Such a screen unit is known from, for example, Offenlegungsschrift DE-2,407,663. In the case of this known screen unit the screen element is cleaned by placing a part of the screen element in front of the backflush channels.

The backflush channels are placed in communication with the environment by opening the shut-off element. The pressure in the backs lush channels is then equal to the atmospheric pressure, with the result that part of the viscous mass present in the interior of the screen element is flushed back through the screen part situated in front of the backflush channels. The screen element of the known screen unit consists of a screen carrier, a screen and a screen plate, parallel rows of drill holes being made in the screen carrier and screen plate. Each row of drill holes is placed in succession in front of the backflush channels, with the result that viscous mass can flush back through the drill holes concerned, so that the impurities present in the drill holes are removed.

The disadvantage of the known screen unit is that in each case only a small part of the screen element is cleaned by backflushing. If the viscous mass is greatly

polluted, the apertures of the screen element will become blocked more quickly than the cleaning of the screen element occurs. The pressure in the supply channel of the screen unit will therefore become increasingly high and the capacity of the screen unit will decrease.

The object of the invention is to provide a screen unit of the type mentioned in the preamble which does not have this disadvantage.

This object is achieved with a screen unit according to the in the introductory part mentioned type, which is characterized in that the screen element is provided in a predetermined fixed position in the housing and that the means for cleaning the screen element further comprise a pressure element which can be operated to flush the viscous mass back through the screen element at a pres- sure which is higher than the pressure in the supply channel, and which interacts with the shut-off element in order to discharge the solid particles screened off by the screen element.

In this way it is possible to clean the screen element in its entirety at any desired time. If, for example, the pressure in the supply channel rises too high, the pressure element can be operated to clean the screen element.

Preferred embodiments of the screen unit according to the invention are described in the subclaims.

The invention will be explained in greater detail in the description which follows of an exemplary embodiment of the screen unit according to the invention, with refer- ence to the drawing, in which: Fig. 1 is a diagrammatic illustration in section of a screen unit according to the invention; Fig. 2 is a view on an enlarged scale of an axial side of one of the rings from which the screen casing of the device of Fig. 1 is composed; Fig. 3 is a perspective view of a part of the ring of Fig. 2; Fig. 4 is a view in the radial direction of a part of the screen casing composed of rings according to Fig. 2;

Fig. 5 is a diagrammatic illustration in section of another embodiment of the screen unit according to the invention; Fig. 6 is a perspective view of a component for forming a flat screen element; Fig. 7 is a perspective view of further embodiment of a component for forming a flat screen element; Fig. 8 is a diagrammatic illustration of a stack of conical rings.

Fig. 1 shows diagrammatically a screen unit for removing solid particles from a viscous mass, for example a molten plastic. The screen unit comprises a housing 1. A supply channel 2 for viscous mass polluted with solid particles is provided in the housing 1, which supply channel 2 opens out into a central space 3 situated in the housing by way of the radial peripheral wall of the space 3.

In the embodiment illustrated in Fig. 1 a connecting channel 4 is provided in the housing 1 in such a way that it is coaxial with the space 3. In addition, a discharge channel 5 for cleaned viscous mass is provided in the housing 1, the inflow aperture of which discharge channel is provided in the radial peripheral wall of the connecting channel 4. The supply channel 2 is in communication with the discharge channel 5 by way of the space 3 and the connecting channel 4.

A device (not shown) for producing viscous mass can be connected to the supply channel 2, such as, for example the extruder for producing molten plastic. The discharge channel 5 can be connected to a device (not shown) for manufacturing plastic products, such as, for example, recycled granulated material and/or films.

A screen element is placed in the space 3 of the housing 1. The screen element is composed of a positioning aid 7 and a screen casing 8. The positioning aid 7 is formed by a hollow cylinder which is shut off at one axial end and at the other axial end is provided with a radially outward projecting flange 6. The external diameter of the flange 6 corresponds to the diameter of a circular opening

20 in the axial end of the space 3 facing away from the axial side of the housing 1. Slits 9 are cut out at a distance from teach other in the positioning aid 7, which slits extend in the axial direction from the flange 6 to the shut-off axial end of the positioning aid 7. The slits 9 have dimensions which ensure that an optimum relationship is obtained between the strength and the throughput capacity of the positioning aid 7. The slits 9 extending in the axial direction can also be replaced for example by radial drill holes in the wall of the positioning aid 7, which drill holes are distributed in a regular pattern over the wall.

