Push button type ballpoint pen

Abstract

A retractable ballpoint pen is free from leakage of ink from its tip and ink back-flow attributable to shocks produced by retractable operation, and is capable of satisfactory writing performance. The retractable ballpoint pen has a ballpoint pen refill (3) having a ballpoint pen tip (7) disposed in a forward end of an ink-accommodating tube (8), a ball (9) held for free rotation in a forward end part of the ballpoint pen tip (7). The retractable ballpoint pen is provided with a retracting mechanism (5, 6) for projecting a forward end of the ballpoint pen tip (7) and retracting the same through an open forward end (2a) of a barrel (2). A water-based ink (15) contained in the ink-accommodating tube (8) contains a thixotropic agent and has a viscosity at a rate of shear of 1.9 sec−1 at 20° C. in the range of 900 to 2500 mPa·s. A restricting passage (20) is formed in a passage for the water-based ink (15) contained in the ink-accommodating tube (8) to flow to the ball (9) and has a cross-sectional area measured perpendicular to the axis of the ink-accommodating tube (8) in the range of 0.4 to 0.8 mm2. The restricting passage (20) limits the flow of the water-based ink (15).

Dependencies are inferred from the claim text.