JP2012177201A - Method of manufacturing sintered component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method for molding a favorable molding in manufacturing a minute sintered component by pressure molding a plastic raw material within a die cavity and sintering the obtained molding.SOLUTION: The method of manufacturing the sintered component includes: charging the raw material prepared by adding 40-60 vol.% of a binder consisting of a thermoplastic resin and wax to a metal powder and kneading it with heat into the die cavity of a die to pressure mold it to a desired shape; pulling successively out the pressed molding from the die; removing the binder with heat; and then sintering the molding with heat. In the method, the moving speed U of a punch in pressure molding is set lower than a speed obtained by [U=ΔP/(32μ×L)×De] with ΔP: a punch pressing force (Pa), μ: a viscosity (Pa s), L: a length (m), and De: an equivalent pipe diameter (m).

Description

  The present invention relates to a method for manufacturing a sintered part by a powder metallurgy method, and more particularly to a method for manufacturing a fine sintered part having a thin part or a convex part with a width of about 0.01 to 0.2 mm.

  In the powder metallurgy method, a raw material powder is filled in a mold cavity of a pressing mold, and this is pressed with a punch to form a compact and the compact is obtained. The raw powder is mixed with a large amount of binder. The fluidized raw material is pressure-filled into the voids in the mold, and the obtained molded body is heated to remove the binder, and then roughly divided into injection molding methods.

  In the molding method, in order to obtain fluidity of the raw material powder and lubricity with the mold, a molding lubricant of about 1% by mass or less may be mixed into the raw material powder, but the amount of the molding lubricant added is small. Therefore, it is easy to volatilize and remove at the beginning of the sintering process, and there is an advantage that the degreasing process can be shortened. In the pressing method, filling of the raw material powder into the mold is performed by a method of dropping the raw material powder into a space formed by a mold and a lower punch from a powder feeder called a feeder (powder box). It is inevitable that a certain variation occurs in filling. On the other hand, when manufacturing a minute product having a narrow part as described above, this variation is not within an allowable range, and since a narrow part as described above is formed, a minute gap is formed in the mold. When it is intended to fill the gap with the raw material powder, it is necessary to use a raw material powder having a small particle size. In this case, the fluidity of the raw material powder is lowered and the filling property is lowered, so that there is a problem that a stable raw material powder cannot be supplied.

  The injection molding method has an advantage that even a shape having an undercut or the like that cannot be modeled by the above-described mold method can be modeled. However, in order to ensure the fluidity of the raw material, a binder such as 30 to 70% by volume of a thermoplastic resin is added to the raw material powder and kneaded, so the molded product contains a large amount of binder. There is a drawback that it takes time to remove the binder. Moreover, since the cavity of a metal mold | die becomes too small with respect to the thin part about 0.1-0.3 mm thick, it is difficult to fill a metal powder uniformly in a cavity. That is, in the injection molding method, the raw material is injected into the mold through the gate and the runner. However, when the gap of the mold for filling the raw material is very small, the raw material is filled in the gap. However, it is not practical to increase the pressure of the apparatus. That is, separation of the metal powder and the binder and mold burrs are problematic. On the other hand, studies have been made to increase the fluidity of the raw material by increasing the amount of the binder. However, if the amount of the binder is increased, the dimensional shrinkage after sintering becomes large, causing a problem of deformation. From these facts, the limit of the wall thickness that can be practically injection-molded is 0.5 mm.

  Under such circumstances, a modeling method that combines the advantages of the stamping method and the injection molding method has been proposed (Patent Document 1, etc.). This is a method of die-molding using a raw material in which a raw material powder is provided with a larger amount of binder or the like than that provided by a normal die-molding method. Patent Document 1 is an invention relating to an electrode for a cold cathode fluorescent lamp. A metal powder made of molybdenum powder or tungsten powder is added with 40 to 60% by volume of a binder made of a thermoplastic resin and wax, and heated and kneaded to prepare a raw material. A raw material adjustment step for adjusting, a filling step for filling a predetermined amount of the raw material into the mold cavity, a pressure forming step for pressing the raw material in the die with a punch to form a bottomed cylindrical shape, and this pressurization The bottomed cylindrical molded body obtained after the molding process is extracted from the mold, the bottomed cylindrical molded body extracted from the mold is heated to remove the binder, and the binder is removed. A sintered part having a cylindrical portion with a thickness of 0.1 to 0.2 mm and a narrow sintered portion by heating the bottomed cylindrical molded body and performing a sintering step in which powders are diffusion-bonded to each other. Can be manufactured It describes that you can. In Patent Document 1, the raw material is heated to a temperature equal to or higher than the softening point of the thermoplastic resin to perform a molding step, and the raw material is cooled to a temperature equal to or lower than the softening point of the thermoplastic resin and equal to or higher than the softening point of the wax. And performing the extraction step.

