MIXING ATTACHMENT FOR A SCREW

Abstract

A mixing attachment for a screw, in particular an extruder screw in an extruder or in an injection-molding machine, comprising an essentially rotationally symmetrical lateral surface, the diameter of which increases from a first end to a second end of the lateral surface, and a securing device, which is provided in the region of the first end of the lateral surface, for securing the mixing attachment at a downstream end of the screw, wherein the lateral surface is perforated by a plurality of apertures, in particular bores.

Claims

1. A mixing attachment (10) for a screw (12), in particular an extruder screw (12) in an extruder or in an injection-moulding machine, comprising an essentially rotationally symmetrical lateral surface (18), the diameter of which increases from a first end to a second end of the lateral surface (18), and a securing device, which is provided in the region of the first end of the lateral surface (18), for securing the mixing attachment (10) at a downstream end of the screw (12), wherein the lateral surface (18) is perforated by a plurality of apertures (20), in particular bores (20).

2. The mixing attachment (10) according to claim 1, wherein the securing device comprises a thread.

3. The mixing attachment (10) according to claim 1, wherein the lateral surface (18) is formed at least partly in a frustoconical shape.

4. The mixing attachment (10) according to claim 1, wherein the apertures (20) are configured as bores (20).

5. The mixing attachment (10) according to claim 1, wherein at least a portion of the apertures (20) or bores (20) perforates the lateral surface (18) essentially orthogonally.

6. The mixing attachment (10) according to claim 1, wherein the diameter (D.sub.Boh) of the apertures (20) or bores (20) is essentially identical.

7. The mixing attachment (10) according to claim 1, wherein the apertures (20) or bores (20) have different diameters (D.sub.Boh).

8. The mixing attachment (10) according to claim 1, wherein the edge of the lateral surface (18) has at its second end an essentially circular circumference.

9. The mixing attachment (10) according to claim 1, wherein the edge of the lateral surface (18) has at its second end a flow-influencing pattern.

10. The mixing attachment (10) according to claim 9, wherein the flow-influencing pattern comprises at least one notch (22) or at least one tooth.

11. The mixing attachment (10) according to claim 1, wherein the thickness of the lateral surface (18) decreases from its first end to its second end.

12. The mixing attachment (10) according to claim 1, the axial length (L) of which from the first end to the second end of the lateral surface (18) is essentially 1.5 times the diameter (D.sub.max) of the lateral surface (18) at its second end.

13. A screw (12), comprising a mixing attachment (10) according to claim 1.

14. An extruder or injection-moulding machine, comprising a screw (12) according to claim 13.

Description

[0017] An embodiment of the invention will be explained below with the aid of the single FIGURE as a non-restrictive example. Herein:

[0018] FIG. 1 shows a lateral view, partly in section, of an embodiment of the mixing attachment according to the invention with use on a screw of an extruder or respectively injection-moulding machine.

[0019] FIG. 1 shows a lateral view, partly in section, of a mixing attachment 10 according to the invention. It is secured by screwing at the downstream end, indicated partly in section on the left in the FIGURE, of a horizontally oriented screw 12 of an extruder or respectively injection-moulding machine. The cylinder 14 of the extruder or respectively injection-moulding machine is likewise illustrated in section, in which the screw 12 rotates, in order to discharge melted and compacted extrusion material through the discharge opening 16 on the right in FIG. 1 into a tool which is to be filled.

[0020] Above the rotation axis X of the screw 12, indicated by a dot-and-dash line in the FIGURE, and of the mixing attachment 10, the mixing attachment 10 is illustrated in section, beneath the rotation axis X not in section, i.e. the FIGURE shows here the outer side of its lateral surface 18.

[0021] In the embodiment illustrated in the FIGURE, the lateral surface 18 has essentially the shape of a horizontal truncated cone with the rotation axis X as central symmetry axis. At the left-hand first axial end of the lateral surface 18 in the FIGURE, its diameter corresponds essentially to that of the screw 12. The diameter of the lateral surface 18 therefore increases from its first axial end, on the left in the FIGURE, close to the downstream end of the screw 12, to its second axial end, on the right in the FIGURE, close to the discharge opening 16, essentially continuously up to a maximum diameter D.sub.max. This is slightly smaller than the internal diameter D.sub.Zyl of the cylinder 14.