A screen casing 8 is fitted on the radial outer periphery of the positioning aid7. The screen casing 8 has an internal diameter which corresponds to the external diameter of the positioning aid 7. The external diameter of the screen casing 8 corresponds to the diameter of the opening 20. The external periphery of the screen casing 8 is situated at a distance from the peripheral wall of the space 3, so that the screen casing 8 can be circumfused fully by viscous mass coming from the supply channel 2. The positioning aid 7 and the screen casing 8 are fixed in such a way that they are immovable in the axial and radial direction in the space 3 by a shut-off means 10. The positioning aid interacts with the shut-off means 10 to attend to the positioning of the screen casing 8 in the space 3.

The screen casing 8 is formed by a substantially hollow cylinder with radial screen channels which extend from an inflow face of the screen casing 8 to an outflow face of the screen casing. The screen channels diverge from the inflow face towards the outflow face. In the illustrated exemplary embodiment of the screen unit according to the invention the screen casing 8 is constructed of coaxially stacked rings 22, ribs 23, 24 being provided on each ring 22 for forming the radial screen channels (see Fig. 2).

Backflush channels 11 are provided in the shut-off means 10, which backflush channels open out into the space

3 at one side. At the other side the backflush channels 11 open out into a central opening 12 in the shut-off means 10. The central opening 12 is in communication with an outflow channel 16 situated in a shut-off element 13 and can be shut off by the shut-off element 13, in which a shut-off ball 14 is situated. The shut-off ball 14 can be moved in the direction of the central opening 12 by a control element 15. The outflow channel 16 is in communi- cation with the environment.

Instead of the shut-off ball 14 and the control element 15, a plunger which shuts off the outflow channel 16 and the backflush channels 11 can also be used or another suitable shut-off element.

Coaxially with the screen element, a connecting channel 4 is provided in the housing 1, the diameter of which connecting channel corresponds to the internal diameter of the positioning aid 7. The connecting channel 4 extends from the space 3 to the other axial end of the housing 1. The discharge channel 5 extends from the connecting channel 4 to the outer periphery of the housing 1.

In the exemplary embodiment shown a drive cylinder 18 is fixed coaxially with the connecting channel 4 on the housing 1. A backs lush plunger 17 fitted in the drive cylinder 18 projects into the connecting channel 4 and can be moved by the drive cylinder 18 until it is inside the screen element. The drive cylinder 18 is provided with a stop 21. Interacting with a stop 19 fitted on the backflush plunger 17, the stop 21 determines the maximum distance over which the plunger 17 can move through the connecting channel 4 and into the screen element.

Fig. 2 shows an axial view of a ring 22 from which the screen casing 7 is constructed. On one axial side of the ring 22, radial ribs 23 are provided at regular distances from each other. The other axial side of the ring 22 is flat. The ribs 23 are of a substantially triangular shape in cross-section. The height and the width of the ribs 23 decrease from the outer periphery of the ring 22 towards the inside. A number of distance ribs 24

distributed over the ring 22 have the same height over the entire length. The distance ribs 24 ensure that when several rings 22 are placed coaxially on each other the rings 22 remain at a distance from each other over the entire width. The decreasing height and width of the ribs 23 has the result that the throughflow surface of the screen casing increases in its entirety in the direction of flow of the viscous mass during the screening of the viscous mass.

The height and shape of the ribs 23, 24 on the outer periphery of the ring 22 and their distance from each other determine the screen apertures in the screen casing 8. The screen apertures here determine the smallest throughf low apertures and are situated in the inflow face of the screen casing 8. The ribs 23, 24 can rest against each other on the outer periphery of the ring 22. The ribs 23, 24 can have a rectangular or a bow-shaped cross-section or be of any other shape by means of which screen channels substantially diverging from the outside towards the inside are formed. The ribs 23 and distance ribs 24 may be provided in a spiral shape on the rings if desired.

In the exemplary embodiment shown the screen channels diverge from the outer periphery of the screen casing in the direction of the inner periphery. Of course, it is possible to construct the device according to the invention in such a way that the mass to be screened flows from the inside to the outside through the cylindrical screen casing. In that case the screen channels are divergent from the inner periphery of the screen casing 8 in the direction of the outer periphery. In addition, the screen channels in the screen casing according to the invention diverge in such a way that the screen channels merge into each other at a distance from the screen apertures, with the result that screen channels situated adjacent to each other are connected to each other in a direction at right angles to the direction of throughflow.

