In order to provide a writing ball capable of suppressing wear of a ball seat of a ballpoint pen tip and obtaining stable ink outflow, a writing ball that is a sphere, the sphere comprising, at least: a first hard component containing a first metal; a second hard component containing a second metal; and a binding component, the writing ball comprising: a first outer surface from which the first hard component is exposed, the first outer surface being an outermost surface of the sphere; and a recess including, as a bottom surface, a second outer surface from which the second hard component is exposed, the second outer surface being recessed inward from the first outer surface, wherein the first outer surface includes a smooth surface is provided.

To make it possible to easily change a writing line width.

A ballpoint pen chip that brings a front half part side of a transfer ball into contact with an inward annular protrusion on a front end side in a chip body from a backward side of the inward annular protrusion and receives a rear half part side of the transfer ball with a front end part of a receiving member. The receiving member includes, on a front end side thereof, a reduction part gradually reduced in a cross-sectional area forward. A front-most end part of the reduction part is disposed backward from the transfer ball. A portion of the receiving member further on a rear side than the reduction part is unretreatably fixed to the chip body and configured to elastically contract when a pressing force is received from the transfer ball.

A writing ball capable of suppressing wear of a ball seat of a ballpoint pen tip and obtaining stable ink outflow, a writing ball that is a sphere, the sphere comprising, at least: a first hard component containing a first metal; a second hard component containing a second metal; and a binding component, the writing ball comprising: a first outer surface from which the first hard component is exposed, the first outer surface being an outermost surface of the sphere; and a recess including, as a bottom surface, a second outer surface from which the second hard component is exposed, the second outer surface being recessed inward from the first outer surface, wherein the first outer surface includes a smooth surface.

Embodiments of the invention are directed to a roller ball marking instrument for use with paint, a method for assembling the roller ball marking instrument, and a system for delivering paint. The body of the marking instrument includes a reservoir for storing paint. The body is coupled to a fibrous roller ball via a retaining ring. The roller balls sits within a partially-enclosed cavity in the center of the retaining ring. A least part of the roller ball protrudes through an opening in the retaining ring. Paint in the reservoir moves first through an opening at one end of the body and then through an open center of the retaining ring. The paint is transferred to the roller ball. The roller ball rotates freely within the cavity of the retaining ring and transfers the paint to a writing surface.

A ballpoint pen 10 includes: a ballpoint pen tip 20 having a writing ball 30 and a holder 21 holding the writing ball 30; a shaft tube 12 to which the rear end part of the ballpoint pen tip 20 is mounted; and ink 40 accommodated in the shaft tube 12. The holder 21 has an ink guiding hole 26 formed from the rear end thereof toward the head end thereof and a ball house 22 formed with the inner circumference near the head end of the holder 21 expanded. The writing ball 30 is formed of a zirconia sintered body with the content of an aluminum element being less than 0.1 weight %, and the ink 40 contains inorganic particles 41.