Cooling device for plastic tubular films

Abstract

The invention relates to a cooling device for plastic tubular films. In order to increase the cooling capacity of a known cooling device while simultaneously reducing bubble instabilities, the operating distance between the support air unit and the film bubble can be adjusted using at least two driven lifting elements, wherein the lifting elements are coupled together via a coupling element such that the lifting elements produce the same displacement distance upon adjusting the operating distance.

Description

[0001] Cooling device for plastic tubular films

[0002] The invention relates to a cooling device for plastic tubular films. Furthermore, the invention relates to a blown film line for producing plastic tubular films or plastic film webs with a corresponding cooling device.

[0003] Such blown film lines are known, for example, from WO 2021 / 069474 A1 and, in their basic design, consist of one or more extruders, a film blowing head, a cooling device, a take-off device and one or more winders.

[0004] A corresponding film blowing head for such blown film systems is known, for example, from WO 2002 / 070230 A1.

[0005] Corresponding cooling devices for such blown film systems are known, for example, from DE 1 960 962 A1, from WO 2008 / 025451 A1 and from EP 2 498 970 B1.

[0006] The extruder essentially consists of a metal cylinder in which a rotating extruder screw rotates. The plastic granules fed to the extruder screw are melted and homogenized by friction and pressure in the metal cylinder, and then fed to the film blowing head.

[0007] The film blowing head forms the melt streams from the extruders into a multilayer film composite, which then exits as a thermoplastic film tube from an annular die gap. The annular die gap is formed by an outer and an inner mouth gap ring.

[0008] The thermoplastic film tube is inflated with air and fed into the cooling device, where it is cooled by cooling air from both the outside and inside. The width and thickness of the film are also determined in this way in the cooling device. At the so-called frost line or solidification line, the melt then transitions from the thermoplastic to the thermoelastic state.

[0009] The film tube, which has cooled in the cooling device, is then flattened and wound into a film roll on a winder. Typically, the flattened film tube is split at the edges using two knife blades, creating two separate film webs, which are also wound separately on two winders. The wound film rolls are then further processed in the desired manner, for example, into bags or garbage sacks.

[0010] Depending on the number of connected extruders, there are films that consist of only a single layer (so-called mono-blown film) or films that are composed of several layers (so-called co-extrusion blown film). Co-extrusion blown film offers the possibility of combining the positive properties of different materials in a single film. For example, a typical food packaging consists of five different layers:

[0011] • Outside: a printable layer, usually polypropylene or polyethylene

[0012] • between outside and middle: adhesion promoter made of ethylene vinyl alcohol or ethylene vinyl acetate

[0013] • Middle: a barrier layer made of polyamide, to "lock in" the aroma

[0014] • between middle and inside: adhesion promoter

[0015] • Inside: a food-safe layer with good sealing properties to weld the film to the bottom of the packaging, usually made of polyethylene In the blown film extrusion described, the cooling device is of particular importance, since the cooling of the thermoplastic film tube emerging from the die gap significantly influences both the film properties and the efficiency of the extrusion process.

[0016] The cooling devices known from the prior art usually consist of an external cooling ring and an internal cooling ring. These supply cooled air to the film tube emerging from the die gap from the outside and inside to remove heat from the thermoplastic melt. An internal exhaust pipe is located inside the film tube to extract the air blown in by the internal cooling ring. A control loop maintains a constant internal pressure within the film tube, allowing the bubble shape to be kept stable with respect to the withdrawal speed.

[0017] In principle, the cooling system should have the highest possible cooling capacity, as this increases the mass throughput and thus the production speed of the system. On the other hand, the thermoplastic film tube has only limited strength, so there is a risk of bubble breakage at excessively high draw-off speeds.

