Screw elements with improved mixing effect and improved heat transfer, and the use of same

Abstract

The invention relates to a pair of three-lead screw elements for a multi-shaft screw machine having screw shafts rotating in the same direction and at the same speed. The two screw elements of the pair of screw elements according to the invention, opposing each other directly adjacently on two directly adjacent screw shafts, are mutually cleaned in a practical manner. The invention also relates to the use of the pair of screw elements according to the invention in a multi-shaft screw machine, to a multi-shaft screw machine equipped with a pair of screw elements according to the invention, and to a method for extruding plastic or viscoelastic compounds using the pair of screw elements according to the invention.

Description

[0001]2018PF30224a Screw elements with improved mixing effect and improved heat transfer. The invention relates to a pair of triple-flighted screw elements for a multi-screw extruder with screw shafts rotating in the same direction and at the same speed. The two screw elements of the pair of screw elements according to the invention, which are located directly adjacent to each other on two directly adjacent screw shafts, essentially clean each other. The invention also relates to the use of the pair of screw elements according to the invention in a multi-screw extruder, as well as to a multi-screw extruder equipped with a pair of screw elements according to the invention, and to a method for extruding plastic or viscoelastic masses using the pair of screw elements according to the invention.For the purposes of the present invention, a multi-screw extruder is understood to mean a screw extruder with more than one screw shaft, for example a screw extruder with two, three, or four screw shafts or an extruder with eight to sixteen, in particular twelve, screw shafts arranged in a ring. With more than two screw shafts, the axes of rotation of the screw shafts can be arranged side by side or, for example, as in a so-called ring extruder, in a ring to one another. In multi-screw extruders, the axes of rotation of the screw shafts are generally arranged parallel to one another. This parallel arrangement of the axes of rotation is also preferred according to the invention. The screw elements according to the invention are preferably arranged opposite one another on the screw shafts in a number that corresponds to the number of screw shafts of the respective extruder.Such a screw extruder with more than one screw shaft is also referred to below as a multi-screw extruder, multi-screw extruder, or multi-screw extruder. A twin-screw extruder is also referred to below as a twin-screw extruder. For the purposes of the present invention, the term "screw extruder" is used synonymously with the term "extruder." Modern extruders have a modular system in which various screw elements can be mounted on a core shaft to form a screw shaft; such a screw shaft is therefore segmented. This allows the expert to adapt the extruder to the respective process task. However, a screw shaft can also be made from a single piece, i.e., have only one screw element that extends essentially over the entire length of the screw shaft, or be only partially segmented.The present invention relates both to 2018PF30224a screw elements that can be mounted on a core shaft and to the screw shafts described above, which are manufactured from a single piece. Co-rotating twin-shaft screw extruders, whose screw shafts precisely clean each other, have long been known, e.g., from DE 862668 C. In polymer production and processing, screw extruders with screw shafts, whose screw elements are based on the principle of precisely cleaning screw cross-sectional profiles, have been used in a variety of ways. This is primarily due to the fact that polymer melts adhere to surfaces and degrade over time under normal processing temperatures. This is prevented by the self-cleaning effect of pairs of screw elements that precisely clean each other in multi-shaft screw extruders.Rules for generating screw cross-sectional profiles for screw elements that precisely clean each other are presented, for example, in [1] ([1] = Klemens Kohlgrüber: “The co-rotating twin-screw extruder”, 2nd edition, Hanser Verlag Munich 2016, pages 107 to 120). It is also described here that for screw elements that precisely clean each other, a predetermined screw cross-sectional profile on the first shaft of a twin-screw extruder determines the screw cross-sectional profile on the second shaft of the twin-screw extruder ([1], page 108). A screw cross-sectional profile, also referred to as screw profile for short in the context of this invention, is understood to be the outer contour of a screw element in a planar section perpendicular to the axis of rotation of the screw element, corresponding to the axis of rotation of the associated screw shaft. The screw profile for the screw element on the first shaft is referred to as the generating screw profile.The screw profile for the screw element on the second screw shaft follows from the screw profile of the first screw shaft of the twin-screw extruder and is therefore referred to as the generated screw profile. In a multi-screw extruder, the screw element with the generating screw profile and the screw element with the generated screw profile on adjacent shafts are always used alternately. A person skilled in the art of screw elements will, of course, understand that a single screw element or screw profile cannot be exactly raking or practically raking on its own, but that a pair of such elements is always required.A distinction must be made here between two things: the precisely cleaning screw profile, a mathematical construct in which two screw elements, which are located opposite one another on two directly adjacent screw shafts, clean one another without any play, and screw profiles for screw elements which are actually used as intended, i.e. technically designed screw elements. 2018PF30224a It is known to those skilled in the art and is stated, for example, in [1], on pages 39 to 41 and on pages 113 to 121, that technically designed screw elements must have clearances - namely both screw element-screw element clearances ^ and screw element-inner wall of the housing bore ^ (referred to in the context of the present invention as gap between screw element and inner wall of the housing bore ^) - in order to ensure the functionality of the extruder.This is necessary to avoid metallic "seizure", manufacturing tolerances, roughness, angular deviations as well as uneven thermal expansion and excessive extrudate stress due to insufficient distances between two screw elements that are located directly adjacent to each other on two directly adjacent screw shafts. The aforementioned pages also explain methods for determining the exact geometry of the element to be manufactured from the clearances and the exact scraping contour. These methods are called clearance strategies. For the purposes of the present invention, a clearance - also designated by ^ in the context of the present invention - is understood to be the distance between the closest points on the screw profiles of two screw elements that are essentially cleaning each other. The rules mentioned in [1] for producing screw profiles with clearances are applicable to the screw elements according to the invention.For the purposes of the present invention, a gap - also designated by ^ in the context of the present invention - is understood to be the distance between a crest of a screw profile and the closest point on the inner wall of the barrel bore of an extruder. The inner wall of the barrel bore of an extruder is also referred to below as the inner barrel wall for short. If the term "precisely cleaning" is used in the context of the present invention, this means - unless otherwise stated - the mathematical construct of a precisely cleaning screw profile or the corresponding screw element having this screw profile. If the term "practically cleaning" is used in the context of the present invention, this means - unless otherwise stated - the technically designed screw element orIts screw profile, wherein this practically cleaning screw profile was derived from a precisely cleaning screw profile, preferably by applying one of the following strategies: increased axial distance, longitudinal section equidistant, circular equidistant, or spatial equidistant, particularly preferably by applying one of the following strategies: longitudinal section equidistant, circular equidistant, or spatial equidistant, as explained in more detail below. 2018PF30224a The longitudinal section equidistant, circular equidistant, and spatial equidistant strategies are also referred to below as the longitudinal section equidistant calculation rule, circular equidistant calculation rule, and spatial equidistant calculation rule. Multi-screw extruders, in particular twin-screw extruders, are known to introduce mechanical energy into an extrudate through dissipation.This has both desirable and undesirable consequences because, on the one hand, the energy input is required to perform process-related tasks such as mixing and devolatilization, and on the other hand, the introduced mechanical energy is consumed and also leads to temperature increases in the extrudate, which can lead to undesirable chemical reactions that damage the extrudate. Mixing is also known to be a basic operation in multi-screw extruders, particularly twin-screw extruders. Inhomogeneities in the extrudate due to incomplete mixing are known to lead to problems in further processing of the extrudate and in the final properties. In a twin-screw extruder, the extrudate is sheared particularly strongly between a screw crest and the inner wall of the barrel bore of an extruder. This is where a particularly large amount of energy is dissipated into the extrudate, leading to severe local overheating in the extrudate.This is shown, for example, in [1] on pages 416 to 423, figures 4.80 to 4.84. This local overheating can lead to damage in the extrudate, such as changes in smell, color, chemical composition or molecular weight, or to the formation of inhomogeneities in the extrudate, such as gel bodies or specks. A large crest angle, and in particular a large sum of the crest angles of a pair of screw elements that are located opposite each other on immediately adjacent screw shafts and thereby scrape each other, is harmful in this case. Screw elements that aim to improve the mixing effect have long been the subject of technical development. Numerous known geometries neglect the fact that screw elements should advantageously clean each other, as is