Extrusion screws for multi-screw extruders

Abstract

An extrusion screw (100) for a multiple screw extruder (200) includes at least an inlet metering section (30) and a rotor body (50) having a diameter enlarged compared to the inlet metering section (30) and having a plurality of satellite screws (20), the satellite screws (20) being arranged on the outer periphery of the rotor body (50) while being exposed over at least a portion of their length, a conical section (11) and a drive zone following the conical section (11) are formed between the inlet metering section (30) and the rotor body (50), and in the drive zone, the satellite screws (20) are engaged, via drive pinions (21), with external teeth provided on the rotor body (50) or internal teeth provided on a stator ring (244) or the inner wall of the extruder housing (240) of the multiple screw extruder (200). Between at least two adjacent grooves (15) for the drive pinion (21), at least one circumferential flow passage (13) is formed, each closed in the circumferential direction, and the circumferential flow passage (13) extends from an inlet opening (12) provided in the conical portion (11) to an outlet opening (14) arranged downstream of the drive pinion (21) in the flow direction.

Description

[Technical field]

[0001] The present invention relates to an extrusion screw for a multiple screw extruder having the features according to the preamble of claim 1.

[0002] For conditioning plastic melts, in particular polyesters, a multi-rotation system (MRS), which is basically described in WO 2003033240, has proven advantageous. The multi-rotation system comprises an extrusion screw, which has a so-called multi-rotation unit with a rotor body shaft between an inlet zone, in which the plastic is drawn in and melted, and a discharge zone. The rotor body shaft has a significantly enlarged diameter compared to the other zones and furthermore several rotating satellite screws. With the multi-rotation system, a significantly improved degassing performance is achieved compared to single-screw and double-screw systems. The residence time of the melt in the multi-rotation unit can therefore be very short. Known drive concepts envisage a drive zone for the satellite screws, which is located in the treatment space provided for degassing. The melt leaving the metering zone is guided through the drive zone. In some applications, the introduction of energy via the shear occurring there can be advantageous, since this promotes homogenization of the plastic melt. On the other hand, shearing of the polymer in the drive zone can be detrimental to product properties.

[0003] It is therefore an object of the present invention to improve an extrusion screw for an MRS system or for a multiple screw extruder equipped with an MRS system, in order to reduce the effect of shear on the polymer being processed by the extrusion screw.

[0004] The solution according to the invention consists in an extrusion screw with the features of claim 1 or in a multiple screw extruder with the features of claim 6 or 7.

[0005] In the present invention, the main part of the melt flow is not guided in the drive zone via the pinion driving the satellite screw, but in a closed circumferential flow channel formed inside the support bearing for the satellite screw and passing by the pinion. This has the advantage that the melt is not heated by shear. For example, in the processing of PET, it is advantageous if the melt is not completely plasticized and therefore has a relatively low temperature, so that excessive reduction of the melt can be avoided already in the introduction area.

[0006] With regard to the extrusion screw itself, a further advantage arises that the risk of damage to the extrusion screw in the event of contamination of the melt is reduced. The screw can be supported in plain bearing bushes before and after the pinion.

[0007] This support prevents the tips of the pinion teeth from contacting the bottom of the grooves of the support bearing member, thereby avoiding possible wear.

[0008] The key to the effectiveness of the peripheral passage is to select it large enough so that the main part of the volume flow of the polymer conveyed and processed by the extrusion screw is not guided through the drive zone. A sufficient cross section is also important to prevent the not fully plasticized material in the introduction screw from clogging the passage and causing a significant pressure drop. Both of these would result in a high pressure head at the end of the introduction and thus a significantly higher energy input, which would damage the melt.

[0009] In particular, it is desirable for the passageways to provide an open cross section in any dimension of at least 5mm, preferably between 8mm and 10mm.

[0010] A second solution of the invention consists in that the peripheral flow passage is guided around the outside of the tooth row, i.e. is formed in the wall of the extruder housing and / or in the stator ring.

[0011] The annular gap between the outer surface of the support bearing member and the inner surface of the extrusion hole in the housing preferably has a radial width of 1 mm to 3 mm with a maximum of 5 mm, for example for a diameter of 130 mm an annular gap of 1.6 mm to 2.0 mm is set.

[0012] Preferably, an annular gap is formed between the outer circumferential surface of the extrusion screw in the drive zone and the inner circumferential surface of the extrusion bore, the cross-sectional area of ​​which is set to a maximum of 20% of the sum of the cross-sectional areas of all the circumferential passages, so that a much larger amount is guided past the pinion via the circumferential passages.

[0013] This predetermines that only a small volume flow passes through the outer periphery, which allows the polymer to act as a lubricant in the drive zone, while the majority of the volume flow is distributed to the respective circumferential flow passages and thus does not undergo shear in the drive zone.Furthermore, it is advantageous if the size of the annular gap is selected to be small, so that foreign particles in the melt flow, the size of which could lead to serious mechanical damage to the teeth, are retained.

