feedback signal can include a control signal, in particular a control signal for a heating device for warming the material layer, as described above in connection with the method according to the invention.

[0048] To solve at least one of the aforementioned problems, a computer program is provided according to a third aspect of the invention. This program includes instructions that cause the method according to the first aspect to be executed when the computer program is run 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.

[0049] To solve at least one of the aforementioned problems, a thermoforming machine is provided according to a fourth aspect of the invention. The thermoforming machine comprises the thermoforming tool described above according to the second aspect for forming a plurality of articles in a single layer of material; and a heating device for heating the layer of material before it is formed using the thermoforming tool.

[0050] The heating device can be controlled using the feedback signal provided by the evaluation unit of the thermoforming tool. Brief description of the drawings

[0051] Further details and advantages of the invention are explained with reference to the following drawings. They show: Fig. 1a / 1b Views of an exemplary thermoforming tool as known from the prior art; Fig. 2 a diagram showing a correlation between the temperature of a material layer and the stretching force required; Fig. 3 a flowchart which represents a method according to the invention for monitoring a thermoforming process; Fig. 4 a diagram showing the time course of the stretching force during the pre-stretching of a layer of material in the thermoforming tool; Fig. 5 schematic representations of mold cavities of a thermoforming tool and a material layer with different temperature distributions; Fig. 6 schematic representations of different temperature distributions in the thickness direction of the material layer; Fig. 7 A schematic representation of a thermoforming tool according to the invention, which is used to implement the function associated with Fig. is trained in the procedures described in 2; Fig. 8a / 8b Three-dimensional views of a thermoforming tool according to the invention with a measuring device for recording measured values ​​indicative of stretching forces; and Fig. 9a / 9b schematic views of tool components of the in the Fig. 8a and Fig. Thermoforming tool shown in 8b. Detailed description

[0052] In connection with the Fig. 1a and Fig. 1b First, a thermoforming tool 1000 is described, as is known from the prior art, in which the inventive technique described here for monitoring a thermoforming process can be implemented. Fig. 1 shows a section view, while Fig. Figure 1b shows an isometric representation of the thermoforming tool 1000.

[0053] The thermoforming tool 1000 is a multi-cavity thermoforming tool that has at least two mold cavities 160 and at least two pre-stretchers 232 cooperating with the mold cavities 160. In the Fig. 1a and Fig. Figure 1b shows, by way of example, ten forming 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 forming cavities 160 and pre-stretchers 232. The thermoforming tool 1000 can also have more than 10 forming cavities 160 / pre-stretchers 232 or fewer than 10 forming cavities 160 / pre-stretchers 232, but at least two forming cavities 160 / pre-stretchers 232.

[0054] 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 vertical direction) and are movable relative to each other. The first thermoforming tool part 100 is located in the area described in the Fig. 1a and Fig. 1b the thermoforming tool 1000 is designed as the lower thermoforming tool part 100; accordingly, the second thermoforming tool part 200 is designed as the upper thermoforming tool part 200.

[0055] 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 displaceable at the axial lower end of a respective mold insert 140. The mold base 130 and the mold insert 140 together each form a mold cavity 160 into which a layer of material supplied to the thermoforming tool 1000 can be formed to create an article, in particular a packaging article. The article can be a cup, container, tray, or capsule. The geometry of the formed 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 base 130.

[0056] The mold base 130 of the thermoforming tool 1000, which is assigned to each mold insert 140, is coupled to a common ejector bar 136 via a corresponding ejector rod 134. The ejector bar 136 is actuated and can, in particular, be axially raised (moved upwards), 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.

[0057] The upper thermoforming tool part 200 has a block-shaped upper tool carrier 210 in which at least two recesses 240 are formed. A hold-down device 220 is provided in each of the at least two recesses 240 for holding the material position 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, each pre-stretcher 232 being axially displaceable and arranged 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 at its end facing away from the mold cavity 160 to a pre-stretcher rod 234. Each pre-stretcher rod 234 is in turn coupled to a common pre-stretcher plate 236.The pre-stretcher plate 236 is axially actuated so that the at least two pre-stretchers 232 can be actuated simultaneously. In particular, with the aid of the actuated 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 re-engaged and moved back to their initial position (i.e., moved upwards). Fig. 1a The pre-stretchers 232 are in their starting position, in which they are received in the corresponding hold-down recess. By extending the respective pre-stretchers 232 into the mold cavities 160, a layer of material arranged between the lower thermoforming tool part 100 and the upper thermoforming tool part 200 can be pre-stretched into the respective mold cavity.

