Improved cleaning device for filters of filtering devices for plastic materials

Abstract

Cleaning device (40) for filters (32, 34) of filtering devices (10, 110) for a plastic material in the molten state, comprising a rotary main body (42) provided with scraping elements (48, 50) that cooperate with the filtering surfaces (32A, 34A) of two filters (32, 34) facing each other. On the two opposite sides (44, 46) of said main body (42) there are channels (60A, 60B, 60C, 70A, 70B, 70C) suited to receive the impurities collected by said scraping elements (48, 50) and to convey them into a discharge chamber (68), towards which they converge tangentially, and wherein the discharge openings (66) of said channels are made on a side of said discharge chamber (68) and arranged on the middle plane (M) of the main body (42).

Description

[0001] IMPROVED CLEANING DEVICE FOR FILTERS OF FILTERING DEVICES FOR PLASTIC MA TERIALS DESCRIPTION

[0002] The present patent concerns systems for filtering plastic materials in the molten state, in particular systems for filtering molten plastics during plastic recycling processes.

[0003] More specifically, the present patent concerns a new cleaning device for filters suited to be used in filtering devices for plastic materials in the molten state, in particular in filtering devices for molten plastics during plastic recycling processes.

[0004] The present patent also concerns the filtering device for plastic materials comprising the new filter cleaning device.

[0005] The problem of recovering / recycling plastic materials in order to reuse them for manufacturing new products is known and particularly important.

[0006] The plastic material to be recycled undergoes a melting process and simultaneous filtering intended to remove impurities. Usually, the melted and filtered material is then appropriately treated to produce, for example, a granular product that can be reintroduced in the known plastic production processes.

[0007] A known device that filters raw plastic material in the molten state comprises a cylindrical filtration chamber equipped with one or more filtering elements interposed between a feed inlet for the raw plastic material to be filtered and an outlet, located downstream of the filtering elements, for the filtered molten plastic material.

[0008] Known filtration chambers comprise a first part with closed bottom,preferably a first cylindrical part with closed bottom, and a cover that is associated with the mouth of the first part. With the cover associated with / fixed to the mouth of the first part, the filtration chamber is closed and the filtering device is ready for use. The cover is preferably fixed to the first part by means of screws and nuts that can be conveniently unscrewed for normal maintenance and / or repair operations.

[0009] According to the known technique, the raw plastic material in the molten state is introduced in the filtration chamber and passed, under pressure, through the filtering elements, which typically consist of substantially flat (or cylindrical) filters provided with filtration through openings of appropriate size suited to ensure the desired filtration capacity.

[0010] During operation with continuous feeding of raw material, impurities tend to gradually accumulate on the surface of the filter. For this reason, a cleaning system is provided inside the filtration chamber for continuous cleaning of the filter surface.

[0011] According to the known technique, the cleaning system comprises at least one fixed or movable (preferably rotating) scraper, equipped with scraping elements that slide on the surface of the filter to remove the accumulated impurities. At the same time, the removed impurities are conveyed outside the filtration chamber. The impurities collected by the scraping elements are preferably conveyed towards the centre of the filter along a curved path and from there, through a channel, conveyed towards the outside by a worm screw / screw feeder or by an on / off valve that extends from the centre of the filter and exits from the filtration chamber.

[0012] According to a first known technique, the scraper element and the screwfeeder are rotated together around a common rotation shaft, which is conveniently driven by motorized means, normally an electric motor.

[0013] A drawback associated with this type of filtering device lies in the inability to optimize and minimize waste material, as it is not possible to manage the rotation speed of the screw feeder and that of the scraper separately.

[0014] According to a second known technique, instead, the scraper element is fixed while the filtering elements are set rotating. Furthermore, each filtering element is associated with an independent screw feeder.

[0015] The drawbacks associated with this second known technique are the presence of only one scraper for each filtering surface and the need for a screw feeder for each filtering surface.

[0016] The same applicant is the owner of patent 102021000021380, which concerns a filtering device for plastic materials that overcomes the above-mentioned drawbacks, as it is equipped with a specially configured filter cleaning device, which improves the cleanliness of the filtering elements and minimizes waste.