The distance ribs 24 can be replaced by spacers, for example lugs, situated near the inner diameter of the ring 22. Such spacers can be distributed in a suitable

manner over the ring, in order to keep two adjacent rings at equal distance from each other.

Fig. 3 shows a perspective view of a part of the ring 22. In this examplary embodiment it can be seen here that the cross-section of the ribs 23, 24 is substantially triangular in shape and that the distance rib 24 has the same height over the entire length.

Fig. 4 shows in radial view a part of the screen casing 8 which is composed of several rings 22. It can be seen here that the screen apertures of the screen casing 8 are determined by apertures 25 between the ribs 23, 24 on the outer periphery of the ring 22 and the outer periphery of the flat axial side of the adjacent ring 22, inscribed circles with a diameter of 50 ijm, for example, being possible in the apertures 25. The orientation of the rings 22 is the same for all rings 22. The screen casing 8 is resistant to high pressures both from the inside and from the outside. The positioning aid 7 ensures that the rings 22 remain coaxial. For a screen casing constructed in this way it is not necessary to place a supporting element on the outer periphery, in order to support the screen casing when a pressure is being exerted from the inside for cleaning the screen casing by means of backflushing.

It is also possible to keep the rings 22 coaxial by providing at least two drill holes at a distance from each other in each ring. The rings can subsequently be pushed over positioning pins which are provided in a fixed position in the axial end of the space 3 of the screen unit and correspond to the number of drill holes. The shut-off element 10 can subsequently be placed over the free ends of the positioning pins, in order to hold the positioning pins in place.

It is also possible to provide recesses in one axial side of the rings and projections on the other axial side. When the rings are placed on each other in order to form the screen casing, the projections of one ring fall into the recesses of the adjacent ring. The projections and the recesses interact here to ensure a fixed position of the rings relative to each other.

The screen unit works as follows: A viscous mass, for example molten plastic, containing impurities is conveyed by way of the supply channel 2 to the space 3. The viscous mass is forced through the screen casing 8, impurities remaining behind on the outside of the screen casing. The impurities remaining behind are greater in size than the apertures 25 on the outer periphery of the screen casing 8. If an impurity is smaller than or equal to the aperture 25 on the outer periphery of the screen casing 8, this impurity will pass through the screen casing. Since the screen channels widen inwards from the apertures 25 on the outer periphery of the screen casing 8, an impurity will no longer be able to stick in the screen channels 25 once it has passed the aperture 25. The cleaned viscous mass flows by way of the slits 9 into the interior space of the positioning aid 7.

The viscous mass then flows in the axial direction to the connecting channel 4 and by way of the discharge channel 5 to a device (not shown) for processing the viscous mass.

It is also possible to guide the viscous mass flowing out of the screen unit through a further screen unit. The screen apertures of the second screen unit can then be smaller, so that impurities with a smaller diameter than the impurities screened off in the first screen unit are screened off by the second screen unit. In this way it is possible to place several screen units according to the invention in series, in order to obtain a viscous mass of a desired purity.

The impurities remaining behind on the outer periphery of the screen casing 8 cause blockage of the screen casing which causes the increase of the resistance of the screen casing 8 and subsequently the loss of pressure across the screen casing 8. Consequently the pressure in the supply channel 2 will have to rise, in order to be able to guarantee that the same volume flow is still produced through the discharge channel 5. When the pressure in the supply channel 2 reaches a critical value, it is necessary to clean the screen casing. However, if the pressure in the supply channel 2 is held constant, the

volume flow through the discharge channel 5 will decreaase.

This can eventually generate a control signal indicating that the screen casing has to be cleaned.

To clean the screen casing 8 the backflush plunger 17 is moved in the direction of the screen element. With that the discharge channel 5 is first of all shut off by the backflush plunger 17. The backflush plunger 17 is then pushed with such force and at such speed into the interior space of the screen casing that this produces a pressure inside the screen casing 8 which is higher than the pressure in the supply channel 2, and preferably two to four times the level of the pressure in the supply channel 2. At the same time, the shut-off element 13 is put into operation, in order to place the backflush channels 11 in communication with the outflow channel 16 by way of the central opening 12. The pressure in the space 3 will consequently be virtually equal to the atmospheric pressure.

