1. Field of the Invention
The present invention relates to marking instruments, and specifically to a scheme for metering capillary flow of marking fluid in a marking instrument.
2. Description of the Prior Art
Two problems common to many prior art ink pens or marking instruments are the undesired discharge of ink prior to use (i.e., shelf leakage) and the uncontrolled or uneven release of ink during use.
Various schemes have been proposed to prevent undesired or premature ink leakage. For example disposable and sealed ink cartridges have been used. Such ink containers are pierced or tapped for use. Koh-I-Noor Rapidograph, Inc. of Bloomsbury, N.J. has manufactured a completely disposable two-section pen. One section of the pen contains ink in a free-flowing state. The second section of the pen, which is movable with respect to the first section, contains the pen marking tip. A cap or seal is positioned between the two pen sections and by moving the second section toward the first section, the cap or seal is dislodged to permit the ink to flow to the marking tip.
The rate of flow of ink from a pen is a function of the viscosity of the ink along with other variables, such as temperature, pen tip size, etc. To avoid creating a vacuum (and thus stopping ink flow) as ink is drawn out of a pen's ink reservoir, ink reservoirs are usually vented to ambient air to permit air to be drawn into and occupy the space vacated by the ink. General examples of marking instruments constructed in this fashion are shown in the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 3,355,239 Albrecht 11/28/67 3,377,124 Matsumoto 4/9/68 3,397,939 Berry 8/20/68 3,966,336 Lotfallah 6/29/76 4,017,870 Hubbard et al. 4/12/77 4,017,871 Hubbard 4/12/77 4,145,148 Fukuoka 3/20/79 4,317,639 Kato 3/2/82 4,382,707 Anderka 5/10/83 ______________________________________
These U.S. patents are hereby incorporated by reference.
Various schemes for ink flow regulation are shown in the incorporated patents. For example, the Anderka patent shows a felt tip pen with cylindrical inner and outer pressure equalization chambers. The Hubbard et al. patent shows a pen having an ink reservoir wherein pressure regulation and ink feed is obtained by manipulation of a thin disk. The Berry patent discloses a pen wherein a metering valve is provided between a reservoir of liquid ink and the pen tip for measuring and controlling the flow of liquid ink to the tip. The metering valve in Berry is a cap with an aperture and shape designed to create an air pressure differential within the pen.
As noted above, air vents are often employed as a means to control the flow rate of ink. The major problem with using an air vent to control ink flow is that different inks have different viscosities, and thus different sizes of vents are necessary (all other variables being the same) to obtain identical flow rates for each different ink. In addition, such pens are often used on precision instruments such as plotters. The air vents of such pens must thus be very precise themselves in order to achieve the desired ink flow rate. To obtain a precise ink flow rate dependent upon the size of an air vent requires careful machining and expensive manufacturing costs in making the pen and air vent arrangement.