Method and film-extruding machine for producing a plastics film

Abstract

The invention describes a method for producing a plastics film by a film-extruding machine, wherein at least two plastics materials from at least two reservoirs are fed together to one extruder or are each fed to a respective extruder, the plastics materials are melted in the extruder or in the extruders and are conveyed as a melt strand or as melt strands into a die head, the melt strand or the melt strands are spread out flat in the die head and is / are extruded from a die of the die head as a melt film, and the melt film and / or the film sheet formed by cooling is / are drawn off by at least one take-off roll. It should particularly be noted that measured values are recorded by means of a measuring device for at least one property of at least one plastics material and / or at least one melt strand and / or the melt film and / or the film sheet and / or for a machine parameter of the film-extruding machine and that a first plastics material is a plastics material with varying material properties and a second plastics material is a plastics material with substantially constant material properties, wherein the first and the second plastics materials are fed to the extruder or the extruders in relative proportions, the relative proportions being varied according to the measured values for the property.

Description

[0001] Process and film extrusion machine for producing a plastic film

[0002] The invention relates to a method and a film extrusion machine for producing a plastic film according to the preambles of claims 1 and 6.

[0003] To produce a plastic film, at least two plastic materials are often provided in at least two storage containers. These plastic materials are usually in a solid but pourable form, for example, in granular form, in some cases also as flakes or a mixture of both. These at least two plastic materials can be fed together into an extruder. Alternatively, each plastic material is fed to a separate extruder.

[0004] In the extruder(s), the plastic materials are then melted, usually under the influence of heat energy and mechanical pressure. If two plastic materials are processed within an extruder, they are mixed and, in particular, homogenized. However, mixing can also occur during the process from the storage containers to the extruder.

[0005] After melting, the flowable melt strand from each extruder is fed to a die head. The melt strand is spread widthwise to create a flat melt stream whose width is many times greater than its thickness, in particular at least 20 times the size. The melt strand can be distributed into a melt stream linearly or annularly. If several melt strands are fed to the die head, they can be combined within the die head to form a multi-layer melt stream. This melt stream is then forced out of a nozzle in the die head to create a melt film. This film then cools and forms a film web, which can be a flat web, for example, or, if the melt strand was distributed in a ring shape, a tubular web. The melt film and / or the already formed film web are then drawn off by at least one take-off roller.

[0006] This process and this film extrusion machine often allow for the production of film webs on a large scale. However, the quality of the film web produced over a longer period of time may not be consistent.

[0007] The object of the present invention is therefore to propose a method and a film extrusion machine with which the quality of the film webs can be maintained consistently even over a longer production period.

[0008] According to the invention, this object is achieved by all features of claim 1. Possible embodiments of the invention are specified in the dependent claims.

[0009] According to the present invention, it is provided that measured values ​​are recorded for at least one property of at least one plastic material and / or at least one melt strand and / or the melt film and / or the film web and / or for a machine parameter of the film extrusion machine using a measuring device. This measure makes it possible to carry out quality monitoring not or not only on the finished film web, but also on pre-products and intermediate products. For example, infrared or radiation measurements can be used to obtain information about the composition of the plastic materials. This allows changes in the materials that occur over the production period to be observed. However, changes in a plastic material over time also lead to changes within the extruder during the melting of this material.For example, the temperature of the plastic material within the extruder, particularly within the extruder zones, can change, particularly because the material's heat capacity can fluctuate. A change in a plastic material can influence the pressure curve in the extruder, with the pressure curve resulting in particular from the material's backpressure. The duty cycle of the extruder zones can also be monitored. One easily observable parameter is the motor torque required to drive a given speed of an extruder screw located within the extruder. Measurements for all of the aforementioned parameters can be obtained using suitable measuring devices, such as a temperature measuring device, a pressure measuring device, or a torque monitoring device.

[0010] Furthermore, the invention provides that a first plastic material is a plastic material with fluctuating material properties and a second plastic material is a plastic material with substantially constant material properties, wherein the first and the second plastic material are fed to the extruder or extruders in a quantity ratio, wherein the quantity ratio is varied depending on the measured values ​​for the property.

