DEVICE AND METHOD FOR MONITORING THE MATERIAL PROPERTIES OF PLASTICS

Description

[0001] The invention relates to a device for monitoring material properties of plastics during the production process.

[0002] Methods and corresponding devices for testing and measuring the physical parameters and material properties of plastics are known.

[0003] For example, in plastics production, especially in film production, a pressure filter test is used to determine the dispersion of additives or impurities in a polymer, a rheometer to determine viscosity properties, optical film inspection, color measurements, near-infrared spectroscopy (NIR), Fourier transform infrared spectrometry, mass spectrometry or mechanical strength measurements (e.g. tensile, elongation measurements, scratch resistance of the surface and the like) to determine the properties of the product.

[0004] These examination methods are largely standardized.

[0005] WO 2016 / 037 205 A1 discloses a method and a device for modular material analysis for plastics comprising at least two different measuring devices that are mechanically and / or functionally connected to each other.

[0006] DE 38 26 095 A1 discloses a device for taking melt samples, in particular from continuously operating plastics processing plants.

[0007] As a rule, these tests are carried out offline in various measuring devices after sampling during the production process or after the product has been manufactured, usually externally in analysis and quality control laboratories.

[0008] The corresponding results are then usually only available after the production process has been completed, which makes it almost impossible to react to quality deviations during the production process.

[0009] This increases the number of defective products that do not meet quality requirements. This reject must then be sorted out and / or reworked.

[0010] It is therefore advantageous to analyze the relevant properties of the product to be manufactured or of the raw material or preliminary material during the production process in order to be able to intervene in the production process in a timely manner and thus reduce the amount of finished product that does not meet the quality requirements.

[0011] The object of the invention was to provide a device and a method for quality control and quality assessment for plastics, which allows certain parameters and properties of the product and the preliminary product to be determined inline, i.e. already during the production process, in order to be able to react promptly to deviations from the target state and thus largely avoid the production of defective products.

[0012] The invention solves this problem by the device and method of the independent claims. Advantageous further developments are presented in the dependent claims. The subject matter of the invention is therefore a device for inline quality control of plastics during the production process, comprising at least two different measuring devices selected from the group consisting of measuring devices for determining rheological properties, measuring devices for color measurement, measuring devices for detecting inclusions and gel particles and the like, measuring devices for assessing optical properties, and measuring devices for determining mechanical parameters. The device comprises a device for removing plastic melt from the production extruder, a device for forming a flat film from this plastic melt, and devices for transporting the flat film to the measuring devices and for converting the flat film.wherein the device is coupled to a side strand of the extruder via an adapter and the plastic melt is removed after a valve is opened, and the plastic melt is removed by a displacement body which is positioned in the melt stream and around which the melt is guided to a bore in the cylinder housing of the extruder.

[0013] A further subject of the invention is a method for quality control of plastics by means of a device during the production process, wherein the device is coupled to a side strand of a production extruder via an adapter, wherein the method is characterized by opening a valve, removing a defined amount of plastic melt from the production extruder through a displacement body which is positioned in the melt stream and around which the melt is guided to a bore in the cylinder housing of the extruder, forming this plastic melt into a flat film, transporting the flat film to measuring devices situated in the device, and examining and assessing the flat film.

[0014] By removing the melt inline directly from the production extruder and inspecting the flat film formed from the melt during the production process, quality deviations can be responded to immediately and promptly, thus avoiding unnecessary product scrap.

[0015] The device according to the invention comprises a device for taking a sample of the plastic melt from the production extruder. This device is implemented, in the simplest case, by an adapter, which in the simplest case has a bore to which a melt line to the device according to the invention is attached, or by a bore directly in the cylinder housing of the extruder's discharge area.

[0016] However, these solutions have the disadvantage that the flow velocity at the edge is very low due to edge effects, and there is therefore a risk of correlating samples that have already been altered, degraded or are no longer relevant in terms of time with the material flow.

[0017] To avoid this, adapters or intermediate pieces can be installed according to the invention to ensure that melt samples are taken over the entire or at least from or in the middle of the main flow.

[0018] In a simple design, the discharge opening is simply a pipe end that is directed into the melt stream.

[0019] A displacement body is introduced into the melt stream, designed to contain a discharge point for the melt stream. The discharge point can be either a bore within the displacement body that directs the melt stream out of the extruder, or a melt channel that diverts the melt stream along the surface of the displacement body.

[0020] In both cases, the sampling point must be designed in such a way that the melt is discharged independently due to the pressure and flow conditions prevailing in the extruder. The melt channel design is advantageous in that a representative sample can be taken radially from the melt flowing in the extruder barrel.

[0021] A simple embodiment of such a displacement body can be a fluidically optimized spoke running diagonally through the cylinder of the extruder with a melt channel attached to the front side.

