Ballpoint pen tip

Abstract

To provide a ball-point pen tip which allows an intermediate ball to be easily rotated even at a stage where the rotational force received by the rotation of the writing ball accompanying writing is small, and which gives a light feeling to the first writing, which is repeated especially in character writing.SOLUTION: There is provided a ball-point pen tip in which the diameter of an intermediate ball 4 is 70% or more of the diameter of a writing ball 3, the intermediate ball 4 is arranged in the rear hole behind the inward protrusion, and the arithmetic mean height (Sa) of the surface of the intermediate ball 4 is 2 (nm) or more and 20 (nm) or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a ballpoint pen tip comprising a writing ball, a ball holder that rotatably holds the writing ball with a portion of the writing ball protruding from an opening at the tip, and an intermediate ball that supports the rear of the writing ball. [Background technology]

[0002] In recent years, there has been a trend toward preference for ballpoint pens that require less force to write with and have a light, smooth writing feel. As a result, the ink viscosity is set low or surfactants are added, resulting in an ink with high permeability.In order to prevent ink from seeping out or leaking from the tiny gap between the front edge of the ball holder tip opening and the writing ball, a ballpoint pen tip is known that has a structure in which a resilient member such as a coil spring located behind the writing ball urges the writing ball forward, pressing the writing ball against the inner surface of the front edge of the tip opening. Also known are ballpoint pens (Patent Document 1) that have an intermediate ball interposed between the writing ball and a coil spring to bias the center of the writing ball forward and ensure a secure seal between the front edge of the ball holder tip opening and the writing ball, with the front surface of the intermediate ball abutting against the rear surface of the writing ball and an inward protrusion that restricts the backward movement of the writing ball when writing, and an applicator (Patent Document 2) that has an inward protrusion that restricts the backward movement of the writing ball and avoidance body (corresponding to the intermediate ball) when writing. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-221787 [Patent Document 2] Japanese Patent Application Publication No. 8-228830 Summary of the Invention [Problem to be solved by the invention]

[0004] The ballpoint pen disclosed in Patent Document 1 has an inward protrusion that restricts the backward movement of the writing ball during writing, and an intermediate ball that is positioned within the center of the ink flow hole and is set to be smaller than the inner diameter of the center of the ink flow hole. Furthermore, since the inner diameter of the center of the ink flow hole is inversely proportional to the relative width of the ball receiving seat, if the inner diameter is made too large, the area of ​​the ball receiving seat will be insufficient, making it more likely for the ball to sink. As a result, the intermediate ball has an outer diameter significantly smaller than the writing ball. Therefore, the friction acting between the writing ball and the intermediate ball due to the rotation of the writing ball does not generate a large rotational torque for the intermediate ball, making it difficult to generate a force that exceeds the frictional resistance between the intermediate ball and the coil spring that urges it forward. This causes slippage and sliding friction, which can easily result in a heavy writing feel, especially at low writing speeds such as when starting to write.

[0005] Furthermore, if writing is continued over a long distance while the writing ball and the intermediate ball are in a slipping state, wear will occur at the point of contact between the writing ball and the intermediate ball, increasing the contact area and making it more difficult for the ink, which provides lubrication, to penetrate into the contact area, which will increase resistance to the rotation of the writing ball and raise concerns that the writing will feel heavy.

[0006] The applicator disclosed in Patent Document 2 has an inward protrusion that restricts the backward movement of the writing ball and avoidance body (corresponding to the intermediate ball) during writing, and also has an avoidance body (corresponding to the intermediate ball) with the same diameter as the writing ball. When writing, the writing ball actively rotates due to contact with the paper surface, but the avoidance body (intermediate ball) barely rotates because it is in contact with the ball receiving seat while receiving writing pressure. For this reason, the friction acting between the writing ball and the intermediate ball due to the rotation of the writing ball does not generate a large rotational torque for the intermediate ball, and slippage occurs, resulting in a heavy writing feel, especially at low writing speeds such as when starting to write. [Means for solving the problem]