the case, for example, with all variants of toothed mixing elements, for example DE 4134026 A1, DE 19706134 A1 or WO 2004009326 A1.This class of mixing elements is essentially characterized by the fact that a screw thread is equipped with openings or grooves that interrupt the material transport and ensure improved mixing. However, the surfaces in the openings or grooves are not kinematically cleaned, so that material can adhere to these areas, degrade, and become a source of contamination for the extrudate – in this case, the plastic or viscoelastic mass to be extruded. However, mixing elements with complete self-cleaning have also been known for a long time. For example, DE 940 109 C already disclosed three-flight kneading disks that provided an improved mixing effect compared to continuously running screw flights. DE 3 412 258 A1 teaches how to design gaps between the screw crests and the inner barrel wall for three- and four-flight screw elements for twin-screw extruders.The extrudate is sheared in a targeted manner by varying the gap at the screw crests. For this purpose, a symmetrical arrangement of three- or four-flight screw profiles of a twin screw, which a priori have the same gap S (referred to therein as ^) to the inner barrel wall on all screw crests, is displaced parallel from the centers of rotation with an eccentricity ^ that is less than ^. EP 2131 A1 discloses a method for producing pairs of self-cleaning screw elements, wherein the individual screw crests of these screw elements have a different gap to the inner barrel wall. The gap width of an individual screw crest can be increased to up to half the flight depth ^. The aim here, too, is to create a material exchange between the individual screw flights and to shear the material in a targeted manner as it passes over the screw crest.The resulting two-flight screw elements have no axes of symmetry and different crest angles at the two screw crests. Three- and four-flight screw elements are also subject to testing. In the three-flight screw elements, the gap is increased at one screw clearance (Figure 4). The flight depth ^ is defined as half the distance, which is the difference between the outer diameter da of the screw element and the core diameter di of the screw element, i.e. ^ = (^^ – ^^) / 2. This application teaches nothing about preventing extrudate damage or improving heat transfer. DE 4239220 A1 describes three-flight screw elements that have different gaps and different crest angles at the three crests, with the screw crest with the smallest gap to the inner barrel wall having the largest crest angle.This allows the design of three-flight screw profiles with a ratio of outer diameter da to core diameter di of greater than 1.366. However, screw elements designed according to this design principle are disadvantageous because the screw crest with the narrowest gap and simultaneously the largest crest angle creates a zone of high shear stress for the polymer being processed, in which damage can easily occur due to the high shear and temperature stress. WO 0209919 A2 describes, among other things, three-flight and four-flight screw elements, wherein the crest angles can be designed differently on each screw crest of a three- or four-flight screw element. However, WO 02 09 919 A2 does not teach which embodiments are advantageous in terms of their mixing and dispersing effect or their behavior during pressure buildup.The design specification for these elements is limited to transition elements (referred to there as "fractional elements") between single- and triple-flighted elements or between two- and four-flighted elements, which limits the design freedom. EP 1 093 905 A2 describes screw profiles for triple-flighted, pairwise self-cleaning screw elements for twin-screw extruders with high distributive and dispersive mixing efficiency. However, the screw elements described there have the disadvantage of having a wide crest angle at the point with the narrowest gap to the inner barrel wall, resulting in a zone of high energy dissipation and a high local temperature peak, which can cause damage to sensitive polymers.The object of the invention was to provide a screw element with which an improved mixing and dispersing effect compared to the prior art can be achieved while simultaneously achieving good heat transfer. The pair of triple-flighted screw elements according to the invention should also provide the extrudate in the barrel bore with more volume than conventional triple-flighted screw elements. Furthermore, the two screw elements of the pair of screw elements according to the invention should practically scrape each other off when used as intended. Surprisingly, it has now been found that this object is achieved by a pair of screw elements having the features of the main claim. Within the scope of the present invention, the following terms apply: A screw profile is a closed convex curve.A worm profile is made up of several different curves which, depending on their geometric properties, are referred to as a "crest", a "flank" or a "groove". A crest is always adjacent to a flank on both sides. A groove is always adjacent to a flank on both sides. In this case, crests and grooves, each separated from each other by a flank, always alternate in a worm profile and follow the same direction. This creates the sequence crest – flank – groove – flank – crest – etc. A curve is an uninterrupted line with a length but no width, whereby a curve has a first endpoint and a second endpoint that are not the same point; i.e. the first endpoint does not coincide with the second endpoint.A curve can be composed of several curve segments, where a first curve segment has a common point of contact with a second curve segment that is immediately adjacent to the first curve segment. However, a curve can also consist of exactly one curve segment. A curve segment is a segment of a curve, where the curve segment has a first endpoint and a second endpoint that are not one and the same point; that is, the first endpoint does not coincide with the second endpoint. The mathematical expressions underlying a curve segment are preferably selected from the group of mathematical expressions comprising the following members: circular arc, elliptical arc, parabolic arc, longitudinal section equidistant calculation rule according to [1], pages 117 to 121, circular equidistant calculation rule, and spatial equidistant calculation rule.To achieve a constant clearance during mutual cleaning of the screw elements, the longitudinal equidistant calculation rule or the circular equidistant calculation rule is preferred. For a curve section, ^ applies. ^ also that it is a line which, in a parametric representation of its arc length ^ can be represented and at the ^ ^,^ ( ^ ) and ^ ^,^(^) are analytic functions and ^ and ^ are the coordinates of the line in the plane, and thus representable by infinite power series, continuous, arbitrarily differentiable, and thus kink-free. The longitudinal equidistant calculation rule is disclosed in [1], pages 117 to 121. The circular equidistant rule assumes an exactly scraping screw profile in the xy plane of a Cartesian coordinate system, with a perpendicular to the pivot point DP being drawn at each point of the screw profile. The point that is shifted by half the play along this perpendicular to the pivot point then belongs to the technically implemented screw profile. If, for example, a section of an exactly scraping screw profile is a circular arc with a radius ^ ^ the corresponding section of the corresponding technically designed screw profile is a circular arc with the same center point and the radius The spatial equidistant is mentioned in [1], page 41; a spatial equidistant can be obtained, for example, through a parametric representation. The spatial equidistant is explained in detail in the international patent application number PCT / EP2023 / 058951. A closed, convex curve is a continuous line with a non-zero length but no width, composed of one or more curve segments, which in turn are composed of one or more curve segments. It has no distinct starting and end points. Starting from any point on the curve, one can determine the length of the curve by adding the lengths of the curve segments once around the curve. Every tangent to a closed, convex curve lies outside the region enclosed by the curve.Since all the curved segments of a worm profile lie in one plane, a closed curve, which is a worm profile, divides the area of ​​this plane into an area inside the closed curve and an area outside the closed curve. A circular arc is a curved segment in which all points of the arc are the same distance, called the radius, from a common center. A circular arc has a starting point and an end point that are not the same point. An arc is considered a circular arc only if and when all points on this arc have the same center and the same radius and the points on this arc form a continuous curved segment; that is, two immediately adjacent arcs that have a common point of tangency are only considered two circular arcs if they have a different center or a different radius.A circular arc with center coordinates ^. ^ , ^ ^ and the radius ^ has, as is well known, the parametric representation ^ = ^ ^ + ^ ∙ cos ^ ^ = ^ ^ + ^ ∙ sin ^ where the parameter ^ is: 2018PF30224a ^ ^ ≤ ^ ≤ ^ ^ whereby the initial angle of a circular arc ^ ^ and the final angle of a circular arc ^ ^ of which are defined and ^ ^ < ^ ^ + ^ in radians. For a point with the coordinates ^, ^ it is defined that with respect to a designated point with the coordinates ^ ^ , ^ ^ the initial angle is equal to the final angle, so that according to the above equations, ^ ^ = ^ = ^ ^ A circular arc has the starting point AP with the coordinates with the coordinates ^ ^ and ^ ^ ^ ^ = ^ ^ + ^ ∙ cos ^ ^ ^ ^ = ^ ^ + ^ ∙ sin ^ ^The central angle of a circular arc (also called angle of a circular arc) is ^ = ^ ^ − ^ ^By specifying the center coordinates, the coordinates of the starting point AP, and the central angle, a circular arc is fully defined. The radius ^ is then the distance of the center from the starting point AP. In the tables below, the radius is also specified for clarity to characterize the circular arcs. The radius of curvature is the magnitude of the inverse of the curvature of the circle of curvature at a given point on a curve segment. The circle of curvature at this point is the circle that best approximates the curve at this point, as described, for example, on Wikipedia (https: / / de.wikipedia.org / wiki / Kr%C3%BCmmungskreis, accessed 2023-05-04). The pivot point DP of a screw profile is the intersection point of the axis of rotation of a screw element with the cross-sectional plane perpendicular to this axis of rotation X. The pivot point of the screw profile, hereinafter also referred to as the pivot point,also coincides with the bore center of the housing bore in which the respective screw element is located or for which the respective screw element is designed. In relation to a screw profile, a pivot point is the point around which a screw profile rotates as a cross-sectional image of a screw element. 