DEVICE AND METHOD FOR PRODUCING A DYED AND AN UNDYED PLASTIC MELT

Abstract

A device for producing a dyed plastic melt and an undyed plastic melt includes a multi-shaft screw extruder, a first metering installation, a second metering installation, and a control installation for selecting between a first operating mode for producing the dyed plastic melt and a second operating mode for producing the undyed plastic melt. The first metering installation serves for feeding an undyed plastic material through a first infeed opening into a housing of the multi-shaft screw extruder, and the second metering installation serves for feeding at least one dyeing agent through a second infeed opening into the housing. In order for the undyed plastic melt to be produced, the plastic material is fed exclusively via the first infeed opening such that residual dyeing agent which is still located in the second metering installation or in the region of the second infeed opening does not contaminate the undyed plastic melt.

Claims

1. A device for producing a dyed plastic melt and an undyed plastic melt, the device comprising: a multi-shaft screw extruder for providing a plastic melt, the multi-shaft screw extruder having a housing, a first infeed opening, a second infeed opening, at least two treatment element shafts and at least two mutually penetrating housing bores configured in the housing, the first infeed opening and the second infeed opening opening into the at least two mutually penetrating housing bores, the at least two treatment element shafts being rotatingly drivable in the at least two mutually penetrating housing bores; a first metering installation for feeding an undyed plastic material through the first infeed opening into the at least two mutually penetrating housing bores; a second metering installation for feeding at least one dyeing agent through the second infeed opening into the at least two mutually penetrating housing bores; and a control installation for selecting between a first operating mode for producing the dyed plastic melt and a second operating mode for producing the undyed plastic melt.

2. The device as claimed in claim 1, wherein the control installation is configured such that: in the first operating mode at least the dyeing agent is capable of being fed through the second infeed opening into the at least two mutually penetrating housing bores via the second metering installation, and the undyed plastic material is capable of being fed via at least one of the first metering installation and the second metering installation; in the second operating mode the undyed plastic material is capable of being fed through the first infeed opening into the at least two mutually penetrating housing bores via the first metering installation.

3. The device as claimed in claim 1, wherein the second metering installation comprises an infeed extruder which opens into the second infeed opening.

4. The device as claimed in claim 3, wherein the infeed extruder is configured as a single-shaft mixing extruder, the single-shaft mixing extruder being assigned a dyeing agent metering unit and a plastic material metering unit upstream of the single-shaft mixing extruder.

5. The device as claimed in claim 3, wherein the infeed extruder is configured as a dual-shaft screw extruder, the dual-shaft screw extruder being assigned a masterbatch metering unit upstream of the dual-shaft screw extruder.

6. The device as claimed in claim 1, wherein a gap is defined between the at least two treatment element shafts, and a width of the gap in relation to an external diameter D of the at least two treatment element shafts is equal to or greater than 0.003 and equal to or less than 0.05.

7. The device as claimed in claim 1, wherein the at least two treatment element shafts and a housing internal wall of the housing define a gap, and a width of the gap in relation to an external diameter of the at least two treatment element shafts is equal to or greater than 0.004 and equal to or less than 0.03.

8. The device as claimed in claim 1, wherein the second infeed opening is disposed downstream of the first infeed opening in a conveying direction.

9. The device as claimed in claim 8, wherein each of the at least two treatment element shafts comprises one shaft to provide at least a plurality of shafts, and screw elements are rotationally fixed on the shafts between the first infeed opening and the second infeed opening.

10. The device as claimed in claim 8, wherein each of the at least two treatment element shafts comprises one shaft to provide at least a plurality of shafts, and kneading elements are rotationally fixed on the shafts between the first infeed opening and the second infeed opening.

11. The device as claimed in claim 1, wherein each of the at least two treatment element shafts comprises one shaft to provide at least a plurality of shafts, and screw elements and kneading elements are rotationally fixed on the shafts downstream of the first infeed opening and the second infeed opening.

