JP3967305B2 - Manufacturing method of piston for internal combustion engine - Google Patents

Description

  The present invention relates to a method of manufacturing a piston for an internal combustion engine by a die casting method.

Forged pistons for internal combustion engines used in automobile engines and the like have been mainly made of aluminum alloy forgings and gravity castings. However, recently, an aluminum alloy die-cast material has been used from the viewpoint of cost.
Although the die casting method is advantageous in terms of cost, gas entrainment defects are likely to occur because casting is performed at a high speed. In particular, since a piston for an internal combustion engine has a shape having a thick part, a cast hole is likely to occur at a thick part as shown by A in FIG. Moreover, since the plastic working is not performed after casting, the entrained gas and the cast hole cannot be crushed, and there is a problem in strength.

In order to improve the strength of the piston cast by the die casting method, Patent Document 1 proposes a method of locally pressurizing the top of the piston during die casting to improve the strength of the top by densifying the metal structure. .
In the method proposed in this document, a secondary pressurizer for pressurizing a molten metal cast into a die cavity and a hydraulic cylinder for driving the molten metal are incorporated into a die of a die casting machine. After being cast and before it solidifies, the secondary pressurizer is advanced from the direction perpendicular to the top surface of the head wall of the piston toward the die cavity, so that the molten metal in the molten metal reservoir is melted in the molten metal in the die cavity. The metal structure is to be pressed into the metal to make it dense. Since the die cavity is almost sealed, secondary pressurization is possible, and the metal structure densification is most advanced in the head wall, coinciding with the location and direction in which the explosion energy in the cylinder is directly applied to the piston Therefore, a piston for an internal combustion engine having excellent pressure resistance characteristics can be obtained.

JP 2001-232454 A

By the way, the technique described in Patent Document 1 intends to pressurize the top of the piston locally. For this reason, even if the strength of the piston top portion is improved, the effect of improving the strength cannot be obtained because the pin boss portion with a heavy mechanical load is away from the local pressurizing portion. In general, a casting hole, which is a typical example of a casting defect, is more likely to occur in a thick part than in a thin part. The top of the piston is thin and the pin boss is thick.Therefore, even if pressure is applied from the top of the piston, no pressure is transmitted to the thick pin boss, and molten metal is applied to the place where the cast hole is about to occur. Since it cannot be replenished, it is impossible to prevent the formation of a cast hole. In other words, since the thin top part solidifies in a short time, the solidified part hinders the transmission of force, so pressure is not transmitted to the unsolidified molten metal in the thick part. As a result, the molten metal cannot be replenished and a cast hole is generated and remains as it is. Furthermore, when local pressurization is performed, the surface solidified wall of the locally pressurized portion is mixed into the piston and the strength of the portion is reduced. Furthermore, burrs remain on the externally pressurized outer surface, and the dimensional accuracy also decreases. However, since the top of the piston is the place where the dimensional accuracy is most required, when pressure is applied from the top, it is necessary to cut and remove a considerable amount of the piston top. Casting in anticipation of this is unavoidable, resulting in a decrease in yield and an increase in cost.
The present invention has been devised to solve such a problem, and an object of the present invention is to produce a piston having few casting defects such as a cast hole and improved in strength by a die casting method with a high yield.

In order to achieve the object, the method for manufacturing a piston for an internal combustion engine according to the present invention casts a piston for an internal combustion engine having a substantially disc-shaped head wall and a pin boss portion and a skirt portion extending from the head wall by a die casting method. Before the molten metal is poured into the die cavity of the mold, a pressurizing member that applies local pressure is protruded into the cavity in advance and the molten metal is poured into the die cavity of the mold. and coagulating the surface layer of the poured been molten in a state that remains unsolidified portion inside, and characterized in that said further protruding the pressing member presses locally pressurizing the pin boss of the piston To do.
In the die casting method , the inside of the cavity of the mold is vacuum-sucked, and then the active gas is injected from the sleeve into the cavity by overlapping with vacuum suction, and the atmospheric pressure in the cavity is increased to atmospheric pressure or higher, and then the molten metal is added to the sleeve. It is preferable to apply a method in which the molten metal is injected into the cavity from the sleeve by advancing the plunger after the vacuum is sucked again and the pressure in the cavity is reduced to 30 kPa or less.

