DE102010048925B4 - Extruder for processing plastics - Google Patents

Abstract

Extruder with a screw (3) provided with at least one screw web (1, 2), which is rotatably mounted in a cylinder (4), the cylinder (4) having a feed zone (5) extending over part of its axial length, which is provided with at least one groove (7) formed between adjacent cylinder webs (6), a melting zone (8) being provided adjacent to the feed zone (5), characterized in that the cylinder (4) adjoins the feed zone () in the conveying direction ( 5) has a diameter which is equal to the diameter in the base of the grooves (7), that the screw (3) in the conveying direction adjacent to the feed zone (5) has an abruptly enlarged diameter than in the area of the feed zone (5), and that the feed zone (5) is formed with a constant diameter.

Description

The invention relates to an extruder for processing plastics, according to the features of the preamble of claim 1, as it is for example in the DE 199 28 870 A1 is described. In particular, the invention particularly relates to a single-screw extruder, but is not limited thereto.

Grooved-nut extruders have been known for many years and are used to achieve a high and sometimes back pressure-independent conveying effect by increasing the solids friction on the cylinder or increasing the transferable shear forces. In order to be able to transmit as high a shear force as possible to the cylinder, axially parallel or helical tapered grooves (decreasing groove depth) with different cross sections (eg rectangular, crescent-shaped) are provided over a length corresponding to about three to five times the screw diameter , half round) incorporated. The grooves are used here in the solids conveying zone and usually run out before the end of the solids conveying zone. The worm has a constant outer diameter over its entire length. Due to the high conveying effect and the geometric narrowing of the available volume at the end of the grooves, a high pressure can be built up and a largely counter-pressure-independent delivery can be realized.

The high pressure at the end of the grooves in turn causes increased wear on the worm and grooved bushing. Due to the high pressure build-up, especially at high screw speeds, there is a heating of the plastic, which can lead to the formation of a melt film and a reduction of the specific output (output per screw revolution).

One approach to reducing this pressure increase is to make the groove continuous and to provide not only the solids conveying zone but also the melting zone and the discharge zone with grooves in the cylinder. Such a system is in the DE 199 28 870 A1 described. However, this concept has the disadvantage that, especially in the region of the discharge zone, stagnations of the molten polymer can occur in the grooves, which can lead to increased residence time and degradation of the polymer.

An extruder screw is usually divided into three zones, the solids conveying zone, the melting zone and the melt conveying zone. In the solids conveying zone, the polymer to be processed is present as a solid and is conveyed forward in the screw flight. In the melting zone, which can be designed conventionally or with a barrier web on the screw, the polymer is melted by means of shear and heat until at the end of this zone there is only melt. In the melt conveying zone, the homogenization of the melt takes place. If necessary, there are also shearing and / or mixing parts on the screw.

The terms front and rear refer to the material flow: at the rear from the filling opening along the screw towards the front of the screw. At the front end of the extruder, a shaping tool is usually arranged via a flange.

The JP S60-210 420 A describes an extruder in which grooves are provided in the feed zone, whose radial inner diameter is constant. Accordingly, the worm also has a constant outside diameter in this area. At the transition from the feed zone to the melting zone, the cylinder begins to conically expand. In this case, the worm is conical.

The JP S60-210 419 A describes an extruder design in which the cylinder and the screw in the melting zone each have a constant diameter, while the screw is formed conically in the region of the feed zone. Accordingly, the depth of the grooves decreases continuously, resulting in a conical envelope.

From the DE 34 21 634 A1 It is known to reduce the depth of the grooves in the region of the transition zone, so that the inner diameter of the grooves in the axial direction decreases and the grooves in the transition zone become flatter.

The US 5 909 958 A shows a cylinder of constant diameter and a screw with a constant diameter. However, an adjustable element is provided with which the effective in the range of this element inner diameter of the cylinder can be changed.

From the DE 199 28 870 A1 It is known to change the outer diameter of the screw by different diameters of the screw flights.

The invention has for its object to provide an extruder of the type mentioned, which has a high efficiency and avoids the disadvantages of the prior art with a simple structure and simple, cost-effective manufacturability.

According to the invention the object is achieved by the combination of features of claim 1, the dependent claims show further advantageous embodiments of the invention.

The aim of the invention is thus to realize a high specific output over a wide speed range, to keep the pressure rise especially at the end of the grooves and the temperature increase of the melt low. A further goal is therefore an optimal self-cleaning of the grooves, so that no material stagnates and degrades in the grooves. In addition to avoiding material degradation, good self-cleaning also plays an important role in the processing of various formulations and the necessary material and color changes. Likewise, it should be ensured by the novel extruder that sufficient plastic granules are drawn and thus a high output at the appropriate extruder speed is possible.

