GB1566835A

GB1566835A - Slide filters

Info

Publication number: GB1566835A
Application number: GB27086/77A
Authority: GB
Original assignee: Berlyn Corp
Current assignee: Berlyn Corp
Priority date: 1976-06-28
Filing date: 1977-06-28
Publication date: 1980-05-08
Also published as: FR2356445B3; JPS5311367A; FR2356445A1; DE2729152A1

Description

(54) SLIDE FILTERS (71) We, THE BERLYN CORPORA TION, of 93 West Main Street, Millbury, Massachusetts 01527, United States of America, a corporation organised under the laws of the State of Massachusetts, one of the United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the follow mg statement: In the manufacture of articles from rubbery, viscoelastic and thermoplastic working materials (e.g.: polymers, and certain thermosettiilg resins, vulcanizable materials and polymerizable monomers) all of which are sometimes called plastic stock, it is common practice to filter the working materials. This may be done, for example, in a reclaiming process, to prepare re-claimed materials for use in a product; it may also be done, for example, in an extrusion apparatus, to make certain that no foreign matter will be introduced into an extrusion die.

An extrusion process is a continuous process in which the quality and quantity of the extrudate are proportional to the stability and consistency of the melt. A change in operating conditions changes the quality or quantity or both of the end product, and some of the conditions which can change are affected by conventional filtering operations. The practice of filtering the working material in an extrusion process requires the positioning of a filter medium across the path of material flow resulting in a pressure drop. The size of the pressure drop is related to the difficulty with which working material passes through the filter medium.

When the filter medium is clean the pressure drop across the medium is low as compared to the pressure drop which exists when the filter medium is clogged with collected contaminants. For example, a 1500 psi drop is typical for a clean filter, whereas a pressure drop of 4000 psi is typical for a clogged filter medium.

It is desirable to have a constant and preferably low pressure drop across the filter medium for reasons relating to the quality and quantity of the working material. When the pressure drop across the filter medium increases, the shear on the working material increases, which in turn increases the temperature of the working material. The temperature of the working material is a basic operating parameter which determines a particular quality of the material. In addition, an increase in pressure drop across the filter medium reduces the rate of production of the entire line as well as requiring additional extruder pump horsepower to overcome the rising back pressure. Of course, an increase in costs also results.

The need to minimize transients in operating conditions caused by the collection of contaminants on the filter medium has been recognized for more than 75 years. A rigid slide plate filter, not unlike filters in use today, is shown in U.S. Patent 642,814 issued to Cowen in 1900. It has two filters on one slide plate, and when one of the filters becomes clogged the slide plate is moved to remove the first filter entirely and replace it with the second filter, bringing about an immediate dramatic change in operating pressure. Other forms of rigid slide plate filter like Cowen's are shown in Garrahan's U.S. Patent No. 1,195,576; Voight 3,684,419; Paquette 3,797,655; Cooper et al 3,804,758; and Heston 3,983,038.

It is common practice to use slide plate filters in a process where the working material is highly pressurized, sometimes as high as 10,000 pounds per square inch or more. There is, therefore, a need to seal the slide filter apparatus against leakage of working material from the melt stream along surfaces of the slide plate. On the other hand, there is a need to move the filter medium across the melt stream at such a rate that a low, constant pressure drop is present, and if the sealing means used presents high friction between the slide plate and the filter housing (which up to now has been the common practice in the art), then the ability to move the filter medium across the melt stream is made more difficult. Indeed, the main obstacle to providing a filter that is useable at such high operating pressure with continuous screen changing capabilities is the failure of industry to develop a workable seal arrangement which can be maintained continuously under the high operating pressure while permitting the filter medium to be moved easily and continuously across the melt stream.

According to the present invention there is provided a rigid slide-plate type filter apparatus for filtering a pressurized fluid working material that can stiffen in response to temperature change while maintaining the pressure drop through the apparatus substantially constant with respect to time during use of the apparatus, the apparatus comprising (a) a defined path for the fluid working material and rigid slide plate means providing the sole and entire support for carrying on an upstream surface replaceable filter media means from an inlet port passageway across said path to an outlet port passageway, the cross-section of the outlet port passageway being larger than the crosssection of the slide plate means so that the slide plate means fits loosely in the outlet port passageway and forms with it a gap; (b) thermally-operable means in said gap to form and maintain continually during use of the apparatus a seal preventing leakage of fluid working material between the slide plate means and walls bounding said gap; (c) means to advance the slide plate means into said inlet port passageway, across said path and out through said outlet port passageway for changing the filter media means in a mode approximating substantially continual change during use of the apparatus, the advancing means including means operative during use of the apparatus to apply a force to move the slide plate means into said inlet port passageway and unidirectionally across said path against the retarding force of said seal so that during use of the apparatus the slide plate means advances in a mode approximating substantially continual advance across said path while said seal is continually maintained; and (d) means to restrain the increase in the pressure drop across the filter media means due to build up of contaminants collected on the portion of the filter media means resident in said defined path during use of the apparatus to a range of pressure values which is a minor fraction of the operating pressure on said working material.

