Method and apparatus for testing preforms

Abstract

Method for testing the quality of preforms (2) for the manufacture of containers immediately following the manufacturing process of the preforms using passive thermography, wherein the preforms are made of plastic by injection molding and these preforms are subsequently further processed into the finished containers, comprising the steps - Transporting the preforms to be tested (2) by means of a first transport device (3), - Transporting the tested preforms (2) using a second transport device (5), - Transfer of the preforms (2) from the first to the second transport device (3, 5) at a transfer point (4), wherein the preforms (2) are transferred in free fall or along an inclined plane, - Taking thermal images of the preforms (2) using a thermal imaging camera (6) during the transfer of the preforms (2), - Processing the thermal images with a processing device (7) such that the temperature distribution on the surface of the preforms (2) is determined and anomalies in the temperature distribution on the surface of the preforms (2) are detected by comparing the temperature distribution of the recorded thermal image with a reference thermal image for the respective preform to be produced and if deviations occur that exceed certain tolerance values ​​stored in the processing device (7), this is detected by the processing device (7), whereby such deviations indicate that excessively high or excessively low temperatures occur in the manufacturing process.

Description

[0001] The invention relates to a method for testing the quality of preforms immediately following the manufacturing process of the preforms, comprising the steps - Transporting the preforms to be tested using a first transport device, - Transporting the tested preforms using a second transport device, - Transferring the preforms from the first to the second transport device at a transfer point.

[0002] The invention also relates to a device for testing the quality of preforms immediately following the manufacturing process of the preforms, comprising - a first transport device set up for transporting the preforms to be tested, - a second transport system was set up for transporting the tested preforms, - a transfer for passing the preforms from the first to the second transport facility.

[0003] In the production of containers, especially bottles, so-called preforms are generally manufactured first, and these preforms are then further processed into the finished containers. These preforms are typically made of plastic, primarily PET (polyethylene terephthalate), using injection molding.

[0004] PET bottles are typically manufactured in two process steps. In the first step, the preforms are produced using injection molding. In the second step, the PET bottles are manufactured from the preforms using stretch blow molding.

[0005] Both manufacturing steps can be carried out directly in the plastics processing plant in a single-stage process. However, the two-stage process, which separates the injection molding process from the stretch blow molding process, is more common.

[0006] Because defective preforms necessitate an interruption of production, prior art practices involve inspecting the preforms for defects before they proceed to the second processing step. Potential defects in the manufactured preforms include, in particular, inaccurate dimensions, areas in the container wall that are too thin or too thick, pinholes, burn marks, inclusions, or defective color.

[0007] Image processing systems have become the standard for inspecting preforms. The preforms are moved past a digital camera, capturing one or more images of each preform. An electronic processing unit compares the images with a reference image and determines whether the respective preform is free of defects or exhibits specific flaws. If a defect is detected, the image processing system can reject the identified defective preforms via a separate device.

[0008] Such an image processing system is described by INTRAVIS GmbH, Rotter Bruch 26 a, 52068 Aachen, in their brochure “INTRAVIS Vision Systems, Image Processing Systems - Preforms” version 09 / 2013 A. Configured there as an inline system, the image processing system inspects the preforms for all geometric features and material defects in both the body and the mouth area of ​​the preform at speeds of up to 72,000 objects per hour.

[0009] Furthermore, WO 2014 / 147176 A1 discloses a method and a device for testing the color quality of preforms, in which the items to be tested can be fed to a digital image acquisition device in a random order. This allows the device to be easily integrated into existing preform manufacturing machines that lack corresponding devices for aligning and feeding the preforms to the digital image acquisition device in a random order. The preforms are transferred in a random order into a receiving container as they leave a transport device, with the image being captured between the point of exiting the transport device and the receiving container. The testing method achievable with the described device makes it possible to detect preforms with color defects without the need for prior alignment. Moreover, it requires only minor modifications to the existing preform manufacturing machine.

[0010] DE 100 47 269 A1 discloses a method and a device for determining the temperature distribution of bulk materials. During the handling and processing of such bulk materials, particles agglomerate, and these particles retain very high temperatures internally, sometimes for extended periods. If these agglomerates enter an oxygen-rich environment, fires can occur, potentially resulting in significant consequential costs. Therefore, it is proposed to determine the temperature distribution of the bulk material with spatial resolution within a measuring section of a drop path in order to prevent fires in bulk materials.

