GB2068249A

GB2068249A - A screw press for processing thermoplastic and elastomeric materials

Info

Publication number: GB2068249A
Application number: GB8102691A
Authority: GB
Original assignee: Hermann Berstorff Maschinenbau GmbH
Current assignee: KraussMaffei Extrusion GmbH
Priority date: 1980-02-01
Filing date: 1981-01-29
Publication date: 1981-08-12
Also published as: JPS56126143A; IT8026787A0; GB2068249B; IT1134809B; NL8006782A; FR2474946A1; DE3003615A1; SE435824B; FR2474946B1; DE3003615C2; SE8100750L

Abstract

A screw press has a screw (2) with a land (3) thereon and is rotatable in a cylinder (1). Pegs (4) project inwardly from the wall of the cylinder (1) towards the axis of the screw and pass through a circumferentially extending passage in the land (3). End surfaces (10, 11) of the land formed by the passage therein face one another and are directly opposite one another when considered in a direction axially of the screw thereby to constrain material being processed to pass through narrow gaps between the peg (3) and the end surfaces (10, 11) of the land with consequent introduction of shear forces into the material to improve its homogeneity. The screw (2) may be a multi- start screw or short lengths of multi-start lands may be provided immediately upstream and down- stream of the position at which a plurality of the pegs (4), located in a plane normal to the axis of the screw (2), are disposed, i.e. on either side of the passage. <IMAGE>

Description

SPECIFICATION A screw press for processing thermoplastic and elastomeric materials The invention relates to a screw press for processing thermoplastic and elastomeric materials.

Such a screw press may comprise an extruder with a screw rotatable within a cylinder and a plurality of pegs extending radially inwardly from the inner wall of the cylinder, disposed in a plane normal to the axis of the screw, preferably disposed at approximately even spacings around the periphery and directed towards the axis of the screw, a land or lands of the screw being interrupted at the positions where the pegs project into the cylinder to form a passage through the land or lands for the pegs to pass through.

Extruders with pegged cylinders are known from Published German Specification 2 235 784. In the extruders shown in this specification the screw lands are interrupted at the places where the pegs project through the cylinder into the extruder chamber. With such a construction a satisfactory mixing and hom- ogenising action is said to be obtained for the material to be procesed.

The disadvantage of this construction is that there is virtually no shearing of the material by the pegs, so that the homogeneity of the substance being processed can leave much to be desired. When the material arrives at the pegs there is indeed a dividing into streams and a turning-over action, but there is no significant shearing of the material. This is because the material when it impinges upon a peg, can escape into the adjacent groove between the lands without undergoing any significant shearing between the land and the pegs. By dividing the flow and turning it over, the known construction therefore provides many partial streams and much relayering, but no shearing effect is obtained. The pegs are surrounded by many particles of material flowing around them but without any intensive shearing.

According to the invention there is provided a screw press for processing thermoplastic and elastomeric materials, comprising a screw with a land thereon and mounted for rotation within a cylinder a plurality of pegs extending radially inwardly from the inner wall of the cylinder, disposed in a plane normal to the axis of the screw and directed towards the axis of the screw, and a passage extending circumferentially around the screw through the land of the screw and radially aligned with the pegs, through which passage the pegs pass upon rotation of the screw, wherein end faces of the land of the screw formed by said passage, between which end faces the pegs fixed to the cylinder pass upon rotation of the screw, face one another and are substantially directly opposite one another when considered in a direction axially of the screw.

Preferably on each side of said plane the screw has homogenising portions with multi start screw lands, the diameter of the screw is D, each of said homogenising portions has an axial length of approximately 1/5 D and the passage has a width, measured axially of the screw, of approximately 1/5 D.

Advantageously ends of the lands of said multi-start screw lands of said homogenising portions which are remote from said passage taper substantially to a point.

Since the end faces or respective end faces of the or each interrupted land are disposed directly opposite to one another when consid ered axially of the screw, the material will be subjected to a strong shearing action when ever it passes between a stationary peg and one of the end surfaces.

