Resource-saving twin-screw extruder system with multiple twin-screw extruders for plastics processing

Abstract

Resource-saving twin-screw extruder system with multiple twin-screw extruders (10; 10') for plastics processing; comprising at least one large twin-screw extruder (10') and one small twin-screw extruder (10), each of which includes at least: - an extruder housing (1; 1') with an extruder bore (2; 2') which has a cross-section formed by two overlapping circular cross-sections with an inner diameter d1, d2 in the shape of a figure eight lying on its side and - a pair of meshing extruder screw shafts (6; 6') arranged rotating in the extruder bore (2; 2') with at least one screw web (8) having an outer diameter D1, D2, wherein: - in the large twin-screw extruder (10') the inner diameters d2 of the extruder bore (2') and the outer diameters D2 of the extruder screw shafts (6') are each larger by a wear allowance ΔD than the inner diameters d1 of the extruder bore (2) and the outer diameters D1 of the extruder screw shafts (6) of the small twin-screw extruder (10); - the center distance x of the extruder screw shafts (6; 6') is the same for all twin screw extruders (10; 10') in the system; - the lengths of the extruder screw shafts (6; 6') and the extruder bores (2; 2') are the same for both twin-screw extruders (10; 10'); and - the roller gap in the small twin screw extruder (10) is larger than in the large twin screw extruder (10').

Description

[0001] The invention relates to a resource-saving twin-screw extruder system with several twin-screw extruders for PVC plastic processing.

[0002] Twin-screw extruders are typically used to produce PVC profiles. These extruders each have an extruder barrel with two extruder bores, each with a horizontal figure-eight cross-section formed by two overlapping circular cross-sections. Arranged within these barrels are a pair of rotating extruder screw shafts. Each extruder screw shaft has at least one raised screw flight. The geometry of the screw flights is identical. The extruder screw shafts are connected to a drive gear such that their cross-sections are rotated by a specific angle relative to each other, allowing the screw flights to mesh with each other as the extruder screw shafts rotate.

[0003] In production, twin-screw extruders wear in such a way that the outer diameter of the extruder screw shaft, measured across the flights, decreases and the inner diameters of the two circular cross-sections that together form the extruder bore increase. This causes the gap between the extruder screw shafts and the extruder barrel to widen.

[0004] If this gap becomes too large for a specific production process, in which PVC with a certain viscosity is to be prepared and pressed at a certain pressure through a die tool downstream of the extruder, there will be increased overflow of the screw flights. This will impair the quality of the plastic products or even disrupt the production process, result in insufficiently homogenized thermoplastic melt being ejected, or insufficient pressure being able to build up. For preparation, a hard metal layer must then be applied to the extruder bore and to the extruder screw shafts, which is particularly complex in the extruder bore. Alternatively, the extruder bore can be formed in an insert bushing that can be inserted into the extruder housing, making it easier to replace a worn insert bushing.

[0005] In any case, the reconditioning costs are high and amount to up to 80% of the new price of a pair of extruder screw shafts or 150-200% of an insert bushing compared to a standard cylinder.

[0006] The object of the invention is therefore to reduce the wear-related operating costs of a twin-screw extruder system with several twin-screw extruders.

[0007] This object is achieved by a resource-saving twin-screw extruder system with several twin-screw extruders for PVC plastic processing with the features of claim 1.

[0008] In large PVC plastic processing plants, several identical twin-screw extruders are usually used for similar applications in order to achieve cost savings through uniform purchasing, stocking of spare parts, etc.

[0009] The invention deviates from such a usual commercial measure and starts with the initial investment, in that according to the invention at least one large twin-screw extruder and one small twin-screw extruder are provided in one system.

[0010] It is also possible to provide a group of large twin-screw extruders and an equally large group of small twin-screw extruders.

[0011] The designations “large” and “small” simply express that the respective nominal diameters are graded in size relative to one another, whereby the actual difference in the nominal diameter is small, namely less than 8% of the nominal diameter.

[0012] In both groups of twin-screw extruders, the respective geometry is identical in the following points: - the extruder screw shafts and the extruder bores are of equal length; - the center distance of the extruder screw shafts and the extruder bores is the same for the group of large twin-screw extruders and for the group of small twin-screw extruders.