In order to achieve an increased conveying effect, a positive retention of the granulate is necessary in the solids conveying area. This positive connection is produced by so-called grooves.

Instead of incorporating grooves in the material of the cylinder, webs can protrude from the cylinder and in turn form grooves in the space. Both variants are possible for the invention. For ease of understanding, however, it will be discussed in the following description of grooves.

In the extruder according to the invention not the grooves are leaking, but the webs. The diameter of the cylinder at the end of the grooves corresponds approximately to the diameter before the leakage of the grooves plus twice the groove depth. By this design measure results in the end of the grooves, a larger volume and thus a pressure spike is avoided.

At the same time, the outside diameter of the screw increases in the area where there are no more grooves. The outer diameter of the screw increases approximately twice the depth of the grooves at the end. As a result, corresponds to the outer diameter of the screw, except for a necessary clearance, the cylinder diameter.

The core diameter of the screw can remain the same, and also increase or decrease. Simultaneously with the change in the outer diameter, the flight depth, the pitch, the web width or the flexibility of the screw can be changed.

In a further embodiment variant, the grooved area is also longer and the grooves only run out in the region of the melting zone. The length of the grooved area may be between 3 times and 20 times, preferably between 4 times and 15 times the length or between 5% to 80%, preferably between 10% to 60% of the total length of the screw. It has been shown that, especially at the beginning of the melting zone, the grooves can accelerate the melting process. However, if there is already more melt such as solid, no improvement is achieved by the grooves, so that they are designed to be ending in the second region of the melting zone.

The end of the webs, so the end of the grooved zone, may be stepped, or tapered conically. If the end is graded, there will also be a gradual increase in the external diameter of the screw in the screw, while in the case of a conical outlet, the external diameter of the screw will also increase conically.

The screw flight can pass through and is correspondingly increased in the area of the grooves, so that the outer diameter is equal to the cylinder diameter except for a necessary clearance. But it is also possible to interrupt the screw bridge and to promote further with a new screw bridge. The webs can also be offset from each other.

This area preferably also corresponds to the grooved area of the cylinder. With the conical feed area, a larger amount of plastic granules, powder, semolina or recyclate, can be fed. This is particularly advantageous in so-called high-speed rotors, ie extruders which are operated at a high screw speed. Likewise, this is advantageous in the processing of materials with low bulk density.

The embodiment in the grooved area can be embodied as a grooved bushing, as grooves machined in the cylinder, or by webs which protrude into the cylinder. The depth of the grooves can remain constant over the length, or have any profile of a depth profile. It is also possible, in particular if the grooves are formed by webs, that they are adjustable and thus the conveying action of the extruder is adjustable.

The cross-sectional geometry of the grooves can, as shown in the drawing, be approximately cuboid, but also triangular, half-round or any other arbitrary geometry. The cross-sectional area of the grooves to the cross-sectional area of the webs can take any ratio. Likewise, only one groove can be used. The grooves can be purely axial, or designed as helical grooves. It is possible to make the grooves in a region of the cylinder axially and then coiled, or vice versa.

The screw used can be a classic three- or multi-zone screw or a barrier screw.

The cylinder can be made in one piece, or even consist of several parts, which are connected to a cylinder. The division of the cylinder can be done both axially, as well as radially and also multiple times. By a multi-piece manufacturing results in the possibility of disassembly and thus a simpler screw pulling, if this is necessary.

The inventive design results in no pressure peak, thus less wear on the system. Due to the lower pressures and a significant increase in temperature of the plastic is avoided. With the extruder, the design of the grooves results in an improved conveying effect up to the melting region, in particular because the melting rate is also increased. Due to the parallel leakage of the grooves without reducing the groove depth and the open end cross-section also results in a better self-cleaning of the grooves and a high specific output over a wide speed range.

The grooves provided according to the invention may, as mentioned, not only extend along the feed zone but also into a part of the melting zone. As long as there are still individual solid particles in the system, it proves to be advantageous if grooves are formed in the cylinder.

The invention is particularly advantageous for high-speed screws, which rotate for example at 1500 min ^-1 .

In order to provide a favorable transition between the grooved area and the subsequent area, it is also possible to make the cross-section of the grooves larger, for example, by the webs having a decreasing circumferential width or a decreasing circumferential height.

In the following the invention will be described by means of embodiments in conjunction with the drawing. Showing:

1 a partial axial sectional view of an extruder according to the invention,

2 a sectional view taken along the line AA of 1 , and

three a sectional view along the line BB of 1 ,

According to the invention, the extruder has a screw three on which about a rotation axis 9 is rotatable. The snail three has a core diameter 10 on, which over the in 1 shown range is constant. The core of the snail three is thus cylindrical.