[0017] The subject of the present patent is a new improved cleaning device for the filters of filtering devices for plastic material in the molten state of the type described in patent 102021000021380, which is referred to herein for reference purposes.

[0018] The new cleaning device is configured to improve the outward flow of the impurities obtained from filtration, thus reducing the risk of clogging of the drainage channels.

[0019] It is another object of the present invention to facilitate cleaning operations, as the new cleaning device is easier to remove and manage.The new filter cleaning device can be installed in a filtering device, for example for a plastic material in the molten state, comprising:

[0020] - a hollow body provided with a mouth and a cover suited to be associated with said mouth to define a filtration chamber;

[0021] - at least one inlet way for the introduction of said plastic material in the molten state in said filtration chamber and at least one outlet way for the outlet of said filtered plastic material in the molten state from said filtration chamber;

[0022] - a first filter and a second filter arranged opposite each other inside said filtration chamber in a position between said at least one inlet way and said at least one outlet way, said first filter being suited to define a first surface suited to filter said plastic material and collect any impurities and said second filter being suited to define a second surface suited to filter said plastic material and collect any impurities;

[0023] - said cleaning device arranged between said first filter and said second filter;

[0024] - an evacuation device suited to discharge said impurities collected by said cleaning device towards an evacuation outlet, said evacuation device being arranged centrally with respect to said cleaning device;

[0025] wherein said cleaning device comprises a rotary main body with a first side facing towards said first filter and provided with first scraping elements cooperating with said first filtering surface of said first filter and a second side facing towards said second filter and provided with second scraping elements cooperating with said second filtering surface of said second filter, said main body comprising one or more first axially symmetrical channelscommunicating with said first side and one or more second axially symmetrical channels communicating with said second side and suited to receive the impurities respectively collected by said first scaping elements and said second scraping elements, and to convey them towards said evacuation device, said first one or more axially symmetrical channels and said second one or more axially symmetrical channels being defined in said main body staggered with respect to one another and arranged at the same distance from the middle plane of said main body, in such a way that said impurities pass through said first one or more channels and said second one or more channels along equal paths with the same flow rate and discharge said impurities on said evacuation device in a balanced manner through discharge openings arranged on said middle plane, guaranteeing the introduction of impurities in said evacuation device in completely equivalent positions.

[0026] The discharge openings lie on the middle plane or are arranged on the middle plane, meaning that they are arranged in such a way that said middle plane cuts or crosses them, coinciding or not coinciding with a plane of symmetry of the geometric figure defined by the discharge openings themselves.

[0027] More specifically, said channels converge towards a discharge opening located in a central area of said main body, along directions that are essentially tangential or essentially parallel to a non-radial direction tangential to said discharge opening.

[0028] Said channels are defined by shaped surfaces designed to channel the material towards an area below said scraping elements and then towardssaid discharge opening.

[0029] More preferably, said discharge opening defines a toroidal chamber in which the outlet openings of the channels are positioned.

[0030] Said toroidal chamber is defined by the concave, that is, recessed edge of said discharge opening.

[0031] Said configuration, which is described in greater detail below, promotes better and faster drainage of impurities because it makes them move more uniformly and reduces the risk of blockages.

[0032] Further specific technical operating characteristics of the device that is the subject of the present invention are described in the related dependent claims.

[0033] The characteristics of the new device are better clarified in the following description, making reference to the drawings, which are attached by way of non-limiting example.

[0034] Figure 1 shows an axonometric view of a filtering device 10 according to a preferred embodiment of the invention.

[0035] Figure 2 shows a sectional view of the filtering device 10 of Figure 1 along a horizontal section plane.

[0036] Figures 2A and 2B respectively show a detail of Figure 2 and a sectional view of the filtering device 10 of Figure 1 along a vertical section plane. Figure 3 shows a side view of a first possible embodiment of the new filter cleaning device 40.

[0037] Figure 4 shows an axonometric view of the cleaning device 40 of Figure 3 from a first side 44.

