EP2043837A1

EP2043837A1 - Process for production of a screw for an extruder, and screw

Info

Publication number: EP2043837A1
Application number: EP07765328A
Authority: EP
Inventors: Erwin Schnabl
Original assignee: Theysohn Extrusionstechnik GmbH
Current assignee: Theysohn Extrusionstechnik GmbH
Priority date: 2006-06-16
Filing date: 2007-06-06
Publication date: 2009-04-08
Also published as: AT504385B1; CN101466523A; WO2007144303A1; TW200810911A; US20090162470A1; CA2654011A1; JP2009539653A; AT504385A1

Abstract

At least one section of the screw (11) has a wear-protection layer (13) and at least one other section of the screw preferably has an anti-friction layer (18). In a solid rod (11'), a bed (12) for the wear-protection layer (13) is first formed, and in this the wear-protection layer (13) is then applied, and finally the interstices (14) between the screw flights (15) are formed. According to the invention, the wear-protection layer (13), for example composed of tungsten carbide, is applied by build-up welding, and the interstices (14) are formed with lateral separation (16, 16') with respect to the wear-protection layer (13). For production of the anti-friction layer, the dimension of that/those section(s) of the screw (11) where the anti-friction layer, e.g. composed of molybdenum, is to be applied is reduced below specification, while providing lateral separation (19) with respect to the wear-protection layer. The anti-friction layer is then applied in the under-dimensioned region (17). Finally, the screw (11) is brought to the specified dimension.

Description

description

METHOD FOR PRODUCING A SNAIL FOR AN EXTRUDER AND SNAIL

Technical area

The present invention relates to a method for producing a screw for an extruder, wherein the screw is provided at least in a section with a wear protection layer, wherein initially formed in a full bar a bed for the Verschleißschαtzschicht, in which then the wear protection layer and finally forms the spaces between the screw flights. In particular, it also relates to a method for producing a screw, which is additionally provided in at least one other section with a sliding layer. It also affects such snails.

State of the art

From the

EP 1614502 A -. It is already known to line the housing of an extruder in those areas which are subject to increased wear, with a particularly wear-resistant material. These areas can be limited both axially and radially.

According to the

EP 1614502 A -. For a counter-rotating twin-screw extruder in the radial direction, the areas which correspond to 10 o'clock and 2 o'clock on the clock are particularly critical. In the axial sense, you can an extruder in a feed zone, a

Compression zone, a decompression zone (for degassing, if needed) and a Meteringzone split. From paragraph 25 of the

EP 1614502 A -. is known, the wear-resistant material only in the compression zone and in the

Metering zone to install. This is most easily possible by doing that

Housing two-piece executes. In the accompanying Fig. 1, such an extruder housing is shown in longitudinal section.

The cross section looks like in Fig. 7 of the

EP 1614502 A -.

, The extruder housing consists of the parts 1 and 2, which by means of flanges (not shown) are interconnected. Before connecting the two parts 1 and 2 together, you bring in part 2 from both sides of the wear-resistant material. The wear-resistant material is shown in FIG. 1 in the form of sleeve-shaped inserts 3, 4, that is not restricted in the radial direction. However, this does not matter that the wear-resistant material could also be present only in certain radial areas, as shown in FIG. 3 of EP 1614502 A -. is shown.

Part 1 of the housing is provided for the intake and Vorkompressions- or preheating zone. Here, the material is fed via a funnel (not shown). The insert 3 is located in the compression zone (where high mechanical wear occurs). Here the material is melted. The insert 4 is located in the metering zone (where high chemical wear occurs). Here, the screw ensures a constant volume flow, which is then pressed through the extruder die. Between the inserts 3, 4 is in this embodiment, the decompression zone. In the decompression zone, a vacuum is applied so that the gases escape.

The part 1 is usually made of nitriding steel and is e.g. 1.6 m long. The part 2 is e.g. 2.4 m long and is equipped with inserts 3, 4 made of (very hard) powder metallurgical steel. The part 2 itself can also consist of nitriding steel.