[0011] WO 2013 / 087762 A1 discloses a method and the associated system for inspecting objects coated with a gas barrier layer. An infrared camera takes an image of each object and processes these images using a processing module in such a way that defective objects are detected and eliminated.

[0012] DE 198 46 995 A1 relates to a method for the non-contact detection of structural and / or surface defects in plate-shaped materials. For rapid, non-contact, and non-destructive detection, it is proposed that thermal energy be continuously introduced into the surface of the test specimen, which is continuously moving relative to the heat source, in specific areas, and that subsequently, a thermal image or multiple thermal images are continuously acquired line by line from the previously heated surface of the test specimen.

[0013] WO 2011 / 137264 A1 discloses a system and a method for the non-destructive testing and detection of thickness and structure variations in opaque and semi-opaque non-metallic composite materials and plastic materials using infrared thermography.

[0014] US 2002 / 0033943 A1 discloses a device and a method for detecting defects in colored plastic objects, comprising irradiating the object in the near-infrared range, measuring radiation reflected by or passing through the object, and analyzing the reflected or transmitted radiation to provide defect data for the object.

[0015] US Patent 2008 / 0022632 A1 discloses a method and a device for capturing thermal images of heat-sealed seams produced in heat-sealing stations when sealing containers with tear-off films. To determine the airtightness of the heat-seal seam, a thermal image of the seam is captured while the seam is still hot. By evaluating the image, it can be determined whether the seam temperature is within a range that ensures a sound seal or whether it has exceeded an upper or lower temperature limit, indicating a defective heat-seal seam. Based on the evaluation of the thermal image, defective items can then be rejected.

[0016] Based on this prior art, the invention aims to provide a method for testing the quality of preforms that can be easily integrated into existing manufacturing systems, particularly injection molding systems for preforms, and that enables the early detection of malfunctions in the manufacturing process or the manufacturing system, especially before defects appear in the preforms. Furthermore, a device for carrying out the method is to be specified.

[0017] The solution to this problem relies on the application of passive thermography. Using imaging techniques, the temperature distribution on the surface of the preforms is recorded immediately after the manufacturing process. In contrast to active thermography, the preforms being tested are not thermally excited to generate additional heat flow within them.

[0018] In detail, the problem is solved by a method having the features of independent claim 1 and a device having the features of independent claim 10.

[0019] During the transfer of preforms from one transport unit to a second, thermal images of the preforms are captured using at least one thermal imaging camera. These images are then processed electronically to determine the temperature distribution on the surface of the preforms and to detect any anomalies. This is achieved by comparing the temperature distribution of the captured thermal image with a reference thermal image for the specific preform being manufactured. Any deviations exceeding predefined tolerance values ​​are detected by the processing unit. Such deviations indicate that excessively high or low temperatures are occurring during the manufacturing process.This can occur due to local overheating in the cavities of the injection molding machine or due to insufficient post-molding cooling. The preforms produced in the first step of the injection molding process are selectively cooled using various devices; in particular, the body and the thread located at the neck of the preform are cooled internally and externally. Cooling errors can occur if the position of the preforms relative to a cooling device is incorrect. Therefore, the temperature distribution between the body and neck of each preform can be used, for example, to determine whether the preforms are being cooled properly.

[0020] Although the known testing methods for the absence of defects in preforms using digital image recording devices do not yet signal any defects, the method according to the invention can detect problems in the manufacturing process of the preforms at an early stage.

[0021] In one embodiment of the invention, the preforms to be tested are transported in a random order by the first transport device and transferred to the second transport device in free fall. This method allows for the straightforward integration of the device for testing the quality of preforms into existing preform manufacturing plants. Furthermore, any anomalies in the temperature distribution of the preforms can be detected without the preforms first coming into contact with sorting and / or aligning devices that could influence the surface temperature and thus the thermal image. The thermal imaging camera preferably captures the preforms in free fall immediately at the beginning of the free fall, while the preforms' velocity is still low.

[0022] To increase the time window for acquiring thermal images of the preforms, in one embodiment of the invention the preforms are transferred along an inclined plane. This transfer along the inclined plane is achieved, for example, with a chute that reduces the speed of the preforms during the transfer.