[0011] The first plastic material is one with properties that fluctuate over time during processing in the film extrusion machine, particularly with regard to its intrinsic material composition. Such fluctuating properties arise in particular from the fact that two different volume elements of the first plastic material have different base material compositions. With chemically similar base structures, different base materials can have different densities. For example, polyethylenes can be present in a high density (HDPE) or a lower density (LDPE). Depending on the change in the proportions of these two polyethylenes from volume element to volume element, the overall density of the plastic material can vary.

[0012] Compared to the first plastic material, the second plastic material is a material with essentially constant material properties. Such materials are often manufactured from high-purity starting materials and in strict compliance with specified manufacturing parameters. Therefore, such second plastic materials are often more expensive than the first plastic materials. The invention can thus also contribute to reducing the costs of the produced film web.

[0013] In order to be able to compensate for the fluctuating properties of the first plastic material, the invention further provides that the first and second plastic materials are initially fed to the extruder or extruders in a quantity ratio. In the first case, therefore, it is provided that both materials are fed in a quantity ratio before entering the extruder. For example, the quantity proportion of the first plastic material can be 70% and the quantity proportion of the second plastic material 30%. It is provided that the quantity ratio is varied depending on the above-mentioned measured values ​​for the above-mentioned property. In the second case, in which the melt strands are combined into several layers within the nozzle head, the quantity ratio can be determined by the layer thicknesses assuming a specific ratio to one another.For example, in a total layer thickness, the layer thickness of the first plastic material can contribute 70% and the layer thickness of the second plastic material can contribute 30% to the total layer thickness.

[0014] In both of the aforementioned cases, it can be provided that the second plastic material comprises a quantity of at least 10% of the total plastic material, but in particular at least 20%. If the second plastic material comprises an additive or the second plastic material consists only of an additive, it can be provided that the second plastic material comprises a quantity of at least 0.5% of the total plastic material. An additive can be a secondary component that corresponds to a main component of the second

[0015] plastic material. For certain properties of the first plastic material, it may even be sufficient to use the second

[0016] plastic material only an additive to use.

[0017] As already mentioned above, measured values ​​for the above-mentioned properties are recorded using at least one measuring device. The measured values ​​are in particular fed to a computing and control device. The computing and control device compares the recorded measured values ​​with target values ​​for this property and / or with average values ​​from the previously obtained measured values. If the difference between the measured values ​​and the target or average values ​​exceeds a limit value, the quantitative ratio of the first plastic material to the second plastic material is varied, i.e. in particular adjusted. This means in particular that the supply of the first plastic material is changed or reduced and / or the supply of the second plastic material is also changed, in particular increased, until the measured values ​​are again below the limit value.It is preferable to change the supply of both plastic materials simultaneously in order to keep the total amount constant.

[0018] In an advantageous embodiment of the invention, the first plastic material is a recycled plastic material. A recycled plastic material is characterized by the fact that it contains a plastic material that has already been used by a user. Since materials from various initial applications of the plastic often enter a recycling plant one after the other, the properties within the material can vary greatly. This applies to the material composition, but also to possible contamination by foreign substances, which, although undesirable, can usually not be avoided. Overall, the invention thus offers the possibility of processing recycled material in a film extrusion machine, whereby the quality of the film web produced is increased compared to conventional processes.It is advantageous if the properties for which measured values ​​are recorded with a measuring device include at least one of the following properties:

[0019] Bubble stability, bubble shape, frost line height or frost zone height, film temperature, film thickness, pull-off speed, film width.

[0020] Bubble stability describes whether the melt film moves along a predetermined transport path or whether the actual transport path deviates from it. In practice, this is often referred to as "fluttering" of the film, i.e., periodic deviations from the intended transport path. An optical camera can be used as a measuring device here. The images recorded by this camera form the measured values ​​and can be evaluated with regard to the aforementioned deviations.

[0021] The bubble shape describes whether the geometry of the melt film describes a desired geometry. In practice, it happens that the shape of the melt film changes due to external influences, such as cooling of the environment of the film extrusion machine. Within the scope of the present invention, however, it is possible that such changes are due to the properties of the first material. To assess the bubble shape, an optical camera is again suitable as a measuring device. The camera images are to be understood as measured values, whereby the actual shape of the melt film in the area of ​​the film bubble can be compared with a desired shape for evaluation.