2018PF30224a The outer radius ^^ of a screw profile is the maximum distance of the screw profile from the pivot point; thus, the outer radius ^^ is the radius of the circle that encloses the entire screw profile and whose center is the pivot point associated with this screw profile. The inner radius RI of a screw profile is the minimum distance of the screw profile from the pivot point. A ridge is: (i) either exactly one circular arc of a screw profile that has the pivot point of the screw profile as its center,and in which all points of this circular arc, except for the respective common points of contact with the two curve sections immediately adjacent to the crest, have a greater distance from the pivot point than the two curve sections immediately adjacent to the crest; (ii) or a point of a screw profile that has a greater distance from the pivot point than the two points immediately adjacent to this point; the point that is a crest according to this case (ii) is therefore a local maximum of the screw profile with respect to the distance to the pivot point. Preferably, in case (ii), the point that is a crest is a point of a circular arc whose radius is smaller than the distance between this point and the pivot point. In case (ii), the point that is a crest can be the middle point of a circular arc whose radius is smaller than the distance between this point and the pivot point. In case (ii), the curves that are immediately adjacent to the crest,at the point that is the crest, they merge tangentially into each other. In case (i), the crest angle is the angle of the circular arc, i.e., the difference between the final and initial angles. In case (ii), it is zero. The crest radius is, in case (i), the distance of the respective crest, which is a circular arc, from the pivot point of a screw profile, and in case (ii), the distance of the point that is a crest from the pivot point of the screw profile. A groove is: (iii) either exactly one circular arc of a screw profile, which has the pivot point of the screw profile as its center, and in which all points of this circular arc, except for the common points of contact with the two curve sections immediately adjacent to the groove, are at a smaller distance from the pivot point than the two curve sections immediately adjacent to the groove; 2018PF30224a (iv) or a point of a screw profile that is at a smaller distance from the pivot point than the two points immediately adjacent to this point; the point,which is a crest according to this case (iv), is therefore a local minimum of the screw profile with respect to the distance to the pivot point. Preferably, in case (iv), the point that is a groove is a point of a circular arc whose radius is greater than the distance between this point and the pivot point. In case (ii), the point that is a groove can be the middle point of a circular arc whose radius is greater than the distance between this point and the pivot point. In case (iv), the curves immediately adjacent to the groove merge tangentially into one another at the point that is the groove. In case (iii), a groove angle is the angle of the circular arc, i.e., the difference between the end and start angles. In case (iv), it is zero. In case (iii), a groove radius is the distance of the respective groove, which is a circular arc, from the pivot point of a screw profile, and in case (iv), the distance of the point that is a groove.from the pivot point of the screw profile. A flank is a curve of a screw profile in which all points of this curve, except for the common point of contact with a first curve segment immediately adjacent to the flank, are at a smaller distance from the pivot point than this first curve segment immediately adjacent to the flank. At the same time, all points of this curve, except for the common point of contact with a second curve segment immediately adjacent to the flank, are at a greater distance from the pivot point than this second curve segment of the screw profile immediately adjacent to the flank. A flank can be composed of several curve segments to which the above definition applies. A flank is then represented by a convex curve composed of several curve segments.wherein the radii of curvature of the curve sections are always smaller than the center distance ^. According to the invention, a flank is preferably formed from a convex curve whose curve sections are formed exclusively from circular arcs with a radius less than or equal to the center distance ^, and according to the invention, a flank is particularly preferably formed by exactly one circular arc with a radius less than the center distance ^. According to the invention, it is particularly preferred that all flanks of a screw cross-sectional profile are each formed by exactly one circular arc with a radius less than the center distance ^. 2018PF30224a Two screw profiles are said to be congruent in the same direction if they can be converted into one another by a translation and a rotation. Two screw profiles are said to be congruent in opposite directions if they can be converted into one another by a translation, a rotation, and a reflection. Two screw profiles are said to be congruent without rotation.if they can be converted into one another by displacement. The screw profile according to the invention has exactly six flanks. According to the invention, the following preferably applies: For each of the six flanks, independently of each of the other five flanks: a flank is formed from exactly a single circular arc, wherein this circular arc has a center point that is not the pivot point of the screw profile, or a flank can be formed exclusively from several circular arcs, wherein the center points of all of these circular arcs are different from the pivot point of the screw profile, or a flank can be formed from at least one circular arc and at least one curved section that is not a circular arc, wherein no circular arc has a center point that is the pivot point of the screw profile, or a flank can be formed from only a single curved section that is not a circular arc,or a flank can be formed exclusively from several curve segments, none of which is a circular arc. Alternatively, according to the invention, a flank is preferably formed from a convex curve whose curve segments are formed exclusively according to a longitudinal equidistant, circular equidistant, or spatial equidistant calculation rule, and particularly preferably according to the invention, a flank is formed by exactly one curve segment formed exclusively according to a longitudinal equidistant, circular equidistant, or spatial equidistant calculation rule. Alternatively, according to the invention, it is particularly preferred that all flanks of a screw cross-sectional profile are formed exclusively according to a longitudinal equidistant, circular equidistant, or spatial equidistant calculation rule.and according to the invention, all flanks of a screw cross-sectional profile are each formed by exactly one curve section formed exclusively according to the longitudinal equidistant, circular equidistant, or spatial equidistant calculation rule. Alternatively, very particularly preferably, all curves of a 2018PF30224a screw profile are formed according to the same longitudinal equidistant, circular equidistant, or spatial equidistant calculation rule. For the purposes of the present invention, a screw element is referred to as three-flighted if it has exactly three crests. In particular, the object is achieved by a pair of three-flighted screw elements, wherein the pair of screw elements consists of the screw element SE and the screw element SE', and wherein the pair of screw elements SE and screw element SE' is suitable for a multi-shaft screw machine with ^ screw shafts SW1 to SWm rotating in the same direction and at the same speed.whose adjacent axes of rotation X1 to Xm have an axial distance ^ in a cross-section perpendicular to the axes of rotation and with ^ interpenetrating, circular housing bores, each having an identical housing inner radius ^ and whose bore centers M1 to Mm are at a distance equal to the axial distance ^, and whose bore centers M1 to Mm coincide with the respective associated axes of rotation X1 to Xm of the worm shafts SW1 to SWm, wherein the worm element SE has a worm profile SP and the worm element SE' has a worm profile SP', whereby (1) each of the two worm profiles SP and SP' is a closed convex curve, whereby this closed convex curve is composed only of curve sections whose radii of curvature are less than or equal to the axial distance ^, (2) either neither of the two worm profiles SP and SP' has mirror symmetry,wherein, within the scope of the present invention, a screw profile is mirror-symmetrical if it is imaged onto itself by vertical axial reflection at any axis passing through the respective pivot point, or the screw profiles SP and SP' are not congruent with each other, (3) each of the two screw profiles SP and SP' has exactly three grooves, 2018PF30224a (4) each of the two screw profiles SP and SP' has exactly three crests - designated circumferentially with K1, K2, K3 for the screw profile SP and K1', K2', K3' for the screw profile SP', (6) two immediately adjacent screw elements of the pair of screw elements on two of the screw shafts of the multi-shaft screw machine rotating in the same direction and at the same speed practically clean each other in pairs, (7) a groove is separated from the nearest point of an adjacent crest by a flank, (8) it has exactly six curves, which are flanks,where in particular, (9) ^(^^) is the crest radius of the comb Ki and ^(^^') is the crest radius of the comb Ki', where: ^(^1) > ^(^2) and ^(^1) > ^(^3) and ^(^1') > ^(^2') and ^(^1') > ^(^3') and ^, ( ^2 ) ≥ ^(^3) and ^ ( ^2′ ) ≥ ^(^3′) and at least one of the conditions a) or b), where a) ^ ( ^2 )> (^3) and ^(^2') > ^(^3') b) ^(^2) not equal to ^(^2') and ^(^3) not equal to ^(^3') where r(K1) is the crest radius of the worm crest K1 and r(K2) is the crest radius of the worm crest K2 and r(K3) is the crest radius of the worm crest K3 and r(K1') is the crest radius of the worm crest K1' and r(K2') is the crest radius of the worm crest K2' and r(K3') is the crest radius of the worm crest K3', (10) the worm crest K1 has the crest