[0013] The properties "same length" and "same diameter" refer to nominal diameters, especially to stepped nominal diameters within a series. A deviation in the measurable actual geometric dimensions that lies within the tolerance range is still considered "same" within the meaning of the invention.

[0014] With regard to the same center distance, the system of the invention departs from the usual methods used in industrial plastics processing, particularly PVC plastics processing. For smaller diameter extruder screw shafts, a smaller center distance is usually chosen than for larger diameters, so that the so-called roller gap, i.e., the distance between the outer side of a screw flight on one extruder screw shaft and the shaft core on the other extruder screw shaft, remains constant. However, it has been shown that in the range of diameter differences between large twin-screw extruders and small twin-screw extruders to which the invention relates, increasing the roller gap has no noticeably negative effects.

[0015] According to the invention, it is ensured in terms of process technology that the use of both large twin-screw extruders and small twin-screw extruders leads to a flawless production of plastic extrusion profiles or the like.

[0016] The twin-screw extruder system of the invention further provides that in the large twin-screw extruder, the inner diameter d of the extruder bore and the outer diameter D of the extruder screw shafts are each larger by a wear allowance ΔD than in the small twin-screw extruder.

[0017] For extruder screw shafts, the outer diameter measured across the flights is always considered. The inner diameter, which refers to the core of the extruder screw shafts between the flights, is the same for both large and small twin-screw extruders.

[0018] In the resource-saving twin-screw extruder system of the invention, both cylindrical extruder screw shafts and cylindrical extruder bores can be selected, in which the extruder screw shafts are arranged parallel to each other, as well as conical shapes of extruder bore and screw shafts that are aligned at an acute angle to each other.

[0019] Furthermore, the twin-screw extruder system of the invention is suitable for both co-rotating twin-screw extruders and counter-rotating twin-screw extruders.

[0020] With these geometric conditions, a resource-saving twin-screw extruder system is formed, and a resource-saving operation procedure for utilizing the twin-screw extruder system with multiple twin-screw extruders for PVC plastic processing can be carried out as follows: In a large twin-screw extruder and a small twin-screw extruder configured according to the geometric specifications of the resource-saving twin-screw extruder system, PVC is processed until a wear limit is reached.

[0021] The extruder screw shafts of the large twin-screw extruder are then remachined, with their outer diameter being reduced to the nominal diameter of the small twin-screw extruder. The extruder bore in the extruder barrel of the small twin-screw extruder, or in an insert bushing within it, is enlarged to the nominal diameter of the large twin-screw extruder. These two processing steps can be performed very cost-effectively by simply removing material, e.g., by grinding and honing.

[0022] The reconditioned, reduced-diameter extruder screw shafts from the large twin-screw extruder are reused in a new insert bushing of the small twin-screw extruder with a small nominal diameter. The reconditioned insert bushing or extruder barrel from the small twin-screw extruder is reused as an extruder barrel in the large twin-screw extruder with a large nominal diameter.

[0023] Instead of two pairs of screws and two insert bushings, in the system according to the invention only one pair of screws and one insert bushing need to be disposed of or subjected to cost-intensive processing with hard metal deposition, while the other pair of screws and the other insert bushing can be reused with only very little machining effort.

[0024] The nominal diameter gradations of large twin-screw extruders and small twin-screw extruders are small, typically ranging between 2% and 8% of the nominal diameter. In typical manufacturer series, directly consecutive types with the same center distance are selected.

[0025] The following is an example of a resource-saving twin-screw extruder system with two twin-screw extruders (DSE): Small DSE Large DSE Type of series 1 2 Center distance 88 mm 88 mm Nominal diameter of the extruder bore d 103.5 mm 106.5 mm Nominal diameter of the extruder screw shaft D 103 mm 106 mm

[0026] In this example, the size difference between the two types is 3 mm; this also represents the wear allowance ΔD. The wear occurring during production of both twin-screw extruders must be monitored and remain below the maximum wear allowance ΔD to allow for reprocessing and subsequent reuse with the other type.

