Mixing device in the screw antechamber of a twin-screw extruder

Abstract

The invention relates to a mixing device for a twin-screw extruder, in particular a twin-screw extruder rotating in the opposite direction, comprising a first screw and a second screw, for mixing a melt flow in a screw antechamber, wherein the first screw has an extension in the form of a mixing screw tip connected to the first screw in a fixed manner, and a first melt channel, into which the first screw feeds, is arranged in a flow-connected manner via an elongated slot with a second melt channel, into which the second screw feeds.

Claims

1. A mixing device for a counter-rotating twin-screw extruder for the extrusion of PVC, the mixing device comprising: parallel or conically arranged screws; a first screw and a second screw configured and arranged for mixing a melt stream, the first screw including an extension in the form of a mixing screw tip, at least one screw flight and at least one wing arranged substantially paraxially or helically wound on a tip core of the mixing screw tip, and a pitch (P) of the at least one screw flight is greater than a nominal diameter (ND) of the first screw; a first screw antechamber including a first melt channel with a cylindrical shape and configured and arranged to receive from the first screw; and a second screw antechamber including a second melt channel; wherein the first melt channel is arranged in a flow-connected manner via an elongated slot, the slot is formed by overlap of the first melt channel with the second melt channel, into which the second screw conveys; wherein the pitch (P) is twice to five times as large as the nominal diameter (ND), and in that the mixing screw tip extends over a length (L) in the direction of a longitudinal axis (B) which is equal to or greater than the nominal diameter (ND) of the first screw.

2. The mixing device of claim 1, wherein the length (L) is twice to ten times the nominal diameter (ND).

3. The mixing device of claim 1, wherein the length (L) is twice to five times the nominal diameter (ND).

4. The mixing device of claim 1, wherein the mixing screw tip is manufactured separately and rigidly connected to the first screw.

5. The mixing device of claim 1, wherein the mixing screw tip has a tip core which on an inlet side has a common core diameter (KD) with the first screw, and the tip core tapers conically to a tip in a conveying direction (A).

6. The mixing device of claim 1, wherein the mixing screw tip has a tip core which on an inlet side has a common core diameter (KD) with the first screw, wherein the tip core conically widens at first in a conveying direction (A) to a thickened portion and then tapers conically to a tip.

7. The mixing device of claim 5, further including a static mixing element placed after the mixing screw tip in the conveying direction (A), the static mixing element configured and arranged to stop a forced rotation of the melt through the mixing screw tip.

8. A twin screw extruder comprising: the mixing device of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention will be described in more detail with reference to the non-limiting figures, wherein:

(2) FIG. 1 shows a mixing device according to the invention for a parallel twin-screw extruder in a first embodiment in an oblique view in half section;

(3) FIG. 2 shows the mixing device of the first embodiment in a plan view.

(4) FIG. 3 shows a mixing screw tip of a mixing device according to the invention in a first embodiment in a plan view;

(5) FIG. 4 shows a mixing screw tip in a second embodiment according to FIG. 1 in plan view;

(6) FIG. 5 shows a mixing adapter in an oblique view in half section;

(7) FIG. 6 shows a mixing element of the mixing device according to patent AT 506 577 in an oblique view.

DETAILED DESCRIPTION

(8) The terms “inlet side” and “outlet side” refer to a conveying direction A.

(9) FIG. 1 shows an oblique view of a mixing device. A mixing adapter 2 is shown in half section. This is used instead of the normal adapter of a twin-screw extruder, which is extended by about 1 to 10 nominal diameters. On the inlet side, a first screw 3 and a second screw 4 are shown, here for a so-called parallel twin-screw extruder with counter-rotating screws 3, 4. These screws are part of a twin-screw extruder 1, which is not shown in the figures. Only the outlet side of the two screws is shown. Instead of a first screw tip 5, which is not shown in the figures, of the first screw 3, a mixing screw tip 6 is provided which co-rotates with the first screw 3. A second screw tip 7 of the second screw 4 is unchanged.

