FILTERING DEVICE FOR PVC PLASTIC MELT FOR CONNECTION WITH A TWIN SCREW EXTRUDER AND TWIN SCREW EXTRUDER FOR PVC PROCESSING

Description

[0001] The invention relates to a filtering device for PVC plastic melt having the features of the preamble of claim 1 and a twin-screw extruder for PVC processing having the features of the preamble of claim 8.

[0002] Processing PVC plastic melt is challenging, as it undergoes chemical degradation if exposed to heat for too long. This can release hydrochloric acid, especially at temperatures above 180°C, which has a corrosive effect on processing equipment. Therefore, the flow paths between the melt in the extruder and the discharge at a die must be kept short.

[0003] When recycling PVC, a filter device often has to be inserted between the extruder and the tool to filter out conglomerates and foreign matter from the plastic melt before it reaches the nozzle or other tool. The inserted filter lengthens the flow paths, and there is a risk of insufficient flow areas forming where the plastic melt lingers for too long.

[0004] DE 42 35 523 C1 describes a screening device with a rotating screen disc. Perforations in the housing and screen inserts in the screen disc have a circular base. The melt channel widens in an opening area and forks in front of the screen disc, forming two partial openings at the same pitch as the perforations on the screen disc. The partial flows reunite at the rear. However, this concept serves to increase throughput and is not suitable for filtering PVC plastic melt produced by a twin-screw extruder.

[0005] A similar concept for a filtration device, in which the melt channel divides into two sub-channels in the flow direction upstream of the screen disc, which rejoin behind the screen, is disclosed in US 2008 / 179261 A1. Here, too, the filtration device is neither intended nor objectively suitable for use with a twin-screw extruder.

[0006] DE 40 18 310 C1 describes a method for filtering plastic melts using a screen wheel filter device. To prevent chemical changes in the plastic melt in poorly flowed areas of the melt channel leading through the filter device, the plastic melt remaining in the screen chambers is forced out by compressed gas before reaching the cleaning or filter change position and fed to the outlet end of the melt channel. The object of the invention is therefore to provide a filter device for PVC plastic melt that can be connected to a twin-screw extruder, in which the flow behavior is improved and consequently the risk of chemical changes in the melt is reduced.

[0007] The publications EP 3 160 711 B1 and CN 203 680 783 U describe further filtering devices for PVC plastic melt.

[0008] This object is achieved by a filtering device having the features of claim 1.

[0009] The invention is based on the known concept of a sieve wheel filter and the further approach of converting parts of the inlet side of such a

[0010] To design the filtration device directly as part of the tip area of ​​a twin-screw extruder so that the extruder screws can be inserted with their tips into the filtration device.

[0011] For this purpose, an inlet melt pocket is formed in the inlet block. This pocket has a double extruder screw receptacle for two screw tips of two extruder screws arranged side by side on one inlet side of the housing and opens into an oval or slot-shaped collecting area in front of the screen wheel. By partially integrating the extruder into the filtration device, the conventional pipe flange connections, which are usually used to establish a flow connection between a self-contained extruder and a likewise self-contained filtration device, can be dispensed with. The flow paths within the filtration device are therefore particularly short in the invention, since the tips of the extruder screws can reach very close to the screen wheel with the screen elements inserted therein.

[0012] A further advantage of the invention is the simultaneous widening of the melt channel cross-section in the inlet melt pocket, both in the x- and y-directions. The widening occurs relative to the inlet channel cross-section, the dimensions of which are determined by the geometry of the twin screw, and in particular with a relatively uniform widening horizontally and vertically, i.e., in the x- and y-directions. Compared to the conventional conversion of the cross-section of a twin screw into a cylindrical flow channel, this results in the advantage that the flow is much more uniform and that the sieve elements on the sieve disc can have a smaller radial extent than with a circular cross-section due to the lower height of the oval or elongated hole.The radial expansion determines the sieve wheel diameter and thus the overall size of the filter device, so that conversely the size can be reduced by reducing the radial expansion.