angle ^^1, the worm crest K2 has the crest angle ^^2, the worm crest K3 has the crest angle ^^3 and the worm crest K1' has the crest angle ^^1', the worm crest K2 has the crest angle ^^2' and the screw crest K3 has the crest angle ^^3', where (10.1) (10.1.a) if ^^1 > 0, then ^^1 < ^^2 and ^^1 < ^^3, and (10.1.b)if ^^1' > 0, then ^^1' < KW2 and ^^1 < ^^3, or 2018PF30224a (10.2) (10.2.a) if ^^1 > 0, then ^^1 < ^^2 and ^^1 < ^^3, and (10.2.c) if ^^1' = 0; then ^^2 >= 0 and ^^3 >= 0, or (10.3) (10.3.b)if ^^1' > 0, then ^^1' < KW2 and ^^1 < ^^3, and (10.3.d)if ^^1 = 0, then ^^2 >= 0 and ^^3 >= 0, or (10.4) (10.4.d)if ^^1 = 0, then ^^2 >= 0 and ^^3 >= 0, and (10.4.c) if ^^1' = 0; then ^^2 >= 0 and ^^3 >= 0, where r(K1) is equal to r(K1') is equal to the outer radius ^^ or ^^' of the screw elements and ^ is an integer greater than 1, preferably from 2 to 16, particularly preferably 2, 3, 4, 6, 8, 10, 12 and 16, and ^ can assume the values ​​1, 2 or 3, and the outer radius ^^ is the radius of the circle which encloses the entire screw profile and whose center is the pivot point associated with this screw profile, where the crest K1 is a circular segment of this circle. The following follows for the screw profiles according to the invention: The exactly three screw combs K1, K2, K3 as well as K1', K2' and K3' (summarized as Ki and Ki' respectively) of the screw elements according to the invention have gaps ^1, ^2, ^3 as well as ^1', ^2' and ^3' to the inner wall of the housing, where ^1 and ^1' respectively is the gap between screw comb K1 andK1' and the housing inner wall, ^2 or ^2' is the gap between screw comb K2 or K2' and the housing inner wall, S3 or S3' is the gap between screw comb K3 or K3' and the housing inner wall, the following relations apply: 2018PF30224a In the sense of the present invention, a gap ^ is the distance between a screw comb and the housing inner wall, thus obeys the equation ^1 = ^ - ^(^^) , and ^. ^= ^ – ^(^^) , so ^1 = ^ – ^(^1), ^2 = ^ – ^(^2) and ^3 = ^– ^(^3) as well as ^^′ = ^ – ^(^^′) , so ^1′ = ^ – ^(^1′), ^2′ = ^ – ^(^2′) and ^3′ = ^ – ^(^3′). For the features under number (10), the following preferably applies according to the invention: (10.1) (10.1.a) if ^^1 > 0, then ^^1 < ^^2 and ^^1 < ^^3, and (10.1.b) if ^^1' > 0, then ^^1' < KW2 and ^^1 < ^^3, or (10.4) (10.4.c) if ^^1 = 0, then ^^2 >= 0 and ^^3 >= 0, and (10.4.d) if ^^1' = 0; then ^^2 >= 0 and ^^3 >= 0. Furthermore, the following applies to the screw elements according to the invention: Each crest Ki or Ki' corresponds to a groove Ni' or Ni on the corresponding screw element, i.e. N1' to K1, N2' to K2, N3' to K3, N1 to K1', N2 to K2', N3 to K3'. In this case, for each of the two screw elements SE and SE' according to the invention, the following equation applies to the groove radius ^(^^'): ^(^^') = ^^ + ^^ − ^1 and correspondingly ^(^^) = ^^′ + ^^' − ^1′, and that for the initial angle of the crest ^ and the initial angle of the groove ^' ^ ^,^^ ≥ ^ ^,^^^ + ^ and for the end angle of the comb ^ and the end angle of the groove ^' ^ ^,^^ ≤ ^ ^,^^ ^ + ^ and for the initial angle of the ridge ^' and the initial angle of the groove ^ it applies that ^ ^,^^ ^ − ^ ≥ ^ ^,^^ and ^ ^,^^ ^ − ^ ≤ ^ ^,^^ . It is always true that ^^^ ≤ ^^^ ^ and ^^^ ^≤ ^^^. In other words: a crest Ki of one screw element of a pair of screw elements according to the invention and a groove Ni' of the other screw element of this pair of screw elements according to the invention are located on opposite sides with respect to their respective screw elements. With the pair of screw elements according to the invention, an improved mixing and dispersing effect is achieved compared to the prior art, with simultaneous good shear and good heat transfer, and it is ensured that two screw elements according to the invention, which are directly adjacent to each other on two directly adjacent screw shafts, practically clean each other off. The energy introduction is due to the larger gap of the crests K2 and K3 compared to the gap of the crest K1 in the screw element according to the invention compared to conventional,reduced compared to screw elements known from the prior art. Surprisingly, the pair of triple-flighted screw elements according to the invention nevertheless exhibits an excellent mixing effect. In a preferred embodiment of the triple-flighted pair of screw elements according to the invention, the following also applies to screw profiles with the features set out under (1) to (10): (11) each of the three combs K1, K2, and K3 of the screw element SE is formed from only one curved section, which is a circular arc whose center point is the pivot point DP common to the combs K1, K2, and K3, and each of the three combs K1', K2', K3' of the screw element SE is likewise formed from only one curved section, whose center point is the pivot point DP' common to the combs K1', K2', and K3, and each of the three grooves of the screw element SE is formed from only one curved section, which is a circular arc,whose center is the pivot point DP of the screw element SE common to the three grooves, and wherein there is exactly one groove whose distance from the pivot point DP is smaller than the distances of the other grooves from the pivot point DP, and that each of the three grooves of the screw element SE' is formed from only one curved section, which is a circular arc whose center is the pivot point DP' of the screw element SE' common to the three grooves. For this preferred case according to the invention with the additional feature (11), it also applies that the core radius ^^ is the radius of the groove with the smallest radius - starting from the pivot point DP of the screw profile - of all grooves of the screw element ^^, and likewise, the core radius ^^' is the radius of the groove with the smallest radius - starting from the pivot point DP of the screw profile - of all grooves of the screw element SW'.where ^^ is preferably equal to ^^'. For this preferred case according to the invention with the additional feature (11), it also applies that the outer radius ^^ is the radius of the crest with the largest radius - starting from the pivot point DP of the screw profile - of all crests of the screw element ^^, and likewise the outer radius ^^' 2018PF30224a is the radius of the crest with the smallest radius - starting from the pivot point DP' of the screw profile - of all crests of the screw element SW', where ^^ is preferably equal to ^^'. In this preferred embodiment according to the invention, the circular arcs representing the exactly three grooves, just like the circular arcs representing the exactly three crests K1, K2, and K3, have the same center point, namely the pivot point of one element. This pivot point DP is therefore the common center point of all three crests K1, K2, and K3, as well as all three grooves. This results in,that in this preferred embodiment according to the invention, a screw profile has a kink at all transitions from a screw crest to a flank. A kink in the screw profile means that an edge is formed in the screw element at the corresponding point. Mathematically, a kink means that a curve at the point of the kink is not continuously differentiable. In a preferred embodiment of the pair of three-flight screw elements according to the invention, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) and with the additional feature (11): (12) both in the screw profile SP, the screw crest with the largest crest radius r(Ki), i.e., crest K1, has the smallest crest angle of the crests K1, K2, and K3, and that in the screw profile SP', the screw crest with the largest crest radius r(Ki)', i.e., crest K1',has the smallest crest angle of the crests K1', K2', and K3'. In this way, the energy input is reduced and the thermal stress on the polymer is lessened. For the multi-screw screw extruder described, it follows that the screw crest with the narrowest gap to the inner wall of the housing, i.e., crest K1 with gap S1, has the smallest crest angle. The same applies to crest K1' and gap S1'. According to the invention, two screw elements according to the invention, which are located directly adjacent to each other on two screw shafts of the multi-screw extruder described, practically clean each other in pairs; such two screw elements according to the invention are referred to as a pair of screw elements according to the invention. The screw profiles of these two screw elements according to the invention can be the same or different. 2018PF30224a According to the invention, it is preferred thatthat in a cross-section perpendicular to the screw shafts, all screw elements practically clean each other in pairs. This naturally applies again except for the technically necessary clearances. The screw profiles of these screw elements according to the invention can be the same or different. In a further preferred embodiment of the invention of the pair of three-flight screw elements according to the invention, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (11) or to screw profiles with the features shown under (1) to (12): (13) that both the screw profile SP is continuously differentiable at all of the combs K1, K2 and K3 and the screw profile SP' at all of the combs K1',K2' and K3' is continuously differentiable. In a further preferred embodiment of the inventive three-flight pair of screw elements, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (11) or to screw profiles with the features shown under (1) to (11) and with the additional feature (13): (14) each of the three crests K1, K2 and K3 of the screw element SE is each formed from exactly one point, i.e. ^^1 = ^^2 = ^^3 = 0, and likewise each of the three crests K1', K2' and K3' of the screw element SE' is each formed from exactly one point,i.e. ^^1' = ^^2' = ^^3' = 0. In a further preferred embodiment of the inventive three-flight pair of screw elements, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (11) or to screw profiles with the features shown under (1) to (12) or to screw profiles with the features shown under (1) to (13): (15) the crests K1 and K1' are formed from exactly one point and the crests K2 and K3 are each formed from exactly one circular arc, the respective center point of which coincides with the pivot point DP, and the crests K2' and K3' are each formed from exactly one circular arc, the respective center point of which coincides with the pivot point DP',the crests K2 and K3 and the crests K2' and K3' each have a crest angle greater than zero. 2018PF30224a In a further preferred embodiment of the inventive three-flight pair of screw elements, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (11) or to screw profiles with the features shown under (1) to (12) or