WO2008087534A2

WO2008087534A2 - Filtering assembly for a screw extruder for extruding plastic material

Info

Publication number: WO2008087534A2
Application number: PCT/IB2008/000101
Authority: WO
Inventors: Albino Rossi
Original assignee: Ferioli Filippo S.P.A.
Priority date: 2007-01-17
Filing date: 2008-01-17
Publication date: 2008-07-24
Also published as: ITTO20070025A1; WO2008087534A3

Abstract

In an extruder for extruding plastic material a filtering assembly (1) for filtering the plastic material in the viscous liquid state has a filtering circuit (2, 22a, 22b, 46, 45, 4) along which the plastic material flows in a first direction of circulation; a filter device (7) defining a first and a second path (21a, 21b, 21a', 21b' ) along which the plastic material flows,- and control elements movable between a first position in which the first path (21a, 21b) is connected to the filtering circuit (2, 22a, 22b, 46, 45, 4); and a second position in which the second path (2Ia', 21b' ) is connected to the filtering circuit (2, 22a, 22b, 46, 45, 4).

Description

FILTERING ASSEMBLY FOR A SCREW EXTRUDER FOR EXTRUDING PLASTIC MATERIAL

TECHNICAL FIELD The present invention relates to a filtering assembly for an extruder for extruding plastic material, in particular plastic material derived from a solid waste recovery plant. BACKGROUND ART An extruder for extruding plastic material generally comprises a screw conveyor that moves and fluidizes the plastic material; a die that shapes the cross-section of the semi-machined part obtained using the extruder; and a filter arranged upstream of the die and through which the plastic material flows to prevent any impurities in the flow of plastic material from blocking the passageways in the die and thus impeding the correct operation of the extruder. When a filter is clogged with impurities, it is removed from the filtering position and replaced with a clean filter. The clogged filter is washed ready to be placed in the filtering position again later. These operations are performed automatically by means of a device known in the prior art without interrupting the extrusion process. In order to prevent premature solidification of the material inside the die the plastic material must be kept in the viscous liquid state at a pressure that is higher than a predetermined value by means of the continuous rotation of the screw conveyor.

However, the screw conveyor must be stopped for a few moments while the filter is being replaced. The risk of the die becoming at least partially clogged therefore increases due to the reduction in the pressure upstream thereof . This drawback is of particular importance when using recycled plastic material, which has a high percentage of impurities and the filter has to be replaced more frequently.

DISCLOSURE OF INVENTION The object of the present invention is to produce a filtering assembly for an extruder for extruding plastic material that overcomes the drawbacks described above .

Said object is achieved with a filtering assembly for an extruder for extruding plastic material according to claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the present invention, a non- limiting preferred embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which:

- figure 1 is a schematic cross-sectional view, with some parts removed for clarity, of a filtering assembly according to the present invention in which the single components are shown in cross-sections along respective broken planes as indicated by the letters I-I in the following drawings; figure Ia is a cross-sectional view on an enlarged scale of a detail in figure 1; - figures 2a to 6a are respective front views of single components according to the arrow F;

- figure 6b is a cross-section along the line VIb-VIb in figure 6a,- figure 7a is a front elevation view of a component of the filtering assembly according to a direction opposite to the direction F; figure 7b shows the positions of a valve illustrated in figure 7a; and figures 8a and 8b are respective schematic front elevation views of the filtering assembly in two working positions according to the direction F. BEST MODE FOR CARRYING OUT THE INVENTION With reference to figure 1, number 1 indicates, as a whole, a filtering assembly comprising an inlet flange 2 connected to a screw conveyor 3 and an outlet flange 4 connected to a second screw conveyor 5 which feeds plastic material (essentially polymers) in the viscous liquid state to a die (not illustrated) . In particular, the conveyors 3, 5 are arranged in succession and comprise respective outer casings 3a, 5a which are tubular in shape and aligned along an axis A and two respective shafts 3b and 5b rotatable about the axis A and provided with respective screws 3c and 5c. The rotating shaft 5b has a diameter that increases towards the die, while the outside diameter of the propeller 5c is constant. The filtering assembly 1 is mounted overhanging on the casings 3a, 5a by means of the flanges 2, 4. The filtering assembly 1 comprises a main body 6; a filter device 7 housed in the main body 6; and two distributing means 8 and 9 connected to the main body 6 to selectively distribute the plastic material respectively supplied to the filter device 7 and coming from the filter device 7.

