EP0048571B1

EP0048571B1 - Pencil lead, and methods and apparatus for its manufacture

Info

Publication number: EP0048571B1
Application number: EP19810304138
Authority: EP
Inventors: Kiyoshi Inoue
Original assignee: Inoue Japax Research Inc
Current assignee: Inoue Japax Research Inc
Priority date: 1980-09-24
Filing date: 1981-09-10
Publication date: 1984-08-01
Also published as: DE3165243D1; EP0048571A2; EP0048571A3; JPS5757696A; EP0048571B2

Description

The present invention generally relates to a pencil lead and, more particularly, to an improved pencil lead, as well as to methods of an apparatus for making the pencil lead.
It is desirable that pencil lead for a variety of forms of pencils including wood-sheathed ordinary pencils and especially spare leads for propelling-type pencils be thinner and stronger, i.e. stiffer and of greater strength to resist breaking, in addition to satisfying their general requisites of lower friction coefficient or greater smoothness to write, suitable wear resistance, higher reflectivity or darkness (e.g. blackness) and less dispersion of powder on writing.
The present invention seeks to provide a novel and improved pencil lead suitable for a wide variety of pencils including wood-sheathed pencils and especially for a propelling pencil, which lead is greater in strength than the conventional pencil lead and can be much reduced in diameter while exhibiting a strength equivalent to or even higher than that of the conventional pencil lead.
The present invention also seeks to provide a novel and improved pencil lead which is thin and stiff, and yet presents a sufficiently soft feeling on writing.
In addition, the present invention seeks to provide a method of making the improved pencil lead and to provide an apparatus for carrying out that method.
In accordance with the present invention, there is provided, in a first aspect thereof, a pencil lead constituted by a plurality of (preferably three or more) filaments of a solid marking substance individually having a thickness ranging between 10 and 250 microns, preferably between 10 and 100 microns and optimally between 30 and 50 microns, and bundled and joined together with a binder material. Specifically, the solid marking substance may consist at least in part of finely divided graphite, and the binder material may consist at least in part of finely divided graphite and may contain a synthetic resin. The pencil lead or the bundle of the filaments when joined may advantageously have a thickness ranging between 0.1 and 0.5 mm. The solid marking substance may advantageously consist of a mixture essentially of graphite particles and at least one synthetic resin uniformly mixed and heat-treated. The graphite particles may be particles of a uniform particle size in a range between 10 and 100 or 200 microns and preferably between 30 and 50 microns, but preferably should include particles of a relatively small, uniform particle size (e.g. between 1 and 10 microns) and particles of a relatively large, uniform particle size (e.g. between 10 and 100 microns or between 150 and 200 microns.
The invention also provides, in a second aspect thereof, a method of making a pencil lead, comprising the steps of: (a) preparing a semi-soft solid marking substance; (b) extruding the said substance to prepare a plurality of filaments of the said substance individually having a thickness ranging between 10 and 250 microns and preferably between 10 and 100 microns; and (c) bundling the said filaments and joining them with a binder material to form the said pencil lead.
The method advantageously further comprises the step (d) of heating the said filaments at at least one temperature between 800 and 3500°C subsequent to step (b) and prior to step (c), and further the step (e) subsequent to step (c), of heating the said bundled filaments at a temperature between 1000 and 2000°C. Preferably, each of steps (d) and (e) is carried out in an inert atmosphere (e.g. argon) or in a graphite powder to prevent the filaments from being oxidized.
The invention also provides, in a third aspect thereof, an apparatus for making a pencil lead, comprising: a vessel for retaining a semi-soft solid marking substance; means for extruding the said substance in the said vessel to prepare a plurality of filaments of the said core material having a thickness ranging between 10 and 250 microns; and means for bundling the said filaments and joining them with a binder material to form the said pencil lead.
US patent specification 2 149 905 discloses a lead pencil whose lead is produced from a number of strands. However, the lead is in the form of such strands only during an intermediate step in the manufacturing process, and not in the final product, in which these strands are compacted and thereby merged to form the final lead.
These and other features of the present invention as well as advantages thereof will become more readily apparent from the following description made with reference to the accompanying drawings in which:

Fig. 1 is a cross-sectional view diagrammatically illustrating a texture of a single conventional graphite pencil lead or filament;
Fig. 2 is a graph illustrating the relationship between the thickness of the single graphite pencil lead or filament and the strength thereof;
Figs. 3, 4 and 5 are cross-sectional views diagrammatically illustrating pencil leads embodying the present invention;
Fig. 6 is a diagrammatic view, essentially in elevational or longitudinal section, illustrating an apparatus for making a pencil lead according to the present invention; and
Fig. 7 is a cross-sectional view diagrammatically illustrating a semi-finished lead which develops in the process utilizing the apparatus of Fig. 6.

Referring now to the drawings:

The conventional single graphite-based pencil lead is in the form of a circular rod having a diameter D of 0.2 to 0.5 mm, and electron- micrographical observation has revealed that it has a texture generally as illustrated in Fig. 1. It has been observed that the texture has fine graphite grains shown by fine gathers 11 closer to the peripheral surface S of the core 1 and coarse graphite grains shown by coarse gathers 12 deeper inside the core 1. It has been found that as the diameter of such a graphite-based lead 1 is reduced, the grains 11 closer to the peripheral surface approach in size and appearance the grains 12 deeper inside the core 1. It has thus been assumed that the finer the core 1, the greater its strength (i.e. breaking force per cross-sectional area). This has been experimentally confirmed as shown by the graph of Fig. 2 in which the thickness (diameter 0) of the lead or graphite filament in microns (µ) is plotted along the abscissa and the strength thereof in tons/cm² (alternatively Newtons/m²) plotted along the ordinate. Each of the abscissa and the ordinate is plotted in logarithmic scale. The graph of Fig. 2 shows that the logarithm of strength varies in inverse proportion to the logarithms of thickness.

