DEGASSING EXTRUDER HAVING A MULTI-SCREW UNIT AND METHOD FOR DEGASSING POLYMER MELTS THEREWITH

Abstract

A degassing extruder having a multi-screw unit, which degassing extruder comprises a housing having a feed region having a feed opening, an inner housing recess having an extraction opening extending as far as the outside and an outlet region having an outlet opening. The multi-screw unit rotatably arranged in the housing recess comprises: a rotor element having a main screw web extending over the outer circumference of a rotor shaft core, and a rotationally driven satellite screw, which is mounted in a receiving groove on the rotor element, which receiving groove extends at least along part of the length of the multi-screw unit. At least in the region of the extraction opening, the main screw web above the receiving groove has an respective opening recess for leading the satellite screw through.

Claims

1. A degassing extruder comprising: a housing having an inlet region with an inlet opening, an inner housing recess with a suction opening extending as far as the outside, and an outlet region with an outlet opening; a multi-screw unit that is rotatably arranged in the housing recess and comprises: a rotor element with at least one main screw flight which extends over the outer circumference of a rotor shaft core; and at least one rotationally driven satellite screw, which is mounted in a receiving groove on the rotor element, which receiving groove extends at least along part of the length of the multi-screw unit, wherein, at least in the region of the suction opening: the main screw flight above the receiving groove has a respective opening recess for passing through the satellite screw; the circumference of the satellite screw in the channels formed by the main screw flight is enclosed to an extent of at least 40% and at most 70% in the receiving groove in the rotor shaft core, and the degree of enclosure of the cross section of the satellite screw within the main screw flight is greater than in the channels outside of this and is at most 95%.

2. The degassing extruder as claimed in claim 1, wherein the degree of enclosure of the cross section of the satellite screw at least in one of the axial zones within the main screw flight or in the channels outside the main screw flight is greater than 50% in each case.

3. The degassing extruder as claimed in claim 1, wherein the flight of the satellite screw reaches to the outer circumference of the at least one main screw flight.

4. The degassing extruder as claimed in claim 1, wherein the center axes of the satellite screws are arranged on a pitch circle which is smaller than the diameter of the rotor shaft core.

5. The degassing extruder as claimed in claim 4, wherein more than 80% of the cross section of the shaft core of the satellite screws is arranged within the circumference of the rotor shaft core.

6. The degassing extruder as claimed in claim 1, wherein formed on the rotor element are at least three receiving grooves, in each of which a satellite screw is rotatably mounted.

7. The degassing extruder as claimed in claim 1, wherein the channel depth of the main screw flight is greater than the maximum channel depth of the receiving grooves.

8. The degassing extruder as claimed in claim 1, wherein the satellite screws are driven in rotation in the opposite direction to the direction of rotation of the rotor element and have an orientation which is opposite to the main screw flight.

9. The degassing extruder as claimed in claim 1, wherein the diameter D and the pitch t of the main screw flight and the respective opening width x of the opening recess are coordinated to one another in such a way that the inner wall of the housing recess is passed over completely by the main screw flight when the rotor element is rotating.

10. The degassing extruder as claimed in claim 1, wherein the ratio of the flight width d of the main screw flight to the channel width of the channel is less than 1:4.

11. A method for processing a polymer melt by means of a degassing extruder as claimed in claim 1, the method comprising: feeding a melt stream to the rotor element rotatably arranged in the housing recess and having at least one rotatably mounted satellite screw; surface-area distributing the melt stream on the circumference of the rotor element and the at least one satellite screw; discharging the polymer melt from the rotor element and from the satellite screw to at least one outlet channel; degassing the polymer melt by applying a vacuum to the suction opening; wherein the polymer melt distributed on the rotor element is conveyed over the length of the rotor element by means of at least one main screw flight arranged on the outer circumference of the rotor element, wherein, to loosen up the melt conveyed in the channels of the main screw flight, at least one satellite screw which is arranged in receiving grooves on the outer circumference of the rotor element is used, wherein the volumetric flow of the polymer melt fed to the multi-screw unit and the volumetric flow discharged therefrom are coordinated to one another in such a way that the conveying volume enclosed between the adjacent portions of the main screw flight, the outer side of the rotor element and the inner side of the housing recess is filled to an extent of less than 100% with polymer melt.

12. The method as claimed in claim 11, wherein, during the degassing, the conveying volume available in the channels is filled to an extent of less than 80% with polymer melt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0045] FIG. 1 shows a degassing extruder in a side view;

[0046] FIG. 2 shows a rotor element in a perspective view;

[0047] FIG. 3 shows a detail of a side view of a multi-screw unit;

[0048] FIG. 4 shows a schematic developed view of the circumference of the rotor element;

[0049] FIG. 5 shows the rotor element in section;

[0050] FIG. 6 shows the rotor element of a third embodiment in section; and

[0051] FIG. 7 shows the rotor element of a third embodiment in section.