[0022] Likewise, three or more spokes can be designed in a star shape. In both embodiments, it is advantageous if the displacement body is positioned in the extruder in a torpedo-like manner, counter to the flow direction.

[0023] Using the adapter, the device according to the invention is coupled to a side strand of the extruder. After opening a valve, a melt pump withdraws a defined amount of plastic melt from the extruder. A rheometer, consisting of a measuring nozzle with a defined cross-section, measures the pressure drop over a specific measuring section. The viscosity of the melt can be determined from this pressure drop.

[0024] The melt is then produced into a flat film using a slot die and a casting roll to determine further properties. It is particularly important that the film has homogeneous properties (thickness, color, etc.) to ensure optimal control later on. Therefore, optimal cooling and the quality of the casting roll surface are essential.

[0025] In the simplest case, the surface is glossy. To ensure the release of various materials, the roller can be manufactured with coatings such as bright chrome, non-stick coatings such as Teflon, or other common non-stick coatings, or combinations thereof.

[0026] In order to create better conditions for special investigations, the roller surface can also be formed with other textures or with different textures in the longitudinal and transverse directions, such as matt areas, embossing, different degrees of gloss / mattness, patterns and the like.

[0027] The aim of these structures is, for example, to prepare the surface for subsequent examinations, such as olfactory examinations (smells) or optical examinations.

[0028] The film produced in this way in the device is subjected to an optical inspection.

[0029] For this purpose, inclusions and gel particles are detected using a high-resolution area camera or line scan camera and a downstream high-performance computer. They are then evaluated using suitable evaluation software that recognizes the size, shape and position of the inclusions and defects and can assign them to classes.

[0030] The errors are detected via grayscale or color changes of individual pixels of the recorded image.

[0031] The size of the inclusions and defects can be defined in the simplest case by the number of pixels occupied, but can also be expressed by the area of ​​the enclosing circle or rectangle of the two extreme values ​​(length, width).

[0032] In the latter case, a statement can also be made about the shape of the defect, since this is the aspect ratio of the defect.

[0033] The foil is conveniently illuminated by a dome light and two ring lights arranged parallel above and below the foil, allowing measurements in both reflection and transmission mode. Line, bar, or area illumination can also be used.

[0034] The lighting units are mounted in an easily removable enclosure to prevent stray light from outside.

[0035] White light is usually used for illumination, but depending on the camera and type of illumination, different colors or combinations of colors can also be used in incident and transmitted light.

[0036] In addition, lighting units arranged at different angles to the film surface can help to facilitate defect detection.

[0037] Furthermore, filters such as linear or circularly polarizing filters can be used in front of the camera and / or behind the camera, for example to better detect defects or to supplement the analysis of materials through their optical properties.

[0038] In a special case, UV illumination units (in different wavelengths) can also be used, for example to investigate whether the material itself or impurities fluoresce in certain spectral ranges.

[0039] To measure all these properties, one or more lighting and / or camera units can be combined in parallel or in series.

[0040] The plastic film is then subjected to a color measurement. The film is illuminated with a daylight lamp or with different colors in succession, and the light emitted by the film is split into different wavelength ranges. The colorimetric data is calculated using a sensor or sensor system.

[0041] The determined color value is preferably represented in the L*a*b color space, which is familiar to those skilled in the art. However, other color spaces such as the RGB, CMYK, or Pantone color space can also be used.

[0042] In a further inspection step, the film undergoes an NIR measurement. This process allows for the detection of foreign materials in the plastic film. This is particularly important for recycled plastics to ensure consistent product quality.

[0043] A multispectral sensor is advantageously used, which is very cost-effective in contrast to conventional NIR spectrometers.

[0044] The sensor operates in selected wavelength ranges and measures the absorption of light within this narrow wavelength range. The absorption rate can be used to determine, for example, impurities or changes in composition. Since the sensor only operates in a narrow light spectrum, it is calibrated in advance specifically for the material being examined.

[0045] Before the mechanical testing, the film is cut to a defined width.

[0046] The remaining film strip is measured for its thickness directly online, for example, purely mechanically using a probe, optically (e.g., IR), capacitively, or by radiation (X-rays), and then stretched or extended over two pairs of rollers. The torque required for stretching is measured, and an equivalent to the modulus of elasticity (E-modulus) is calculated from this.

[0047] The stretching can be carried out using two pairs of rollers running at different speeds or using a combination of two pairs of rollers running at the same speed and a stretching roller arranged between them that changes the web path during a measurement.

[0048] If the second pair of rollers is set into an accelerated movement, a uniaxial tensile test with continuously increasing strain, such as that carried out in a laboratory test, can also be simulated.

[0049] If necessary, all common optical measuring methods can be used in the device according to the invention, such as gloss measurement, haze measurement, UV-VIS, ellipsometry, X-ray fluorescence and the like.

