Screw elements for extruding plastic compositions

1. Screw elements for multi-screw extruders with screws co-rotating in pairs and being fully self-wiping in pairs, the screw elements comprising

a pair of screw elements having tip angles and tip zones,

wherein a first screw element of the pair of screw elements comprises a generating screw profile and a second screw element of the pair of screw elements comprises a generated screw profile, and wherein the generating screw profile and the generated screw profile are convex profiles consisting of arcs having non-infinite radiuses,

wherein the tip zones are equal to an outer screw radius,

wherein a sum of all the tip angles of the pair of screw elements is greater than 0 and less than 2*π−8*arccos(0.5*a/ra), and further

wherein a is the centreline distance of the first and second screw elements and ra is the outer screw radius of the first and second screw elements,

the number of tip zones of the pair of screw elements is greater than 4,

each screw element of the pair of screw elements comprises exactly two screw profile closing zones, consisting, in each case, of the sequence tip zone, flank zone, tip zone,

each screw element of the pair of screw elements comprises exactly two screw profile channel zones, consisting, in each case, of flank zones and grooved zones, and, in each case, beginning with a flank zone and ending with a flank zone, and in which the sum of the flank and groove angles of each channel zone is in each case greater than π/2, and

the grooved zones are equal to a core radius and the flank zones are smaller than the outer screw radius and larger than the core radius.

2. The screw elements according to claim 1 , wherein, after a rotation of π/2, the generated screw profile of the second screw element is identical in shape and orientation to the generating screw profile of the first screw element.

3. The screw elements according to claim 1 , wherein, after a rotation of π/2, the generated screw profile of the second screw element is different in shape and orientation from the generating screw profile of the first screw element.

4. The screw elements according to claim 1 , wherein the generating screw profile and the generated screw profile each comprise a sequence of channel zone-closing zone-channel zone-closing zone.

5. The screw elements according to claim 4 , wherein a closing angle of at least one of the closing zones is greater than the 1.2 times a tip angle δ_kw of a double-flighted Erdmenger screw profile, wherein the tip angle δ_kw of a double-flighted Erdmenger screw profile with 2 flights is calculated as δ_kw=π/z−2*arccos(0.5*a/ra), wherein π (π≈3.14159) is the circle constant.

6. The screw elements according to claim 1 , wherein the outer screw radius of the screw elements normalized to the centreline distance lies in the range from 0.51 to 0.66.

7. The screw elements according to claim 1 , wherein the screw elements are configured as conveying elements or mixing elements by extending the screw profiles helically in the axial direction, or as kneading elements by extending the screw profiles in portions in an offset manner in axial direction.

8. The screw elements according to claim 1 , further comprising clearances between at least one of (i) each screw element and a barrel and (ii) neighbouring screw elements.

9. The screw elements according to claim 1 , wherein the number of tip zones of a pair of screw elements is equal to 8.

10. A method of using screw elements in multi-screw extruders, the method comprising:

producing a pair of screw elements, having tip angles and tip zones, wherein the tip zones are equal to the outer screw radius, wherein a sum of all the tip angles of the pair of screw elements is greater than 0 and less than 2*π-8*arccos(0.5*a/ra), and a total number of tip zones of the pair of screw elements is greater than 4, wherein each screw element of the pair of screw elements comprises exactly two screw profile closing zones, consisting, in each case, of the sequence tip zone, flank zone, tip zone, wherein each screw element of the pair of screw elements is provided with exactly two screw profile channel zones, consisting, in each case, of flank zones and grooved zones, and, in each case, beginning with a flank zone and ending with a flank zone, and in which a sum of the flank and groove angles of each channel zone is, in each case, greater than π/2, wherein the grooved zones are equal to a core radius and the flank zones are smaller than the outer screw radius and larger than the core radius, and wherein each screw element has a convex screw profile consisting of arcs having non-infinite radiuses; and

providing a multi-screw extruder comprising the pair of screw elements, wherein the pair of screw elements are co-rotatable in pairs and fully self-wiping in pairs.

11. A process for extruding plastic compositions in multi-screw extruders having screw elements, the process comprising:

providing a pair of screw elements in a multi-screw extruder, wherein the pair of screw elements are co-rotatable in pairs and fully self-wiping in pairs, wherein the pair of screw elements has a sum of all tip angles greater than 0 and less than 2*π−8*arccos(0.5*a/ra), wherein a number of tip zones of the pair of screw elements is greater than 4, wherein the tip zones are equal to an outer screw radius, wherein each screw element of the pair of screw elements comprises exactly two screw profile closing zones, consisting, in each case, of the sequence tip zone, flank zone, tip zone, wherein each screw element of the pair of screw elements are provided with exactly two screw profile channel zones, consisting, in each case, of flank zones and grooved zones, and, in each case, beginning with a flank zone and ending with a flank zone, and in which a sum of the flank and groove angles of each channel zone is, in each case, greater than π/2, wherein the grooved zones are equal to a core radius and the flank zones are smaller than the outer screw radius and larger than the core radius, and wherein each screw element has a convex screw profile consisting of arcs having non-infinite radiuses; and

extruding one or more plastic compositions in the multi-screw extruder using the pair of screw elements.

12. The process according to claim 11 , wherein the one or more plastic compositions are thermoplastics or elastomers.

13. The process according to claim 12 , wherein the thermoplastics used are polycarbonate, polyamide, polyester.

14. The process according to claim 12 , wherein the thermoplastics used are polybutylene terephthalate and polyethylene terephthalate, polyether, thermoplastic polyurethane, polyacetal, fluoropolymer.

15. The process according to claim 12 , wherein the thermoplastics used are polyvinylidene fluoride, polyether sulfones, polyolefin.

16. The process according to claim 12 , wherein the thermoplastics used are polyethylene and polypropylene, polyimide, polyacrylate.

17. The process according to claim 12 , wherein the thermoplastics used are one of poly(methyl)methacrylate, polyphenylene oxide, polyphenylene sulfide, polyether ketone, polyarylether ketone and styrene polymers.

18. The process according to claim 12 , wherein the thermoplastics used are polystyrene or styrene copolymers.

19. The process according to claim 12 , wherein the thermoplastics used are styrene-acrylonitrile copolymer, acrylonitrile-butadiene-styrene block copolymers, polyvinyl chloride or a blend of at least two of the stated thermoplastics.

20. The process according to claim 12 , wherein the elastomers used 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, ethylenevinyl acetate rubber, polyurethane rubber, thermoplastic polyurethane, gutta percha, acrylate rubber, fluororubber, silicone rubber, sulfide rubber, chlorosulfonyl-polyethylene rubber or a combination of at least two of the stated elastomers.

21. The process according to claim 11 , wherein filler or reinforcing materials or polymer additives or organic or inorganic pigments or mixtures thereof are added to the one or more plastics compositions.