[0038] Figure 4A shows a partial exploded view of Figure 4.Figure 5 shows a second axonometric view of the cleaning device 40 of Figure 3 from a second side 46.

[0039] Figure 5 A shows a partial exploded view of Figure 5.

[0040] Figures 5B and 5C show a detail of Figure 5 and a detail of Figure 5 A. Figure 6A shows a plan view of a side 44 of the main body 42 without a central part 42A, while Figure 6B shows the opposite side 46 without the central part 42B.

[0041] In Figure 6C, which shows the same view of Figure 6A, the path followed by the impurities when flowing out is highlighted.

[0042] Figure 6D shows a view including only the external part 42C of the main body 42.

[0043] Figure 7 shows a second possible embodiment of the invention, in which the main body 42 is lightened.

[0044] Figure 8 shows a sectional view of the filtering device 10 according to the invention.

[0045] Figure 9 shows a variant embodiment 110 of the filtering device according to the invention.

[0046] Figure 10 shows a view of the first side 44' of the cleaning device 40' in a further possible embodiment, while Figure 11 shows the first side 44' of the main body 42' of the cleaning device 40' of Figure 10, without the central part 42A, and with the path followed by the impurities when flowing out highlighted.

[0047] The new cleaning device can be installed in a filtering device 10 that is suitable for use in a plastic recycling process.

[0048] The filtering device 10 has the function of filtering / stopping the impuritiespresent in the raw plastic material to be recycled, with a degree of purity that depends on the mesh size of the filtering element used.

[0049] To perform this filtering operation, the raw plastic material to be recycled is first heated and melted to be subsequently conveyed to the filtering device 10, specifically to a filtration chamber 12 of the device 10 itself.

[0050] The filtering device 10 preferably has a hollow body 14 provided with a mouth 16 and a cover 20 suited to be associated with the mouth 16 to define the filtration chamber 12 (Figures 2, 2b).

[0051] According to the preferred embodiment illustrated, the hollow body 14 is preferably cylindrical in shape and consequently the cover 20 has a preferably circular cross section.

[0052] The hollow body 14 is associated with an inlet way 22 for the introduction of the molten plastic material to be filtered into the filtration chamber 12. The hollow body 14 is also associated with an outlet way 24 for the outlet of the filtered plastic material in the molten state from the filtration chamber 12.

[0053] Preferably, conveyor means, not shown and of a known type per se, are associated with the inlet way 22 for channelling the pressurized molten material to be filtered towards said inlet way 22.

[0054] Filtering means 30 are preferably arranged inside the filtration chamber 12 in a position between the inlet way 22 and the outlet way 24.

[0055] According to an aspect of the present invention, the filtering means 30 comprise a first filter 32 and a second filter 34 arranged opposite each other inside the filtration chamber 12 in a position between the inlet way 22 and the outlet way 24.The first filter 32 and the second filter 34 preferably comprise two flat filtering discs arranged in parallel. In variant embodiments, the shape may be different and follow the internal shape of the filtration chamber.

[0056] The first filter 32 defines a corresponding first filtering surface 32 A designed to collect impurities during the filtration process, and the second filter 34 defines a corresponding second filtering surface 34 A designed to collect impurities during the filtration process (shown in Figure 2A). Downstream of the two filters 32, 34, two respective collection channels 25, 26 for the filtered material are defined, which preferably join at the final outlet way 24.

[0057] The channels 25 and 26 follow identical and symmetrical paths to help achieve equal flow rates and pressures.

[0058] The new cleaning device 40, described in greater detail below and having the function of cleaning the first filtering surface 32A of the first filter 32 and the second filtering surface 34 A of the second filter 34, is arranged between the first filter and the second filter 32, 34.

[0059] Preferably, there is also an evacuation device 80 for discharging the impurities collected by said cleaning device 40 towards an evacuation outlet 82, shown in Figure 2b. The evacuation device 80 is preferably positioned centrally with respect to the cleaning device 40.