[0058] The following describes a standardized thermoforming process in connection with the one described in the Fig. 1a and Fig. The thermoforming tool 1000 shown in Figure 1b is described in more detail below. In the open state of the thermoforming tool 1000 (i.e., when the two thermoforming tool parts 100 and 200 are moved apart → in Fig. (1 not shown) A heated layer of material can be positioned 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. Using the pre-stretchers 232, the layer of material, which is held (clamped) between the two thermoforming tool parts 100, 200 by means of the hold-downs 220, is stretched or pre-stretched into the respective mold cavities 160, thus producing pre-formed parts that do not yet have their final product shape.

[0059] For complete shaping, channels are used (these are in the Fig. 1a and Fig. (1b not shown) Compressed air is introduced into the respective mold cavities 160, pressing the pre-formed parts against the contact surfaces 132 and 162 of the mold base 130 and the mold insert 140, thus forming them into the final articles. Contact with the contact surfaces 132 and 162 cools the formed material layer, resulting in a stable article.

[0060] Subsequently, the thermoforming tool 1000 can be vented (i.e., the molding air that has built up in the respective mold cavities 160 can be released) and the thermoforming tool 100 can be opened, i.e., the two thermoforming tool parts 100 and 200 can be moved apart. By lifting the mold bases 130 with the aid of the ejector bar 136, the formed articles can be ejected.

[0061] To produce articles of the desired quality at the at least two mold cavities 160, it is essential that the same thermoforming process is carried out 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 formed articles is the temperature / forming temperature to which the material layer is heated for the thermoforming process. The forming temperature can fluctuate in material layers with a large forming area, which are provided for multi-cavity thermoforming tools; such temperature fluctuations can be caused, for example, by an unexpected failure of heating elements in a heating device intended to heat the material layer, or by a feeding device intended to supply the material layer to the heating device and the thermoforming tool.In order to detect and counteract fluctuations in the process parameters, especially in the forming temperature of the material layer, at an early stage, it is therefore desirable to continuously monitor the process parameters in the thermoforming tool 1000 and, if possible, with mold cavity resolution.

[0062] Test runs have shown that in a thermoforming tool 1000, as in connection with Fig. 1a and Fig. As described in Figure 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 shown in the diagram in Fig. 2 shown.

[0063] The diagram in Fig. Figure 2 shows the curve of the stretching force as a function of the temperature / forming temperature of a polypropylene material layer formed using a thermoforming tool as described in the following sections. Fig. 1a and Fig. As described in section 1b above, the material was thermoformed. The diagram shows the stretching force recorded during thermoforming of the material layer as a function of continuous thermoforming machine runtime, with the thermoforming machine runtime on the X-axis and the recorded stretching force on the Y-axis. The polypropylene layer was fed intermittently to the thermoforming tool to continuously form articles. For each thermoforming cycle (machine cycle, with several cycles performed per minute), the maximum stretching force required during pre-stretching of the material layer was recorded and shown in the diagram as a function of continuous thermoforming machine runtime.

[0064] In these thermoforming tests, the temperature / forming temperature of the material layer was varied several times. During a warm-up phase in the thermoforming time interval from t0 to t1, the material layer was first heated to a reference temperature T using a heating device. ref The temperature was continuously heated and then maintained at this value until time t2. During this heating phase between t0 and t1 and the subsequent first temperature holding phase between t1 and t2, a large number of test articles were formed. For each forming cycle, the maximum stretching force applied by the pre-stretcher to pre-stretch the material layer was recorded and plotted in the diagram. As can be seen from the diagram, the recorded stretching force decreases continuously with increasing forming temperature of the material layer and remains almost unchanged during the first temperature holding phase between t1 and t2.