JP 2006-344581 A

As described in the above-mentioned Patent Document 1, the present invention is excellent in producing a small sintered part by pressure-molding a plastic raw material in a mold cavity and sintering the obtained molded body. It aims at providing the manufacturing method which can shape | mold a molded object.

In the present invention , a metal powder is added with 40 to 60% by volume of a binder made of a thermoplastic resin and a wax, a raw material adjusting step for adjusting the raw material by heating and kneading, and a predetermined amount of the raw material in the mold die hole. A filling step for filling, a pressure forming step for pressing the raw material filled in the die with a punch to form it into a desired shape, and an extraction of the molded body obtained after the pressure forming step from the die A baking process comprising: a step of removing the binder by heating the molded body extracted from the mold; and a sintering step of diffusion bonding the powder by heating the molded body that has been debindered. In the method for producing a bonded part, the pressure forming step is expressed as follows: ΔP: punch pressing force (Pa), μ: viscosity (Pa · s), L: length (m), De: equivalent pipe diameter (m) When the punch movement speed U is It is characterized in that the pressure molding is performed at a speed equal to or less than the speed obtained by the formula 1].
U = ΔP / (32 μ × L) × De ² ... [Formula 1]
In the present invention, the punch moving speed U is preferably formed at a speed equal to or higher than 80% of the value obtained by [Equation 1].

According to the present invention, when a plastic raw material is pressure-molded in a mold cavity, and the resulting molded body is sintered, a fine sintered part can be manufactured. There is an effect that a method is provided.

FIG. 4 is a schematic diagram (upper diagram) of a molding process according to the present invention and a graph (lower photograph) showing a relationship between an upper punch drop amount of the process and a molding load. It is a photograph which shows the molded object for every shaping | molding process AE. It is a graph which shows the correlation of the amount of swelling of the thin part at the time of pressure molding, and the load required for swelling by an actual value and a theoretical value. BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows one Embodiment of the metal mold | die which implements the manufacturing method of this invention suitably, and the molded article single piece | mold structure using this metal mold | die. It is a control block diagram which shows the control system of the metal mold | die shown in FIG. It is a figure which shows operation | movement of the metal mold | die shown in FIG. It is a figure explaining the time which 1 molding cycle of one Embodiment requires. It is a cross-sectional photograph of the sintered part obtained by one Embodiment.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
"Material properties"
First, the results of experiments conducted to investigate the behavior of raw material powder during pressure molding will be described.

  Tungsten powder having an average particle diameter of 2 μm is prepared as a metal powder, and 56 vol% of a polyacetal-based resin and a resin binder mainly composed of paraffin wax are mixed, a cylindrical pellet having an outer diameter of 1.88 mm and a total length of 2.97 mm. A raw material was prepared. Instron type testing machine (Shimadzu Autograph) with a band heater attached to the outer periphery of a die having a circular die hole with a diameter of 2.08 mm, and a lower punch slidably fitted into the die die hole. 2 types of upper punches with diameters of 1.68 mm and 1.88 mm are prepared, and the die hole and the upper punch are arranged so that they are concentric and attached to the Instron type testing machine. The mold was constructed. The raw material is put into the die hole of this die, and the die and the raw material are heated to 433 K by a band heater, and then the upper punch is lowered at a speed of 0.08 mm / s to perform pressure forming of the raw material, and the wall thickness is 0 Press molding into bottomed cylindrical shapes of 1 mm and 0.2 mm. The change in the molding load at this time is shown in FIG. 1 shows the process of pressure molding in the order of A to E. Reference numeral 31 is a raw material pellet, 32 is a die having a mold hole 32a, 33 is a lower punch, and 34 is an upper part. Each punch is shown. Moreover, FIG. 2 has shown the photograph of the molded object for every shaping | molding process AE.