Owing to the high pressure in the interior of the screen element relative to the pressure at the outside of the screen casing, the viscous mass present in the interior of the screen element flows back to the space 3 through the radial screen channels in the screen casing 8. During this process the impurities situated on the outer periphery in front of the apertures of the screen casing are dislodged over the entire outer periphery of the screen casing. Under the influence of the pressure generated by the backflush plunger 17 and the viscous mass flowing out of the supply channel 2, the flushed-back viscous mass present in the space 3 and having a high concentration of impurities flows out of the screen unit to a receptacle (not shown) by way of the backflush channels 11 and the outflow channel 16.

After the polluted viscous mass has flowed out of the screen unit, the central opening 12 is shut off by the shut-off element 14, with the result that the viscous mass present in the supply channel 2 again flows through the screen casing 8 and the apertures 9 into the interior space of the screen element. At the same time, the pressure in the drive cylinder 18 is brought to a value which is lower

than the pressure in the screen element. As a result of this, under the influence of the viscous mass flowing through the screen element, the plunger is forced back to its initial position, and the discharge channel 5 is opened again.

Since the backflush plunger 17 is forced back into the initial position by the viscous mass, no vapour bubbles will occur in the viscous mass. Were the plunger to be actively moved back to the initial position, a partial vacuum could occur in the transition region between the plunger and the viscous mass, thereby promoting the formation of vapour bubbles. Vapour bubbles in the viscous mass flowing out of the screen unit are undesirable, since they adversely affect the quality of the end product to be manufactured.

The whole cleaning process can be carried out in a very short time, one to two seconds. Between two successive cleaning actions of the backflush plunger 17, the plunger 17 can be used as a pressure-regulating means for keeping the pressure constant in the discharge channel 5.

It is possible to provide the housing with a second bore, in which a second plunger can be placed and which is in communication with the discharge channel 5.

Said bore can be filled with viscous mass during operation, so that during the cleaning action of the backflush plunger 17 it is possible to make this viscous mass flow into the discharge channel 5 with the aid of the second plunger, as a result of which the pressure in the discharge channel 5 can be kept constant during the cleaning process.

Another possibility is to place two screen units according to the invention in parallel, so that one of the backflush plungers can keep the pressure in the discharge channel constant, while the other backflush plunger cleans the corresponding screen element.

In the screen unit according to the invention the backflush plunger 17 and the shut-off element 13 are the only moving parts. This makes the manufacture and main- tenance of the screen unit particularly advantageous.

Moreover, no complex control is necessary to carry out the

cleaning procedure.

The capacity of the screen unit can be adapted as desired by altering the dimensions of the screen element.

In another embodiment (not shown) of a screen unit according to the invention, the capacity of the screen unit is increased by placing several screen elements in parallel, each screen element being situated in a corresponding space in the housing. For discharging the cleaned viscous mass each space is in communication with a common channel, into which the backflush plunger is inserted. When the backflush plunger is operated to clean the screen elements, all screen elements are cleaned simultaneously by means of backflushing.

In this embodiment the backflush plunger does not move into a screen element. The backflush plunger does shut off a common discharge channel, after which the pressure in the common channel, and consequently in all channels in communication therewith, rises. All corresponding screen elements are therefore cleaned simultaneously.

Of course, it is also possible to have each screen element of the embodiment described above cleaned by a corresponding backflush plunger. In such a screen unit the backflush plungers can be operated simultaneously or in succession for cleaning the screen element belonging to the backflush plunger concerned.

Fig. 5 shows a variant of a screen unit according to the invention. The unit comprises a housing 101, which is provided with a space 103. A supply channel 102 opens into the space 103. A communication channel 104 also opens into the space 103, the communication channel 104 being in line with the supply channel 102. The communication channel 104 extends from the space 103 to an outside of the housing 101. A drive cylinder 118 with a backflush plunger 117 is provided coaxially with the communication channel 104 on the housing 101. The backflush plunger 117 projects into the communication channel 104. A discharge channel 105, which extends from the communication channel 104 to an outside of the housing 101 and can be shut off by the backflush plunger 117, is also provided in the housing 101.

A substantially flat screen element 108, assembled by means of components consisting of flat plates 122 on which ribs 123 and 124 are provided, is provided in the space 103. The screen element 108 divides the space 103 into two parts. The screen element 108 is held in place by a shut-off means 110. A backflush channel 111, which can be shut off by means of a shut-off element 113, is provided in the shut-off means 110. For this purpose, the shut-off element 113 comprises a plunger 115 which can move a shut- off ball valve 114 in the direction of the backflush channel 111. When the shut-off element 113 is in the open position, the backflush channel 111 is in communication with an outflow channel 116.