[0065] At time t2, the temperature of the material layer was increased by a predetermined amount relative to the reference temperature T using the heating device. ref The temperature was further increased (in this case by 2 Kelvin) and held constant until time t3. During this second temperature holding phase between t2 and t3, a large number of test articles were again formed, and the maximum stretching force applied by the pre-stretcher to pre-stretch the material layer in each forming cycle was recorded. As can be seen from the diagram, the recorded stretching force decreases in steps due to 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).

[0066] At times t3 and t4, the temperature of the material layer was increased again by the same amount (2 Kelvin each time) using the heating device, and the maximum stretching force during pre-stretching of the material layer was recorded. As a result of the temperature increase, the recorded maximum stretching force decreased by a further 100 N at times t3 and t4 and subsequently remained at a lower force level.

[0067] As the diagram clearly shows, the measured 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 accurately resolved by measuring the stretching force. As the diagram illustrates, even small temperature changes in the material layer lead to significant changes in the stretching force (100 N for temperature changes of 2 Kelvin). Incidentally, similar relationships are observed when a material layer made of a different thermoplastic material, such as PET or polystyrene (PS), is used instead of a polypropylene layer.

[0068] Based on these tests and their results, the inventors recognized that the stretching force required for a mold cavity correlates with the actual forming temperature of the material layer in the mold cavity area, and that by recording the stretching force or the stretching energy, or a measurement indicating 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 recognized that the stretching force required for pre-stretching or stretching the material layer in each mold cavity 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 at the mold cavity.Thus, the required stretching force 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.

[0069] Furthermore, the inventors recognized that the correlation between stretching force or stretching energy and the temperature / forming temperature can be used to record and compare the thermoforming process, in particular the temperature / forming temperature of the material layer in a multi-cavity thermoforming tool, as a function of the mold cavity. This makes it possible to monitor the thermoforming process in a multi-cavity thermoforming tool as a function of 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 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.

[0070] In connection with the Fig. Sections 3 to 6 now describe in more detail a method according to the invention for monitoring thermoforming processes in a thermoforming tool designed for forming a plurality of articles in a single layer of material. The method is based on the invention related to Fig. 2. Correlation described above between the actual temperature / forming temperature of the material layer and the stretching force or stretching energy required for pre-stretching or stretching the material layer.

[0071] Fig. Figure 3 shows a flowchart illustrating the steps of the method according to the invention.

[0072] In a first step S10 of the procedure, at least two measured values ​​are recorded at at least two mold cavities 160a, 160b, 160c (see also Fig. 5 and Fig. 6), wherein at least one measurement 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.

[0073] The at least one measured value recorded at each of the at least two mold cavities 160a, 160b, 160c can be a single measurement, for example, a maximum value indicating a maximum stretching force to be applied when pre-stretching the material layer into the respective mold cavity (for example, by a 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 a single measurement, such as a maximum value indicating the maximum stretching force to be applied, but also to record a multitude of measurements during the pre-stretching process or during the entire thermoforming cycle at the respective at least two mold cavities 160a, 160b, 160c. This allows a measurement profile to be obtained for each molding cycle or pre-stretching process, indicating the stretching force profile at the respective mold cavities.

[0074] In the diagram in Fig. Figure 4 shows such a measurement curve, where the recorded measurements were converted into corresponding stretching force values ​​and plotted as a function of the mold cycle time (see X-axis) (see values ​​on the Y-axis). It is clearly evident that the stretching force depends on the pre-stretching distance or pre-stretching time, and that with increasing pre-stretching distance (pre-stretching time), the recorded stretching force increases continuously up to a maximum value (time range 610 in Fig. 4) The measured value profile or stretching force profile in measuring range 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 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 decreases again in the time range 620 following time range 610, since the maximum pre-stretching distance has been reached and forming air has been introduced into the respective mold cavity for further forming of the material layer (range 620 in Fig. 4).

