JP4920341B2 - Catalyst carrier for exhaust gas purification - Google Patents

Description

  The present invention relates to a catalyst carrier for exhaust gas purification. That is, the present invention relates to an exhaust gas purifying catalyst carrier having a honeycomb structure to which a catalyst substance is adhered.

《Technical background》
For example, exhaust gas from automobile diesel engines and other engines contains particulate matter PM, nitrogen oxides NO _x , and other harmful substances, which are harmful if discharged directly into the outside air.
Therefore, an exhaust gas purification device that removes harmful substances contained in the exhaust gas is interposed in the exhaust pipe of the engine.
As this type of exhaust gas purifying apparatus, a catalyst carrier having a honeycomb structure and having a catalytic substance attached thereto is frequently used.

<Conventional technology>
FIG. 4B is a front sectional view of this type of conventional example. As shown in the figure, the conventional catalyst carrier 1 for purifying exhaust gas A has a honeycomb structure 4 in which a corrugated sheet 2 made of a metal foil having a strip shape and a flat plate 3 are wound in multiple layers. The structure is inserted into the outer cylinder 5.
The exhaust gas A from the engine passes through each cell space 7 of the honeycomb structure 4 of the catalyst carrier 1 interposed in the exhaust pipe 6. The harmful substances contained in the exhaust gas A come into contact with the catalytic substances attached to the corrugated plates 2 and the flat plates 3 forming the cell spaces 7 of the honeycomb structure 4 and are reacted and removed. Therefore, the exhaust gas A was purified and discharged to the outside air.

The catalyst carrier 1 is thus exposed to the high-temperature exhaust gas A and is also subjected to vibrations from the engine or the like, and is used in a very severe environment. Therefore, various devices have been made to cope with this.
For example, regarding the bonding between the corrugated sheet 2 and the flat plate 3 of the honeycomb structure 4 and the bonding between the honeycomb structure 4 and the outer cylinder 5, various types such as brazing bonding, solid phase diffusion bonding, laser bonding, high-frequency heating bonding, etc. The method was developed and devised.
In addition, various methods such as full-surface bonding and specific-portion bonding have been developed and devised with respect to the bonding points. Furthermore, various developments and devices have been made for winding the corrugated plate 2 and the flat plate 3 of the honeycomb structure 4 and the shape of the cell space 7.
As described above, various ideas and developments have been made on the bonding method for restricting the movement in the axial direction X of the honeycomb structure 4 inserted into the outer cylinder 5.

《Information on prior art documents》
As a conventional example of such a catalyst carrier 1, for example, the one shown in the following Patent Document 1 can be cited.
JP 2001-321678 A

By the way, the following problems have been pointed out with respect to such a conventional catalyst carrier 1.
About the problem
As described above, this kind of conventional catalyst carrier 1 has been pointed out that various troubles are still generated despite various developments and ingenuity as countermeasures against high temperatures and vibrations. That is, abnormal oxidation, thermal stress failure, fatigue failure, etc. occurred at the joints, which were problematic.
More specifically, first, the temperature distribution of the exhaust gas A, the velocity distribution of the exhaust gas A when flowing into the catalyst carrier 1, the temperature rise due to the reaction with the catalyst substance, the natural frequency of the catalyst carrier 1 itself, the exhaust gas All of the conditions such as the natural frequency of the system vary depending on the engine / exhaust system, and it is difficult to cope with it by simply selecting one of the above-mentioned methods.
That is, due to temperature unevenness and speed unevenness of the high-temperature exhaust gas A, temperature rise due to the catalyst material, etc., the joined portion of the catalyst carrier 1 is abnormally oxidized, or the joined catalyst carrier 1 is caused by internal temperature unevenness. It was easy to break by thermal stress. Further, since the vibration of the engine overlaps with the natural frequency of the catalyst carrier 1 and the exhaust system, the joined catalyst carrier 1 is easily fatigued at an early stage.
In addition, such abnormal oxidation, thermal stress failure, fatigue failure, etc., become troubles either singly or in combination, and it has been extremely difficult to identify and deal with the cause. Actually, it is necessary to make a prototype, test, and select the catalyst carrier 1 having a specification adapted to a specific engine, and it is not easy to cope with it.

