DE202017000276U1 - Pressure build-up screw - Google Patents

Abstract

Extrusion device for processing plastic materials composed of a cylinder with two overlapping holes and at least two intermeshing, counter-rotating screws, which are both driven synchronously, characterized in that at least 15% of the screw length is designed so that at least one flank angle at the screw land at least 35th ° and the web head width is less than 15% of the screw diameter.

Description

• 1. Spalt zwischen Schneckendurchmesser und Zylinderdurchmesser, der sog. Kopfspalt
• 2. Spalt zwischen Kerndurchmesser der einen Schnecke und Außendurchmesser der zweiten Schnecke, der sog. C-Spalt oder auch Walzenspalt
• 3. Spalt zwischen den einzelnen Flanken der einen und der zweiten Schnecke, der sogenannte A-B Spalt oder auch Flankenspalt.

The invention relates to counter-rotating, close-meshing twin-screw extruder for processing plastic materials. The heart of twin-screw extruders are the twin screws themselves. Known such twin screws are designed trapezoidal for the counter-rotor application. The conveying principle of this processing machine is based on a chamber conveying principle, wherein the contents of each two C-shaped chambers are forced with each revolution forcibly according to the screw pitch to a slope to the front. The connection of the C-shaped chambers with each other or with the preceding or underlying chambers is given by three types of columns. Through these gaps, the plasticized material can flow from a higher to a lower pressure range. The columns are described as follows:

• 1. gap between screw diameter and cylinder diameter, the so-called. Head gap
• 2. Gap between core diameter of a screw and outer diameter of the second screw, the so-called. C-gap or nip
• 3. gap between the individual flanks of one and the second screw, the so-called AB gap or flank gap.

Each type of flow of a plastic mass, here a plastic melt through a constriction or through a gap, leads to a Friktionserwärmung. In addition to the gap width, the speed and the differential speed of the adjacent walls also play a major role.

Usual counter-rotating twin screws have a trapezoidal design. The AB columns are z. B. executed according to the screw design for pressure build-up between 1 mm and 5 mm. It has also been attempted to make these flank angles negative, thereby widening the tread of the webs. These running surfaces of the webs are supported on one side against the cylinder with the head gap and on the other side they form with the screw core of the second screw the so-called nip. Since the core diameter of one screw and the diameter of the tread differ significantly from the other screw, the relative velocity in this shear gap is very different.

Accordingly, in this wide gap, a high Dissipationsleistung and, accordingly, a large warming of the mass. A wider version of the screw flights leads to an increase in dissipation performance.

It is state of the art to carry out the widest possible bridge head width in the pressure buildup area in order to thereby make the supporting surface of the bridge head as large as possible against the cylinder wall. It is assumed in the prior art that the wider the web, the lower the wear. In addition, it is state of the art to say that increasing the C-gap results in a reduction in wear. However, a large web head width always requires a low flank angle. In conventional pressure build-up zones in counter-rotating twin screws, the flank angle is between 1 ° and 6 °.

According to the state of the art in 1 are in a cylinder ( 1 ) two counter-rotating twin screws ( 2 ) and ( 3 ). Due to the opposite rotation between the two snails spreading forces arise between 10-12 h ( 4 ) and 12-14 h ( 5 ) lead to wear. These spreading forces are caused by the forces in the nips ( 2a ) and ( 3a ).

For the special design of the screws of counter-rotating twin-screw extruders, there are a variety of publications or patent applications EP 0 983 836 A1 , Here, in particular, the execution of the screw geometry with equidistant flank columns by an involute form may be mentioned. This generated flank design is similar to the transmission technology. In contrast to the gear technology, in which the metals of the individual screws or gears roll on each other, they are executed in this screw design with a gap.

At the plastics fair in Düsseldorf, K 2016, a screw geometry from Hans Weber Kunststofftechnik was shown, in which the flanks are executed in a wave shape in the circumferential direction. As a result, the close-meshing edge gap is lengthened and improved plasticization is to be achieved.

In 2 is a conventional counter-rotating twin-screw pair ( 2 ) and ( 3 ) in a cylinder ( 1 ). This pair of twin screws usually consists of several zones, each with different shapes and functions.

In 3 are the prior art twin screws ( 2 ) and ( 3 ) or gap areas ( 6 ) 7 ) and ( 8th ). The nip ( 6 ) remains constant during rotation, but the AB columns, theoretically planned in parallel, change ( 7 ) and ( 8th ), due to production in their shape to such an extent that between the opposite flanks a lens or o-shaped cross section, as in 4 represented with ( 9 ) and ( 10 ) arises. It quickly becomes clear that with appropriate rotation A variation of the flank gaps inevitably leads to uneven shearing about the axes of the individual screws and, moreover, hardly any very narrow flank gaps are operated.

The representation according to the prior art, the cut version with equidistant edge columns 5 can compensate this. However, practical experience has shown that the execution of the snails ( 2 ) and ( 3 ) with an involute flank ( 11 ) and ( 12 ) can often be produced insufficiently in their precision and execution with narrow edge gaps in practical application is difficult to implement.