[0060] The evacuation device 80 preferably comprises an evacuation channel 85 that houses a rotary discharge element 84, preferably a worm screw or screw feeder, suited to receive and move the impurities collected in the filtration chamber 12 along a longitudinal direction towards the evacuation outlet 82. In a variant embodiment, shown in Figure 9 and described in greater detailbelow, the evacuation device 180 preferably comprises an evacuation channel 85 and an on / off valve 190 suited to open to allow the impurities collected in the filtration chamber 12 to move towards the evacuation outlet 82 under the action of pressure.

[0061] Suitable rotation means 28, preferably an electric motor 28A with controlled speed, enable the controlled rotation of the rotary discharge element 84 during operation of the filtering device 10.

[0062] According to an aspect of the present invention, the cleaning device 40 comprises a rotary main body 42, preferably circular in shape, having a first side 44 facing towards the first filter 32 and a second side 46 facing towards the second filter 34. The first side 44 is provided with first scraping elements 48 cooperating with the first filtering surface 32A of the first filter 32 and the second side 46 is provided with second scraping elements 50 cooperating with the second filtering surface 34 A of the second filter 34. Therefore, the rotary main body 42 can be rotated around its rotation axis while it is not constrained axially.

[0063] The first scraping elements 48 and / or the second scraping elements 50 preferably comprise scraping blades rigidly connected to the respective sides 44, 46 of the main body 42, more preferably flexible scraping blades, suited to be arranged in contact with the respective first filtering surface 32 A of the first filter 32 and second filtering surface 34 A of the second filter 34. The flexibility of the scraping blades 48, 50 causes them to be compressed by the two filters 32, 34 at the moment when the device 10 is closed by the cover 20, and the parts of the scraping blades 48, 50 in contact with the filtering surfaces 32A, 34A provide the pressure necessary toscrape and remove impurities from the surfaces 32A, 34A themselves. This is facilitated by the fact that the main body 42 is not constrained axially and can therefore be centred thanks to the pressure exerted by the scraping blades 48, 50. The scraping blades 48, 50 are preferably arranged symmetrically on the two sides 44, 46 of the main body 42 with respect to the middle plane M of the main body 42 itself. Advantageously, the scraping blades 48, 50 deform essentially in the same way on both sides 44, 46 of the main body 42 and thus exert the same pressure on the surfaces 32A, 34A of the filters 32, 34, ensuring better filtration quality (same degree of cleanliness as the filters 32, 34) as well as regular and equivalent wear of the filters 32, 34 and / or the scraping blades 48, 50. The scraping blades 48, 50 are fixed to the respective sides 44, 46 of the main body 42 by means of fixing screws.

[0064] According to the preferred embodiment illustrated, there are three scraping blades 48, 50 for each side 44, 46. In other variant embodiments, however, the number of these blades can be different.

[0065] Suitable rotation means 27 enable the controlled rotation of the rotary main body 42 during operation of the filtering device 10.

[0066] The rotation means 27 preferably comprise an electric motor 27A, more preferably with controlled speed, which rotates a drive shaft 27B. The end of the drive shaft 27B engages in a corresponding seat 44A created centrally in the first side 44 of the main body 42 of the cleaning device 40 (best seen in Figures 4 and 4A).

[0067] The end of the drive shaft 27B and the corresponding seat 44A of the main body 42 are preferably polygonal, more preferably hexagonal in shape.According to another aspect of the present invention, the main body 42 comprises one or more first channels 60A, 60B, 60C communicating with the first side 44 and one or more second channels 70A, 70B, 70C communicating with the second side 46.

[0068] Said first channels 60A, 60B, 60C are conveniently created in the main body 42 to receive the impurities collected by the first scraping elements 48 and convey them centrally towards the evacuation device 80, more preferably to convey them onto the external surface of the rotary discharge element 84 or, in other cases, towards the evacuation device 180.