[0018] To increase the cooling performance, it is known from EP 2 498 970 B1 to extend the outer cooling ring of the cooling device with a supporting air unit. The supporting air unit of EP 2 498 970 B1 consists of three hollow cylinders with different diameters. The hollow cylinder with the smallest diameter is placed on the outer cooling ring. The hollow cylinder with the medium diameter is arranged concentrically above it, and the hollow cylinder with the largest diameter, in turn, forms the end of the supporting air unit. The flow conditions thus created between the three hollow cylinders arranged one above the other and the film bubble can increase the cooling performance on the outer wall of the film bubble. To optimize the flow conditions, the axial distances between the three hollow cylinders can also be adjusted, with EP 2 498 970 B1 also mentioning a motorized adjustment option.

[0019] Extensive tests with the supporting air unit known from EP 2 498 970 B1 have now shown that while the supporting air unit can increase cooling performance, it cannot reduce the risk of bubble breakage. Furthermore, it was observed that when the cooling performance is increased with the supporting air unit, film quality deteriorates due to inhomogeneities.

[0020] The object of the invention is therefore to increase the cooling performance of a known cooling device while simultaneously reducing bubble instabilities.

[0021] This problem is solved by the features of patent claims 1 and 4.

[0022] The solution according to the invention consists in that the operating distance between the supporting air unit and the film bubble is adjustable with at least two driven lifting elements, wherein the lifting elements are coupled to one another via a coupling element in such a way that the lifting elements effect the same adjustment path when adjusting the operating distance.

[0023] The solution according to the invention eliminates the mechanical inaccuracies that arise when adjusting the three hollow cylinders according to EP 2 498 970 B1. The adjustment options known from EP 2 498 970 B1 cannot namely ensure the centering and horizontal alignment of the three hollow cylinders with respect to the axial axis of symmetry of the annular nozzle gap to a sufficient extent. This results in different distances and alignments to the three hollow cylinders of the supporting air unit along the circumference of the film bubble. Areas with a large flow gap then lead to lower flow velocities and thus to poorer cooling performance than areas with smaller flow gaps. In turn, the film is stretched longer in the extrusion direction in areas with a large flow gap, which leads to thin spots in the film and thick spots in other areas. This results in inhomogeneities and instabilities in the film.

[0024] In the support air unit according to the invention, however, the motorized height adjustment is so precise that the hollow cylinders of the support air unit remain centered and horizontally aligned along the entire adjustment range. This allows the operator to precisely adjust the support air unit to the bubble geometry, both during machine setup and during operation. This allows the adjustment of small to very small air gaps between the support air unit and the bubble geometry. Smaller air gaps, in turn, increase the flow velocity, thereby reducing the local air pressure. By increasing the flow velocity while simultaneously decreasing the local air pressure, both heat transfer and bubble stability can be increased.In addition, the reduced local air pressure allows earlier expansion of the film bubble within the supporting air unit, thus providing a larger film surface for heat dissipation.

[0025] Further details and advantages of the invention are described with reference to the accompanying drawings, in which:

[0026] Fig. 1 is a schematic 3D view of the cooling device according to the invention,

[0027] Fig. 2 is a side view of the cooling device according to the invention, and

[0028] Fig. 3 shows the cooling device according to the invention according to Fig. 1 with a detailed representation of the mechanical components.

[0029] Fig. 1 shows a schematic 3D view of a film blowing head 101 for extruding a film tube 102 with a cooling device 103 mounted on the film blowing head 101. Between the cooling device 103 and the film blowing head 101 there is a cooling ring housing 104 in which an outer cooling ring for supplying the film tube 102 with external cooling air and an inner cooling ring for supplying the film tube 102 with internal cooling air are accommodated in a manner not shown in detail.

[0030] Inside the film tube 102 is an internal exhaust pipe 105 for extracting the air blown in by the internal cooling ring. A control loop maintains a constant internal pressure within the film tube, allowing the bubble shape to be kept stable with respect to the withdrawal speed.

