Method for producing plastic film and film extruder

Abstract

The invention relates to a method for producing a plastic film by means of a film extruder, in which at least two plastic materials are supplied together from at least two storage containers to one extruder or to one extruder in each case, the plastic materials are melted in one extruder or in a plurality of extruders, the invention relates to a method for producing a thin film web, in particular a thin film web, which is formed by cooling and is fed as a melt strand or a plurality of melt strands into a nozzle head, the melt strand or the plurality of melt strands are distributed in a planar manner in the nozzle head and are pressed out of a nozzle of the nozzle head as a melt film, and the melt film and / or the thin film web formed by cooling is pulled by means of at least one pulling roller. In particular, measured values are recorded by means of a measuring device for at least one property of at least one plastic material and / or at least one melt strip and / or melt film and / or film web, and / or for machine parameters of the film extruder, and the first plastic material is a plastic material having undulating material properties, and the second plastic material is a plastic material having undulating material properties. The first plastic material is a plastic material having a substantially constant material property, and the second plastic material is a plastic material having a substantially constant material property, the first plastic material and the second plastic material being fed to one or more extruders in a quantity ratio, the quantity ratio being varied as a function of a measured value for the property.

Description

Technical Field

[0001] The invention relates to a method for producing a plastic film and a film extruder according to the preambles of claims 1 and 6. Background Art

[0002] To produce plastic film, at least two plastic materials are often provided in at least two storage containers. The plastic materials are usually present in a solid but pourable form (e.g., granules), in some cases also as flakes, or a mixture of the two. The at least two plastic materials can be fed together to the extruder. In one variant, each plastic material is fed separately to the extruder.

[0003] In one or more extruders, the plastic material is typically melted under the action of heat and mechanical pressure. If two plastic materials are processed in one extruder, they are mixed and, in particular, homogenized. However, this mixing can also take place on the way from the storage container to the extruder.

[0004] After melting, the flowing melt strands from each extruder are fed to the nozzle head. Here, the melt strands are distributed in width, thereby producing a flat melt stream whose width is many times greater than its thickness, in particular at least 20 times its size. The distribution of the melt strands into melt streams can be carried out linearly or annularly. If multiple melt strands are fed to the nozzle head, these can merge into a multi-layer melt stream within the nozzle head. Subsequently, the melt stream is extruded from the nozzle of the nozzle head, thereby producing a melt film. The melt film now cools and forms a film web, which can be, for example, a flat web or, if the melt strands are distributed annularly, a hose web. Subsequently, the melt film and / or the already formed film web is pulled by at least one pulling roller.

[0005] With this method and this film extruder, film webs can usually be produced on a large scale. However, it may happen that the quality of the film webs produced over a longer period of time is not constant. Summary of the Invention

[0006] The object of the present invention is therefore to provide a method and a film extruder with which the quality of the film web can be maintained constantly even over a relatively long production period.

[0007] This object is achieved according to the invention by all the features of claim 1. Possible embodiments of the invention are specified in the dependent claims.

[0008] According to the present invention, measurement values are recorded using a measuring device for at least one property of at least one plastic material and / or at least one melt strand and / or melt film and / or film web, and / or for machine parameters of a film extruder. This measure makes it possible to monitor quality not only on the finished film web, but also on the preceding or intermediate products. This allows, for example, infrared or radiometric measurements to be used to obtain information about the composition of the plastic material. This allows for monitoring material changes occurring over the production period. However, changes in the plastic material over time also lead to changes within the extruder during the material's melting process. Thus, within an extruder, particularly within multiple extruder zones, the plastic material temperature may vary, particularly due to fluctuations in the material's heat capacity. Changes in the plastic material may influence the pressure profile in the extruder, particularly due to the material's backpressure. The duration of the extruder zones' switching on time can also be monitored. One easily monitored variable is the motor torque required to drive the extruder screw at a predetermined speed within the extruder. For all of the above-mentioned parameters, measured values can be obtained using suitable measuring devices, such as temperature measuring devices, pressure measuring devices or torque monitoring devices.

[0009] Furthermore, according to the invention, it is provided that the first plastic material is a plastic material with fluctuating material properties and the second plastic material is a plastic material with essentially constant material properties, wherein the first plastic material and the second plastic material are supplied to the extruder or extruders in a quantity ratio, wherein the quantity ratio is varied depending on measured values for the properties.