The present invention also provides a method of operating a rigid slide-plate type filter apparatus of the invention, which method comprises generating a train of force pulses, applying the pulses to force the slide plate means to move unidirectionally across said defined path while continuously maintaining said seal, and continuing to apply said force pulses substantial without interruption so as to restrain the increase in the pressure drop across the filter media due to build up of contaminants collected on the portion of filter media resident in said defined path to a range of pressure values which is a minor fraction of the operating pressure on said working material.

In the present invention a filter machine of the kind using a rigid slide plate permeable to flow therethrough of the material being filtered to provide the sole and entire support for carrying filter media across the melt stream includes a body defining a melt stream passage and inlet and outlet ports through which the slide plate can be passed and moved, continuously or incrementally in steps of any size as desired, to advance the filter media and introduce different parts thereof across the melt stream passage, at least the outlet port being formed as an elongate conduit bounded by smooth walls defining a conduit cross-section that is larger than the cross-section of the slide plate therein, to provide a gap of a size to permit movement of the slide plate and flow of fluid working material therethrough, and at least the outlet port being fitted with thermally-operable means for forming a seal of the working material filling the gap therein.

This invention makes possible a filter of the kind described in which a wide variety of working materials which can be put into a fluid state can be filtered while changing the filter medium continuously or incrementally in steps of any desired size, the working materials including by way of example and not limitation, silicone rubber, natural rubber, synthetic rubber, thermoplastic materials (polyesters, epoxides, and the like), oils, aqueous liquids, and molten metals.

The sealing means at the outlet port may include heat exchange means, the purpose of which is to control the temperature of the working material which flows into the gap between the slide plate with screens in place and the confronting inner walls of the outlet port conduit. The temperature of the workmg material filling this gap is controlled so that the more viscous, adequately hardened working material itself forms a seal to prevent working material leaking to the outside environment. Cooling or heating means, such as temperature-controlled air, can be employed with the heat exchange means.

The invention is useful with materials which stiffen or at least partially solidify, that is, become more viscous, upon cooling, such as thermoplastic polymers and the like; and the invention is generally described herein in relationship with such materials which substantially increase in viscosity upon cooling. The invention is also useful with materials which can stiffen irreversibly upon being heated, such as natural and synthetic rubbers, and silicone rubber, among others. Some such materials may flow readily at an intermediate temperature such as room temperature and their viscosities increase substantially when refrigerated to form an acceptable but reversible seal, and stiffen irreversibly upon heating to form a permanently solid or stiffened seal. In such instances one seal, for example at an exit port, may be controlled by heating and another seal, for example at an inlet port, may be controlled by cooling or by a mechanical seal. Suitable mechanical sealing means for the inlet port passage which are adjustable so as to minimize frictional drag therein are described and claimed in U.S. Patent Specification No. 4,059,525.

The formation of working material seals provides an inexpensive sealing mechanism by utilizing the thermodynamic characteristics of the working material itself. A thermoplastic material will experience a drastic reduction in shear characteristics when heated to its softening range. In this softer state, the thermoplastic material will provide little resistance to a positive shear force yet it will not flow through a long narrow gap. These thermodynamic characteristics exist over a fairly broad and attainable temperature range. In addition, when the working material is solidified against a smooth surface by chilling or heating, it can be slid off that surface relatively easily. It is, therefore, possible to utilize the working material to form an effective seal in the outlet port and a mechanical or a working material seal in the inlet port, yet enable the slide plate carrying the filter medium to move across the path of working material flow in a smooth continuous manner.

The provision of a slide plate conduit that is larger in cross section than the slide plate, providing a gap wherein sealing means can be provided confined exclusively to the inlet port and the outlet port passageways, has the advantage that the slide plate is more easily moved, so that a clean screen or screens, sandpack, or other media can be moved across the melt stream into the path of flow of the working material continuously or incrementally in small steps of any desired size, resulting in maintaining relatively low pressure drop across the filter medium, and in minimizing variations (AP) in that pressure drop. Low pressure drop across the filter medium reduces the amount of extruder horsepower needed to push the working material through the filter medium.