[0014] The invention will now be explained in more detail with reference to the drawings, in which: FIG. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a perspective view showing each component of the extrusion screw. [Diagram 2] FIG. 4 is a perspective view showing a satellite screw support member. [Diagram 3] FIG. 2 is a perspective view showing each component of the extrusion screw. [Figure 4] FIG. 2 is a cross-sectional side view showing the components of a multi-screw extruder. [Diagram 5] FIG. 2 is a perspective cross-sectional view showing the components of a multi-screw extruder.

[0016] FIG. 1 shows the components of an extrusion screw for a multi-screw extruder in a perspective view, and in this case the transition region between an inlet metering section 30 with a screw web 31 and a multi-screw section with several satellite screws 20. Between the inlet metering section 30 and the multi-screw section a conical section 11 is formed, in which the diameter of the extrusion screw increases in the flow direction. The conical section 11 is part of a support bearing member 10, on which the end sections with the pinions 21 of the several satellite screws 20 are supported. Between adjacent pinions 21, an elongated axial section of the support bearing member 10 is provided, in which a tubular circumferential flow passage 13 is formed, which is closed in the circumferential direction. The circumferential flow passage 13 runs from an inlet opening 12 in the conical section 11 to an outlet opening 14 arranged downstream of the pinion 21 as viewed in the axial direction of the extrusion screw.

[0017] 2 is a perspective view of the support bearing member 10 seen from the back side. The support bearing member 10 has a groove 15 for supporting a pinion for each satellite screw, and has a bearing support portion 16 in front of the head as a recess into which the bearing step of the satellite screw or pinion can be inserted. The support portion of the satellite screw such as the pinion is lubricated via the polymer conveyed by the extrusion screw. The bearing support portion 16 is lubricated by the flow through the cone portion, so that a number of holes 17 are provided which each extend to the bearing support portion 16.

[0018] 2, the triangular or trapezoidal cross section of the tubular peripheral flow passage 13 can be clearly seen. Due to the apex of the triangular cross section or the short side of the trapezoid pointing towards the central axis and the wide base of the triangle being located on the outer circumferential surface, the space between the grooves 15 is optimally utilized for the pinion. The outlet openings 14 of the peripheral flow passages 13 are not located at the end of the respective support bearing member 10, but rather the peripheral flow passages 13 extend in the axial direction approximately the same distance as the pinion reaches.

[0019] The advantages of this arrangement are evident from FIG. 3, which shows an extrusion screw with the inlet metering section 30, the satellite screw support element 10 and the satellite screw 20 in a perspective view. In addition, a part of the rotor body 50 is shown adjoining the satellite screw support element 10. The grooves 15 of the support bearing element 10 are each adjoining grooves 52 in the rotor body 50, in which the satellite screws 20 are guided, the satellite screws 20 being located with their outer surfaces exposed. Between each groove 52, the rotor body 50 has a section of its own main screw web 51. Since the outlet openings 14 do not reach the end of the support bearing element 10, the melt emerging from the outlet openings 14 immediately flows transversely into the inlet region of the webs 22 of the satellite screws and the main screw web 51.

[0020] In Figure 4, the components of a multiple screw extruder 200 are shown in cross-sectional side view. Shown is the same section of the extruder screw shaft 100 shown in Figure 3, rotatably supported within an extruder housing 240 having an extrusion bore 241. The extruder housing 240 has a housing portion 242 which supports the cone section 11 and a housing portion 243 which has a reduced diameter to support the inlet metering section 30 of the extruder screw.

[0021] In the drive zone, a stator ring 244 is inserted into the extrusion bore 241, the stator ring 244 having an inner toothing with which the pinion 21 of the satellite screw 20 meshes. Furthermore, a retaining ring 245 is inserted, which limits the annular gap between the inner wall of the housing and the outer circumferential surface of the extrusion screw 100 at this position and makes it possible to adjust its width.

[0022] In Fig. 5, the multi-screw extruder 200 is again shown in a perspective cross-sectional view. The multi-screw extruder 200 is configured as a degassing extruder. The extruder housing 240 therefore comprises, in addition to the further housing components 242, 243, a connecting flange 247, which comprises a suction opening 248. The suction opening 248 is arranged at the beginning of the main screw web 51, i.e. the plastic melt can already be degassed downstream of the fan-out of the cone 11. Further suction openings can also be connected downstream.

[0023] The path of the plastic melt from the inlet metering section 30, through the cone section 11 and the circumferential flow passage 13 is shown by dashed arrows. It can be seen that the majority of the plastic melt flows past the stator ring 244, which has a row of teeth with which the pinion (not visible here) of the satellite screw 20 meshes.

