Method for manufacturing internal combustion engine piston

Abstract

To provide a method for manufacturing an internal combustion engine piston that replaces a conventional wet defatting process without requiring a large-capacity water washing tank, a defatting tank, or thermal energy for heating the tanks, while still maintaining adhesion of a resin coat.SOLUTION: A method for manufacturing an internal combustion engine piston comprises: a process to prepare a piston 10 which has a piston body and two skirt sections 18 and is made of aluminum alloy as a base material; a laser irradiation process to irradiate an external surface of the skirt section 18 with pulse laser beams 41 while the piston 10 rotates around its axis, with air being injected onto the surface through an air injection nozzle 35a; and a surface treatment process which forms a resin coat on the external surface of the skirt section 18 after laser irradiation. In the laser irradiation process, the air is injected from the air injection nozzle 35a at an angle deviated in the counter-rotation direction of the piston relative to a radial direction R of the piston regarding the inject direction of the air injection nozzle 35a.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a piston for an internal combustion engine. [Background technology]

[0002] In an internal combustion engine such as an engine installed in a vehicle such as an automobile, a piston reciprocates in a cylinder bore extending along a linear longitudinal axis in a direction along the longitudinal axis. At this time, the outer periphery of the piston slides against the inner periphery of the cylinder bore. Typically, the piston includes a piston body having an outer periphery that is slidable against the inner periphery of the cylinder bore, and two skirts extending from the outer periphery of the piston body to the bottom side of the cylinder bore, and further, in the piston, a resin coating is formed on the outer surface of each skirt in order to reduce the frictional resistance of the outer periphery of the piston against the inner periphery of the cylinder bore.

[0003] Patent Document 1 describes a method for manufacturing an internal combustion engine piston having excellent adhesion to a resin coating while maintaining the general shape of the streaks, the method including: an alkaline etching step of alkaline etching the skirt portion to form a plurality of recesses on the outer surface of the skirt portion that are finer than the grooves while maintaining the configuration of the grooves and intermediate surface provided on the outer surface of the skirt portion; a cleaning step of removing smut generated on the outer surface of the skirt portion by alkaline etching; and a surface treatment step of forming a resin coating on the outer surface of the skirt portion from which the smut has been removed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2023-019522 A Summary of the Invention [Problem to be solved by the invention]

[0005] Conventionally, in order to form a resin coat on the skirt portion of a piston, it was necessary to perform a number of steps, such as a first water washing step, a first degreasing step, a second degreasing step, a second water washing step, and a drying step, before the resin coat printing step, in order to wash off the cutting oil that adhered to and remained on the surface of the skirt portion in the machining step and obtain adhesion strength of the resin coat. In the first water washing step, dirt other than the cutting oil is mainly removed before the first degreasing step. It is to be noted that the method is not limited to water washing, and hot water washing or a combination of these may be used. In the first degreasing step, the cutting oil on the skirt surface is removed using a neutral or alkaline degreasing agent. In this step, it is desirable to immerse the piston (skirt surface) in the degreasing liquid and perform ultrasonic cleaning. In the subsequent second degreasing step, degreasing is performed using the same or a different degreasing agent as in the first degreasing step, and the cutting oil adhered to the skirt surface is further removed. If necessary, a water washing step may be performed between the first degreasing step and the second degreasing step. In the second water washing step after the second degreasing step, the degreasing liquid adhered to the skirt surface is removed. This step may be either water washing or hot water washing, or a combination of both. After the second water washing step, a drying step is carried out to remove moisture, especially that adhering to the skirt surface. If moisture remains on the skirt surface, there is a concern that the adhesive strength of the resin coat printed in the subsequent resin coat printing step may decrease.

[0006] However, such conventional wet degreasing methods including multiple washing and degreasing steps require large-capacity washing and degreasing tanks, and also require a large amount of thermal energy to heat these tanks, so improvements were required.

