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A solenoid valve includes a solenoid coil, a yoke of magnetic material, an armature of magnetic material to move in an axial direction of the solenoid coil when the solenoid coil is energized, a plunger to be moved with the armature, a valve seat member including a seat portion formed with a valve h

A solenoid valve includes a solenoid coil, a yoke of magnetic material, an armature of magnetic material to move in an axial direction of the solenoid coil when the solenoid coil is energized, a plunger to be moved with the armature, a valve seat member including a seat portion formed with a valve hole to be closed and opened by a forward end portion of the plunger, and a coil spring which is disposed between a spring receiving portion of the plunger and the valve seat member, around the plunger. The plunger includes a pressure receiving portion (such as a flat portion) which extends in a radial direction of the plunger at a position between the forward end portion and the spring receiving portion of the plunger, and which faces to the seat portion.

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1. A normally-open solenoid valve comprising: a solenoid coil to produce a magnetic field when energized;a yoke of magnetic material encasing the solenoid coil;an armature of magnetic material surrounded by the yoke, and arranged to move in an axial direction of the solenoid coil when the solenoid c

1. A normally-open solenoid valve comprising: a solenoid coil to produce a magnetic field when energized;a yoke of magnetic material encasing the solenoid coil;an armature of magnetic material surrounded by the yoke, and arranged to move in an axial direction of the solenoid coil when the solenoid coil is energized;a plunger of nonmagnetic nonmagnetic material arranged to move with movement of the armature;a valve seat member including a seat portion formed with a fluid passage to be closed and opened by a forward end portion of the plunger moving toward and away from the seat portion; anda coil spring disposed between a spring receiving portion of the plunger and the valve seat member, around the plunger, and arranged to urge the plunger in a direction toward the armature;the plunger including a flat portion which extends in a radial direction of the plunger at a position between the forward end portion of the plunger and the spring receiving portion, which faces to the seat portion, and which has an outer circumference so sized that an outside diameter of the flat portion is smaller than an inside diameter of the coil spring;wherein the forward end portion of the plunger includes a hemispherical end, the flat portion of the plunger is formed continuously between a larger diameter portion of the forward end portion and an outside circumferential surface of the plunger, the flat portion extends from an inner corner which is formed between the flat portion and the larger diameter portion, to an outer corner which is formed between the flat portion and the outside circumferential surface, and the inner and outer corners are formed so that a radius of curvature of the inner corner is smaller than or equal to R0.03 or a radius of curvature of the outer corner is smaller than or equal to R0.1. 2. The normally-open solenoid valve as recited in claim 1, wherein the forward end portion of the plunger is hemispherical, and the inner and outer corners are formed so that the radius of curvature of the inner corner is smaller than or equal to R0.03 and the radius of curvature of the outer corner is smaller than or equal to R0.1. 3. The normally-open solenoid valve as recited in claim 2, wherein the outside diameter of the flat portion is greater than a hole diameter of a hole of the seat portion against which the forward end portion of the plunger abuts. 4. The normally-open solenoid valve as recited in claim 3, wherein the length from the forward end portion to the flat portion of the plunger is equal to the hole diameter of the seat portion multiplied by 1.5. 5. The normally-open solenoid valve as recited in claim 3, wherein the length from the forward end portion to the flat portion of the plunger is equal to the hole diameter of the seat portion multiplied by 1.2. 6. The normally-open solenoid valve as recited in claim 1, wherein the plunger is a resin molding. 7. The normally-open solenoid valve as recited in claim 6, wherein the plunger is the resin molding formed by injection molding of a resin with a mold having a parting line at the position of the flat portion. 8. A normally-open solenoid valve comprising: a resin molded plunger urged by a coil spring in a valve opening direction;a drive section to actuate the plunger electromagnetically; anda valve seat member including a fluid passage to be closed and opened by a forward end portion of the plunger and a seat portion arranged, so that the forward end portion of the plunger abuts against the seat portion and thereby closes the fluid passage;the plunger including a fluid pressure receiving portion extending in a radial direction of the plunger at a position between the forward end portion of the plunger and a spring receiving portion of the plunger, facing to the sea portion, and having an outside diameter smaller than an inside diameter of the coil spring;wherein the forward end portion of the plunger is hemispherical, the pressure receiving portion of the plunger is formed continuously between a larger diameter portion of the hemispherical forward end portion and an outside circumferential surface of the plunger, and a radius of curvature of a corner which is formed between the pressure receiving portion and the larger diameter portion is smaller than or equal to R0.03. 