Multi-screw system and process for processing polymer melts

Abstract

Multi-screw system (100) for processing polymer melt, at least comprehensively: - a housing (30) with o an inlet area (20) with an inlet opening, o a housing recess and o a discharge area (40) with a discharge opening; as well as - a polyrotation unit (10) which is rotatably arranged in the housing recess and which comprises at least: o a rotor element (11; 11'), with a main worm web (12; 12') extending over the outer circumference of a shaft core (15; 15') and o at least one satellite screw (16) rotatably mounted in a receiving groove (13; 13') which extends at least along at least a part of the length of the polyrotation unit (10; 10'), wherein the satellite screw (16) is arranged completely within the circumference of the main screw web (12; 12'); characterized in that - that the degree of enclosure of the satellite screw (16) varies over the length of the rotor element (11; 11'), wherein the satellite screws (16) have an orientation of their screw web opposite to that of the main screw web (12; 12'); - that the housing (30) has at least one intake opening (32) extending into the housing recess, which is to be connected to a vacuum pump and - that the satellite worms (16) are provided at their ends with pinions which are in direct engagement with a toothed ring which is formed or attached to the rotor element (11; 11') and are driven rotating in the opposite direction to the rotor element (11; 11').

Description

[0001] The invention relates to a multi-screw system for processing polymer melt with the features of claim 1 and a method for processing polymer melt therewith.

[0002] In the processing of polycondensates, especially polyesters, it is important to continuously remove the byproduct – usually water – in order to increase or at least maintain the polymer chain length and prevent further degradation of the molten polymer. This is particularly important in demanding manufacturing techniques such as the production of fine spun fibers, which require high polymer quality and, in particular, high intrinsic viscosity.

[0003] WO 2013 180 941 A1 describes a process for producing bulk continuous carpet filament (carpet-BCF) from recycled polyester. The process involves splitting a stream of molten polymer into at least two separate streams. These streams are to be degassed separately by applying and maintaining a vacuum in separate extruders before being recombined into a single melt stream, which is then fed directly to a downstream spinning unit. However, splitting the process into multiple streams processed in parallel extruders results in significant equipment complexity in terms of cost, space requirements, synchronization, etc.To carry out the process, a multi-screw extruder is further described as an exemplary embodiment. This extruder is intended to replicate the function of several separate extruders in a single unit and to degas the separated melt streams independently of one another before they are recombined. For this purpose, a multi-rotation unit rotates around a central axis, in which several satellite screws are rotatably mounted. A housing with an opening connected to a vacuum pump is provided for degassing. While the division into separate partial melt streams has the advantage over a single stream of increasing the polymer surface area available for gas exchange,The degassing of the individual melt streams, which are guided via the satellite screws, can only occur during the brief moment when, during the rotation of the polyrotation unit, the respective satellite screw with its partial melt stream passes the opening in the housing. It remains unclear how a consistently low pressure is to be maintained for degassing in each partial melt stream, given that the corresponding satellite screw is only briefly moved through the housing opening and is then outside the vacuum for the remainder of the rotation.

[0004] WO 2003 033 240 A1 describes a multi-screw system comprising several satellite screws. This significantly increases the surface area of ​​the polymer melt and considerably improves melt quality. Crossflow across the rotor element, from one satellite screw to the next, is possible and desirable. The satellite screws are responsible for the majority of the polymer melt conveying, while a shallow screw web on the rotor element contributes a smaller portion, primarily serving to surround the rotor element with melt, thus providing lubrication within the housing.

[0005] The object of the present invention is to provide a multi-screw system with which the quality of the polymer melt processed therein can be increased. In particular, a large polymer surface area is to be generated and / or a high surface exchange in the polymer melt is to be achieved.

[0006] This problem is solved by a multi-screw system with the features of claim 1.

[0007] In the context of the present invention, the “enclosure degree” is defined as the ratio between the proportion of the circumference of the satellite screw which is shielded by walls of the rotor element and its main screw web, in relation to the total circumference.

[0008] The approach according to the invention is exactly contrary to the prior art described above in WO 2013 180 941 A1. The invention does provide for a similar design with a rotor element rotating in a housing bore and with several satellite worms rotating independently therein, each with at least one worm helix on the outer circumference.

[0009] In every case, the conveying action is achieved predominantly by at least one main screw web, which is formed on the outside of the rotor element. The main screw web is positioned high enough above a shaft core of the rotor element to ensure conveying action, making the multi-screw system suitable as a replacement for a single- or twin-screw extruder in melt preparation and conveying in a plastics processing plant.