In the present invention, as shown in FIG. 2, a pin boss portion for opening a pin hole into which a pin connected to a piston and a connecting rod is inserted is locally pressurized ((a) in FIG. 2 is a pressurizing material before pressurization). (B) shows the position of the pressurizing material P after pressurization at the position of P).
And the problem of the pressurization method from a top part can be eliminated by adopting the local pressurization method from a pin boss part instead of the local pressurization from a top part. In other words, since the periphery of the pin boss is a thick part where a void is likely to occur in the piston, the molten metal can be efficiently replenished to the formed void by pressurizing this portion. Can be suppressed. In addition, since the thick portion is slower to cool than the thin portion, the structure tends to be larger than that of the thin portion. However, in the present invention, not only the top part and the skirt part but the pin boss part which is a thick part is locally pressurized, so that the whole structure can be refined and the strength can be further improved. Further, since the pressure is not applied from the top, the surface shape of the die is transferred as it is, and a top-shaped piston with excellent dimensional accuracy is obtained.
A pin boss | hub part is a location from which the hole for a connection with a connecting rod is originally opened by cutting. Therefore, even if the surface properties of the piston pin boss part cast by local pressurization using the pressurizing member are deteriorated, the surface adjustment is performed simultaneously with the opening of the hole by the normal cutting operation, which increases the cost. Must not.

  When the piston is locally pressed by die casting, as described above, it is preferable to locally pressurize the pin hole portion that can be removed by post-processing. However, in order to pressurize the pin boss portion when die-casting the piston, it is necessary to pressurize in the direction perpendicular to the movable direction of the mold. Therefore, in order to apply local pressurization, as shown in FIG. 3A, the surface portion is solidified to some extent (the solidified surface layer portion is hereinafter referred to as “shell S”), and an unsolidified portion remains inside. When pressure is applied in this state, since the interior is not completely solidified, the shell S is pushed and a part thereof is broken. As a result, the molten metal inside is smeared into the sliding portion between the mold and the pressure member (indicated as R in FIG. 3B). If many spots appear, the spots may not be removed during pin hole processing. Further, the spotted portion prevents sliding between the pressurizing material and the mold, and may cause galling between the pressurizing material and the mold. Furthermore, it becomes difficult to take out the cast material from the mold. In particular, when the pin boss portion is pressurized, the pressure is applied in a direction orthogonal to the movable direction of the mold, making it difficult to remove the cast material.

On the other hand, in a preferred embodiment of the present invention, before the molten metal is poured into the cavity of the molten metal mold, a pressurizing body for applying local pressure is projected in advance into the cavity (see FIG. 4), and the surface layer of the piston. When the part is solidified, the pressurizing material is further projected to pressurize the pin boss part.
When the pressurizing material is protruded in advance, a shell S is formed along the surface of the pressurizing material as seen in FIG. When pressure is applied in a state where the shell S is formed, the shell S formed at the tip of the pressure material is destroyed as seen in FIG. 5B, but formed on the side surface of the pressure material. The shell S that has been left remains. This shell S suppresses the spotting and prevents the spotting from reaching the outside of the cutting part. Since no galling occurs between the pressurizing material and the mold, the cast material can be easily taken out.

By the way, in WO / 01/051237, the cavity of a die-casting mold is vacuum-sucked to 10 kPa or less, and then overlapped with vacuum suction, the active gas is blown from the sleeve into the cavity to adjust the atmospheric pressure of the cavity to atmospheric pressure ( The molten aluminum alloy is injected into the sleeve while continuing to blow the active gas, and then the cavity is vacuumed again through the gas supply / exhaust pipe opened to the overflow of the cavity, and the pressure is lower than atmospheric pressure. In this state, a die casting method has been proposed in which the plunger is advanced and the molten aluminum alloy is pressed into the cavity through the runner from the sleeve.
Also in the die casting method of this invention, it is preferable to employ | adopt the die casting method proposed by the said gazette. When this method is adopted, it is preferable to press-fit the molten metal into the cavity after vacuum suction again to a pressure 70 kPa or more lower than the atmospheric pressure (about 100 kPa), that is, 30 kPa or less.

