SCREEN UNIT FOR VISCOUS MASS

Abstract: During the processing of viscous masses, for example recycled molten plastic, it is important that impurities in the form of solid particles should be removed from the viscous mass. A screen unit with a substantially cylindrical hollow screen element (8) is used for this purpose. The viscous mass flows through the screen element in the radial direction. The screen element (8) can be cleaned by backflushing. The means for the backflushing comprise backflush channels (11) which can be placed in communication with an outflow channel (16) by way of a shut-off element (14) which can be opened. The means for backflushing further comprise a pressure element (17) which can be operated in order to flush back the viscous mass at a pressure which is higher than the pressure in the supply channel (2). The pressure element (17) interacts with the shut-off element (14) in order to discharge the solid particles screened off by the screen element.

Description: Short title: Screen unit for viscous mass The invention relates to a screen unit for removing solid particles from a viscous mass, in particular molten plastic, comprising a housing provided with a supply channel and a discharge channel for the viscous mass, a screen element which is suitably fitted in the housing between the supply channel and discharge channel and can be flowed through by the viscous mass, and means for cleaning the screen element by means of backflushing at least during operation, which means comprise backflush channels which are intended to discharge during the backflushing solid particles screened off by the screen element, which backflush channels can be placed in communication with an outflow channel by way of a shut -off element which can be opened. 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 backflush 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 pressure 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 - A - 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 -each 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 communication 