US3371383A - Continuous casting apparatus - Google Patents

Description

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March 5, 1968 o. H. HELLSUND 3,371,383

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Y V5 V l l l 1 n@ @Il ffl@ Ffa. 5. BY a@ nk United States Patent O 16 Claims. (Cl. 18-26) ABSTRACT F THE DHSCLOSURE Apparatus for continuous casting of polymerizable monomers comprising a first endless belt disposed above a second endless belt to define a casting zone wherein the material-receiving end of the casting zone is at a higher elevation than the material discharging end thereof such that the belts are inclined and wherein the path of the belts through this incline is defined by the natural curvature thereof. Preferably, support means and backing means defining a path -or the belts in accordance with the natural curvature thereof are provided and tne upper belt is maintained under a tension greater than that used to determine the natural curvature and the lower belt is maintained at a tension less than that used to determine the natural curvature. Heating means are provided in the `casting zone and seal means are provided between the edges of the belts in the casting zone. The difference in elevation between the material receiving end and the material discharging end of the casting zone is sufcient to incline the belts such that a liquid material being cast in the casting zone will develop suiiicient iluid pressure to maintain the spacing between the belts such that the belts are parallel and spaced at a predetermined distance in the casting zone` Background of the invention This application is a division of application Ser. No. 490,897, filed Sept. 28, 1965.

The present invention relates to the continuous casting of polymeric compositions. There has been substantial interest in casting polymeric compositions `for many years and many processes have been proposed and developed for such casting. Much of this development work has concerned the casting of acrylic and other polymerizable unsaturated organic materials.

While the present invention is particularly applicable to the polymerization and casting of unsaturated compounds such as monomers, it is understood that it is applicable to a wide range of polymerizable materials such as prepolymers and the like, all of which materials will be referred to in the specification and claims for convenience as monomers.

In general, the casting processes which have been previously developed comprise subjecting the polymerizable compositions to heat and/or light with or without the application of pressure. A highly successful and widely used process comprises casting liquid monomer between a pair of vertical rigid sheets which sheets are provided with a peripheral gasket of flexible material to compensate for the shrinkage which occurs during polymerization, the assembly being immersed in a hot water bath or hot air oven for the length of time required to carry out the thermal polymerization. The hot water bath is, of course, the source of both heat and pressure. This process has come to be known as cell casting. Cell casting is, as is apparent `from the foregoing description, a batch process and, although widely used, is subject to all of the deficiencies of any batch process, viz., a substantial amount of down time between casting operations, size limitations, quality control diiculties due to inadvertent changes in pressure, temperature and composition, etc.

Thus, there has long been substantial interest in the development of a continuous casting process, but, prior to the present invention, a commercially practicable continuous casting process has not been developed. In this regard, it should be pointed out [that the ditiiculties involved in continuous casting become increasingly greater as the thickness of the cast sheet is increased.

One proposal with regard to continuous casting is that disclosed in United States Patent No. 2,500,728, wherein it is suggested that a pair of endless belts arranged such that they are vertically spaced from each other with the lower run of the upper belt and the upper run of the lower belt substantially parallel to each other might be used as a continuous casting apparatus. While this proposal appeared to have substantial possibilties from a theoretical standpoint, it was found that the apparatus specifically disclosed in Patent No. 2,500,728 was not suitable for commercial operation because of its short length and the extremely slow speed of travel required 'to bring the necessary polymerization to completion such that a iinal continuous, rigid sheet would be formed. It was then, of course, proposed that an apparatus of the type disclosed in Patent No. 2,500,728 be produced in which the belts were of substantially greater length- However, when the apparatus was designed such that it would be suitable for commercial operation from a size standpoint, e.g., ve feet wide and one hundred feet long, it was found that the spacing between the belts could not be maintained if used in the manner described in Patent No. 2,500,728. This, of course, resulted in loss of control over the thickness of the final product and caused nonuniformity in this thickness. This problem was found to be particularly acute when stainless steel belts, which are preferred, were used. In general, this inability to maintain proper spacing between the moving belts results from the lack of support for the lower run of the upper belt which tends to bow downwardly toward the upper run of the lower belt.