12. The device as claimed in claim 1, further comprising a closure element for closing the second infeed opening.

13. The device as claimed in claim 12, wherein the closure element is activatable via a closure element drive.

14. The device as claimed in claim 12, wherein the closure element has a closure wall, the closure wall being shaped to correspond to a housing internal wall and the closure wall defining at least one of the at least two mutually penetrating housing bores.

15. A method for producing a dyed plastic melt and an undyed plastic melt, the method comprising the following method steps: providing a multi-shaft screw extruder having a housing, at least two mutually penetrating housing bores configured in the housing, at least two treatment element shafts, a first infeed opening and a second infeed opening, the first infeed opening and the second infeed opening opening into the at least two mutually penetrating housing bores, the at least two treatment element shafts being rotatingly drivable in the at least two mutually penetrating housing bores; providing a first metering installation and a second metering installation; selecting between a first operating mode for producing the dyed plastic melt and a second operating mode for producing the undyed plastic melt via a control installation, wherein in the first operating mode at least one dyeing agent is fed through the second infeed opening into the at least two mutually penetrating housing bores via the second metering installation, and an undyed plastic material is fed via at least one of the first metering installation and the second metering installation, wherein in the second operating mode the undyed plastic material is fed through the first infeed opening into the at least two mutually penetrating housing bores via the first metering installation; and operating the multi-shaft screw extruder, the first metering installation and the second metering installations in a selected operating mode.

16. The device as claimed in claim 1, wherein a gap is defined between the at least two treatment element shafts, and a width of the gap in relation to an external diameter D of the at least two treatment element shafts is equal to or greater than 0.004 and equal to or less than 0.035.

17. The device as claimed in claim 1, wherein a gap is defined between the at least two treatment element shafts, and a width of the gap in relation to an external diameter D of the at least two treatment element shafts is equal to or greater than 0.011 and equal to or less than 0.02.

18. The device as claimed in claim 1, wherein the at least two treatment element shafts and a housing internal wall of the housing define a gap, and a width of the gap in relation to an external diameter of the at least two treatment element shafts is equal to or greater than 0.005 and equal to or less than 0.025.

19. The device as claimed in claim 1, wherein the at least two treatment element shafts and a housing internal wall of the housing define a gap, and a width of the gap in relation to an external diameter of the at least two treatment element shafts is equal to or greater than 0.012 and equal to or less than 0.019.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the drawings:

[0026] FIG. 1 is a partially sectional view of a device for producing a dyed and an undyed plastic melt according to a first exemplary embodiment;

[0027] FIG. 2 is a partially sectional plan view of a multi-shaft screw extruder of the device in FIG. 1;

[0028] FIG. 3 is a sectional view of the multi-shaft screw extruder along a section line III-III in FIG. 2;

[0029] FIG. 4 is a sectional view in the region of a second infeed opening of the multi-shaft screw extruder of a device according to a second exemplary embodiment;

[0030] FIG. 5 is a sectional view in the region of a second infeed opening of the multi-shaft screw extruder of a device according to a third exemplary embodiment;

[0031] FIG. 6 is a sectional view in the region of a second infeed opening of the multi-shaft screw extruder of a device according to a fourth exemplary embodiment;

[0032] FIG. 7 is a sectional view in the region of a second infeed opening of the multi-shaft screw extruder of a device according to a fifth exemplary embodiment;

[0033] FIG. 8 is a partially sectional view of a device for producing a dyed and an undyed plastic melt according to a sixth exemplary embodiment; and

[0034] FIG. 9 is a partially sectional plan view of a multi-shaft screw extruder of the device in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Referring to the drawings, A first exemplary embodiment of the invention is described hereunder by means of FIGS. 1 to 3. A device 1 for producing a dyed plastic melt S.sub.1 and an undyed plastic melt S.sub.2 comprises a multi-shaft screw extruder 2, a first metering installation 2, a second metering installation 4, and a control installation 5. The multi-shaft screw extruder 2 has a housing 6 in which two mutually penetrating housing bores 7, 8 are configured. Treatment element shafts 9, 10 are disposed so as to be rotatable about associated rotation axes 11, 12 in the housing bores 7, 8. The treatment element shafts 9, 10 by means of a drive motor 13, and by way of a transfer gearbox 14, are rotatingly drivable in the same direction, thus in identical directions. A clutch 15 is disposed between the drive motor 13 and the transfer gearbox 14.

