US 4,753,546 AGranted

Pressure balanced stylographic pen

AssigneeKoh-I-Noor Rapidograph, Inc.

InventorsWolfgang Witz, Fortunato J. Micale, John P. Leuenberger

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Abstract

Improvements in stylographic technical writing pens, particularly through a venting channel design which provides for a pressure balancing, or equalization, between an ultimate ink reservoir pressure and the total pressure at the writing tip; as writing depletes ink within the reservoir. Particularly, a venting channel, extending from the reservoir to ambient air, has the size of its cross-sectional shape vary as a function of the distance from its communication with the reservoir, with the variation calculated to offset gravitational forces attendant to a moving ink meniscus inside the vent channel. The balancing of total pressures also provides a constant ink flow through the writing tip when writing since the total pressure at the tip is maintained constant by the vent channel configuration. The present invention is characterized by the novel approach, of beginning with the insight that a varying static pressure, from a varying level of ink to a vent channel, must exactly offset by the capillary forces at the meniscus of that ink level in the vent channel, and then creating a total vent structure that can follow that relationship.

Claims8

Dependent claims

In a stylographic drafting pen of the type comprising a housing which surrounds an ink reservoir, a writing nib which communicates ink, in a longitudinal direction, from said reservoir to form a drop at the tip of a writing tube, and a venting channel which communicates, at a first end, with said ink reservoir through a vent hole and, at a second end, with ambient air, the improvement comprising:

(a) a venting channel which has a cross-sectional area variation from said first to said second end, as a function of longitudinal distance of a given vent cross-section, h.sub.3, in elevation above said vent hole which is spaced a longitudinal distance L.sub.2 above said tip (whereat h.sub.3 =0), wherein said vent cross-sectional variation is defined so that variations in the hydrostatic pressure, which is experienced at the pen tip, as a consequence of ink travel within the venting channel to a given value of h.sub.3, are balanced and offset by variations in the capillary force which then acts upon the air/ink interface of a meniscus of ink formed as a result of the cross-sectional value of said vent cross-section which is defined at said given value of h.sub.3,
and
(b) wherein, said venting channel improvement further comprises, for a given value of a preferred mean radius of curvature of a droplet at the pen point (r.sub.m.sbsb.2), a given ink surface tension (.gamma.) and density (.rho.), a height of the vent hole above the tip of the writing pen (L.sub.2), the gravity acceleration constant (g), a given contact angle (.theta.) between said ink and the material defining the walls of said venting channel, a venting channel cross-sectional area variation which is a decreasing function as h.sub.3 increases away from said tip, wherein at any h.sub.3 the cross-section value will ensure that the mean radius of curvature of an ink meniscus (r.sub.m.sbsb.3) at that h.sub.3 is governed by the relationship, as follows:
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A pressure balanced stylographic drafting pen of the type comprising a housing which surrounds an ink reservoir, a writing nib which communicates ink from said reservoir, in a longitudinal direction, to the tip of a writing tube, and a venting channel which communicates, at a first end, to an ink reservoir through a vent hole and, at a second end, to an ambient air port, which comprises:

(a) a venting channel which has a cross-sectional area variation as a function of the distance (h.sub.3) of a given cross-section from said first end of the venting channel, wherein said cross-sectional area variation is a function of a mean radius of curvature (r.sub.m.sbsb.3) for a meniscus of ink which forms at an ink air interface of an ink column at a given value of h.sub.3 ;
wherein
(b) said vent hole, at the first end of said venting channel, is located a distance (L.sub.2) from the tip of said writing tube and said vent hole is also proximate to the writing nib end of said ink reservoir;
and
(c) said venting channel extends longitudinally, for increasing values of h.sub.3, in a double-spiral fashion, wherein said venting channel first end communicates with said vent hole, with an initial cross-sectional area thereat which then decreases in cross-sectional area as h.sub.3 increases away from said writing tip, to a position on said housing whereat said venting channel returns in a second spiral, towards the second end of said venting channel, said second end communicating with said ambient air port and being proximate the writing tube of said pen, wherein said variations in vent channel cross-section enable hydrostatic pressure increases, as ink fills said venting channel, to be directly balanced and offset by a variable capillary force upon the ink/air interface at the meniscus of said ink column, with said capillary force being a direct function of h.sub.3,
(d) wherein the initial value for the venting channel cross-section of h.sub.3 =0 is defined by a relationship, for an ink having a given surface tension (.gamma.), a given density (.rho.), and a given wetting angle between said ink and the material of which the venting channel is constructed (.theta.) so that a desired mean radius of curvature for an ink droplet at the writing tip (r.sub.m.sbsb.2) will be maintained stable, and not break away, in response to increased values of hydrostatic pressure from increasing columns of ink within said venting channel according to a relationship where g is the acceleration due to gravity, as follows:
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Dependencies are inferred from the claim text.