[0069] Similarly, said second channels 70A, 70B, 70C are conveniently created in the main body 42 to receive the impurities collected by the second scraping elements 50 and convey them centrally towards the evacuation device 80, more preferably to convey them onto the external surface of the rotary discharge element 84 or, in other cases, towards the evacuation device 180. Said channels 60A, 60B, 60C, 70A, 70B, 70C are defined by surfaces that are shaped in such a way as to channel the material towards an area below said scraping elements 48, 50 and then towards a discharge opening, where said evacuation device 80 is located.

[0070] For example, as shown in Figure 4A, said surfaces may comprise slides 610 inclined towards the area 611 where the scraping elements 48, 50 are fixed, so as to define said channels 60A, 60B, 60C, 70A, 70B, 70C on the surface of the main body 42.

[0071] According to the preferred embodiment illustrated, there are three axially symmetrical channels 60A, 60B, 60C associated with the respective three scraping blades 48 of the first side 44 (Figures 4A and 6A) and three axiallysymmetrical channels 70A, 70B, 70C associated with the respective three scraping blades 50 of the second side 46 (Figures 5A and 6B). In variant embodiments, however, the number of said channels can be different.

[0072] Said first channels 60A, 60B, 60C and said second channels 70A, 70B, 70C are preferably defined in the main body 42, staggered with respect to one another and arranged all at the same distance from the middle plane M in such a way that the impurities pass through said first channels 60A, 60B, 60C and said second channels 70A, 70B, 70C along equal paths and discharge the impurities on the evacuation device 80 in an equivalent and balanced manner.

[0073] Each channel 60A, 60B, 60C, 70A, 70B, 70C preferably comprises a first section 62 defined at and below a respective scraping blade 48, 50 (see Figures 6A and 6B) and a second section 64 which, through a corresponding discharge opening 66, ends into a discharge chamber 68 defined by a central, for example circular, opening which is created in said main body 42 and in which the rotary discharge element 84 (screw feeder 84) is inserted. The first section 62 preferably comprises a rectilinear section, while the second section 64 is preferably curved so as to reach the discharge opening 66.

[0074] The channels 60A, 60B, 60C, 70A, 70B, 70C, therefore, preferably have a non-radial configuration.

[0075] More specifically, said second curved section 64 of each channel is oriented in such a way that it is essentially tangential or parallel to a non-radial direction tangential to said discharge chamber 68 at said discharge opening 66, as shown, for example, in Figure 6C.Said configuration promotes the flow of impurities towards the discharge opening 66 with the favourable rotation direction of the cleaning device 40, indicated by R in Figures 6 A and 6B.

[0076] The discharge openings 66 of all channels 60A, 60B, 60C, 70A, 70B, 70C, meaning six openings 66 in the preferred embodiment illustrated, are arranged around the discharge chamber 68 at equal distances and all lie on the middle plane M of the main body 42, ensuring that material with impurities is introduced in the evacuation device 80 in completely equivalent positions.

[0077] In this way, advantageously, it is possible to maintain equivalent and balanced flow rates through the channels 60A, 60B, 60C, 70A, 70B, 70C and to have corresponding equivalent and balanced pressures downstream of the two filters 32, 34.

[0078] Furthermore, preferably, the channels 60A, 60B, 60C, 70A, 70B, 70C have an increasing cross section in the direction towards the evacuation device 80, in order to promote the flow of the material towards the discharge openings 66.

[0079] According to the present invention, therefore, it is possible to carry out the cleaning device 40 in a symmetrical configuration, which makes it possible to maintain the flow rates of the removed material coming from both filters 32, 34 equivalent and balanced, using a single evacuation device 80 or, in other cases, 180.

[0080] According to the illustrated embodiment, the rotary main body 42 is made up of three modular parts 42A, 42B and 42C for ease of construction, disassembly and cleaning, as shown in Figures 4A and 5A: an external part42C and two central parts 42A, 42B arranged symmetrically with respect to the middle plane M.

[0081] The external part 42C is provided with a central hole 421C in which said two central parts 42A and 42B are partially housed and located opposite each other, each of them being also provided with a central hole 421A and 421B, wherein said holes 421A and 421B define said discharge chamber 68. Said channels 60A, 60B, 60C, 70A, 70B, 70C are partially defined in said external part 42C, while their terminal part, that is, their part close to the discharge opening 66, is defined by said central parts 42A and 42B coupled together.