However, it was not until the process and apparatus described and claimed in copendin-g application Serial No. 490,897, tiled Sept. 28, 1965 and Ser, No. 605,111 filed Sept. 15, 1966, in the names of Opel et al. and assigned to the same assignee as the present application was developed that continuous casting of monomers on a commercial scale became possible. Certain problems continued to be experienced with regard to the use of the Opel et al. process and apparatus including the need for extremely tall buildings to house the inclined portion of an apparatus of substantial length and the difficulty in maintaining the inclined portion in the desired position. Furthermore, the inclined portion of the Opel et al. apparatus required rather massive supporting structures, and it was desired to simplify this portion of the apparatus.

It is a primary object of the present invention to provide a process and apparatus for the continuous casting of polymerizable organic monomers wherein the liquid organic monomer is cast between a pair of moving endless belts which are so arranged and constructed that the spacing between them is controlled and maintained at a desired predetermined distance wherein the belts follow an inclined path deiined by their natural curvature.

It is another object of the present invention to provide a process and apparatus `for continuous casting of acrylic and other polymerizable monomers comprising a pair of moving endless belts which are inclined in such a manner that the pressure of the materials between the belts is sufcient to maintain the spacing between them at a desired predetermined distance and wherein the belts follow an inclined path defined by their natural curvature.

Other objects and advantages of the present invention, it is believed, will be apparent from the following detailed description of specific embodiments thereof.

Broadly, the apparatus of the present invention cornprises a pair of endless belts mounted on rollers in such a manner that the lower run of the upper belt is substantially parallel to the upper run of the lower belt for a substantial portion of their lengths. These belts are arranged such that at least a portion of the lower run of the upper belt and the upper run of the lower belt follow an inclined path defined by the natural curvature of the belts. Preferably, the upper belt is held under sufhcient tension to tend to cause the lower run thereof to be held against support rollers positioned above this run while the lower belt is held under less tension to permit the upper run thereof to relax against rollers positioned below this run. In operation, each of the belts is caused to move around its supporting rollers such that the lower run of the upper belt and the upper run of the lower belt move in the same direction and such that there is substantially no relative movement between the lower run of the upper belt and the upper run of the lower belt. Liquid polymerizable monomer is fed to the intake ends of these belts and is passed through at least one heating station, In the preferred embodiment of the process of the present invention, a liquid acrylic monomer composition is fed into the casting apparatus and is first heated to a suicient extent to cause the monomer to polymerize (or cure) and is then subjected to a further heatinsy step to postcure the polymerized composition. The initial heating step may be performed by hot air heating, by causing the moving belts to pass through a zone which is liooded with hot water, or preferably, may be accomplished by spraying hot water on the external surfaces of the moving belts. In any event, the critical requirement of the apparatus and process of the present invention is that the belts follow an inclined path deiined by their natural curvature whereby a pressure head of the liquid monomer is developed which is suhcient to maintain the spacing between the belts at the desired distance until the liquid has polymerized.

Referring now to the drawings, FIGURE la illustrates a side view of a portion of the apparatus of the present invention wherein the path defined by the moving belts is the natural curvature of these belts.

FIGURE lb illustrates the remainder of the apparatus illustrated in FIGURE la.

FIGURE 2 is a vertical cross-section taken on line 2-2 of FIGURE 1a which illustrates the spray means used as a source of heating medium.

FIGURE 3 is a Vertical cross-section taken on line 3-3 of FIGURE 2 which illustrates a side view of the spray means and moving belts.

FIGURE 4 is a vertical cross-section taken on line 4 4 of FIGURE 1b and shows the moving belts passing through the postcuring oven.

FIGURE 5 is a vertical cross-section taken on line 5 5 of FIGURE 4 and illustrates a side View of the moving belt passing through the postcuring oven.

As shown in FIGURE 1a and FIGURE lb, the apparatus of the present invention comprises an upper belt 11 which is positioned above a lower belt 12. Upper belt 11 is supported on rollers 13 and 14 while lower :belt 12 is supported on rollers 15, 16 and 17. As is seen most clearly in FIGURE 1a, lower belt 12 is somewhat longer than upper belt 11 so as to provide exposed area 18 which serves as a convenient means Ifor feeding liquid monomer between rollers 13 and 16. Support posts 19 which are provided with rollers 20 and rollers 21 provide additional support for the belts 11 and 12. Roller 14 is mounted on movable support 22and support 22 is in turn connected to piston 23. Thus, the tension exterted by the rollers on belt 11 may be adjusted by actuating piston 23 so as to move support 22 and roller 14. Roller 15 is similarly mounted on movable support 24 which support is connected to piston 25 so that the tension exerted by the rollers on belt 12 may also be adjusted. Rollers 14 and 17 are driven by motor 26 through chains 27 and 28.