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 backflush 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 ω ω to to F 1 H LΠ o LΠ o LΠ O LΠ ti F- a LQ t to • ω cz Ω tr CQ TS PJ Ω Φ ti CQ Ω PJ tl tr Φ Pi F- CQ rt ti F- tr ςz rt φ Pi F- ts P ) CQ cz F- rt 0 ) tr Φ 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 μm, 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 t to rt 0 Ti rt 0 0 ra Ω TS ct 0 HI TS 0 rt m TS Ω Ω Ω 0 t TS Ti rt rt rt ι-3 tr Ω tr tr TS V tr < ti J tr Ω ti 0 Φ 0 TS • 0 d TS d tr tr tr tr 0 Φ ti Φ cz Φ rt PJ PJ d Φ Φ 0 0 Φ Φ d d 3 φ d φ PJ ra φ PJ 1— ' rt Φ ra ct rt Ω ts tl d Ω F- rt φ ra ra rt 3 ti CQ F- Ω tr rt F- rr ra d tr Φ Φ tr 0 Φ tl LQ F- rt φ PJ tr ra φ CQ Φ ti d J rt m Φ Φ tr ra 3 ti Φ m ti 0 Ω Φ Φ Φ ti rt Φ Ω d d Λ PJ d rt rt d Φ PJ rt rt Ω φ rr r ti tr 0 ti ti d P- rt F- 0 Ω tr 3 tr PJ < CQ Ω Φ tr rt Φ ra CQ φ φ Φ Φ Ω 0 tr 3 Φ rt £ ?r Φ 0 Φ d Φ Ω PJ Φ > PJ tr PJ t F> cz rt Ω Ω φ O • d 0 PJ d Φ Φ tr F- < ti ts Φ tl Ω Φ tl - -J Φ Φ rt tr d Φ s: rt TS rt - CQ F- φ rt rt Φ 0 ra Φ 0 Ω φ rt 77 d d tr φ Φ φ 3 F- 1 Φ d tr tr d Ω :> ^ φ t rt Φ cz Φ rt J Ω rt φ Φ φ Ω φ d X d 0 J rt ra φ ra Ω ti Φ ti LQ tr 3 ti H Ω tr tr ts Φ d d PJ d LQ d d 3 tr rt Ω tr Ω Φ d rt φ Φ cs ti PJ Φ PJ rt tr d Φ Φ tr ti d tr d Φ Φ rt tr ti φ oo tl rt rt φ <; 0 Φ TS TS Ω φ F- rt Φ LQ £ 0 Φ Φ Ω Ω TS - PJ d d F rt tr tr φ rt φ d tr d i-i 0 rt F 1 φ tr rt d 0 Φ φ PJ cz Φ ts ts tr 0 t ti PJ Φ LQ Φ <_ rt φ d φ d LQ tr Ω d ii 3 ti TS rt Ω LQ cz Φ rt d rt d 3 - rt to tr Φ 3 <_ d φ d φ Φ rt PJ LQ φ TS tr TS tr cz tr rt C rt d φ PJ tr • ti Φ φ TS Ω d 0 LQ tr ra ^< tr d - Φ 0 Pi ti PJ tr <_ Φ Φ ti Φ d rt rt Φ rt rt - F- ' TS J φ Ω Φ rt rt X ti Φ ti Φ φ rt tr F- d >-3 tr rt Ω Ω tr F- d LQ rt . tr s; Ω ti rt tr TS m Φ < tr PJ tr Φ 0 rt 0 X PJ Φ PJ LQ Φ tr Φ Ω F- φ c Φ Ω ti TS Φ F- Φ tr l-h d d Φ Ω d Φ > tr rt ti rt rt LQ tr O φ Φ o <: LQ 0 TS Φ Ω LQ PJ d Ω tr Φ rr PJ PJ tr rt Φ ra 0 tr PJ cz ra ti F- d 0 F- rr trr ^ TS d rt tr d d Φ rt φ PJ H tr TS ti ti Ω ra ra rt s: CQ 0 rt ra tr J LQ 1— ' φ rt Φ ti t PJ F 1 PJ Ω r d TS rt d rt ra Ω TS Φ Φ Ω ^rr Φ d X 0 d rt ra o tr ti Φ tr <_ F 1 0 3 F tr tr