9. The normally-open solenoid valve as recited in claim 8, wherein a radius of curvature of a corner which is formed between the flat portion and the outside circumferential surface is smaller than or equal to R0.1. 10. The normally-open solenoid valve as recited in claim 9, wherein an outside diameter of the pressure receiving portion is greater than an inside diameter of a hole of the seat portion against which the forward end portion of the plunger abuts. 11. The normally-open solenoid valve as recited in claim 10, wherein the length from the forward end portion to the pressure receiving portion of the plunger is equal to the inside diameter of the hole of the seat portion multiplied by 1.5. 12. The normally-open solenoid valve as recited in claim 10, wherein the length from the forward end portion to the pressure receiving portion of the plunger is equal to the inside diameter of the hole of the seat portion multiplied by 1.2. 13. The normally-open solenoid valve as recited in claim 8, wherein the plunger is a resin molding formed by injection molding of a resin, with a mold having a parting line at the position of the pressure receiving portion. 14. A normally-open solenoid valve comprising: a solenoid coil to produce a magnetic field when energized;a yoke of magnetic material encasing the solenoid coil;an armature of magnetic material surrounded by the yoke, and arranged to move in an axial direction of the solenoid coil when the solenoid coil is energized;a plunger of nonmagnetic material arranged to move with movement of the armature;a valve seat member including a seat portion formed with a hole to be closed and opened by a forward end portion of the plunger moving toward and away from the seat portion; anda coil spring disposed between a spring receiving portion of the plunger and the valve seat member, around the plunger, and arranged to urge the plunger in a valve opening direction away from the seat portion;the plunger including a pressure receiving portion which extends in a radial direction of the plunger at a position between the forward end portion of the plunger and the spring receiving portion, and which faces to the seat portion;wherein the forward end portion of the plunger includes a hemispherical end, the pressure receiving portion of the plunger is formed continuously between a lager diameter portion of the forward end portion and an outside circumferential surface of the plunger, the pressure receiving portion extends from an inner corner which is formed between the pressure receiving portion and the larger diameter portion, to an outer corner which is formed between the pressure receiving portion and the outside circumferential surface, and the inner and outer corners are formed so that a radius of curvature of the inner corner is smaller than or equal to R0.03 or a radius of curvature of the outer corner is smaller than or equal to R0.1. 15. The normally-open solenoid valve as recited in claim 14, wherein the plunger includes a shaft portion extending from the spring receiving portion to the forward end portion, and the shaft portion and the forward end portion of the plunger and the seat portion of the valve seat member are disposed in the coil spring. 16. The normally-open solenoid valve as recited in claim 15, wherein the pressure receiving portion includes an annular shoulder surface facing axially toward the seat portion and extending radially outwards from an inner circumference forming an inner recessed corner with an outside circumferential surface of the forward end portion, to an outer circumference forming an outer projected corner with an outside circumferential surface of the shaft portion. 17. The normally-open solenoid valve as recited in claim 14, wherein the pressure receiving portion includes a shoulder surface facing axially toward the seat portion and extending radially outwards from an imaginary inner cylindrical surface defining an inner circumference of the shoulder surface, to an imaginary outer cylindrical surface surrounding the inner cylindrical surface coaxially and defining an outer circumference of the shoulder surface, and the forward end portion projects axially from the inner circumference of the shoulder surface to a tip end within the imaginary inner cylindrical surface. 18. The normally-open solenoid valve as recited in claim 14, wherein the seat portion of the valve seat member projects to an annular top end confronting the pressure receiving portion of the plunger, and includes a concave conical surface which is formed around the hole and surrounded by the annular top end and which extends radially outwards from a circular inner circumference and terminates at a circular outer circumference, and the plunger includes the forward end portion including a hemispherical convex tip end abutting on the concave conical surface of the seat portion to close the hole in a valve closing position, and the pressure receiving portion including an annular shoulder surface extending radially outwards from a circular inner circumference having a diameter greater than a diameter of the circular inner circumference of the concave conical surface, to a circular outer circumference having a diameter which is greater than a diameter of the circular outer circumference of the conical surface.