[0075] As described above, in the first step S10, at least one measured value (a single measured value or a plurality of measured values ​​to record a measured value profile during the pre-stretching process) is recorded at at least two mold cavities 160a, 160b, 160c. For example, the at least one measured value indicating a stretching force or stretching energy can be recorded at at least three mold cavities 160a, 160b, 160c, as shown in Fig. Figure 5 illustrates this. The three mold cavities 160a, 160b, 160c, in each of which at least one measured value indicating a stretching force or stretching energy is recorded, are shown in Fig. 5 marked by an “X”.

[0076] Fig. Figure 5 shows a lower thermoforming tool part 100 in both the left and right illustrations (indicated by a dashed rectangle in Fig. 5), which has a plurality of forming cavities 160 arranged in a 5x3 matrix shape (see dashed circles). Furthermore, both illustrations show a material layer 10 that feeds the thermoforming tool in the feed direction 22 (vertical direction in Fig. 5) is supplied intermittently and is arranged above the lower thermoforming tool part 100. In the left illustration, the material layer exhibits a temperature gradient in the direction transverse to the supply direction 22 (i.e., in the transverse direction 24), while in the right illustration, the temperature is homogeneous over the entire forming surface and exhibits no temperature variation or temperature gradient.

[0077] 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 spaced apart from each other 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 the different temperatures of the material layer 10 in the area of ​​the mold cavities 160a, 160b and 160c (see grayscale 30 in Fig. 5) During pre-stretching of the material layer 10 into the respective mold cavities 160, 160b, 160c, different measured values ​​or measured value profiles are recorded, indicating the stretching force or stretching energy. The situation differs 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.

[0078] In the exemplary presentation in Fig. 5. Measurements are taken at three mold cavities 160a, 160b, and 160c, arranged diagonally. This allows for an assessment of the temperature of the material layer 10 in the center and in the respective outer corner areas, thus providing a reliable indication of the homogeneity of the temperature profile across the entire material layer 10. It is understood that, depending on the size of the mold, and especially the number of mold cavities, measurements can also be taken at more than two or three cavities to accurately resolve local temperature changes or deviations from a target temperature.However, it is usually 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 to obtain a valid statement about the homogeneity of the temperature / forming temperature over the entire forming surface of the material layer 10.

[0079] Additionally, the measuring principle described here can also be used to determine the actual temperature / forming temperature across the material layer thickness, as in connection with Fig. 2. As indicated above and subsequently in connection with Fig. Section 6 is described further.

[0080] Fig. Figure 6 shows the thermoforming tool from Fig. 5 comprising the lower thermoforming tool part 100 with the forming cavities 160; the material layer 10 has a homogeneous surface temperature, which corresponds, for example, to a target temperature / target forming temperature, as in Fig. Figure 6 is shown on the left. Nevertheless, the measured values ​​at the mold cavities 160a, 160b and 160c may indicate stretching forces or stretching energies that deviate from the target stretching force or target stretching energy corresponding to the target temperature of the material layer 10.

[0081] To illustrate this fact, refer to the cross-sectional view of material layer 10 along line AA in the middle of Fig. Reference is made to section 6 and to the cross-sectional views BT1, BT2, and BT3 shown on the right, which depict different temperature profiles in the cross-sectional area B of the material layer 10. While in the upper view BT1 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 therefore homogeneous, views BT2 and BT3 each show a negative temperature gradient in the thickness direction. In BT2, the temperature decreases sharply towards the center of the material layer, while in BT3 the temperature of the material layer decreases from the surface 10a, albeit more slowly, but continuously towards the back surface of the material layer 10. The measured value or measured value profile recorded at the respective mold cavities 160a, 160b, and 160c, which relates to the stretching force to be applied during the pre-stretching of the material layer 10, is shown.As indicated by the stretching energy, the measured value will only correspond to the target value or target value profile for the homogeneous temperature profile in representation BT1, while for the inhomogeneous temperature profile in representation BT2 a strong deviation of the measured value from the target value is to be expected, and for the inhomogeneous temperature profile in representation BT3 a moderate deviation of the measured value from the target value. Therefore, the measured values ​​at the at least two mold cavities 160a, 160b, 160c can also be used to determine whether the temperature / forming temperature of the material layer 10 exhibits a desired (homogeneous) temperature profile in the thickness direction of the material layer.