Screws are usually coated with molybdenum. Molybdenum is a good friction partner for nitriding steel, it acts as a sliding layer, but it wears very fast in the field of powder metallurgical steel inserts. On the other hand tungsten carbide is a good friction partner for the powder metallurgical applications, it acts as a wear protection layer, but it leads to rapid wear of nitriding steel. (Since snails are cheaper than the extruder housings, tungsten carbide is a very poor solution on nitriding steel screws because it often requires replacement of the extruder housings, and it is better if the screws wear out and need replacing more often, and the housings for them last longer.)

So that screws are suitable for an extruder housing according to FIG. 1, they must be coated differently in the axial direction, as for example from DE 10161363 A -. is known. According to FIG. 2 of this document, a helical groove is first milled into a rod. In this groove a tape with the desired layer is inserted. Thereon There is a heat treatment through which this band enters into a solder joint with the base body. Thereafter, the spaces between the screw flights are formed. In the end, the entire surface of the screw flights is armored by a wear protection layer or with a

Slip layer provided (depending on the tape used). This method has several disadvantages. On the one hand, a solder joint is not a very good connection. Tungsten carbide as wear protection layer is therefore usually applied by build-up welding on the screw, resulting in a much better connection results (melt bond). In hardfacing, it is problematic that the material of the worm heats up strongly, which causes it to distort slightly. (The demands on the concentricity are extremely high, because the screws are several meters long and should deviate a maximum of 0.1 mm.) If you then form the spaces between the screw flights, you must inevitably remove the layer of tungsten carbide , But tungsten carbide is so hard that this is hardly possible with machining, as the cutting tools are blunt in no time.

With screws, which have different coatings in different sections, further results in this known method, the disadvantage that the coatings must all be the same thickness, which is also not optimal: molybdenum as a sliding layer is usually of lesser thickness (eg 0 , 4 mm) applied as tungsten carbide (eg 1 mm) as a wear protection layer.

Presentation of the invention

It is an object of the present invention to provide a method for producing screws, which is easy to perform and yet creates very good dimensionally stable screws. It should also be provided a method by which these screws are only partially provided with a wear protection layer and coated in the remaining parts with a sliding layer.

This object is achieved by a method of the type mentioned in the present invention, characterized in that the wear protection layer, e.g. made of tungsten carbide, applied by build-up welding and that forms the gaps with a lateral distance to the wear-resistant layer.

By the build-up welding a high-strength welded joint (a melt bond) is produced. By being involved in the production of Interspaces a lateral distance to the wear protection layer complies, the screw land is not fully occupied in the end with the wear protection layer, but the wear protection layer is also on the finished screw in a bed, that is laterally surrounded by material of the main body of the screw. This does not interfere with the use of the screw, but manufacturing technology has the advantage that the wear protection layer, such as tungsten carbide, must not be mechanically machined laterally. Nevertheless, you get a sharp edge.

The preparation of the spaces is best done by a process which is referred to as "whirling". In this case, a cutter head is moved around the screw to be produced. This is much cheaper than milling, but this process is extremely difficult to machine extremely hard coatings.

As already mentioned, some extruder housings require the screw to be differently coated in different sections. In addition to a wear-resistant layer is often a sliding layer necessary to reduce the wear of the housing in the non-hardened housing sections. Such a sliding layer, for example molybdenum, is applied, for example, by thermal spraying using a plasma method. The base material heats up only moderately (eg to 150 ^° C), so there is no danger that the screw will warp. In thermal spraying, in contrast to build-up welding, there is no fusion bond, but the adhesion is nevertheless sufficient.

The problem is the location where the overlay adjoins the wear protection layer. The build-up welding (PTA welding) namely, the base material of the screw is mixed with, for example, carbides, whereby the sliding layer does not adhere well at this point and easily breaks off. In addition, the sliding layer is usually much thinner, e.g. only half the thickness of the wear protection layer. A continuous bed, as in DE 10161363 A -. provided, is not optimal for these reasons.

For this reason, it is provided according to an embodiment of the invention that, when providing a sliding layer in at least one other section of the screw one steps after the process of claim 1, the screw in the section or in the sections where the overlay, eg made of molybdenum, to be applied to undersize brings, but it provides a lateral distance to the wear protection layer that then applied in the area with undersize the sliding layer and that finally brings the screw to the specified size. It can be seen - in contrast to DE 10161363 A -.