It may be advantageous to make the core diameter in the catchment area variable. An increasing flight depth of the screw will cause a further reduction in pressure. A decreasing flight depth, on the other hand, can increase the counterpressure-independent conveying effect, but also increase the pressure in the catchment area.

On the outside of the core of the snail three are snail bars 1 . 2 formed, which extend helically or helically, as is known from the prior art. The 1 shows a screw bridge 1 with an outer diameter 11 which is in a feed zone 5 a cylinder 4 of the extruder is arranged.

Adjacent to the intake zone 5 is a melting zone 8th intended. In this is at least a Schneckensteg 2 formed, which has an outer diameter 12 which is larger than the outer diameter 11 ,

The snail bars 1 and 2 have the smallest possible marginal gap to the inner wall of the cylinder 4 on, as is also known from the prior art.

At the in 1 shown embodiment (see also 2 ) are on the circumference of the cylinder 4 in the feed zone 5 evenly distributed around the circumference, several grooves 7 provided, which between raised, radially inwardly projecting cylinder webs 6 are formed. The grooves 7 are thus through the cylinder webs 6 educated. The radial depth of the grooves is through the groove depth 13 or determined by the resulting height of the cylinder webs, see 1 ,

According to the invention, the radial inner diameter of the grooves 7 in the area of the intake zone 5 equal to the radial inner diameter in the region of the melting zone 8th , At the end of the groove 14 or at the end of the cylinder webs 6 , which the longitudinally or helically extending in the circumferential direction grooves (see 2 ), thus results, as in 1 shown, a diameter jump, which is also in the different outer diameters 11 and 12 the snail bars 1 and 2 reflects.

The 2 shows in the sectional view only a screw web 1 , It is understood that in the context of the invention, a multi-start screw can be provided.

The three shows a sectional view taken along the line BB and thus a view of the front end of the cylinder with the webs 6 provided area of the feed zone 5 , It can be seen in particular that in the in three embodiment variant shown, the cylinder webs 6 may have a decreasing width in the circumferential direction, so that as it were flared grooves 7 resulting in a more uniform transition to the region of the melting zone having a larger free diameter 8th result.

LIST OF REFERENCE NUMBERS

1, 2

flighting

3

slug

4

cylinder

5

feed zone

6

cylinder dock

7

groove

8th

melting

9

axis of rotation

10

core diameter

11, 12

outer diameter

13

groove depth

14

groove end

Claims (9)

Extruder with one with at least one screw flight ( 1 . 2 ) provided snail ( three ), which in a cylinder ( 4 ) is rotatably mounted, wherein the cylinder ( 4 ) a feed zone extending over part of its axial length ( 5 ), which with at least one between adjacent cylinder webs ( 6 ) formed groove ( 7 ), adjacent to the intake zone ( 5 ) a melting zone ( 8th ), characterized in that the cylinder ( 4 ) in the conveying direction adjacent to the feed zone ( 5 ) has a diameter which is equal to the diameter in the bottom of the grooves ( 7 ), that the snail ( three ) in the conveying direction adjacent to the feed zone ( 5 ) has an abruptly increased diameter, as in the region of the feed zone ( 5 ), and that the intake zone ( 5 ) is formed with a constant diameter. Extruder according to claim 1, characterized in that the grooves ( 7 ) having a constant cross-section over its length or having a varying cross section, in particular with a conveying direction towards the end of the grooves ( 7 ) are provided enlarged cross-section. Extruder according to one of claims 1 or 2, characterized in that the radial height of the cylinder webs ( 6 ) over the length of the intake zone ( 5 ) is constant. Extruder according to one of claims 1 to 3, characterized in that the diameter of the screw ( three ) from the intake zone ( 5 ) to the adjacent area about twice the depth of the grooves ( 7 ) increases. Extruder according to one of claims 1 to 4, characterized in that the grooves ( 7 ) have an axial length which is equal to four to fifteen times the outer diameter of the screw ( three ) or between 5% to 80%, preferably between 10% and 60% of the total screw length. Extruder according to one of claims 1 to 5, characterized in that the grooves ( 7 ) in the axial direction parallel to the axis of rotation ( 9 ) of the snail ( three ) are formed running or coiled. Extruder according to one of claims 1 to 6, characterized in that the grooves ( 7 ) by cylinder webs ( 6 ) are formed, which the cylinder ( 4 ) of the intake zone ( 5 ), wherein the cylinder webs ( 6 ) are preferably adjustable in the radial direction. Extruder according to one of claims 1 to 7, characterized in that the cylinder webs ( 6 ) into at least part of the catchment zone ( 5 ) adjacent melting zone ( 8th ). Extruder according to one of claims 1 to 8, characterized in that the grooves ( 7 ) are formed with a parallel open Nutenauslauf without reducing the groove depth.

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