[0023] To capture the temperature distribution of as many preforms as possible in the thermal images, the preforms are preferably aligned during transfer and only then photographed with the thermal imaging camera. The electronic processing unit is preferably configured such that longitudinal alignment of the preforms is sufficient, regardless of whether the dome or the neck of the preform is at the front in the transport direction. Such alignment of the preforms can be achieved, for example, by means of a chute arranged between the first and second transport directions, which has longitudinal grooves designed for aligning the preforms. The at least one thermal imaging camera is positioned for the transfer process such that only thermal images of the aligned preforms are captured, preferably immediately after they leave the chute.

[0024] To minimize distortion of the thermal images caused by the atmosphere between the preforms and the thermal imaging camera, the images are captured at a distance of no more than two meters between the camera and the preforms. Furthermore, the thermal imaging camera preferably operates within a limited wavelength range in which the atmosphere emits and absorbs very little radiation.

[0025] To prevent distortion of the thermal images by external heat sources, the images are preferably acquired against a background with a surface exhibiting a homogeneous temperature distribution. A plate with fluid-carrying channels is a suitable example of such a background. It is not necessary for the plate to be cooled below ambient temperature by the fluid. The crucial factor is that the fluid-filled plate has a homogeneous surface temperature distribution. The surface is preferably non-reflective to prevent reflections of surrounding warm objects into the thermal image.

[0026] Every preform emits thermal radiation above absolute zero with an emissivity ε < 1, which depends on the preform material, in this case PET. The spectrum of the emitted thermal radiation is temperature-dependent. As the temperature increases, the emitted spectrum shifts to shorter wavelengths (Wien's displacement law). Depending on the material-dependent emissivity and the temperature-dependent emission spectrum of the preforms under investigation, the preferably limited wavelength range of the thermal imaging camera is matched to the emitted spectrum of the preforms, in particular such that the limited wavelength range of the thermal imaging camera at least partially, and preferably completely, overlaps with the expected emission spectrum of the preforms.

[0027] For preforms with surface temperatures between 20 and 100 degrees Celsius, the thermal imaging camera has a limited wavelength range of approximately 8 to 14 µm (long-wave infrared - LWIR). A thermal imaging camera with a wavelength range between 3 and 5 µm (mid-wave infrared - MWIR) would also be suitable in principle. However, in this wavelength range, the radiant power is significantly lower at the specified preform temperatures. Therefore, thermal imaging cameras preferably utilize the long-wave infrared spectral range.

[0028] Thermal imaging cameras can be either uncooled or cooled. Cooled cameras count photons of energy at a specific wavelength, typically in the infrared range between approximately 3 and 5 µm. These photons strike the individual sensors of the array, which are electronically controlled. The thermal sensitivity (temperature resolution) of cooled cameras is significantly higher than that of uncooled cameras. The detectors of a cooled camera must be cooled to their operating temperature. If the cooling system fails, the thermal imaging camera cannot function properly. Furthermore, the cooling process requires a start-up time. The advantage of higher thermal sensitivity is offset by the higher purchase and operating costs of cooled cameras.

[0029] For economic reasons, an uncooled thermal imaging camera is preferably used in a device for carrying out the method according to the invention. The sensors of the uncooled camera consist of a material whose resistance changes significantly with temperature. Such uncooled thermal imaging cameras do not require costly cooling devices. They are therefore significantly smaller and less expensive than cooled thermal imaging cameras.

[0030] The invention is explained in more detail below with reference to the figures. The figures show: Fig. 1 A schematic side view of a device for testing preforms according to a first embodiment of the invention, Fig. 2 a schematic side view of a device for testing preforms according to a second embodiment of the invention as well as Fig.3a a schematic perspective view of a third embodiment of the invention with devices for aligning the preforms at the transfer as well as Fig. 3b a schematic representation of the aligned preforms during the transfer.

[0031] Fig. Figure 1 shows a device (1) for testing the quality of preforms (2) that are produced using an injection molding machine (not shown) and subsequently selectively cooled. The device (1) comprises a first transport unit (3), which in this embodiment is designed as a conveyor belt. The upper run of the conveyor belt of the transport unit (3) transports the preforms (2) to be tested from the injection molding machine (not shown) to a transfer point (4) for transferring the preforms (2) from the first transport unit (3) to a second transport unit (5) for removing the tested preforms (2).