  As shown in FIG. 1, the upper punch and the raw material abut on the relationship between the movement distance of the upper punch and the molding load (A), the raw material is deformed as the upper punch is lowered, and the raw material comes into contact with the mold hole wall surface (B). After the initial deformation (A to B), after the raw material comes into contact with the mold hole wall surface (B), the raw material is further deformed and the middle stage of deformation (B to C) in which the mold cavity is filled, and the raw material is It can be seen that after filling the mold cavity (C), the thin-walled portion is extruded (D) and undergoes the three stages of deformation (C to E) until the press molding is completed (E). Further, in each stage of the initial stage of deformation (A to B), the middle stage of deformation (B to C), and the latter stage of deformation (C to E), the molding load may increase at a constant rate with respect to the movement distance of the upper punch. Recognize. Further, in the case where the thickness of the thin portion is 0.1 mm and 0.2 mm, the load in the formation of the thin portion is higher than the thickness of 0.2 mm when the thickness is 0.1 mm, It can be seen that the amount of increase in the load increases as the moving distance of the upper punch increases.

Further, as shown in FIG. 1, the load required to obtain the target bottomed cylindrical shape is 1.6 N when the wall thickness is 0.2 mm, and about 2.6 N when the wall thickness is 0.1 mm. It can be seen that molding is possible with a pressure much smaller than that of the method. In this experiment, 2.217Mm ² if the sectional area is the thickness of the upper punch is 0.1 mm, since the wall thickness is about ² 2.776mm in the case of 0.2 mm, the molding pressure of the upper punch, meat When the thickness is 0.2 mm, it is 0.72 MPa, and when the thickness is 0.1 mm, it is about 0.94 MPa.

  By the way, when the bulk metal is extruded backward at a steady pressure, the extrusion pressure (forming load) shows a constant value because the raw material and the die do not slide at the portion in contact with the container (die) or the punch wall surface. It becomes. However, in the experiment described above, the molding load required for the extrusion of the thin portion occurring in the later stage of deformation (C to E) increases at a constant rate as the moving distance of the upper punch increases. The behavior is clearly different from the case where the bulk metal is extruded backward.

  This behavior is due to the fact that in the later stage of deformation, where the thin part is pushed out, the thin part is always formed with the sliding of the die and the raw material. Furthermore, after the raw material is filled in the mold cavity (C), it is considered that such a slip occurs because the semi-molten raw material behaves as a liquid. Therefore, the deformation resistance of the kneaded product is shown from the initial stage to the middle stage (A to C), that is, plastic deformation of the solid. It is considered that the rheological deformation behavior becomes dominant.

Therefore, based on this idea, the pressure loss equation (Hagen-Poiseuille equation) is used for the behavior in the later stage of deformation when the thickness of the thin portion is different from 0.1 mm and 0.2 mm. The load required for the bulge of the thin portion relative to the bulge amount was calculated. The result is shown in FIG. The pressure loss equation is as shown in [Equation 2] below.
ΔP = 32 μ × L × U / De ² [Formula 2]

  Here, ΔP: pressure loss (Pa), μ: viscosity (Pa · s), L: length (m), U: flow velocity (m / s), and De: equivalent pipe diameter (m) in this experiment. Is (diameter of mold cavity−diameter of upper punch), that is, twice the thickness t of the thin portion.

  From FIG. 3, it can be seen that the correlation between the amount of bulging of the thin portion and the load required for the bulging of the thin portion substantially matches the calculated value by the above pressure loss equation and the actually measured value. From this result, it was confirmed that a raw material that is sufficiently heated to the melting point of the binder component and in a fluid state can be handled as a fluid.

  From the above experimental results, when a raw material obtained by adding a binder composed of a thermoplastic resin and a wax to metal powder and heating and kneading is heated to a temperature equal to or higher than the softening point of the thermoplastic resin and pressure-molded, the raw material is The knowledge that it can be handled as a fluid and that the load required for the bulge amount in a narrow thin wall portion can be obtained by the above-described pressure loss equation was obtained.