This can result in substantial savings in equipment requirements and energy costs, and the quality of the resulting product can be improved when the variation in pressure drop is held to a small value.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a perspective view of a slide-plate filter with temperaturecontrolled sealing means at both the inlet port conduit and the outlet port conduit for the slide plate; Figure 2 is a section view along line 2-2 in Figure 1; Figure 2A is an enlarged section view along line 2A-2A in Figure 1; Figure 3 is an enlarged perspective partial view of two sequential slide plates; Figure 4 is a perspective view of a slide-plate filter with mechanical sealing means at the inlet port conduit for the slide plate and temperature-controlled sealing means at the outlet port conduit for the slide plate; Figure 5 is a section on line 5-5 of Figure 4; Figure 6 is a partial section on line 6-6 of Figure 4, showing a mechanical filter means located within the inlet port conduit; Figure 7 is an end view of the inlet port on line 7-7 of Figure 4; Figure 8 is a section on line 8-8 of Figure 6; Figure 9 is a view similar to Figure 6 of another mechanical filter means; Figure 10 is a view on line 10-10 of Figure 9; Figure 11 is a side sectional view of a slide plate as shown in Figure 3 or Figure 4 modified to carry a sandpack filter medium; Figure 12 is a perspective view of another form of slide plate that can be used in filters according to the invention; Figure 13 is a graphical comparison of the prior art with the present invention; and Figure 14 is a schematic illustration of control means for advancing a filter across a melt stream.

The filter device shown in Figures 1, 2, and 2A is primarily adapted for operation with a heat softening material such as a thermoplastic polymer or the like. This device has a rigid slide plate 11 on which are carried filter screens 12, upstream and downstream housing members 15A and 15B respectively, defining a working material or melt-stream path 16, an upper guideway member 21, a lower guideway member 22, inlet heat exchange devices 18 and outlet heat exchange devices 19. The housing members 15A and 15B are attached to the upper and lower guideway members 21 and 22, respectively, by bolts 31, as shown; these members can be attached to each other by other means, such as welding, copper brazing or the like. The guideway members 21 and 22 are fixed between the housing members 15A and 15B, above and below the working material path 16 so as to form inlet and outlet passageways 14A and 14B, respectively for the slide plate 11. The members 15A and 15B may be heated or cooled, as desired, via bores 45.

The inlet and outlet passageways 14A and 14B have smooth walls and provide a conduit for the slide plate 11 which is dimensionally larger in cross-section than the slide plate at both the inlet and outlet ends. As will presently become apparent, this dimensional relationship provides operational flexibility not found in prior art rigid slide plate filters.

The inlet heat exchange devices 18 are secured to the guideway members 21, 22 nearby the housing members 15A. and 15B so as to provide with the guideway members a continuation elongating the inlet passageway. Similarly, the outlet heat exchange devices 19 are secured to the guideway members nearby the housing members so as to provide with the guideway members a continuation elongating the outlet passageway. As shown, the heat exchange devices 18 and 19 are heat radiators, capable of exchanging heat with surrounding air, and thereby cooling the regions between them.

The slide plate 11 can be forced to move through the passageways 14A and 14B and to advance across the melt-stream path 16 by a pusher means 13 having a push rod 34 which in cooperation with a piston 13' makes contact with the slide plate 11. The push rod 34 and piston 13' can be activated by hydraulic means such as a fixed or variable displacement pump or a pump driven by a variable speed electric motor or the push rod itself can be activated by mechanical means driven by an electric motor. A variety of pushing means are known to those skilled in the art.

As is indicated in Figures 2 and 2A, the slide plate 11 is smaller in cross-sectional size than the passageways through which it moves in its path transverse to the meltstream path 16. Figure 2 illustrates a small gap, which may be in the range 0.001 inch to 0.004 inch, but this gap may be larger. In the operation of this filter, the fluid working material is caused to flow along the melt stream path 16 in the direction of arrow 17 (Figure 2A) to be filtered by the screen of filter medium 12 carried by the slide plate 11, which is permeable to the flow of the fluid working material, and which is movable across the melt stream path by the pusher mechanism 13. The slide plate 11 can be advanced continuously or incrementally across the path 16, the choice being made by the operator, depending on the extent to which it is desirable continuously to present clean screens or filter media to the melt stream. The slide plate advancing means may cooperate with sensing means (see Figure 14) which detects pressure drop across the filter medium 12 and activates the pusher means 13, as may be required to maintain a desired limit on the variation of working pressure (AP) applied to the melt stream.