[0032] The device (1) further comprises a thermal imaging camera (6) which is directed towards an upper area of ​​the transfer (4), that is, the area immediately below the deflection (3a) of the conveyor belt. The thermal imaging camera (6) is connected to an electronic processing device (7), for example a personal computer, which is configured to process the thermal images of the preforms (2) acquired by the thermal imaging camera (6).

[0033] The second transport device (5) is also designed as a conveyor belt. Other continuous conveyors, such as roller conveyors or vibratory conveyors, are of course also suitable as transport devices. The first transport device (3) is arranged on a first transport level, and the second transport device (5) is arranged on a second transport level below the first transport level.

[0034] The transfer of the preforms (2) takes place in free fall, as is shown by the free-falling collective (2a) of preforms (2) in Fig. As indicated in Figure 1, the thermal images are acquired using the thermal imaging camera (6) against a background (8) with a surface (8a) exhibiting a homogeneous temperature distribution. The homogeneous temperature distribution of the surface (8a) is generated by a fluid-permeable cavity (8b) arranged on the back of a plate, through which a temperature-controlled medium can be continuously flowed. Preferably, the temperature control by the medium is such that the surface (8a) has a surface temperature corresponding to the ambient conditions.

[0035] The embodiment of the device for testing the quality of preforms (2) according to Fig. 2 differs from the device according to Fig.1. This is achieved essentially by transferring the preforms (2) to the second transport device (5) along an inclined plane. In this embodiment, the background (8) simultaneously forms the inclined plane on which the preforms (2) slide. Due to the temperature control of the surface (8a) of the background (8) by means of the fluid-permeable cavity (8b), a homogeneous temperature distribution on the surface (8a) is permanently ensured. Local heating of the background (8), and thus a heat signature on the surface (8a) of the background (8) that would distort the thermal images, is avoided by maintaining the homogeneous surface temperature. Furthermore, the temperature control allows the use of a less expensive thermal imaging camera with lower resolution.

[0036] Both in the device according to Fig. 1 as well as the device according to Fig.2. The transfer of the preforms (2) from the first transport device (3) to the second transport device (5) occurs in a disordered manner, as can be seen in the schematically depicted group (2a) of preforms located in the transfer area (4). The thermal image of individual preforms (2) within the group (2a) may therefore be incomplete, as it is wholly or partially obscured by other preforms. Furthermore, individual preforms (2) in the thermal image may be positioned at an angle to the background (8), resulting in the temperature distribution of only a portion of the surface being visible in the thermal image.

[0037] In the embodiment according to Fig.2 By transferring the preforms (2) along the inclined plane created by the background (8), it is already largely ensured that the preforms are detected by the thermal imaging camera over their entire length, since the longitudinal axis of the preform is usually aligned parallel to the surface (8a) of the background at the moment of thermal imaging.

[0038] However, the foregoing explanations make it clear that the devices according to the Fig. 1 and Fig. 2. It is not possible to subject all preforms (2) to testing, since some preforms are always in a position during the recording of the thermal image during free fall or sliding on the inclined plane that precludes meaningful evaluation by the processing device.

[0039] During the processing of the thermal images, those preforms whose position on the thermal image is suitable for comparison with a reference thermal image are identified. Deviations from the reference thermal image, for example, due to local overheating of the preform, can indicate a faulty manufacturing process in the injection molding machine and / or inadequate post-cooling.

[0040] In the Fig. In the device shown in 3a, b, the preforms (2) are aligned during the transfer, immediately after leaving the first transport device (3), by means of an inclined chute (9) with grooves (9b) running in the longitudinal direction (9a).

[0041] At the end of the slide, the aligned preforms (2) leave the slide (9) and fall freely against the background (8) from the Fig.3a not shown. Thermal imaging camera (6) recorded. The collective (2a) of aligned preforms (2) appears, for example, as in Fig. 3b indicated. All preforms are aligned longitudinally (9a) in three rows, whereby it is irrelevant for the thermal imaging whether the neck (2b) or the dome (2c) of the preform (2) points downwards.

[0042] Due to the parallel alignment of the longitudinal axes of the preforms (2) to the surface (8a) of the background (8), a flawless thermal imaging of the aligned preforms (2) of the collective (2c) is ensured.