[0007] The first gist of the present invention is a ballpoint pen tip, which has an inward protrusion formed in the middle of an ink flow hole of a ball holder to restrict the backward movement of a writing ball, and the writing ball is disposed at the tip side of the inward protrusion, and an intermediate ball is disposed behind the inward protrusion to support the writing ball via a center hole, the intermediate ball having a diameter of 70% or more of the diameter of the writing ball and an arithmetic mean height (Sa) of the surface of 2 (nm) to 20 (nm), The second feature is that a coil spring is provided behind the intermediate ball to bias the writing ball forward via the intermediate ball, and the inner diameter of the contact point between the coil spring and the intermediate ball is 70% or more and 95% or less of the diameter of the intermediate ball, The third gist is that when writing, with the writing ball seated on the inward protruding portion that serves as the seating surface, the intermediate ball abuts the writing ball and, at the position where it contacts the inner wall of the rear hole, the angle formed by the line connecting the center of the intermediate ball and the center of the writing ball and the axis of the ball holder is greater than or equal to 1 degree and less than 10 degrees. [Effects of the Invention]

[0008] In the ballpoint pen tip of the present invention, the diameter of the intermediate ball that supports the rear of the writing ball is sufficiently large, at 70% or more of the diameter of the writing ball, allowing the intermediate ball to exert a large rotational torque. Furthermore, by arranging the intermediate ball in the rear hole behind the inward protrusion, the inward protrusion receives the writing pressure from the writing ball during writing, thereby avoiding contact with the inward protrusion, which would inhibit the rotation of the intermediate ball. Therefore, the intermediate ball can easily rotate even when the rotational force exerted by the rotation of the writing ball during writing is small, making it more likely that rolling friction, with a small coefficient of friction, will occur between the writing ball and the intermediate ball. In particular, when writing characters with a writing instrument in a diary or notebook, for example, short strokes of several millimeters are written for kanji, while continuous strokes of several tens of millimeters are written for cursive alphabets. In either case, the beginning of a sentence is repeated many times, providing the exceptional effect of continuously achieving a light writing feel at the beginning of writing. Furthermore, when considering the abrasion resistance due to contact between the writing ball and the intermediate ball, by setting the arithmetic mean height (Sa) of the surface of the intermediate ball to between 2 (nm) and 20 (nm), the intermediate ball rotates more easily, and when writing over long distances, the friction between the writing ball and the intermediate ball generated by the rotation of the writing ball reduces wear on the writing ball and the intermediate ball, resulting in a smooth writing feel for a long period of time, which is an exceptional effect.

[0009] Furthermore, a coil spring is provided behind the intermediate ball, which urges the writing ball forward via the intermediate ball and brings the writing ball into close contact with the inner edge of the ball holder when not writing, and the inner diameter of the coil spring's contact point with the intermediate ball is between 70% and 95% of the diameter of the intermediate ball, thereby preventing the intermediate ball from shifting from the inner periphery of the coil spring or from becoming buried in the inner periphery of the coil spring, and minimizing conditions that would hinder the rotation of the intermediate ball.

[0010] Furthermore, when writing, with the writing ball seated on the inward protrusion that serves as the seat, the angle formed by the line connecting the center of the intermediate ball and the center of the writing ball at the position where the intermediate ball abuts the writing ball and contacts the inner wall of the rear hole and the axis of the ball holder is between 1 degree and 10 degrees, thereby reducing the contact area between the intermediate ball and the wall surface of the rear hole, suppressing vibration of the intermediate ball within the rear hole, and providing a smooth writing feel. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a longitudinal cross-sectional view of the ballpoint pen tip of the present invention. [Figure 2] Enlarged view of part I in Figure 1. [Figure 3] Variation 1 of the present invention. [Figure 4] Variation 2 of the present invention. [Figure 5] FIG. 10 is an explanatory diagram for calculating the inner diameter of the contact portion between the coil spring and the intermediate ball. DETAILED DESCRIPTION OF THE INVENTION

[0012] The ball holder that holds the writing ball has an ink flow hole, which is a through hole. The tip of the ink flow hole is formed with a diameter smaller than the diameter of the writing ball by crimping or other processing, and has a tip opening through which a portion of the writing ball protrudes while preventing the writing ball from falling out. An inward protrusion is formed in the middle of the ink flow hole to restrict the writing ball's backward movement during writing, and the space between the tip opening and the inward protrusion serves as the writing ball holding portion. The inward protrusion also has a center hole and radial grooves for supplying ink to the writing ball, and a rear hole with an inner diameter larger than the inner diameter of the center hole is formed behind the center hole. When forming the rear hole, to reduce wear on the cutting tool, the cutting tool gradually reduces the cutting diameter from the rear end of the ball holder, resulting in a rear hole with a gradually smaller diameter toward the tip. The cutting edge angle of the cutting tool is approximately 120°. That is, the wall surface of the inward protrusion on the rear hole side is formed as a mortar-shaped tapered portion at an angle of about 120° corresponding to the angle of the cutting tool tip, and is connected to the minimum diameter portion of the rear hole.