[0022] Therefore, only an annular gap with a width of (D.sub.ZylD.sub.max)/2 remains for the melt conveyed by the screw 12, in order to flow past the downstream end of the lateral surface 18 of the mixing attachment 10 to the discharge opening 16. The majority of the melt flows through apertures 20 in the lateral surface 18 from its outer side to its inner side and from there to the discharge opening 16. In the present example embodiment, the apertures 20 are configured as bores 20. As the lateral surface 18 of the mixing attachment 10 according to the invention rotates together with the screw 12 during operation of the extruder or respectively injection-moulding machine, the material stream of the melt is divided hereby into a plurality of individual strands which are mixed with one another. This is indicated in the FIGURE by individual arrows. Hereby, the material- and temperature homogeneity of the melt are improved according to the invention.

[0023] In the embodiment illustrated in the FIGURE, the lateral surface 18 has in axial direction seven bores approximately in a row, the diameter D.sub.Boh of which is greater than the width of the annular gap (D.sub.ZylD.sub.max)/2. Depending on the rheological characteristics of the extrusion material and/or resistance of the tool which is to be filled, the number of the bores 20 and/or their diameter D.sub.Boh can also be selected differently. Likewise, it is possible to provide the bores 20, differently to in the FIGURE, not in a row in axial direction but rather for example running helically around the lateral surface 18.

[0024] The lateral surface 18 has at its second axial end, on the right in the FIGURE, a flow-influencing pattern in the form of several notches 22. These bring about an additional intermixing of the portion of the melt which flows through the annular gap between the lateral surface 18 and the inner wall of the cylinder 14. Alternatively or additionally to such notches 22, the flow-influencing pattern can also comprise one or more teeth which project from the lateral surface 18 radially outwards in the direction of the inner wall of the cylinder 14.

[0025] The mixing attachment 10 according to the invention can expediently have an axial length L in the direction of flow of the melt of approximately L=1.5*D.sub.max. In practice, e.g. cylinders 14 with a diameter D.sub.Zyl of approximately 90 mm are used. In this case, the diameter D.sub.max of the lateral surface 18 can be expediently approximately 87 mm at its downstream second end, so that an axial length of the mixing attachment 10 of approximately 130 mm is expedient. At the same time, an annular gap of width (D.sub.ZylD.sub.max)2=2.5 mm remains between the lateral surface 18 and the inner wall of the cylinder 14 for a direct flowing past of the melt, bypassing the bores 20.

[0026] The total number of bores 20, which perforate the lateral surface 18 depends essentially on the diameter of the screw 12, which corresponds to the diameter of the lateral surface 18 at its first end, and on the length L of the lateral surface 18 in axial direction. With the above-mentioned exemplary values, expediently approximately 100 bores 20 can be provided distributed over the entire lateral surface 18. In practice, in extruders or respectively injection-moulding machines, screws are used in a diameter range of approximately 25 mm to approximately 265 mm, and mixing attachments 10 according to the invention which are suitable for this can be produced for this entire range, if applicable with a correspondingly smaller or greater total number of bores 20.

[0027] The mixing attachment 10 according to the invention makes possible in all embodiments an improved intermixing of the melt and therefore a better material- and temperature homogeneity of the extrusion material. For this, the lateral surface 18 does not imperatively have to have the frustoconical shape illustrated in the FIGURE, as long as its diameter increases in the direction of flow of the melt, which is also ensured in the case of a rotationally symmetrical lateral surface 18 with a stepped surface line. In addition, for reasons of ease of maintenance and the possibility of providing an extruder or respectively an injection-moulding machine for the processing of a different extrusion material with a different mixing attachment, it is in fact preferred if the mixing attachment 10 can be connected to the screw in a detachable manner by a detachable securing device such as e.g. a thread. However, a non-detachable securing device is basically also included by the invention, e.g. in the form of a welded connection, by which a single-piece configuration of a mixing attachment 10 according to the invention is realized as an integral tip of a screw 12.