[0010] The first plastic material has properties that fluctuate over time during processing in a film extruder, particularly regarding its inherent material composition. Such fluctuating properties arise, in particular, from the fact that the two different volume elements of the first plastic material have different base material compositions. While the base structures are chemically similar, the different base materials may have different densities. For example, polyethylene can exist as high-density (HD-PE) but also as low-density (LD-PE). The overall density of the plastic material can vary depending on the proportions of the two polyethylenes between the volume elements.

[0011] The second plastic material is a material with substantially constant material properties compared to the first plastic material. Such a material is often produced from a high-purity starting material and strictly adheres to predefined manufacturing parameters. Consequently, such a second plastic material is often more expensive than the first plastic material. Therefore, the present invention also contributes to reducing the costs of the produced film web.

[0012] To compensate for the fluctuating properties of the first plastic material, the present invention further provides that the first and second plastic materials are initially supplied to one or more extruders in a quantitative ratio. Thus, in the first case, the two materials are supplied in a quantitative ratio before entering the extruder. Thus, for example, the quantitative proportion of the first plastic material is 70%, and the quantitative proportion of the second plastic material is 30%. The quantitative ratio is varied based on the aforementioned measured values for the aforementioned properties. In the second case, in which the melt strands are combined into multiple layers within the nozzle head, the quantitative ratio can be determined so that the layer thicknesses have a specific ratio to one another. Thus, for example, the first plastic material layer thickness is 70% of the total layer thickness, and the second plastic material layer thickness is 30% of the total layer thickness.

[0013] In both cases, the second plastic material can be present in a quantitative proportion of at least 10%, but in particular at least 20%, of the total plastic material. If the second plastic material contains additives or consists solely of additives, the second plastic material can be present in a quantitative proportion of at least 0.5% of the total plastic material. The additives can be minor components mixed into the main component of the second plastic material. For specific properties of the first plastic material, it may even be sufficient to use only additives as the second plastic material.

[0014] As already mentioned, for the aforementioned properties, measured values are acquired using at least one measuring device. In particular, the measured values are supplied to a computing and control device. The computing and control device compares the acquired measured values with a theoretical value for the property and / or an average of previously acquired measured values. If the difference between the measured value and the theoretical value or the average exceeds a limit value, the quantity ratio of the first and second plastic materials is altered, i.e., adapted. In particular, this means altering or reducing the supply of the first plastic material and / or similarly altering, in particular increasing, the supply of the second plastic material until the measured value again falls below the limit value. Preferably, the supply of both plastic materials is altered simultaneously to keep the total quantity constant.

[0015] In an advantageous embodiment of the present invention, the first plastic material is a recycled plastic material. Recycled plastic materials are distinguished by the fact that they contain plastic material that has already been used by the user. Since materials, often originating from different first-time applications of plastics, arrive at the recycling facility one after another, the properties within the materials may differ significantly. This is true both in terms of the material composition and in terms of possible contamination by foreign matter (which, while undesirable, is often unavoidable). Overall, the present invention thus offers the possibility of processing recycled materials in film extruders, thereby improving the quality of the film web produced compared to conventional methods.

[0016] Advantageously, the characteristic for which the measurement value is acquired by means of the measuring device comprises at least one of the following characteristics:

[0017] Bubble stability, bubble shape, cold solidification line height or cold solidification zone height, film temperature, film thickness, pulling speed, film width.

[0018] Bubble stability describes whether the melt film follows the intended transport path or whether the actual transport path deviates from it. In practice, this is often referred to as film "flutter," meaning periodic deviations from the intended transport path. An optical camera can be used as a measuring device. The images captured by the camera form the measured values and can be evaluated with respect to these deviations.

[0019] The bubble shape describes whether the geometry of the melt film describes the set geometry. In practice, the shape of the melt film can change due to external influences, such as ambient cooling in the film extruder. However, within the scope of the present invention, it is possible to attribute such changes to the properties of the first material. An optical camera, as a measuring device, is also suitable for evaluating the bubble shape. The camera image is also to be understood as a measured value, wherein the actual shape of the melt film in the film bubble region can be compared with the target shape for evaluation.

