Claims
- 1. A writing instrument comprising an ink reservoir and an air hole, the air hole being spanned by barrier means, formed as a porous non-absorbent ink repellent membrane of substantially uniform pore size comprising a membrane formed from microfibers fixed together at their intersections, non-porously sealed along its entire periphery to the wall of the hole, for allowing the macroscopic flow of air therethrough in both directions while preventing the escape of ink therethrough and therearound.
- 2. A writing instrument according to claim 1, in which the barrier means is formed of a hydrophobic fluorinated polymeric material.
- 3. A writing instrument according to claim 2, in which the barrier means is formed of polytetrafluoroethylene.
- 4. A writing instrument according to claim 1, in which the pore radius of the barrier means is not more than 10 .mu.m.
- 5. A writing instrument according to claim 1, in which the wetting angle for ink on the barrier means is not less than 100.degree..
- 6. A writing instrument according to claim 1, in which the thickness of the barrier means is not more than 200 .mu.m.
- 7. A writing instrument according to claim 1, in which the effective central area of the barrier means is between 0.25 mm.sup.2 and 25 mm.sup.2.
- 8. A writing instrument according to claim 1, in which the surface tension and the viscosity of ink in the reservoir are respectively not less than 30 mN m.sup.-1 and not more than 10 cp.
- 9. A method of making a writing instrument comprising heat sealing the entire periphery of a barrier formed as a macroscopically porous non-absorbent ink repellent membrane of substantially uniform pore size, comprising a membrane formed from microfibers fused together at their intersections, in such a manner that a central part of the barrier spans an air hole to allow air to pass freely therethrough in both directions but to prevent escape of ink from an ink reservoir with the peripheral part of the barrier being non-porously sealed to the wall of the hole.
- 10. A method according to claim 9, in which the act of applying heat causes a projecting seating to melt and permeate the pores at the periphery of the barrier.
- 11. A method according to claim 10, in which the projecting seating is tapered, before the heating starts, and is thinner nearer the barrier.
- 12. A method according to claim 9, in which sealing pressure is applied during the heating.
- 13. A method according to claim 9, in which all other routes, except a writing tip, through which ink could escape from the reservoir are sealed.
- 14. A method according to claim 13, in which the sealing of the other ink escape routes involves the use of an adhesive.
- 15. A method according to claim 9, in which the barrier is formed of a fluorinated material.
- 16. A method according to claim 15, in which the barrier is formed of polytetrafluoroethylene.
- 17. A method according to claim 9, in which the pore radius of the barrier is not more than 10 .mu.m.
- 18. A method according to claim 9, in which the wetting angle for ink on the barrier is not less than 100.degree..
- 19. A method according to claim 9, in which the thickness of the barrier is not more than 200 .mu.m.
- 20. A method according to claim 9, in which the effective central area of the barrier is between 0.25 mm.sup.2 and 25 mm.sup.2.
- 21. A method according to claim 9, in which the surface tension and the viscosity of ink in the reservoir are respectively not less than 30 mN m.sup.-1 and not more than 10 cp.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 8207675 |
Mar 1982 |
GBX |
|
Parent Case Info
This application is a continuation, of application Ser. No. 474,523, filed 3/11/83 now abandoned.
The present invention relates to writing instruments, of the kind including an air hole allowing ingress and egress of air to and from an ink reservoir, as well as to methods of making such writing instruments.
It is well known that, as a result of a temperature or pressure change, ink may be forced out of the reservoir. A collector for temporarily retaining this displaced ink is therefore usually provided. However, with excessive changes, such as repeated impacts, flicking, placing close to a radiator or altitude changes as experienced in flying, the collector is filled. Excess displaced ink is then ejected through the air hole. In the absence of a collector, displaced ink would be ejected directly through the air hole. This can cause considerable damage to clothing, if unexpected, and is in any event a nuisance and a waste of ink.
Many attempts have been made over the past 90 years or so to reduce leakage of ink through the air hole.
Initially, the attempts were rather crude and U.S. patent application Ser. No. 511,134 merely proposed the use of a perforated disc. The hope was that the perforations would allow air to enter the reservoir but the disc would retain ink within the reservoir. Even though this did not work, the same kind of arrangement was proposed in U.S. Pat. No. 2,618,239 with the proviso that the perforations would need to be sufficiently small, and the ink would need to be sufficiently viscous. More recently, U.S. Pat. No. 4,108,559 has acknowledged that ink seepage has still occurred through a variety of materials which are porous, rather than perforated, and has instead proposed a complicated braided bundle of filaments encased in an elongate sheath. It is thus clear that, although the theory of operation is well established, no practical and economic answer to the problem has been found.
The aim of the present invention is to provide such good immunity from leakage through the air hole, even by a low viscosity ink, that it is necessary to seal all other ink escape routes, except of course a writing tip, to prevent the air hole being by-passed.