The working material being filtered is maintained in a fluid state in the melt stream 16 where it flows under the working pressure through the screen or filter medium 12, and through breaker plate passageways 38 in the slide plate 11. The working pressure also forces the fluid material toward the outside of the device through the inlet and outlet passageways 14A, 14B, where the working material becomes progressively cooler as it moves away from the melt stream path 16. In the case where the working material is a thermoplastic polymer, for example, it can be cooled to a substantially solid state within the heat exchangers 18 and 19, forming seals in the gaps 40 40' between the slide plate 11 and the smooth inner walls or surfaces of the inlet and outlet passageways 14A and 14B, respectively. Ambient air passing over fins 20 is adequate to form highly viscous, substantially solid seals of many working materials. In order to enhance the cooling capabilities of the heat exchange devices 18 and 19, the thermal-isolation gaps 43 and 44 are provided between the heat exchange devices and the housing members. If desired, temperature controlled air (not shown) can be caused to flow along the surfaces of the fins 20 of the heat exchange devices 18 and 19, in the direction of arrows 27, as shown in Figure 1. The amount of cooling may be controlled by a temperature control damper 24 (shown in Figure 1) for each of the heat exchange devices 18 and 19, enclosing the surfaces of the fins 20 of each heat exchange device 18 and 19, so as to regulate the amount of air reaching the fins.

The full width of each gap 40 and 40' is filled with cooled working material so that solidifying or substantially solidified working material may be in contact with the housing members 15A, 15B, as well as the slide late 11 and the heat exchange devices 18,1.The widths of the gaps 40 and 40' can be made sufficiently small so that only a small quantity of the working material is used for sealing purposes. Typically, the upstream gap 40 may be approximately 0.001 to 0.004 inches wide and the downstream ga 40' may be approximately 0.001 inches wide. The difference in the widths of gaps 40 and 40' is the result of fluid material flow in the melt stream path 16 forcing the slide plate 11 to move slightly in the downstream direction. The substantially solidified working material seal formed in the gaps 40 and 40' surrounds the slide plate 11 and is attached to it; in the outlet passageway 14B this seal 25 is continuously being formed and then passed out of the filter machine with the advancing slide plate 11, to be replaced by more hardening working material from the melt stream, as the filter is changed. Smooth walls inside the passageways 14A and 14B facilitate sliding the seal material through the passageways when the slide plate 11 is pushed. The working material nearer to the melt stream path 16, viscosity of which may be intermediate between fluid and substantially solid, is shearable. Seal material from the inlet passageway 14A will, upon being advanced mto the melt stream path 16, melt and join the filter melt, so that the only material lost from the filter machine is the seal material 25 attached to the slide plate 11 emerging from the outlet passageway 14B. That material is removable in known ways, so that a filter plate 11 can be re-used, with new screen means 12 if desired. As figure 3 illustrates, a second slide plate can follow a first slide plate into the filter machines, so that a filtering process can carry on continuously.

In the present invention, the filter medium positioned in slide plate 11 can be moved across the path 16 of fluid material flow without introducing heat to soften the working material seal. This can be accomplished by operating the heat exchangers 18, 19 at relatively cold temperatures for most thermoplastic polymers or at intermediate temperatures for substantially all thermoplastic polymers. Typically, when the work mg material is a thermoplastic polymer, the temperature in the melt stream is in the range of 375"F to 475"F, while the temperature of the cooled working material in the inlet and outlet passageways 14A, 14B at an intermediate range is 200"-250"F, and at a relatively cold range is approximately that of room temperature. The inner surfaces of members bounding the outlet passageway 14B can be coated with a material providing a slippery surface, such as polytetrafluoroethylene or other fluorocarbons so that substantially solidified working material seals can less readily adhere to them, and the force required to push the slide plate 11 will be reduced.

Working material is prevented from leaking out of the top and bottom of the slide filter device 10 by tightening the bolts 31 so that the guideways 21, 22 and housing members 15A, 15B are pressed tightly against each other. Gaps 41 and 41' are provided between the slide plate 11 and the guideways 21, 22 in the region of the melt stream path 16, as well as between slide plate and housing members 15A, 15B in the inlet and outlet passageways 14A, 14B, to reduce friction between the slide plate 11 and those members. Typically, each such gap 41, 41' is in the order of 0.001" wide.

The flow of fluid working material through these gaps 41 and 41' is effectively prevented by the narrow width of the gaps, and the working material flows preferentially through the filter medium 12. In use, the down-stream surface 30 of the slide plate 11 is pressed against the confronting surface 32 of the downstream housing member 15B, so as to render the down-stream gap 40' virtually impassable to the fluid working material.

Thus, there is provided a filter machine of the rigid slide plate type wherein the slide plate can be passed across the path of the fluid melt stream through inlet and outlet ports, and through intermediate guiding members, forming a guideway across the melt-stream path which is larger in crosssection than the slide plate, with means located exclusively in the inlet and outlet passageways to form seals preventing loss of working material to the surrounding environment, the sealing means being controllable to provide stable conditions under which the slide plate can be advanced continuously across the melt stream path, the slide plate carrying a filter medium or filter media and providing the sole support for the filter medium or media in the melt stream path.