[0013] Materials that can be used for such ball holders include stainless steel, copper alloys such as nickel silver and brass, and highly wear-resistant synthetic resins such as polyoxymethylene resin. Considering wear resistance during writing and corrosion resistance of ink, stainless steel is preferred, with austenitic stainless steel SUS304 and ferritic stainless steel SUS430 being preferred. Furthermore, to maintain good workability while suppressing deformation such as dents and wear at the tip of the ball holder, it is preferable to set the Vickers hardness (HV) to 150 or more and 300 or less. Furthermore, in consideration of the natural environment, it is preferable to replace lead, which is added to improve machinability, with bismuth, which provides similar workability.

[0014] The writing ball is arranged to rotate freely with a portion of it protruding from the tip of the ballpoint pen, and when pressed against the surface being written on, such as paper, it moves backward, causing ink to flow out of the gap formed between the ball holder or be transported out as the writing ball rotates, and transfer it to the surface being written on.

[0015] The size of the writing ball that can be used is the same as that used in ordinary ballpoint pens, with a diameter of 0.18 mm to 2.0 mm, but a range of 0.3 mm to 1.0 mm is more preferable.

[0016] Within the rear hole of the ball holder are arranged an intermediate ball that can come into contact with the writing ball through the middle hole, and a coil spring that urges the intermediate ball forward.The intermediate ball urges the writing ball forward by receiving the urging force of the coil spring, and is rotatably held by the front end of the coil spring.

[0017] By making the size of the intermediate ball at least 70% of the diameter of the writing ball, friction generated between the writing ball and the intermediate ball as the writing ball rotates creates a large rotational torque for the intermediate ball, and by adjusting the pressure load of the coil spring that urges the intermediate ball forward, a force that exceeds the frictional resistance caused by contact between the intermediate ball and the coil spring is generated against the rotation of the intermediate ball, making it easier for the intermediate ball to rotate in response to the rotation of the writing ball during writing and providing a light writing feel. It is more preferable for the diameter of the intermediate ball to be at least 80% of the diameter of the writing ball, and even more preferably at least 100%. There is no particular upper limit to the diameter of the intermediate ball, but using an intermediate ball that is too large compared to the diameter of the writing ball will result in a thin wall between the rear hole behind the inward protrusion where the intermediate ball is placed and the ball holder, which may reduce the tip's strength against writing pressure and drops. Furthermore, if the outer diameter of the ball holder is increased in order to ensure the thickness of the ball holder, the ballpoint pen tip will be too large compared to the diameter of the writing ball, which may result in problems such as poor visibility of the pen tip when writing. Therefore, it is preferable that the diameter of the intermediate ball be 125% or less of the diameter of the writing ball.

[0018] By placing the intermediate ball in the rear hole behind the inward protrusion and positioning the center of the intermediate ball in the rear hole behind the inward protrusion, an intermediate ball with a larger diameter than the writing ball can be used, and the gap between the writing ball and the writing ball holding portion and the gap between the intermediate ball and the rear hole can be set independently, so that the amount of ink supplied from the gap between the intermediate ball and the rear hole to the writing ball holding portion can be adjusted according to the viscosity of the ink. Furthermore, when the intermediate ball is subjected to writing pressure, it is pushed back by the writing ball, causing the intermediate ball and the inward protrusion to move further apart and become non-contacting. As a result, the large rotational torque that can be obtained from the intermediate ball due to its size is not hindered, and the intermediate ball can rotate easily. The friction caused by contact between the writing ball and the intermediate ball is rolling friction with a small coefficient of friction, and the writing feel is light even at a stage where the rotational force received from the rotation of the writing ball accompanying writing is smaller.