[0020] The cold setting zone is a transition zone where the melt film solidifies into a film web. Therefore, the film web no longer forms a melt, but forms a solid film web, which is particularly unable or at least no longer able to be significantly shaped. The cold setting zone is often also referred to as a cold setting line. The cold setting zone height refers to the distance between the cold setting zone and the nozzle along the conveying path. Although the term "height" originates from blown film extrusion, the described distance should also be determined in a flat film extruder. The cold setting zone height may change in the following ways, especially during the extrusion operation, that is, especially for the first plastic material heat capacity changes over time. Knowing the cold setting zone height is very important for adjusting or positioning machine parts, especially for the position of the calibration basket. The cold setting zone can be carried out by temperature measurement at the film bubble, for example, by an infrared camera. If it is confirmed that the temperature increases or decreases compared to the theoretical temperature or the average temperature, this means that the cold setting zone has been moved in the conveying direction or against the conveying direction.

[0021] In film extruders, separate control circuits are often also provided for calibrating the basket height and / or the cold-setting zone height, so that the measured variables of these control circuits can be set to measured values for properties of the first plastic material.

[0022] Typically, the film temperature can also be determined at other locations by means of temperature measuring devices. In particular, as already described in connection with the cold setting zone, the film temperature depends on the heat capacity of the first plastic material and thus on its properties (such as chemical composition).

[0023] Film thickness is also a property of the produced film web, which can depend on the properties of the plastic material used. Thus, variations in the properties of the first plastic material can lead to variations in film thickness. Film thickness can thus preferably be determined using a measuring device operating with ultrasound or radiation waves or based on tactile measurement. Combining such a measurement with a capacitance measurement offers particular advantages. Capacitance measurement using electrodes provides results that, for a given film thickness, depend on the material density of the film web.

[0024] If the thickness of the film web also varies in addition to the density, both must be determined independently of one another. Therefore, the thickness and density can be determined separately using two measuring devices based on different physical measurement principles. The density of the film web, in turn, allows inferences to be drawn about the density of the first plastic material, so that in the event of deviations from the theoretical density, corrections can be made to the quantitative composition of the first and second plastic materials.

[0025] The film width can also be determined using the width measuring device, and the film width also provides information on the properties of the first plastic material. The film width depends in particular on the extensibility of the film and is therefore directly dependent on the properties of the first plastic material.

[0026] The pulling speed of film web, ie the setting circumferential speed of pulling roller under given torque depends on the properties of first plastic material, especially the density of first plastic material. When the outer circumference of pulling roller is known, circumferential speed can be measured by angle sensor.

[0027] The properties of the first plastic material can be determined using the measuring devices described above and below (this list is not exhaustive), wherein the measured values of the measuring devices can be considered individually or collectively. Preferably, these measured values are compared with theoretical values and / or average values in the computing and control device. If a deviation exceeds a threshold value, according to the present invention, the quantitative ratio of the first plastic material and the at least one second plastic material is changed. It can be provided that, if the computing and control device considers multiple measured values, these measured values are weighted, for example, to account for their influence. Furthermore, it can be provided that the weights are repeatedly checked and adapted based on their influence. The weights can be stored or stored in a memory device of the film extruder and / or the computing and control device. Thus, data from previous film production processes can be used during the current film production, thereby enabling faster material production when starting a new film production. Of course, such data can be stored in conjunction with a recipe, wherein the recipe includes not only the type of plastic material and the quantity to be supplied for producing the intended film, but also machine parameters (such as the positioning of the calibration basket) and process parameters (such as the speed of the extruder screw). It is particularly advantageous if the computing and control device varies the quantity ratios, in particular within a predefined range of variation, and records the effects on the measured values. This allows for the generation and storage of mixing rules, which can be formulation-specific and, in particular, specific to different first plastic materials. Such variations can also be performed by providing a test extruder outside the film extruder, feeding the first plastic material and the at least one second plastic material in different quantity ratios, and determining the measured values.

[0028] The measuring device can be used to measure not only the properties of the melt film and / or film web, but also the properties of the melt strand. These properties include:

[0029] Pressure curve in the extruder,

[0030] Temperature in the extruder zone,

[0031] density,

[0032] Viscosity,

[0033] Back pressure,

[0034] Conveying rate.

[0035] This list is also not exhaustive.