When the working material is one that can be reversibly solidified, as a thermoplastic material, the process of solidification of working material at the inlet heat exchange devices 18 provides a prefill operational advantage without the need for special ducting or other air or gas venting means.

The gaps 40 and 40' are filled between slide plate 11 and housing members 15A and 15B so as to fill the breaker plate recesses with initially fluid working material from the melt stream path 16, thereby reducing the opportunity for air to be carried on the slide plate into the melt stream path 16 as the slide plate 11 is advanced across that path.

Figure 3 illustrates one embodiment of a slide plate 11 wherein two sequential slide plate members 11A and 11B are joined together by a locking mechanism 50. The locking mechanism fits into a slot 51 and secures one slide plate member 11A to a successive slide plate member 11B. It is desirable to secure successive slide plate members closely together at their confronting ends for continuous screen changing operation in a manner such that when working material is filling the gaps 40 and 40' as previously described, it does not also force the slide plates apart, and flow between them and thus bypass the filter medium 12. Using a suitable locking mechanism 50, when the ends of slide plate members 11A and 11B are registered in the filtering mode in the melt stream path 16, fluid working material will not flow directly between them.

The apparatus of Figures 1-2A can be utilized with working materials such as silicone rubber and other materials which stiffen or harden when heated. Such use of the apparatus can be accomplished by suitable control of inlet heat exchange devices 18 and outlet heat exchange devices 19 (see, for example Figure 1) to provide heated air over one of them. and cool air over the other. In the case where the working material stiffens irreversibly upon being heated, heat is furnished to the outlet passageway 14B, to form a substantially solidified seal of the working material which fixes itself to the slide plate 11 and emerges from the outlet passageway like the solid material 25 shown in figure 1 when the slide plate is advanced across the melt stream path 16. Working materials like silicone rubber are maintained fluid in the melt stream path by cooling the housing members 15A, 15B, via the temperature control bores 45. In order to prevent the formation of a permanentlystiff seal of such a material in the inlet passage 14A, which would introduce the possibility of blinding the filter medium 12 by carrying permanently-stiffened working material into the melt stream path upon advancing the slide plate 11, the heat exchangers 18 at the inlet passageway 14A may be cooled to thicken the working material into a more viscous, substantially solid seal that is reversibly stiffened, and will return to the fluid state when carried into the melt stream.

The filter machine shown in figures 4-8 incorporates a mechanical seal in its inlet passageway (corresponding to the inlet passageway 14A in figure 1). Figure 4 is a perspective view of the machine, the housing 117 of which is illustrated only in block form, and includes a bore 121 defining the melt stream path 111. This machine employs a slide plate 110, arranged to traverse the melt stream path 111 from an inlet passage 118 defined as a passage through an inlet passage member 160 to an outlet passageway (not shown) which is the same as passageway 14B in figure 1, and the heat exchange devices 19 of Figure 1 are fitted at the outlet passageway in figure 4, which differs from the embodiment of figure 1 essentially only in that mechanical seals as shown in Figures 6-8, or in Figures 9 and 10, are fitted within the inlet passageway 118, around the slide plate 110 therein.

In Figure 5 the slide plate 110 is shown in cross section, with fluid-permeable breaker plate apertures 126 in the melt stream path 111. The slide plate is fitted within a transverse passageway 118' (an extension of the inlet passageway 118) which is larger in cross-section than the slide plate. As in the embodiment of Figure 1, the gaps 142 and 143 between the slide plate and the confronting walls of the passageways 118, 118' and 14B (which is not shown) are small.

Typically, gaps 142, above and below the slide plate 110, are approximately 0.001", while the gaps 143 between the front surface 112 and rear surface 113 of the slide plate and the passageway walls may be greater than 0.001 inch thick, providing an effective seal against fluid materials bypassing the filter medium in the melt stream. This seal is enhanced by the fact that slide plate 110 is pressed against surface 14Q under pressure of the fluid material in the melt stream path 111. The slide plate 110 is thus fitted loosely within the transverse passageway guiding it across the melt stream, and this structure permits the slide plate to be forced through the passageways 118 and 118' more easily than against a metal-to-metal friction contact as is typical of the prior art mentioned above.