[0019] When writing, with the writing ball seated on the inward protrusion that serves as the seat, the intermediate ball abuts the writing ball and, at the position where it contacts the inner wall of the rear hole, the angle formed by the line connecting the center of the intermediate ball to the center of the writing ball and the axis of the ball holder is between 1 degree and 10 degrees, thereby reducing the contact area between the intermediate ball and the wall of the rear hole, suppressing vibration of the intermediate ball within the rear hole, and providing a smooth writing feel. Setting the angle to 1 degree or more prevents the intermediate ball from making circumferential contact with the rear hole wall, reducing the contact area between the intermediate ball and the rear hole wall so that rotation of the intermediate ball is not hindered. Setting the angle to 10 degrees or less reduces the radial movement range of the intermediate ball within the rear hole, causing rolling friction and sliding friction to act simultaneously on the intermediate ball during writing, causing the intermediate ball to slide in the direction of rotation of the writing ball, and the spring's pressure repeatedly pressing the intermediate ball against the rear hole wall. This suppresses vibrations and provides a smooth writing feel. On the other hand, if the angle exceeds 10 degrees, the radial range of movement of the intermediate ball within the rear hole becomes larger than necessary, causing the intermediate ball to vibrate significantly due to sliding friction when writing, making it difficult to achieve a smooth writing feel.

[0020] The position where the intermediate ball abuts against the writing ball and contacts the inner wall of the rear hole can be either the inner wall portion parallel to the axis of the rear hole or the rear, conical tapered wall portion of the inward protrusion connected to the middle hole. The position can be adjusted by changing the taper angle and shape of the rear side cone-shaped tapered wall surface of the inward protrusion.

[0021] The arithmetic mean height (Sa) of the surfaces of the writing ball and intermediate ball is preferably between 2 nm and 20 nm, taking into consideration the writing feel between the writing ball and the paper surface, the increase in the coefficient of friction due to contact between the writing ball and intermediate ball, contact between the writing ball and ball holder, and contact between the intermediate ball and the front end of the coil spring, and wear resistance. If the arithmetic mean height of the ball surface is less than 2 nm, the ball surface is almost flat and the ink acting as a lubricant is not sufficiently adhered, resulting in high frictional resistance. If the arithmetic mean height of the ball surface is more than 20 nm, the ball surface is too uneven, resulting in high frictional resistance. The arithmetic mean height (Sa) conforms to the international standard ISO 25178 and is equivalent to a parameter that expands the arithmetic mean roughness (Ra) of a roughness curve to a surface. It is a parameter that represents the average absolute value of the height difference at each point relative to the average surface of a given area of ​​the surface.

[0022] The materials for the writing ball and the intermediate ball include cemented carbide alloys primarily composed of tungsten carbide, metals such as stainless steel, aluminum, and steel, ceramics such as silicon carbide, silicon nitride, titanium nitride, chromium carbide, alumina, and zirconia, resin materials such as polyethylene resin, polypropylene resin, polyacetal resin, and polyamide resin, and glass. However, cemented carbide and ceramics are preferred for the writing ball in consideration of their wettability to ink, corrosion resistance, sliding properties with the intermediate ball, and abrasion resistance. The intermediate ball may be made of either the same material as the writing ball or a different material, but it is preferable to use a combination of high-hardness materials, such as cemented carbide alloys primarily composed of tungsten carbide. Furthermore, from the perspective of suppressing adhesive wear, it is more preferable to use a combination of materials with low mutual solubility, such as silicon carbide or chromium carbide, for the intermediate ball, rather than the same material as the writing ball.

[0023] If the pressure load of the coil spring that biases the intermediate ball forward is too small, the sealing caused by the pressure between the writing ball and the tip opening of the ball holder will become unstable, which may lead to ink seeping or leaking.If the pressure load is too strong, the contact load between the writing ball and the intermediate ball will increase, which will accelerate wear on the intermediate ball and make the writing feel heavy.Therefore, it is preferable to set the pressure load to between 5 (gf) and 30 (gf).

[0024] By bringing the windings of the front end of the coil spring into contact with the intermediate ball and setting the inner diameter of the contact point to between 70% and 95% of the diameter of the intermediate ball, it is possible to minimize the possibility of the intermediate ball being embedded in the inner periphery of the coil spring, and to ensure that stable rotation of the intermediate ball is not hindered. Note that the inner diameter of the contact point between the front end of the coil spring and the intermediate ball can be adjusted by appropriately selecting the inner diameter of the windings of the coil spring and the diameter (thickness) of the wire.