to screw profiles with the features shown under (1) to (13): (16) all crests of the screw profile SP and all crests of the screw profile SP' have a crest angle greater than zero. The sequence of crests and grooves can be designed differently according to the invention. For characterization, starting from the crest K1 on the worm shaft SW,the sequence of crests and grooves is listed in a mathematically positive sense. In a further preferred embodiment of the invention of the three-flight pair of screw elements according to the invention, the following also applies to the screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) with an additional feature of the features (11) to (16) or to screw profiles with the features shown under (1) to (10) with more than one additional feature of the features (11) to (16): (17) for a given barrel inner radius ^ in relation to a given axial distance ^ and a given barrel inner diameter ^ = 2 ^ the distance ^(^1) of the crest K1 from the pivot point is selected such that for ^(^1) or ^(^1') the range applies: ^(^1) is less than or equal to ^ - 0.002 ^ and ^(^1) is greater than or equal to ^ - 0.05 ^ and ^, ( ^1 ^)is less than or equal to ^ – 0.002 ^ and ^(^1′) is greater than or equal to ^ – 0.05 ^. In a further preferred embodiment of the invention of the three-flight pair of screw elements according to the invention, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) with an additional feature of the features (11) to (17) or to screw profiles with the features shown under (1) to (10) with more than one additional feature of the features (11) to (17): 2018PF30224a (18) for a given barrel inner radius ^ in relation to a given axial distance ^ and a given barrel inner diameter ^ = 2 ^ the distance ^(^1) of the crest K1 from the pivot point is selected such that for the crests K2 or K2' the range preferably applies: ^(^2) is less than or equal to ^(^1) - 0.002 ^ and r(K2) is greater than or equal to ^(^1) - 0.08 ^ and ^(^2′) is less than or equal to ^(^1′) – 0,002 ^ and r(K2') is greater than or equal to ^(^1′) – 0.08 ^ where additionally ^(^2) is greater than (^(^1) + ^(^1)) / 2 and r(K2') is greater than (^(^1') + ^(^1')) / 2. In a further preferred embodiment of the invention of the three-flight pair of screw elements according to the invention, the following also applies to the screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) with an additional feature of the features (11) to (18) or to screw profiles with the features shown under (1) to (10) with more than one additional feature of the features (11) to (18): (19) for a given barrel inner radius ^ in relation to a given axial distance ^ and a given barrel inner diameter ^ = 2 ^ the distance ^(^1) of the crest K1 from the pivot point is selected such that for the crests K3 and K3' the range preferably applies: r(^3) is greater than or equal to ^(^1) - 0,09 ^ and ^(^3) is less than or equal to ^(^2) and ^(^3) is greater than or equal to ^(^2) – 0.05 ^ and r(^3′) is greater than or equal to ^(^1′) – 0.09 ^ and ^(^3′) is less than or equal to ^(^2′) and ^(^3′) is greater than or equal to ^(^2′) – 0.05 ^ and (^(^1) + ^(^1)) / 2 and r(K3') is greater than (^(^3') + ^(^1')) / 2. From this, according to the invention, for a multi-shaft screw machine 2018PF30224a with m screw shafts SW1 to SWm rotating in the same direction and at the same speed, whose adjacent axes of rotation X1 to Xm have an identical center distance ^ in a cross-section perpendicular to the axes of rotation and with m interpenetrating, circular housing bores, each having an identical housing inner radius ^ and whose bore centers M1 to Mm have a distance equal to the center distance ^, and whose bore centers M1 to Mm coincide with the respective axes of rotation X1 to Xm of the screw shafts SW1 to SWm,the gap ^1 in relation to the housing inner diameter ^ is preferably from ^1 / ^ = 0.002 ^^^ ^1 / ^ = 0.05, likewise the gap ^1' in relation to the housing inner diameter ^ is preferably from ^1' / ^ = 0.002 to ^1' / ^ = 0.05, the gap ^2 in relation to the housing inner diameter ^ is preferably from ^2 / ^ = 0.004 ^^^ ^2 / ^ = 0.082, likewise the gap ^2' in relation to the housing inner diameter ^ is preferably from ^2' / ^ = 0.004 to ^2' / ^ = 0.082, the gap S3 in relation to the housing inner diameter D is preferably from S3 / D = 0.004 to S3 / D = 0.1, likewise the gap S3' in relation to the housing diameter D is preferably from S3' / D = 0.004 to S3' / D = 0,1. In a further preferred embodiment of the invention of the triple-flighted pair of screw elements according to the invention, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) with an additional feature of the features (11) to (19) or to screw profiles with the features shown under (1) to (10) with more than one additional feature of the features (11) to (19): (20) the crest angle KW10 degrees < KW1 < 10 degrees and preferably 2 degrees < KW1 < 8 degrees and the crest angle KW1' is 0 degrees < KW1' < 10 degrees and preferably 2 degrees < KW1' < 8 degrees. Alternatively, the crest angle KW1 is preferably 0 degrees if the screw profile is continuously differentiable at the crest,i.e. the screw profile has no kink at the crest. In a further preferred embodiment of the inventive three-flight pair of screw elements, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) with an additional feature of the features (11) to (20) or to screw profiles with the features shown under (1) to (10) with more than one additional feature of the features (11) to (20): (21) the screw profiles SP and SP' of the two screw elements SE and SE' of a pair of screw elements are congruent without rotation and can be converted into one another by a shift from the pivot point of one screw profile to the other screw profile. This leads tothat the screw elements are identical. The sequence of the crests and grooves of the screw profile SP is then K1 N2 K3 N1 K2 N3 or K1 N3 K2 N1 K3 N2. In a further preferred embodiment of the invention of the three-flight pair of screw elements according to the invention, the following also applies to screw profiles with the features shown under (1) to (10) or to screw profiles with the features shown under (1) to (10) with an additional feature of the features (11) to (20) or to screw profiles with the features shown under (1) to (10) with more than one additional feature of the features (11) to (20) that (22) the screw profiles SP and SP' of the two screw elements SE and SE' of a pair of screw elements are congruent in opposite directions and are determined by the sequence of operations - displacement of the screw profile from the pivot point of one screw profile to the pivot point of the other screw profile, - reflection along the line,which connects the pivot points to each other, - rotation of one of the screw profiles through an angle other than zero. Screw elements designed as conveying elements are preferably designed according to the rule of spatial equidistant and have crest angles that are all greater than zero. Screw elements designed as kneading disks are preferably designed according to the rule of circular equidistant, and the profiles consist solely of circular arcs. In a particularly preferred embodiment of such screw elements, the screw profiles are composed of circular arcs with two different radii, and all crest angles are 0. The present invention further relates to the use of a pair of the screw elements according to the invention in a multi-shaft screw machine. The pair of 2018PF30224a screw elements according to the invention is preferably used in a twin-shaft screw machine,i.e., in a twin-screw extruder. The present invention thus also relates to a multi-screw extruder equipped with a pair of the screw elements according to the invention. The multi-screw extruder is preferably equipped with at least as many screw elements as the screw extruder has shafts. The screw elements according to the invention are arranged on the screw shafts in such a way that each of the screw elements according to the invention practically cleans itself with at least one other of the screw elements according to the invention. The pair of screw elements according to the invention can be present in a multi-screw extruder in the form of kneading, conveying, or mixing elements. It is possible to combine kneading, conveying, and mixing elements in one screw extruder. The pair of screw elements according to the invention can also be combined with other screw elements, which are known, for example, from the prior art.be combined. A conveying element is known to be characterized by the fact that a screw profile is continuously twisted and continued in a helical manner in the axial direction. The conveying element can be right- or left-handed. The pitch of a conveying element according to the invention is preferably in the range of 0.5 to 5 times the axial distance A, and the axial length of a conveying element according to the invention is preferably in the range of 0.25 to 2 times the pitch. The length of the conveying element is particularly preferably equal to the pitch; thus, the conveying element represents a complete rotation of the screw profile. A kneading element is known to be characterized by the fact that a screw profile is continued in the axial direction intermittently and parallel to the axis in the form of kneading disks. The arrangement of the kneading disks can be either right- or left-handed.which results in either a conveying effect or a return conveying effect. With an offset angle of 45° between two axially adjacent kneading disks, a neutral arrangement without a conveying effect results for three-flight screws. The axial length of the kneading disks is preferably in the range of 0.05 to 0.5 times the center distance ^. The axial distance between two adjacent kneading disks is preferably in the range of 0.0005 to 0.02 times the center distance ^. Mixing elements are known to be formed, among other things (see, for example, [1], pages 148-151), by designing conveying elements with openings in the screw combs. The mixing elements can be right- or left-flighted. Their pitch is preferably in the range of 0.1 to 10 times the center distance ^, and the axial length of the elements is preferably in the range of 0.5 times to 5 times the axial distance ^. The openings are preferably U- or V-shaped and are preferably arranged counter-conveying or axially parallel. The sequence of the screw elements consisting of conveying elements and / or kneading elements and / or mixing elements on a screw shaft is also referred to as the screw configuration. The screw element according to the invention can also be designed as a transition element, i.e. the screw profile at one point of