[0082] Said terminal parts and said discharge openings 66 of the channels, on the other hand, are preferably created in corresponding positions on the faces 422A and 422B of said central parts 42A and 42B intended to be facing each other, so that said terminal parts and said discharge openings 66 are completely defined by mounting said central parts 42A and 42B on said external part 42C.

[0083] In the preferred solution, said discharge chamber 68, which is defined between the main body 42 of the cleaning device 40 and the evacuation device 80, has an at least partially toroidal shape, being defined by an external annular wall 681 with a concave, semicircular or in any case arched shape, as shown in Figure 5.

[0084] In an example of embodiment, the combination of the two central parts 42A and 42B defines the concave external annular wall 681 of the discharge chamber 68. Specifically, therefore, the perimeter edges 423 A, 423B of said openings 421 A and 42 IB of the central parts 42 A and 42B are arched inshape.

[0085] Consequently, the channels 60A, 60B, 60C, 70A, 70B, 70C are shorter than those present in the known cleaning devices, because the position of the discharge openings 66 is set back inside the toroidal chamber 68. Advantageously, this makes it easier and quicker for impurities to flow out, reducing the risk of blockages.

[0086] The solution, therefore, improves the cleanliness of the filters 32 and 34 as well as the quality of filtration, both when using an evacuation system with a screw feeder 84 and when using an evacuation system with an on / off valve 190.

[0087] Furthermore, according to the illustrated embodiment, the rotary main body 42 and the rotary discharge element 84 are rotated independently through the respective rotation means 27, 28. Advantageously, the respective rotation directions and rotation speeds can be adjusted independently of each other and in such a way as to ensure that impurities are removed with the greatest possible efficiency.

[0088] Preferably, during operation the rotary main body 42 and the rotary discharge element 84 rotate in the same direction. Furthermore, preferably, the rotation speed of the rotary discharge element 84 is higher than the rotation speed of the rotary main body 42 by such a value as to generate a tangential component of the speed of the material flowing out of the channels 66 that is concordant with the tangential speed of the rotary discharge element 84. In this way, the material flowing out of the channels 66 comes into contact with the rotary discharge element 84 with good fluid dynamic efficiency.According to the invention, the cleaning device 40 is lighter than the known cleaning devices, as it comprises one or more thinner parts or openings 47 or discharge slots 471 created on said main body 42, for example on said external part 42C, as shown in Figure 7. The lightening openings 47 and slots 471 are, for example, made on the external part 42C of the main body 42, in the spaces free from the scraping elements 48, 50 and the channels. In the solution shown in Figures from 4 to 7, the size of said discharge chamber 68 is such that it is completely covered by the central parts 42A and 42B. As shown, for example, in Figure 7, when said central parts 42A, 42B are correctly assembled with the external part 42C, the discharge chamber 68 can be accessed only through the channels 60A, 60B, 60C, 70A, 70B, 70C. Therefore, the impurities that accumulate in the channels 60A, 60B, 60C, 70A, 70B, 70C can flow towards the discharge chamber 68, essentially parallel to the surface of the main body 42.

[0089] In the solution shown in Figures 10 and 11, instead, the discharge chamber 68, shown in Figure 11 with a dotted line, is larger, to the point that when said central parts 42A, 42B are correctly assembled with the external part 42C, the discharge chamber 68 can be accessed even crosswise with respect to the surface of the main body 42 through openings 682 visible in Figure 10.

[0090] The impurities that accumulate in the channels 60A, 60B, 60C, 70A, 70B, 70C flow towards the discharge chamber 68 running parallel to the surface of the main body 42, and furthermore they can flow towards the discharge chamber 68 even through said openings 682.

[0091] As shown in Figure 11, which schematically illustrates the path of theimpurities, said path and, therefore, the flow of material are maximized, reducing the risk of blockages and the number of cleaning / maintenance operations required.