[0022] The frost zone is the transition area in which the melt film solidifies into a film web. The film web no longer forms a melt, but a solid film web that can no longer be shaped, or at least not to a significant extent. The frost zone is often referred to as a frost line. The frost zone height refers to the distance of the frost zone from the nozzle along the transport path. Even though the term "height" is derived from blown film extrusion, the described distance can also be determined in a flat film extrusion machine. The frost zone height can vary, particularly during extrusion operation, because the heat capacity of the first plastic material varies over time. Knowledge of the frost zone height is important for the adjustment or positioning of machine components, for example for the position of the calibration basket.The frost zone can be determined by measuring the temperature of the film bubble, for example, using an infrared camera. If an increase or decrease in temperature is detected compared to a target temperature or an average temperature, this means that the frost zone has shifted in or against the direction of transport.

[0023] In film extrusion machines, a separate control loop is often provided for the height of the calibration basket and / or for the height of the frost zone, so that the measured variable of this control loop can be provided as a measured value for the property of the first plastic material.

[0024] In general, the film temperature can also be determined at other locations using a temperature measuring device. As already described in connection with the frost zone, the film temperature can depend, in particular, on the heat capacity of the first plastic material and thus on its properties, such as its chemical composition.

[0025] The film thickness is also a property of the resulting film web, which can depend on the properties of the plastic materials used. For example, the varying properties of the first plastic material can lead to a varying film thickness. The film thickness can therefore preferably be determined using a measuring device that operates on the basis of ultrasonic or radiometric waves or one that operates on the basis of tactile measurement. Particular advantages arise when such a measurement is carried out in combination with a measurement of the electrical capacitance. Capacitance measurement with electrodes delivers results that, for a given film thickness, depend on the density of the materials in the film web. If the thickness of the film web changes alongside the density, both must be determined independently of one another.Consequently, both the thickness and the density can be determined separately using two measuring devices based on different physical measurement principles. The density of the film web, in turn, allows conclusions to be drawn about the density of the first plastic material, so that in the event of deviations from a target density, corrections must be made to the quantitative composition of the first and second plastic materials.

[0026] The film width can also be determined using a width measuring device and also provides an indication of the properties of the first plastic material. The film width depends primarily on the film's stretchability and thus directly on the properties of the first plastic material.

[0027] The film web's pull-off speed, i.e., the resulting peripheral speed of the pull-off roller at a given torque, also depends on the properties of the first plastic material, particularly its density. The peripheral speed can be measured using an angle sensor with a known outer circumference of the pull-off roller.

[0028] Using these measuring devices mentioned above, but also the ones mentioned below, the lists of which are not exhaustive, properties of the first plastic material can be determined, wherein the measured values ​​from the measuring devices can be taken into account individually or jointly. The measured values ​​are preferably compared with target values ​​and / or with average values ​​in a computing and control device. In the event of deviations beyond a threshold value, the quantitative ratios of the first and the at least one second plastic material are changed according to the invention. It can be provided that, if several measured values ​​are taken into account by the computing and control device, these are weighted in order, for example, to be able to take their influence into account. It can also be provided that these weightings are repeatedly checked and adjusted with regard to their influence.These weightings can be stored in a memory device of the film extrusion machine and / or the computing and control device. This allows data from previous film production runs to be accessed during a current film production run, so that when a new film production run begins, a satisfactory result can be achieved more quickly. Naturally, such data is stored in conjunction with a recipe, whereby a recipe includes not only the type of plastic materials and their input quantities for the production of the intended film, but also the machine parameters, such as the positioning of the calibration basket, and the process parameters, such as the rotational speed of an extruder screw.In particular, it is advantageous if the computing and control device varies the quantity ratio, in particular over a predetermined variation range, and records the influence on the measured values. From this, mixing rules can be generated and stored, which can be done recipe-specifically and in particular specifically for different first plastic materials. Such variation can also be achieved by providing a test extruder outside the film extrusion machine, to which the first plastic material and the at least one second plastic material are fed in different quantity ratios, and the measured values ​​are determined.