Also illustrated in FIGURES la and 1b are the housing 29 for the spray means which are used to provide the heat for polymerization of the monomer and housing 30 of the postcuring oven. These heating means are illustrated in more detail in FIGURES 2-5.

As shown in FIGURES 2 and 3, housing 29 encloses a series of spray means, some of which are positioned above the lower run of the upper belt and are indicated generally by numeral 31 and some of which are positioned below the upper run of the lower belt and are indicated generally by numeral 32. Preferably, spray nozzles 33 are so constructed such that a square pattern is sprayed by each nozzle on belts 11 and 12 in the manner indicated by the broken lines emanating from each nozzle 33. Each of the nozzles 33 is mounted on one of pipes 34 and the pipes 34 are connected to conduits 35. Conduits 35 are connected to a source of hot water or other suitable liquid (not shown). As shown in FIGURE 2, housing 29 is provided with supporting gusset 36. Gaskets 50 and 51 are also provided between belts 11 and 12 to prevent leakage of the polymerizable composition.

As further shown in FIGURES 2 and 3, a series of rollers 37 are provided above the lower run of belt 11 and a series of rollers 38 are provided beneath the upper run of belt 12. Rollers 33 function to provide the necessary support to maintain belt 12 in the desired inclined path. As previously pointed out, the spacing between belts 11 and 12 is maintained by the fluid pressure exerted by thel material being cast between them. However, it has surprisingly been found that even when thin sheets are being cast the fluid pressure is sutiicient to cause irregularities unless rollers 37 are provided to maintain belt 11 in the desired inclined path which is generally parallel to belt 12. Furthermore, since shrinkage generally occurs during polymerization, this shrinkage must be compensated for by causing belts 11 and 12 to assume a path which converges to the same extent as the decrease in thickness caused by polymerization shrinkage. Preferably, this is accomplished by providing precisely adjustable mountings for rollers 37, but any suitable means, such as springloaded mountings, may be used. Rollers 37 and 38 are preferably arranged such that they form a square pattern, i.e., such that each roller is equidistant from its adjacent rollers in both the lateral and longitudinal directions.

Preferably, housing 29 is constructed in sections, two of which are indicated generally by numerals 39 and 40 in FIGURE 3. Each of these sections is supported at one end by support member 19 and at the other end by support member 41 as seen most clearly in FIGURE la. As shown in FIGURE 3, the exit end of each section is provided with wiper means 42 and 43 which substantially prevent the water or other liquid on the surfaces of belts 11 and 12 from leaving section 39. lt is to be understood that the water flooding heating means described and illustratedin FIGURE 7 of the copending Opel et al. application could also be used as could any other suitable heating means.

Referring now to FIGURES 4 and 5, we find the postcuring section which is surrounded by housing 30. As shown in FIGURE 5, housing 30 is adjacent to the last section of housing 29 and wiper means 44 and 45 are provided to substantially prevent the water or other liquid on the surfaces of belts 11 and 12 from entering housing 30. A series of upper rollers 46 and a series of lower rollers 4'7 are also provided in housing 30 to supportv belts 11 land 12, in much the same manner as rollers 37 and 38 1n housing 29. However, rollers 46 and 47 are so arranged that the belts 11 and 12 assume a converging path in order to compensate for the shrinkage which might occur during postcuring. If the product shrinks away from the belts, certain defects including surface irregularities will occur in the product. Preferably, rollers 46 are spring mounted in order to urge belt 11 toward belt 12 and thereby compensate for the shrinkage. It is also preferred to provide rollers 38 and 47 with lixed mountings. Hous ing is preferably provided with an electrical heating means, but any suitable heating means, such as a gas burning heater, may be used.