[0027] In principle, the twin-screw extruder system of the invention also allows for cascading models with more than two sizes of twin-screw extruders, in which the extruder bores are enlarged at least a second time and the extruder screw shafts are reduced in diameter several times, so that such a system comprises, in addition to a large twin-screw extruder and a small twin-screw extruder, at least one medium-sized twin-screw extruder. In this case, two coupling diameters are provided, with a first coupling diameter being selected between the small twin-screw extruder and the medium-sized twin-screw extruder, and a second coupling diameter being selected between the medium-sized twin-screw extruder and the large twin-screw extruder.In this variant of the system, a subsystem is then provided that comprises the small twin-screw extruder and the medium twin-screw extruder, the latter being considered a large twin-screw extruder in this subsystem for the purpose of applying the invention. A second subsystem comprises the medium twin-screw extruder and the large twin-screw extruder, the medium twin-screw extruder being considered a small twin-screw extruder in this subsystem for the purpose of applying the invention.

[0028] The direction of passage through the system is the same as in the simple version with only two steps: - A new extruder barrel can be used first in the twin-screw extruder with the smallest inner diameter of the extruder bore and finally in the twin-screw extruder with the largest inner diameter of the extruder bore. - At the same time, a new pair of extruder screw shafts can be used first in the twin-screw extruder with the largest inner diameter of the extruder bore and finally in the twin-screw extruder with the smallest inner diameter of the extruder bore.

[0029] The limiting factor for post-processing and reuse in any case, whether in a system with only two stages or in a cascading system with more than two stages, is the relationship between the fixed center distance and the screw flight height reduced by the post-processing steps. This height must still be sufficiently large after each post-processing step to ensure the feed, plasticization, and pressure build-up of the processed plastic.

[0030] The invention is explained in more detail below with reference to the exemplary embodiment shown in the drawings. The figures show in detail: Fig.1 a twin-screw extruder in a highly simplified longitudinal section; Fig. 2 a partial section of a twin-screw extruder with a pair of extruder screw shafts; Fig. 3 a cross-section through an extruder bore; and Fig. 4 the twin-screw extruder system with several twin-screw extruders according to the invention in a schematic overview.

[0031] In Fig.Figure 1 shows a conventional design of a twin-screw extruder 10 in a highly simplified longitudinal section in a vertical plane. An extruder bore 2 extends longitudinally through an extruder housing 1. Arranged within this bore are a pair of rotatable extruder screw shafts 6, each with a tip 7 and a screw flight (not shown here). Solid, particularly powdered, plastic is drawn in through an intake opening 3 and, due to the orientation of the flight of the rotating extruder screw shaft 6, is conveyed toward the tip 7 and melted in the process. The plasticized mass is degassed via a suction line connected to the degassing opening 4.

[0032] Fig.Figure 2 shows a partial longitudinal section of a twin-screw extruder in a horizontal plane, revealing the pair of adjacent extruder screw shafts 6. The extruder screw shafts 6 mesh with each other by the screw flight 8 of one extruder screw shaft 6 engaging the valleys 9 between adjacent sections of the screw flight 8 on the other extruder screw shaft 6. There is an axial distance x between the extruder screw shafts 6.

[0033] Fig. Figure 3 shows a cross-section through the extruder barrel 1. The inner contour is formed by two intersecting circular cross-sections with an inner diameter d1 and has the shape of a horizontal figure eight. This contour corresponds to the extruder bore 2 in an unworn small twin-screw extruder.

[0034] Also in Fig.Figure 3 shows the outer contour of an extruder bore 2' with a larger inner diameter d2. This contour corresponds to the extruder bore 2 in an unworn large twin-screw extruder. The area between the two contours of the extruder bores 2, 2' corresponds to a layer 5 available for wear and reworking.

[0035] The twin-screw extruder system according to the invention and an operating method for its use are described with reference to Fig. 4 described.

[0036] In a preparation step 101, a pair of extruder screw shafts 6' with an outer diameter D2 is combined with an extruder housing 1' with an extruder bore 2' with inner diameter d2 to form a large twin-screw extruder 10'.

[0037] At the same time, in a further preparation step 111, a pair of extruder screw shafts 6 with an outer diameter D1 is combined with an extruder housing 1 with an extruder bore 2 with an inner diameter d1 to form a small twin-screw extruder 10.

[0038] During a first production phase, the outer diameter of all extruder screw shafts 6, 6' decreases. In step 102, the worn extruder screw shafts 6 of the small twin-screw extruder 10 are removed from the system.