J tr 0 rt F _P tn Φ d d £ rt tr LΠ d rt Φ Ω cz Φ rt Φ F- ti tl < PJ Φ Φ Φ rt Φ tr d tr Φ d Q 0 rt T) Φ tr tr rt φ Ω 0 tr LQ d 0 Pi Φ Ω ra F Φ ra Φ PJ d s: rt 0 PJ PJ i tr ti to ti TS Ω 3 Ω φ ra LQ X ra ra Ω tr 0 Φ Ω rt tr ra m tr rr Φ φ φ O Φ t Φ Ω Ω 77 3 TS d Φ Ω φ ra to tr d PJ Φ Ω LQ 3 PJ ra ts C TS rt 0 d 0 ti 0 ti PJ ti rt tr rt Ω Ω rt d 0 Φ rt φ LQ c ra rt rt ti d Φ φ d Φ ct ra Φ Φ O tr d tr tr 77 tr LQ Ω d 0 PJ tι tr PJ PJ ra 0 ti φ φ 0 d d PJ Φ PJ φ F- 1 rr Ω rt rt LQ 3 Φ Ω rt ≤ PJ rt d rt d F- rt d Ω Φ d ra ra ∞ Φ tr PJ F- ti J F- 3 F- rt tr tr ct TS d d tr rt Φ ?? Φ & tr TS d rr ra PJ 0 PJ 3 tl ct 0 Φ 0 PJ ts Φ Φ Ω Φ Ω tr ii ti φ tr _— ' 3 F- F- TS Φ ra Ω rt d d <! Φ rt tr to ra £ tl LQ PJ PJ Φ φ Φ rt Φ φ d S LQ Φ tr 0 ti tr φ PJ 0 PJ _- ^ d F 1 rt ra 0 ra tr P d d tr 0 Φ Φ ra Φ 0 rt F- tr ts 0 0 tr Ω F- d φ J Φ rt ra rt rt Φ Φ TJ Φ cz rt rt s: ra PJ 0 TS cz X ti d rt d TS d rt 3 TJ tr tr ti d F 1 PJ d tr LQ Φ tr tr ra rt ra 0 rt ti ct φ LQ Φ LQ PJ rt F- 0 J tr Φ Φ Ω d d J d LΠ Φ Φ tr ≤ F- φ ct φ F Ω rt d m Ω X Ω rt Φ rt d φ Ω PJ s; rt Φ tr m 3 ra o tr d J ∞ Φ F- tr rt J φ tr ra J tr ra 0 LQ 77 ΪS tr TS F- tr z Ω tr TS φ l-h Φ Ti • d Φ Φ t pi F- d r J Φ Φ F- ti ti ti <! rt 0 tr O cz ts Ω Φ Φ ti Φ PJ d ra TS d PJ rt F tr 3 Ω Φ rt 0 F- s; PJ ≤ rt rt tr J O rt o F- ^ d TS X tr cz φ d d tr ra Φ d ra o Φ ts ts tr PJ ra d rt tr d O £ Φ TS rt S φ rt d ra ra i LQ Ω ts — rt Φ Ω F- Ti ti tr Φ rt Ω 0 d ^ Ct tr rt tr F- P F- z F- tr 0 Φ F- t 77 d ar tr F- ii ra rt . φ φ F- 0 • Ω Φ CQ ti 0 cz 0 tr Φ CQ LQ Φ d 0 F- F- 0 Ω 0 ra d TS PJ Ω Φ rt tr c * : ra rt cz • 0 LQ d d rt tr rt ra HI ≤ rt tr X ct F 1 tr TS d Pi X LQ rt Φ tr d PJ rt d Φ F- d F- Φ 0 Φ 3 σ_ €. Φ TS LQ rt tr Φ rt Φ rt d 0 h rt tr rt d d PJ Ξ t! Tl PJ ct 0 • PJ F 1 tr ti φ F- tr 0 tr 0 d Φ F- tr ^ tr LQ LQ PJ Φ PJ tr ^ 0 X d rt φ d Φ 0 Φ φ ra ti Ω Φ Φ ra tr 0 d φ Φ ti Φ F, oo oo to to F 1 F 1 LΠ o LΠ o LΠ o LΠ rt tr PJ d rt tr Φ TJ ii φ CQ CQ d ii Φ F- d rt tr Φ ra Ω H Φ Φ d Φ Φ 3 Φ d rt • > ra PJ F Φ ra d rt 0 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 backflush 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 