If desired, several backflush channels can be provided at equal intervals on the outer periphery of the housing 101, which backs lush channels can be shut off by corresponding shut-off elements. In this way the viscous mass with a high concentration of impurities can be removed rapidly and efficiently during backflushing.

The screen element 108 comprises an inflow face on the side facing the supply channel 102 and an outflow face on the side facing the communication channel 104. The throughput channels extend from the inflow face in the direction of the outflow face, the flow cross-section of the throughput channels increasing constantly in the direction of the outflow face. The throughflow channels preferably meet each other at a distance from the outflow face and therefore merge into each other.

The total surface area of the throughflow apertures in the outflow face of the screen element 108, which is assembled from the components shown in Fig. 6, is greater than the total surface area of the throughflow apertures in the inflow face. This causes the flow resistance to decrease in the direction of the outflow face.

Fig. 7 shows a component for forming a screen element, in which two adjacent ribs determine a substan- tially semi-circular interspace. When assembled, the screen element concerned has a relatively large throughflow surface on the inflow side.

Conventional screen elements for screening viscous masses which are cleaned by backflushing are assembled from two supporting elements with a wire mesh screen or perforated screen wedged between them. The supporting elements prevent deformation of the wire mesh screen or perforated screen as a result of the pressure in the viscous mass both during the screening and during the backflushing.

In the device according to the invention the positioning aid has purely the function of positioning the screen casing, and has no significance for preventing deformation of the screen casing. The screen casing according to the invention is sufficiently strong to withstand the forces acting upon the screen casing during the screening and backflushing.

Fig. 6 shows a component for forming a screen element which consists of a flat plate with ribs provided thereon. When several of such plates are stacked on each other, a flat screen element which is strong enough per se and has good screening characteristics is formed.

Another advantage of the screen element according to the invention is that the 0-pressure, in other words, the pressure needed to overcome the flow resistance of the screen element, is lower than in the case of the conventional screen elements. This is a result of the flow cross-section of the throughflow channels increasing in the direction of flow. As soon as the flow resistance of the inflow face has been overcome, the resistance decreases rapidly.

The screen element is advantageously assembled from components which are made of resilient material and are formed in such a way that the components are under initial tension when in the assembled state. The components for forming the flat screen element can be, for example, bow- shaped or slightly undulating. The components are placed under initial tension by a tensioning means, in order to form the screen element. The state is retained during the screening operation of the unit in question. When the screen element is soiled to such a degree that the pressure

in the supply channel reaches a predetermined value or the flow rate has decreased, the cleaning procedure is put into operation. The tension is removed from the screen element here. The components will assume their tension-free position, so that the components move apart and the total throughflow surface area is increased. The enlarged throughflow surface area of the inflow face ensures that impurities which have become lodged in the throughflow apertures can be entrained easily by backflushing viscous mass.

For the formation of such a screen element, the components forming it are deformed alternately in opposite directions. At least two components differing from each other are necessary for forming such a screen element.

In the case of the components for forming a flat screen element a first component is bent in such a way that the ribs of the first component are situated on the outer peripheral face of the bent component. In the case of a second component the ribs are situated on the inner peripheral face of the bent component.

In the case of the cylindrical screen element the rings forming the screen element can be slightly conical in shape. When the screen element is being assembled, the rings are placed alternately on each other, so that two adjacent rings rest against each other either with the inner peripheral edges or with the outer peripheral edges, as shown diagrammatically in Fig. 8. The rings are tension- free here.

Here again, two types of rings are necessary: a first type of ring with ribs and spacers on the outside of the conical ring and a second type of ring with the ribs and spacers on the inside of the conical ring. The term outside means the side facing away from the axis, while the inside is the side facing the axis.

During the backflushing a pressure difference will occur between the inside and the outside of each conical ring. This pressure difference causes a resulting force on each conical ring. This force is directed in such a way that during the cleaning of the screen casing the outer

peripheral edges of the conical rings resting against each other are forced apart, so that impurities lodged there are easily dislodged and can be removed.

When the initial tension is applied, the rings are pressed into a substantially flat state, so that two adjacent rings are at equal axial distances from each other both on the inner periphery and on the outer periphery.

The ribs are provided on the rings in such a way that in the state under initial tension all rings have the same orientation.

The embodiments described above are given as non- limiting examples. It will be clear to a person skilled in the art that various changes and modifications to the exemplary embodiment are possible without deviating from the scope of the invention, as defined in the appended claims.

For instance, it is possible to provide the components for forming the screen element with ribs and spacers on both axial sides.