[0039] The worn extruder screw shafts 6' of the large twin-screw extruder 10' are reconditioned in the reconditioning step 103, e.g., by cylindrical grinding, so that they now have the outer diameter D1 suitable for the small twin-screw extruder 10 and can be used in a further production phase together with a new extruder housing 1 or an insert bushing therefor.

[0040] Furthermore, in the first production phase, the extruder bores 2, 2' of both twin-screw extruders 10; 10' are worn, causing the diameters d of the bores therein to be enlarged. In step 112, the extruder barrel 1' of the large twin-screw extruder 10' is rejected due to its worn extruder bore 2'.

[0041] The extruder housing 1 of the small twin-screw extruder 10 with the worn extruder bore 2 is reconditioned in the reconditioning step 113 by honing the extruder bore 2 and then having the diameter d2, so that it can be used as extruder housing 1' with the extruder bore 2' with the inner diameter d2 in the large twin-screw extruder 10'.

[0042] In the system according to the invention, only two extruder barrels 1, 1' and two pairs of extruder screw shafts 6, 6' of the same size are ordered or manufactured from a selection of different extruders of a series with the same center distance for the first configuration. Subsequently, only one extruder barrel 1 with the smallest diameter of the extruder bore 2 and one pair of extruder screw shafts 6' with the largest diameter are ordered or manufactured.

[0043] From the sectional views in Fig. 4 or in Fig.3 also clearly shows why the same center distance x is required for all types of twin-screw extruders that can be integrated into the system: Although the extruder screw shafts reduced to diameter d1 after the reprocessing step 103 could also be used in an extruder bore with a smaller diameter and, at the same time, a smaller center distance, the recycling of an extruder housing 1 or an insert bushing for it is only possible if the center distance is the same in both usage phases and a symmetrical widening of the extruder bore is carried out as a reprocessing measure. This means that the respective center of the circular cross-sections of the extruder bores is retained during reprocessing.

Claims

[1] Resource-saving twin-screw extruder system with several twin-screw extruders (10; 10') for plastics processing; at least comprising a large twin-screw extruder (10') and a small twin-screw extruder (10), each of which comprises at least: - an extruder housing (1; 1') with an extruder bore (2; 2') which has a cross-section in the shape of a horizontal figure eight formed by two overlapping circular cross-sections with an inner diameter d1, d2 and - a pair of intermeshing extruder screw shafts (6; 6') arranged to rotate in the extruder bore (2; 2') with at least one screw flight (8) having an outer diameter D1, D2, wherein: - in the large twin-screw extruder (10'), the inner diameters d2 of the extruder bore (2') and the outer diameters D2 of the extruder screw shafts (6') are each larger by a wear allowance ΔD than the inner diameters d1 of the extruder bore (2) and the outer diameters D1 of the extruder screw shafts (6) of the small twin-screw extruder (10); - the axial distance x of the extruder screw shafts (6; 6') is the same for all twin-screw extruders (10; 10') in the system; - the lengths of the extruder screw shafts (6; 6') and the extruder bores (2; 2') are the same in both twin-screw extruders (10; 10'); and - the roller gap in the small twin-screw extruder (10) is larger than in the large twin-screw extruder (10'). [2] Twin-screw extruder system according to claim 1, characterized bythat the inner diameter and the outer diameter of the small twin-screw extruder (10) are 2% to 8% of the nominal diameter smaller than that of the large twin-screw extruder (10'). [3] Twin-screw extruder system according to claim 1 or 2, characterized by that the extruder screw shafts (6; 6') and the extruder bores (2; 2') are cylindrical and arranged parallel to one another. [4] Twin-screw extruder system according to one of claims 1 to 3, characterized by that the extruder screw shafts and the extruder bores are each conical with the same cone angle and are arranged at an acute angle to each other. [5] Twin-screw extruder system according to one of the preceding claims, characterized by , - that more than two sizes of twin-screw extruders are provided, with adjacent sizes each forming a subsystem with a large twin-screw extruder and a small twin-screw extruder, - that a new extruder housing can be used first in the twin-screw extruder with the smallest internal diameter of the extruder bore and finally in the twin-screw extruder with the largest internal diameter of the extruder bore and - that a new pair of extruder screw shafts can be used first in the twin-screw extruder with the largest inner diameter of the extruder bore and finally in the twin-screw extruder with the smallest inner diameter of the extruder bore.

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