According to the prior art and in the embodiments in 2 to 5 is common that the bridge head is very wide and is responsible for the large spreading of this screw geometry and the flank angle is therefore quite small.

Due to the chamber conveying principle, each counter-rotating twin screw also has a strong pulsation in the antechamber. To reduce this pulsating effect, the A-B columns or the C-column are often opened at the end of the metering zone. An alternative to this is the use of so-called Mischigeln at the output of Meteringzone. However, the Mischigel have the fundamental disadvantage that they are very susceptible to wear and not infrequently breaks off one of the mixing teeth.

Both mixing gel and the opening of the column must be considered in terms of design in order to reduce the pulsation effect sufficiently.

The inventive task is to develop an execution of the screw geometries for counter-rotating, closely intermeshing twin screws, which allows to build up pressure with the lowest possible friction heating and to minimize the spreading forces during pressure build-up and the pressure pulsation through the base geometry. This object is achieved according to the invention by a device for processing plastic materials, which are equipped with at least two counter-rotating, each driven, intermeshing twin screws solved, these twin screws are designed at least in a pressure-building zone with ridge widths and flank angle of the inventive type.

In general, counter-rotating twin screws are designed with three to eight different zones for plasticization, u. a. is spoken of different compression ratios in these zones. Each of these zones has a specific plasticizing task, such as shearing, homogenizing, compressing, pressurizing, degassing, etc., or a combination of the above objects.

The inventive task is to perform at least one of the zones of twin screws inventive. By modifications of the inventive solution, as described later, above-mentioned Aufgagen can be fulfilled by the screw geometry according to the invention.

Over a longer period of experiments carried out with different web shapes, in particular with very large flank angles and small web head width, as in 6 surprisingly, have led to very good pressure build-up with low increase in mass and low wear. Are 2 ) and ( 3 ) the respective screws as well as ( 14 ) and ( 15 ) the flank column and ( 13 ) the nip. By theoretical considerations models were found, which have an explanation of the results from the attempts described and further developed concepts, in order to further optimize the geometry.

In 7 is a part of a WW screw according to the invention and to recognize its cross section. The snails are with ( 26 ) and ( 27 ) designated. The web head width ( 17 ) is very narrow and the flank angle ( 16 ) in comparison to conventional counter-rotating twin screws made very large. The flanks in themselves ( 18 ) and ( 19 ) are being executed, this straight line extending through the center of the WW screw.

In 7 is a partial section of a corresponding inventive screw shown. Due to the very narrow head width ( 17 ), usually with a maximum of 15% of the diameter, and at a flank angle ( 16 ) greater than 30 ° arise low spreading forces, since only a very small area in the C-gap causes large spreading forces. Consequently, the counter-rotating twin screws are not pressed so much against the cylinder wall, and the wear is significantly reduced. The spreading forces from the flank gaps are not very wear-promoting despite the large angle.

Second, the flanks ( 18 ) and ( 19 ) linear and the screws can by this type of execution with a very narrow edge gap ( 14 ) and ( 15 ) are manufactured and operated.

The nip is responsible for the overwhelming proportion of the mass temperature increase due to friction, said nip only playing a minor role in the inventive design of the screw. By the specific Flank shape, the inventive screw geometries are referred to as WW screws.

Due to the embodiment according to the invention with a small web head width and a large flank angle, the length of the gap region is reduced as the sum of the flank and nip (FIG. 13 ) 14 ) and ( 15 ) through which an exchange can take place with the neighboring connected chambers.

Short lengths of the total column ( 13 ) 14 ) and ( 15 ) lead to lower leakage flows with less friction heating. As a result, a required back pressure can be built up with a shorter pressure build-up zone. As a result, this invention WW screw design is not only more stable in pressure, but can also be reduced in the practical design in its overall length to build up pressure. This is an additional economic argument.

It has been shown that the solution according to the invention without correction of the gap dimensions at the end of the screw leads to very small pressure pulsation effects in the screw antechamber.

In 8th are the twin screws according to the invention ( 26 ) and ( 27 ) shown as a section AA or in longitudinal section. Here, the typical WW shape of the snails ( 26 ) and ( 27 ) visible in section AA.

The 9 shows the snails ( 26 ) and ( 27 ) in a typical embodiment. The sections DD and EE show the course of the melt-filled volumes ( 20 ) 21 ) 22 ) 23 ) 24 ) and ( 25 ), as it exists in a slice cut out in the cutting area in each case. Due to the fact that the web head width ( 17 ) is very narrow and the flank angle ( 16 ) is very large, the C-shaped volumes change very strongly with their centers of gravity with each rotation.

In comparison of the 9 with the 2 It becomes clear that the spreading forces of the screws ( 26 ) and ( 27 ) in 9 unlike a conventional screw geometry like in 2 shown not act constantly on a narrow range of the cylinder, but according to the rotational position, these spreading forces vary in a larger circumferential range on the cylinder. The actual pressure build-up in the nip is also greatly reduced by the small head width and the spreading forces out of the gaps of the flanks are also low. Clearly, the hiking of the C-shaped chambers in 9 With ( 20 ) to ( 23 ) and ( 24 ) respectively. ( 21 ) and ( 22 ) to ( 25 ) in the sections DD and FF. The complete C-shaped volumes in the area of rotation are divided into two segments. The resulting spreading forces acting in the radial direction are small and only a fraction of the spreading forces caused by conventional screw geometries in the flank gap.