[0025] From the viewpoint of manufacturing costs, it is desirable for the coil spring to have a straight tubular shape with the same outer diameter along its entire length, but it may also have a shape in which the winding diameter at the end is reduced to a smaller diameter to match the size of the intermediate ball, a stepped shape in which a small-diameter winding portion is connected to a large-diameter winding portion, or a sloping shape in which the outer diameter gradually decreases toward the end. Furthermore, by making the front and rear ends of the coil spring the same shape, it is not necessary to determine the insertion direction when inserting the coil spring into the ball holder, which simplifies the manufacturing process and is effective in terms of manufacturing costs, etc. By making the ends of the coil spring tightly packed, adjacent windings are less likely to slip and behave like a cylinder, which increases the stability of the seating between the coil spring and the intermediate ball and stabilizes the pressure applied to the intermediate ball without dispersing it. The number of closely packed windings is preferably two or more, and it is even more preferable to grind both ends of the coil spring flat. The coil spring can be made of stainless steel wire such as SUS304, hard steel wire, piano wire, or resins such as polycarbonate or polyether ether ketone. To prevent corrosion or leaching into the ink, nickel-plated coil springs are also suitable. Nickel-plated coil springs are preferred for their lubricity in the rotation of the intermediate ball.

[0026] The coil spring can be fixed in place in the rear hole of the ball holder by creating a portion whose inscribed circle diameter is smaller than the outer diameter of the coil spring. For example, protrusions can be formed at equal intervals on the same circumference of the inner wall surface of the rear hole to prevent it from slipping out of the ball holder. These protrusions can also be formed by cutting chips from the inner wall surface of the rear hole of the ball holder using a broaching process, or by creating a recess in the side wall of the ball holder using a punching process or the like to create protrusions on the inner wall surface. Alternatively, a separate member connected to the rear end of the ballpoint pen tip can be used, and the method and shape for preventing the coil spring from slipping out can be selected as appropriate.

[0027] The ballpoint pen tip of the present invention can be configured as a writing instrument by connecting it directly to an ink tank containing ink or via an intermediary member, and can be installed inside the exterior body as a ballpoint pen refill. When installed in the exterior body, it may be equipped with a cap that covers the ballpoint pen tip portion protruding from the front end of the exterior body, or it may be a so-called knock-type ballpoint pen in which a ballpoint pen refill is inserted and retracted from the front end of the exterior body by a knock operation, or it may be a so-called multi-color ballpoint pen in which multiple refills are stored inside the exterior body and ballpoint pen refills are inserted and retracted from the front end of the exterior body by a slide operation.

[0028] The ink used to form the handwriting can be any of water-based inks, which use water as the main medium, oil-based inks, which use organic solvents such as alcohol as the main medium, and water-based or oil-based gel inks with shear thinning properties, which may contain pigments and / or dyes as coloring components, high-boiling organic solvents for anti-freezing purposes, resin components that provide adhesion to the writing surface, surfactants or polysaccharides that adjust surface tension, viscoelasticity, lubricity, etc., rust and mildew inhibitors, etc. Correction fluid compositions containing white-opacifying components such as titanium oxide may also be used. [Example]

[0029] An example will be described below with reference to the drawings. FIG. 1 is a vertical cross-sectional view showing an example of a ballpoint pen tip of the present invention, and FIG. 2 is an enlarged view of part I in FIG.

[0030] The ballpoint pen tip 1 includes a ball holder 2, a writing ball 3 as a writing member, an intermediate ball 4, and a coil spring 5.

[0031] Ball holder 2 has a through-hole formed in it as an ink flow hole, consisting of, from the tip side, a tip opening 6, a writing ball holding portion 7, a middle hole 8, and a rear hole 9, and holds writing ball 3 rotatably within ball holder 2 with a portion of it protruding from tip opening 6. Tip opening 6 is crimped from the tip side to reduce its diameter. When crimping, the inner edge of tip opening 6 is pressed against writing ball 3, transferring the curved surface of writing ball 3 and giving it a mirror finish. When writing, writing ball 3 is biased forward and comes into close contact with the periphery, preventing ink leakage and air inflow.