[0013] Preferably, the twin extruder screw receptacle at the inlet melt pocket is formed by two laterally intersecting, parallel cylinder bores, with a cross-section in the shape of a horizontal figure-eight. This surrounds the end sections of the satellite screws at the periphery in the same way as is the case within the extruder's extruder bore.

[0014] A further preferred embodiment of the filtration device provides that the inlet melt pocket is formed in a channel bushing detachably connected to the inlet block and / or that the outlet melt pocket is formed in a channel bushing detachably connected to the outlet block. This allows the respective channel bushing to be quickly replaced and cleaned outside the housing of the filtration device.

[0015] This is a particular advantage because the funnel section widens from a smaller inlet opening toward the screen wheel, creating an undercut. Cleaning the funnel area would be difficult, either through the inlet opening or after completely disassembling the filter assembly. If the channel bushing is removed, it can be easily cleaned from the inside. Furthermore, access to the screen disc is made easier when cleaning.

[0016] In addition, the interchangeable channel bushings of the filter device allow easy adaptation of the melt pockets to different screw geometries while using the same housing.

[0017] To prevent any increase in the viscosity of the plastic melt due to cooling within the filtration device, the inlet block and / or the outlet block are preferably provided with flow channels for liquid temperature control. Heating is preferably provided for a start-up process or during a downtime of the extrusion device.

[0018] Liquid temperature control not only enables heating to operating temperature, but also, and in particular, cooling. This allows the heat generated by the frictional forces within the flowing melt to be dissipated. Of particular importance here are the area of ​​the screw tip in the inlet - shear energy introduced by the movement of the extended screws - and the passage through the filter element itself - high shear within the small passage openings in the screen element. This means, for example, that the filter system can be integrated into an existing, thermally optimized processing process without filtration, without the need for fundamental changes to the upstream and downstream components. The additional heat introduced by the filter element and, if applicable, a screw extension can be dissipated by liquid temperature control.

[0019] Removable channel bushings also allow flexible adaptation to different extruders, but also to downstream tools, e.g. oval channels on the outlet side for subsequent wide slot tools or other geometry for profile tools adapted to the profile shape.

[0020] Furthermore, a preferred embodiment provides that the housing has at least one opening area on its upper side, in which the sieve wheel protrudes from the inlet block and the outlet block and in which at least one sieving point on the sieve wheel is accessible from outside the housing.

[0021] A twin-screw extruder, which is particularly suitable for PVC processing, usually provides an extruder housing with an extruder bore in which two axially parallel, rotationally driven extruder screws are arranged, whereby the extruder screws extend to a funnel zone at the end of the extruder bore, which is in flow connection with an extruder outlet opening.

[0022] According to the invention, the filtration device can be functionally integrated directly into the twin-screw extruder. For this purpose, a rotatable screen wheel, which has a plurality of screen openings, each of which can accommodate at least one screen element through which plastic melt can flow, is guided through the hopper zone in the front area of ​​the extruder bore, dividing it into two hopper zone sections.

[0023] Such a solution is provided by a twin-screw extruder with the features of claim 8. In addition to a fully integrated version of an extruder with a built-in screen wheel, it is also possible to connect a conventional twin-screw extruder with only minor adjustments to the front end to a filtration device according to the invention. In this case, the screen wheel is part of the previously described filtration device, which is firmly connected to the twin-screw extruder, with the inlet melt pocket in the housing of the filtration device forming an end section of the extruder bore.

[0024] It may also be possible to provide two tip extension elements for the extruder screws. These are attached to the front of the actual extruder screws, but no longer have screw flights. The tip extension elements are then inserted into the twin extruder screw receptacle in place of the actual extruder screw tips. The advantage is that with an existing twin-screw extruder, the extruder housing does not have to be shortened, nor do additional longer screws have to be purchased to allow the screw tips to protrude slightly from the extruder bore and engage with the twin extruder screw receptacle of the filtration device.

[0025] The sieve wheel can be driven via a freewheel or a ratchet drive, with a drive lever engaging in recesses on the outer edge of the sieve disc.