  In the die casting method of the present invention, the molten aluminum alloy is preferably press-fitted into the cavity while overlapping vacuum suction, active gas blowing, and re-vacuum suction. For this reason, since the amount of gas entrained at the time of casting decreases, the casting hole decreases. Further, by pressing the molten aluminum alloy into the cavity that has been reduced to 30 kPa or less by vacuum suction again, the unreacted active gas remaining in the cavity is removed from the cavity, and the unreacted active gas becomes aluminum. It is possible to obtain a die-cast piston which is prevented from being taken into the molten alloy and whose gas component is greatly reduced. The die-cast piston obtained in this way is excellent in mechanical properties because the amount of casting defects such as a casting hole and variation due to gas are suppressed.

Next, the present invention will be specifically described by way of examples with reference to the drawings.
In the preferred die casting method of the present invention, the cavity is vacuum-evacuated to a reduced-pressure atmosphere having a degree of vacuum of 10 kPa or less, then the cavity is brought to an atmospheric pressure of atmospheric pressure or more by blowing oxygen, and the cavity is again vacuum-evacuated when the molten aluminum alloy is injected. A method (hereinafter referred to as “DVO process”) is adopted.
In this DVO process, for example, a die casting machine schematically shown in FIG. 6 is used. A cavity 30 corresponding to the piston shape manufactured between the fixed mold 10 and the movable mold 20 is formed. Further, a pressurizing mechanism such as a pressurizing material for locally pressurizing the molten metal in the portion and a hydraulic cylinder for driving the same is provided on both sides of the cavity 30, in FIG. The pressurizing material is arranged in advance so as to protrude into the cavity 30. A runner 11 communicating with the sleeve 40 is formed in the fixed mold 10, and the molten aluminum alloy M injected from the hot water supply port 41 is press-fitted into the cavity 30 through the runner 11 by the plunger 42. The runner 11 communicates with the cavity 30 via the gate 12 divided into a plurality of parts according to the product shape so that the molten aluminum alloy M is supplied to each part of the cavity 30.

The cavity 30 includes an overflow part 31 formed on the fixed mold 10 or the movable mold 20 side, and a chill vent 32 is provided outside the overflow part 31. The overflow part 31 stabilizes the flow of the molten aluminum alloy M in the cavity 30. The chill vent 32 is an uneven or corrugated mating portion formed between the fixed mold 10 and the movable mold 20 as shown in the figure, and promotes the solidification of the molten aluminum alloy M in contact with the chill vent 32 to provide a vacuum system. Prevents metal from being sucked into the surface. By providing the chill vent 32, the cavity 30 can be vacuum-sucked during the injection of the molten aluminum alloy M without inserting metal.
An ejector pin 21 is incorporated in the movable mold 20 so as to be able to advance and retract in order to remove the die-cast casting after casting.

In order to keep the cavity 30 airtight with respect to the outside air, a packing 51 such as an O-ring is interposed between the mating surfaces of the fixed mold 10 and the movable mold 20. The packing 51 is filled in a groove surrounding the cavity 30, and blocks air from entering through the gap between the fixed mold 10 and the movable mold 20. The packing 52 is also inserted into the pin hole 22 through which the ejector pin 21 is pushed, and the airtightness between the inner wall of the pin hole 22 and the ejector pin 21 is maintained. By using the seals 51 and 52, the cavity 30 can be depressurized to a vacuum atmosphere of 10 kPa or less, and vacuum suction can be performed while the molten aluminum alloy M is being injected.
When the groove in which the packing 51 is mounted is connected to the vacuum suction device 60 and vacuum suction is also performed from the groove, intrusion of outside air is more effectively suppressed.

An exhaust pipe 61 connected to the vacuum suction mechanism 60 opens in the runner 11 in order to vacuum the cavity 30. A vacuum valve 63 that is opened and closed by a drive cylinder 62 is provided at the opening of the exhaust pipe 61 facing the runner 11. Further, an exhaust pipe 65 branched from a gas supply / exhaust pipe 64 opened to the mating surface of the fixed mold 10 and the movable mold 20 between the chill vent 32 and the packing 51 is connected to the vacuum suction mechanism 60 via the vacuum valve 66. Has been.
In order to detect the atmospheric pressure of the cavity 30, a pressure gauge 67 is attached to the gas supply / discharge pipes 61 and 64. In order to manage the humidity in the cavity 30, it is preferable to attach a hygrometer 68 to the gas supply / exhaust pipe 64.
Since the gas supply / discharge pipe 64 is also used to send compressed air into the cavity 30, the branched supply pipe 71 is connected to the compressed air ejection mechanism 70 via the check valve 72. After the die opening is completed, the mold is opened, and then compressed air is blown into the gas supply / exhaust pipe 64 to remove foreign matters attached to the vacuum suction mechanism.