DETAILED DESCRIPTION

[0052] FIG. 1 illustrates a degassing extruder 100 in a side view. Said degassing extruder comprises an inlet region 20, which has a long form in the exemplary embodiment illustrated, laterally alongside a housing 30. Said inlet region comprises a rotating screw shaft 21 in an internal inlet channel. Furthermore, on the other side of the housing 30, there follows an outlet region 40 with an internal outlet channel in which a rotating screw shaft 41 is likewise arranged. The housing 30 is illustrated in FIG. 1 as seen from that side which has two housing openings 32 next to one another which are arranged within a common flange area 31, to which in turn a vacuum suction line can be attached. Parts of a multi-screw unit 10 can be seen in the interior through the housing openings 32, shown in particular being the channel depth or flight height, enlarged to a quite substantial extent in comparison with the prior art, of a main screw flight 12, which extends over the outer circumference of a rotor element 11.

[0053] FIG. 2 illustrates the rotor element 11 in a perspective view. A shaft core 15 is surrounded on its outer circumference by the main screw flight 12. Moreover, in total eight receiving grooves 13 for satellite screws that are arranged offset by 45° to one another in each case are formed on the outer circumference. It can already be seen in FIG. 2 that a relatively large channel depth is provided in the channel 18 formed in the main screw flight 12.

[0054] This is also particularly clear in FIG. 3, which shows a detail of a side view of the multi-screw unit 10, specifically the end which is at the front in the flow direction and transitions into the discharge screw 41 at a transition cone 42. Only immediately in front of the transition cone 42 does the rotor element 11 have a channel 14.1 with a small depth. In the other areas to the right of this, the channel 14 is incised significantly deeper, the channel depth being measured radially from the outer circumference of the main screw flight 12 to the outer circumference of the shaft core 15.

[0055] The diameter of the rotor element 11 is denoted by D; t denotes the pitch of the main screw flight 12, the pitch t generally being specified as a dimensionless number which indicates the relation of the axial spacing of the flight portions at the same angular position with respect to the diameter D. In that case, the channel pitch is the distance, measurable on the main screw flight, from one flight edge to the next, measured at the same angular position, and is calculated as the product of diameter D and pitch t. The width of the channels 14 is consequently determined as the difference between the channel pitch D*t and a flight width d.

[0056] According to this definition, a pitch of t=1 means that the axial spacing, measured at the same angular position on the circumference, from one screw-flight leading edge to the next screw-flight leading edge is exactly the same as the diameter. For the purpose of polymer degassing, t<D, in order that the residence time of the polymer melt is long and the action of gas suction can develop. Furthermore, it can be clearly seen in FIG. 3 that the main screw flight 12 has an orientation in the opposite direction to the satellite screw flights 17. The rotor element 11 and the satellite screws 16 rotate in opposite directions, since they are toothed and directly in engagement with one another.

[0057] The features of the invention, described in relation to the cross section, with regard to the degree of enclosure of the satellite screws 16 are significantly associated with further features which relate to the profile of the rotor element in the longitudinal direction that can be seen in FIG. 2. In the case of a multi-screw unit equipped with the multi-screw unit 10 of the invention, the increased enclosure of the cross section of the satellite screws 16 compared to the prior art leads to a self-cleaning effect, since the main screw flight 12 scrapes over the entire length and the entire circumference of the housing recess on the inner walls in the housing and thus eliminates any adhering polymer residues.

[0058] The degrees of enclosure of the satellite screws 16 provided according to the invention accordingly result in the interruptions in the main screw flight 12 being short enough that the self-cleaning effect mentioned is provided. This relationship is explained with reference to FIGS. 4 and 5.

[0059] FIG. 5 shows a cross section of the rotor element 11. The main screw flight 12, which is interrupted for each of the eight receiving grooves 13, has a respective opening recess 12.1 above the receiving groove 13. The receiving groove 13 located at the top is illustrated as empty. The solid thick line characterizes the degree of enclosure of the satellite screws 16 within the channels 14, as in the axial regions between parallel portions of the main screw flight 12. The dashed line next to the receiving groove 13 on the right characterizes the degree of enclosure at the points where the satellite screws 16 pass through the main screw flight 12.

[0060] FIG. 4 shows a schematic developed view of the outer circumference of the rotor element 11 with the main screw flight 12 and opening recesses 12.1 for a satellite screw. The bore diameter or the external diameter D of the main screw flight 12 are determined in advance from aspects such as the desired throughput of the extruder or the viscosity of the polymer to be processed. The external diameter D is therefore regarded as a constant for the further structural design of the multi-screw unit. From this, the length of the circumference is obtained:

U=D.Math.π

[0061] The opening width x of the opening recess 12.1 is determined by the opening angle α (cf. FIG. 5):

[00001]$x=\alpha \frac{D}{2}$

[0062] The functional requirement in terms of the self-cleaning action of the multi-screw unit 10 determines that there must be a slight overlap in the axial direction between the edges 12.2 and 12.3, which delimit the opening recess 12.1; this is labelled in FIG. 4 as the overlap zone 12.4.