[0029] Not only the properties of the melt film and / or film web, but also the properties of the melt strand can be measured with measuring devices. These properties include: pressure profile in the extruder, temperature of the extruder zones, density, viscosity number, backpressure, and feed rate.

[0030] This list is not exhaustive. The pressure profile in the extruder depends directly on the properties, particularly the densities, of the plastic materials used and can be measured with pressure gauges. In particular, the pressure drop across a screening device can also be determined. These measured values ​​are then fed into the computing and control system.

[0031] The temperature of the melt stream in individual zones of the extruder (extruder zones) can also be determined using known temperature sensors and provide an indication of the properties of the first plastic material.

[0032] Density can be determined at various locations within the film extrusion machine and using density measuring devices. However, the density can also be measured directly on the unmelted plastic material. This can include not only the density within a granule or flake of the first plastic material, but also the bulk density of the material, particularly within a storage container.

[0033] The viscosity number of the melt strand, which can also be referred to as MFI (abbreviation for "melt flow index"), can be measured within the extruder or extruders with at least one viscosity number measuring device and / or, for example, within pipes between the extruder and the die head and / or within the die head.

[0034] A counterpressure generated by the melt strand can also be determined and provides an indication of the properties of the first plastic material.

[0035] The feed rate, i.e., the conveyed volume or weight per unit of time, can be determined using a dedicated measuring device. Furthermore, it is advantageous if the properties include at least one of the following machine parameters: duty cycle of the extruder zones,

[0036] Motor torque of the extruder drive,

[0037] Supply and / or removal of outside and / or inside cooling air.

[0038] The duty cycle refers to the time it takes from the start of the extruder(s) until the extruder or its individual zones reach operating temperature or operating pressure. This can also include the time it takes for an extruder screw to reach its nominal speed. The operating temperature or operating pressure can be determined using temperature or pressure sensors.

[0039] The motor torque can be determined in a simple way, for example, by determining the current consumption at a specified speed. The motor torque can be an indication of the density of the first and / or second plastic material. In particular, the motor torque can be very sensitive to changes in the material fed into the extruder.

[0040] The supply of internal cooling air via an internal cooling device serves, on the one hand, to shape the film bubble into a predetermined shape, and on the other hand, to remove heat from the melt film so that it solidifies into a film web. In order to remove the removed heat, the internal cooling air must be removed again. Fans are usually available for the supply and removal of the cooling air. Control loops for the bubble shape and / or for the temperature of the film bubble are often provided so that the measured variables for this control loop and / or the manipulated variables 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 absorb heat from the melt film. The only difference from the internal cooling device is that the external cooling air does not need to be removed, as it escapes into the environment.In contrast, the outside cooling air is often tempered, so a control loop is also provided for this. Here, too, the manipulated variable can be used to determine the properties of the first plastic material.

[0041] The object mentioned at the outset is also achieved by a film extrusion machine for producing a plastic film according to a method according to the invention, with

[0042] • at least two storage containers, each for providing a plastic material

[0043] • at least one extruder, to which the plastic materials from both storage containers can be fed, or at least two extruders, each of which can be fed with a plastic material, wherein the plastic materials can be melted in the extruder or in the extruders and formed into one melt strand or two melt strands

[0044] • a nozzle head to which the melt strand or strands can be fed and in which the melt strand or strands can be distributed over a large area by means of a distributor

[0045] • a nozzle arranged on or in the nozzle head, from which the melt strands formed into a melt film can be pressed out

[0046] • a take-off roller with which the melt film and / or the film web formed by cooling can be pulled off the nozzle.

[0047] This film extrusion machine 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 the melt film and / or the film web and / or for a machine parameter of the film extrusion machine, that a first plastic material is a plastic material with fluctuating material properties and a second plastic material is a plastic material with essentially constant material properties, and that a computing and control device is provided and configured such that the first and the second plastic material can be fed to the extruder or extruders in a quantity ratio, wherein the quantity ratio can be varied depending on the measured values ​​for the property in order to keep production parameters of the film extrusion machine at essentially constant values.

[0048] This achieves the same advantages as those already described in connection with the method according to the invention.