[0026] The invention is explained in more detail below with reference to the exemplary embodiment of a filtering device shown in the drawings. The figures show in detail: Figure 1 shows a perspective view of a filtering device; Figure 2 shows a perspective view of the filtering device with parts of a twin-screw extruder; Figure 3 shows the rear side of the filtering device from above; Figure 4 shows a channel bushing in perspective view; Figure 5 shows an inlet block in perspective view; Figure 6 shows a horizontal section through the filtering device; and Figure 7 shows a plan view of the cutting plane from Figure 6Figure 8 shows a perspective sectional view of a vertical section through the filtration device; Figure 9A shows the size distribution between a twin extruder screw receptacle and a cylindrical hopper section according to the prior art; Figure 9B shows the size distribution between the twin extruder screw receptacle and the hopper section of the invention.

[0027] Figure 1 shows a perspective view of a filtration device 100 with a housing 10, which is essentially formed by an inlet block 11 and an outlet block 12, as well as a rotatably mounted sieve wheel 30 arranged therebetween. The sieve wheel 30 is driven in rotation by a drive 20.

[0028] The housing 10 is cuboid-shaped only in its lower area on its outside. As indicated by the dashed lines, it does not extend upwards far enough to completely enclose the screening wheel 30. Rather, an opening area 13, represented by the dashed lines, is formed in which, in the illustrated embodiment, four screening points 31 on the screening wheel 30 are freely accessible simultaneously for maintenance purposes. Since the housing contains a double extruder screw receptacle for the screw tips of two extruder screws 201, 202, and these must be arranged laterally next to one another, it is advantageous to provide the opening area 13 at the top of the housing 10, as this allows the overall housing dimensions to be smaller.

[0029] A special feature of the filtering device 100 according to the invention is when looking at a Figure 1visible rear side 14 of the housing 10. Two extruder screws 201, 202 of a twin-screw extruder are guided into an inlet melt pocket in the inlet block 11. Thus, the inlet melt pocket of the filtration device 100 forms part of the extruder bore of the twin-screw extruder, which can be connected to the housing 10 of the filtration device 100 via a flange 17.

[0030] Figure 2 is a perspective view of the filtration device 100 with a front part of a twin-screw extruder 200 positioned in front of it, which is not yet connected to the flange 17 on the housing 10 of the filtration device 100. A twin extruder screw receptacle 53 can be seen in the central recess in the flange 17.

[0031] In Figure 3The filtration device 100 is shown in perspective from the rear 14 and without the flange 17. It is clearly visible that there is a greatly expanded outlet channel, referred to as the inlet melt pocket 51. The inner contour of the inlet melt pocket 51, which is formed within a removable channel bushing 50, is adapted to the two intermeshing extruder screws and therefore initially has the shape of a horizontal figure eight in the area facing the outside of the housing. This area forms the double extruder screw receptacle 53, which must be adapted to the outer contour of the pair of extruder screws. Behind it is a funnel section 52, which leads to the sieve elements 32 in the sieve wheel 30.

[0032] The drive 20 consists of a hydraulic cylinder 24, whose piston rod is connected to a drive lever 22 via a joint 23. The drive lever 22 is mounted on an axle 21 on the housing, which includes a freewheel, and is connected via the freewheel to a pinion (not shown here), which engages from the outside with a toothing 33 on the sieve wheel 30.

[0033] Figure 4 shows in perspective view a channel bushing 50 removed from the inlet block 11, in which the inlet melt pocket 51 with its two sections 52, 53 is formed.

[0034] Figure 5shows the inlet block 11 in a perspective view, looking from the inside facing the screen wheel through the inlet melt pocket 51 to the outside. It can be seen here that the inlet block 11 has an opening 18 with a kidney-shaped contour, to which the channel bushing 50 inserted behind it in the inlet block 11, with its opening in the shape of a horizontal figure eight, seamlessly connects.

[0035] Figure 6 is a horizontal section through the filter device 100, wherein the cutting plane is arranged at the level of the tips of the extruder screws 201, 202, and thus approximately halfway up the channel bushing 50 with the inlet melt pocket 51, which contains the funnel section 52.