In the DVO process, since an active gas such as oxygen is blown after the cavity 30 is vacuumed, an active gas supply mechanism 80 is provided. The active gas is fed into the sleeve 40 from the active gas supply mechanism 80 through the gas supply pipe 81 and the air supply port 82. The gas supply pipe 81 incorporates a dryer 83 that dehumidifies the active gas in order to keep the humidity of the cavity 30 low.
The atmospheric pressure and humidity in the cavity 30 are detected by a pressure gauge 67 and a hygrometer 68 provided in the gas supply / exhaust pipes 61 and 64. The detected value from the pressure gauge 67 is sent to a control system that controls the driving of the vacuum suction mechanism 60, the compressed air ejection mechanism 70, and the active gas supply mechanism 80, and is used for timing control of vacuum suction → oxygen blowing → vacuum suction. used. When the detected value from the hygrometer 68 becomes 15% RH or less and the pressure in the cavity 30 becomes atmospheric pressure (about 100 kPa) or more, the supply of the molten aluminum alloy M to the sleeve 40 is started.

The DVO process according to the present invention will now be described.
The movable mold 20 is put together with the fixed mold 10 and the mold is closed, and the cavity 30 is vacuumed through the runner 11. For vacuum suction of the cavity 30, a gas supply / exhaust pipe 64 opened at the mating surface between the chill vent 32 and the packing 51 is also used. The vacuum suction is continued until the atmospheric pressure of the cavity 30 detected by the pressure gauge 67 becomes 10 kPa or less. At this time, the plunger tip 43 is positioned between the hot water supply port 41 and the air supply port 82 of the sleeve 40 to prevent air from entering from the hot water supply port 41. Since the suction is performed through the runner 11, the lubricant from the sleeve 40 is discharged out of the system without reaching the cavity 30.
In vacuum suction, it is preferable to set the suction speed to 50 kPa / second or more. Even when the cavity 30 has a complicated shape, the gas is removed from every corner of the cavity 30 by setting the suction speed to preferably 50 kPa / second or more. Further, when the cavity 30 is vacuum-sucked at a suction speed of 50 kPa / second or more, the moisture contained in the release agent or the like adhering to the inner surfaces of the molds 10 and 20 bumps, and the moisture in the cavity 30 is greatly increased. To decrease.

The vacuum suction is preferably continued for about 1 to 2 seconds with the plunger 42 closing the hot water supply port 41. In this regard, the suction time is set relatively long as compared with the conventional vacuum die casting method in which the hot water supply port 41 is not blocked and the suction time is less than 1 second. The cavity 30 is decompressed to a vacuum degree of 10 kPa or less by vacuum suction. Moisture contained in the mold release agent or the like adhering to the inner surface of the mold becomes water vapor by vacuum suction, separated from the inner surface of the mold, and discharged out of the system.
However, in the vacuum suction where the degree of vacuum does not reach 10 kPa, a relatively large amount of air remains in the cavity 30 and is not replaced by the active gas in the subsequent active gas injection process, and is entrapped in the product. May cause defects. On the other hand, when the ultimate degree of vacuum is set to 10 kPa or less, the evaporation of moisture contained in the release agent or the like is effectively promoted to be taken out of the system as water vapor. In particular, when vacuum suction is performed at a high speed of 50 kPa / sec or more, moisture evaporation is accelerated from the inside of the release agent or the like due to a bumping phenomenon, and the residual moisture is greatly reduced. The upper limit of the suction speed is about 80 kPa / second in consideration of the capability of the vacuum suction device.

