a marking pen comprises a hollow cylindrical ink reservoir and a removable writing tip . it may also include a pressure means which , upon hand actuation , applies pressure to the viscous ink within the reservoir . the writing tip is a hollow plastic member having an internal flange to position a spring and means to hold a hollow cylindrical metal member . the spring is held between the flange and a ball , the ball partly protruding from the orifice of the inwardly flanged valve - type seat of the metal member .