[0007] In view of the above problems, an object of the present invention is to provide a method for manufacturing a piston for an internal combustion engine, which can replace the conventional wet degreasing method, while maintaining the adhesive strength of the resin coat, and which does not require a large-capacity water washing tank or degreasing tank, and does not require thermal energy for heating the tank. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides a method for manufacturing a piston for an internal combustion engine having a resin coating on a skirt portion, the method comprising the steps of: preparing a piston for an internal combustion engine, the piston having an outer periphery configured to be slidable in the longitudinal direction relative to an inner periphery of a cylinder bore; and two skirt portions that face each other in the radial direction of the piston body and extend from the outer periphery of the piston body toward a bottom of the cylinder bore, the piston body and the skirt portions being constructed using an aluminum alloy as a base material; a laser light irradiation step of irradiating an outer surface of the skirt portion of the piston for the internal combustion engine with a pulsed laser beam while injecting air from an air injection nozzle while rotating the piston for the internal combustion engine about its axis; and a surface treatment step of forming a resin coating on the outer surface of the skirt portion irradiated with the pulsed laser beam, wherein in the laser light irradiation step, the injection direction of the air injection nozzle is shifted relative to the radial direction of the piston for the internal combustion engine in a direction opposite to the rotation direction of the piston for the internal combustion engine. Effect of the Invention

[0009] According to the present invention, the conventional wet degreasing method can be changed to the dry degreasing method, which eliminates the need for the large-capacity washing tank and degreasing tank required in the wet degreasing method, and also eliminates the need for thermal energy to heat the tank, while maintaining the adhesive strength of the resin coat. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a front view showing a schematic diagram of an example of a piston for an internal combustion engine. [Diagram 2] FIG. 1 is a flow chart for explaining an embodiment of a manufacturing method for a piston for an internal combustion engine according to the present invention. [Diagram 3] FIG. 2 is a schematic diagram illustrating an example of a laser light irradiation device that performs a laser light irradiation step in the method for manufacturing a piston for an internal combustion engine according to the present invention. [Figure 4] 3 is a schematic diagram for explaining circumferential overlap of irradiation spots in the laser light irradiation step shown in FIG. 2. FIG. [Diagram 5] 3 is a schematic diagram for explaining overlapping of irradiation spots in the longitudinal direction in the laser light irradiation step shown in FIG. 2. FIG. [Figure 6] 3 is a schematic diagram illustrating the injection direction (tilt angle α) of the air injection nozzle in the laser light irradiation step shown in FIG. 2. FIG. [Figure 7] 3 is a schematic diagram for explaining the injection direction (direction deviation angle β) of the air injection nozzle in the laser light irradiation step shown in FIG. 2. FIG. [Figure 8] 3 is a schematic diagram for explaining the injection direction (direction deviation angle β) of the air injection nozzle in the laser light irradiation step shown in FIG. 2. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of a method for manufacturing a piston for an internal combustion engine according to the present invention will be described with reference to the drawings.

[0012] First, the piston 10 for an internal combustion engine will be described. The piston 10 for an internal combustion engine of this embodiment (hereinafter, simply referred to as "piston 10") is a member that reciprocates inside a cylinder bore (not shown), and is made of an aluminum alloy. The aluminum alloy contains silicon (Si) as a component that contributes to wear resistance and aluminum adhesion resistance. Examples of such aluminum alloys include AC materials such as AC4A, AC8A, and AC9, ADC materials such as ADC10 to ADC14, and A4000 series materials for pistons.

[0013] An internal combustion engine piston 10 has a generally cylindrical shape and, as shown in Fig. 1, has on its outer circumferential surface, in order from the piston crown surface 11 side, a first ring groove 13, a second ring groove 15, and an oil ring groove 17. On the outer circumferential surface, the portion between the piston crown surface 11 and the first ring groove 13 is called a first land 12, the portion between the first ring groove 13 and the second ring groove 15 is called a second land 14, the portion between the second ring groove 15 and the oil ring groove 17 is called a third land 16, and the portion beyond the oil ring groove 17 is called a skirt portion 18.

[0014] Therefore, it can be said that the piston 10 for an internal combustion engine comprises a piston body from the piston crown surface 11 to the oil ring groove 17, and a skirt portion 18 extending from the outer periphery of the piston body toward the bottom of the cylinder bore. The skirt portion 18 is composed of two skirt portions opposed to each other in the radial direction of the piston body, and is disposed on both sides with a pin boss portion 19 therebetween, as shown in Fig. 1. The piston 10 reciprocates inside the cylinder bore while the outer surface of the skirt portion 18 slides on the inner wall surface of the cylinder bore.