[0092] Another advantageous aspect lies in that the inlet way 22 is shaped so that it has an inlet portion 22A (Figure 8) that is inclined and tangent to the inner diameter of the filtration chamber 12 so that the fluid is rotated in a direction concordant to the rotation direction R of the cleaning device 40.

[0093] Figure 9 shows a variant embodiment of the filtering device 110 according to the invention. The components corresponding or equivalent to those present in the first embodiment are identified by the same reference numbers.

[0094] As already mentioned, this embodiment differs from the embodiment described above in that the evacuation device 180 preferably comprises an evacuation channel 85 and an on / off valve 190 suited to open to allow the impurities collected in the filtration chamber 12 to move towards the evacuation outlet 82 under the action of pressure. The evacuation device 180, therefore, is not provided with the rotary discharge element (screw feeder). Even the corresponding rotation means are therefore absent.

[0095] According to said embodiment, therefore, the impurities collected by the first and the second scraping elements 48, 50 are conveyed centrally towards the evacuation device 180 through the respective channels 60A, 60B, 60C, 70A, 70B, 70C, and there, under the action of pressure, the on / off valve 190 opens to allow the impurities to move along the evacuation channel 85 towards the evacuation outlet 82.

[0096] Therefore, with reference to the above description and the attacheddrawings, the following claims are made.

Claims

CLAIMS1. Cleaning device (40) for filters (32, 34) of filtering devices (10, 110) for a plastic material in the molten state, comprising:- a rotary main body (42) with a first side (44) facing towards a first filter (32) and provided with first scraping elements (48) cooperating with a first filtering surface (32A) of said first filter (32) and a second side (46) facing towards a second filter (34) and provided with second scraping elements (50) cooperating with a second filtering surface (34A) of said second filter (34),one or more first axially symmetrical channels (60 A, 60B, 60C) obtained in said main body (42), communicating with said first side (44) and suited to receive the impurities collected by said first scraping elements (48) and convey them towards an evacuation device (80; 180),one or more second axially symmetrical channels (70A, 70B, 70C) obtained in said main body (42), communicating with said second side (46) and suited to receive the impurities collected by said second scraping elements (50) and convey them towards said evacuation device (80; 180),said first channels (60A, 60B, 60C) and said second channels (70A, 70B, 70C) being defined in said main body (42) staggered with respect to one another and arranged at the same distance from the middle plane (M) of said main body (42) in such a way that said impurities pass through said first channels (60A, 60B, 60C) and said second channels (70A, 70B, 70C) along equal paths with the same flow rate and discharge said impurities in abalanced manner on said evacuation device (80; 180) through discharge openings (66) arranged on said middle plane (M), guaranteeing the introduction of impurities in said evacuation device (80; 180) in completely equivalent positions, characterized in that one or more of said first channels and second channels (60A, 60B, 60C, 70A, 70B, 70C) converge so that they are essentially tangential or essentially parallel to a non-radial tangential direction towards a central opening of said main body (42) that defines a discharge chamber (68) on one side of which there are said discharge openings (66) arranged on said middle plane (M), and wherein said discharge chamber (68) is delimited by an external concave annular wall (681) on which said discharge openings (66) of said channels (60A, 60B, 60C, 70A, 70B, 70C) are created.

2. Cleaning device (40) according to the preceding claim, characterized in that it comprises one or more thinner parts or one or more lightening openings (47) or discharge slots (471) created on said main body (42).

3. Cleaning device (40) according to the preceding claim, characterized in that the cross section of one or more of said first channels (60A, 60B, 60C) and / or of said second channels (70A, 70B, 70C) increases in the direction towards said discharge chamber (68).

4. Cleaning device (40) according to the preceding claim, characterized in that said main body (42) is made up of three modular parts (42A, 42B, 42C), wherein a first external part (42C) is provided with a central hole (421C) in which two central parts (42 A, 42B) are partially housed and located opposite each other, said central parts (42A, 42B) being arranged symmetrically with respect to said middle plane (M) and each of them beingalso provided with a central hole (421A, 421B), said central holes (421A, 42 IB) defining said discharge chamber (68).