Referring once again to FIGURE la, washing station 48 may be provided for upper belt 11 and washing station 49 may be provided for lower belt 12. These washing stations simply function to remove any materials which remain on the surfaces of belts 11 and 12 and may comprise any conventional means, such as spray means, ro-tating brushes, solvent solution, etc. It is, of course, desirable to remove any deposits which may be present on the surfaces of belts 11 and 12 since such deposits would otherwise result in the formation of irregularities on the surface of the cast sheet.

In the operation of the apparatus of the present invention, a fluid composition comprising a polymerizable organic monomer is fed to the inlet end of the casting apparatus and is polymerized while being carried between the moving belts which are substantially parallel for a large portion of the time during which they are in contact with the composition being polymerized and cast. These belts have substantially no relative motion during the time they are in contact with the composition being poiymerized and cast and are inclined at an angle to the horizontal.

The configuration of the curved path dened by the lower run of the upper belt and the upper run of the lower belt will be the natural sag or natural curvature of these belts. As used herein, natural curvature means that curvature which a given belt would assume when the ends thereof are supported at a given height under a given tension. It has been found preferable to calculate the natural curvature for a given tension and to arrange support rollers 37 and 38 such that they assume the path of this natural curvature and then to hold the upper belt under greater tension than the given tension and the lower belt under less tension than Ithe given tension such that the tension on the upper belt tends to pull the lower run thereof against rollers 37 and the upper run of the lower belt can relax against rollers 38. Thus, in a preferred embodiment, the stainless steel belts are held under a tension of about 5100 p.s.i. for the lower belt and 7000 p.s.i. for the upper belt, these being somewhat less than maximum tension, maximum tension being defined as the greatest tension to which the belts may be subjected without reaching the yield point. The concept of natural curvature and the methods of calculating this curvature are known to those skilled in the art and, as such, do not forma par-t of the present invention. For example the considerations are discussed in The Sandvik Steel Band Conveyor Handbook, published in 1956 by The Sandvik Steel Works Co., Ltd., which is incorporated by reference herein.

In the preferred embodiment of the present invention, the curved incl-ine of the belts extends over only a portion of their path between the rollers. Thus, as shown in FIGURE lb, the path of the belts is substantially horizontal from a point shortly before they enter the postcuring unit housing 30 until they reach rollers 14 and 17 and the curved path extends between the last section of the housing 29 and rollers 13 and 16. The curved portion of Ithis path is tangent to the horizontal portion and this relationship taken with the length of the path beween rollers 13 and 16 and the horizontal portion plus the natural curvature of the belts under a given tension will determine the vertical distance above rollers 14 and 17 to which rollers i3 and 16 must be raised.

The advantages of using the natural curvature configuration are many. Among these advantages are the reduction of supporting structure to a minimum because the belts assume their natural position, greater ease in maintaining control over the spacing between the belts, and, of Igreat importance in commercial operations, a substantial reduction in height of rollers 13 and 16 under that which would be necessary if the belts followed a straight inclined path. This reduction in height is of great importance because it reduces the need to build extremely tall buildings to house the apparatus of the present invention. This interest in keeping the height of rollers 13 and 16 to a minimum is also the primary reason for maintaining the path of the belts through the postcuring ovens on a horizontal level. It has been found that by the time the polymerized material has reached housing 30 it is sufiiciently rigid that it is capable of resisting downward movement of the upper belt and is thus capable of maintaining the spacing between the belts. Unexpectedly, it has been found that the use of the natural curvature configuration does not in any way interfere with the action of the fluid pressure developed by the composition being cast from maintaining the spacing between the belts in the inclined portion of the path lthereof at the desired distance.

The compositions which may be cast according to the present invention may be various types of liquids including monomeric compounds in the liquid state, preferably when at normal atmospheric pressure, partially polymerized monomeric -compounds and solutions of soluble polymeric resins in Amonomeric compounds. In addition, pastes may be employed including mixtures of liquid monomeric and solid polymeric organic compounds which are at least partially soluble in the monomeric compounds. Monomeric compounds which are in the liquid state at normal atmospheric pressure may contain one CH2=C group per molecule, eg., methyl methacrylate, butyl methacrylate, styrene and vinyl acetate, or more than one CH2=C group per molecule, e.g., glycol dirnethacrylate, allyl methacrylate, diallyl oxalate, etc. Monomeric compounds which contain more than one CH2=C group per molecule when polymerized alone, or in the presence of a compound containing more than one CH2=C group per molecule and which is not an overwhelmingly major component of the mixture, e.g., not more than about first form viscous liquids, then gels which have only a small solubility in monomers and then insoluble solid polymeric resins. Thus, not all partially polymerized or fully polymerized monomeric compounds are suitable for use in the present invention particularly when the monomers contain more than one CH2=C group per molecule.