<< About the present invention >>
The catalyst carrier for purifying exhaust gas of the present invention has been developed in order to solve the above-described problems of the conventional examples in view of such circumstances.
An object of the present invention is to propose a catalyst carrier for purifying exhaust gas that can avoid troubles such as abnormal oxidation, thermal stress failure, fatigue failure and the like.

<About Claim>
The technical means of the present invention for solving such a problem is as follows. First, claim 1 is as follows.
That is, in the exhaust gas purifying catalyst carrier according to claim 1, the honeycomb structure having a roll shape in which a corrugated metal plate made of a metal foil and a flat plate are wound in multiple layers is inserted into the outer cylinder. The catalytic material is adhered to the corrugated plate and the flat plate.
The outer cylinder includes a main body portion having substantially the same axial length as the honeycomb structure, and both end portions protruding from the main body portion in the front-rear axial direction. The honeycomb structure is positioned and held immovably in the axial direction in the main body portion of the outer cylinder by the restriction section.
The restriction section uses a slit provided at the boundary between the main body portion and both end portions of the outer cylinder, and a portion corresponding to the slit is pressed and bent toward the inside in the radial direction. It is characterized by being formed.
Next, claim 2 is as follows. That is, the catalyst carrier for purifying exhaust gas according to claim 2 is characterized in that, in claim 1, the restriction section of the metal outer cylinder has a protruding end portion that is pushed inward in the radial direction and is bent between each other. Alternatively, the outer cylinder is in contact with and welded to a portion where the slits are not formed at both ends of the outer cylinder.

《Action etc.》
Since the present invention comprises such means, the following is achieved.
(1) Exhaust gas from the engine is supplied to a catalyst carrier for exhaust gas purification.
(2) The catalyst carrier has a structure in which a honeycomb structure is inserted in an outer cylinder, and the honeycomb structure has a corrugated plate and a flat plate wound in multiple layers in a roll shape, and a catalyst substance is attached thereto.
(3) Therefore, when the exhaust gas passes through the honeycomb structure, the contained harmful substances come into contact with, react with, and are removed from the catalyst substance, and are purified.
(4) The honeycomb structure of the catalyst carrier is positioned and held immovably in the axial direction in the main body of the outer cylinder by the restriction section.
That is, for the outer cylinder, a slit is provided at the boundary between the main body portion and both end portions protruding in the axial direction, and a restriction section is formed by using this slit, and the restriction section is a portion corresponding to the slit at both ends of the outer cylinder. Is pressed and bent inward in the radial direction.
(5) As described above, the honeycomb structure is restricted from moving in the axial direction between the corrugated plate and the flat plate or between the outer cylinders by adopting the regulation section. In particular, when the protruding end of the restriction section is welded, such a stationary rule is further ensured. Also, the movement in the radial direction is restricted by the main body portion of the outer cylinder. The circumferential movement is regulated by the frictional force between them.
(6) Therefore, in this catalyst carrier, brazing or other joining methods are not employed for the inside of the honeycomb structure or between the outer cylinders. Therefore, abnormal oxidation, thermal stress failure, fatigue failure, etc. do not occur at the joint.
In other words, the catalyst carrier is exposed to high-temperature exhaust gas, and the exhaust gas is accompanied by temperature unevenness and speed unevenness. Further, the temperature rises due to the reaction with the catalyst material, but because the brazing material is not used, there is an abnormality at the joint. Oxidation does not occur. Further, since there are no joints, thermal stress failure does not occur even under such temperature conditions, and vibration from the engine or the like is added, but fatigue failure is also avoided.
(7) The catalyst carrier of the present invention exhibits the following effects.

As described above, the catalyst carrier for purifying exhaust gas of the present invention employs the restriction section using the outer cylinder, so that the honeycomb structure is positioned and held immovably in the axial direction. No brazing or other joining methods are employed between the outer cylinder and the outer cylinder.
Therefore, unlike the above-described conventional catalyst carrier in which various joining methods are employed, abnormal oxidation, thermal stress failure, fatigue failure, etc. do not occur at the joining point, and troubles can be avoided. .
Further, these are realized with excellent cost by slit processing, bending of the outer cylinder, and further welding of the protruding end portion of the restriction section. In other words, unlike the above-described conventional brazing joint method, there is no need for brazing material costs and equipment costs such as a vacuum furnace. No cost is required, and further, the laser and high-frequency equipment costs are not required as in the laser bonding method and the high-frequency heating bonding method.
As described above, the effects exerted by the present invention are remarkably large, such as all the problems existing in this type of conventional example are solved.