(10) The mixing screw tip 6 is accommodated by a cylindrical first screw antechamber 8.

(11) A first melt (not shown), which is ejected from the first screw 3, is ejected substantially into this cylindrical first screw antechamber 8 and would flow without mixing screw tip 6 mainly in the axial direction of the longitudinal axis B. The rotating mixing screw tip 6 with attached screw flights 9 or wings cause additional rotation of the melt about the longitudinal axis B, which superimposes the longitudinal flow.

(12) A second melt (not shown) of the second screw 4 is ejected into an approximately obliquely conical second screw antechamber 10. With decreasing cross-section of this second screw antechamber 10, the melt is displaced in the direction of the cylindrical first screw antechamber 8 of the first screw 3 and, as it were, is wound up on the outside of the rotating melt stream of the first screw 3. A total melt stream at the end 11 of the mixing screw tip 6 thus consists of a slightly less mixed inner region and of several wound, thin layers in the outer region. As a result of heat conduction and over short distances, temperature differences in adjacent layers are compensated in a short time, so that there is a substantially uniform temperature in the outer region of the melt stream at an outlet 12 of the twin-screw extruder 1.

(13) Following the mixing screw tip 6, a static mixing element 13 can also be provided, which is likewise shown in FIG. 1. This mixing element 13 causes an additional mixing effect, which is particularly desirable for the inner region of the total melt stream.

(14) In addition, the rotation of the melt forced by the mixing screw tip 6 is thereby stopped, so that the melt has a linear flow after flowing through this mixing element 13.

(15) FIG. 2 shows the same situation as FIG. 1, in plan view. The two screws 3, 4 have an opposite direction of rotation, a first screw thread 14 and a second screw thread 15 move apart on the upper side. The two screws 3 and 4 have an outer diameter ND at the screw tip, which is referred to as the nominal diameter of the twin-screw extruder. The core diameter KD denotes the diameter of the screw core at the screw tip. The cylindrical first screw antechamber 8 of the first screw 3 can be clearly recognized here, in which the mixing screw tip 6 with the integrally formed screw flights 9 or wings are rotating.

(16) The second screw 4 is associated with an approximately oblique tapered screw antechamber 10. The melt stream from the second screw 4 is introduced obliquely into the melt stream of the first screw 3.

(17) In FIG. 3, a first embodiment of the mixing screw tip 6 is shown in plan view. In FIG. 3 on the left, a thread 26 is shown, with which the mixing screw tip 6 is screwed to the first screw 3, so that it is rigidly connected to the screw 3. On the inlet side, based on the conveying direction A of the melt in the twin-screw extruder 1, a tip core 16 is flush with a core 17 of the first screw 3, whereafter the tip core 16 tapers conically and eventually ends in a tip 18.

(18) Integrally formed on the tip core 16 are in this first embodiment steeply running, relatively thin screw flights 9 with a pitch P of about five nominal diameters ND. Instead of these screw flights, axially extending wings can be provided, which are, however, not shown in the figures. These screw flights 9 or wings fulfill an important task: They force a superposition of the flow originally taking place in the direction of the longitudinal axis B with a rotational movement which is essential for the mixing effect. In addition, screw flights 9 instead of the wings contribute to the longitudinal flow due to a similar mode of operation as in a single-screw extruder. The number of screw flights 9 or wings is preferably to be chosen unequal to the number of threads of the screws 3 and 4 in the discharge zone, thus three mixing blades of the mixing screw tip 6 at double-threaded screws 3, 4 and vice versa. The reason for this is the avoidance of regular superimposition of various effects.

(19) The first screw antechamber 8 has a diameter d, which corresponds to the nominal diameter ND. The outer circumference of the screw flights 9 or wings fills the cylindrical first screw antechamber 8 by leaving a small gap, which is about 1 mm in this case. Optionally, this gap can also be chosen to be larger or increase in the conveying direction A from initially about 1 mm to 10 mm or more. To force the rotation flow, it is not necessary in the extrusion of PVC that the screw flights 9 or wings reach particularly closely to the cylinder wall of the screw antechamber 8, because PVC melts tend to slide on the wall.