the axial extension of the screw element is different from that at another point of the axial extension of the screw element, whereby these different screw profiles cannot be converted into one another by rotation. The screw element according to the invention is suitable for the extrusion of plastic and viscoelastic masses, e.g. suspensions, pastes, glass, ceramic masses, metals in the form of a melt, plastics, plastic melts, polymer solutions,Elastomer and rubber compositions. The present invention therefore also relates to a multi-screw extruder equipped with at least one pair of the screw elements according to the invention. The present invention therefore also relates to a process for extruding plastic or viscoelastic compositions using a pair of screw elements according to the invention or using a screw extruder equipped with a pair of screw elements according to the invention. A plastic composition is understood to mean a deformable composition. Examples of plastic compositions are polymer melts, especially thermoplastics, as well as elastomers, mixtures of polymer melts or dispersions of polymer melts with solids, liquids or gases. Preference is given to using thermoplastic polymers, also called thermoplasts, or mixtures of thermoplastic polymers from the following series: polycarbonate, polyamide, polyester,in particular polybutylene terephthalate and polyethylene terephthalate, as well as polyether, thermoplastic polyurethane, polyacetal, fluoropolymer, in particular polyvinylidene fluoride, as well as polyethersulfones, polyolefin, in particular polyethylene and polypropylene, as well as polyimide, polyacrylate, in particular poly(methyl)methacrylate, as well as polyphenylene oxide, polyphenylene sulfide, polyether ketone, polyarylether ketone, styrene polymers, in particular polystyrene and styrene copolymers, in particular styrene-acrylonitrile copolymer and acrylonitrile-butadiene-styrene block copolymers, as well as polyvinyl chloride 2018PF30224a. Also preferably used are so-called blends of the listed plastics, which the person skilled in the art understands to be a combination of two or more plastics. Viscoelastic masses are understood to be materials and mixtures which have a time-, temperature- and frequency-dependent elasticity. The viscoelasticity is characterized by a partially elastic,Partially viscous behavior is characteristic. The material relaxes only incompletely after the external force is removed; the remaining energy is dissipated in the form of flow retardation. Examples of viscoelastic materials are styrene-butadiene rubber, natural rubber, butadiene rubber, isoprene rubber, ethylene-propylene-diene rubber, ethylene-propylene rubber, butadiene-acrylonitrile rubber, hydrogenated nitrile rubber, butyl rubber, halobutyl rubber, chloroprene rubber, ethylene-vinyl acetate rubber, polyurethane rubber, thermoplastic polyurethane, gutta-percha, arylate rubber, fluororubber, silicone rubber, sulfide rubber, and chlorosulfonyl polyethylene rubber. A combination of two or more of the listed rubbers,Or a combination of one or more rubbers with one or more plastics is of course also possible. The plastic or viscoelastic masses to be extruded can be used in pure form or as mixtures with fillers and reinforcing materials, such as, in particular, glass fibers, as mixtures with one another or with other polymers, or as mixtures with conventional polymer additives. Additives can be added to the extruder as solids, liquids, or solutions together with the polymer, or at least some or all of the additives can be fed to the extruder via a side stream. Additives can impart a variety of properties to a polymer. These can include, for example, plasticizers, colorants, pigments, processing aids, fillers, antioxidants, reinforcing materials, UV absorbers and light stabilizers, extender oils, metal deactivators, peroxide scavengers, basic stabilizers, nucleating agents,Benzofurans and indolinones acting as stabilizers or antioxidants, mold release agents, flame-retardant additives, antistatic agents, colorants, and melt stabilizers. Examples of fillers and reinforcing materials include carbon black, glass fiber, clay, mica, graphite fiber, titanium dioxide, carbon fiber, carbon nanotubes, ionic liquids, and natural fibers. 2018PF30224a The pair of screw elements according to the invention, as explained above, is particularly suitable for extruding viscoelastic masses. The process steps that can be carried out using this pair of screw elements include, for example, the mixing or dispersing of solids, liquids, or gases. Solids can be, for example, the above-mentioned solid additives. Liquids can be, for example, the above-mentioned additives in liquid form.but also, for example, water. Gases can be, for example, nitrogen or carbon dioxide. In particular, a pair of screw elements according to the invention or a single-screw or multi-screw machine equipped with at least one pair of screw elements according to the invention can be advantageously used for compounding thermoplastics, in particular polycarbonates or thermoplastic polyurethanes, with colorants, pigments, or additives. The present invention thus relates both to a process for compounding thermoplastics, in particular polycarbonates or thermoplastic polyurethanes, with colorants and additives using a pair of screw elements according to the invention and to the use of a pair of screw elements according to the invention for compounding thermoplastics, in particular polycarbonates or thermoplastic polyurethanes,with colorants and additives. The invention is explained below by way of example with reference to the attached figures using preferred exemplary embodiments, wherein the features presented below can represent an aspect of the invention both individually and in combination. It should be noted that, for the purposes of the present invention, a pivot point DP is the cross-sectional profile of a rotational axis X. Directly opposite screw elements of a twin-screw extruder with the rotational axes X1 and X2 thus have the pivot points DP1 and DP2, hereinafter also referred to as DP and DP'. Figure 1 shows a pair of three-flight screw elements for a co-rotating twin-screw extruder in cross section, thus showing the screw cross-sectional profiles of these screw elements. To clarify the geometric dimensions, the values ​​RI (inner radius of SP), RA (outer radius of SP), RI' (inner radius of SP'),RA' (outer radius of RA'), the gaps S1, S2 and S3 as well as S1', S2' and S3' as well as the pivot point DP and the pivot point DP' are shown. Furthermore, the crests K1, K2 and K3 as well as K1', K2' and K3 and grooves N1, N2 and N3 as well as N1', N2' and N3 are shown as well as the crest radii r(K1), r(K2) and r(K3). 2018PF30224a Figure 1 shows the sequence of crests and grooves of the left screw profile K1 N1 K2 N2 K3 N3. Figure 2A shows a top view of an inventive pair of three-flight screw elements for a co-rotating twin-screw extruder, which practically scrape off one another. The barrel wall is indicated in Figure 2A by vertical lines. In this inventive pair of screw elements, the two screw profiles are identical. The screw combs are labelled K1 to K3 for the left screw element and K1' to K3' for the right screw element,The grooves are designated N1 to N3 and N1' to N3', respectively. The screw crest K1 cleans the inner housing wall with the gap S1, the screw crest K2 with the gap S2, etc. The ratio ^ / ^ of the center distance ^ to the inner housing diameter ^ is 0.91. The ratio ^1 / ^ is 0.005, the ratio ^2 / ^ is 0.01, and the ratio ^3 / ^ is 0.015. The screw elements SE and SE' have an identical screw profile. The ratio of the pitch to the inner housing diameter ^ / ^ is 1. The crest angles are, given in radians, ^^1 = ^^1, ^= 0.1224, ^^2 = ^^2' = 0.1362 and ^^3 = ^^3′ = 0.2306. The groove angles are, also in radians, ^^1 = ^^1' = 0.17453, ^^2 = ^^2' = 0.18789 and ^^3 = ^^3' = 0.28184. The sequence of crests and grooves for the screw profile is K1 N2 K3 N1 K2 N3 and K1' N2' K3' N1' K2' N3' respectively. Figure 2B shows the screw profiles of the screw elements from Figure 2A. The two screw profiles can be converted into one another by shifting the pivot point DP of one screw element to the pivot point DP' of the other screw element. The coordinates of the screw profiles are given in Table 2B. Table 2B: Radial coordinates ^ ( ^ ) / ^ of the screw profiles in Figure 2B, which practically clean each other, read in 2° steps for ^, plus the bends, based on the inner diameter of the casing ^. ^ ^ ( ^ ) / ^ ^ ^(^) / ^ ^ ^(^) / ^ 0,000 0,49500 120,000 0,48500 243,306 0,49000 2,000 0,49500 122,000 0,48500 244,000 0,48751 3,492 0,49500 124,000 0,48500 246,000 0,48060 4,000 0,49304 125,803 0,48500 248,000 0,47406 6,000 0,48556 126,000 0,48426 250,000 0,46789 2018PF30224a 8,000 0,47847 128,000 0,47696 252,000 0,46209 10,000 0,47177 130,000 0,47006 254,000 0,45663 12,000 0,46544 132,000 0,46354 256,000 0,45151 14,000 0,45947 134,000 0,45739 258,000 0,44672 16,000 0,45386 136,000 0,45160 260,000 0,44226 18,000 0,44860 138,000 0,44616 262,000 0,43811 20,000 0,44367 140,000 0,44106 264,000 0,43427 22,000 0,43907 142,000 0,43630 266,000 0,43074 24,000 0,43480 144,000 0,43187 268,000 0,42750 26,000 0,43083 146,000 0,42775 270,000 0,42455 28,000 0,42717 148,000 0,42394 272,000 0,42189 30,000 0,42381 150,000 0,42044 274,000 0,41951 32,000 0,42075 152,000 0,41723 276,000 0,41740 34,000 0,41797 154,000 0,41431 278,000 0,41556 36,000 0,41547 156,000 0,41167 280,000 0,41399 38,000 0,41325 158,000 0,40931 282,000 0,41268 40,000 0,41130 160,000 0,40723 284,000 0,41163 42,000 0,40961 162,000 0,40542 286,000 0,41085 44,000 0,40820 164,000 0,40387 288,000 0,41032 46,000 0,40704 166,000 0,40259 290,000 0,41004 48,000 0,40615 168,000 0,40156 291,140 0,41000 50,000 0,40551 170,000 0,40080 292,000 0,41000 52,000 0,40513 172,000 0,40029 294,000 0,41000 53,990 0,40500 174,000 0,40003 296,000 0,41000 54,000 0,40500 176,000 0,40000 298,000 0,41000 56,000 0,40500 178,000 0,40000 300,000 0,41000 58,000 0,40500 180,000 0,40000 302,000 0,41000 60,000 0,40500 182,000 0,40000 304,000 0,41000 62,000 0,40500 184,000 0,40000 306,000 0,41000 64,000 0,40500 186,000 0,40003 307,288 0,41000 64,755 0,40500 188,000 0,40029 308,000 0,41002 66,000 0,40505 190,000 0,40080 310,000 0,41024 68,000 0,40534 192,000 0,40156 312,000 0,41071 70,000 0,40588 194,000 0,40259 314,000 0,41144 72,000 0,40668 196,000 0,40387 316,000 0,41243 74,000 0,40774 198,000 0,40542 318,000 0,41369 76,000 0,40906 200,000 0,40723 320,000 0,41520 78,000 0,41064 202,000 0,40931 322,000 0,41698 80,000 0,41249 204,000 0,41167 324,000 0,41903 82,000 0,41461 206,000 0,41431 326,000 0,42136 84,000 0,41700 208,000 0,41723 328,000 0,42396 