[0049] Further advantages, features, and details of the invention will become apparent from the following description, in which various exemplary embodiments are explained in detail with reference to the figures. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination of mentioned features. Within the scope of the entire disclosure, features and details described in connection with the method according to the invention naturally also apply in connection with the film extrusion machine according to the invention, and vice versa, so that with regard to the disclosure, reference is always made to the individual aspects of the invention. The individual figures show:

[0050] Fig. 1 Schematic diagram of a first film extrusion machine

[0051] Fig. 2 Schematic diagram of a second film extrusion machine

[0052] Figure 1 shows a schematic diagram of a first film extrusion machine, which is designed as a blown film extrusion machine 100. This machine initially comprises a dosing hopper 101, into which a first plastic material is fed. This first plastic material is in particular a recycled plastic material with varying properties, which is in granulated form. This material can therefore also be referred to as “ReGran”. This material has first material properties, in particular a first density and a first melt flow index. Within the dosing hopper 101, at least one second plastic material can be admixed with the first plastic material, which can be fed via the dosing hopper 102 and / or the dosing hopper 103. The second plastic material has certain properties, in particular a second density and a second melt flow index.The mixture of the first and at least one second plastic material has a mixture density that is adjusted, for example, using the method according to the invention. The respective mass flow of the plastic materials can be influenced, so that the mixture has a variable proportion of the plastic materials depending on the desired properties.

[0053] The first and at least the second plastic material can be fed to an extruder 104, which comprises a rotating extruder screw. It is driven by an electric motor (not shown in detail), whose torque can be measured, in particular by measuring the motor current.

[0054] The mixture is melted and homogenized in the extruder 105. It then forms a melt strand. The energy required for melting and / or adjusting the mixture to specific temperatures can, in addition to the temperature itself, represent a measured variable for the process according to the invention.

[0055] The melt strand then passes through a screen 105, resulting in a pressure difference between the melt strand before and after the screen 105. The inlet pressure and the outlet pressure are material-dependent and therefore represent suitable measurement variables.

[0056] The melt strand passes through another pipe 106 into a die head 107. The melt flow index of the melt strand can be measured within the pipe 106. A further pressure change can occur within the melt strand along the length of the pipe, whereby a measurable inlet pressure can prevail at the die head. In the die head 107, the melt strand is distributed in a ring shape and pressed out through a nozzle. The resulting film tube 108 is drawn off in the transport direction A by draw-off rollers (not shown). During this transport, the film tube 108 solidifies to such an extent that its dimensions, in particular its diameter and the thickness of the film, no longer change; this transition region is represented by the frost line 109.The diameter of the film tube 108 in the area of ​​the frost line 109 and also the distance of the frost line from the die of the blowing head in the transport direction are characteristic of the material mixture used and can therefore also represent measured variables within the meaning of the invention. In general, the film contour and / or the film temperature can represent measured variables depending on the distance from the die (as seen in the transport direction).

[0057] In order to be able to influence the shape and / or temperature of the film tube 108 and / or the thickness of the film and thus also the height of the frost line, at least one cooling ring 110 is provided, which can be supplied with a cooling fluid via the at least one nozzle 111. The volume flow and / or the temperature of the cooling fluid can be influenced, preferably locally differently.

[0058] The shape and / or temperature of the film tube 108 can be influenced alternatively or additionally by means of an internal cooling device 112. A cooling fluid for internal cooling can be supplied to this internal cooling device via a nozzle 113. Since the supplied cooling fluid for internal cooling cannot escape into the environment, it is necessary to discharge the cooling fluid for internal cooling. This is done via a nozzle 114.

[0059] In the context of the described blown film system 100, some of the parameters mentioned are adjustable variables (control variables). These control variables include, among others, the mass flow, the density and the melt flow index of the at least one second plastic material, the speed of the drive motor for the extruder screw and / or the volume flows and / or temperatures of the cooling fluids. With these control variables, it is possible to influence the properties of the film. In particular, these control variables influence the density of the mixture, the temperature of the mixture, the inlet pressure at the die head, the melt flow index of the melt strand, the diameter of the film tube at the position of the frost line and / or the frost line height. In particular, these control variables are measurable and can be compared with target variables in order to establish at least one control loop.