[0031] A supporting air unit is mounted on the cooling ring housing 104. It consists of three concentrically arranged hollow cylinders 106, 107, and 108, each of which has different diameters. The hollow cylinder with the smallest diameter, 106, is mounted on the cooling ring housing 104. The hollow cylinder with the medium diameter, 107, is arranged concentrically above it, and the hollow cylinder with the largest diameter, 108, forms the end of the supporting air unit. The flow conditions thus created between the three stacked hollow cylinders 106, 107, and 108 and the film bubble 102 can increase the cooling performance on the outer wall of the film bubble 102. To optimize the flow conditions, the axial distances between the three hollow cylinders can also be adjusted by motor. Fig. 3 shows a detailed illustration of the mechanical components for motorized adjustment of the axial distances.First, the flow conditions in the cooling device according to the invention are explained in accordance with Fig. 2.

[0032] Fig. 2 shows a side view of the cooling device according to the invention. Corresponding components from Fig. 1 are designated by the same reference numerals, so that reference can be made to the description of Fig. 1 in this regard. For better illustration, the view according to Fig. 2 is divided into two sections with respect to the vertical axis of the internal exhaust air pipe 105. The left side shows the cooling device during operation of the blown film line, while the right side shows the cooling device in the idle state.

[0033] During operation of the blown film system, the film tube 102 emerges from an annular nozzle gap (not shown) of the film blowing head, is inflated onto a film bubble 102 in the manner shown, and is drawn off from a take-off device in the transport direction y.

[0034] The cooling ring housing 104 transports cooling air to an outer cooling ring 201 and an inner cooling ring 202. The outer cooling ring directs the outer cooling air to the outer skin of the film bubble 102 via a dam 203. The dam 203 also divides the outer cooling air so that the outer cooling air hits the outer skin of the film bubble 102 in two opposing flow directions. This creates a point-like negative pressure that fixes the film bubble 102 between the outer cooling ring 201 and the inner cooling ring 202.

[0035] The outer cooling ring 201 is adjustable in height by means of a linear drive 204 so that the exit point of the film bubble 102 above the outer cooling ring 201 can be adapted to the properties of the respective film.

[0036] The supporting air unit illustrated in Fig. 1, with the three hollow cylinders 106, 107, and 108 arranged one above the other, is mounted on the outer cooling ring 201. The three hollow cylinders are each motor-adjustable in the vertical direction. During operation of the blown film line, the three hollow cylinders are moved toward the film bubble (left side of Fig. 2). When the blown film line is at rest, the outer cooling ring 201 is lowered via the linear drive 204. Furthermore, the three hollow cylinders 106, 107, and 108 are pushed together as shown (right side of Fig. 2).

[0037] Fig. 3 shows the cooling device according to the invention according to Fig. 1 with a detailed representation of the mechanical components for adjusting the three hollow cylinders 106, 107 and 108. Corresponding components from Fig. 1 are marked with the same reference numerals, so that in this respect reference can be made to the description of Fig. 1.

[0038] Essentially, the three hollow cylinders 106, 107, and 108 for vertical adjustment are each equipped with the same mechanical components. For the sake of simplicity, the description will therefore only refer to the uppermost hollow cylinder 108. A corresponding functionality also applies to the other two hollow cylinders 106 and 107.

[0039] The hollow cylinder 108 is supported on the underlying hollow cylinder 107 via three telescopic guides evenly distributed around the circumference. Two of these telescopic guides are visible and are designated by reference numerals 301 and 302. Each of these telescopic guides comprises an integrated spindle that is drivable via a spindle gear. Two of these spindle gears are visible and are designated by reference numerals 303 and 304. Driving the spindle gear and thus also the spindle causes the respective telescopic guide to move closer together or apart.

[0040] A revolving chain 305 is mounted on the upper edge of the hollow cylinder 108, into which each of the three spindle gears engages, so that movement of the chain 305 drives the respective spindle gear. This also drives the respective spindles in the same way, allowing the hollow cylinder 108 to be precisely adjusted in the vertical direction.