[0015] Next, a manufacturing method 20 of a piston for an internal combustion engine according to the present embodiment will be described. As shown in Fig. 2, the manufacturing method 20 of a piston for an internal combustion engine includes a step 21 of casting a piston, a step 22 of heat treating the cast piston, a step 23 of machining the heat treated piston, a step 24 of irradiating a skirt portion of the piston with laser light, a step 25 of printing a resin coat on the skirt portion of the piston, and a step 26 of baking the piston with the printed resin coat.

[0016] The above-mentioned casting, heat treatment and machining steps 21, 22 and 23 are similar to those used in manufacturing a general piston for an internal combustion engine, and therefore a detailed description thereof will be omitted here.

[0017] The laser light irradiation step 24 is a step of irradiating the outer surface of the skirt portion 18 of the piston 10 with a pulsed laser light while injecting air from an air injection nozzle, in order to degrease the cutting oil remaining on the outer surface of the skirt portion 18 of the piston 10 in the machining step 23 and to roughen the outer surface of the skirt portion 18. To perform the laser light irradiation step 24, for example, a laser light irradiation device 30 shown in FIG. 3 can be used.

[0018] As shown in Fig. 3, the laser light irradiation device 30 includes a protection box 31 that houses the piston 10, and a laser light oscillator 40 that irradiates the skirt portion 18 of the piston 10 with a pulsed laser light 41 through a laser light passage opening 34 provided in the protection box 31. The protection box 31 further includes a holding member 52 that holds the piston 10, an air injection nozzle 35 that injects air into the skirt portion 18 of the piston 10, a suction nozzle 38 that removes scattered material generated by the laser light irradiation, and an air curtain unit 42 that forms an air curtain 43 at the laser light passage opening 34. The air injection nozzle 35 is connected to a vacuum flow 37 through an air supply line 36. The suction nozzle 38 is connected to a dust collector 39.

[0019] The laser beam oscillator 40 is disposed so as to irradiate the pulsed laser beam 41 in a direction perpendicular to the outer surface of the skirt portion 18 of the piston 10. The type of laser used for the laser beam oscillator 40 is not particularly limited as long as it is a laser for metal processing, and for example, a fiber laser, a semiconductor laser, a CO2 laser, a solid-state laser, or the like can be used alone or in combination.

[0020] In the laser light irradiation step 24, the first land portion of the piston 10 is held by the holding member 52 by gripping it with the chuck 53, and the outer surface of the skirt portion 18 of the piston 10 is irradiated with the pulsed laser light 41 from the laser light oscillator 40 while rotating the piston 10 about its axis. At this time, in order to irradiate the outer surface of the skirt portion 18 evenly with the pulsed laser light 41 over the entire surface, the laser light irradiation step 24 preferably repeats a series of scanning steps, including a first scanning step in which the piston 10 is rotated in one direction to scan the irradiation spot of the pulsed laser light 41 from one end to the other end in the circumferential direction of the skirt portion 18, a second scanning step in which the holding member 32 is moved by about one spot in the axial direction of the piston 10 to scan the irradiation spot in the longitudinal direction of the skirt portion 18, and a third scanning step in which the piston 10 is rotated in the direction opposite to the first scanning step to scan the irradiation spot from the other end to one end in the circumferential direction of the skirt portion 18.

[0021] When scanning the irradiation spots on the outer peripheral surface of the skirt portion 18, if the irradiation spots are scanned so that the outer edges of adjacent spot diameters are in contact with each other, degreasing and roughening may be insufficient in the areas where the laser energy is sparse at the outer edges of the spot diameters, and the adhesive strength of the resin coat may decrease. Therefore, in the first scanning step, as shown in Figure 4, it is preferable to scan the irradiation spots 50 irradiated with the pulsed laser light so that they overlap along the circumferential direction C of the skirt portion. At this time, the distance (pitch P) between the centers Ca and Cb of two adjacent irradiation spots 50a and 50b in the circumferential direction C is set to 0.05 mm. C It is more preferable that the pitch P be equal to or smaller than the spot radius R of the irradiation spot 50. C is more preferably 90% or less of the spot radius R, further preferably 70% or less, and further preferably 40% or less. C If the lower limit is too short, the processing time becomes too long, so the lower limit is preferably 20% or more of the spot radius R.