[0082] Back to Fig. 3. After at least two measured values ​​have been acquired at at least two mold cavities 160a, 160b, 160c in a first process step, the measured values ​​acquired at the at least two mold cavities 160a, 160b, 160c are compared in a subsequent step S20 to obtain a comparison value. This comparison step can include comparing the measured values ​​acquired for the at least two mold cavities 160a, 160b, 160c with each other and / or comparing the measured values ​​acquired 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 when pre-stretching the material layer 10 at a desired forming temperature.

[0083] Furthermore, the comparison step can include determining a deviation between the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c and / or determining a deviation of the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c from the reference measured value.

[0084] The determined deviation between the measured values ​​recorded for the at least two mold cavities 160a, 160b, 160c is a measure (indicator) of how much 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 to 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. For large thermoforming tools with many mold cavities, it can be advantageous to compare the measured values ​​at at least three, at least four or more spaced-apart mold cavities 160a, 160b, 160c and to determine their deviation from each other in order to monitor the homogeneity of the temperature distribution / forming temperature distribution over the forming surface of the material layer 10a.

[0085] The determined deviation of the measured values ​​recorded for the at least two forming 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.

[0086] If, in the first step S10, an individual measurement value is recorded for each of the at least two mold cavities 160a, 160b, 160c, such as a maximum value (measurement maximum), these values ​​are compared with each other and / or with a corresponding reference measurement value (target measurement value); any deviations detectable during the comparison can be provided as a comparison result. If, on the other hand, in the first step S10, a measurement profile is recorded for each of the at least two mold cavities 160a, 160b, 160c by recording a large number of measurements during the pre-stretching process, the recorded measurement profiles can be compared with each other and / or with a target measurement profile. For example, slopes of the recorded measurement profiles that indicate an increase in the stretching force to be applied during the pre-stretching process (see Fig. 4. The slope of the measured value profiles in measuring range 610) can be compared with each other. Alternatively, it is also conceivable that the recorded measured value profiles over a specific pre-stretching distance (pre-stretching time in Fig. 4) are integrated and the integration values, which indicate the required distribution energy, are compared.

[0087] The comparison result provided by the comparison (i.e., the determined deviation(s)) can be compared with a predefined threshold value (tolerance value) that represents a tolerance dimension that is not critical for the thermoforming process. If the determined deviation(s) exceed the threshold value, this indicates that the thermoforming process, in particular the forming temperature of material layer 10 across the mold cavities of the multi-cavity 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.

[0088] 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 occur if the determined deviation(s) exceed the tolerance value, as described above.

[0089] In connection with Fig. 7 A thermoforming tool 1000a according to the invention is described, which is designed for implementing the method described above.

[0090] The thermoforming tool 1000a is in Fig. Figure 7 is only schematically indicated. 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 forming 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 forming cavities 160. Each pre-stretcher 232 of the at least two pre-stretchers 232 is designed to form a layer of material (not shown in Figure 7). Fig. 7) to extend into the corresponding mold cavity 160.

[0091] In Fig. 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 Section 7. It is understood that the at least two pre-stretchers 232 can be coupled to a pre-stretcher bar for common actuation via respective pre-stretcher rods, as described in connection with the thermoforming tool 1000 in [reference]. Fig. 1 described. Similarly, the at least two mold cavities 160 can each have a mold base at their axial lower end, which are coupled with respective ejector rods and a common ejector bar.

[0092] The thermoforming tool 1000a further 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, indicating a stretching force or stretching energy to be applied when pre-stretching the material layer into the respective mold cavity 160. For this purpose, the measuring device 300 has at least two sensors 320, which are oriented in the transverse / lateral direction (see Fig. 7) and / or are arranged at intervals from each other in the thermoforming tool 1000a in the longitudinal direction. A specific implementation of measuring sensors 320 will be discussed in connection with the Fig. 8, Fig. 9a and Fig. 9b is described in more detail below. Regardless of the specific implementation, each of the at least two measuring sensors 320 of the measuring device 300 is designed to record 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.