- Initially, the bed only in the area in which the wear protection layer is to be applied. After this has been done and the interspaces between the screw flights have been made, the screw is undersized in the areas where the overlay is to be applied. (This can - as will be explained - at least in part again be achieved by providing a bed, but which need not have the same depth as the bed of the wear protection layer.) But this does not make up directly to the wear protection layer, but only up to one certain minimum distance (eg 2 mm) to this. In the areas in which the screw now has undersize, bring the sliding layer on. Due to the minimum distance to the wear protection layer, the sliding layer does not come into contact with the carbides, so that there is no danger that the sliding layer breaks away at this point. In addition, there is a clean level at the end, where the sliding layer is applied laterally, which additionally improves the adhesion. After the overlay has been applied, the entire screw is dragged to the specified size.

It is useful if, after applying the wear protection layer and before bringing the screw for applying the overlay on undersize, the screw nitrided. This has two advantages: on the one hand, nitriding is desirable for the screw flanks and the screw base; On the other hand, the sliding layer adheres poorly to nitrided material. The fact that the screw is brought to undersize after nitriding, the nitrided layer at these points (ie on the head of the screw flights) removed, so that the sliding layer adheres well here; of the remaining areas (where no overlay is desired) the overlay is easier to remove.

As an alternative to nitriding or in addition, it is possible to areas of

To mask a worm on which no sliding layer is to be applied. Again, the adhesion of the sliding layer is prevented on the screw material, so that it is easily removable.

In the method described so far has the sliding layer on the front side, which faces the wear protection layer, a corner with an acute angle. Such corners may break away under the highest loads of the screw under certain circumstances. In order to avoid this, it is possible to provide a bed for the sliding layer, at least in the area adjoining the section with the wear protection layer Milling with lateral bridge, so that the bottom of the bed has the undersize. In this way you have in this critical area a bed with a rounded corner (the rounding corresponds to the radius of the router), and also is the corner of the

Slip layer bordered on both sides by the basic material of the screw. Screws according to the invention of the type mentioned above are characterized in that the sliding layer, e.g. of molybdenum, a lateral distance to the wear protection layer, e.g. made of tungsten carbide. This lateral distance reduces the risk of the sliding layer breaking out. It is further favorable if the sliding layer in the end region, which faces the wear protection layer, is enclosed in a bed and has a rounded corner.

Brief description of the drawings

Reference to the accompanying drawings, the invention is explained in detail. 1 shows a housing of an extruder with different sections; FIG. 2 shows a main body for a milled-bed screw for a wear protection layer; FIG. Figure 3 shows this body, wherein in this bed the wear protection layer is applied. Fig. 4 shows this main body after the spaces between the screw flights have been formed; 5 shows this screw after the screw has been undersized in the area adjacent to the wear-resistant layer; Fig. 6 shows the finished screw; Fig. 7 shows a variant of Fig. 5; and FIG. 8 shows the corresponding variant of FIG. 6.

Best way to carry out the invention

The extruder housing of FIG. 1 consists of the parts 1 and 2, by means of

Flanges (not shown) are interconnected. Before connecting the two parts 1 and 2 together, you bring in part 2 from both sides of the wear-resistant material. The wear-resistant material is shown in FIG. 1 in the form of sleeve-shaped inserts 3, 4.

An associated screw is in the area of the inserts 3 and 4 with a

Wear protection layer, e.g. coated with tungsten carbide, but in the other areas with a sliding layer, for example with molybdenum.

The production of such a screw is explained with reference to FIGS. 2-6. In these figures, an axial region of the screw (or the main body of the screw) is shown, where the transition from tungsten carbide to molybdenum is (or should be).

First, in the cylindrical base body 11 'of the screw, a bed 12th milled, and only in the area where the wear protection layer is to be applied later. In this bed 12, a wear protection layer 13 is then introduced by build-up welding (see Fig. 3). The hardfacing can be carried out manually, for larger series, of course, a welding robot can be used.

Since there is no smooth surface during build-up welding, you bring first a little more wear protection layer than necessary and later grinds them off to the nominal size. For example, Apply 1.5 mm tungsten carbide and sand it down to 1 mm.