[0022] In the second scanning step, as shown in FIG. 5, it is preferable that the irradiation spots 50 irradiated with the pulsed laser beam are scanned so that the spot diameters D of the spots 50 overlap each other along the longitudinal direction L of the skirt portion. That is, the distance (pitch P) between the centers Ca and Cb of two adjacent irradiation spots 50a and 50b in the longitudinal direction L is L ) is set to be less than the spot diameter D of the irradiation spot 50. In particular, the pitch P L is preferably 90% or less of the spot diameter D. L The lower limit is preferably 50% of the spot diameter D or more.

[0023] In the third scanning step, the skirt portion 18 is irradiated with the pulsed laser light under the same conditions as in the first scanning step, except that the rotation direction of the piston 10 is reversed. As a result, as shown in Fig. 5, it is possible to irradiate an irradiation spot 50 adjacent to the irradiation spot 50 in the circumferential direction C of the skirt portion 18 irradiated in the first scanning step at a position moved by about one spot in the longitudinal direction L of the skirt portion 18. Therefore, by repeatedly performing a series of scanning steps such as the first scanning step, the second scanning step, the third scanning step, the second scanning step, and the first scanning step, it is possible to irradiate the outer surface of the skirt portion 18 evenly over the entire surface with the pulsed laser light 41.

[0024] In addition, in the laser light irradiation process 24, scattered matter called fumes, which contain aluminum alloy components sublimated by the heat of the pulsed laser light, is generated. If the scattered matter reattaches to the surface of the skirt portion 18, the desired cleanliness and processing state cannot be obtained, and the adhesive strength of the resin coat may decrease. Therefore, in this embodiment, as shown in FIG. 3, the pulsed laser light 41 is irradiated while air is sprayed from an air spray nozzle 35 onto the outer surface of the skirt portion 18. The scattered matter blown away by the air is removed by a suction nozzle 38 arranged on the opposite side of the air spray nozzle 35 with respect to the outer surface of the skirt portion 18.

[0025] In the laser light irradiation step 24, since the outer surface of the skirt portion 18 is irradiated with the pulsed laser light 41 while rotating the piston 10 as described above, it is preferable that the air injection direction (bold arrow) of the air injection nozzle 35 is inclined toward the piston crown surface side with respect to a plane X (i.e., the irradiation direction of the pulsed laser light 41) perpendicular to the axis of the piston 10 as shown in Fig. 5. The inclination angle α of the injection direction of the air injection nozzle 35 with respect to the perpendicular plane X can be appropriately set within a range from more than 0° to less than 90°, but is preferably 30° or more, and more preferably 45° or more.

[0026] In addition, in Figures 3 and 6, the air injection nozzle 35 is located on the crown surface side of the piston 10 and the suction nozzle 38 is located on the skirt side of the piston 10, so the injection direction of the air injection nozzle 35 is inclined toward the piston crown surface side with respect to the vertical plane X. However, the present invention is not limited to this. For example, when the positions of the air injection nozzle 35 and the suction nozzle 38 are reversed with respect to the skirt portion 18 in Figure 3 (or when the piston 10 is held from above by the holding member 32), the injection direction of the air injection nozzle 35 is inclined toward the opposite side to the piston crown surface with respect to the vertical plane X.

[0027] 7, in the laser light irradiation process 24, the air injection direction (thick arrow) of the air injection nozzle 35a is shifted to a direction opposite to the rotation direction of the piston 10 with respect to the radial direction R of the piston 10 (i.e., the irradiation direction of the pulsed laser light 41). As a result, the air injected from the air injection nozzle 35a opposes the rotation of the piston 10, and therefore the injection pressure of the air applied to the surface of the skirt portion 18 is increased compared to the case where air is injected from the radial direction R of the piston 10 with the same injection pressure, and the removal of the scattered material caused by the irradiation of the pulsed laser light 41 is promoted. In addition, the scattered cutting oil can also be removed by the air injection, so that the cleanliness can be significantly improved.

[0028] The directional deviation angle β of the injection direction (bold arrow) of the air injection nozzle 35a relative to the radial direction R of the piston 10 can be set appropriately within a range from more than 0° to less than 90°, but is preferably within a range of, for example, 10° or more and less than 60°.