substantially semi -circular interspace. When assembled, the screen element concerned has a relatively large throughflow surface on the inflow side. CQ n Mi d cn Mi rt Hi Ω ti F- Pi Ω Ω CQ rt rt PJ 0 rt φ TS tr <! TS Φ S rt 3 Ω Ω 0 d tr 0 Φ 0 O PJ d F, 0 Ω tr 0 d rt tr F 1 0 PJ F- φ φ a PJ ti ii Pi PJ F, d F 3 S Mi 0 d F Φ Pi tr Φ φ CQ Ω CQ ti Φ F, 0 ra Φ Φ 3 φ TS 3 cn 3 TS F- Φ CQ <! Φ rt Φ ti 3 F- 7? Ω Hι 3 Hi ra Φ φ ti φ F- F- Φ 0 Pi 0 Ω ra φ Φ TS tr tr F Φ φ rt Hi 0 0 φ 0 cn φ d d rt Pi d 0 d F" s: rt d d ti φ PJ >• 0 d F- d ti d F d ra F- tr F- LQ d Φ X F- ω rt φ CQ d rt 0 d J rr PJ S Φ d Φ d 0 F- d Hi 0 Φ F- Φ CQ F- PJ d CQ rt CQ rt Ti ≤ LQ F- ii rt S. d rt X PJ d Ω 0 F 1 CQ d > LQ a F- tr 3 Φ Φ 0 tr π Φ CQ rt tr tr en tr Ω rt d Φ d < d 0 Hi S tr d d F- PJ i TS Pi ii F- 0 3 0 Ω F- ra φ Φ CQ φ Φ 0 F- PJ 3 ti φ 0 0 tr F- LQ d CQ ti rt Ω d φ TS ti P ) F 1 d d :> Hi 0 F 1 φ φ d rt Pi PJ Φ Ω rt LQ CQ CQ Φ cn F- tr <: d Φ Φ F- Mi Ω tr cn tr d d rr tr rt d tr PJ tr Ω <_ Ω d Φ rt ti Φ LQ F- tr F- Ω P ) Hi CQ rt d F- Φ ra F- φ tr ti Φ LQ i d PJ d rt tr PJ d Ω F, cn F" 0 Ω - φ 0 F, Ω CQ ra Ω 0 Φ d Φ F, rt F- rt φ rt rt PJ tr Φ 0 F φ d H Ω Φ o Pi rt Φ rt φ Φ Φ CQ F- Φ d F 1 rt φ tr : > Φ F- α PJ Φ ti <! d tr φ tr d d 0 O X cn tr ≤ PJ d Φ rt Φ CQ φ F- Pi Φ φ φ PJ < Φ Ω d ra d Φ F- Ω Φ PJ Φ tr d P. CQ < d φ ti F- CQ F- PJ Φ 5L 3 F> PJ 0 3 0 d F X Φ Φ d > Φ F 1 PJ F- d PJ CQ Ω d CQ Hi Φ φ Φ F ' F- 0 Φ d d Φ PJ CQ Φ 0 rt rt d d rt TJ φ ti 0 d J h- ' Hi d Φ CQ ct 3 φ 0 rt s; O tr rt LQ φ CQ d F- J F LQ rt d ra Φ rt tr d d CQ tr PJ 3 <_ CQ tr Φ Φ PJ PJ 0 F, PJ d 3 Φ cn φ Ω rt . X φ SU i φ Φ 0 H 3 F 0 rt LQ Ω Φ Hi 0 Φ Ω LQ F PJ P Pi F tr PJ Φ g rt Φ d F, O 0 φ <! Φ tr 3 Hi Ω Φ rt S Φ rt Φ Ml PJ rt rt Ω d d d rt φ rt J φ ra X 0 rt F- tr F- tr Φ 0 tr F- Φ F 1 rt 0 O LQ rt tr ii tr 0 ti d d PJ ti tr d 0 Φ rt X d d Φ rt d 3 Φ tr Hi F- 3 PJ tr CQ PJ Ω φ J rt Ω ct Pi φ d rt tr cn d Φ LQ Φ P. 