《About drawing》
Hereinafter, the catalyst carrier for purifying exhaust gas of the present invention will be described in detail based on the best mode for carrying out the invention shown in the drawings. 1 and 2 serve to explain the best mode for carrying out the present invention.
1 (1) is a front view and a side view of the first example, FIG. 1 (2) is a side view of the second example, and FIG. 3 (3) is a side view of the third example. (4), (5) figure is the front view 2 form of a 3rd example. 2A is a front view and a side view of the fourth example, and FIG. 2B is a front view and a side view of the fifth example.
FIG. 3 is a perspective view for explaining an exhaust gas purifying catalyst carrier. (1) FIG. 3 shows corrugated plates and flat plates used, and (2) FIG. 3 shows corrugated plates and flat plates to be wound. (3) The figure shows a catalyst carrier. FIG. 4A is an enlarged side sectional view of the main part.

<< About catalyst carrier 1 >>
First, the exhaust gas A purification catalyst carrier 1 will be generally described with reference to FIG. 3 and FIG. 4 (1).
Particulate matter PM, nitrogen oxides NO _x , and other harmful substances are contained in the exhaust gas A exhausted from engines of automobile engines, internal combustion engines such as generators, locomotives, and various mechanical equipment. If it is discharged as it is, it is harmful to the human body and the environment.
Therefore, the exhaust pipe 6 of the engine (see FIG. 4 (2)) includes a catalyst carrier for purifying exhaust gas A as part of an exhaust gas purifying apparatus for removing harmful substances contained in exhaust gas A. 1 is used.
The catalyst carrier 1 includes a corrugated sheet 2 made of a metal foil having a strip shape and a flat plate 3 wound in multiple layers, and a honeycomb structure 4 having a roll shape is inserted into the outer cylinder 5 and the corrugated sheet. 2 and the flat plate 3 are attached with the catalyst substance B.

Such a catalyst carrier 1 will be further described in detail. First, as shown in FIG. 3 (1), the corrugated plate 2 and the flat plate 3 are made of, for example, stainless steel foil or other metal foil and have a strip shape. The corrugated plate 2 has corrugated irregularities that are linearly parallel to the short direction perpendicular to the long side of the band and repeatedly bent continuously at a predetermined pitch and height in the longitudinal direction. .
Then, as shown in FIG. 3B, the corrugated plate 2 and the flat plate 3 are wound in multiple layers while being alternately superposed around the axis.
Therefore, as shown in FIG. 3 (3), the corrugated sheet 2 and the flat plate 3 constitute a honeycomb structure 4. The honeycomb structure 4 has, for example, a substantially cylindrical roll shape as a whole, and includes corrugated plates 2 and flat plates 3 as cell walls, and hollow columnar cell spaces 7 that are partitioned into independent spaces along the axial direction X. It consists of a planar assembly and its both end faces are open.
In the catalyst carrier 1, such a honeycomb structure 4 is inserted into an outer cylinder 5 which is a case.
Further, as shown in FIG. 4A, the catalytic substance B is attached to the outer surfaces of the corrugated plate 2 and the flat plate 3 which are cell walls. That is, using the cell walls of the honeycomb structure 4 having the feature that the surface area per unit volume is large, that is, the outer surfaces of the corrugated plate 2 and the flat plate 3, the catalytic substance B is attached in a film form, Therefore, a wide contact area with the exhaust gas A is secured. As the catalyst substance B, for example, platinum for oxidation reaction or other noble metals, or a substance for reduction reaction is used.
The catalyst carrier 1 is roughly as described above.

<< About the present invention >>
The features of the catalyst carrier 8 for purifying exhaust gas A according to the present invention will be described below with reference to FIGS.
The catalyst carrier 8 includes a honeycomb structure 9 and an outer cylinder 10. The outer cylinder 10 includes a main body 11 having substantially the same length in the axial direction X as the honeycomb structure 9, and both end portions 12 extending from the main body 11 in the front-rear axial direction X and projecting. The honeycomb structure 9 is positioned and held in the main body portion 11 of the outer cylinder 10 by the restriction section 13 so as not to move in the axial direction X.
The restriction section 13 is formed by using a slit 14 provided at the boundary between the main body portion 11 and the both end portions 12 of the outer cylinder 10. That is, at both end portions 12 of the outer cylinder 10, portions corresponding to the slits 14 are formed by being pressed and bent toward the inside in the radial direction Y.
The catalyst carrier 8 of the present invention is as described above.