[0029] In the first and third scanning steps of the laser light irradiation step 24, the rotation direction of the piston 10 is reversed, so that it is preferable to use two air injection nozzles 35 in order to inject air at the above-mentioned direction deviation angle β in both scanning steps. As shown in FIG. 7, when the piston 10 rotates rightward, air is injected from the first air injection nozzle 35a, whose injection direction is arranged in a direction opposite to the rotation direction of the piston 10, onto the outer surface of the skirt portion 18. As shown in FIG. 8, when the piston 10 rotates leftward, air is injected from the second air injection nozzle 35b, whose injection direction is arranged in a direction opposite to the rotation direction of the piston 10, onto the outer surface of the skirt portion 18. When the first air injection nozzle 35a injects air, the second air injection nozzle 35b does not inject air, and when the second air injection nozzle 35a injects air, the first air injection nozzle 35b does not inject air. By thus linking the first and third scanning processes with the first and second air injection nozzles 35a, 35b, air can be injected onto the outer surface of the skirt portion 18 at the above-mentioned directional deviation angle β, regardless of whether the piston 10 rotates to the right or left.

[0030] The installation position of the suction nozzle 38 that sucks and removes the scattered material blown away by the air is preferably appropriately arranged according to the above-mentioned inclination angle α and directional deviation angle β. Also, in order to prevent the scattered material from adhering to the lens of the laser light oscillator 40, it is preferable to form an air curtain 43 at the laser light passage opening 34. Also, by making the laser light passage opening 34 as small as possible, the flow rate of the air curtain 43 can be suppressed, and the effect on the injection and suction of air to remove the scattered material can be reduced.

[0031] In addition, the air supplied to the air inlet (not shown) of the vacuum flow 37 is preferably passed through a filter (such as an air dryer or mist separator) to remove moisture and oil components. Since air generally used in factories contains moisture and oil components, if the air is used without removing the moisture and oil components, the moisture and oil components may adhere to the outer surface of the skirt portion 18 that has been cleaned by the irradiation of the pulsed laser light 41 due to the air injection, which may reduce the adhesive strength of the resin coat. Therefore, it is preferable to use air from which moisture and oil have been removed as much as possible.

[0032] The protective box 31 is provided with an opening (not shown) for inserting and removing the piston 10, and it is preferable to make this opening as small as possible and install an air curtain at the opening. It is also preferable to make the inside of the protective box 31 positive pressure. This generates an air flow from inside the protective box 31 to the outside via the opening for inserting and removing the piston and the laser light passage port 34, making it possible to prevent scattered objects from adhering again to the outer surface of the skirt portion 18.

[0033] It should be noted that the cutting oil remaining on the surface of the skirt portion 18 in the machining process 23 can be degreased in the laser light irradiation process 24. However, if there is too much cutting oil remaining in the machining process 23 and degreasing by laser light irradiation is insufficient, a degreasing process such as neutral degreasing or alkaline degreasing (not shown) may be performed before the laser light irradiation process 24, if necessary.

[0034] In the laser light irradiation step 24, since the laser light is irradiated only onto the skirt portion 18, a masking step (not shown) for masking the piston body other than the skirt portion 18 may be performed before the laser light irradiation step 24. Also, the laser light irradiation step 24 may be performed without masking.

[0035] In addition, if necessary, a water washing step (not shown) may be performed after the laser light irradiation step 24 in order to remove dust particles that have adhered to the internal combustion engine piston 10 by the laser light irradiation. The water washing step may be, for example, ultrasonic water washing or other simple steps. Note that, if the water washing step is performed, it is necessary to perform a drying step thereafter.

[0036] The laser light irradiation step 24 is not limited to the embodiment in which a series of scanning steps including the above-mentioned first to third scanning steps are repeatedly performed, and for example, the irradiation spot may be scanned in a spiral shape on the outer peripheral surface of the skirt portion 18. This can be performed by rotating the piston 10 in one direction about its axis while irradiating the outer peripheral surface of the skirt portion 18 with a pulsed laser light 42 from the laser light oscillator 40, and at the same time, moving the holding member 32 in the axial direction of the piston 10 by about one spot during one rotation of the piston 10. By rotating the piston 10 about its axis while also moving it in the axial direction in this way, the outer peripheral surface of the skirt portion 18 can be irradiated with the pulsed laser light 41 in a spiral shape, and the entire outer peripheral surface of the skirt portion 18 can be irradiated with the pulsed laser light.