Accordingly, a raw material having a viscosity of μ (Pa · s) in a molten state of the binder is used, and a molded body having a length of L (m) and an equivalent tube diameter De (m) by the thin-walled portion t is molded. In this case, if the punch moving pressure U (m / s) is adjusted to be equal to or lower than the speed obtained by the following [Equation 1], the raw material that behaves as a fluid is sufficiently thin It is possible to swell and fill the thin portion, so that a good molded body can be formed.
U = ΔP / (32 μ × L) × De ² ... [Formula 1]

  Although a good molded product can be obtained when the moving speed U of the punch is slower than the speed obtained by [Equation 1], if the moving speed U of the punch is extremely slow, the pressure forming step is performed. Since it becomes long, productivity will fall. In view of this, it is preferable that the moving speed U of the punch is at least 80% of the speed obtained by [Equation 1] at the latest.

  Further, if the punch pressure ΔP is increased, the punch moving speed U can be increased, thereby shortening the time required for the molding process.

  By the way, when a sintered product having a general density is manufactured by the mold method, the molding pressure is about 500 to 800 MPa, and when a sintered product having a high density is manufactured, it may exceed 1 GPa. On the other hand, when the raw material in a fluidized state as in the present invention is pressure-molded, as can be seen from FIG. 1, the molding pressure of the upper punch is 0.72 MPa when the thickness is 0.2 mm, When the thickness is 0.1 mm, the molding can be performed at a pressure of about 0.94 MPa, which is much smaller than that of the mold method. Therefore, when press-molding a raw material in a fluidized state, there is no need for a high-load press device having a press capacity of several tens to hundreds of t, which is used in a pressing method, and the press device is small. Can be Further, in the case of pressure forming a raw material in a fluidized state, a press apparatus capable of precisely controlling the punch pressing force ΔP and the punch moving speed U is suitable. For this reason, it is preferable to use a single-axis servo press type capable of precise stroke control as the press device.

"Improved mold"
As described above, the pressing mold is preferably one in which the punch is driven by a servo mechanism and precisely controlled in the pressure molding step, but further, the time required for heating the raw material and cooling the molded body after molding is reduced. For this reason, it is preferable to provide a pressing die having a configuration in which the cooling means that is relatively difficult to control is arranged inside the pressing die, and the heating means by high-frequency induction that is easy to control is arranged outside thereof. That is, since the heating means by high frequency induction is used, the surface of the mold cavity is directly heated by the eddy current generated in the vicinity of the surface, so that the heating time is greatly shortened. In addition, since the cooling means for flowing the refrigerant is embedded in the vicinity of the surface of the mold cavity of the mold, it is possible to rapidly cool the surface of the mold cavity of the mold by flowing the refrigerant. By adopting such a configuration, the control for shortening the cycle time of heating and cooling of the die is facilitated.

  FIG. 4 is a cross-sectional view showing one embodiment of a mold suitable for carrying out the production method of the present invention and a single-piece molded structure with this mold, (1) is a raw material filling state, (2) shows the state when the molding is completed. In the figure, reference numeral 1 is a raw material to be molded, 2 is a bottomed cylindrical molded product after the raw material 1 is molded, 3 is a fixed mold for molding (lower mold) in which the raw material is inserted, 4 Indicates a movable mold (upper mold) which is installed above the fixed mold 3 and molds the raw material 1.

  The fixed die 3 is provided with a cylindrical pressing die 5 having a hollow hole 5a in the center. The die 5 is a mold part for extruding a raw material, is a metal having good thermal conductivity, and is made of a magnetic material (such as iron) that is heated by high frequency induction, and makes the heat capacity as small as possible. Therefore, it is formed in a small mass and a small volume. The movable mold 4 is provided with an upper inner punch 6 for lowering the raw material 1 by extrusion into the hollow hole 5a of the pressing die 5 and an upper outer punch 7 for determining the height of the extruded raw material 1. Yes.

  Reference numeral 9 in FIG. 4 is a cooling means embedded in the vicinity of the molding surface (mold hole wall surface) 5b of the die 5, and is made of a tubular body through which a coolant such as cold water flows. In order to eliminate the temperature unevenness of the molding surface 5b during cooling, as shown in FIG. 4 (2), the distance A to the molding surface 5b of the tubular body of the cooling means 9 and the pitch B in the longitudinal direction of the tubular body are as follows. It is set almost the same. When cooling the tubular body of the cooling means 9, a coolant (cold water) flows, but when the molded product is discharged (described later), the cold water is drained and discharged by blowing pressurized air into a hollow state. The state is used so as to be maintained during heating.