[0036] A first infeed opening 16 and a second infeed opening 17 which open into the housing bores 7, 8 are configured in the housing 6. The second infeed opening 17 in a conveying direction 18 is disposed downstream of the first infeed opening 16. The first metering installation 3 opens into the first infeed opening 16 and serves for feeding an undyed plastic material M through the first infeed opening 16 into the housing bores 7, 8. The first metering installation 3 is configured as a gravimetric metering unit, for example.

[0037] The second metering installation 4 comprises an infeed extruder 19, a dyeing agent metering unit 20, and a plastic material metering unit 21. The second metering installation 4, or the infeed extruder 19, respectively, opens into the second infeed opening 17. The undyed plastic material M and/or a dyeing agent F is capable of being fed through the second infeed opening 17 into the housing bores 7, 8 by means of the second metering installation 4.

[0038] The infeed extruder 19 is configured as a single-shaft mixing extruder. The infeed extruder 19 comprises a housing 22 having a housing bore 23 in which a mixing shaft 24 is rotatably disposed. The mixing shaft 24 is rotatingly drivable by means of a drive motor 25. The dyeing agent metering unit 20 and the plastic material metering unit 21 which open into the housing bore 23 are disposed upstream of the infeed extruder 19. To this end, a dyeing agent infeed opening 26 and a plastic material infeed opening 27 are configured in the housing 22, for example. The dyeing agent metering unit 20 serves for metering the dyeing agent F into the infeed extruder 19. Accordingly, the plastic material metering unit 20 serves for metering the undyed plastic material M into the infeed extruder 19. The metering units 20, 21 are configured as gravimetric metering units, for example.

[0039] The screw extruder 2 in the conveying direction 18 has in succession a drawing-in zone 28, a melting and mixing zone 29, and a discharge zone 30. The housing 6 at the end side of the discharge zone 30 has a discharge opening 31. The infeed openings 16, 17 are disposed in the drawing-in zone 28. The treatment element shafts 9, 10 comprise in each case an associated shaft 32, 33, screw elements 34, 34 and kneading elements 35, 35 in succession in the conveying direction 18 being disposed in pairs so as to be rotationally fixed on said shafts 32, 33. Screw elements 34, 34 are exclusively disposed so as to be rotationally fixed on the shafts 32, 33 in the drawing-in zone 28. Screw elements 34, 34 and kneading elements 35, 35 are disposed so as to be rotationally fixed on the shafts 32, 33 in the melting and mixing zone 29. Screw elements 34, 34 are in turn exclusively disposed so as to be rotationally fixed on the shafts 32, 33 in the discharge zone 30. The kneading elements 35, 35 are configured, for example, as individual kneading disks and/or as kneading blocks having a plurality of integrally interconnected kneading disks.

[0040] The treatment element shafts 9, 10 are configured so as to mutually mesh in a tight manner and so as to scrape a housing internal wall 36 of the housing 6. To this end, the treatment element shafts 9, 10 are mutually disposed in such a manner that a gap 37 delimited by the treatment element shafts 9, 10, when viewed in the cross section, has a width A.sub.1, where the width A.sub.1 in relation to an external diameter D of the treatment element shafts 9, 10 is: 0.003A.sub.1/D0.05, in particular 0.004A.sub.1/D0.035, and in particular 0.011A.sub.1/D0.02.