[0063] For the mathematical determination, it applies that the edges 12.2, 12.3 must be at least axially at the same height in order that no axial gap is obtained, because in the event of a gap, it would not be possible to clean that region of the inner wall of the housing recess which is passed over by this portion of the rotor element.

[0064] This results in the following relationship for the flight width d of the main screw flight 12 in relation to the opening angle α (see FIG. 5) and to the pitch t:

[00002]$d=\frac{\propto .Math.D.Math.t}{2\pi }$

[0065] The flight width d should be as small as possible in order that the conveying volume, determined by the flight width and flight height, of the channels 14 between the sections of the main screw flight 12 is as large as possible. As already stated above, the ratio of flight width d to channel width should be selected as follows:

[00003]$\frac{d}{D.Math.t-d}\le \frac{1}{4}$

[0066] In relation to the pitch t, the following is obtained for the flight width d:

[00004]$d\le \frac{D.Math.t}{5}$

[0067] Based on the flight width limitation of 20% of the pitch mentioned above, the following is obtained for the opening angle α:

[00005]$\propto \le \frac{2\pi }{5}$

[0068] Further relationships for the multi-screw unit 10 according to the invention are obtained from the sectional views in the following figures.

[0069] The multi-screw unit 10 is illustrated in section in FIG. 5, specifically in the region of the line IV-IV in FIG. 3. A pitch circle 19, which predefines the position of the centers of receiving grooves 13 and satellite screws 16, has a diameter which is approximately as large as the diameter of the shaft core 15 of the rotor element 11. Part of the core cross section of the satellite shafts 16* protrudes respectively beyond the circumferential line of the rotor shaft core 15. This is necessary in order, on the one hand, to keep the size of the satellite screws 16 limited in order that they do not develop any significant conveying action, and, on the other hand, to allow the satellite screw flights 17 to reach to the outer edge of the main screw flight 12 or at least to the vicinity thereof. Such a wide radial extent of the satellite screw flights 17 outward is selected in order that the proportions of the openings in the main screw flight that are not covered again by the projected cross-sectional area of the satellite screws including their flights 17 remain small. The resulting degree of enclosure E.sub.G1 of the satellite screws 16 in the channel is shown by a solidly drawn, arcuate line. In this example it is less than 50%. The channel depth of the channel 14 in the main screw flight 12 is denoted by T.sub.G1. At the same time, it can be seen in FIG. 4 that the proportion of the opening in the main screw flight 12 that is not covered again by the projected surface of the satellite screw 16 remains small.

[0070] FIG. 5 shows a very similar illustration of a multi-screw unit 10′ to that in FIG. 4. The external diameter of the main screw flight 12′, the external diameter of the satellite screws 16′ and the pitch circle 19′, on which receiving grooves 13′ and satellite screws 16′ are arranged, are each identical, for example according to FIG. 4. What is different in comparison to this is that the channel 14′ formed in the main screw flight 12′ has a greater channel depth T.sub.G2>T.sub.G1, which consequently lowers the diameter of the rotor shaft core 15′. The degree of enclosure E.sub.G2 in the channel 14′ that is characterized by the boldly drawn arcuate line is thus likewise smaller, whereas the degree of enclosure Est of the satellite screw 16′ remains constant as it passes through the main screw flight 12′.

[0071] Milled in on the shaft core 15″ of the multi-screw unit 10″ illustrated in FIG. 7 are five round receiving grooves 13″, which are arranged offset to one another by 72° on a common pitch circle 19″. In this example, the pitch circle 19″ is smaller than the diameter of the shaft core 15″. This achieves a situation in which the cross sections of the respective core regions of the satellite screws 16″ lie almost completely within the circumferential line of the rotor shaft core 15″; that is to say the conveying volume in the channel 14″ in the main screw flight 12″ is almost completely retained and is scarcely constricted by the satellite screws 16″.

[0072] The flanks of the receiving grooves 13″ formed in the rotor shaft core 11″ each extend over more than 180°. A degree of enclosure of more than 50% is achieved. As a result, the satellite screws 16″ are mounted in a form-fitting manner in the receiving grooves 13″. With the remaining proportion of their circumference, the satellite screws 16″ lie open within the channel 14″ in the main screw flight 12″. The satellite screw flights 17″ of the satellite screws 16 can therefore effectively loosen up the melt from the base of the channel 14″. Since at the same time the satellite screw flights 17″ reach to the outer circumference of the main screw flights 12″ and rotate in opposite directions, the circulation is particularly effective.

[0073] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.