A writing instrument, in accordance with a first aspect of the present invention, comprises an ink reservoir and an air hole, the air hole being spanned by a barrier formed as a porous non-absorbent ink repellent membrane of substantially uniform pore size non-porously sealed along its entire periphery to the wall of the hole, which barrier allows air to pass freely therethrough in both directions but prevents escape of ink therethrough and therearound.
Requiring the barrier to be porous (preferably a pore radius of not more than 10 .mu.m), non-absorbent in that the pores remain empty when in contact with ink at normal operating pressures, ink repellent in that the wetting angle for ink on the barrier is greater than 90.degree. (preferably not less than 100.degree.) and of substantially uniform pore size rather than random pore sizes, allows the barrier to be as thin as a membrane (preferably not more than 200 .mu.m).
The writing instrument is intended for use with an ink of high surface tension and low viscosity (preferably not less than 30 mN m.sup.-1 and not more than 10 cp respectively) and can therefore be used with, merely as examples, an extruded plastics tip, a ball point or a fountain nib.
Preferably, the barrier is situated at the only point from which air can enter the reservoir or ink can escape from the writing instrument other than through its writing tip.
Other independently preferred embodiments are that:
The barrier presents a membrane which may be formed from microfibres which are fused together at their intersections. Such a membrane is available commercially under the trade name Mitex (RTM) from Millipore (UK) Ltd., of Abbey Road, London, England. Another suitable barrier is available commercially under reference SM 11842 from Sartorius Instruments Ltd., of Belmont, Surrey, England. These and other similar fluorinated polymeric membranes are marketed as filters for the purpose of removing solid particulate material from unidirectional fluid flow therethrough.
It may be thought that, in general terms, most porous materials (e.g. porcelain) can be used as a barrier to stop leakage of a fluid therethrough, while allowing free passage of air. Unless the fluid is mercury, for example, it will be absorbed in the pores of the barrier and will, sooner or later, saturate the barrier. There is then no further protection against leakage other than by normal capillary forces. Moreover, while the barrier is being saturated, there cannot be free passage of air through the barrier because the air first has to displace the fluid from the pores of the barrier. If used in a writing instrument, the lack of free passage of air to the reservoir will result in a reduction in the flow of ink to the writing tip. In the present invention, the porous material is also required to have a number of further physical characteristics so that ink will not penetrate the pores until a considerable pressure has been applied.
The head of liquid, corresponding to the pressure, required to force ink into the pores (assuming cylindrical capillaries) is given by the usual equation for capillary rise: ##EQU1## where .DELTA.P is the liquid head
.theta. is the wetting angle
Preferably, the pore radius is small (e.g. 10 .mu.m or less), the wetting angle is high (necessarily over 90.degree. to be described as ink repellent), and the surface tension is high (e.g. 30 mN m.sup.-1 or more), to increase the liquid head necessary to force ink into the pores.
For example, if the pore radius is 2.5 .mu.m, the wetting angle is 100.degree., and the ink surface tension is 40 mN m.sup.-1, the barrier will support a liquid head of 566 mm.
It will thus be clear that the present invention permits a high degree of immunity to be achieved from ink leakage. The immunity from leakage through the air hole may be so great that it is necessary to seal (e.g. by use of an adhesive) any other ink escape routes to prevent the air hole being by-passed. Indeed it is also important that the air hole be properly spanned and sealed on the periphery otherwise the barrier might itself be by-passed.
A method of making a writing instrument in accordance with a second aspect of the present invention comprises heat sealing the entire periphery of a barrier formed as a porous non-absorbent ink repellent membrane of substantially uniform pore size in such a manner that a central part of the barrier spans an air hole to allow air to pass freely therethrough in both directions but to prevent escape of ink from an ink reservoir with the peripheral part of the barrier being non-porously sealed to the wall of the hole.
Preferably, the act of applying heat causes a projecting seating to melt and permeate the pores at the periphery of the barrier. The best bond is produced by a thermoplastics material which is melted to a high temperature to produce low viscosity and is under a sealing pressure to induce good penetration. The melting of the seating can be induced either by directly heating or indirectly by for example welding ultrasonically. As integrity of the seal is of greater significance than its strength, the seating is preferably tapered, and is thinner nearer the barrier. The effective central area of the barrier (not obstructed by the heat sealing) is preferably between 0.25 mm.sup.2 and 25 mm.sup.2.
US Referenced Citations (6)
Foreign Referenced Citations (5)
| Number |
Date |
Country |
| 465591 |
Mar 1975 |
AUX |
| 715042 |
Sep 1954 |
GBX |
| 1019675 |
Feb 1966 |
GBX |
| 1053750 |
Jan 1967 |
GBX |
| 1191674 |
May 1970 |
GBX |
Continuations (1)
|
Number |
Date |
Country |
| Parent |
474523 |
Mar 1983 |
|
Patent Grant
4954002