[0041] Furthermore, the treatment element shafts 9, 10, conjointly with the housing internal wall 36, form a respective gap 38 which, when viewed in the cross section, has an associated width A.sub.2, where the width A.sub.2 to external diameter D ratio is: 0.004A.sub.2/D0.03, in particular 0.005A.sub.2/D0.025, and in particular 0.012A.sub.2/D0.019.

[0042] The control installation 5 serves for selecting between a first operating mode B.sub.1 for producing the dyed plastic melt S.sub.1 and a second operating mode B.sub.2 for producing the undyed plastic melt S.sub.2. To this end, the control installation 5 in terms of signaling is connected to the multi-shaft screw extruder 2, the first metering installation 3, and the second metering installation 4. The control installation 5 is configured in such a manner that in the first operating mode B.sub.1 at least the dyeing agent F by means of the second metering installation 4 is capable of being fed through the second infeed opening 17 into the at least two housing bores 7, 8, and the undyed plastic material M by means of the first metering installation 3 and/or the second metering installation 4 is capable of being fed through the first infeed opening 16 and/or the second infeed opening 17. The control installation 5 is furthermore configured in such a manner that in the second operating mode B.sub.2 the undyed plastic material M by means of the first metering installation 3 is capable of being fed through the first infeed opening 16 into the at least two housing bores. No dyeing agent F is capable of being fed into the housing bores 7, 8 in the second operating mode B.sub.2.

[0043] The functioning mode of the device 1 is as follows:

[0044] A selection between the first operating mode B.sub.1 and the second operating mode B.sub.2 is possible by means of the control installation 5. The production of the dyed plastic melt S.sub.1 is performed in the first operating mode B.sub.1, whereas the production of the dyed plastic melt S.sub.2 is performed in the second operating mode B.sub.2.

[0045] The undyed plastic material M and the dyeing agent F are fed to the multishaft screw extruder 2 in the first operating mode B.sub.1. The undyed plastic material M and the dyeing agent F in the drawing-in zone 28 make their way into the housing bores 7, 8 and are conveyed in the conveying direction 18 into the melting and mixing zone 29. The plastic material M is melted and the dyeing agent F is mixed with the melted plastic material M in the melting and mixing zone 29 such that the dyed plastic melt S.sub.1 is created. The dyed plastic melt S.sub.1 is subsequently discharged through the discharge opening 31 in the discharge zone 30. The dyed plastic melt S.sub.1 is subsequently pelletized in the usual manner, and dyed plastic granulate is generated, for example.

[0046] The undyed plastic material M, thus the plastic material M of natural color, is a bulk material and is configured so as to be pulverulent and/or in the shape of granulate, for example. Accordingly, the dyeing agent F is present as bulk material and is configured so as to be pulverulent and/or in the shape of granulate, for example. The dyeing agent F is pulverulent carbon black, for example. The dyed plastic melt S.sub.1 comprises between 0.5% by weight and 10% by weight, in particular between 1% by weight and 6% by weight, and in particular between 1.5% by weight and 4% by weight dyeing agent F, and between 90% by weight and 99.5% by weight, in particular between 94% by weight and 99% by weight, and in particular between 96% by weight and 98.5% by weight undyed plastic material M. Additionally, other additives than the dyeing agent F can be mixed with the dyed plastic melt S.sub.1, said additives not being considered in the weight proportions stated above.