The active gas is fed into the cavity 30 from the air supply port 82 after the cavity 30 is vacuumed to 10 kPa or less. The vacuum suction is stopped after a slight overlap with the injection of the active gas. Due to this overlap, the sent active gas spreads to every corner of the cavity 30, and the entry of outside air from the mating surfaces of the molds is suppressed. The injection of the active gas is continued until the atmospheric pressure of the cavity 30 detected by the pressure gauge 67 becomes atmospheric pressure (about 100 kPa) or higher.
When injecting the active gas, the humidity in the cavity 30 is measured by the hygrometer 68, and the humidity is controlled so that the humidity of the cavity 30 does not exceed 15% RH. As a result, the amount of moisture that is brought into the cavity 30 along with the active gas and generates hydrogen gas by reaction with the molten aluminum alloy M is reduced. In order to lower the humidity in the cavity 30, it is preferable to inject the active gas that has passed through the dryer 83 into the cavity 30.

After the atmospheric pressure in the cavity 30 becomes equal to or higher than atmospheric pressure (about 100 kPa), the plunger tip 43 is retracted to the hot water supply position and the hot water supply port 41 is opened. Next, an amount of molten aluminum alloy M required for one die casting is poured into the sleeve 40. At this time, since the cavity 30 is maintained at an atmospheric pressure equal to or higher than the atmospheric pressure, intrusion of outside air is prevented by the active gas ejected from the hot water supply port 41. The hot water supply port 41 is blocked from communicating with the cavity 30 by moving the plunger 42 forward after the injection of the molten aluminum alloy M is completed.
After pouring, the cavity 30 is again vacuumed through the overflow part 31. It is preferable to slightly overlap the injection of the active gas and the vacuum suction again. The injection of the active gas can be continued until after the casting is finished. Due to this overlap, excess active gas is exhausted from the cavity 30, and moisture derived from the mold release agent and the sleeve lubricant is effectively removed from the cavity 30 along with the active gas. In addition, intrusion of outside air that tends to occur when vacuum suction is performed after the active gas injection is eliminated.

The plunger 42 is advanced while sucking the vacuum again, and the plunger tip 43 moves forward at a low speed over the hot water supply port 41 to the high-speed injection start position. At this time, when the vacuum start position is reached, another vacuum suction is started.
Next, after the pressure in the cavity becomes 30 kPa or less, the plunger 42 is advanced at a high speed from the high-speed injection start position to the injection limit position, and the molten aluminum alloy M is press-fitted into the cavity 30. At this time, since the cavity 30 is vacuumed, the unreacted active gas is effectively removed from the cavity 30 as the molten aluminum alloy M is injected. Therefore, the unreacted active gas is not taken into the metal. If the inside of the cavity is not lower than 30 kPa, it is difficult to stably obtain a material having a large variation in porosity and a small porosity, as will be understood from examples described later. The vacuum suction is continued until the cavity 30 is filled with the molten aluminum alloy M.

When the cavity 30 is filled with the molten aluminum alloy M and the surface layer portion of the molten aluminum alloy in contact with the cavity 30 is solidified, a pressure mechanism (not shown) is operated and the pressure material disposed on the pin boss portion. Is pushed toward the inside of the cavity. By pressing with the pressure material, solidification of the thick-walled unsolidified portion is promoted, and the solidified structure is refined. Moreover, even if a cast hole is about to be formed, the molten metal is replenished, and as a result, a die-cast piston without a cast hole is cast.
After the completion of casting, the vacuum suction is stopped, the pressurizing material of the pin boss part is retracted to the rear end, the mold is opened, and compressed air is blown from the compressed air blowing mechanism 70 to clean the inside of the gas supply / discharge pipe 64. Then, the ejector pin 21 is advanced in the direction of the cavity 30 to remove the die-cast piston.
As described above, the example in which the DVO process is adopted has been described as the best embodiment of the die casting method. However, if the pin boss part is locally pressurized, the casting is not limited to the DVO process but is performed by other PF methods, vacuum die casting methods, and VO methods. It goes without saying that a piston with few casting defects such as a nest can be obtained.

As the aluminum alloy, an Al-Si-Ni-Cu-Mg based alloy (proposed in Japanese Patent Application No. 2002-187582) developed by the present applicants for a high-load piston was used.
An aluminum alloy molten metal that has been subjected to a molten metal cleaning process such as a degassing process was cast into a simple piston shape shown in FIG. 7 by a die casting method. Casting was performed not by the so-called DVO method, but by a pore-free (PF) die casting method in which the air in the mold cavity was replaced with oxygen before casting, and then the molten metal was pressed into the cavity. During casting, the pin boss portion was directly pressed from the side in the die casting of FIG. 7A, and pressed from below the skirt portion in the die casting of FIG. 7B. Moreover, the casting which pressurizes from a top part and the casting which does not pressurize at all were also performed.
Then, the density of each obtained casting material was measured by Archimedes method. The difference between the measured value and the density of the cast material cast by gravity casting into a rectangular parallelepiped shape in which the cast hole was difficult to be found was determined, and the porosity of each cast material was calculated. The results are shown in Table 1.
From this result, it can be seen that the pressure applied to the pin boss portion has the smallest porosity, that is, there are few casting defects such as a cast hole.