In particular, the main body 6 supports the flanges 2, 4 and defines an inlet duct 10 to connect the flange 2 to the distributing means 8 and an outlet duct 11 to connect the flange 4 to the distributing means 9. The filter device 7 comprises a hollow shaft 12 supported by the distributing means 8, 9 so as to rotate about an axis B and a drum 13 defining a plurality of passageways P (figure Ia) dimensioned so as to trap the impurities in the plastic material that could hinder the extrusion process or affect the quality of said plastic material. With reference to figure Ia, the drum 13 comprises a plurality of annular disks 14a having a respective shaped peripheral edge with a first profile and a plurality of annular disks 14b having a respective shaped peripheral edge with a second profile. Figure 2a illustrates a disk 14a which has a first plurality of radial projections 15a and a second plurality of radial projections 15b having a smaller radial dimension than that of the radial projections 15a. Moreover, the radial projections 15a alternate with the radial projections 15b and the radial projections 15a and 15b are evenly distributed about the axis B and are separated by a recess 16. Each radial projection 15b is of a shape and dimensions that are substantially identical to the shape and dimensions of a projection 15a except for the fact that the radial projection 15b has no end portion. Figure 2b illustrates a disk 14b defining a plurality of radial projections 17, which are evenly- distributed about the axis B, are of the same dimensions and same shape as the radial projections 15a and define a plurality of recesses 18. Moreover, each of the disks 14a, 14b has a central hole 19 to house the shaft 12 and a notch 20 suitable to define a rotationally rigid connection with the shaft 12. The recesses 16 and 18 are angularly equally spaced and delimited by respective bow-shaped profiles with a circumference having a same diameter.

With reference to figure Ia, the disks 14a and 14b are packed on the shaft 12 so that each radial projection is perfectly superimposed on a radial projection 17 and each radial projection 15b is superimposed on a radial projection 17 leaving one end of the radial projection 17 free. In this way, the passageways P are delimited by the opposite faces of two consecutive radial projections 17 and by the end face of a radial projection 15b comprised between two consecutive radial projections 17 and by the main body 6. The disks 14a, 14b are alternately mounted on the shaft 12 so that the apertures 16 and 18 are coaxial along respective axes parallel to the axis B.

With reference to figure 8a, the disks 14a, 14b define sixteen straight and angularly equally spaced channels 21. The drum 13 is housed inside a specific cylindrical seat defined by the main body 6 and surrounded by a plurality of ducts 22 defining respective longitudinal apertures Q opposite the drum 13. In particular, the projections 15b define the passageways P when the drum 13 is mounted in the cylindrical seat and the projections 15a and 17 approximate by defect to the radial dimension of the cylindrical seat. Moreover, the ducts 22 are angularly equally spaced, they are provided in a number corresponding to the channels 21, they are parallel to the axis B and are evenly distributed about the axis B. The ducts 22 are identical to one another and closed in the axial direction by means of the distributing means 8, 9 as described more fully later on in this document .