[0036] The pressure profile in the extruder is directly dependent on the properties of the plastic material used, in particular its density, and can be measured using a pressure measuring device. In particular, the pressure drop across the screen arrangement can also be determined. These measured values are then fed to the computing and control system.

[0037] The melt stream temperature in the individual zones (extruder zones) of the extruder can also be determined using known temperature sensors and provide information about the properties of the first plastic material.

[0038] The density can be determined at various locations within the film extruder using a density measuring device. However, the density can also be measured directly on the unmelted plastic material. This can be influenced not only by the density within the granules or flakes of the first plastic material but also by the bulk density of the material, particularly within a storage container.

[0039] The viscosity of the melt strand, which can also be referred to as MFI (“Melt-Flow-Index”), can be measured in an extruder or in multiple extruders using at least one viscometer and / or, for example, in the line between the extruder and the nozzle head and / or in the nozzle head.

[0040] The counterpressure generated by the melt strand can also be determined and provides information about the properties of the first plastics material.

[0041] The delivery rate, ie the volume delivered or the weight delivered per unit time, can be determined using a measuring device provided for this purpose.

[0042] Furthermore, it is advantageous if the machine parameters in the characteristics include at least one of the following parameters:

[0043] The duration of the extruder zone being switched on,

[0044] Extruder drive motor torque,

[0045] Supply and / or exhaust of external and / or internal cooling air.

[0046] The "on-time" refers to the time from the start of one or more extruders until the extruders or their respective zones reach the operating temperature or pressure. The "on-time" also includes the time until the extruder screw reaches its rated speed. The operating temperature or pressure can be determined using temperature or pressure sensors.

[0047] The motor torque can be determined in a simple manner, for example, by determining the current consumption at a set speed. The motor torque can be an indicator of the density of the first plastic material and / or the second plastic material. In particular, the motor torque reacts sensitively to changes in the material supplied to the extruder.

[0048] The supply of internal cooling air via an internal cooling device serves, on the one hand, to form the film bubble into the desired shape, and, on the other hand, to extract heat from the melt film so that it solidifies into a film web. To dissipate the extracted heat, the internal cooling air must be exhausted again. A blower is typically provided for supplying and exhausting the cooling air. A control circuit for the bubble shape and / or the temperature of the film bubble is often provided, so that the measured and / or controlled variables used in this control circuit can be used as measured values for determining the properties of the first plastic material. The same applies to the supply of external cooling air via an external cooling device, which also serves to extract heat from the melt film. The only difference from an internal cooling device is that the external cooling air does not need to be exhausted, as it escapes into the surroundings. Instead, the external cooling air is often temperature-controlled, so a control circuit is also provided for this purpose. Here, the controlled variables can also be used to determine the properties of the first plastic material.

[0049] The object stated at the outset is also achieved by a film extruder for producing a plastic film according to the method according to the invention, the film extruder having

[0050] At least two storage containers for providing plastic material

[0051] at least one extruder to which the plastic material from the two supply containers can be fed, or at least two extruders to which the plastic material can be fed, respectively, wherein the plastic material can be melted in the extruder or in the extruders and can be formed into one melt strand or two melt strands

[0052] a nozzle head to which the melt strand or strands can be supplied and in which the melt strand or strands can be distributed over a surface area by means of a distributor

[0053] A nozzle arranged at or in the nozzle head, from which a melt strand that is deformed into a melt film is extruded

[0054] A pulling roll, with which the melt film and / or the film web formed by cooling can be pulled from the nozzle.

[0055] The film extrusion is characterized in that a measuring device is provided, with which measured values can be recorded for at least one property of at least one plastic material and / or at least one melt strand and / or melt film and / or film web, and / or for machine parameters of the film extruder, the first plastic material being a plastic material with fluctuating material properties and the second plastic material being a plastic material with essentially constant material properties, and a computing and control device is provided and is designed to feed the first plastic material and the second plastic material in a quantitative ratio to one or more extruders, wherein the quantitative ratio can be varied as a function of the measured values for the properties in order to maintain the production parameters of the film extruder at essentially constant values.