[0093] The measured values ​​recorded by the at least two sensors 320 can be provided to an evaluation unit 400 via a (wireless or wired) communication interface 340 of the measuring device 300.

[0094] The evaluation unit 400 is configured to compare the measured values ​​acquired at the at least two forming cavities 160 or pre-stretchers 232 in order to obtain and provide a comparison result. In particular, the measuring unit 400 can be configured to compare the measured values ​​acquired at the at least two forming cavities 160 and to determine any deviation of the at least two acquired measured values ​​from each other or from a reference measured value (target measured value) and to provide this as a comparison result, as described above in connection with the procedure (step S20).

[0095] The evaluation unit 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 occur if the determined deviation in the comparison result exceeds a predefined threshold (tolerance value). In such a case, the evaluation unit can generate and output the feedback signal 500. The generated feedback signal 500 can, for example, include a warning signal for a user; additionally or alternatively, the generated feedback signal 500 can include a control signal, in particular a control signal for a heating device for heating the material layer. Thus, a heating device can be automatically adjusted.

[0096] While 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 unit 400 can be located outside the thermoforming tool 1000a and communicate wirelessly or wired with the sensors 320 installed in the thermoforming tool 1000a via a communication interface 440. The communication interface 440 can be coupled with the communication interface 340 of the measuring device 300 and read the sensor measurements provided by the respective sensors 320 in real time during the thermoforming process.

[0097] The evaluation unit 400 can be implemented as a combined software and hardware module and may include at least one processor 420 for performing the above-mentioned functionalities.

[0098] In connection with the Fig. 8a and Fig. 8b and the Fig. 9a and Fig. Section 9b describes a specific implementation of a measuring device 500 in connection with a thermoforming tool 1000 according to the invention. Fig. Figure 8a shows an isometric representation of the thermoforming tool 1000, while Fig. 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 via respective pre-stretcher rods 234 to a common, actuable pre-stretcher plate 236.

[0099] The thermoforming tool 1000 of the Fig. 8a and Fig. 8b essentially corresponds to the thermoforming tool 1000 from Fig. 1. The following only outlines the differences to the thermoforming tool 1000 from Fig. 1. Describe. Regarding components that are identical in construction and function, refer to the description in connection with Fig. Referenced above.

[0100] Unlike the thermoforming tool of the Fig. 1a / 1b The thermoforming tool 1000 additionally includes 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.

[0101] The bending elements can be realized directly in the pre-stretcher plate 236 by means of slot structures 312 formed in the pre-stretcher plate 236, as shown in Fig. Figure 2b shows that this implementation of the bending elements 310 is simple and space-saving, and allows for 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 the Fig. 8a and Fig. 8b shown. In addition to or alternative to the one shown in the Fig. 8a and Fig. In the sensor arrangement shown in 8b, the sensors 320 can also be arranged on the underside of the respective bending elements 310, facing away from the top.

[0102] Each bending element 310 exhibits 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 used in connection with the Fig. 9a and Fig. 9b is described further.

[0103] In the Fig. 9a and Fig. Figure 9b shows, as examples, a bending element 310 and the pre-stretcher 232 coupled to the bending element 310 via the pre-stretcher rod 234. Fig. 9a No force acts on the pre-stretcher 232, so the bending element 310 is not deformed. In Fig. 9b A force acts on the pre-extensioner 232. The force acting on the pre-extensioner 232 is in Fig.9b is indicated by an arrow, the direction of which 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. The stretching force acting on the pre-stretcher 232 elastically deforms or bends / flexes the bending element 310 proportionally to the magnitude of the force.

[0104] This bending / deflection can be detected by the sensors 320 on the respective bending element 310, with the measured value being 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 is itself proportional to the temperature / forming temperature of the material layer 10, as described above. Thus, the measured values ​​detected by the sensors 320 on the respective bending elements 310 are proportional to the stretching force applied to the respective pre-stretcher, and therefore also proportional to the actual forming temperature of the material layer.

[0105] Strain gauges, for example, can be used as sensors 310, which change their electrical resistance depending on the deformation (bending / deflection) 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; 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 bending elements 310 is also conceivable.

