Abstract: In a writing instrument including an ink reservoir and an air hole, the air hole is spanned by a barrier formed as a porous non-absorbent ink repellent membrane of substantially uniform pore size. The barrier is non-porously sealed, for example by direct heating or ultrasonic welding, along its entire periphery to the wall of the air hole, the barrier allowing air to pass freely therethrough in both directions, but preventing escape of ink therethrough and therearound. The barrier is preferably formed of polytetrafluoroethylene with a pore radius of not more than 10 .mu.m and a thickness of not more than 200 .mu.m. The ink for use in the writing instrument preferably has a surface tension of not less than 30 mN m.sup.-1, a viscosity of not more than 10 cp and a wetting angle with the barrier of not less than 100.degree..

Abstract: The lead reservoir and feed mechanism of a knock type mechanical pencil is secured to the tubular barrel in which it is housed by a resilient locking sleeve positioned in a groove near the front of the feed mechanism and a spring compressed between a rearwardly facing internal abutment on the feed mechanism. The spring urges the lead reservoir and feed mechanism rearwardly into the barrel to a position where an external flange on the locking sleeve abuts the front end of the barrel.

Abstract: A writing ink composition having a pseudoplasticity index value between about 0.02 and 0.18, a low-shear viscosity between about 25,000 and 120,000 cP, and a high-shear viscosity between about 6 and 26 cP can be used in a hybrid, ball-point marking instrument which combines advantageous features of both paste-ink and liquid-ink ballpens. The writing ink can be prepared using a colloid milling procedure.

Abstract: A continuous feed mechanical pencil includes a lead cartridge having a lead advancing mechanism responsive to forward axial movement of an actuator button protruding from the rear end of the instrument, the actuator button being fixedly attached to a pocket clip and extending a relatively short distance from the rear of the instrument housing so that the rear end of the actuator is shrouded by the rear end of the housing when the actuator button is fully depressed.