[0056] Thus, the same advantages are achieved as already described in conjunction with the method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Further advantages, features, and details of the present invention are apparent from the following description, in which various exemplary embodiments are explained in detail with reference to the accompanying drawings. The features mentioned in the claims and the description may be essential to the present invention individually or in any combination thereof. Within the scope of the entire disclosure, the features and details described in conjunction with the method according to the invention also apply to the film extruder according to the invention, and vice versa, so that the disclosure of the various aspects of the present invention is or can be mutually referenced. Among them:

[0058] Figure 1 Schematic diagram showing the first film extruder

[0059] Figure 2 Schematic diagram showing the second film extruder DETAILED DESCRIPTION

[0060] Figure 1 The schematic diagram shows a first film extruder, which is configured as a blown film extruder 100. The machine first includes a metering hopper 101 into which a first plastic material is fed. The first plastic material is, in particular, a recycled plastic material with varying properties, present in the form of pellets. Therefore, this material is also referred to as "ReGran." This material has first material properties, in particular a first density and a first melt flow index.

[0061] In metering hopper 101, at least one second plastic material can be mixed with a first plastic material. This second plastic material can be supplied via metering hopper 102 and / or metering hopper 103. The second plastic material has specific properties, in particular a second density and a second melt flow index. The mixture of the first plastic material and the at least one second plastic material has a mixed density that can be adjusted, for example, using the method according to the present invention. The corresponding mass flows of the plastic materials can be influenced so that the mixture has a variable proportion of plastic materials depending on the desired properties.

[0062] The first plastic material and at least the second plastic material can be fed to an extruder 104 comprising a rotating extruder screw. The drive is carried out by an electric motor (not shown in detail), the torque of which can be measured, in particular by measuring the motor current.

[0063] The mixture is melted and homogenized in the extruder 105. The mixture then forms a melt strand. In addition to the temperature itself, the energy required to melt the mixture and / or to set a specific temperature of the mixture can also be a measured variable for the method according to the invention.

[0064] The melt strand then passes through the screen 105, wherein a pressure difference is generated in the melt strand before and after the screen 105. The inlet pressure and outlet pressure are material-dependent and are therefore suitable measured variables.

[0065] The melt strand passes through a further line 106 into a blow head 107. The melt flow index of the melt strand can be measured in line 106. Further pressure changes can occur in the melt strand over the length of the line, wherein a measurable input pressure can be present at the blow head.

[0066] In the blow molding head 107, the melt strand is distributed annularly and extruded through a nozzle. The resulting film hose 108 is pulled in the conveying direction A by pull-off rollers (not shown). During conveying, the film hose 108 solidifies to such an extent that its dimensions, in particular its diameter and film thickness, no longer change, with this transition region being indicated by the cold consolidation line 109. The diameter of the film hose 108 in the region of the cold consolidation line 109 and the distance of the cold consolidation line from the blow molding head nozzle, viewed in the conveying direction, are characteristic of the material mixture used and can therefore also be measured variables within the meaning of the present invention. In general, the film profile and / or film temperature can be measured variables that are correlated with the distance from the nozzle (viewed in the conveying direction).

[0067] In order to be able to influence the shape and / or temperature of the film hose 108 and / or the thickness of the film and also the height of the cold consolidation line, at least one cooling ring 110 is provided, which can be supplied with cooling fluid via at least one connection 111. The volume flow and / or the temperature of the cooling fluid can be influenced, preferably in a locally different manner.

[0068] Alternatively or additionally, the shape and / or temperature of the film hose 108 can be influenced by means of an internal cooling device 112. This internal cooling device can be supplied with cooling fluid for internal cooling via a connection 113. Since the supplied cooling fluid for internal cooling cannot escape into the environment, it is necessary to drain the cooling fluid for internal cooling. This is done via a connection 114.

[0069] With regard to the blown film device 100 described, several of the parameters are adjustable parameters (adjustment parameters). In particular, these adjustment parameters include the mass flow, density and melt flow index of at least one second plastic material, the speed of the drive motor for the extruder screw and / or the volume flow and / or temperature of the cooling fluid. It is possible to influence the properties of the film using these adjustment parameters. In particular, these adjustment parameters can be used to influence the density of the mixture, the temperature of the mixture, the input pressure at the blow molding head, the melt flow index of the melt strand, the diameter of the film hose at the location of the cold consolidation line and / or the cold consolidation line height. In particular, the parameters are measurable and can be compared with theoretical parameters in order to construct at least one control loop.