[0047] In a first variant of the operating mode B.sub.1 the undyed plastic material M is fed both by means of the first metering installation 3 as well as by means of the second metering installation 4. The undyed plastic material M is fed through the first infeed opening 16 into the housing bores 7, 8 by means of the first metering installation 3. Between 40% by weight and 90% by weight, in particular between 45% by weight and 85% by weight, and in particular between 50% by weight and 80% by weight, of the entire undyed plastic material M is fed by means of the first metering installation 3. The residual undyed plastic material M by means of the plastic material metering unit 21, and the dyeing agent F by means of the dyeing agent metering unit 20 are fed into the infeed extruder 19 and mixed with one another therein. To this end, the mixing shaft 24 is rotatingly driven at a rotating speed between 200 rpm and 3500 rpm, in particular between 350 rpm and 3000 rpm, and in particular between 500 rpm and 2500 rpm. The mixture from the undyed plastic material M and the dyeing agent F is fed into the housing bores 7, 8 by way of the second infeed opening 17.

[0048] In the case of a second variant of the operating mode B.sub.1, the second metering installation 4 is exclusively operated and the entire undyed plastic material M by means of the plastic material metering unit 21, and the dyeing agent F by means of the dyeing agent metering unit 20 are fed to the infeed extruder 19. The mixture generated from the entire plastic material M and the dyeing agent F is fed into the housing bores 7, 8 by way of the second infeed opening 17.

[0049] In the case of a third variant of the first operating mode B.sub.1, the entire undyed plastic material M by means of the first metering installation 3 is fed by way of the first infeed opening 16, and the dyeing agent F is fed exclusively by way of the second infeed opening 17.

[0050] The second metering installation 4 is deactivated when the second operating mode B.sub.2 is subsequently selected by means of the control installation 5. In order for the undyed plastic melt S.sub.2, thus the plastic melt S.sub.2 of natural color, to be produced the multi-shaft screw extruder 2 is fed the undyed plastic material M exclusively by means of the first metering installation 3. No dyeing agent F is fed in the second operating mode B.sub.2. Self-cleaning of the multi-shaft screw extruder 2 takes place in a transition period after the changeover. Residual dyeing agent F which is located in the housing bores 7, 8, in particular on the treatment element shafts 9, 10 and on the housing internal wall 36, by virtue of the tightly meshing configuration of the treatment element shafts 9, 10 and of the scraping of the housing internal wall 36 by means of the treatment element shafts 9, 10, is removed from the multi-shaft screw extruder 2. An impure plastic melt S.sub.3 which is capable of being utilized neither as dyed plastic melt S nor as undyed plastic melt S.sub.2 is created in said transition period, said impure plastic melt S.sub.3 therefore having to be separated. The self-cleaning is completed after the transition period such that the undyed plastic melt S.sub.2 is generated in the desired manner in that the plastic material M that is fed through the first infeed opening 16 is conveyed to the melting and mixing zone 29 and therein is melted and subsequently discharged in the discharge zone 30.

[0051] A changeover from the second operating mode B.sub.2 to the first operating mode B.sub.1 is possible in a simple manner since no self-cleaning of the multi-shaft screw extruder 2 is required for producing the dyed plastic melt S.sub.1.

[0052] A second exemplary embodiment of the invention is described hereunder by means of FIG. 4. As opposed to the preceding exemplary embodiment, the device 1 has a closure element 39 which closes the second infeed opening 17 in the second operating mode B.sub.2 and opens said second infeed opening 17 in the first operating mode B.sub.1. The closure element 39 comprises a support component 40, a closure component 41, and a closure element drive 42 having a first drive motor 43 and a second drive motor 44. The support component 40 has two passage openings 45, 46, wherein the closure component 41 is disposed so as to be repositionable in the passage opening 46. The support component 40 by means of the first drive motor 42 is repositionable in a horizontal direction such that either the free passage opening 45 or the passage opening 46 that is provided with the closure component 41 is aligned with the second infeed opening 17. In the first operating mode B.sub.1, the support component 40 by means of the drive motor 43 is repositioned such that the free passage opening 45 is aligned with the second infeed opening 17 such that the infeed through the second infeed opening 17 into the housing bores 7, 8 is possible. In the transition to the second operating mode B.sub.2 the support component 40 by means of the first drive motor 43 is repositioned in the horizontal direction such that the passage opening 46 is aligned with the second infeed opening 17. The closure component 41 by means of the second drive motor 44 is subsequently repositioned in a vertical direction such that the second infeed opening 17 is closed by the closure component 41. The closure component 41 has a closure wall 47 which is shaped so as to correspond to the housing internal wall 36 and to complement the housing internal wall 36 in the region of the second infeed opening 17 such that the housing bores 7, 8 are delimited by the housing 6 and the closure component 41. In the second operating mode B.sub.2, no residual dyeing agent F can make its way into the housing bores 7, 8 through the second infeed opening 17, for example by virtue of vibrations. In the transition to the first operating mode B1 the closure component 41 and the support component 40 are repositioned in the reverse order. Reference in terms of the further construction and of the further functional mode is made to the first exemplary embodiment.