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Φ tr Φ d rt Pi lP F- d Ω . d Φ rt ti PJ F- φ CQ ti F- F- CO PJ rt rt F- Φ Φ CQ Φ d CQ LQ 0 tr φ d PJ φ 0 d ra Hi d φ Ω ra rt F- tr d ≤ 3 3 tr LQ φ 3 φ Ml d rt LQ CQ s: d Ω ii φ ra PJ Φ 0 Φ LQ TS F- φ F- Hi ii ra F 3 Ti - tr rt Φ F- Φ PJ PJ 0 φ d ra ti d ti ra d 0 Φ rt φ Φ 0 Φ CQ 3 0 CQ F, CQ s: - F- rt Φ ≤ F- PJ Φ Φ ■-3 tr LQ F Φ F- rt PJ d l-h PJ d rt Φ ra ti φ φ CQ F, F- 0 d d ra tr 0 PJ d Φ 0 Ω PJ rt CQ J ra φ F, F- F J 0 CQ rt H, < i CQ 3 Φ ra PJ ra rt d F- tn d F 0 ti Φ r— ' d F- F- CQ 0 φ d Φ Hi d rt tr Φ d Φ Ω F Ω tr d ^ rt 3 Φ ti X Ω cn d d Hi tn 3 0 LQ J TS d Pi F cn ti Φ J φ CQ φ TS F- Φ rt Ω F- 0 φ F Ω ti 0 rt d Φ tr d φ φ rt s; Pi F- X 0 d PJ PJ J ti rt rt CQ rt d 3 φ 77 F- ra F- ii TS φ PJ φ tr d P ) PJ d d CQ d Φ tr rt tr rt Φ d Φ σ F- 0 F- F- ra TS d ra d rt Φ 3 φ Pi CQ Φ Ω PJ 0 Φ PJ Φ 0 α CQ rt d d d Ω 0 F- tr d d 0 Φ TS d Φ PJ Φ Ω Φ ra Hi d F J • d F- LQ F, ti 0 φ d Φ F, ti rt i d ^ ti F- Ω Ω LQ 0 TS 0 ct rt φ rt ii LQ rt F- Pi TS φ CQ Φ 0 d rt φ Ξ Ω tr d F, d tr rt tr φ F- O' S tr d φ ^ F- PJ Hi LQ tr o 0 0 F- Φ tr Φ d d F> <! ti Φ LQ ti PJ l-h d PJ Φ ra φ 0 r ti TJ Φ <_ d φ LQ Φ F- Φ Ω tr 0 F- F, φ rt F- Hi Pi Pi Φ PJ F- LQ 0 P ra CQ ft rt Φ 0 F rt φ ra tr d Hi ra F- F, Ω P. Ω ra tr 0 α ≤ F- Hi Φ rt ^ d tr Φ rt 0 d φ PJ F, rt 0 tr LQ CQ tr d ti O tr c Φ Φ φ 0 ra φ 3 Φ 3 oo oo t to H F 1 LΠ o LΠ o LΠ o LΠ rt Φ F 0 F- 0 PJ tr PJ F- Ω d d d PJ Ω d Ω CQ rt Pi rt r LQ d F- CQ . ti F- ra Pi Ω φ Pi TS d 0 X tr Φ PJ ti d tr Φ F- Ω F- F- F- rt σ m 3 φ d Ω CQ ≤ d φ F, LQ J Φ F, rt PJ m TS Φ F- tr d rt ti d d Φ CQ o tr ti Φ LQ d φ F- cn rt CQ rt d cn tr rt F- tr rt Ω LQ d Φ tr φ tr ti Φ Φ Φ φ φ F- ra PJ H d tr Hi F- F- d tr Pi n PJ P> d F- F- F- F- Ω Ω φ m d M Hi 7? F- F- LQ Hi φ Hi d Hi Pi Mi Φ LQ PJ Φ 0 0 F PJ d Ω Hi ti Φ d CQ rt F- O Ω d tr tr d Hi rt Φ Ω F- Φ LQ tr Φ rt d rt φ F- tr LQ PJ J tr cn Ω Φ X £, φ CQ PJ PJ F- J Ω Pi d 0 CQ X Ω ti F- ra d TS . 0 φ ti Φ rt F, Hi d φ φ CQ CQ Φ F F- d Ω F- CQ 0 Ω rt PJ Pi CQ 3 ) Ω Φ φ d I- 1 PJ P F F, rt ra φ 0 Φ tr ti F- F- CQ Hi Φ F- d d d d Q F 1 φ F- P ) LQ ra rt PJ Mi X • rt d F- Ω Mi F- tr Ω d tr Φ m X φ tr LQ F - tr Ω Φ Φ 0 PJ Hi 0 d ≤ d 0 d Ω tr rt rr s: F, F- Φ F- φ Φ PJ Ω Ω ti ii X Φ PJ Φ 3 F" F- 0 F 1 rt d F tr Φ 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.