About the details
Such a catalyst carrier 8 will be further described in detail. First, the slits 14 are respectively formed at the boundary between the main body portion 11 of the outer cylinder 10 and both end portions 12 protruding in the front and rear axial direction X (that is, in the vicinity of the front and rear end face positions of the honeycomb structure 9 inserted therein). Is done. The formation is performed using a press, a saw board, a water jet, a laser, and other various methods.
The slits 14 are appropriately formed on each side of the both end portions 12, but about one to three are typical. In the example of FIG. 1 (1), FIG. (2), and FIG. 2 (2), two are formed, and in the example of FIG. 1 (3), (4), and (5), One is formed, and three are formed in the example of FIG.
Further, the formation length of the slit 14, that is, the drilling length along the outer periphery of the cylindrical outer cylinder 10 can be various. For example, in the case where two or more are formed per one side of the both end portions 12, as shown in the example of (2) in FIG. It is good also as a length dimension common to each book like the example of illustration.

And the both ends 12 which protruded from the main-body part 11 of the outer cylinder 10 to the front-back axial direction X utilize the slit 14 formed in this way, and are located in the part (it is located outside the slit 14). (1), (2), (3), and (1), (2) in FIG. 2 from the outside toward the center in the radial direction Y, as shown in FIG. It is pushed and bent, so that it becomes the regulation section 13.
Therefore, the number of restriction sections 13 corresponding to the number of slits 14 is formed at both ends 12. That is, two regulation sections 13 are formed on both sides of the both ends 12 of the outer cylinder 10 in the example of FIG. 1 (1), (2), and FIG. 2 (2), respectively. In the example shown in FIGS. (3), (4), and (5), one is formed, and in the example shown in (1) in FIG. 2, three are formed.

And the shape of the regulation section 13 formed by pushing and bending using the slit 14 in this way can be various. For example, as shown in FIG. 1 (1), FIG. (2), FIG. 2 (1), and (2), as shown in FIG. As shown in the figure, a shape in which the rounded portions are continuous is also possible.
Further, the number and position of the slits 14 and the number, position, and shape of the restriction sections 13 to be formed do not need to be the same for both surfaces of the both end portions 12 of the outer cylinder 10. For example, in the example shown in FIG. 5 (5), the positions of the slit 14 and the regulation section 13 formed are shifted 180 degrees around the circumference between the one surface and the other surface of the both ends 12.
Of the both end portions 12 of the outer cylinder 10, a portion that does not correspond to the slit 14, that is, a portion that is not pushed in and that does not form the regulation section 13 is left as it is, that is, a state that protrudes on the circumference of the outer cylinder 10. Become.

Now, in the honeycomb structure 9 inserted into the outer cylinder 10, the front and rear end faces are locked from the outside in the axial direction X in the restriction section 13 thus pushed toward the center in the radial direction. Therefore, it is positioned and held immovably in the front and rear axial directions X.
Note that the front and rear end faces of the honeycomb structure 9 are not limited to flat faces, that is, flat faces parallel to the radial direction Y. For example, as shown in the example of FIG. Alternatively, it is conceivable to form both end faces in a central convex shape regardless of the illustrated example. In this case, the boundary between the main body portion 11 and the both end portions 12 of the outer cylinder 10 is also formed in a corresponding curved shape, so that the slit 14 is curved at the boundary, and the regulation section 13 is also formed with curvature inside.
The catalyst carrier 8 is as described above.

<About the action>
The catalyst carrier 8 for purifying the exhaust gas A of the present invention is configured as described above. Therefore, it becomes as follows.
(1) The exhaust gas A from the engine contains harmful substances and is supplied to the catalyst carrier 8 for purifying the exhaust gas A.

  (2) The catalyst carrier 8 has a structure in which the honeycomb structure 9 is inserted into the outer cylinder 10. The honeycomb structure 9 includes a corrugated sheet 2 made of metal foil and a flat plate 3 each having a strip shape and wound in multiple layers in a roll shape, so that a large number of cell spaces 7 are formed by the corrugated sheet 2 and the flat plate 3. The catalytic substance B is adhered to the corrugated plate 2 and the flat plate 3 (see FIGS. 3 and 4 (1)).