[0053] A third exemplary embodiment of the invention is described hereunder by means of FIG. 5. As opposed to the preceding exemplary embodiment, the support component 40 has only the first passage opening 45, and by way of a region laterally beside the passage opening 45 configures the closure component 41. In order for the second infeed opening 17 to be closed and opened, the support component 40 by means of the drive motor 43 is laterally repositioned in the horizontal direction such that either the closure component 41 or the passage opening 45 is disposed above the second infeed opening 17. Reference in terms of the further construction and of the further functional mode is made to the preceding exemplary embodiments.

[0054] A fourth exemplary embodiment of the invention is described hereunder by means of FIG. 6. The closure element 39 comprises a guide 48 by means of which the closure component 41 is repositionable in a guided manner into the second infeed opening 17. The guide 48 is only schematically indicated in FIG. 6. The guide 48 can be configured in an arbitrary manner, for example as a pivot joint. In order for the second infeed opening 17 to be opened and closed, the closure component 41 by means of the drive motor 43 is repositioned along the guide 48. Reference in terms of the further construction and of the further functional mode is made to the preceding exemplary embodiments.

[0055] A fifth exemplary embodiment of the invention is described hereunder by means of FIG. 7. The closure component 41 is manually assembled for closing the second infeed opening 17, and is manually disassembled for opening the second infeed opening 17. The housing 6 in the region of the second infeed opening 17 configures a detent 49 for the closure component 41 such that the latter in the assembled state is exactly positioned. Reference in terms of the further construction and of the further functional mode is made to the preceding exemplary embodiments.

[0056] A sixth exemplary embodiment of the invention is described hereunder by means of FIGS. 8 and 9. As opposed to the preceding exemplary embodiments, the second metering installation 4 has a masterbatch metering unit 50 which is disposed upstream of the infeed extruder 19. The infeed extruder 19 is configured as a dual-shaft screw extruder. Said infeed extruder 19 comprises a housing 51 having two mutually penetrating housing bores 52, 53 configured therein, two screw shafts 54, 55 being disposed in said housing bores 52, 53 so as to be rotatable about associated rotation axes 56, 57. The screw shafts 54, 55 by means of a drive motor 58, and by way of a transfer gearbox 59, are rotatingly driven in the same direction, thus in identical rotation directions. An infeed opening 60 into which the masterbatch metering unit 50 opens is configured in the housing 51.

[0057] The multi-shaft screw extruder 2 in the conveying direction 18 configures in succession a first drawing-in zone 28, a melting zone 29, a second drawing-in zone 28, the melting and mixing zone 29, and the discharge zone 30. Screw elements 34, 34 are exclusively disposed so as to be rotationally fixed on the shafts 32, 33 in the region of the first drawing-in zone 28. By contrast, kneading elements 35, 35 are in turn exclusively disposed so as to be rotationally fixed on the shafts 32, 33 in the melting zone 29. The second infeed opening 17 is configured so as to be downstream of the first infeed opening 16 in the second drawing-in zone 28. The melting zone 29 is thus disposed between the first infeed opening 16 and the second infeed opening 17. Accordingly, screw elements 34, 34 and kneading elements 35, 35 are disposed so as to be rotationally fixed on the shafts 32, 33 between the infeed openings 16, 17. The second infeed opening 17 is configured laterally in the housing 6. The infeed extruder 19 is configured as a dual-shaft side-feeder screw extruder.