(20) The conveyance of the melt in the direction of the longitudinal axis B is mainly caused by the screws 3, 4. The longitudinal flow causes a certain pressure drop in the conveying direction A to overcome the friction and flow resistance. As a result of the conveying action of the helical screw flights 9 due to a pitch P, this pressure drop is slightly reduced. The first screw 3 and the second screw 4 have double-threaded screws in this case whose pitch P1 is smaller than the pitch P. Since the pitch P is significantly larger than can be drawn in FIG. 3, the measure is entered in a symbolically shortened manner. Instead of the helical screw flights 9, wings or flights extending in the axial direction of the longitudinal axis B can be integrally formed on the tip core 16, as a result of which, however, the conveyance of the melt in the direction of the longitudinal axis B is not supported.

(21) FIG. 4 shows a modified embodiment of the mixing screw tip 6 according to FIG. 3. In this case, the nominal diameter KD after the first screw 3 increases conically, then remains a short distance unchanged and then tapers conically, so that the tip core 16 then expires in the tip 18.

(22) Between the thickened tip core 16 and the wall of the cylindrical first screw antechamber 8, a constriction 19 is formed, which causes an increased flow resistance for the first melt stream. A part of this melt stream is therefore deflected to the second melt stream. After the constriction 19, said stream in turn flows to the first cylindrical screw antechamber 8 and is thereby subjected to the desired mixing effect.

(23) In FIG. 5, the mixing adapter 2, thus the housing for the two screw antechambers 8, 10 and for receiving the static mixing element 13, is shown in an oblique view in half section. The plane of intersection is at the same time the mirror plane for forming the other half of the mixing adapter 2. On the inlet side, the inner contour of the mixing adapter 2 is adapted to a so-called eyeglass bore of an extrusion cylinder, which are two intersecting circles on the front side.

(24) The mixing zone in the mixing adapter 2 has three characteristic areas: a cylindrical first melt channel 23 as a continuation of the first screw 3, an approximately oblique conical-shaped second melt channel 24 as a continuation of the second screw 4 and a cylindrical recess 25 for the mixing element 13.

(25) The screw antechamber 8 for the first screw 3 extends cylindrically co-linearly in the axial direction B. This applies to parallel and conical TSE. The one for the second screw 4 narrows in an approximately obliquely tapered manner and runs on the outlet side blade-shaped on a cylindrical surface. The two basic forms of the screw antechambers 8, 10 overlap in a gusset region, so that here an elongate, conically tapering slot 20 remains free for the passage of the second melt stream. The melt streams initially discharged separately from the first screw 3 and the second screw 4 gradually merge over the length of this slot 20 into a total melt stream.

(26) In the second screw antechamber 10, one or more longitudinal flights can be provided, which prevent a rotation of the second melt stream. Without such longitudinal flights, the second melt stream, at the point of contact with the rotating first melt stream, would be excited to rotate in opposite directions of rotation. A pure longitudinal flow of the second melt stream is advantageous for the desired mixing process since in that case similar to the sharpening of a pencil it is not the shell layers lying on the conical surface that are “planed” with relatively uniform temperature but parabolic cross-sectional areas, which include all temperature ranges over the cross-section. In addition, this meets the requirements of symmetry resolution: A first, rotating melt stream and a second, non-rotating melt stream are combined to form a total melt stream.

(27) The mixing adapter 2 can be heated by means of heating jackets.

(28) FIG. 6 shows the inlet-side view of a suitable static mixing element 13. This mixing element 13 is inserted into the corresponding opening of the mixing housing 2. After the total melt stream has flowed through the openings 21 shown, it has a pure longitudinal flow. The purpose of the mixing element 13 is to stop the rotation of the melt forced by the mixing screw tip 6 and to introduce an additional mixing effect.