Device for extruding plastics

Abstract

Device for extruding plastics, preferably recycled plastics, with a screw (2) arranged within a screw housing (1), which has at least a first screw section (2.1) for melting and at least a second screw section (2.2) for degassing, with a damming element (9) between the first screw section (2.1) and the second screw section (2.2) and with a bypass device (5) associated with the damming element (9), which connects the first screw section (2.1) with the second screw section (2.2), characterized in that the bypass device (5) has an adjustable throttle section (14) formed by a cylindrical pin (13) which projects with a free throttle end (13.1) into a bypass channel (12.3) and forms an annular gap (15).

Description

[0001] The invention relates to a device for extruding plastics according to claim 1.

[0002] When processing and melting thermoplastic materials, it is generally known that the material being fed in contains air and water vapor inclusions. Furthermore, gas-containing bubbles can form during the chemical reactions of the process, which are undesirable and must be removed. To obtain a homogeneous and pore-free melt at the end of the extrusion process, it is therefore known to perform degassing during extrusion. Such known devices typically have a screw with at least two screw sections, in which the material is melted in the first screw section. In the second screw section, the polymer melt is then degassed by applying a vacuum to remove the volatile components.However, this requires that the melt pressure within the housing between the first screw section and the second screw section be reduced. It is also known to arrange a pressure relief element with an associated bypass device between the first screw section and the second screw section. Such an extrusion device is known, for example, from DE 38 17 941 A1.

[0003] A screw press for processing thermoplastic materials is known from DD 67 578 A6.

[0004] In the known device, the bypass assembly located between the two screw sections of the screw has a throttle valve to regulate pressure reduction between the zones. However, such throttle valves are very susceptible to contamination within the melt, so that larger quantities of contaminants, such as those commonly found in recycled plastics in the form of metal particles or sand, very quickly lead to clogging of the throttle point.

[0005] Therefore, the object of the invention is to provide a generic device for extruding plastics, preferably recycled plastics, in which a molten recycled material can be continuously degassed.

[0006] Another objective of the invention is to design the generic device for extruding recycled plastics with a bypass device that is as insensitive to dirt as possible.

[0007] This problem is solved according to the invention by the fact that the bypass device has an adjustable throttle section.

[0008] Advantageous embodiments of the invention are defined by the features and combinations of features of the respective dependent claims.

[0009] The invention departs from the concept of regulating pressure reduction within the extruder by adjusting a flow cross-section. To allow even larger contaminant particles to pass through the melt, the invention utilizes an adjustable throttle section to regulate the necessary pressure reduction between the screw sections. This enables larger flow cross-sections to be achieved with correspondingly long throttle sections. The contaminant particles carried in the melt can pass through the throttle section unimpeded.

[0010] For the formation of the throttling section within the bypass device, it has proven particularly effective to use a cylindrical pin as the throttling section. One free end of this pin projects into a bypass channel, forming an annular gap. This ensures a uniform and constant flow cross-section within the throttling section.

[0011] Furthermore, the further development of the invention, in which the cylindrical pin is slidably mounted on the bypass device, offers the possibility of adjusting the length of the throttling section. This allows for precise regulation of the pressure reduction between the screw sections, ensuring optimal pressure relief for the process.

[0012] Additionally or alternatively, the cylindrical pin on the bypass device can be made interchangeable, allowing the annular gap width to be adjusted by replacing the cylindrical pin. Depending on the degree of contamination in the melt, relatively small or larger flow cross-sections can thus be achieved in the annular gap of the throttling section. The length of the throttling section can therefore be influenced by the annular gap width.

[0013] To achieve a stable adjustment mechanism within the bypass unit in the harsh environment of an extrusion device, a preferred embodiment of the invention features a cylindrical pin with a threaded end, which is rotatably held in a threaded sleeve. This allows, for example, the threaded sleeve to be attached directly to the screw housing of the device, so that rotating the cylindrical pin within the threaded sleeve moves the free end of the throttle valve in the bypass channel.

[0014] To ensure that the annular gap formed within the bypass channel in the throttle section is as concentric and uniform as possible, the cylindrical pin is preferably guided within a guide bore that opens coaxially into the bypass channel. This guarantees that the cylindrical pin is aligned coaxially with the bypass channel.