[0010] In contrast, the conveying effect of the satellite screws is significantly reduced or even negligible. Instead, the satellite screws serve to circulate, loosen, and mix the polymer melt. This is due to the fact that, in the multi-screw system according to the invention, the satellite screws have varying degrees of enclosure along their length. Where the satellite screws pass through the main screw web, they are enclosed by at least 80% of their circumference. This allows only minimal flow along the rotor element through the remaining gaps between the main screw web and the satellite screw. The conveying effect of the main screw web is maintained. In the length between adjacent sections of the main screw web, however, the satellite screws are significantly less enclosed by the receiving groove in which they are arranged and are at least half exposed.Thus, they no longer form a conveying element like in an extruder, but are mixing elements for the polymer melt.

[0011] With its large diameter and circumference, over which at least one nearly continuous main screw web runs, the rotor element offers a large surface area for spreading the melt stream. This promotes, for example, the degassing of the polymer melt when it is guided past a housing opening where a vacuum is applied. Furthermore, the screw flights located between adjacent sections of the main screw web carry a uniform melt stream instead of many individual streams. Due to the rotation of the rotor element, this uniform melt stream is guided past the vacuum-controlled housing opening multiple times. This results in a defined residence time within the vacuum zone.

[0012] According to the invention, as already explained, the satellite screws do not serve to convey individual molten streams. Rather, they are positioned so openly within the screw channel of the rotor element that they have little conveying effect, but primarily cause a circulation of those portions of the molten metal located at the bottom of the screw channels of the main screw web. The satellite screws thus loosen, mix, and relocate individual components of an otherwise uniform molten stream flowing through the channel of the main screw, ensuring that the degassing effect of the vacuum is constantly present in the entire molten metal stream conveyed by the rotor element and not just in the outermost components.

[0013] The following considerations apply to the selection of the number of satellite snails: - If the multi-screw system is primarily used for mixing several components or homogenizing one component, the mixing effect can be increased by increasing the number of satellite screws. - If the focus is on degassing, the mixing effect influences the degassing efficiency. With an increased number of mixing elements, the required partial pressure in the gas space can be increased, i.e., the pressure in the vacuum zone can be increased. - In the case of fumigation, the influence of the number of mixing elements is correspondingly reversed.

[0014] Given a specific application, the number of satellite screws is determined, and the desired optimal pressure range for the process is achieved through a simple experiment by gradually applying a pressure from less than 0.1 mbar to over 1 bar and analyzing the product treated in this way for several test stages.

[0015] The cross-sectional area in the main screw channel is selected in relation to the nominal volume flow rate for the degassing extruder such that the cross-section is not completely filled during normal operation. This allows the suction effect of the vacuum to indirectly extend to all those circumferential areas of the rotor element that are not directly adjacent to the housing opening.

[0016] A key aspect of the invention is that the main screw web has only a small opening at the point where the satellite screws pass through it. Either the upper edge of the main screw web is uninterrupted, so that it extends over the satellite screw like a bridge, or it has an opening that is filled by the cross-section of the satellite screw. Here, the satellite screw is enclosed by cross-sectional areas of the rotor element, including the main screw web, for 80% of its circumference. This means that the remaining area of ​​the opening in the main screw web is at most approximately 20% larger than the projected cross-sectional area of ​​the opening that is covered by the satellite screws.The vast majority of the opening is covered, maintaining the conveying effect of the main screw flight; that is, the melt inevitably follows the path of the main screw flight and is guided past the housing opening multiple times. However, the melt can hardly flow forward in a secondary flow over the main screw flight and skip sections of the main screw.

[0017] When considering the axial course of the satellite screws, an alternating sequence of sections emerges in which the respective satellite screw is almost completely surrounded by the main screw web and at the same time by the flank of a receiving groove in the rotor element, and in which the satellite screw is only partially enclosed by the receiving groove and is otherwise free.

[0018] Another important feature of the degassing extruder according to the invention is the coordination of the following parameters: - Outer diameter of the rotor element, measured at the top edge of the main screw web; - Height of the main snail ridge; - Diameter of the pitch circle on the rotor element on which at least one satellite screw is arranged, and - Diameter of the satellite worm webs.

[0019] Even a single satellite screw improves the mixing and surface area of ​​the polymer melt. Preferably, at least three satellite screws are arranged on the rotor element at uniform angular intervals.