[0037] The resin coat printing step 25 is a step of printing a resin coat on the outer surface of the skirt portion 18 that has been degreased and roughened by the laser light irradiation step 24, and can be performed by a method of printing a resin coat on the skirt portion of a general piston. For example, a resin coat agent is applied to the outer surface of the skirt portion 18 by a spray method, a screen printing method, or the like. As the resin coat agent, a known agent used for the skirt portion of a piston can be used, for example, MOLYKOTE D-10-GBL and MOLYKOTE PA-744 manufactured by DuPont-Toray Specialty Materials Co., Ltd. In addition, a resin coat agent (a resin (binder) changed from the polyamideimide that has been widely used in the past, or a resin having microhard particles added thereto) that has been developed in recent years for the purpose of achieving both the wear resistance and friction reduction of the resin coat may be used.

[0038] The baking process 26 is a process for baking the resin coating agent applied in the resin coating printing process 25 to form a resin coating on the outer surface of the skirt portion 18. As a result, the resin coating agent fills the outer surface of the skirt portion 18 that has been degreased and roughened in the laser light irradiation process 24, and the adhesive strength of the resin coating is increased by the anchor effect.

[0039] In the piston 10 for an internal combustion engine manufactured by such a series of processes, the resin coating is applied with the laser light in the laser beam irradiation process 24 while ensuring the same degreasing effect as the conventional method, and the outer surface of the skirt portion 18 is roughened uniformly, and the resin coating fills the uneven parts caused by the roughening, resulting in an anchor effect that improves and stabilizes the adhesion strength. In addition, since the degreasing effect can be ensured, the pre-cleaning process can be changed from the conventional wet degreasing method to the dry degreasing method, which eliminates the need for large-capacity water washing tanks and degreasing tanks that were required in the conventional pre-cleaning process, and also significantly reduces thermal energy, which contributes greatly to carbon neutrality during the manufacture of internal combustion engines. In addition, the area occupied by the conventional pre-cleaning process can be reduced, the man-hours required for managing the liquid in the water washing tank and degreasing tank can be reduced, and waste liquid treatment can be eliminated. [Explanation of symbols]

[0040] 10. Pistons for internal combustion engines 18 Skirt Club 20 Manufacturing method of piston for internal combustion engine 30 Laser light irradiation device 32 Retaining member 33 Chuck 35 Air injection nozzle 37 Vacuum Flow 38 Suction nozzle 39 Dust collector 40 Laser Oscillator 41 Pulsed laser light 42 Air curtain unit 43 Air Curtain 50 Irradiation Spots

Claims

1. A method for manufacturing a piston for an internal combustion engine having a resin coating on a skirt portion, comprising the steps of: preparing a piston for an internal combustion engine, the piston body having an outer periphery configured to be slidable in a longitudinal direction relative to an inner periphery of a cylinder bore, and two skirt portions facing each other in a radial direction of the piston body and extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the piston body and the skirt portions being constructed using an aluminum alloy as a base material; a laser light irradiation step of irradiating an outer surface of the skirt portion of the piston for an internal combustion engine with a pulsed laser light while injecting air from an air injection nozzle while rotating the piston for an internal combustion engine about its axis; and a surface treatment step of forming a resin coating on the outer surface of the skirt portion irradiated with the pulsed laser light, a laser beam irradiation step of the air injection nozzle shifting a direction of the air injection nozzle from a radial direction of the piston to a direction opposite to a rotational direction of the piston, the laser beam irradiation step being performed to inject air from the air injection nozzle.

2. The laser light irradiation step includes: a first scanning step of scanning an irradiation spot of the pulsed laser light from one side to the other side in a circumferential direction of the skirt portion by rotating the piston for an internal combustion engine in one direction; a second scanning step of scanning the irradiation spot in the longitudinal direction of the skirt portion by moving the piston for an internal combustion engine along the longitudinal direction; a third scanning step of rotating the internal combustion engine piston in a direction opposite to the one direction, thereby scanning the irradiation spot from the other direction to the one direction in the circumferential direction of the skirt portion; Repeating a series of scanning steps including 2. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein air is injected from a first air injection nozzle in the first scanning step and from a second air injection nozzle in the third scanning step, with the injection direction shifted.

3. 3. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein the air injection direction of the air injection nozzle is inclined toward the piston crown surface side or toward an opposite side with respect to a plane perpendicular to an axis of the piston body for an internal combustion engine, and air is injected.

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