[0032] An inward protrusion 10 is formed between the writing ball holding portion 7 and the rear hole 9, restricting the backward movement of the writing ball 3. A plurality of equally spaced radial grooves 11 are formed by cutting into the inward protrusion 10. These radial grooves 11 are threaded through the rear hole 9 to ensure ink is supplied to the writing ball holding portion 7, but they may be threaded partway through the middle hole 8 without passing through the rear hole 9 to prevent ink backflow or for other purposes. Furthermore, as shown in Figure 3, if burrs or chips remain at the boundary between the radial grooves 11 and the rear hole 9 when the radial grooves 11 are threaded through the rear hole 9, these can be removed by drilling the rear hole 9 (additional processing portion 9b). A concave ball receiving seat 12 is formed in the inward protrusion 10, which stabilizes the position of the writing ball 3 during writing and contributes to smooth rotation with less unnecessary vibration. When the writing ball 3 is pressed against the ball receiving seat 12, the writing ball 3 and the ball receiving seat 12 come into contact in a generally planar manner, contributing to smooth rotation with less unnecessary vibration of the writing ball 3. The aforementioned radial grooves 11 have openings outside the ball receiving seat 12, ensuring ink supply to the writing ball holding portion 7. In this embodiment, five radial grooves 11 are formed at equal intervals around the circumference, but their size and number are not particularly limited.

[0033] The intermediate ball 4 is rotatably disposed in the rear hole 9 behind the inward protrusion 10 so as to abut against the writing ball 3. When writing, with the writing ball 3 seated on the inward protrusion 10 that serves as the seating surface, the position at which the intermediate ball 4 abuts against the writing ball 3 and contacts the inner wall of the rear hole 9 can be designed so that the intermediate ball 4 contacts the inner wall 9a that is parallel to the axis of the rear hole 9 as shown in Figure 2, or so that the intermediate ball 4 contacts the rear conical tapered wall surface 10a of the inward protrusion that is connected to the middle hole as shown in Figure 4, by adjusting the depth and diameter of the rear hole 9 and the taper angle of the rear conical tapered wall surface 10a of the inward protrusion 10. Furthermore, a coil spring 5 is disposed behind the intermediate ball 4, in contact with the intermediate ball 4 at abutment portion 13. The coil spring 5 is inserted from the rear of the ball holder 2 and pressed in so as to compress its entire length to prevent it from coming out, and the restoring force resulting from this compression urges the intermediate ball 4 forward, which in turn urges the writing ball 3 forward via this intermediate ball 4. The rear end of the coil spring 5 is prevented from coming out from inside the ball holder 2 by protrusions 14 that are formed by broaching and are provided at four equally spaced locations on the same circumference of the inner wall surface of the rear hole 9 of the ball holder 2.

[0034] The material of the ball holder 2 was stainless steel (manufactured by Shimomura Tokusyu Seiko Co., Ltd., product name: SF20T) with a Vickers hardness (HV) of 240. Taking into consideration wettability with ink, corrosion resistance, sliding ability with the intermediate ball, and abrasion resistance, as well as manufacturing costs, the material of the writing ball 3 was a cemented carbide alloy (manufactured by Tsubaki Nakashima Co., Ltd., product name: PB11, arithmetic mean surface height Sa (ISO 25178): 3 (nm)) with tungsten carbide as the main component and cobalt, chromium, etc. as the binder phase. Furthermore, the material of the intermediate ball 4 was a cemented carbide alloy (manufactured by Heraeus Co., Ltd., product name: H3) with tungsten carbide as the main component and cobalt, chromium, etc. as the binder phase. The arithmetic mean height (Sa) of the surfaces of the writing ball 3 and the intermediate ball 4 was calculated from the average value by measuring three arbitrary points in an area of ​​20 (μm) × 20 (μm) using a scanning probe microscope (AFM5100N; manufactured by Hitachi High-Tech Science Corporation).

[0035] The coil spring 5 is made of nickel-plated SUS304 stainless steel wire and has a large diameter coil portion 5a and small diameter coil portions 5b at the front and rear ends, with the ends of the small diameter coil portion 5b forming a tightly wound coil portion 5c. The front and rear ends of the coil spring 5 also form the small diameter coil portion 5b and the tightly wound coil portion 5c.

[0036] The position of the protrusion 12 of the coil spring 5 was changed so that the pressing load would be 20 gf, and the amount of deflection of the coil spring 5 was adjusted. After the ballpoint pen tip 1 was produced, the pressing load was measured using a digital force gauge (ZTA-5N; manufactured by Imada Co., Ltd.) when the writing ball 3 of the ballpoint pen tip 1 was pressed forward and in contact with the tip opening 6 of the ball holder 2, and then moved 0.025 mm backward in the axial direction.