[0020] The advantages achieved with the invention consist not only of improved mixing and degassing. Furthermore, the shear of the polymer is reduced. The reduction of mechanical shear alone leads to improved melt quality. In addition, lower shear reduces the heat input into the melt, thus preventing overheating.

[0021] Because the satellite screws protrude a large proportion of their surface area from the rotor element, the surface area of ​​the main screw is correspondingly increased. A larger surface area leads to greater dispersion and improves the degassing effect.

[0022] The sum of the diameter of the pitch circle and the diameter of the satellite worm webs should not exceed the outer diameter of the rotor element. This means that the worm webs never extend radially beyond the top edge of the main worm web. As a result, the gap between the outer edge of the main worm web and the housing recess can be kept very small.

[0023] On the other hand, the pitch circle and diameter of the satellite worms must be chosen so that their webs extend sufficiently beyond the bottom of the worm passages in the main worm web to achieve their effect.

[0024] Preferably, the arrangement is such that the satellite worms are surrounded by the receiving groove with a maximum of 50% of their circumference outside of those points where they are guided through the main worm web, and otherwise lie freely in the worm channel outside of it.

[0025] It is further preferred that, when using satellite screws, the screw web has an opposite orientation and rotation compared to the rotor element. This allows the satellite screws to be equipped at their ends with pinions that mesh directly with a toothed ring formed or attached to the rotor element.

[0026] When using the multi-screw system according to the invention for processing polymer melt, at least the following process steps are provided: - Supplying a molten stream to the rotor element rotatably arranged in the housing recess, which has several rotatable satellite screws around its circumference; - area distribution of the melt flow on the circumference of the rotor element and on the satellite screws by rotation of the rotor element relative to the housing; - Discharge of the polymer melt from the rotor element and from the satellite screws to at least one outlet channel.

[0027] The polymer melt distributed on the rotor element is conveyed along the length of the rotor element by means of at least one main screw web arranged on the outer circumference of the rotor element. If several satellite screws are present, this also allows for the exchange of polymer melt between the adjacent satellite screws.

[0028] In this process, it is advantageous if the volume flow rate of the polymer melt supplied to the multi-screw system and the volume flow rate discharged from it are coordinated such that the conveying volume enclosed between the adjacent sections of the main screw web, the outer surface of the rotor element, and the inner surface of the housing recess, or the conveying cross-section considered in the longitudinal section of the rotor element, is less than 100% filled with polymer melt, and in particular less than 80%. Reducing the so-called fill level ensures that sufficient space is available to swirl the polymer melt through the satellite screws, increase the surface area, and intensify the mixing of adjacent portions of the melt stream.

[0029] The processing method using a multi-screw system according to the invention can be used to process, in particular, the following polymer melts: - Polyester for various applications, in particular polyester in different fiber forms such as bulk continuous filament (BCF), which is suitable for carpet manufacturing. The polyester treated in a multi-screw system according to the invention can be directly introduced into a spinning process. - Polyamide. - Polystyrene, which is mixed with butane, in particular to form a polymer foam, without applying a vacuum in this case.

[0030] The invention is explained in more detail below with reference to the drawings. The figures show, in detail: Fig. 1 a multi-screw system in side view; Fig. 2 the multi-screw system in perspective view; Fig. Three parts of a rotor element in perspective view; Fig. 4. Sectional view of parts of the rotor element according to a first embodiment; and Fig. 5 parts of a rotor element according to a second embodiment in section.

[0031] In Fig. Figure 1 shows a side view of a multi-screw system 100 according to the invention. This system comprises, in addition to a housing 30 in the center, an inlet area 20, which in the illustrated embodiment is long and has a rotating screw shaft 21 in an internal inlet channel. Furthermore, an outlet area 40 with an internal outlet channel adjoins the housing 30 on the other side, in which a rotating screw shaft 41 is also arranged. The housing 30 is shown from the side that has two housing openings 32 next to each other, which are arranged within a flange area 31 to which a vacuum suction line can be attached. Through the housing openings 32, parts of a polyrotation unit 10 are visible inside, in particular the significantly increased channel depth compared to the prior art.The height of the web of a main screw web 12, which extends over the outer circumference of a rotor element 11, is noticeable.

[0032] Fig. Figure 2 shows parts of the multi-screw system 100 in perspective view, with the housing 30 removed and only indicated to allow a view into the interior of the housing 30 and the polyrotation unit 10 located there.