  Reference numeral 8 in FIG. 4 is a heating means provided around the cooling means. The heating means 8 is configured by winding a high-frequency induction heating coil 8a around the cooling means in a state of being embedded in an insulator 8b. Since the high frequency induction heating has a large heating capacity and is easy to control, it is disposed outside the cooling means. Reference numeral 10 denotes a lower inner punch that receives the raw material 1 to be pressure-molded and cooperates with the upper inner punch 6 to apply a pressing force in the axial direction to the raw material 1. The lower inner punch 10 also has a function of pushing up the molded product 2 that has been cooled and solidified after molding from the pressing die 5, and discharges the molded product 2 from the hollow hole 5a by moving in the upward arrow direction.

  Further, reference numeral 11 in FIG. 4 is a fixed-side warming heater that keeps the entire fixed mold 3 at 120 ° C., and 12 is a movable-side warming heater that keeps the entire movable mold 4 at 80 ° C. Accordingly, during the molding operation, the pressing die 5 is kept warm at 120 ° C. together with the fixed die 3 and the movable die 4 is kept warm at 80 ° C. Reference numeral 14 denotes a heat insulating plate disposed on the outer periphery of the pressing die 5. By this heat insulating plate 14, the pressing die 5 is thermally separated from the fixed die 3, and therefore the heat capacity of the entire lower die is minimized. In addition, since the fixed die 3 is always kept at 120 ° C., the pressing die 5 is kept at the same temperature by air propagation or the like.

  The above is the structure of the mold, and since this mold has a simple structure, it is inexpensive and has improved maintainability.

  FIG. 5 is a block diagram showing the configuration of the mold control system. In the figure, reference numeral 20 denotes a mold control mechanism including a drive mechanism for both movable and fixed molds 3 and 4, and 21 denotes heating for controlling the fixed-side heat retaining heater 11, the movable-side heat retaining heater 12, and the high-frequency induction heating coil 8a. It is a controller. The heating controller 21 drives the high frequency induction heating coil 8a at a frequency of about 25 kHz. 5 is a refrigerant / air controller for supplying refrigerant or air to the tubular body of the cooling means 9, 23 is a mold controller for controlling the operation of the mold control mechanism 20, and 24 is for controlling each controller. It is a control unit.

  The operation of the mold having the above configuration will be described with reference to FIGS. In FIG. 4 (1), the raw material 1 is inserted into the hollow hole 5a of the pressing die 5 (FIG. 6, raw material supply), and at the same time, heating of the pressing die 5 of the fixed mold 3 is started (FIG. 6: T1 to T2). Since this heating is performed by high frequency induction heating, the entire surface including the surface of the pressing die 5 is directly heated by an eddy current generated near the surface of the pressing die 5 which is a magnetic body inside the heating coil 8a. Therefore, since the molding surface 5b of the die 5, that is, the surface in contact with the raw material 1 is directly heated, heat can be transferred to the raw material 1 with high efficiency.

  At this time, the tubular body of the cooling means 9 is in a hollow state drained by the air blowing of the refrigerant (cold water) by the refrigerant / air controller 22, and therefore the heat capacity of the die 5 is small. Coupled with efficient heat transfer, the time to rise from 120 ° C. to 30 ° C. is greatly reduced (FIG. 6: heating steps T1 to T4). According to the examples, 120 ° C. to 150 ° C. could be reached in 2 to 3 seconds.

  When the raw material 1 reaches a state where it is heated to 150 ° C. at the timing of T4 in FIG. 6, the movable mold 4 descends in the direction of the arrow to reach the bottom dead center shown in FIG. The raw material 1 is extruded and the upper outer punch 7 defines the height of the molded product 2 (FIG. 4: T4 to T5).

