Piston for internal combustion engine, and production method and production device for piston for internal combustion engine

A production device for a piston for an internal combustion engine, the piston having a crown portion having a crown surface that defines a combustion chamber, thrust-side and anti-thrust-side skirt portions formed integrally with the crown portion and sliding on a cylinder wall surface and a pair of apron portions joined to the pair of skirt portions in a circumferential direction, each of the apron portions having a pin boss portion provided with a piston pin hole, the production device comprising:

• a lower mold that molds inner surfaces of the both skirt portions and the both apron portions and a back surface that is an opposite side to the crown surface of the crown portion, molds a recessed portion on the back surface between the pair of skirt portions and molds a plurality of protrusions integrally with an inner surface of the recessed portion, the plurality of protrusions being formed along an arrangement direction of the pair of skirt portions or an arrangement direction of the pair of apron portions, wherein a middle portion is formed in a middle of a bottom surface of the recessed portion in a longitudinal direction of the bottom surface of the recessed portion, and has a predetermined span in the longitudinal direction of the bottom surface, the middle portion dividing the plurality of protrusions into two groups, the predetermined span being greater than a predetermined clearance between two adjacent protrusions in each group of the two groups, the middle portion defining the predetermined span, and at least one end edge in a longitudinal direction of each of the protrusions being integrally connected to an inner side surface of the recessed portion, the inner side surface facing the at least one end edge;
• and an upper mold that is located on an upper side of the lower mold and molds a crown surface side of the crown portion, and the lower mold being provided with:
• a protruding portion for molding the recessed portion, the protruding portion being formed on an upper surface of a middle portion of an inner surface forming portion that forms each surface of the both skirt portions;
• and a plurality of groove portions for molding the protrusions, the plurality of groove portions being formed on an upper surface of the protruding portion, and the lower mold being configured so that a height of the middle portion is set to be higher than those of the other portions of the inner surface forming portion of the lower mold by a height of the protruding portion, and a depth of each of the groove portions is set to be shallower than the height of the protruding portion, a height of an opening formed on at least one end side in a longitudinal direction of each of the groove portions is set to be lower than or the substantially same as a bottom surface of the groove portion, and molten metal poured in the lower mold flows to the bottom surface of each of the groove portions from the opening of the groove portion.

A method of producing a piston for an internal combustion engine, the piston having a crown portion having a crown surface that defines a combustion chamber;

• thrust-side and anti-thrust-side skirt portions formed integrally with the crown portion and sliding on a cylinder wall surface;
• a pair of apron portions joined to the pair of skirt portions in a circumferential direction, each of the apron portions having a pin boss portion provided with a piston pin hole;
• a recessed portion formed on a back surface that is an opposite side to the crown surface of the crown portion and extending between the pair of skirt portions along a substantially longitudinal direction;
• and a plurality of protrusions formed integrally with a bottom surface of the recessed portion and extending along an arrangement direction of the pair of apron portions, and at least one end edge in a longitudinal direction of each of the protrusions integrally connected to an inner side of the recessed portion, the inner side surface facing the at least one end edge, wherein a middle portion is formed in a middle of the bottom surface of the recessed portion in a longitudinal direction of the bottom surface of the recessed portion, and has a predetermined span in the longitudinal direction of the bottom surface of the recessed portion, the middle portion dividing the plurality of protrusions into two groups, the predetermined span being greater than a predetermined clearance between two adjacent protrusions in each group of the two groups, the middle portion defining the predetermined span, the method comprising:
• a clamping process clamping a lower mold that molds a back surface that is an opposite side to the crown surface of the crown portion and inner surfaces of the both skirt portions and the both apron portions, and setting an upper mold that molds the crown surface of the crown portion to a predetermined upper position of the lower mold;
• a pouring process pouring molten metal into a cavity formed between the lower mold and the upper mold by a gravity casting process;
• a flowing process, during the pouring process, making the molten metal flow to a bottom surface of each of a plurality of groove portions of the lower mold, which mold the plurality of protrusions, through an opening formed at one end portion of each of the groove portions;
• and a taking-out process, after filling the cavity with the molten metal and cooling and solidifying the molten metal, separating the upper mold and the lower mold and taking out the piston from the cavity.