With reference to figure 1, the distributing means 8 guide the flow of plastic material from the duct 10 towards the ducts 22 and the channels 21. In particular, the distributing means 8 comprise a closure plate 23 defining an axial end of the filtering assembly 1 and an intermediate plate 24 arranged between the closure plate 23 and the main body 6. With reference to figure 3, the closure plate 23 defines within the thickness thereof a shaped recess 25 comprising an annular portion 26 having a mean circumference C identical to the circumference defined by the axes of the ducts 22 and an elongated portion 27 in communication with the annular portion 26. The annular portion 26 is coaxial to the axis B and surrounds a through hole 28 suitable to house the shaft 12 and having a diameter that is smaller than the outside diameter of the disks 14a, 14b. With reference to figure 4, the intermediate plate 24 has a central hole 29 that is identical to the hole 28, a hole 30 through which, when in use, the plastic material from the duct 10 flows and a series of four through holes 31 arranged along a circumference D identical to the circumference C. Moreover, the intermediate plate 24 has four blind holes 32 having respective centres arranged along the circumference D and connected two by two by means of a pair of grooves 33 preferably parallel to one another. The pairs of blind holes 32 thus connected are further connected to one another by means of respective grooves 34 slantingly arranged with respect to the grooves 33 and converging towards a common radial aperture 35 along the outer face of the plate 24. The blind holes 32 and the grooves 33 and 35 are obtained in such a way as to prevent any communication with the holes 31.

With reference to figure 1, the distributing means 9 comprise in sequence starting from the main body 6 a distribution plate 36, a manifold element 37 and a closing element 38 axially opposed to the closure plate 23.

With reference to figure 5, the distribution plate 36 has a central hole 37 that is identical to the holes 28 and 29 and a hole 38 through which the molten plastic material flows towards the duct 11. Moreover, the distribution plate 36 defines four holes 39 and four holes 40 alternately arranged along a circumference E that is identical to the circumference D. In particular, each hole 39 and 40 is arranged so as to be angularly inserted between a hole 31 and a blind hole 32 when the intermediate plate 24 is mounted on the main body 6. Each hole 39 is angularly arranged between two holes 40 and connected to a single one of the latter by means of a duct 41 so that each hole 39 is connected to a single hole 40. Moreover, each hole 40 is connected to the central hole 37 by means of a radial duct 42. The radial ducts 42 also communicate with one another via an annular groove 43 radially open towards the centre of the hole 37 .

Conveniently, the entrance to each hole 40 is controllable by means of a valve V preferably of the poppet type and which works as described later on in this document.

The manifold element 37 (figure 6a) is mounted in series to the distribution plate 36 and defines an outlet 44 communicating with the hole 38, a blind hole 45 coaxial to the axis B and a group of four ducts 46 configured so as to conduct the molten plastic material from the holes 39 to the blind hole 45.

Moreover, the outlet 44 receives the flow from two ducts 47, 48 arranged parallel to one another. The ducts 47, 48 are partially defined by an extractable drawer C for housing respective additional filters F arranged downstream of the filter device 7.

Moreover, the manifold element 37 defines a group of four seats for the valves V.

Figure 7a illustrates the closure plate 38 that defines an upturned T-shaped groove 49 for conveying the flow of molten plastic material from the blind hole 45 towards the ducts 47, 48. Moreover, the closure plate 38 comprises a valve 50 to control the flow entering the ducts 47, 48.

Figure 8a shows the ducts 22 which are divided into four functional groups each consisting of four channels and the channels in each group are connected in a fluidly equivalent manner to the intermediate plate 24 and to the distribution plate 36. In the first group, the channels are open towards the respective holes 31 at one end, they are closed by the distribution plate 36 at the other end and are indicated by number 22a; in the second group the channels are open towards the respective holes 39, they are closed by the intermediate plate 24 and are indicated by number 22b; in the third group the channels are closed by the distribution plate 36, they are open towards the respective blind holes 32 and indicated by number 22c ; and in the fourth group the channels are shut-off by the respective valves V, they are closed by the intermediate plate 24 and indicated by number 22d. For the sake of clarity in figure 8a the reference number of the hole or valve V to which the respective channel is coaxial is also shown in brackets .