[0058] In the first operating mode B.sub.1 the undyed plastic material M by means of the first metering installation 3 is fed through the first infeed opening 16 into the first drawing-in zone 28. The plastic material M is conveyed to the melting zone 29 and melted therein by means of the kneading elements 35, 35. In the second drawing-in zone 28, color masterbatch granulate G is fed to the molten plastic material M by means of the second metering installation 4. The masterbatch granulate G comprises plastic material M and bound dyeing agent F which is present in a concentrated form in the masterbatch granulate G. The masterbatch granulate G by means of the masterbatch metering unit 50 is fed into the infeed extruder 19 and by means of the latter is fed through the second infeed opening 17 into the housing bores 7, 8. The masterbatch granulate G is melted in the melting and mixing zone 29 and is homogenized conjointly with the plastic material M that is present as a melt. The dyed plastic melt S.sub.1 is subsequently discharged in the discharge zone 30. The dyed plastic melt S.sub.1 comprises between 0.5% by weight and 10% by weight, in particular between 1% by weight and 6% by weight, and in particular between 1.5% by weight and 4% by weight dyeing agent F, and between 90% by weight and 99.5% by weight, in particular between 94% by weight and 99% by weight, and in particular between 96% by weight and 98.5% by weight undyed plastic material M. Additionally, other additives than the dyeing agent F can be mixed with the dyed plastic melt S.sub.1, said additives not being considered in the weight proportions stated above.

[0059] No dyeing agent F, or no masterbatch granulate G, respectively, is fed by means of the second metering installation 4 in the second operating mode B.sub.2. The second metering installation 4 is deactivated and the infeed extruder 19 is disassembled in a first variant of the second operating mode B.sub.2. The second infeed opening 17 is closed by means of a closure element or a closure component, respectively. Since the second infeed opening 17 is configured laterally, the closure wall is shaped according to a housing bore 7 or 8, respectively, and delimits the latter. The exemplary embodiments according to FIGS. 4 to 7 can be applied in an analogous manner to the present exemplary embodiment. The undyed plastic material M by means of the first metering installation 3 is subsequently fed through the first infeed opening 16 and melted in the melting zone 29 such that self-cleaning is carried out in the transition period by means of the molten plastic material M in the second drawing-in zone 28, in the melting and mixing zone 29, and in the discharge zone 30. The undyed plastic melt S.sub.2 is produced after self-cleaning. The masterbatch metering unit 50 is deactivated and the infeed extruder 19 is operated empty in a second variant of the second operating mode B.sub.2, such that the molten plastic material M in the second drawing-in zone 28 is kept in the housing bores 7, 8 by the screw shafts 54, 55. Disassembling the infeed extruder 19 is not required in the case of the second variant. Reference in terms of the further construction and of the further functional mode is made to the preceding exemplary embodiments.

[0060] The device 1 according to the invention enables the selective production of a dyed plastic melt S.sub.1 and an undyed plastic melt S.sub.2 by way of only a single multi-shaft screw extruder 2. The complexity in terms of machine technology is comparatively minor on account thereof. When the first operating mode B.sub.1 is changed over to the second operating mode B.sub.2, an impure plastic melt S.sub.3 is thus created during self-cleaning in a transition period. The impure plastic granulate produced therefrom, depending on the economics, is sold or in a subsequent production of the dyed plastic melt S is re-fed to the multi-shaft screw extruder 2 by way of the second infeed opening 17 and is further processed. The device 1 thus enables a simple, flexible and economical production selectively of a dyed plastic melt S.sub.1 and an undyed plastic melt S.sub.2.

[0061] While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.