[0015] To prevent the polymer melt guided in the throttle section from escaping in the opposite direction around the circumference of the cylindrical pin, a seal is provided in the area between the guide bore and the threaded sleeve, surrounding the circumference of the cylindrical pin. This allows for easy adjustment of the cylindrical pin from the outside using a suitable tool, such as an Allen key.

[0016] To allow the pressure reduction between the screw sections within the screw housing to be adjusted according to specifications for the respective process, the bypass device has a pressure measuring port located upstream of the throttling section in the flow direction. A pressure gauge can be connected to this port, enabling continuous pressure monitoring by an operator.

[0017] For the adjustability and handling of the bypass device, a further development of the invention has proven particularly effective, in which the bypass device is arranged in a bypass housing that is held to the screw housing by a flange connection. Here, the bypass device is connected to the screw section via an outlet opening and an inlet opening in the screw housing. Furthermore, this allows the bypass device to be advantageously used in extrusion devices that have multiple screws within the screw housing, such as a twin-screw extruder.

[0018] A flow control element, particularly a flow control ring held around the circumference of the screw, has proven effective, forming a passage gap with the housing. This allows for further control over the flow rate conveyed between the two screw sections. The flow control ring is preferably dimensioned such that only a small fraction of the melt passes directly from the first screw section to the second within the screw housing.

[0019] The invention is explained in more detail with reference to an embodiment of the device according to the invention for extruding recycled plastics, with reference to the accompanying figures.

[0020] They represent: Fig. 1 Schematic overview of an embodiment of the device according to the invention for extruding recycled plastics Fig. 2 schematically a partial cross-section of the exemplary embodiment from Fig. 1 Fig. 3 schematically a cross-section of another embodiment of a bypass device with adjustable throttle section

[0021] In the Fig. Figure 1 schematically depicts a first embodiment of the device according to the invention for extruding recycled plastics. Only the components essential to the invention for melting and degassing recycled plastic are shown. The embodiment includes a screw 2 rotatably arranged in a screw housing 1. The screw 2 is connected at one end to a drive unit 3. Adjacent to the drive unit 3, a first screw section 2.1 is formed on the screw 2. A feed nozzle 4 is associated with the first screw section 2.1, through which a recycled material, for example, bottle-grade chips, is fed to the screw 2. The first screw section 2.1 extends to a damming element in the form of a damming ring 9, which is arranged on the circumference of the screw 2. The damming ring 9 is dimensioned such that a sealing gap is formed with respect to the screw housing 1.

[0022] In the extrusion direction, the first screw section 2.1 is followed by a second screw section 2.2, which extends to an outlet 8. Several degassing openings 7 in the screw housing 1 are associated with the second screw section 2.2. Both the first screw section 2.1 and the second screw section 2.2 of the screw 2 have different screw geometries. The screw geometries within screw sections 2.1 and 2.2 can also vary to form multiple zones. For example, the first screw section 2.1 of the screw 2 typically includes a feeding zone, a compression zone, and a metering zone. Similarly, the second screw section 2.2 of the screw 2 has multiple zones to enable degassing and discharge of the melt. These zones could include, for example, a degassing zone, a compression zone, a metering zone, and a discharge zone.

[0023] In the first screw section 2.1, up to the dam ring 9, a melt is produced from the recycled material. The throughput per hour is a function of the screw size, screw geometry, rotational speed, polymer properties, and the melt pressure at the dam ring. The second screw section 2.2 of screw 2, located in a degassing zone, is designed to remove volatile components, such as water vapor, from the melt using an applied vacuum. The screw channel depth in the degassing zone of the second screw section 2.2 is deeper compared to the other screw sections, so that screw 2 is only partially filled in this area. This allows the vacuum to act on a large free surface of the melt. Further along the screw 2, the channel depth is reduced until the screw channel is completely filled. From this point onward, the second screw section 2.2...2. The screw also builds up a melt pressure again. The two screw sections must be coordinated; that is, with the necessarily identical rotational speed of the two screw sections 2.1 and 2.2, the degassing zone, which immediately adjoins the dam ring 9, must remain partially filled, and further along the screw section 2.2, the melt pressure required at the outlet 8 must be restored. This coordination must then function over a certain flow rate range and with a range of fluctuations in viscosity and melt pressure. For this coordination of flow rate and melt pressure between the first screw section 2.1 and the second screw section 2.2, a bypass device 5 is arranged next to the dam ring 9.