[0050] However, combined analyses, such as scratch measurement with optical detection of the defect, or other mechanical parameters such as tear propagation test, necking behavior or temperature-dependent tensile properties can also be provided in the device according to the invention.

[0051] Testing for odors is also particularly important for the recycling sector. Since no real odor sensors exist yet, a simple measurement can be carried out in a module using a simple gas chromatograph.

[0052] To do this, the film is heated either immediately during production, i.e. during the casting process, or later, for example by infrared, hot air or via a contact roller, and the resulting vapors are extracted and fed into a gas chromatograph.

[0053] Using a very short measuring column, for example, 10 cm, a rapid measurement can be achieved, enabling the detection of various ingredients, both odorous and neutral. The longer the column, the better the resolution, and the longer the measurement time.

[0054] In Fig. 1 a device according to the invention is shown.

[0055] The device consists of a housing 1 which has a device 2 for removing the plastic melt, which is transported through a measuring nozzle for determining the rheological properties, in particular the viscosity of the melt, into the interior of the housing of the device.

[0056] Finally, the plastic melt is formed into a flat film 6 through the slot die 3 and a casting roller 5. It is then guided over deflection rollers for the first optical inspection using an area scan camera 7 to detect inclusions and gel particles. The film is guided through a housing to exclude stray light and illuminated with a dome light and ring lights.

[0057] The flat film is then passed on to color measurement 8 and NIR measurement 9 using a multispectral sensor.

[0058] Before the subsequent examination, the film is cut into strips by an edge trim 10.

[0059] The middle strip preferably has a width between 20 and 25 mm in order to correspond to the standard test specimen from the film tensile test (ISO 527-3), but can also be made wider or narrower.

[0060] The middle strip is stretched between two pairs of rollers 11, 12 to determine the modulus of elasticity. The two edge strips 13 are not used for the tensile test.

[0061] All measuring instruments and processing equipment are located in the housing to save space.

[0062] This allows the required parameters of the plastic or plastic film to be determined directly in one place during production, allowing intervention in the production process if necessary. This significantly reduces the amount of material that does not meet quality criteria.

[0063] Furthermore, by taking a sample once at a single point in the production process, there is no need for measuring points located at different locations along the production facility.

Claims

1. A device for in-line quality control for plastic materials during the production process, comprising at least two different measuring instruments selected from the group of measuring instruments for determining the rheological properties, measuring instruments for color measurement, measuring instruments for detecting inclusions and gel particles and the like, measuring instruments for assessing the optical properties and measuring instruments for determining the mechanical parameters, examination of the olfactory parameters, wherein the device has an installation for removing plastic melt from the production extruder, an installation for forming a flat film from this plastic melt, and installations for transporting the flat film to the measuring devices and for finishing the flat film, wherein the device is coupled to a side branch of the extruder via an adapter and the removal of the plastic melt is performed after opening a valve, and the removal of the plastic melt is performed by means of a displacement body which is positioned in the melt flow and around which the melt is directed to a bore in the cylinder housing of the extruder.

2. The device according to claim 1, characterized in that the removal of the plastic melt is performed through a bore in the cylinder housing of the exit region of the extruder.

3. The device according to one of claims 1 to 2, characterized in that the device comprises a wide slit die and a casting roller for forming a flat film.

4. The device according to one of claims 1 to 3, characterized in that the device comprises two pairs of rollers for stretching the flat film.

5. The device according to one of claims 1 to 4, characterized in that the device comprises a measuring die for determining the rheological properties.

6. The device according to one of claims 1 to 5, characterized in that the device comprises an area scan camera for detecting inclusions and gel particles in the flat film.

7. The device according to one of claims 1 to 6, characterized in that the device comprises a multispectral sensor for NIR measurement.

8. The device according to one of claims 1 to 7, characterized in that the device comprises an illumination device and a sensor for color measurement.

9. A method for the quality control of plastic materials by means of a device during the production process, wherein the device is coupled via an adapter to a side branch of a production extruder, wherein the method comprises: opening a valve, removing a defined amount of plastic melt from the production extruder by means of a displacement body which is positioned in the melt flow and around which the melt is directed to a bore in the cylinder housing of the extruder, forming this plastic melt into a flat film, transporting the flat film to measuring devices situated in the device, and examining and evaluating the flat film.

10. The method according to claim 9, characterized in that the removal of the plastic melt is performed through a bore in the cylinder housing of the exit region of the extruder.

11. The method according to one of claims 9 to 10, further comprising one or more of: forming a flat film by means of a wide slit die and a casting roller, stretching the flat film using two pairs of rollers, determining the rheological properties using a measuring nozzle, detecting inclusions and gel particles in the flat film using an area scan camera, NIR measurement using a multispectral sensor, and color measurement using an illumination device and a sensor.