86,000 0,41968 210,000 0,42044 330,000 0,42685 88,000 0,42264 212,000 0,42394 332,000 0,43002 90,000 0,42589 214,000 0,42775 334,000 0,43349 92,000 0,42943 216,000 0,43187 336,000 0,43727 94,000 0,43328 218,000 0,43630 338,000 0,44134 2018PF30224a 96,000 0,43744 220,000 0,44106 340,000 0,44574 98,000 0,44192 222,000 0,44616 342,000 0,45046 100,000 0,44672 224,000 0,45160 344,000 0,45550 102,000 0,45186 226,000 0,45739 346,000 0,46089 104,000 0,45734 228,000 0,46354 348,000 0,46662 106,000 0,46317 230,000 0,47006 350,000 0,47271 108,000 0,46936 232,000 0,47696 352,000 0,47917 110,000 0,47593 234,000 0,48426 354,000 0,48600 112,000 0,48287 235,502 0,49000 356,000 0,49322 112,592 0,48500 236,000 0,49000 356,477 0,49500 114,000 0,48500 238,000 0,49000 358,000 0,49500 116,000 0,48500 240,000 0,49000 118,000 0,48500 242,000 0,49000 Figure 2C shows the precisely scraping pair of three-flight screw elements from Figures 2A and 2B in cross-section, thus showing the screw cross-sectional profiles of these screw elements. Figure 2C shows the screw profiles of two screw elements that precisely clean each other, ie there is no play between the screw elements during mutual cleaning of the screw elements. In practice, a play different from zero will be set between the screw elements. The procedure for this is, as already mentioned above, shown, for example, in [1] on pages 40 and 41 and 117 to 121. The circular arcs that make up the screw profiles,are designated with the numbers 1 to 12 for the left screw profile and 1' to 12' for the right screw profile. The gaps between the screw combs and the inner housing wall are designated S1 to S3 for the left screw element and S1' to S3' for the right screw element. Also shown as dashed lines are the construction lines for the circular arcs that make up the screw profiles. The screw profiles of the screw elements in Figure 2C consist only of circular arcs. In the following Table 1B, the ratio of the radii ^, ^ to the center distance ^, the central angles, the coordinates of the circle centers (^ ^,^ and ^ ^,^ ) and the coordinates (^ ^ and ^ ^) of the starting points AP relative to the center distance ^ of the respective circular arcs. The numbering of the circular arcs is counterclockwise for the generating screw profile, here the left one, and clockwise for the generated screw profile, here the right one. All length measurements are normalized to the distance ^ between the pivot points DP and DP'. The coordinate origin is located at the pivot point of the respective screw profile. The information on the central angles ^ ^ - are given in radians. Since the two screw profiles are identical, only one profile is given for each. 2018PF30224a Table 2C: Details of the circular arcs of the screw profiles in Figure 2C 1, 1' 0,55220 0,17453 0,55010 -0,04813 0,00000 0,00000 2, 2' 1,00000 0,42980 0,55009 0,04813 -0,32142 -0,44224 3, 3' 0,45330 0,18789 0,26651 0,36668 0,00000 0,00000 4, 4' 1,00000 0,40261 0,19333 0,41000 -0,23316 -0,49449 5, 5' 0,54121 0,28184 -0.19518 0.50479 0.00000 0.00000 6. 6' 1.00000 0.41192 -0.32787 0.43059 0.55010 -0.04813 7. 7' 0.44780 0.17453 -0.44610 0.03903 0.00000 0.00000 8. 8' 1.00000 0.42524 -0.44610 -0.03903 0.55010 0.04813 9. 9' 0.54670 0.18789 -0.32142 -0.44224 0.00000 0.00000 10, 10' 1.00000 0.40694 -0.23316 -0.49449 -0.19518 0.50479 11, 11' 0.45879 0.28184 0.16546 -0.42792 0.00000 0.00000 12, 12' 1.00000 0.42081 0.27794 -0.36502 -0.32787 0.43059 The profiles in Figures 2A and 2B are the practical cleaning screw profiles which were derived from the profile in Figure 2C during the manufacture of the screw elements.These screw profiles were calculated so that the screw elements spatially clean each other with equidistant clearance, i.e., they were calculated using the spatial equidistant calculation rule. Alternatively, such screw profiles that practically clean each other can also be calculated using the longitudinal equidistant calculation rule. Figure 3 shows the screw profiles of another pair of screw elements according to the invention. The screw profiles are composed of circular arcs. Also shown as dashed lines are the construction lines for the circular arcs that make up the screw profiles.The screw profile of the left screw element SE can be converted into the screw profile SE' by following the following steps: - shifting the screw profile from the pivot point of the left screw profile DP to the pivot point of the right screw profile DP', - mirroring it across the connecting line of the two pivot points, - rotating it in a mathematically positive direction by 15°. The screw profile of the left screw element SE and the screw profile of the right screw element SE' are therefore congruent in opposite directions. The ratio of the center distance to the housing inner diameter is A / D=0.82, the ratio of the gaps ^1 and ^1' to the housing inner diameter ^1 / ^ = ^1' / ^ = 0.018, the ratio of the gap 2018PF30224a ^2 and ^2' to the housing inner diameter ^2 / ^ = ^2' / ^ = 0.05 and the ratio of the gaps S3 and S3' to the housing inner diameter ^3 / ^ = ^3' / ^ = 0.077136 and the ratio of the clearance between the screw profiles SW and SW' ^ / ^ = 0.022.All crest angles of this screw profile are KW1=KW2=KW3=KW1'=KW2'=KW3' = 0. The sequence of crests and grooves on the screw element is K1 N2 K2 N1 K3 N2. Table 3A contains the list of circular arcs of the screw profile SW, relative to the x and y coordinates relative to the pivot point DP. Table 3A ^^^ / ^ ^^ ^^ / ^ ^^ / ^. ^ ^,^ / ^ ^ ^,^ / ^ 1 0.068239 1.05238 0.473976 -0.032106 0.413761 020.729761 1,13916 0.471485 0.036394 -0.145823 -0.352813 3 0.068239 1.13916 -0.241076 0.370705 -0.232169 0.30305 4 0.729761 1.05238 -0.297345 0.323264 0.399663 0.10708950.068239 0,95005 -0.133478 -0.391221 -0.083624 -0.344625 6 0.729761 0.95005 -0.074717 -0.41228 -0.16997 0.311238 Table 3B shows the list of circular arcs of the screw profile SW', relative to the coordinates x', y', relative to the pivot point DP'. Table 3B ^^^ / ^ ^^ ^^ / ^ ^^ / ^ ^ ^,^ / ^ ^ ^,^ / ^1‘ 0,729761 1,05238 -0,410185 0,343344 0,413761 0 2‘ 0,068239 1,13916 -0,383547 -0,389207 -0,145823 -0,3528133‘ 0,729761 1,13916 -0,316916 -0,420468 -0,232169 0,303054‘ 0,068239 1,05238 0,284839 0,086875 0,399663 0,107089 5‘ 0,729761 0,95005 0,269516 0,153685 -0,083624 -0,3446256‘ 0,068239 0,95005 -0,358877 0,378893-0.16997 0.311238Figures 4A to 4H show various screw profiles according to the invention with different sequences of crests and grooves. For all these screw profiles, the ratio of the axial distance to the inner barrel diameter is ^ / ^ = 0.82, the ratio of the gaps ^1 and ^1' to the inner barrel diameter is ^1 / ^ = ^1' / ^ = 0.018, and the ratio of the clearance between the screw profiles SW and SW' to the diameter is ^ / ^ = 0.022. All crest angles of the screw elements according to the invention in Figures 4A to 4H are zero. The screw profile in Figure 4E corresponds to that of Example 2. Table 4 provides an overview of the various 2018PF30224a screw profiles; the crests and grooves Ki, Ni, Ki', and Ni' are also shown there. Figure 4B shows screw profiles that are not congruent and have a mirror symmetry to the x-axis. Table 4: Screw profiles of Figures 4A to 4H.Figure S2 / D S3 / D S2' / D S3' / D Sequence Comb Congruence Symmetry Grooves 4A 0.05 0.05 0.077136 0.077136 K1 N1 K2 N2 K3 none none N3 4. B 0.05 0.05 0.077136 0.077136K1 N2 K2 N1 K3no mirrors N3 4C 0.05 0.077136 0.05 0.077136 K1 N1 K2 N2 K3 no no N3 4D 0.05 0.077136 0.05 0.077136 K1 N1 K2 N3 K3 opposite no N2 4 E0.05 0.077136 0.05 0.077136K1 N2 K2 N1 K3opposite direction none N3 4F 0.05 0.077136 0.05 0.077136 K1 N2 K2 N3 K3 none none N1 4G 0.05 0.077136 0.05 0.077136 K1 N3 K2 N1 K3 non-rotational none N2 4H 0.05 0.077136 0.05 0.077136 K1 N3 K2 N2 K3 opposite direction none N1 Figure 5 shows the screw profiles of a further pair of screw elements according to the invention. The screw profiles are composed of circular arcs. Also shown as dashed lines are the construction lines for the circular arcs from which the screw profiles are composed. The screw profile of the left element SW can be converted into the screw profile SW' by the sequence of steps - displacement of the screw profile from the pivot point of the left screw profile DP to the pivot point of the right screw profile DP', - reflection on the connecting line of the two pivot points (here designated Y) rotation in the mathematically positive direction by 15.72025°, whereby the line Y is converted into Y' 2018PF30224a.The ratio of the center distance to the crest angle is A / D = 0.8, the ratio of the gaps S1 and S1' to the housing inner diameter is ^1 / ^ = ^1 / ^ = 0.025, the ratio of the gaps S2 and S2' to D is ^2 / ^ = ^2' / ^ = 0.062 and the ratio of the gaps S3 and S3' to D is ^3 / ^ = ^3' / ^ = 0.095. The crest angles are, given in radians, ^^1 = ^^1' = 0, ^^2 = ^^2' = 0.102871 and ^^3 = ^^3' = 0.162301. The groove angles, also in radians, are ^^1 = ^^1' = 0, ^^2 = ^^2' = 0.114586 and ^^3 = ^^3' = 0.171879. The left and right screw profiles of Figure 5 are composed of circular arcs whose radii are related to the inner diameter of the casing ^. ^ / ^ or ^^ ^ / ^ , central angle, coordinates of the starting points relative to the coordinate origin ^, ^ or ^', ^' related to the housing inner diameter ^^ ^,^ / ^ and ^^ ^,^ / ^ or ^^′ ^,^ / ^ and ^^′ ^,^ / ^, coordinates of the center of the circular arc relative to the diameter ^^ ^,^ / ^ and ^^ ^,^ / ^ shown in Tables 5A and 5B. Table 5A ^ ^ ^ ^ ^ ^^ ^,^ / ^ ^^ ^,^ / ^ ^ ^,^ / ^ ^ ^,^ / ^ ^ 1 0.060252 1.083640 0.467734 -0.028684 0.414748 0.0000 2 0.790000 0.538798 0.464899 0.033394 -0.192662 -0.404457 3 0.342000 0.114586 0.147077 0.308759 0.0000 0.0000 4 0.790000 0.464125 0.110810 0.323551 -0.145155 -0.423833 5 0.438000 0.102871 -0.250827 0.359068 0.0000 0.0000 6 0.719748 1.076264 -0.286373 0.331413 0.399235 0.112372 7 0.405000 0.162301 -0.118966 -0.387133 0.0000 0.0000 8 0.790000 0.267960 -0.054845 -0.401269 -0.164230 0.381121 9 0.375000 0.171879 0.148401 -0.344387 0.0000 0.0000 10 0.790000 0.495861 0.205116 -0.313931 -0.226995 0.347417 Table 5B 2018PF30224a ^ ^ ^^ ^′ ^ ^^ ^^ / ^ ^^ ^^,^ / ^ ^ ^,^ / ^ ^ ^,^ / ^ ^ 1’ 0,719748 1,076264 -0,219406 0,340420 0,414748 0,0000 2’ 0,438000 0,102871 -0,185869 -0,396607 0,0000 0,0000 3’ 0,790000 0,464125 -0,144159 -0,413597 -0,254559 0,368651 4’ 0,342000 0,114586 0,194328 -0,281426 0,0000 0,0000 5’ 0,790000 0,538798 0,225231 -0,257362 -0,295039 0,337129 6’ 0,060252 1,083640 0,456558 0,093815 0,399235 0,112372 7’ 0,790000 0,495861 0,442467 0,154340 -0,124375 -0,395924 8’ 0,375000 0,171879 0,112387 0,357763 0,0000 0,0000 9’ 0,790000 0,267960 0,049541 0,371713 -0,054826 -0,411363 10’ 0,405000 0,162301 -0,161514 0,371400 0,0000 0,0000