[0106] The electrical resistance of the respective strain gauges 320 on the respective bending elements 310 can be continuously measured. The resistance values ​​measured at the respective bending elements 310 can be directly compared with each other. Alternatively, the measured resistance values ​​can be converted into corresponding stretching force values ​​and compared with each other or with a target measured value. The deviation(s) between the resistance values ​​determined during the comparison indicate whether the thermoforming process is proceeding uniformly (and thus homogeneously) at the respective mold cavities.

[0107] The technique described here enables reliable and cost-effective monitoring of a thermoforming process in a multi-cavity thermoforming tool. In particular, the homogeneity of the temperature / forming temperature of the material layer fed to the multi-cavity thermoforming tool can be reliably monitored (in real time) across the entire forming surface of the material layer as well as in the thickness direction of the material layer. Furthermore, the technique according to the invention can be used for the automatic adjustment of a heating device in the thermoforming machine when deviations in the temperature distribution of the material layer are detected.

Claims

[1] Method for monitoring a thermoforming process designed to form a large number of articles in a single 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 when pre-stretching the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c); and Comparing the measured values ​​recorded at the at least two forming cavities (160, 160a, 160b, 160c) to obtain a comparative result, the comparative result being an indicator of the homogeneity of the thermoforming process. [2] Method according to claim 1, wherein the at least two measured values ​​are acquired at at least two forming cavities (160, 160a, 160b, 160c) which are spaced apart from each other 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] Method according to claim 1 or 2, wherein the measured value recorded at each of the at least two mold cavities (160, 160a, 160b, 160c) is a measured value that is proportional to the stretching force or stretching energy applied when pre-stretching the material layer (10) into the respective mold cavity (160, 160a, 160b, 160c). [4] Method according to any one of claims 1 to 3, wherein the comparison result is obtained by determining a deviation of the at least two recorded measurements from each other and / or a deviation of the at least two recorded measurements from a predetermined reference value. [5] Method according to any one of claims 1 to 4, wherein the sensing step comprises the simultaneous sensing of at least one measured value or a 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). [6] 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 profiles acquired at the at least two mold cavities (160, 160a, 160b, 160c). [7] Method according to claim 6, wherein the comparison result is obtained by comparing the increases or maxima of the measured value profiles recorded at the at least two mold cavities (160, 160a, 160b, 160c) during the pre-stretching of the material layer (10). [8] Method according to claim 6, wherein the comparison result is obtained by integrating the recorded measurement profiles at the at least two mold cavities (160, 160a, 160b, 160c) and comparing the integration values. [9] 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] Method according to any one of claims 1 to 9, further comprising generating and providing at least one feedback signal (500) when the comparison result indicates an inhomogeneous thermoforming process, in particular an inhomogeneous temperature distribution of the material layer (10). [11] 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 regulation signal, in particular a control or regulation signal for a heating device for heating the material layer (10), which is generated depending on the comparison result. [12] Thermoforming tool (1000, 1000a) for forming a plurality of articles in a layer of material (10) supplied to the thermoforming tool (1000, 1000a), wherein the thermoforming tool (1000, 1000a) comprises: 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 at at least two mold cavities (160, 160a, 160b, 160c) which 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 indicating 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 indicating 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-extensioner (232) to the first mold cavity (160, 160a, 160b, 160c) respectively.is spaced apart 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). [14] Thermoforming tool (1000, 1000a) according to claim 13, wherein the first sensor (320) is configured to measure an elongation or deflection of a first bending 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 bending 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 a top and / or on a bottom of the first bending element (310), and wherein the second sensor (320) is arranged on a top and / or on a bottom 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 designed to limit the elongation 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 configured to compare the measured values ​​recorded for the at least two forming 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 configured to generate and provide a feedback signal (500) when 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] Thermoforming machine comprising: the thermoforming tool (1000, 1000a) according to one of claims 12 to 19 for forming a plurality of articles in one layer of material (10); and a heating device for heating the material layer (10) before the material layer (10) is formed using the thermoforming tool (1000, 1000a).

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