[0060] Figure 2 shows a further embodiment, which differs from the embodiment shown in Figure 1 in that the at least one second plastic material can be fed via the dosing hopper 120 and melted in the extruder 121. This second melt strand is also fed to the die head and distributed in a ring shape. This melt strand then comes into contact with the melt strand originating from the first extruder 104 and distributed in a ring shape, thus forming a multilayer film, wherein the properties of the individual layers can be varied.

[0061] It should be noted that an embodiment according to Figure 1 can also be expanded such that multi-layer films can be produced. As is known, an extruder is provided for each layer. The melt strands from the individual extruders are then distributed in a ring shape and brought together. At least one melt strand can comprise a mixture of a first plastic material with varying properties and at least a second plastic material with essentially constant properties, as described in connection with Figure 1. Further features of the description of Figure 1 can of course be transferred to an extrusion line for producing a multi-layer film. Furthermore, the features essential to the invention, which have been presented in connection with the descriptions of the figures for blown film lines, can also be transferred to so-called flat film lines.The differences mainly concern the components located downstream of the nozzle head. A slot nozzle is comparable to a...

[0062] Ring die in the blown film line; a chill roll handles both cooling the melt and drawing off the resulting film. These functions are provided in a blown film line by a drawing-off device and the cooling ring or internal cooling.

Claims

Patent claims Method for producing a plastic film with a film extrusion machine, wherein • at least two plastic materials from at least two storage containers are fed to an extruder together or each to an extruder, • the plastic materials are melted in the extruder or extruders and are conveyed as a melt strand or strands into a die head, • the melt strand or strands are distributed over the entire surface of the nozzle head and are pressed out of a nozzle of the nozzle head as a melt film • the melt film and / or the film web formed by cooling is or are drawn off with at least one draw-off roller, characterized in that • that measured values ​​are recorded for at least one property of at least one plastic material and / or at least one melt strand and / or the melt film and / or the film web and / or for a machine parameter of the film extrusion machine by means of a measuring device and • that a first plastic material is a plastic material with fluctuating material properties and a second plastic material is a plastic material with substantially constant material properties, wherein the The first and second plastic materials are fed to the extruder(s) in a quantity ratio, the quantity ratio being varied depending on the measured values ​​for the property. A method according to claim 1, characterized in that the first plastic material is a recycled plastic material. A method according to any one of the preceding claims, characterized in that the properties of the melt film and / or the film web include at least one of the following properties: • Bladder stability, • Bubble shape, • Frost zone height, • Film temperature, • Film thickness • Trigger speed, • Film width. Method according to one of the preceding claims, characterized in that the properties of at least one melt strand include at least one of the following properties: • Pressure curve in the extruder, • Temperature of the extruder zones, • Density, • Viscosity number, • Back pressure, • Delivery rate. Method according to one of the preceding claims, characterized in that the properties include at least one of the following machine parameters: • Duty cycle of the extruder zones, • Motor torque of the extruder drive, • Supply and / or removal of the external and / or internal cooling air. Film extrusion machine for producing a plastic film according to the method according to one of claims 1 to 5, with • at least two storage containers, each for providing a plastic material • at least one extruder, to which the plastic materials from both storage containers can be fed, or at least two extruders, each of which can be fed with a plastic material, wherein the plastic materials can be melted in the extruder or in the extruders and formed into one melt strand or two melt strands • a nozzle head to which the melt strand or strands can be fed and in which the melt strand or strands can be distributed over a large area by means of a distributor • a nozzle arranged on or in the nozzle head, from which the melt strands formed into a melt film can be pressed out • a take-off roller with which the melt film and / or the film web formed by cooling can be pulled off the nozzle, characterized in that • 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 the melt film and / or the film web and / or for a machine parameter of the film extrusion machine, and that a first plastic material is a plastic material with fluctuating material properties and a second plastic material is a plastic material with essentially constant material properties, and that a computing and control device is provided and is designed to measure the first and the second plastic material in a quantity Ratio can be fed to the extruder or extruders, whereby the quantity ratio can be varied depending on the measured values ​​for the property in order to keep production parameters of the film extrusion machine at essentially constant values.

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