Claims

2018PF30224a Claims 1. Pair of triple-flighted screw elements, wherein the pair of screw elements consists of the screw element SE and the screw element SE', and wherein the pair of screw elements SE and screw element SE' is suitable for a multi-shaft screw machine with ^ co-rotating and co-speed rotating screw shafts SW1 to SWm, whose respective adjacent axes of rotation X1 to Xm have an axial spacing ^ in a cross-section perpendicular to the axes of rotation and with ^ mutually penetrating, circular housing bores, each having an identical inner housing radius ^ and whose bore centers M1 to Mm have a spacing equal to the axial spacing ^, and whose bore centers M1 to Mm coincide with the respective associated axes of rotation X1 to Xm of the screw shafts SW1 to SWm, wherein the screw element SE has a screw profile SP and the screw element SE' has a screw profile SP', wherethat (1) each of the two screw profiles SP and SP' is a closed convex curve, whereby this closed convex curve is composed only of curve sections whose radii of curvature are less than or equal to the center distance ^, (2) either neither of the two screw profiles SP and SP' has mirror symmetry, or the screw profiles SP and SP' are not congruent with each other, (3) each of the two screw profiles SP and SP' has exactly three grooves, (4) each of the two screw profiles SP and SP' has exactly three crests - designated circumferentially with K1, K2, K3 for the screw profile SP and K1', K2', K3' for the screw profile SP', (6) two immediately adjacent screw elements of the pair of screw elements on two of the screw shafts of the multi-shaft screw machine rotating in the same direction and at the same speed practically clean each other in pairs,(7) a groove is separated from the nearest point of an adjacent ridge by a flank, 2018PF30224a (8) it has exactly six curves which are flanks, characterized in that (9) ^(^^) is the crest radius of the crest Ki and ^(^^') is the crest radius of the crest Ki', where: ^(^1) > ^(^2) and ^(^1) > ^(^3) and ^(^1') > ^(^2') and ^(^1') > ^(^3') and ^ ( ^2 ) ≥ ^(^3) and ^ ( ^2′ ) ≥ ^(^3′) and at least one of the conditions a) or b), where a) ^ ( ^2 ) > (^3) and ^(^2') > ^(^3') b) ^ ( ^2 ) not equal to ^ ( ^2' ) and ^ ( ^3 ) not equal to ^ ( ^3' )where r(K1) is the crest radius of the screw crest K1 and r(K2) is the crest radius of the screw crest K2 and r(K3) is the crest radius of the screw crest K3 and r(K1') is the crest radius of the screw crest K1' and r(K2') is the crest radius of the screw crest K2' and r(K3') is the crest radius of the screw crest K3', (10) the screw crest K1 has the crest angle ^^1, the screw crest K2 has the crest angle ^^2, the screw crest K3 has the crest angle ^^3 and the screw crest K1' has the crest angle ^^1', the screw crest K2 has the crest angle ^^2' and the screw crest K3 has the crest angle ^^3', where (10.1) (10.1.a) applies if ^^1 > 0, then ^^1 < ^^2 and ^^1 < ^^3, and (10.1.b) if ^^1' > 0, then ^^1' < KW2 and ^^1 < ^^3, or (10.2) (10.2.a) if ^^1 > 0, then ^^1 < ^^2 and ^^1 < ^^3, and (10.2.c) if ^^1' = 0; then ^^2 >= 0 and ^^3 >= 0, or (10.3) (10.3.b) if ^^1' > 0, then ^^1' < KW2 and ^^1 < ^^3, 2018PF30224a and (10.3.d)if ^^1 = 0, then ^^2 >= 0 and ^^3 >= 0, or (10.4) (10.4.d)if ^^1 = 0, then ^^2 >= 0 and ^^3 >= 0, and (10.4.c) if ^^1' = 0; then ^^2 >= 0 and ^^3 >= 0, where r(K1) is equal to r(K1') is equal to the outer radius ^^ or ^^' of the screw elements and ^ is an integer greater than 1, preferably from 2 to 16, particularly preferably 2, 3, 4, 6, 8, 10, 12 and 16, and ^ can assume the values ​​1, 2 or 3, and the outer radius ^^ is the radius of the circle which encloses the entire screw profile and whose center is the pivot point associated with this screw profile, where the crest K1 is a circular segment of this circle.

2. Pair of screw elements according to claim 1, characterized in that each of the three combs K1, K2 and K3 of the screw element SE is formed from only one curved section, which is a circular arc, the center of which is the pivot point DP common to the combs K1, K2 and K3, and each of the three combs K1',K2' K3' of the screw element SE ' is likewise each formed from only one curved section, the center of which is the pivot point DP' common to the combs K1', K2' and K3', and each of the three grooves of the screw element SE is formed from only one curved section, which is a circular arc, the center of which is the pivot point DP of the screw element SE common to the three grooves, and wherein there is exactly one groove whose distance from the pivot point DP is smaller than the distances of the other grooves from the pivot point DP, and that each of the three grooves of the screw element SE' is formed from only one curved section, which is a circular arc, the center of which is the pivot point DP' of the screw element SE common to the three grooves. 2018PF30224a 3. A pair of screw elements according to claim 1 or 2, characterized in that both the screw profile SP, the screw crest with the largest crest radius r(Ki), i.e., crest K1, has the smallest crest angle of the crests K1, K2, and K3, and the screw profile SP', the screw crest with the largest crest radius r(Ki)', i.e., crest K1', has the smallest crest angle of the crests K1', K2', and K3'.

4. A pair of screw elements according to one of the preceding claims 1 to 3, characterized in that both the screw profile SP is continuously differentiable at all of the crests K1, K2, and K3, and the screw profile SP' is continuously differentiable at all of the crests K1', K2', and K3'. 5.A pair of screw elements according to one of claims 1, 2 or 4, characterized in that each of the three crests K1, K2 and K3 of the screw element SE is each formed from exactly one point and likewise each of the three crests K1', K2' and K3' of the screw element SE' is each formed from exactly one point.

6. A pair of screw elements according to one of claims 1 to 4, characterized in that the crests K1 and K1' are formed from exactly one point and the crests K2 and K3 are each formed from exactly one circular arc whose respective center coincides with the pivot point DP, and the crests K2' and K3' are each formed from exactly one circular arc whose respective center coincides with the pivot point DP'.

7. A pair of screw elements according to one of claims 1 to 4, characterized in that all crests of the screw profile SP and all crests of the screw profile SP' have a crest angle greater than zero. 8.Pair of screw elements according to one of claims 1 to 7, characterized in that for a given housing inner radius ^ in relation to a given axial distance ^ and a given housing inner diameter ^ = 2 ^ the distance ^(^1) of the comb K1 from the pivot point is selected such that for ^(^1) or ^(^1') the range applies: ^(^1) is less than or equal to ^ - 0.002 ^ and ^(^1) is greater than or equal to ^ - 0.05 ^ and ^. ( ^1 ^) is less than or equal to ^ – 0.002 ^ and 2018PF30224a ^(^1′) is greater than or equal to ^ – 0.05 ^.

9. Pair of screw elements according to one of claims 1 to 8, characterized in that for a given housing inner radius ^ in relation to a given axial distance ^ and a given housing inner diameter ^ = 2 ^ the distance ^(^1) of the comb K1 from the pivot point is selected such that for the combs K2 and K2' the range applies: ^(^2) is less than or equal to ^(^1) - 0.002 ^ and r(K2) is greater than or equal to ^(^1) - 0.08 ^ and ^(^2′) is less than or equal to ^(^1′) - 0.002 ^ and r(K2') is greater than or equal to ^(^1′) - 0.08 ^, wherein additionally ^(^2) is greater than (^(^1) + ^(^1)) / 2 and r(K2') is greater than (^(^1') + ^(^1')) / 2. 10.Pair of screw elements according to one of claims 1 to 9, characterized in that for a given housing inner radius ^ in relation to a given axial distance ^ and a given housing inner diameter ^ = 2 ^ the distance ^(^1) of the crest K1 from the pivot point is selected such that for the crests K3 and K3' the range preferably applies: r(^3) is greater than or equal to ^(^1) - 0.09 ^ and ^(^3) is less than or equal to ^(^2) and ^(^3) is greater than or equal to ^(^2) - 0.05 ^ and r(^3′) is greater than or equal to ^(^1′) - 0.09 ^ and ^(^3′) is less than or equal to ^(^2′) and ^(^3′) is greater than or equal to ^(^2′) - 0.05 ^ and (^(^1) + ^(^1)) / 2 and r(K3') is greater than (^(^3') + ^(^1')) / 2.

11. Pair of screw elements according to one of claims 1 to 10, characterized in that for the screw profiles SP and SP' independently of one another, the crest angle KW10 degrees < KW1 < 10 degrees and preferably 2 degrees < KW1 < 8 degrees. 2018PF30224a and the crest angle KW1' is 0 degrees < KW1' < 10 degrees and preferably 2 degrees < KW1' < 8 degrees.

12. A pair of screw elements according to one of claims 1 to 11, characterized in that the screw profiles of the two screw elements SE and SE' are either rotationally congruent or the screw profiles SP and SP' of the two screw elements SE and SE' are congruent in opposite directions.

13. Use of a pair of screw elements according to one of claims 1 to 12 in a multi-screw extruder.

14. A multi-screw extruder equipped with a pair of screw elements according to one of claims 1 to 12.

15. A process for extruding plastic or viscoelastic masses using a pair of screw elements according to claim 1 or using a screw extruder according to claim 14.

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