The liquids and pastes used in the present invention may contain plasticizers such as tricresyl phosphate or dibutyl phthalate; fillers such as finely divided inorganic materials; reinforcing materials such as fabrics, glass fibers, wire, etc.; dyes and pigments, providing that these materials do not prevent the polymerization of the monomeric compounds contained in these mixtures. The monomeric material may also contain a parting agent to faciiltate separation of the belts from the cast product and a polymeriztion catalyst such as benzoyl peroxide or acetyl peroxide.

Patterned or contoured surfaces may be produced on the cast sheet by providing a matrix having a surface which is the negative of that desired on the surface of the cast sheet between one or both of the moving belts and the composition being cast. This matrix may be attached to the surface of the moving belt or may be mounted on separate rollers operated at such a speed so as to move the matrix material at substantially the same linear speed as that of the moving belts.

The present invention is further illustrated by the following example in which all proportions are by weight:

'3" EXAMPLE 1 A viscous syrup comprising 98% methyl methacrylate, 1.5% lauroyl peroxide (catalyst) and .00Ol% Zelec (parting agent), 0.5% ethylene glycol dimethacrylate and 0.05% Tinuvin P (ultraviolet absorber) is fed to the inlet end of an apparatus of the type illustrated in FGURES la and lb. The length of the curved portion of the path of these belts is approximately 135 feet While the length of the horizontal portion of these belts is approximately 30 feet. The vertical distance between the top and bottom of the curved portion was 6 feet. The lbelts are 54 inches wide. The temperature in housing 29 is maintained at 150 F. by hot water spray means of the type illustrated in FIGURES 2 and 3 and the temperature in housing 30 is maintained at 250 F. by electric heating means to postcure the cast sheet. The speed of the belts is 3 feet per minute. The belts are set 1/s-inch apart to produce a cast sheet having a thickness of approximately 1/s-inch. These sheets are transparent, have extremely smooth surfaces, are free from internal voids or bubbles and are substantially free from internal strain.

The belts used were fabricated from highly polished stainless steel.

lt is to be expected that certain variations of the specic embodiments disclosed herein will be devised by those skilled in the art. Thus, the present invention is of the scope of the appended claims.

I claim:

1. An apparatus for continuously casting a polymerizable organic monomer comprising a first endless liexible belt and a second endless flexible belt, said belts being disposed such that the lower run of said first belt is positioned above the upper run of said second belt, said lower and upper runs passing through a casting zone, said casting zone having a material-receiving end and a materialdischarging end, means for feeding liquid material to said material-receiving end, said material-receiving end being at a higher elevation than said material-discharging end, said lower and said upper runs being inclined throughout at least a substantial portion of said casting zone, the path of said incline being defined by the natural curvature of said belts, said natural curvature being substantially that which said belts would assume if supported only at the elevation of the material-receiving end and at the elevation of the material-discharging end, seal means positioned between the edges of said runs in said casting zone, means for heating said casting zone and means for driving said belts through said zone.

2. The apparatus of claim 1 including support means for said upper run, said support means defining a path substantially the same as that of said natural curvature of said run, backing means for said lower runs, said backing means defining a path substantially the same as that of said natural curvature of said run.

3. The apparatus of claim 2 wherein said belts are fabricated from stainless steel.

4. The apparatus of claim 1 wherein said heating means comprise spray means.

5. T he apparatus of claim wherein said support means and said backing means comprise rollers.

6. The apparatus of claim 2 including tension means maintaining said lower run of said first belt at a tension greater than that used to define the natural curvature and tension means maintaining said upper run of said second belt at a tension less than that used to define said natural curvature.