The longitudinal corners 127, 128, 133, and 134 of the slide plate 110 are rounded in the present embodiment as are the corresponding longitudinal corners 135, 136, 137, and 138 of the passageways 118 and 118' (and 14B) to accomodate the mechanical seal 150 in the inlet passageway 118, which i seal 150 is thereby thickened and forced to press against the breaker plate 110, at the confronting surfaces of bars 132 at the front and rear surfaces of the slide plate, as well as against the top and bottom surfaces of the slide plate 110. Although fluid working materials being filtered will fill the gaps 143 in the region between the seal 150 and the melt stream passage, it is blocked from leaking out of the machine through the inlet by the seal 150. In the illustrated embodiment employing a slide plate with breaker plate recesses 130 surrounded by bars or frames 132, the seal 150 should be longer than the length of a recess 130 so that at least one of the bars 132 will be present at all times in the inlet passageway 118 to provide effective sealing against material leakage.

The use of a mechanical seal at the inlet end of the slide plate 110 permits continuous filtering of materials that flow freely when only moderately heated or not heated at all, for example, a silicone, which if heated can harden irreversibly.

As is shown in Figure 7, the bolts 153 are spaced around the end flange member 152 so as to impose a substantially uniform pressure on the seal 150.

Figure 8 illustrates how the seal 150 effectively fills the gaps between the slide plate 110 and the inlet passageway 118, all around the periphery of slide plate 110.

Another seal configuration for the inlet passageway 118 is shown in Figures 9 and 10, where a wedge-form end seal 155 is made up of two wedges 156 and 157, for each side of the slide plate 110. Under pressure from the end flange member 152, the wedges are forced to slide one on the other, thereby expanding to fill the space between the slide plate 110 and the walls of the passageway 118. One wedge 156 may be made of steel while its mating wedge 157 may be made of a low coefficient of friction material such as bronze, copper, iron or carbon. Figure 10 illustrates this seal in cross section. Using the wedge seal 155, it is not necessary for the corners 160, 161, 162, and 163 of either the slide plate or the seal means to be rounded, as in the elastomeric seal of Figure 6.

The present invention can readily be adapted for continuous filtering using sand pac filter media. Figure 11 shows a typical sandpack filter-recess configuration adapted to a filter plate 10, as used in the embodi ment of Figure 1, but it will be understood that a similar adaptation may be made of the filter plate 110 as used in the embodiment of Figure 4. Between the bars 32 bounding a recess in the slide plate 10 and the breaker plate portion of that recess with passage ways 26 there are fitted, in order, a lower screen 36 and an upper screen 37, with sand 38 located between the screens 36 and 37.

Located above the upper screen 37 is a perforated retainer plate 39. The retainer plate 39 has sufficiently open perforations, which can be of any desired geometric configuration, to be permeable to the fluid material being filtered. The retainer plate 39 can be secured to the bars 32 by any suitable means; for example, the retainer plate 39 can be bolted directly to the bars 32 as shown. Alternatively, it can be fixed to the inside surface of the bars 32 while covering substantially all of screen 37 and held in place by a retainer, slot-ring configuration or the like, (not shown). The bars 132 of the slide plate 110 have grooves that would be suited to that use.

Figure 12 shows an embodiment of the slide plate 11 that is adaptable for use with a roll or belt type filter screen 61 in a filter machine according to Figure 1. A recess 60 runs the full length of the slide plate, to position the filter medium 61 on the upstream surface of the breaker plate portion 63. In practice, the filter medium 61 may be continuously supplied to each of a series of successive slide plates before or during the insertion of each slide plate into the inlet passageway 14A.

Figure 13 illustrates the improved performance capability of filter machines incorporating filter media (e.g.: screen) changers according to the invention. Curve 71 represents the build-up of pressure (psi) across the filter medium of a typical prior art filter machine employing intermittent screen changing. A typical extrusion line might operate with a pressure drop of 1500 psi in the period immediately following the insertion of a clean filter by a slide plate screen changer. Line 72 represents the ideal situation (not achieved in practice) wherein the pressure remains constant at 1500 psi. As time passes with the filter remaining in the melt stream collecting contaminants, the filter begins to blind, and the pressure increases, to (typically) 4000 psi, over some period of time varying from minutes to days depending upon the application, an example of 11 hours being shown in Figure 13.

Insertions of a new filter at that time causes an abrupt drop (AP) in pressure across the filter medium to 1500 psi, causing irregularities in operation, as is discussed above. The present invention making possible continuous screen changing, or screen changing in small steps or increments, brings about the possibility to reduce the pressure variation AP to a small fraction of what has heretofore been tolerated, as is illustrated by curve 73. This can be done by manually presetting the change rate, or by installation of an automatic closed-loop control system, as is described in connection with Figure 4.

In Figure 14 a speed control 81, comprising an interval timer which can be manually set to provide individual electrical pulses over a widely variable frequency range, feeds such pulses at a set frequency to a control system 82 which converts the electrical pulses to hydraulic pressure pulses, and these pulses in turn are fed to the hydraulic piston 13'. For automatic control a pressure transducer 84, pressure controller 85, and variable pulse generator 86 are substituted for the manual speed control 81.