  In addition, when pressing the pin boss part from the side, a pressurizing body that applies local pressure is protruded into the cavity in advance, and when the surface layer part of the pin boss part is solidified, the pressing material is further protruded. The pin boss was pressurized. And when the external appearance of the cast material was observed, no stain was observed in any of the pressurized portions. As a comparison, casting was performed in which the molten metal was press-fitted with the tip of the pressurizing material being flush with the inner wall of the cavity without protruding the pressurizing material into the cavity, and then the pin boss portion was pressurized. Spots entered the sliding part of the mold and the pressure material, and it was difficult to remove the cast material from the mold. After the cast material was taken out, the appearance of the pressure material was observed, and a trace of galling was observed.

The same molten aluminum alloy as used in Example 1 was subjected to molten metal cleaning treatment such as degassing treatment, and then cast into a piston shape by a die casting method.
Casting is performed by vacuuming the cavity of the mold to 10 kPa before injecting the molten metal, and then injecting oxygen into the cavity. The pouring port was opened, and the molten metal was poured into the pouring port. After pouring the molten metal, the plunger is advanced to close the pouring port of the sleeve, vacuum suction is started again, and when the pressure in the cavity reaches a predetermined pressure, the plunger is further advanced to Die casting was performed to inject molten metal into the inside.
The pressure in the cavity when the plunger is further advanced in order to press the molten metal into the cavity is changed to three levels of 30 kPa, 40 kPa, and 50 kPa, which are lower than the atmospheric pressure by about 70 kPa, about 60 kPa, and about 50 kPa. Each was cast five times.
Thereafter, in the same manner as in Example 1, the density of each obtained casting material was measured by the Archimedes method. The difference between the measured value and the density of the cast material cast by gravity casting into a rectangular parallelepiped shape in which the cast hole was difficult to be found was determined, and the porosity of each cast material was calculated. The results are shown in Table 2 and FIG.
From this result, when the vacuum is sucked again and the pressure in the cavity when the molten metal is injected is reduced to 30 kPa or less, the porosity is smaller than that exceeding that, and the variation in porosity is small. It turns out that a casting is obtained.

The figure explaining the defect occurrence place when manufacturing the piston by the die casting method The figure explaining the aspect which locally pressurizes the pin boss | hub part of this invention A diagram explaining the occurrence of stains due to the action of the solidified shell during local pressurization The figure explaining the aspect which makes a pressurizing material project before solidification shell formation The figure explaining the aspect which prevents a stain out by the action of the solidification shell Diagram explaining the outline of the die casting machine The figure explaining the simple piston shape and local pressurization position which were used in Example 1 Diagram explaining the relationship between pressure and porosity when vacuum is again drawn

Claims (2)

When casting a piston for an internal combustion engine having a substantially disc-shaped head wall and a pin boss portion and a skirt portion extending from the head wall by the die casting method, before pouring the molten metal into the die cavity of the mold , A pressurizing member for applying pressure is protruded into the cavity in advance, and after pouring the molten metal into the die cavity of the mold, the surface layer portion of the poured molten metal is solidified, and the unsolidified portion is inside. A method for manufacturing a piston for an internal combustion engine, characterized in that, in the remaining state, the pressure member is further protruded to locally pressurize the pin boss portion of the piston. In the die casting method, the inside of the cavity of the mold is vacuum-sucked, and then the active gas is injected from the sleeve into the cavity by overlapping with vacuum suction, and the atmospheric pressure in the cavity is increased to atmospheric pressure or higher, and then the molten metal is added to the sleeve. The piston of the internal combustion engine according to claim 1, wherein after the vacuum is sucked again and the inside of the cavity is brought to a pressure of 30 kPa or less, the plunger is advanced to press the molten metal into the cavity from the sleeve. Production method.

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