5. Cleaning device (40) according to the preceding claim, characterized in that one or more of said first channels and second channels (60A, 60B, 60C, 70A, 70B, 70C) are created in part on said external part (42C) and in part on said central parts (42A, 42B), in such a way that the assembly of said two central parts (42A, 42B) defines at least said discharge openings (66) of the channels.

6. Cleaning device (40) according to claim 4, characterized in that the size of said discharge chamber (68) is such that it is completely covered by said central parts (42A, 42B), so that the impurities that accumulate in the channels (60A, 60B, 60C, 70A, 70B, 70C) can enter the discharge chamber (68) only by flowing essentially parallel to the surface of the main body (42).

7. Cleaning device (40) according to claim 4, characterized in that the size of said discharge chamber (68) is such that it is partially covered by said central parts (42A, 42B), defining access openings (682) through which the impurities can enter said discharge chamber (68).

8. Filtering device (10; 110) for a plastic material in the molten state, said filtering device (10; 110) comprising:• a hollow body (14) provided with a mouth (16) and a cover (20) suited to be associated with said mouth (16) to define a filtration chamber (12);• at least one inlet way (22) for the introduction of said plastic material in the molten state in said filtration chamber (12) and at least oneoutlet way (24) for the outlet of said filtered plastic material in the molten state from said filtration chamber (12);• a first filter (32) and a second filter (34) arranged opposite each other inside said filtration chamber (12) in a position between said at least one inlet way (22) and said at least one outlet way (24), said first filter (32) being suited to define a first surface (32 A) suited to filter said plastic material and collect any impurities and said second filter (34) being suited to define a second surface (34A) suited to filter said plastic material and collect any impurities;• said cleaning device (40) arranged between said first filter (32) and said second filter (34), according to one or more of the preceding claims;• an evacuation device (80; 180) suited to discharge said impurities collected by said cleaning device (40) towards an evacuation outlet (82), said evacuation device (80; 180) being arranged centrally with respect to said cleaning device (40).

9. Device (10; 110) according to claim 8, characterized in that it comprises first rotation means (27) suited to rotate said main body (42) of said cleaning device (40).

10. Device (10) according to claim 8 or 9, characterized in that said evacuation device (80) comprises an evacuation channel (85) that houses a rotary discharge element (84), preferably a worm screw, suited to receive said impurities collected in said filtration chamber (12) and move them along a longitudinal direction towards said evacuation outlet (82).

11. Device (10) according to claim 10, characterized in that it comprisessecond rotation means (28) suited to rotate said rotary discharge element (84).

12. Device (10) according to claim 10 or 11, characterized in that said main body (42) of said cleaning device (40) and said rotary discharge element (84) rotate in the same rotation direction, wherein the rotation speed of said rotary discharge element (84) is preferably higher than the rotation speed of said main body (42) by such a value as to generate a tangential component of the speed of the material flowing out of said discharge openings (66) that is concordant with the tangential speed of said rotary discharge element (84).

13. Device (10; 110) according to any of the claims from 8 to 12, characterized in that said first scraping elements (48) and / or said second scraping elements (50) comprise scraping blades, preferably flexible scraping blades, suited to be arranged in contact with said first filtering surface (32A) of said first filter (32) and / or with said second filtering surface (34A) of said second filter (34).

14. Device (10; 110) according to claim 13, characterized in that said flexible scraping blades (48) maintain said rotary main body (42) in an operating position centered between said first filter (32) and said second filter (34), said rotary main body (42) not being constrained axially.

15. Device (10; 110) according to claim 8, characterized in that said evacuation device (180) comprises an evacuation channel (85) and an on / off valve (190) suited to open to allow the impurities collected in said filtration chamber (12) towards said evacuation outlet (82) to move under the action of pressure.

16. Device (10; 110) according to any of the claims from 8 to 15, characterized in that in the hollow body (14) closed by the cover (20) all the scraping elements (48, 50) are subjected to the same degree of compression.

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