[0033] The polyrotation unit 10 comprises the rotor element 11 with the main screw web 12. The rotor element 11 thus forms itself an extruder screw with a large outer diameter and wide, deeply cut screw flights and a corresponding conveying effect.

[0034] In addition, several satellite screws 16 are arranged around the circumference, parallel to the rotor element 11. Some of the satellite screws 16 are recessed into receiving grooves in the rotor element 11, while others protrude from them. In this embodiment, they are positioned so deeply that they penetrate the main screw web 12 at the recesses, but the area around the upper edge of the main screw web 12 remains uninterrupted radially outside each recess.

[0035] The orientation of the respective helical spirals on the satellite spirals 16 is in particular opposite to the main spiral web 12.

[0036] Fig. Figure 3 shows the rotor element 11 alone in a perspective view. A shaft core 15 is surrounded on its outer circumference by the main worm web 12. In addition, a total of three receiving grooves 13, offset from each other by 120°, are formed on the outer circumference. These grooves are open in the worm flights and are supplemented by additional recesses 14 in the main worm web 12. The main worm web 12 is located in the Fig. 3 illustrated embodiment of a rotor element 11 is not completely interrupted at any point along its entire course.

[0037] The essential relationships at the polyrotation unit 10 according to the invention are shown in the sectional view in Fig. 4.

[0038] Three receiving grooves 13 are milled into the shaft core 15 of the rotor element 11, which are arranged offset by 120° to each other on a common pitch circle 19: - The lower left intake groove 13 and an adjacent recess 14 in the main screw web 12 are in Fig. Figure 4 is shown open for illustration. A line 13.1 marks the flank of the opening formed by the receiving groove 13. - In the upper intake groove 13, the shape of the recess 14 in the main screw web 12 is marked by hatching. - In the lower right receiving groove 13, a satellite screw 16 with a screw bridge 17 is inserted into the receiving groove 13.

[0039] The centers of the recesses 13 and the satellite worm 16 lie on the pitch circle 19, which is slightly smaller than the diameter of the shaft core 15. This ensures that the flank 13.1 of the receiving groove extends over approximately 180°, so that the satellite worm 16 is exactly half covered and the other half is not enclosed within the worm webs of the main worm web 12, i.e., it lies open. At the points where the satellite worm 16 is to pass through the main worm web 12, the additional recess 14 must be provided in the main worm web 12. This recess is crescent-shaped and, together with the opening cross-section of the receiving groove 13, forms an open circular cross-section. This circular cross-section is almost completely filled by the projected cross-sectional area of ​​the satellite worm 16 together with its worm web 17.This means that - apart from a small annular gap at the circumference of the satellite snail 16, which ensures its unimpeded rotation - there is no remaining opening that would allow an uncontrolled longitudinal flow through the main snail web 12.

[0040] In Fig. Figure 5 shows a further embodiment of a polyrotation unit 10' in section. The difference from the previously described variant is that a pitch circle 19', which defines the position of the centers of the receiving grooves 13' and satellite worms 16', has a diameter larger than the diameter of the shaft core 15'. With otherwise identical geometric relationships between the shaft core diameter, satellite worm diameter, and the height of the main worm web, this results, firstly, in a shallower receiving groove 13'. Its flank 13.1' extends over less than 180°, so that correspondingly less than half the circumference of the satellite worm 16' is enclosed. Secondly, the satellite worm 16' moves radially outwards to such an extent that the outer edge of its own worm web 17 exactly touches the outer circumferential line of the main worm web 11'. This represents the maximum outer radial position of the satellite worm 16'.Due to the outward displacement of the satellite screw 16' and the reduced cross-section of the receiving groove 13', the recess 14' in the main screw web 12' must be correspondingly larger. Furthermore, the recess extends all the way to the outside, so that the main screw web 11' is completely interrupted by the recess 14'. However, even in this embodiment, the aim is to minimize the remaining opening areas in the main screw web 11'. Therefore, the illustrated embodiment provides an omega-shaped contour for the recess 14', which remains as close as possible to the outer circumference of the satellite screw 16'. The Ω-shaped contour is particularly evident at the recess 14' at the bottom left. Fig. 5 recognizable.