The novel pencil lead according to the invention has been devised with this discovery in mind and, as shown in Figs. 3, 4 and 5 by way of example, comprises a plurality of elementary filaments 10 of a solid marking substance individually having a thickness ranging between 10 and 250 microns, preferably between 10 and 100 microns, and bundled and firmly joined together with a binder material 13. Three, seven and five elementary filaments 10 are used in the embodiments of Figs. 3, 4 and 5, respectively. In each example, the filaments 10 circular in section may have a diameter and hence a thickness of 30 to 50 microns. This makes a composite pencil lead having an overall thickness of 0.1 mm in Fig. 3 and composite pencil leads having an overall thickness of 0.3 mm in Figs. 4 and 5. Electron-micro- graphical observation shows that a graphite-based pencil filament or elementary lead of such a thickness has a uniform graphite grain distribution throughout its body. Measurements show also that each of the composite leads to Figs. 3, 4 and 5 has a strength approaching or even greater than 10 tons/cm² (i.e. 99.64x 1 07 N/m²). This represents five times or more increase in strength than conventional single leads of 0.2 mm diameter which have at most a strength of 2 tons/cm² (i.e. 19.928 x 1 07 N/m²). It has been found that the composite lead of Fig. 3 of 0.1 mm in diameter constituted with three filaments of elementary leads 10 has a strength of 28 tons/cm² (i.e. 278.99 x 10⁷ N/m²), thus being 14 times stronger than the conventional single lead structure of the same diameter.
Referring to Fig. 6 which shows an apparatus for making a novel pencil lead according to the invention, raw solid marking substance 2 retained in a receptacle 3 is forced through a plurality of small extrusion dies or spinneret openings 4 (e.g. individually circular in cross-section) to produce a plurality of continuously formed filaments 10 of the material ,individually having a uniform thickness as described. The material 2 may be a semi-soft solid or solution of a synthetic resin R (e.g. polyacrylonitrile resin monomer) in which graphite particles G are uniformly distributed and as conventionally is heated, say, at a temperature of 200°C during extrusion. The graphite particles may be of a uniform particle size of, say, 30 to 50 microns but, according to a preferred feature of the invention, should preferably include particles of a relatively small, uniform particle size of around 5 microns, e.g. 1 to 10 microns, and particles of a relatively large, uniform particle size of 10 to 100 microns or 150 to 200 microns.
The filaments 10 continuously extruded through the openings 4 in parallel with one another are passed as a group 5 through a heating stage 6 comprising a primary baking heater 6a and a further heater 6b, then through a joining stage 7 and a final heating stage 8, and are finally squeezed between a pair of compression rollers 9a and 9b which serve to apply a sufficient tension to each of the filaments 10 being drawn from the receptacle 3.
The die openings 4 are formed spaced apart from one another with a suitable spacing in a bottom wall of the receptacle 3. The heater 6a is provided to heat filaments 10 being drawn from the openings 4 at a temperature of, say, 1000°C, thereby baking the filaments 10. The heater 6b is used to heat-treat the baked filaments 10 at a temperature of, say, 3000°C, thereby homogenizing graphite crystallization therein.
The parallel-running filaments are bundled in the joining stage 7 which includes an opening 7a, typically circular in section, defined by the wall of a metal frame 7b and an outlet nozzle 7c of an injector 7d. The latter includes a hopper 7e for feeding a binder material 13, e.g. a mixture of graphite particles and 10% by weight stylene solutioned in toluene, into a chamber 7f in which a plunger 7g formed with a screw 7h extends toward the nozzle opening 7c and is rotationally driven by a motor 7m. As the plunger 7g is rotated by the motor 7m, the binder material 13 is continuously fed into the nozzle 7c and in turn to opening 7a to distribute itself around the individual filaments 10 and to fill the interstices therein. Thus, through the opening 7a, a roughly joined composite lead 20' with a cross-section as shown in Fig. 7 is formed. The coarse lead 20' is then passed through the heater 8 and baked there at a temperature of, say, 1200°C, and thereafter squeezed between the compression rollers 9a and 9b rotating in the directions shown by arrows to yield a densified or finished composite core 20. Such a composite pencil lead made in the manner so far described has practically the same appearance as a conventional single lead and yet has much greater strength, as described already.
The finished lead 20 is cut by a blade 15 disposed downstream of the compaction stage 9 and intermittently operated by a motor 16 to provide successive pieces of the finished lead of a desired length.
It is desirable to conduct heat-treatment at each of the stages 6a, 6b and 8 in an inert atmosphere (e.g. argon) or in a graphite powder to prevent the filaments 10 or the core 20' from being oxidized. The coarse or finished pencil lead 20' or 20 may be further subjected to impregnation with paraffine to improve its quality.
As mentioned previously, it is desirable that graphite particles in the solid marking substance 2 have particles of a smaller uniform particle size around 5 microns and a larger uniform particle size between 10 and 100 microns of 150 to 200 microns. With these different size grades of particles mixed uniformly to constitute the solid marking substance 2, it has been found that a composite pencil lead excellent in strength and yet relatively soft and smooth to write with is provided.

Claims

1. A pencil lead (20) constituted by a plurality of filaments (10) of a solid marking substance (2) individually having a thickness ranging between 10 and 250 microns, and bundled and joined together with a binder material (13).

2. The pencil lead defined in Claim 1 wherein said solid marking substance (2) consists at least in part of finely divided graphite.

3. The pencil lead defined in Claim 2 wherein said solid marking substance (2) consists of a mixture essentially of graphite particles (G) and at least one synthetic resin (R) uniformly mixed and heat-treated.

4. The pencil lead defined in Claim 3 wherein said graphite particles (G) have a uniform size of 30 to 50 microns.

5. The pencil lead defined in Claim 3 wherein said graphite particles (G) include particles of a relatively small, uniform particle size, and particles of a relatively large, uniform particle size.

6. The pencil lead defined in Claim 5 wherein said relatively small, uniform particle size is in the range 1 to 10 microns.