This arrangement can provide very large active surfaces for imparting a large quantity of shearing energy to the material since the material is drawn through the narrow gaps at increased speed without being able to escape.

In prior art screw presses, when the mate rial being processed comes into contact with the stationary pegs, it is always allowed to escape without being greatly stressed. With a large number of rows of pegs there is indeed a good mixing action, but the shearing of the material, which is finally responsible for its homogeneity, is sorely neglected.

Several sets of pegs, each in a respective peg plane can be provided, with respective homogenising portions formed by multi-start screw lands arranged upstream and down stream of them having regard to the general direction of flow of the material.

If a plurality of homogenising units of this kind are provided in succession, then an ex cellent mixing and homogenising effect can be obtained with only a very short extruder.

The price of acquiring an extruder unit is thus much reduced.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is a diagrammatic longitudinal sec tion through a single-screw screw press ac cording to the invention; Figure 2 is a longitudinal section through part of a screw press according to the invention having two homogenising units; Figure 3 shows pointed beginnings and ends of lands of a screw of a screw press according to the invention; Figure 4 is a diagram illustrating the basic idea behind the invention; and Figure 5 is a diagram similar to Fig. 4 showing an embodiment in accordance with prior art.

Referring to the drawings and firstly to Fig.

1, an extruder cylinder 1 contains a screw 2 which has a land 3 helically disposed thereon.

Pegs 4 extend through the extruder cylinder 1 and are locked in position by screw means 5.

The pegs 4 are disposed in the extruder cylinder 1 with their axes directed towards the axis of the screw 2. The number of the pegs 4 extends radially through the cylinder 1 into the extruder chamber 6, will largely depend on the diameters of the cylinder 1 and the screw 2. A plurality of the pegs 4 are preferably disposed in a common plane which extends normal to the axis of the screw 2, such a plane is referred to as a peg plane.

To allow passage of the pegs 4 through the land 3, the land is interrupted by a passage extending through the land 3, the passage forming end faces 10, 11 on the land 3 upstream and downstream of the peg plane respectively having regard to the general direction of movement of the material.

Fig. 5 shows a prior art arrangement in stylised form, the curved wall of the screw being represented as being rolled out. Here a through-running helical land has been interrupted, to provide a gap or passage for the stationary pegs 14 to pass through. End faces 12, 13 of the land face one another and are opposite one another in a plane angled at the helix angle of the land. When the radial end surface 13 of the land passes the peg 14, the material can escape unimpeded to the other side without undergoing any shearing. When, in the course of further rotation of the screw, the end surface 12 of the land passes the peg 14, the material is again allowed to escape in the other direction without undergoing any shearing stress.

Referring to Fig. 4 however, it will be seen that end faces 10, 11 of the land, on opposite sides of the gap or passage through which the pegs 4 pass, face one another and are directly opposite one another when considered in a direction axially of the screw.

Whenever, due to rotation of the screw 2, opposed end surfaces 10 and 11 of the land pass a stationary peg 4, the material between the end surfaces 10, 11 of the land and the peg 4 is subjected to considerable shearing.

If the modes of operation of the two diagrammatic Figs. 4 and 5 are compared, the superior shearing action of the screw press of the invention will be very evident.

A plurality of the peg planes may be provided at positions spaced axially along the extruder, and lengths of single-start or multistart lands may be disposed between the various planes.

Part of an extruder with two homogenising sections 7 is shown in Fig. 2. The homogenising sections 7 comprise multi-start screw portions 8 and 9 disposed on opposite sides of a respective peg plane.

The shearing action on the material to be processed, between opposing radial interruption surfaces 10, 11 of the lands 3, is considerably increased by the multiple-thread construction of the homogenising sections 7.

Fig. 3 shows that the ends of the lands of the multi-start screw portions 8, 9 which are remote from the pegs 4 have pointed ends to improve the flow of material into and out of the grooves between the lands of the multistart screw portions 8, 9 respectively.