[0070] Figure 2 Another embodiment is shown, which is similar to the embodiment according to Figure 1 The embodiment of the present invention differs in that at least one second plastic material can be supplied via a metering hopper 120 and melted in an extruder 121. The second melt strand is also supplied to the nozzle head and distributed in an annular shape. This melt strand then comes into contact with the annularly distributed melt strand from the first extruder 104, thereby producing a multilayer film in which the properties of the individual layers can be varied.

[0071] It should be pointed out that according to Figure 1 The embodiment can also be expanded so that multilayer films can be produced. In this case, as is known per se, an extruder is provided for each layer. The melt strands from the individual extruders are then distributed annularly and brought together. At least one melt strand can comprise a mixture of a first plastic material with variable properties and at least one second plastic material with substantially constant properties, such as those already combined. Figure 1 As stated. Figure 1 The further features of the description can of course be transferred to the extrusion device for producing a multilayer film.

[0072] Furthermore, the features essential to the present invention, described in conjunction with the accompanying drawings for blown film systems, can also be applied to so-called flat film systems. The differences primarily concern the components downstream of the nozzle head. The slot nozzle is similar to the annular nozzle in blown film systems, while the cooling rollers are responsible for cooling the melt and drawing the resulting film. In blown film systems, these tasks are performed by the drawing device and the cooling ring or internal cooling system.

Claims

1. A method for producing a plastic film using a film extruder, wherein: supplying at least two plastic materials from at least two storage containers jointly to one extruder or separately to one extruder, melting the plastic material in the extruder or extruders and conveying it as a melt strand or strands to a nozzle head, distributing the melt strand or the melt strands over the surface of the nozzle head and pressing them out of the nozzles of the nozzle head as a melt film, pulling the melt film and / or the film web formed by cooling by means of at least one pulling roll, It is characterized by: recording measured values for at least one property of at least one plastic material and / or at least one melt strand and / or melt film and / or film web, and / or for a machine parameter of the film extruder by means of a measuring device, and The first plastic material is a plastic material with fluctuating material properties and the second plastic material is a plastic material with substantially constant material properties, wherein the first plastic material and the second plastic material are supplied to the extruder or the extruders in a quantity ratio, wherein the quantity ratio is varied depending on measured values for the properties.

2. The method according to claim 1, It is characterized by: The first plastic material is a recycled plastic material.

3. The method according to any one of the preceding claims, It is characterized by: The properties of the melt film and / or the film web include at least one of the following properties: Bubble stability, Bubble shape, Height of cold solidification zone, Film temperature, Film thickness, Pulling speed, Film width.

4. The method according to any one of the preceding claims, It is characterized by: The characteristics of the at least one melt strand include at least one of the following characteristics: the pressure profile in the extruder, the temperature of the extruder zone, ·density, Viscosity, Back pressure, Delivery rate.

5. The method according to any one of the preceding claims, It is characterized by: The machine parameters in the characteristics include at least one of the following parameters: the duration of time the extruder zone is switched on, The motor torque of the extruder drive, Supply and / or exhaust of external and / or internal cooling air.

6. A film extruder for producing a plastic film according to the method according to any one of claims 1 to 5, the film extruder having At least two storage containers for providing plastic material at least one extruder to which plastic material can be supplied from two storage containers, or at least two extruders to which plastic material can be supplied separately, wherein: The plastic material can be melted in the extruder or the extruders and can be deformed into a melt strand or two melt strands a nozzle head to which the melt strand or the melt strands can be supplied and in which the melt strand or the melt strands can be distributed over a surface area by means of a distributor A nozzle arranged at or in the nozzle head, from which a melt strand can be pressed out that is deformed into a melt film a pulling roll, with which the melt film and / or the film web formed by cooling can be pulled from the nozzle, It is characterized by: a measuring device is provided, with which measured values can be recorded for at least one property of at least one plastic material and / or at least one melt strand and / or melt film and / or film web, and / or for a machine parameter of the film extruder, and - the first plastic material is a plastic material with fluctuating material properties and the second plastic material is a plastic material with substantially constant material properties, and A computing and control device is provided and is designed to be able to supply the first plastic material and the second plastic material to the one or more extruders in a quantitative ratio, wherein the quantitative ratio can be varied as a function of measured values for the properties in order to maintain a production parameter of the film extruder at a substantially constant value.

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