[0037] The dimensions of each part were changed to produce ballpoint pen tips of Examples 1 to 22 and Comparative Examples 1 to 16. The dimensions of each part are shown in Tables 1 and 2. As shown in FIG. 5, the inner diameter D of the contact portion 13 between the coil spring 5 and the intermediate ball 4 can be calculated from the following formula (1), where M is the diameter of the intermediate ball 4, S is the diameter of the steel wire of the coil spring 5, and I is the inner diameter of the end of the tight coil portion 5c of the coil spring 5. D = M(S + I) / (M + S) (1) In addition, when the writing ball 3 is seated on the inward protrusion 10, which serves as the seating surface, the angle θ formed by the line connecting the center 4a of the intermediate ball 4 and the center 3a of the writing ball 3 and the axis of the ball holder 2 at the position where the intermediate ball 4 abuts against the writing ball 3 and contacts the inner wall of the rear hole 9 was measured by drawing a diagram using the diameter W of the writing ball 3, the diameter M of the intermediate ball 4, and the dimensions and shape of the rear hole 9 as input, and measuring the angle at which the intermediate ball 4 contacts the inner wall 9a parallel to the axis of the rear hole 9, or the rear, mortar-shaped, tapered wall surface 10a of the inward protrusion 10 connected to the middle hole 8.

[0038] The ballpoint pen tips 1 prepared above in Examples 1 to 22 and Comparative Examples 1 to 16 were attached as ballpoint pen tips to commercially available gel ink ballpoint pens (manufactured by Pentel Co., Ltd., product code BL17-A), the ink tanks were filled with the test inks shown below, the pen tip was positioned facing outward so that centrifugal force would act in the direction of the pen tip, and the ink was centrifuged in a centrifuge (manufactured by Kokusan Centrifugal Machine Co., Ltd., tabletop centrifuge H-103N) to remove unnecessary air from the refill, producing test ballpoint pen samples, which were then subjected to the following tests. The results are shown in Table 1.

[0039] (Test ink) Water Black #256L (14% aqueous solution of black dye, Orient Chemical Industry Co., Ltd.) ) 35.0 parts by weight Water Yellow #1 (CI Acid Yellow 23, Orient Chemical Industry Co., Ltd.) ) 1.2 parts by weight Ethylene glycol 6.0 parts by weight Glycerin 8.0 parts by weight Thiodiglycol 8.0 parts by weight Sarcosinate OHV (N-oleoyl sarcosine, manufactured by Nikko Chemicals Co., Ltd.) 3.0 parts by weight Benzotriazole 0.5 parts by weight Proxel GXL (20% dipropylene glycol solution of 1,2-benzisothiazolin-3-one, manufactured by Avecia Co., Ltd.) 0.2 parts by weight Potassium hydroquinone sulfonate 0.3 parts by weight BC-5.5 (Polyoxyethylene cetyl ether, HLB 11.5, Nikko Chemicals Co., Ltd.) ) 1.0 parts by weight AKP-20 (fine particle alumina, average particle size 0.5 μm, manufactured by Sumitomo Chemical Co., Ltd.) 0.1 parts by weight Sodium hydroxide 0.3 parts by weight Kelzan AR (xanthan gum, manufactured by Sansho Co., Ltd.) 0.4 parts by weight Ion-exchanged water 36.0 parts by weight Of the above ingredients, the entire amount of Kelzan AR was added to 5 parts by weight of water while stirring and stirred for 1 hour to obtain an aqueous solution of Kelzan AR. The remaining ingredients were then mixed and stirred for 1 hour to dissolve uniformly, after which the aqueous solution of Kelzan AR was added and stirred for a further 2 hours to obtain a black test ink. The viscosity of the test ink was measured using an ELD type viscometer (manufactured by Tokimec Co., Ltd.) with an ST type rotor at a temperature of 25°C and a shear rate of 100 s -1 ) and was found to be 200 (mPa·s). The pH value of the test ink was measured using a compact pH meter B-212 (manufactured by Horiba Ltd.) and was found to be 8.7.