Claims

[1] Multi-screw system (100) for processing polymer melt, comprising at least: - a housing (30) with o an inlet area (20) with an inlet opening, o a housing recess and o a discharge area (40) with a discharge opening; as well as - a polyrotation unit (10) which is rotatably arranged in the housing recess and which comprises at least: o a rotor element (11; 11'), with a main worm web (12; 12') extending over the outer circumference of a shaft core (15; 15') and o at least one satellite screw (16) rotatably mounted in a receiving groove (13; 13') which extends at least along at least a part of the length of the polyrotation unit (10; 10'), wherein the satellite screw (16) is arranged completely within the circumference of the main screw web (12; 12'); characterized by , - that the degree of enclosure of the satellite screw (16) varies over the length of the rotor element (11; 11'), wherein the satellite screws (16) have an orientation of their screw web opposite to that of the main screw web (12; 12'); - that the housing (30) has at least one intake opening (32) extending into the housing recess, which is to be connected to a vacuum pump and - that the satellite worms (16) are provided at their ends with pinions which are in direct engagement with a toothed ring which is formed or attached to the rotor element (11; 11') and are driven rotating in the opposite direction to the rotor element (11; 11'). [2] Multi-screw system (100) according to claim 1, characterized by , that the main snail web (12; 12') has a recess (14; 14') above a receiving groove (13; 13') for the passage of the satellite snail (16) through the main snail web (12; 12'). [3] Satellite screw (100) according to claim 2, characterized by , that the satellite mixing element (16) is enclosed to more than 80% when passing through the main screw web (12; 12'). [4] Multi-screw system (100) according to one of claims 1 to 3, characterized by , that the upper edge of the main snail web (12) is not interrupted at at least one recess (14) and extends in a bridge-like manner over the satellite snail (16), wherein the recess (14) in the main snail web (12) has a crescent-shaped cross-sectional area. [5] Multi-screw system (100) according to one of claims 1 to 3, characterized by , that the upper edge of the main snail web (12') on the satellite snail (16) is interrupted by a recess (14'). [6] Multi-screw system (100) according to claim 5, characterized by , that the boundary of the recess (14') in the main snail web (12') is Ω-shaped. [7] Multi-screw system (100) according to one of the preceding claims, characterized by , that the degree of enclosure of the satellite snail (16) outside the passage through the main snail web (12; 12') is less than 50% and the receiving groove (13; 13') is not undercut. [8] Multi-screw system (100) according to one of the preceding claims, characterized by , that the pitch circle (19) of the receiving groove (13) is smaller than the diameter of the shaft core (15). [9] Multi-screw system (100) according to any one of claims 1 to 8, characterized by , that the pitch circle (19') of the recording groove (13') is larger than the diameter of the shaft core (15'). [10] Multi-screw system (100) according to any one of the preceding claims, characterized by , that several receiving grooves (13; 13') are formed on the rotor element (11; 11') and several satellite worms (16) are rotatably mounted on the rotor element (11; 11'). [11] Multi-screw system (100) according to any one of the preceding claims, characterized by , that the height of the main snail stave (12; 12') is greater than the height of the respective stave (17) on the satellite snail (16). [12] Multi-screw system (100) according to any one of the preceding claims, characterized by , that the height of the main snail rib (12; 12') is greater than the radius of the satellite snail (16). [13] Multi-screw system (100) according to one of the preceding claims, characterized by , that the height of the main worm web (12; 12') is greater than the maximum passage depth of the receiving grooves (13; 13'). [14] Method for processing polymer melt with a multi-screw system (100) according to one of the preceding claims, comprising at least the following steps: - Supplying a molten stream to the rotor element (11; 11') rotatably arranged in the housing recess, which has at least one rotatable satellite screw (16) on its circumference; - planar distribution of the melt flow on the circumference of the rotor element (11; 11') and on the at least one satellite screw (16); - Discharge of the polymer melt from the rotor element (11; 11') and from the satellite screw (16) to at least one outlet channel, - Degassing the polymer melt by applying a vacuum to the housing recess; wherein the polymer melt distributed on the rotor element (11; 11') is conveyed over the length of the rotor element (11; 11') by means of at least one main screw web (12; 12') arranged on the outer circumference of the rotor element (11; 11') and wherein at least one satellite screw (16) is used which is partially exposed on the outer circumference of the rotor element (11; 11'). [15] Method according to claim 14, characterized by , that the volume flow of the polymer melt supplied to the multi-screw system (100) and the volume flow discharged from it are coordinated such that the conveying volume enclosed between the adjacent sections of the main screw web (12; 12'), the outside of the rotor element (11; 11') and the inside of the housing recess is less than 100% filled with polymer melt. [16] Method according to claim 15, characterized by that the conveying volume is less than 80% filled with polymer melt.

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