An estimate frequency is set on the variable pulse generator 86 and the desired AP is set on the pressure controller 85. The true AP is read by the pressure transducer 84, which is inserted into the melt stream, and a signal from the pressure transducer is compared with the desired AP in the pressure controller 85. Any resulting deviation signal is transmitted to the variable pulse generator 86, and the pulse generator adjusts its frequency to reduce the deviation to zero.

No prior closed loop control system has been available to the art of slide plate screen changers, for the reason that it has not heretofore been possible to establish a programmable variable pushing force for the slide plate.

WHAT WE CLAIM IS: 1. A rigid slide-plate type filter apparatus for filtering a pressurised fluid working material that can stiffen in response to temperature change while maintaining the pressure drop through the apparatus substantially constant with respect to time during use of the apparatus. the apparatus comprising (a) a defined path for the fluid working material and rigid slide plate means providing the sole and entire support for carrying on an upstream surface replaceable filter media means from an inlet port passageway across said path to an outlet port passageway, the cross-section of the outlet port passageway being larger than the crosssection of the slide plate means so that the slide plate means fits loosely in the outlet port passageway and forms with it a gap; (b) thermally-operable means in said gap to form and maintain continually during use of the apparatus a seal preventing leakage of fluid working material between the slide plate means and walls bounding said gap; (c) means to advance the slide plate means into said inlet port passageway, across said path and out through said outlet port passageway for changing the filter media means in a mode approximating substantially continual change during use of the apparatus, the advancing means including means operative during use of the apparatus to apply a force to move the slide plate means into said inlet port passageway and unidirec- tionally across said path against the retarding force of said seal so that during use of the apparatus the slide plate means advances in a mode approximating substantially continual advance across said path while said seal is continually maintained; and (d) means to restrain the increase in the pressure drop across the filter media means due to build up of contaminants collected on the portion of the filter media means resident in said defined path during use of the apparatus to a range of pressure values which is a minor fraction of the operating pressure on said working material.

2. An apparatus according to claim 1, wherein said advancing means is able to apply said force in the form of pulses.

3. An apparatus according to claim 2, including (a) means to generate pulses, (b) means to establish a desired frequency of said pulses, and (c) means to apply said pulses to said advancing means.

4. An apparatus according to claim 1, including pressure responsive means to monitor the magnitude of said increase in the pressure drop and to control said advancing means for adjusting said force so as to minimize said increase in said pressure drop.

5. An apparatus according to claim 4, including (a) means to generate pulses, (b) means including said pressure-responsive means to adjust the frequency of said pulses, and (c) means to apply said pulses to adjust said force.

6. An apparatus according to claim 5, including (a) a variable-frequency pulse generator having a control terminal for receiving a signal to control the pulse frequency, (b) pressure controller means to establish a desired limit of said pressure drop, (c) means responsive to said pressure controller means for comparing the actual pressure drop monitored by said pressureresponsive means with said desired limit and for providing a signal representing the difference between them, and (d) means to couple said signal to said control terminal of said variable frequency pulse generator for altering the pulse frequency in the direction tending to reduce the magnitude of said difference, thereby to establish a programmable variable force for advancing said slide plate means.

7. An apparatus according to claim 1, wherein said advancing means comprises pusher means adjacent said inlet port passageway for pushing said slide plate means into said inlet port passageway with said force.

8. An apparatus according to claim 7 in combination with drive means to push said slide plate means across said path, wherein the combination includes (a) means to generate pulses, (b) means to establish a desired frequency of said pulses, and (c) means to apply said pulses to said drive means for pushing said slide plate means incrementally in steps at said frequency.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. set to provide individual electrical pulses over a widely variable frequency range, feeds such pulses at a set frequency to a control system 82 which converts the electrical pulses to hydraulic pressure pulses, and these pulses in turn are fed to the hydraulic piston 13'. For automatic control a pressure transducer 84, pressure controller 85, and variable pulse generator 86 are substituted for the manual speed control 81. An estimate frequency is set on the variable pulse generator 86 and the desired AP is set on the pressure controller 85. The true AP is read by the pressure transducer 84, which is inserted into the melt stream, and a signal from the pressure transducer is compared with the desired AP in the pressure controller 85. Any resulting deviation signal is transmitted to the variable pulse generator 86, and the pulse generator adjusts its frequency to reduce the deviation to zero. No prior closed loop control system has been available to the art of slide plate screen changers, for the reason that it has not heretofore been possible to establish a programmable variable pushing force for the slide plate. WHAT WE CLAIM IS:

1. A rigid slide-plate type filter apparatus for filtering a pressurised fluid working material that can stiffen in response to temperature change while maintaining the pressure drop through the apparatus substantially constant with respect to time during use of the apparatus. the apparatus comprising (a) a defined path for the fluid working material and rigid slide plate means providing the sole and entire support for carrying on an upstream surface replaceable filter media means from an inlet port passageway across said path to an outlet port passageway, the cross-section of the outlet port passageway being larger than the crosssection of the slide plate means so that the slide plate means fits loosely in the outlet port passageway and forms with it a gap; (b) thermally-operable means in said gap to form and maintain continually during use of the apparatus a seal preventing leakage of fluid working material between the slide plate means and walls bounding said gap; (c) means to advance the slide plate means into said inlet port passageway, across said path and out through said outlet port passageway for changing the filter media means in a mode approximating substantially continual change during use of the apparatus, the advancing means including means operative during use of the apparatus to apply a force to move the slide plate means into said inlet port passageway and unidirec- tionally across said path against the retarding force of said seal so that during use of the apparatus the slide plate means advances in a mode approximating substantially continual advance across said path while said seal is continually maintained; and (d) means to restrain the increase in the pressure drop across the filter media means due to build up of contaminants collected on the portion of the filter media means resident in said defined path during use of the apparatus to a range of pressure values which is a minor fraction of the operating pressure on said working material.

9. A combination according to claim 8,

wherein said means to generate pulses is a variable-frequency pulse generator having a control terminal for receiving a signal to control the pulse frequency, and wherein the combination includes (a) pressure responsive means to monitor the magnitude of said increase in the pressure drop across the filter media means in said fluid flow path, (b) pressure controller means to establish a desired limit of said pressure, (c) means responsive to said pressure controller means for comparing the actual pressure drop with said desired limit and for providing a signal representing the difference between them, and (d) means to couple said signal to said control terminal of said variable frequency pulse generator for altering the pulse frequency in the direction tending to reduce the magnitude of said difference, thereby to establish a programmable variable force for advancing said slide plate means.

10. An apparatus according to any of claims 1 to 9 for use to filter a thermoplastic or other working material which undergoes a reversible increase in viscosity upon being cooled, wherein said thermally-operable means is cooling means provided at said outlet port passageway to form said seal from said fluid working material entering said gap.

11. An apparatus according to any of claims 1 to 9 for use to filter a material which undergoes an irreversible increase in viscosity as a consequence of being heated, wherein said thermally-operable means is heating means provided at said outlet port passageway to form said seal from said fluid working material entering said gap.

12. An apparatus according to claim 10 or 11, wherein said slide plate means is fitted with means projecting from said upstream surface for carrying said stiffened seal material out of said gap in said outlet port passageway with said advancing slide plate means.

13. An apparatus according to any of claims 1 to 12, wherein the cross-section of the inlet port passageway is larger than the cross-section of the slide plate means so that the slide plate means fits loosely in the inlet port passageway and forms with it a second gap into which fluid working material can escape from said path for pre-filling the filter media with said fluid working material, and wherein seal means is provided in said second gap.

14. An apparatus according to claim 13, including (a) a mechanical seal means positioned within said second gap in said inlet port passageway, said slide plate means being insertable in and through said mechanical seal means, and (b) seal adjustment means cooperatively functioning with said mechanical seal means to limit leakage of fluid working material therethrough.

15. An apparatus according to any of claims 1 to 14, wherein said inlet and outlet port passageways are of similar crosssectional dimensions throughout their respective lengths, and the cross-section of said slide plate means is uniform throughout its length.

16. A rigid slide-plate type filter apparatus, substantially as hereinbefore described with reference to, and as shown in, Figures 1, 2 and 2A, Figure 3, Figures 4, 5, 6, 7 and 8, Figures 9 and 10, Figure 3 as modified by Figure 11, Figures 4, 5, 6, 7 and 8 as modified by Figure 11, or Figure 12, of the accompanying drawings.

17. A method of operating a rigid slideplate type filter apparatus as claimed in claim 1, which method comprises generating a train of force pulses, applying the pulses to force the slide plate means to move unidirectionally across said defined path while continuously maintaining said seal, and continuing to apply said force pulses substantial without interruption so as to restrain the increase in the pressure drop across the filter media due to build up of contaminants collected on the portion of filter media resident in said defined path to a range of pressure values which is a minor fraction of the operating pressure on said working material,

18. A method according to claim 17, including the step of measuring said increase in pressure drop continually during use of said filter and controlling said pulses so as to maintain a prescribed minimum valve of said increase.

19. A method of operating a rigid slideplate type filter apparatus for filtering a fluid working material that can stiffen in response to a temperature change, substantially as hereinbefore described with reference to the accompanying drawings.