Claims

1. An extrusion screw (100) for a multiple screw extruder (200), comprising at least an inflow metering section (30); a rotor body (50) having an enlarged diameter compared to said inlet metering section (30) and having a number of satellite screws (20); the satellite screw (20) is disposed on the outer periphery of the rotor body (50) in an exposed state over at least a portion of its length, In the extrusion screw (100), a conical section (11) and a drive zone following the conical section (11) are formed between the inflow metering section (30) and the rotor body (50), and in the drive zone, the satellite screws (20) are engaged, via drive pinions (21), with an outer tooth row provided on the rotor body (50), or with an inner tooth row provided on a stator ring (244) or an inner wall of an extruder housing (240) of the multiple screw extruder (200), 1. An extrusion screw (100), characterized in that at least one circumferential flow passage (13), closed in the circumferential direction, is formed between at least two adjacent grooves (15) for the drive pinion (21), said circumferential flow passage (13) running from an inlet opening (12) in the conical section (11) to an outlet opening (14) arranged downstream of the drive pinion (21) in the flow direction.

2. The drive zone is formed in a support bearing member (10) on which an end section of the satellite screw (20) having the drive pinion (21) is supported, 2. The extrusion screw (100) according to claim 1, characterized in that the outlet opening (14) of the peripheral passage (13) is not located at the end of the support bearing member (10) and the peripheral passage (13) extends in the axial direction approximately the same distance as the drive pinion (21) reaches.

3. The extrusion screw (100) of claim 2, characterized in that the support bearing member (10) has, for each satellite screw (20), one groove (15) supporting the drive pinion (21) and / or a bearing step portion or a bearing support portion (16) supporting a bearing attached to the end side of the satellite screw (20).

4. The support bearing member (10) is continuous with the rotor body (50), and the grooves (15) of the support bearing member (10) are each continuous with a groove (52) provided in the rotor body (50), The rotor body (50) has a portion of a main screw web (51) between each of the grooves (52), the peripheral flow passages (13) are each tubular over at least a portion of their length and extend from an inlet opening (12) in the conical section (11) to an outlet opening (14) arranged downstream of the drive pinion (21) in the flow direction and located in the inlet region of a web (22) of the satellite screw (20) and of the main screw web (51), The extrusion screw (100) of claim 3.

5. 5. The extrusion screw (100) according to claim 4, characterized in that the circumferential flow passage (13) is formed with a triangular or trapezoidal cross section, in which the apex of the triangle or the short side of the trapezoid is directed towards the centre line of the support bearing member (10) and the base located on the opposite side is arranged on the outer circumferential surface of the support bearing member (10).

6. 6. The extrusion screw (100) according to claim 3, wherein the outer peripheral surface of the support bearing element (10) is provided with at least one radial hole for each of the satellite screws (20), said hole extending into the groove (15) or into the bearing support (16) for the drive pinion (21).

7. A multiple screw extruder (200), characterized in that it has at least an extruder housing (240) with an extrusion hole (241) in which at least an extrusion screw (100) according to any one of claims 1 to 6 is rotatably supported.

8. A multi-screw extruder (200) comprising at least: - an extruder housing (240) with extrusion holes (241); an extrusion screw rotatably supported within said extrusion bore, said extrusion screw comprising an inlet metering section (30) and a rotor body (50) having an enlarged diameter compared to said inlet metering section (30) and carrying a number of satellite screws (20); the satellite screw (20) is disposed on the outer periphery of the rotor body (50) in an exposed state over at least a portion of its length, A multi-screw extruder (200) in which a conical section (11) and a drive zone following the conical section (11) are formed between the inflow metering section (30) and the rotor body (50), and in the drive zone, the satellite screws (20) are engaged with a stator ring (244) or an internal tooth row provided on the inner wall of the extruder housing (240) via drive pinions (21), 2. A multi-screw extruder (200), characterized in that in the housing wall in the region of the drive zone, at least one circumferential flow passage is formed between at least two adjacent drive pinions, in each case, the circumferential flow passage extending in the longitudinal direction from an inlet opening arranged upstream of the internal tooth row to an outlet opening arranged downstream of the internal tooth row.

9. 9. The multi-screw extruder (200) according to claim 8, characterized in that the circumferential flow passage is formed in a stator ring (244) and / or a retaining ring (245) inserted in the extrusion hole (241) in the drive zone.

10. 10. The multi-screw extruder (200) according to any one of claims 8 to 9, characterized in that an annular gap is formed between the outer circumferential surface of the extrusion screw (100) in the drive zone and the inner circumferential surface of the extrusion hole (241), the radial width of the annular gap being at most 5 mm.

11. 11. The multi-screw extruder (200) according to claim 10, characterized in that an annular gap is formed between the outer peripheral surface of the extrusion screw (100) in the drive zone and the inner peripheral surface of the extrusion hole (241), the cross-sectional area of ​​the annular gap being at most 20% of the sum of the cross-sectional areas of all the circumferential flow passages (13).

12. 12. The multi-screw extruder (200) according to claim 10 or 11, wherein the inner teeth are formed on a stator ring (244) inserted in the extrusion hole (241) in the drive zone, and a retaining ring (245) is attached to the upstream side of the stator ring (244) in the longitudinal direction, the retaining ring (245) defining the annular gap.

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