[0043] By the device according Fig. 3a. It is therefore possible to check the temperature distribution on the surface of all preforms. This further increases the reliability of early detection of any problems in the upstream manufacturing machine used to produce the preforms. Nr. Designation 1. device 2. Preforms 2a. Collective 2b. Neck 2c. dome 3. First transport device 3a. Deflection 4. handover 5. Second transport device 6. thermal imaging camera 7. Processing facility 8. background 8a. surface 8b. cavity 9. slide 9a. Longitudinal direction 9b. Gutters

Claims

[1] Method for testing the quality of preforms (2) for the manufacture of containers immediately following the manufacturing process of the preforms using passive thermography, wherein the preforms are made of plastic by injection molding and these preforms are subsequently further processed into the finished containers, comprising the steps - Transporting the preforms to be tested (2) by means of a first transport device (3), - Transporting the tested preforms (2) using a second transport device (5), - Transfer of the preforms (2) from the first to the second transport device (3, 5) at a transfer point (4), wherein the preforms (2) are transferred in free fall or along an inclined plane, - Taking thermal images of the preforms (2) using a thermal imaging camera (6) during the transfer of the preforms (2), - Processing the thermal images with a processing device (7) such that the temperature distribution on the surface of the preforms (2) is determined and anomalies in the temperature distribution on the surface of the preforms (2) are detected by comparing the temperature distribution of the recorded thermal image with a reference thermal image for the respective preform to be produced and if deviations occur that exceed certain tolerance values ​​stored in the processing device (7), this is detected by the processing device (7), whereby such deviations indicate that excessively high or excessively low temperatures occur in the manufacturing process. [2] Method according to claim 1, characterized by , that the preforms (2) to be tested are transported in an unordered manner using the first transport device (3). [3] Method according to claim 1 or 2, characterized by, that the preforms (2) are first aligned during the transfer and only then are thermal images of the aligned preforms taken. [4] Method according to any one of claims 1 to 3, characterized by , that the thermal images are taken with a maximum distance of 2 meters between the thermal imaging camera (6) and the preforms (2). [5] Method according to any one of claims 1 to 4, characterized by , that the thermal images are taken against a background (8) with a surface with a homogeneous temperature distribution. [6] Method according to any one of claims 1 to 5, characterized by that the wavelength range of the thermal imaging camera is limited in the infrared spectrum. [7] Method according to claim 6, characterized by , that the wavelength range of the thermal imaging camera (6) overlaps at least partially with the emission spectrum of the preforms (2), which depends on the emissivity and the expected temperature window. [8] Device (1) for testing the quality of preforms (2) for the production of containers immediately following the production process of the preforms using passive thermography, wherein the preforms are made of plastic by injection molding and these preforms are subsequently further processed into the finished containers, comprising - a first transport device (3) set up for transporting the preforms (2) to be tested, - a second transport device (5) set up for transporting the tested preforms (2), wherein the first transport device (3) is arranged in a first transport level and the second transport device (5) is arranged in a second transport level below the first transport level, - a transfer (4) for transferring the preforms (2) from the first to the second transport device (3, 5), wherein the transfer (4) is configured for a free fall of the preforms (2) between the first and second transport device or includes a chute (9) configured for transferring the preforms along an inclined plane between the first and second transport plane, - a thermal imaging camera (6) set up to take thermal images of the preforms (2) during the transfer of the preforms, - a processing device (7) connected to the thermal imaging camera (6) is set up to process the thermal images in such a way that the temperature distribution on the surface of the preforms (2) is determined and anomalies in the temperature distribution on the surface of the preforms are detected by comparing the temperature distribution of the recorded thermal image with a reference thermal image for the respective preform to be produced and if deviations occur that exceed certain tolerance values ​​stored in the processing device (7), this is detected by the processing device (7), such deviations indicating that excessively high or excessively low temperatures occur in the manufacturing process. [9] Device according to claim 8, characterized by , that the first and / or second transport device (3, 5) are designed as a conveyor belt. [10] Device according to claim 8, characterized by, that the slide (9) has longitudinally extending channels (9b) which are set up for the longitudinal alignment of the preforms (2) and the thermal imaging camera (6) is aligned to the transfer (4) in such a way that only thermal images of the aligned preforms (2) are recorded. [11] Device according to any one of claims 8 to 10, characterized by , that the thermal imaging camera (6) is positioned at a maximum distance of 2 meters from the transfer point (4). [12] Device according to any one of claims 8 to 11, characterized by that it has a background (8) with a surface (8a) with a homogeneous temperature distribution. [13] Device according to claim 12, characterized by , that the background (8) is an integral part of the transfer (4).

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