  At the molding timings T4 to T5, the heating operation of the heating unit 8 is stopped in parallel and the cooling operation of the cooling unit 9 is switched. That is, cold water at 4 ° C. is supplied as a refrigerant to the tubular body of the cooling means 9. Since this tubular body is embedded close to the molding surface 5b of the mold 5, cooling starts from the inside of the mold 5 and spreads over the entire mold 5 (FIG. 6: cooling steps T5 to T7). In this cooling step, the temperature of the die 5 is lowered from 150 ° C. to 120 ° C. by 30 ° C. However, since the die 5 has a small heat capacity and is cooled from the inside, the molded product 2 is cooled with high efficiency, and therefore, for a short time. The temperature can be lowered. According to the example, when cold water of 4 ° C. was used as the refrigerant, the temperature could be lowered by 30 ° C. in about 3 seconds.

  Next, when the molded product 2 is cured by cooling, the movable mold 4 is raised and returned to the top dead center shown in FIG. 4A, and then the lower inner punch 10 is moved in the upward arrow direction (FIG. 6: T8). To T9), the molded product 2 is pushed upward from the hollow hole 5a (FIG. 6: T9 to T10) and discharged out of the mold by a mechanism (not shown) (FIG. 6: T10). These timings T8 to T10 are the molded product removal step.

  The coolant in the cooling means 9 is drained substantially in parallel with the timing of T8 to T10 of the molded product removal step. Specifically, under the control of the refrigerant / air controller 22, air is blown into the tubular body of the cooling means 9 to drain and discharge the cold water, thereby making the tubular body hollow. After the draining, the next raw material 1 is supplied to the stamping die 5 to prepare for the next heating step. In the next heating step, the cold water in the tubular body of the cooling means 9 is heated by the heat retaining heat. There is also an effect that the danger of boiling can be prevented.

  The above timings T1 to T11 constitute one molding cycle, and heating and cooling are performed at high speed, so that the molding cycle time can be shortened. In addition, the operation of the mold is simple with only up and down movement, so that the workability and maintenance of molding are improved, and the mold can be handled easily.

  Further, when a bottomed cylindrical molded body having a wall thickness of 0.2 mm is obtained through the raw material adjustment step, the filling step, and the pressure molding step, the lowering speed of the upper punch according to the above [Formula 1] Was reduced to 5 mm / sec to shorten the time required for molding, and the time required for the heating and cooling steps by the above-described mold configuration was reduced. As shown in FIG. It was confirmed that the time of one molding cycle comprising the steps of heating-molding-molded body cooling-extraction can be achieved to about 10 seconds / cycle as in the case of a normal stamping method.

  Further, FIG. 8 shows a cross-sectional photograph of a sintered part obtained by removing the binder from the bottomed cylindrical molded body formed by the above cycle and sintering it. From the photograph in FIG. 8, it was confirmed that a good sintered part having a thickness of 0.2 mm was obtained.

1 ... Raw material 2 ... Molded product 3 ... Fixed mold (lower mold)
4 ... Moveable mold (upper mold)
DESCRIPTION OF SYMBOLS 5 ... Stamping die 6 ... Upper inner punch 7 ... Upper outer punch 8 ... Heating means 8a ... High frequency induction heating coil 9 ... Cooling means 10 ... Lower inner punch 11 ... Fixed side heat retention heater 12 ... Movable side heat insulation heater

Claims (2)

A raw material adjustment step of adding 40 to 60% by volume of a binder made of a thermoplastic resin and wax to the metal powder, and adjusting the raw material by heating and kneading,
A filling step of filling a predetermined amount of the raw material into the mold cavity of the mold;
A pressure forming step of pressing the raw material filled in the pressing mold with a punch to form a desired shape;
An extraction step of extracting the molded body obtained after the pressure molding step from the pressing die;
A binder removal step of removing the binder by heating the molded body extracted from the die;
In a method for manufacturing a sintered part, comprising a sintering step of diffusion bonding the powders by heating the debindered molded body,
When the pressure forming step is ΔP: pressing force of the punch (Pa), μ: viscosity (Pa · s), L: length (m), De: equivalent pipe diameter (m), the moving speed of the punch A method for producing a sintered part, wherein U is pressure-molded at a speed equal to or less than a speed obtained by the following [Formula 1].
U = ΔP / (32 μ × L) × De ² ... [Formula 1] 2. The method for manufacturing a sintered part according to claim 1, wherein the punch moving speed U is formed at a speed equal to or greater than 80% of the value obtained by the [Expression 1].

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