Claims: Claims 1. Screen unit for removing solid particles from a viscous mass, for example molten plastic, comprising a housing provided with a supply channel and a discharge channel for the viscous mass, a screen element which is suitably fitted in the housing between the supply channel and discharge channel and can be flowed through by the viscous mass, and means for cleaning the screen element by means of backflushing at least during operation, which means comprise backflush channels which are intended to discharge during the backflushing solid particles screened off by the screen element, which backflush channels can be placed in communication with an outflow channel by way of a shut-off element which can be opened, 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 also comprise a pressure element which can be operated to flush the viscous mass back through the screen element at a pressure which is higher than the pressure in the supply channel (2) , and which pressure element interacts with the shut -off element (13) in order to discharge the solid particles screened off by the screen element . 2. Screen unit according to claim 1, characterized in that said pressure element is equipped to produce a pres- sure for the backflushing which is at least twice the level of the pressure in the supply channel (2) of the screen unit . 3. Screen unit according to claim 1 or 2 , characterized in that means are provided which means interact with said pressure element to shut off said discharge channel . 4. Screen unit according to claim 1, 2 or 3 , characterized in that the screen element comprises a substantially cylindrical hollow positioning aid (7) with radial throughflow apertures and a screen casing (8) which is provided on the external periphery of the positioning aid and has through flow screen channels extending substantially in the radial direction. 5. Screen unit according to claim 4, characterized in that the positioning aid (7) is shut off at one axial end and at the other axial end forms a coaxial outflow aperture for the viscous mass, and in that slit-shaped openings (9), extending in the axial direction, are formed in the wall of the positioning aid (7) . 6. Screen unit according to claim 5, characterized in that a radially outward extending flange (6) is formed on the axial end of the positioning aid (7) forming the outflow aperture. 7. Screen unit according to one of claims 4-6, characterized in that the screen casing (8) is composed of coaxial rings (22) stacked on top of one another, ribs (23, 24) being provided on at least one axial side of each ring (22) , which ribs in the state of the rings (22) combined to a screen casing (8) by interaction with an adjacent ring (22) form the radial through flow channels in the screen casing (8) . 8. Screen unit according to claim 7, characterized in that on the outer periphery of the ring (22) the ribs (23, 24) have a greater height and/or width than on the internal diameter of the ring (22) . 9. Screen unit according to claim 7 or 8 , character- ized in that the ribs (23, 24) have a substantially triangular cross-section, in which the height and the width increase from the internal diameter of the ring (22) towards the outer periphery of the ring (22) . 10. Screen unit according to one of claims 7-9, characterized in that a number of spacers are formed on each ring (22) , in order to hold the adjacent ring (22) at a predetermined distance in the axial direction. 11. Screen unit according to one of the claims 7-10, characterized in that each ring has a substantial conical shape and is manufactured of resilient material. 12. Screen unit according to one of the preceding claims, characterized in that the unit is provided with means which act upon the pressure element and by means of which the pressure of the pressure element on the screened viscous mass flowing out of the screen unit can be regulated . 13. Screen unit according to one of the preceding claims, characterized in that the pressure element is a backflush plunger (17) which is placed coaxially with the screen casing (8) at the side of the outflow aperture. 14. Screen unit according to claim 11, characterized in that the backflush plunger (17) in a particular position can shut off an inflow aperture of the discharge channel (5) of the screen unit. 15. Screen unit according to one of claims 13 or 14, characterized in that said screen unit is designed in such a way that the backflush plunger (17) can act as an excess pressure valve for the screen unit . 16. Assembly for removing solid particles from a viscous mass, in particular molten plastic, characterized in that the assembly comprises at least two screen units according to the invention which are placed in parallel, the screen units interacting in order to keep the volume flow and/or the pressure in the discharge channel of the assembly constant .

Inventors: KERSSEMAKERS MATHIEU FRANCISCU, KERSSEMAKERS, MATHIEU, FRANCISCUS, JOSEPHUS
Applicants: KERSSEMAKERS MATHIEU FRANCISCU, KERSSEMAKERS, MATHIEU, FRANCISCUS, JOSEPHUS
Application year: 1998
Main classification: B29C47
IPC codes: B01D29, B29C47