In addition, it should be noted that due to the shape of these C-chambers in 9 with the narrow screw flights ( 17 ) material is always present between the cylinder wall and the screw flights as a separating or sliding film. Thus, no dry friction between the screw running surface and cylinder and on the other hand, the shear stress in this area is only very short. As a result, these screws can be operated at a significantly higher peripheral speed than conventional counter-rotating twin screws. Experience also shows that the self-cleaning effect of the WW snails is particularly good. There are no dead water areas in which material can deposit or plate-out arises.

Becomes 8th and 9 When viewed in the area of the screw tip, the person skilled in the art immediately recognizes that due to the geometry with large flank angles, there is no or only insignificant pressure pulsation due to the geometry.

While originally the WW twin screws were only used for pressure build-up, it has been shown that a very good kneading effect is created, in particular by the introduction of cutouts in the flank and / or core region. In this case, straight-cut cutouts have proven particularly advantageous.

The cutouts in the screw core region in a straight shape promote a mixing action in the longitudinal direction.

Mixing sequences by straight cutouts on the inventive screw geometry have the decisive advantage that they represent a slowly changing flow form with "shallow inlet" of the change. Thus, in contrast to conventional Knetnuten in twin screws, no pressure jumps or rapidly changing torque changes, which can have a negative impact on the wear and fatigue strength of the screw.

It goes without saying that the screw geometries can be carried out in a catchy, double-flighted or more common form. Of course, this also applies to stepless changes in the pitch, flank angle or number of flights.

A particular embodiment of the idea according to the invention is to increase the screw diameter for a given center distance and, accordingly, the delivery volume per Increase screw rotation. Due to the large flank angles, the so-called WW screws can also be operated with screw diameters which are greater than twice the center distance.

Prior art conventional screw geometries for backlashes, such as those described in U.S. Pat 1 to 5 are shown, are operated with a few exceptions with an open oil cooling or with a closed Wasseremperiersystem. This is necessary to dissipate the friction energy in particular from the metering zone and to prevent overheating. However, since the WW screws build up much less friction energy, it is usually not necessary to cool the screws in this area. This also means that the cooling holes in the metering zone are not necessary. In addition, a cooling of the cylinder in the area of the WW screw geometry is usually also not necessary or is hardly used. Overall, the WW screw contributes significantly to the energy savings in the extrusion.

It is also a considerable advantage if the WW screw geometry is used as a pressure build-up unit in a separate machine technology. In this case, the screw geometry is fed with a melt or a kneadable plasticizing compound and the unit is used exclusively for pressure build-up. In this case, a temperature increase is usually undesirable or should be kept as low as possible, for which the so-called WW screw geometry provides ideal conditions.

Opposing plasticizing screws for plastics are usually used with peripheral speeds of approx. 7 m per minute for profile application and max. Operated 12 m per minute for the pipe application. This results in maximum speeds corresponding to the diameter. These speeds result from a maximum friction heating within the various screw gaps.

The so-called WW screw geometry reduces the influence of shear or friction heating to a minimum and also significantly reduces the spreading forces. Consequently, these screw geometries can be operated at significantly higher peripheral speeds and the resulting rotational speeds. Thus, the corresponding processing window or the processing bandwidth of counter-rotating twin-screw extruders with the screw geometry according to the invention can be considerably increased.

On the one hand this is for thermally sensitive materials, such. As PVC, very interesting. On the other hand, it has been found that with appropriate design of the screw geometry according to the invention also otherwise difficult, especially low-viscosity materials with fillers such as carbon black, can be plasticized and discharged. Thus, in addition to considerable economic advantages of the system, there are also completely new fields of application for the geometry design according to the invention.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0983836 A1 [0007]

Claims (8)

Extrusion device for processing plastic materials composed of a cylinder with two overlapping holes and at least two intermeshing, counter-rotating screws, which are both driven synchronously, characterized in that at least 15% of the screw length is designed so that at least one flank angle at the screw land at least 35th ° and the web head width is less than 15% of the screw diameter. Apparatus according to claim 1, characterized in that both flank angles are identical and at least 35 ° and the web head width is less than 10% of the screw diameter. Apparatus according to claim 1 and 2, characterized in that the screw flank is designed in a straight line. Apparatus according to claim 1 and 2, characterized in that the ratio of screw diameter to the center distance is 1.15 to 2.1. Apparatus according to claim 1 and 2, characterized in that in the flank region straight cutouts are introduced. Apparatus according to claim 1 and 2, characterized in that the worm core are made perpendicular to the axial direction milled cutouts. Apparatus according to claim 1 and 2, characterized in that the worm core at an angle to the axial direction rectilinear cutouts are made. Apparatus according to claim 1 and 2, characterized in that the gap width of the flank gap is narrower in the central region than in the region at the screw head or at the web foot.

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