  (3) Therefore, the exhaust gas A passes through the cell space 7 of such a honeycomb structure 9, and at that time, the contained harmful substance comes into contact with the catalyst substance B and, for example, undergoes oxidation reaction or reduction reaction. Is removed and rendered harmless. Thus, the exhaust gas A is purified and discharged to the outside air.

(4) By the way, the honeycomb structure 9 of the catalyst carrier 8 is positioned and held in the main body part 11 of the outer cylinder 10 by the restriction section 13 so as not to move in the axial direction X.
That is, the outer cylinder 10 of the catalyst carrier 8 includes a main body portion 11 having substantially the same length in the axial direction X as the honeycomb structure 9 and both end portions 12 protruding from the main body portion 11 in the front-rear axial direction X. . When the front and rear end surfaces of the honeycomb structure 9 form a flat surface (see the example in FIGS. 1 and 2 (1)), the main body 11 has the same X length in the axial direction as the honeycomb structure 9. . On the other hand, when the front and rear end faces of the honeycomb structure 9 have a central convex shape (see the example of FIG. 2 (2)), the slits 14 are also curved as shown in the figure, and the honeycomb structure 9 is the main body. It is always longer than the length X of the portion 11 in the axial direction.
The restriction section 13 is formed by using the slit 14 provided at the boundary between the main body portion 11 and the both end portions 12 of the outer cylinder 10. That is, the both ends 12 of the outer cylinder 10 are formed by pressing and bending the portions corresponding to the slits 14 (outer portions of the slits 14) inward in the radial direction Y (see FIGS. 1 and 2).

(5) In this way, in the honeycomb structure 9 of the catalyst carrier 8, the internal relationship surface, that is, the relationship between the corrugated sheet 2 and the flat plate 3, and the external relationship surface, that is, the relationship between the outer cylinder 10, The movement in the axial direction X is regulated so as not to move without using a joining method such as brazing.
That is, in this honeycomb structure 9, the movement in the axial direction X is restricted by the adoption of the restriction section 13 using both end portions 12 of the outer cylinder 10. Further, the movement in the radial direction Y is restricted by the main body portion 11 of the outer cylinder 10, and the movement in the circumferential direction is between each other (between the corrugated sheet 2 and the flat plate 3 or between the honeycomb structure 9 and the outer cylinder 10. Movement such as rotation is regulated by the frictional force between the main body portions 11).

(6) Therefore, according to the exhaust gas A purifying catalyst carrier 8 of the present invention, the following is obtained. That is, in this catalyst carrier 8, brazing or other joining methods are not employed between the corrugated plate 2 and the flat plate 3 of the honeycomb structure 9 and between the honeycomb structure 9 and the outer cylinder 10.
Therefore, abnormal oxidation, thermal stress failure, fatigue failure, etc. at the joints do not occur in the catalyst carrier 8.
That is, the catalyst carrier 8 is exposed to the high-temperature exhaust gas A, and the exhaust gas A is accompanied by temperature unevenness and speed unevenness, and further, the temperature rise due to the reaction with the catalyst substance B is added. Is not used and abnormal oxidation does not occur at the joint. Furthermore, in addition to such temperature conditions, vibration from the engine or the like is also applied, but since there are no joints in this way, internal tensile stress and internal compressive stress are not generated, and thermal stress failure is avoided. Moreover, fatigue failure does not occur at an early stage.

<Other examples>
In addition, this invention is not limited to the example demonstrated above, Furthermore, the following example is also possible. FIG. 5 is a side view of another example, in which two restriction sections 13 are formed in FIGS. (1) and (2), three in (3), and one in (4).
In these examples, the restriction section 13 of the metallic outer cylinder 10 has a plurality of protruding end portions 15 that are pressed and bent inward in the radial direction Y, or both end portions 12 of the outer cylinder 10. The portion where the slit 14 is not formed is abutted and welded.