[0024] Bypass device 5 is in Fig. Figure 1 schematically indicates the bypass device 5, which is connected to the first screw section 2.1 via an inlet opening 10 and to the second screw section 2.2 via an outlet opening 11. The bypass device 5 includes a throttling device 6 to allow for the adjustment of the quantity and pressure between the two screw sections 2.1 and 2.2. For further explanation, reference is now made to the Fig. 2 taken.

[0025] In the Fig. Figure 2 is a schematic cross-sectional view of the embodiment in Fig. Figure 1 shows the section of the transition between the first screw section 2.1 and the second screw section 2.2. The bypass device 5 is formed in a bypass housing 19 and has several bypass channels 12.1, 12.2, and 12.3. Bypass channel 12.1 is connected to the inlet opening 10 on the screw housing 1. Bypass channel 12.3 opens into the outlet opening 11 in the screw housing 1. Bypass channels 12.1 and 12.3 are connected to each other by another bypass channel 12.2.

[0026] A cylindrical pin 13 with a restrictor end 13.1 projects into the bypass channel 12.3. The cylindrical pin 13 is slidably held in a guide bore 16 on the bypass housing 19. The restrictor end 13.1 of the cylindrical pin 13 forms a restrictor section 14, the length of which is adjustable via the immersion depth of the restrictor end 13.1 of the cylindrical pin 13. The restrictor end 13.1 has a smaller diameter than the bypass channel 12.3 and thus forms an annular gap 15.

[0027] A pressure measuring port 17 is formed on the bypass housing 19, which is connected to the bypass channel 12.2, and to which a pressure measuring device 18 is connected.

[0028] As from the Fig. As shown in Figure 2, the melt, guided within the screw housing 1 in the first screw section 2.1, passes substantially through the inlet opening 10 to the bypass device 5. The dam ring 9 is preferably dimensioned such that only a small portion of the melt passes through the dam ring 10 into the second screw section. The majority of the melt is guided through the bypass channels 12.1 and 12.2 to the throttle device 6. The melt then passes through the throttle section 14 in the bypass channel 12.3 to the outlet opening 11 and into the second screw section 2.2. The throttle section 14 is dimensioned such that a generous annular gap 15 prevails for a corresponding length of the throttle section 14, making the throttle section 14 insensitive to contamination particles within the melt. The adjustable throttle section 14 in conjunction with a pressure measurement at the inlet of the throttle section 14 then allows the two screw sections 2 to be controlled.1 and 2.2 must be coordinated.

[0029] In order to be able to change the setting during the ongoing operation of the device according to the invention, in Fig. Figure 3 shows a constructive embodiment of the bypass device 5 in a cross-sectional view, as shown, for example, in the embodiment according to Fig. 1 could be used.

[0030] The schematic structure of the in Fig. The embodiment of the bypass device 5 shown in Figure 3 is essentially identical to the embodiment shown in Figure 3. Fig. 2, so that only the differences will be explained below and otherwise reference will be made to the aforementioned description.

[0031] The bypass device 5 is arranged on a bypass housing 19. The bypass housing 19 is flanged to the screw housing 1 via a flange connection 23. Inside the bypass housing 19, the bypass channel 12.1 is connected to the inlet opening 10 and the bypass channel 12.3 to the outlet opening 11. A guide bore 16 and a threaded sleeve 20 are arranged coaxially with the bypass channel 12.3. The threaded sleeve 20 is attached to the bypass housing 19. Inside the threaded sleeve 20, a retaining end 13.2 of a cylindrical pin 13 is held by a thread 21. The cylindrical pin 13 has an opposing throttle end 13.1 that projects into the bypass channel 12.3. The penetration depth of the cylindrical pin 13 with the throttle end 13.1 forms a throttle section 14 within the bypass channel 12.3. The throttle section 14 has a capillary-shaped annular gap 15, which is formed between the throttle end 13.1 of the cylindrical pin 13 and the bypass channel 12.3.