7. The pencil lead defined in Claim 5 or 6 wherein said relatively large, uniform particle size is in the range 10 to 100 microns.

8. The pencil lead defined in Claim 5 or 6 wherein said relatively large, uniform particles size is in the range 150 to 200 microns.

9. The pencil lead defined in any preceding claim wherein said binder material (13) consists at least in part of graphite.

10. The pencil lead defined in Claim 9 wherein said binder material (13) contains finely divided graphite.

11. The pencil lead defined in Claim 9 or 10 wherein said binder material (13) contains at least one synthetic resin.

12. The pencil lead defined in any preceding claim wherein said filaments (10) individually have a thickness not greater than 100 microns.

13. The pencil lead defined in Claim 12 wherein said filaments (10) individually have a thickness in the range 30 to 50 microns.

14. The pencil lead defined in any preceding claim wherein each of said filaments (10) is substantially circular in cross-section.

15. The pencil lead defined in any preceding claim and having a thickness in the range 0.1 to 0.5 mm.

16. A method of making a pencil lead (20), comprising the steps of:

(a) preparing a semisoft solid marking substance (2);

(b) extruding said substance to prepare a plurality of filaments (10) of said substance (2) individually having a thickness in the range 10 to 250 microns; and

(c) bundling said filaments (10) and joining them together with a binder material (13) to form said pencil lead (20).

17. The method defined in Claim 16 wherein said substance (2) consists at least in part of finely divided graphite.

18. The method defined in Claim 17 wherein said substance (2) consists of a mixture essentially of graphite particles (G) and at least one synthetic resin (R) uniformly mixed and heat-treated.

19. The method defined in Claim 18 wherein said graphite particles (G) have a uniform particle size in the range 30 to 50 microns.

20. The method defined in Claim 18 wherein said graphite particles (G) include particles of a relatively small, uniform particle size, and particles of a relatively large, uniform particle size.

21. The method defined in Claim 20 wherein said relatively small, uniform particle size is in the range 1 to 10 microns.

22. The method defined in Claim 20 or 21 wherein said relatively large, uniform particle size is in the range 10 to 100 microns.

23. The method defined in Claim 20 or 21 wherein said relatively large, uniform particle size is in the range 150 to 200 microns.

24. The method defined in any one of the Claims 16 to 23 wherein said binder material (13) consists at least in part of graphite.

25. The method defined in Claim 24 wherein said binder material (13) contains finely divided graphite.

26. The method defined in Claim 24 or 25 wherein said binder material (13) contains at least one synthetic resin.

27. The method defined in any one of the Claims 16 to 26 wherein said filaments (10) individually have a thickness not greater than 100 microns.

28. The method defined in Claim 27 wherein said filaments (10) individually have a thickness in the range 30 to 50 microns.

29. The method defined in any one of the Claims 16 to 28 wherein each of said filaments (10) is substantially circular in cross-section.

30. The method defined in any one of the Claims 16 to 29 wherein said pencil lead (20) is substantially circular in cross-section and has a diameter in the range 0.1 to 0.5 mm.

31. The method defined in any one of the Claims 16 to 30, further comprising (d) heating said filaments (10) at at least one temperature between 800 and 3500°C subsequent to step (b) and prior to step (c).

32. The method defined in any one of the Claims 16 to 31, further comprising the step (e), subsequent to step (c), of heating said bundled filaments (20') at a temperature between 1000 and 2000°C.

33. The method defined in Claim 31 wherein step (d) is carried out in an inert atmosphere, or in a graphite powder.

34. The method defined in Claim 32 wherein step (e) is carried out in an inert atmosphere, or in a graphite powder.

35. An apparatus for making a pencil lead (20), comprising:

-a vessel (3) for retaining a semisoft solid marking substance (2);

-means (4) for extruding said substance (2) in said vessel (3) to prepare a plurality of filaments (10) of said substance (2) having a thickness in the range of 10 to 250 microns; and

-means (7a, 7b) for bundling said filaments (10) and joining them with a binder material (13) to form said pencil lead (20).