For this to be easily possible, it is expedient first to carry out the entire main body 11 'of the worm 11 with a corresponding excess (that is, for example, +0.5 mm for the radius) and also to make the bed correspondingly deeper, e.g. 1.5 mm. Then you can fill the entire bed during build-up welding.

(Note: the depth of the bed 12 is not drawn to scale in Fig. 2, but greatly exaggerated to make the drawing more apparent.)

Next, one forms the recesses 14 (see Fig. 4) between the

Schneckenstegen 15 off. It is thereby to be noted that between the recesses 14 and the wear protection layer 13 should remain a distance, so that a narrow strip 16, 16 'of the base material of the screw stand on both sides of the wear protection layer 13 remains. This is procedurally advantageous because a removal of the wear protection layer would lead to severe wear of the tool.

Now you grind the screw 11 in the area 17 (see Fig. 5), where the

Slip layer should be applied to undersize (e.g., 0.4 mm). The undersize should be exactly as large as the desired thickness of the sliding layer. It should be noted that here a lateral distance to the wear protection layer 13 is to be maintained, so that a web 19 stops.

Finally, the sliding layer 18 (see FIG. 6) is brought into this area 17. Again, one again applies more than necessary (e.g., 0.5mm-0.6mm) and then grinds to the desired level (e.g., 0.4mm).

The grinding to the nominal size is expediently carried out as the last step for the entire screw.

In the screw according to FIG. 6, it is advantageous that the sliding layer 18 is separated from the wear-resistant layer 13 by a web 19, which consists of the basic material of the screw 11. Thus, the sliding layer 18 does not come with carbides so that the adhesion can not be affected by the carbides. In addition, the sliding layer 18 is laterally on the web 19, so that also the adhesion is improved. FIGS. 7 and 8 show a variant of FIGS. 5 and 6. In the worm according to FIG. 6, the corner 20, which has an acute angle, could be problematic. Such corners break out easily. Therefore, it is provided according to FIG. 7 that initially (eg over a half screw circumference) a bed 17 'is milled whose base has the desired undersize. In doing so, a lateral web 21 is allowed to stand, and it necessarily follows (due to the radius of the milling cutter) a rounded corner 20 '. Thus, then also the sliding layer 18 has a rounded corner 20 '(see Fig. 8), which is bordered by the base material of the screw and is largely protected in this way from breaking.

Claims

claims

Method for producing a screw (11) for an extruder, wherein the

Worm (11) is provided at least in one section with a wear protection layer (13), wherein first in a volen rod (H ') a bed (12) for the wear protection layer (13) is formed, in which then the wear protection layer (13) and finally forms the intermediate spaces (14) between the screw flights (15), characterized in that the wear protection layer (13), eg made of tungsten carbide, applied by build-up welding and that the intermediate spaces (14) with a lateral distance (16, 16 ') to the wear protection layer (13) is formed.

A method according to claim 1, characterized in that when providing a

Sliding layer (18) in at least one other section of the screw (11) according to the method steps of claim 1, the screw (11) in the section or sections where the sliding layer, e.g. made of molybdenum, sol is brought to undersize, but it provides a lateral distance (19) for wear protection layer provides that then in the area (17) with undersize the sliding layer applies and that finally the screw (11) on the Solmaß brings.

A method according to claim 2, characterized in that between the

Process steps of claim 1 and 2, the screw nitrided.

A method according to claim 2 or 3, characterized in that before the

Process steps of claim 2 areas of the screw on which no sliding layer to be applied, masked.

Method according to one of claims 2 to 4, characterized in that for the sliding layer (18) at least in the region which adjoins the portion with the wear protection layer (13), a bed (17 ') with lateral web ( 21), so that the bottom of the bed (17 ') is undersized.

Screw for an extruder, wherein the screw (11) at least in one

Section is provided with a wear protection layer (13) and is provided in at least one other section with a sliding layer (18), characterized in that the sliding layer (18), e.g. of molybdenum, a lateral distance (19) to the wear protection layer (13), e.g. made of tungsten carbide.

Screw according to claim 6, characterized in that the sliding layer (18) in the end region, which faces the wear protection layer (13), in one Bed (17 ') is bordered and has a rounded corner (20').