7. An apparatus for continuously casting a polymerizable organic monomer comprising a iirst endless flexible belt and a second endless flexible belt, said belts being disposed such that the lower run of said first belt is positioned above the upper run of said second belt, said lower and upper runs passing through a casting zone, said casting zone having a material-receiving end and a materialdischarging end, means for feeding liquid material to said material-receiving end, said material-receiving end being at a higher elevation than said material-discharging end, said lower and said upper runs being inclined throughout at least a substantial portion of said casting y'zone, the difference in elevation between said materialreceiving end and said material-discharging end being sufficient to cause a liuid head to be developed by liquid carried between said runs such that the fluid pressure exerted by said liquid is sufficient to maintain said runs substantially parallel to each other and the spacing between said runs at a predetermined distance in said Zone, said irst belt being of such size and weight that the lower run thereof would bow downwardly toward the upper run of said second belt if said runs were horizontal in said zone, the path of said incline being defined by the natural curvature of said belts, said natural curvature being substantially that which said belts would assume if supported only at the elevation of the material-receiving end and at the elevation of the material-discharging end, seal means positioned between the edges of said runs in said casting zones, means for heating said casting zone and means for driving said belts through said zone.

3. The apparatus of claim 7 including support means for said upper run, said support means defining a path substantially the same as that of the natural curvature of said run, backing means for said lower run, said backing `means defining a path substantially the same as that of said natural curvature of said lower run.

9. The apparatus of claim 8 including tension means for maintaining said lower run of said first belt at a tension greater than that used to deline the natural curvature of said run and tension means maintaining said upper run of said second belt at a tension less than that used to define said natural curvature.

10. The apparatus of claim 8 wherein said support means and said backing means comprise rollers.

11. The apparatus of claim 7 wherein said heating means comprises a spray means.

l2. The apparatus of claim 7 wherein said belts are fabricated from stainless steel.

13. The apparatus of claim 7 wherein said incline comprises a first portion of said casting zone and wherein the second and remaining portion of said casting zone extends substantially horizontally from said first portion and is substantially straight longitudinally and transversely.

14. The apparatus of claim 13 wherein the backing means and support means extend through said second portion and said backing means are resiliently mounted in said second portion.

1S. In apparatus for continuously casting a solid sheet from a flowable material, the combination of, a first endless liexible belt, a second endless flexible belt, means mounting said flexible belts with a run of said first belt positioned above a run of said second belt and spaced therefrom to form a thin casting cavity therebetween throughout a casting zone, said casting cavity having a material-receiving end and a product-discharging end, means closing said casting cavity at the opposite edges of said runs, and means to move said runs through said casting zone at identical rates of movement whereby opposed surfaces of said runs move together through said casting zone from said material-receiving end to said product-discharging end, the portion of said casting cavity adjacent said material-receiving end being a fluid cavity into which the material moves and forms a body of the material in Huid condition, said casting cavity between ,said fluid cavity and said product-discharging end being a product-solidifying and solid-product cavity in which the material is cast and forms into the solid sheet, said runs being inclined downwardly from said material-receiving end of said casting cavity whereby said material receiving end is at a higher elevation than the remaining portion of said fluid cavity and is closed at its lower end by the solidied product in said product-solidifying and solid-product cavity whereby said fluid cavity is closed except at said material-receiving end and the material in uid condition exerts fluid head pressures against the surfaces of said runs to maintain said runs in uniform spaced relationship, said runs following a path `defined by the natural curvature of said belts throughout the extent of said uid cavity, said natural curvature being substantially the path which said runs would assume if supported only at said material-receiving end and below the lower portion of said fluid cavity.

16. Apparatus as described in claim 15 wherein each of said flexible belts is a thin metal belt, and wherein said means mounting said belts includes means against which said runs are pressed by said material in fluid condition.

References Cited UNITED STATES PATENTS 1,808,525 6/1931 Cadden 18--2 2,311,617 2/ 1943 Hansen 18-2 2,338,770 l/ 1944 Leguillon 18-2 2,867,843 1/ 1959 Browne et al. 18-4 2,929,793 3/ 1960 Hirsh 18-4 3,007,206 11/ 1961 Griswold.

3,011,211 12/1961 Barnes 18-2 3,016,573 1/1962 Blue 18-26 3,269,882 8/1966 Willy 18-4 3,250,841 5/1966 Reinhart 18-2 3,262,151 7/ 1966 OXel 18-4 3,274,643 9/1966 Oxel 18-4 3,278,659 11/1966 Willy 18-4 WILLIAM I. STEPHENSON, Primary Examiner.

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