[0036] On the outlet side behind the screen wheel 30, a further channel bushing 40 is provided, which is inserted into the outlet block 12 and in which a melt outlet pocket is formed. The melt outlet pocket comprises a funnel section 42 located in front of the screen wheel 30, to which a cylindrical outlet channel 41 is connected.

[0037] Due to the wide design of the funnel section 52 in the inlet melt pocket 51, the plastic melt conveyed by the two extruder screws 201, 202 can not only be distributed simultaneously to several screening positions on the screening wheel 30, but it can also expand evenly in both dimensions of the opening cross-section.

[0038] On the outlet side, the funnel section 42 ensures that the melt flowing out of the several screening points 31 is concentrated again and fed to the outlet channel 41.

[0039] In the housing, between the inlet block 11 and the outlet block 12, spacer elements 15, 16 can be seen, which enclose the rotatably mounted sieve wheel 30 between them and serve to create a tight connection between the inlet block 11 and the outlet block 12, but at the same time to prevent the sieve wheel 30 from jamming.

[0040] Figure 7 represents a section of a top view of the cutting plane Figure 6 to illustrate once again how the funnel section 52 widens slightly convexly outward in front of the screen wheel 30, viewed in the direction of flow. This shape not only serves to distribute the plastic melt over a larger screen surface on the screen wheel 30, but is also optimized to prevent wall flows from adhering to the screen. Especially when processing PVC, plastic portions that remain in the flow space for a longer period of time can be chemically decomposed and reduce the quality of the infiltrated PVC plastic melt.

[0041] As the perspective sectional view of a vertical section through the filter device in Figure 8 As shown, after the twin-screw extruder has been fully assembled on the filtration device 100, the extruder screws 202 extend well into the inlet melt pocket 51. Thus, the inlet melt pocket 51 with its twin extruder screw receptacle 53 functionally forms the front part of a twin-screw extruder, although structurally it is part of the filtration device 100.

[0042] The screw tips of the extruder screws 202 are guided almost directly in front of the screen wheel 30 with its screen elements 32, so that beyond the screw tips only a very short flow distance has to be bridged until the melt passes through the screen element 32 and is then compressed again in the tapered funnel section 42 and discharged through the outlet channel 41.

[0043] Figure 9Ashows the size distribution between a twin extruder screw intake 53 and a hopper section 52', which is designed such that the horizontal figure-eight shape, caused by the adjacent and intermeshing twin screws, is transformed into a cylindrical flow channel. This has two disadvantages: the diameter of the hopper section 52' is determined by the width of the "horizontal figure-eight" and determines the required screen size on the screen wheel. Secondly, there is almost no flow change in the X direction, but there is in the Y direction, so that the uneven flows in the transition area can result in disadvantages such as dead zones and the adverse influence of shear forces on the melt.

[0044] In Figure 9BThe double extruder screw receptacle 53 with the funnel section 52 in the channel bushing of the invention is depicted in the same size. The expansion in the X direction approximately corresponds to the largest expansion in the Y direction, so that the flow from the extruder to the screening point is expanded much more evenly here.

[0045] The expansion in the Y direction can be dimensioned such that y1 is assumed to be the largest extension in the Y direction in the center of the twin extruder screw receptacle 53 and is set equal to the expansion x. Alternatively, an average value y2, shown as a dot-dash line, can be used as a reference.

[0046] As a result, the maximum height of the flow channel, which according to Fig. 9B emerges from the funnel section 52, with a similar cross-section as the funnel section 52' in Fig. 9Aby 30%. This allows a smaller diameter screen wheel and, consequently, a smaller filtering device to be selected to achieve the same filtering effect. List of reference symbols:

[0047] 100 Filtration device 10 Housing 11 Inlet block 13 Opening area 14 Rear 15,16 Spacers 17 Flange 18 Opening 20Drive 21Axle 22Drive lever 23Joint 24Hydraulic cylinder 30Screen wheel 31Screening positions 32Screen elements 33Toothing 40Channel bushing 41Outlet channel 42Funnel section 50Channel bushing 51Inlet melt pocket 52; 52'Funnel section 53Double extruder screw holder 200Twin-screw extruders 201, 202Extruder screws