[0032] At the end of the guide bore 16, a seal 22 is arranged between the threaded sleeve 20 and the bypass housing 19. The seal 22 surrounds the circumference of the cylindrical pin 13 and thus seals the guide bore 16 against the environment.

[0033] The bypass channel 12.2, which connects the bypass channels 12.1 and 12.3, is sealed at the bypass housing 19 by a plug 24.

[0034] In addition to the threaded sleeve 20, a pressure measuring port 17 is provided at the level of the bypass channel 12.1, which is connected to the bypass channel 12.2 via a pressure bore 25. A pressure sensor or pressure gauge can be directly connected to the pressure measuring port 17.

[0035] The function of the in Fig. The embodiment of the bypass control shown in Figure 3 is identical to the aforementioned embodiment. However, in this embodiment, the cylindrical pin 13 can be rotated within the threaded sleeve 20 using a tool, for example, an Allen key, thus changing the length of the throttle section 14. This allows the setting to be varied during operation.

[0036] It is also possible, in the event of a process interruption, to completely unscrew the cylindrical pin 13 from the threaded sleeve 20 and replace it with another cylindrical pin 13, which, for example, has a throttle end 13.1 with a larger or smaller outer diameter relative to the bypass channel 12.3. This also allows the gap width of the annular gap 15 to be advantageously changed.

[0037] The in Fig.The illustrated embodiment of the throttle section shown in Figure 3 is only one possible design option. In principle, other alternatives for moving the cylindrical pin within the bypass channel are also possible, for example, automated by electric or pneumatic drives. Automated control systems can also be integrated via a control unit to achieve the desired coordination between the first and second screw sections.

[0038] The particular advantage of the invention lies in the fact that the degassing of the melt in the second screw section can be carried out intensively and with high uniformity. There is no risk of the vacuum area within the screw housing being flooded with melt. Furthermore, it is possible to subsequently achieve a higher output pressure. Due to its insensitivity to contamination, the invention is particularly suitable for the extrusion of recycled materials.

Claims

[1] Device for extruding plastics, preferably recycled plastics, comprising a screw (2) arranged within a screw housing (1), which has at least a first screw section (2.1) for melting and at least a second screw section (2.2) for degassing, with a damming element (9) between the first screw section (2.1) and the second screw section (2.2) and with a bypass device (5) associated with the damming element (9), which connects the first screw section (2.1) with the second screw section (2.2), characterized by , that the bypass device (5) has an adjustable throttle section (14) formed by a cylindrical pin (13) which extends with a free throttle end (13.1) into a bypass channel (12.3) and forms an annular gap (15). [2] Device according to claim 1, characterized by, that the cylinder pin (13) is held so that it can be moved on the bypass device (5) that the length of the throttle section (14) can be adjusted. [3] Device according to claim 1, characterized by , that the cylindrical pin (13) is held interchangeably on the bypass device (5) and that the gap width of the annular gap (15) can be adjusted by replacing the cylindrical pin (13). [4] Device according to claim 2 or 3, characterized by , that the cylindrical pin (13) has a thread (21) at an opposite retaining end (13.2) and that the retaining end (13.2) of the cylindrical pin (13) is rotatably held in a threaded sleeve (20). [5] Device according to one of claims 3 to 4, characterized by , that the cylindrical pin (13) is guided within a guide bore (16) which opens coaxially into the bypass channel (12.3). [6] Device according to claim 5, characterized by, that in the area between the guide bore (16) and the threaded sleeve (20) a seal (22) is assigned to the circumference of the cylindrical pin (13). [7] Device according to any one of claims 1 to 6, characterized by , that the bypass device (5) has a pressure measuring port (17) which is located upstream of the throttling section (14) in the direction of flow. [8] Device according to any one of claims 1 to 7, characterized by , that the bypass device (5) is arranged in a bypass housing (19) which is held to the screw housing (1) by a flange connection (23), wherein the bypass device (5) is connected to the screw sections (2.1, 2.2) by an outlet opening (11) and an inlet opening (10) in the screw housing (1). [9] Device according to any one of claims 1 to 8, characterized by, that the damming element is formed by a damming ring (9) held on the circumference of the screw (2), which forms a passage gap with the screw housing (1).

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