The filtering assembly 1 according to the present invention works as follows. The filtering assembly 1 has a plurality of filtering circuits and a plurality of flushing circuits to regenerate the channels 21a, 21b of the filter device 7. There is a flushing circuit for each filtering circuit and such circuits have a fixed part comprising the ducts defined by the main body 6 and by the distributing means 8, 9 and a movable part comprising the channels 21a, 21b of the filter device 7.

Each filtering circuit is linked to a respective flushing circuit so as to define four functional units only one of which is described below for the sake of simplicity.

The plastic material flows through the filtering circuit in a predefined direction of circulation along the annular portion 26 of the closure plate 23 and is distributed to enter the hole 31 through the duct 10, the hole 30 and the elongated portion 27. The plastic material reaches the duct 22a and enters the channel 21a through the longitudinal aperture Q as the duct 22a is axially closed by the distribution plate 36. The plastic material flows through the passageways P, reaches the channel 21b immediately adjacent to the channel 21a and flows back along the duct 22b through a second longitudinal aperture Q. In this way, any impurities of dimensions exceeding those of the passageways P are trapped by the projections 15b and accumulate in the channel 21a (figure 8a) .

The duct 22b is axially closed by the intermediate plate 24, and the plastic material therefore flows towards the distributing means 9 and through the holes 39, the ducts 46 and the blind hole

45.

The flow is thus directed towards the outlet flange 4 through the ducts 47, 48 and the additional filters .

The method of operation described above refers to a condition in which the valve V shuts off both the groove 40 and the radial duct 42 and no molten plastic material flows through the flushing circuit.

In order to clean the channel 21a clogged by the impurities (specifically indicated by the hatched area in figures 8a, 8b) the valve V must first be moved back to an intermediate position in which the duct 22d is closed so that the radial duct 42 is placed in communication with the duct 41. In this way, a portion of the flow passing through the hole

39 is directed towards the internal volume of the hollow shaft 12 through the radial duct 45 and a specific radial aperture 51 in said hollow shaft 12. The plastic material accumulates inside the hollow shaft until a piston 52 reaches a predefined backward position so as to accumulate the amount of plastic material necessary for regenerating the filter device 7.

The hollow shaft 12 is then turned by 45° so that the channels 21a, 21b are positioned so as to communicate respectively with a duct 22c and a duct 22d (figure 8b) . The valve V is then switched to a position such that the groove 41 is closed and a flow of molten plastic material passes through the flushing circuit. In particular, the internal volume of the shaft 12 is arranged so as to fluidly communicate with the duct 22d via the annular groove 43, the radial duct 42 and the ducts 22d and 22c. The plastic material flows along the channels 21a, 21b and through the passageways P in a counter- current direction with respect to the direction of circulation in said channels 21a, 21b when the latter are connected to the filtering circuit.

The flow of the flushing circuit carries the impurities towards the outlet 35 and allows the clogged channel 21a to be completely regenerated and cleaned. Simultaneously with the flushing process described above, an adjacent pair of channels 21a, 21b indicated in figures 8a, 8b by the respective reference numbers 21a' , 21b' , filter the flow of plastic material coming from the hole 31 and directed towards the outlet flange 4.

Moreover, the hollow shaft 12 can be filled before the channel 21a is clogged and during the normal operation of the filtering circuit. Subsequent turns of the drum 13 in order to clean the channel 21a can be performed in both directions of rotation.

Moreover, downstream of the blind hole 35, the valve 50 allows the flow to be distributed in the ducts 47, 48 via the additional filters F.

When one of these filters needs replacing, the valve 50 is suitably switched to by-pass the relative duct 47, 48 so that the used filter can be removed and a new filter assembled. With the filtering assembly 1 according to the present invention the following advantages can be achieved.