[0041] The revolving chain 305 is driven by a drive gear 306, which is driven by a stepper motor 309 via a gear unit 307 and a flexible shaft 308. The stepper motor 309 is in turn connected to a control unit for automatically operating the blown film system. The hollow cylinders 106, 107, and 108 are movable relative to the cooling ring blower 104, while the associated stepper motors 109 are fixedly connected to the cooling ring blower 104. The flexible shafts allow the hollow cylinders 106, 107, and 108 to be displaced relative to the stepper motors 109.

[0042] The revolving chain 305 thus forms a coupling element with respect to the telescopic guides 301 and 302, so that the telescopic guides 301 and 302 each achieve the same adjustment path. Due to the operating distance between the hollow cylinder 108 and the film bubble 102 and the resulting air gap, a defined air flow is created for cooling and simultaneous stabilization of the film bubble. The coupled telescopic guides 301 and 302 ensure that the hollow cylinders 106, 107, and 108 remain centered and horizontally aligned along the entire adjustment path. This enables the operator to precisely adapt the support air unit to the bubble geometry, both when setting up the machine and during operation. This enables the adjustment of small to very small air gaps between the support air unit and the bubble geometry.Smaller air gaps, in turn, increase the flow velocity, while decreasing the local air pressure. By increasing the flow velocity while simultaneously decreasing the local air pressure, both heat transfer and bubble stability can be increased. Furthermore, the reduced local air pressure allows for earlier expansion of the film bubble within the support air unit, thus providing a larger film surface for heat dissipation.

[0043] At the same time, the control unit allows for automatic adjustment of the operating distances between the supporting air unit 103 and the film bubble 102. When setting up the blown film line, for example, it is possible to automatically adjust the operating distances depending on the composition of the thermoplastic. During operation of the blown film line, it is also possible to automatically adjust the operating distances depending on the position of the frost line of the film tube.

[0044] All of the measures mentioned increase the cooling capacity of the cooling device while simultaneously reducing bubble instabilities. This increases production speed and simultaneously the reliability of the blown film line.

Claims

Patent claims Cooling device for a film tube made of thermoplastic material, with an external cooling ring surrounding the film tube, wherein the outside of the film tube can be supplied with external cooling air by means of the external cooling ring, and wherein the film tube can be formed into a film bubble following the external cooling ring, with a supporting air unit for stabilizing the film bubble formed following the external cooling ring, wherein the supporting air unit has at least one round body surrounding the film bubble for guiding the external cooling air between the round body and the film bubble, wherein an operating distance between the round body and the external cooling ring is adjustable in the axial direction of the film bubble, wherein the operating distance is adjustable with at least two driven lifting elements, and wherein the lifting elements are coupled to one another via a coupling element in such a way thatthat the lifting elements achieve the same adjustment path when adjusting the operating distance. Cooling device according to claim 1, wherein an internal cooling ring is provided, with which the inside of the film tube can be exposed to internal cooling air. Cooling device according to one of claims 1-2, wherein the round body consists of a concentrically arranged hollow cylinder, wherein three driven lifting elements are provided on the circumference of the hollow cylinder, wherein the coupling element consists of a drive chain that jointly drives the lifting elements, and wherein the drive chain is drivable by a stepper motor.Blown film plant for producing plastic films or plastic film webs, with at least one extruder, with a film blowing head for extruding a film tube made of thermoplastic material from an annular outlet nozzle, with a cooling device according to one of claims 1 - 3 downstream of the outlet nozzle for controlled conversion of the thermoplastic material into a thermoelastic state along a frost line, with a take-off and at least one winder, and with a control unit for automatically operating the blown film plant. Blown film plant according to claim 4, wherein the operating distance of the supporting air unit integrated in the cooling device can be automatically adjusted by the control unit. Blown film plant according to one of claims 4 - 5, wherein the operating distance of the supporting air unit integrated in the cooling device can be automatically adjusted by the control unit. is adjustable during setup of the blown film line depending on the composition of the thermoplastic material. Blown film line according to one of claims 4-6, wherein the operating distance of the supporting air unit integrated in the cooling device is adjustable by the control unit during operation of the blown film line depending on the position of the frost line of the film tube.

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