These will be described in further detail. First, in each example of the present invention described with reference to FIGS. 1 and 2, the restriction section 13 is formed by pressing and bending the portions corresponding to the slits 14 at both ends 12 of the outer cylinder 10 inward in the radial direction Y. However, in the example of FIG. 5, the projecting end portion 15 thus pushed and bent is welded by welding Z.
In the example shown in FIGS. 5A and 5B, the two projecting end portions 15 of the regulation section 13 are brought into contact with each other and welded by welding Z. In the example of FIG. 5 (3), the three projecting end portions 15 of the restriction section 13 are in contact with each other and welded by welding Z.
On the other hand, in the example of FIG. 5 (4), one restriction section 13 is formed, and the protruding end portion 15 of this one restriction section 13 is connected to the slits 14 at both end portions 12 of the outer cylinder 10. The portion that is not formed (that is, the portion that does not correspond to the slit 14 and the portion that does not form the restriction section 13) is welded by welding Z.

And if the protrusion 15 of the regulation section 13 is welded as in the example of FIG. 5, the above-described immobility regulation of the honeycomb structure 9 is further ensured.
That is, the honeycomb structure 9 is continuously, closely, and reinforced by the welding of the protrusions 15 from the outside in the axial direction X, and the front and rear end surfaces are more reliably engaged in the reinforced restriction section 13. Stopped.
Therefore, the movement of the honeycomb structure 9 inserted into the outer cylinder 10 is more reliably regulated in the front-rear axial direction X, and is not displaced in the axial direction X based on the flow of the exhaust gas A. In this way, more reliable stationary positioning is realized by welding the restriction section 13.
The example of FIG. 5 is like this.

The exhaust gas purifying catalyst carrier according to the present invention will be described for the best mode for carrying out the invention. (1) FIG. 1 is a front view and side view of the first example, and (2) FIG. 2 is a side view of the third example, and FIG. 3 (3) is a side view of the third example. (4), (5) figure is the front view 2 form of a 3rd example. For the description of the best mode for carrying out the invention, (1) is a front view and a side view of the fourth example, and (2) is a front view and a side view of the fifth example. FIG. 5 is a perspective view for explaining an exhaust gas purifying catalyst carrier. And (1) figure shows the corrugated sheet and flat plate to be used, (2) figure shows the corrugated sheet and flat plate to be wound, and (3) figure shows the catalyst carrier. For explanation of a catalyst carrier for purifying exhaust gas, FIG. (1) is a side sectional view showing an enlarged main part. (2) The figure is a front sectional view of the whole of this kind of conventional example. The exhaust gas purifying catalyst carrier according to the present invention is used to explain the best mode for carrying out the invention and is a side view of another example. Two restriction sections are formed in the example of (1) and (2), three are formed in the example of (3), and one is formed in the example of (4).

Explanation of symbols

1 Catalyst carrier (conventional example)
2 Corrugated plate 3 Flat plate 4 Honeycomb structure (conventional example)
5 Outer cylinder (conventional example)
6 Exhaust pipe 7 Cell space 8 Catalyst carrier (present invention)
9 Honeycomb structure (present invention)
10 Outer cylinder (present invention)
DESCRIPTION OF SYMBOLS 11 Main-body part 12 Both ends 13 Control section 14 Slit 15 Protruding part A Exhaust gas B Catalytic substance X Axial direction Y Radial direction Z Welding

Claims (2)

A honeycomb structure having a roll shape in which a corrugated sheet and a flat plate made of metal foil forming a strip shape are wound in multiple layers is inserted into an outer cylinder, and a catalytic substance is adhered to the corrugated sheet and the flat plate. A catalyst carrier for exhaust gas purification,
The outer cylinder includes a main body portion having substantially the same axial length as the honeycomb structure, and both end portions protruding in the front-rear axial direction from the main body portion,
The honeycomb structure is positioned and held immovably in the axial direction in the main body portion of the outer cylinder by the restriction section,
The restriction section uses a slit provided at the boundary between the main body portion and both end portions of the outer cylinder, and a portion corresponding to the slit is pressed and bent toward the inside in the radial direction. A catalyst carrier for purifying exhaust gas, characterized in that it is formed.   2. The exhaust gas purifying catalyst carrier according to claim 1, wherein the metal-made outer cylinder restricting section has protrusions that are pushed inward in a radial direction and are bent between each other or the outer cylinder. A catalyst carrier for purifying exhaust gas, characterized in that it is brought into contact with and welded to the portions where the slits are not formed at both ends.

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