Claims

1. Filtering apparatusa (100) for PVC polymer melts for connection to a twin-screw extruder, at least comprising: - a housing (10) which at least has: - an inflow block (11) with at least one inflow channel, - an outflow block (12) with at least one outflow channel; - at least one spacer element (15, 16) located between the inflow block (11) and the outflow block (12); - a screen wheel (30), - which is rotatably mounted in the housing (10), - which is located at least partly between the inflow block (11) and the outflow block (12) and - which has a plurality of screening points (31) that can each be positioned between the inflow channel and the outflow channel and into each of which at least one screen element (32) through which polymer melt can flow can be inserted, - a drive device (20) for rotating the screen wheel (30) in the housing (10); characterized - in that the inflow channel is at least partially formed in an inflow melt pocket (51) in the inflow block (11) and the inflow melt pocket has, on an inflow side on the outer side of the housing (10), a twin extruder screw receptacle (53) for two screw tips of two extruder screws (201, 202) located next to each other, widens in an oval or slot-like funnel portion (52; 52') and emerges in front of the screen wheel (30); - in that the outflow channel is at least partially formed in an outflow melt pocket in the outflow block (12), the outflow melt pocket narrowing in the flow direction from a funnel portion (42) facing towards the screen wheel (30).

2. Filtering apparatus (100) according to Claim 1, characterized in that the twin extruder screw receptacle (53) is formed at the inflow melt pocket (50) by two laterally intersecting, parallel-aligned cylindrical bores which have a cross section in the form of a horizontal eight.

3. Filtering apparatus (100) according to Claim 2, characterized in that the funnel portion (52; 52') has an identical funnel angle around the circumference.

4. Filtering apparatus (100) according to Claim 2 or 3, characterized in that, in the funnel portion (52; 52'), the cross section of the twin extruder screw receptacle (53) widens in an X direction leading through the centre points of the two extruder screws (201, 202) to the same extent as in the Y direction, in relation to a dimension y1 in the centre of the twin extruder screw receptacle (53).

5. Filtering apparatus (100) according to Claim 2 or 3, characterized in that, in the funnel portion (52; 52'), the cross section of the twin extruder screw receptacle (53) widens in an X direction leading through the centre points of the two extruder screws (201, 202) to the same extent as in the Y direction, in relation to an average value y2 at the upper and / or lower edge of the twin extruder screw receptacle (53).

6. Filtering apparatus (100) according to one of Claims 1 to 5, characterized in that the inflow melt pocket (51) is formed in a channel bushing (50) detachably connected to the inflow block (11).

7. Filtering apparatus (100) according to one of Claims 1 to 6, characterized in that the outflow melt pocket is formed in a channel bushing (40) detachably connected to the outflow block (12).

8. Filtering apparatus (100) according to one of Claims 1 to 6, characterized in that the inflow block (11) and / or the outflow block (12) have at least one flow channel for a liquid heating means passing through them.

9. Filtering apparatus (100) according to one of Claims 1 to 8, characterized in that the upper side of the housing (10) has at least one aperture region (13) in which the screen wheel (30) protrudes out of the housing (10) and in which aperture region (13) at least one screening point (31) on the screen wheel (30) is accessible from outside the housing (10).

10. Twin-screw extruder (200) for PVC processing, comprising an extruder housing which has an extruder bore containing two axially parallel, rotationally driven extruder screws (201, 202), wherein the extruder screws extend into an end portion of the extruder bore that is in flow connection with an outflow channel, characterized in that the extruder housing is connected to a filtering apparatus (100) according to at least one of the preceding claims, wherein the inflow melt pocket (51) of the filtering apparatus (100) forms the end portion of the extruder bore.

11. Twin-screw extruder (200) according to Claim 10, characterized by two tip extension elements attached in front of the extruder screws (201, 202) and going into the twin extruder screw receptacle (53).