In connection with the manufacture of the homogenising sections 7, it should be mentioned that these are advantageously made simply by taking the multi-start screw portions 8 and 9, which are constructed as separate components, fitting them onto a central tie rod and appropriately securing them with adjusting springs. It is a great advantage to have the multi-start screw portions 8 and 9 and the length without any lands between them, which length forms the passage, constructed as separate components, because the amount of wear may vary. For example, the first homogenising section 7 will have far more strain on it than the following ones, because the material is initially colder and thus far more abrasive. Another advantage of having the the screw made up of separate interchangeable lengths has to do with manufacture.Since the components are of relatively low volume, they can be produced as cheap precision castings.

According to the material being processed, it may be necessary to impart more or less shearing energy to the material and thus to provide more or less homogenising sections 7.

In practice it has been found, for example, that the multi-start screw portions 8 and 9 should have the following numbers of parallel threads for certain sizes of extruder: 2 threads cxtruder 90 mm screw diameter 3 threads cxtruder 150 mm screw diameter 4 threadsextruder 200 mm screw diameter 5 threads cxtruder 250 mm screw diameter It has been found desirable to have the length of the screw portions 8 and 9 in a determined relationship to the diameter of the pegs 4. The ratio of 1:1 to 1:2 has proved to be a particularly effective relationship between the length of the screw portion 8 and the peg diameter 4; that is to say, if D is the diameter of the screw and if the screw portions 8 have a a length of 0.2 D, the pegs or the length of the landless length between the screw portions 8 and 9 should have an axial length of 0.1 to 0.2 D.

When cold rubber mixes were processed with the land and peg geometry according to the invention, the rubber mix was found to be divided up with minimum resistance and drawn out between the pegs and lands in long partial streams. As a result the rubber mix had excellent homogeneity.

At the same time the use of the homogenising sections enables the conveying portion between them to be cut very deep with single or double thread, with high output per revolution, without there being any fear of nonhomogeneity, such as uneven temperature distribution.

A comparative test on an extruder with a pegged cylinder, as shown in Published German Application 2 235 784, produced the following results: The extruder was equipped with ten successive peg planes. The rubber mix to be processed, which was approximately at room temperature, was fed into the extruder, and its homogeneity was determined after it had passed the ten peg planes.

Next an extruder according to the invention was charged with material having an identical viscosity and temperature to that used previously. When the rubber mix had passed through five homogenising sections 7, its homogeneity was already comparable with that of the material processed in the above mentioned known extruder, with ten successive peg planes and twice the length.

It is clear from this comparative test that an extruder according to the invention can be made 50% shorter than a prior art extruder, or that given the same length, substantially higher outputs can be obtained without there being any fear of the material lacking homogeneity.

Claims

1. A screw press for processing thermoplastic and elastomeric materials, comprising a screw with a land thereon and mounted for rotation within a cylinder, a plurality of pegs extending radially inwardly from the inner wall of the cylinder, disposed in a plane normal to the axis of the screw and directed towards the axis of the screw, and a passage extending circumferentially around the screw through the land of the screw and radially aligned with the pegs, through which passage the pegs pass upon rotation of the screw, wherein end faces of the land of the screw formed by said passage, between which end faces the pegs fixed to the cylinder pass upon rotation of the screw, face one another and are substantially directly opposite one another when considered in a direction axially of the screw.

2. A screw press according to claim 1, wherein, on each side of said plane, the screw has homogenising portions with multi-start screw lands, the diameter of the screw is D, each of said homogenising portions has an axial length of approximately 1/5 D and the passage has a width, measured axially of the screw, of approximately 1/5 D.

3. A screw press according to claim 2, wherein ends of the lands of said multi-start screw lands of said homogenising portions which are remote from said passage taper substantially to a point.

4. A screw press according to claim 2 or claim 3, wherein said homogenising portions and other portions of the screw are constructed as separate components which can be interchanged.

5. A screw press for processing thermoplastic and elastomeric materials substantially as hereinbefore described and illustrated with reference to Figs. 1 to 4 of the accompanying drawings.