The control devices defined by the filter device

7 and by the valve V enable rapid switching without having to deactivate the screw conveyors 3, 5. Moreover, after each switchover, the filtering circuit is connected to channels 21a, 21b that have already been regenerated and are ready for efficient filtering. The filtering assembly 1 thus allows filtering to be performed continuously without having to deactivate the screw conveyors 4 , 5. The pressure is maintained at the optimum level and the die does not become clogged. Moreover, with the filtering assembly according to the present invention the filter device 7 can be used for a longer period.

The use of a counter-current flow to regenerate the filter device 7 makes it possible to make the filtering assembly 1 work automatically.

By means of the duct 41, the filtering circuit and the flushing circuit use the same fluid, making the filtering assembly 1 more compact.

Lastly, it is clear that modifications and variations may be made to the filtering assembly 1 described and illustrated herein without departing from the scope of the present invention, as set forth in the appended claims .

The function of the filtering assembly 1 could be performed by a fixed filter device 7 and a system of valves and ducts configured so as to direct the fluid flow in a first direction along the filtering circuit and in a counter-current direction with respect to the direction of circulation in the filtering circuit to clean the channels 21a in the flushing circuit. Moreover, the filtering assembly 1 could comprise the flushing circuit without the piston 52. In that case, the filtering assembly must be connected to an external pressure source to make the flow circulate in a counter-current direction in the flushing circuit.

Claims

1. Filtering assembly (1) for an extruder for extruding plastic material, comprising a filtering circuit (2, 22a, 22b, 46, 45, 4) to filter the plastic material in the viscous liquid state,- a filter device (7) defining at least a first and a second path (21a, 21b, 21a' , 21b' ) for adducting said flow; and control elements (7, V) movable between a first position to connect the first path (21a, 21b) to the filtering circuit (2, 22a, 22b, 46, 45, 4), and a second position to connect the second path (2Ia', 21b') to the filtering circuit (2, 4).

2. Filtering assembly according to claim 1, characterized in that it comprises a flushing circuit

(43, 45, 22c, 22d, 35) and in that the first path (21a, 21b) is connected to the flushing circuit (43, 45, 22c, 22d, 35) when the control elements (7, V) are in the second position.

3. Filtering assembly according to claim 2 characterized in that it comprises a distribution duct (41) to connect the filtering circuit (2, 22a, 22b, 46, 45, 4) to the flushing circuit (42, 35) .

4. Filtering assembly according to claim 3, characterized in that the control elements comprise the filter device (7) .

5. Filtering assembly according to claim 4, characterized in that the control elements comprise a valve (V) to selectively open and close the distribution duct (41) .

6. Filtering assembly according to claim 3 or 4 , characterized in that the filter device (7) is rotatable about an axis (B) .

7. Filtering assembly according to claim 6, characterized in that the filter device (7) comprises a plurality of shaped disks (14a, 14b) to define the first and the second path (21a, 21b, 21a', 21b').

8. Filtering assembly according to any of the previous claims, characterized in that it comprises a pressure source (52) to supply the flushing circuit (42, 35).

9. Filtering assembly according to claim 8, characterized in that said pressure source (52) is provided on the filter device (7) .

10. Filtering assembly according to claims 7 and 9, characterized in that the filter device comprises a shaft (12) to support the plurality of disks (14a, 14b) and that the pressure source comprises a piston (52) that is movable inside the shaft (12) .

11. Filtering assembly according to any of the previous claims, characterized in that the filtering circuit (2, 22a, 22b, 46, 45, 4) comprises a first and a second duct (47, 48) arranged parallel to one another downstream of the filter device (7) along the first direction to house respective second filters (F) and a second valve (50) to selectively close one of either the first or second duct (47, 48) .

12. Extruding machine comprising at least a conveyor (3, 5) to feed plastic material in the viscous liquid state; the conveyor (3, 5) comprising a duct (3a, 5a) and a filtering assembly (1) to filter the plastic material; the machine being characterized in that the filtering assembly (1) is arranged outside the conveyor (3, 5) .

13. Machine according to claim 12, characterized in that the filtering assembly (1) is produced according to any of the claims from 1 to 11.