[0040] (Test 1: Initial low-speed writing resistance test) Using a static and dynamic friction tester (Tribo-master Type TL201Sa; Trinity Lab Co., Ltd.), the tip of a test ballpoint pen refill was placed against a piece of writing paper specified in JIS S 6039. Writing was performed by pressing the tip of the ballpoint pen against the paper with a force of 100 gf, forming a writing angle of 70° with the paper, and moving the paper 1.25 cm at a speed of 0.5 cm / sec. The load applied to the ballpoint pen refill in the direction of writing movement due to the paper movement was measured with a load cell and recorded as the writing resistance. The writing resistance measurement was performed for 2.5 seconds, during which 500 measurement data points were obtained. The average writing resistance in the direction of paper movement was calculated from the first 200 data points obtained between the start of measurement (when the writing speed was zero, assuming the start of writing) and 1.0 second. Five test ballpoint pen samples were prepared for each level, and the calculation results were the average values ​​of the five samples.

[0041] (Test 2: Long-distance writing followed by low-speed writing resistance test) The ballpoint pen sample used in Test 1 was subjected to 500 m of spiral writing using a writing tester under the following conditions: writing angle 70°, writing load 100 gf (0.98 N), and writing speed 7 cm / sec. After that, the writing resistance value was measured in the same manner as in Test 1, and the difference from the initial value was calculated.

[0042] [Table 1] [Table 2]

[0043] When writing with the ballpoint pen tips of Examples 1 to 22, the diameter of the intermediate ball 4 was 70% or more larger than the diameter of the writing ball 3, and the initial writing resistance was lower than in the comparative examples even at the beginning of writing when the rotational force caused by the rotation of the writing ball 3 was small. This is presumably because a large rotational torque is generated in the intermediate ball 4 due to the friction acting between the writing ball 3 and the intermediate ball 4 caused by the rotation of the writing ball 3, and because the arithmetic mean height (Sa) of the surface of the intermediate ball 4 is set to be 2 (nm) or more and 20 (nm) or less, friction at the contact portion 13 between the coil spring 5 and the intermediate ball 4 is small, and the rotation of the intermediate ball 4 is not hindered, making it easy to rotate. Furthermore, when writing with the ballpoint pen tips of Examples 1 to 22, the difference in writing resistance between the initial stage and after long-distance writing was smaller than that of the comparative example, demonstrating the exceptional effect of providing a smooth writing feel over a long period of time. In particular, Examples 8 to 16, 18, 19, 21, and 22 showed superior results in terms of the initial writing resistance and the difference in writing resistance between the initial stage and after long-distance writing. This is presumably because, by setting the inner diameter of the contact portion 13 between the front end of the coil spring 5 and the intermediate ball 4 to be between 70% and 95% of the diameter of the intermediate ball 4, the intermediate ball 4 is prevented from shifting from the inner periphery of the coil spring 5 or from being buried in the inner periphery of the coil spring 5, thereby minimizing the disruption of the rotation of the intermediate ball 4. Furthermore, when writing, when the writing ball 3 is seated on the inward protrusion 10, which serves as the seating surface, the intermediate ball 4 abuts the writing ball 3 and, at the position where it contacts the inner wall of the rear hole 9, the angle θ formed by the line connecting the center 4a of the intermediate ball 4 and the center 3a of the writing ball 3 and the axis of the ball holder 2 is set to 1 degree or more. This increases the difference between the curvature of the intermediate ball 4 and the curvature of the rear hole 9, reduces the contact area between the intermediate ball 4 and the wall surface of the rear hole 9, and does not hinder the rotation of the intermediate ball 4. Furthermore, it is presumed that by setting the angle θ to 10 degrees or less, the radial range of movement of the intermediate ball 4 within the rear hole 9 is limited, thereby suppressing vibration of the intermediate ball 4.

[0044] In Comparative Examples 1 and 13, the arithmetic mean height (Sa) of the surface of the intermediate ball 4 was within the range of 2 nm to 20 nm, but the diameter of the intermediate ball 4 was less than 70% of the diameter of the writing ball 3, resulting in higher initial writing resistance values ​​compared to the Examples. Furthermore, the increase in writing resistance values ​​after long-distance writing is presumably the result of the intermediate ball 4 being unable to rotate easily, which made sliding friction more likely to occur between the writing ball 3 and the intermediate ball 4, and thus causing wear of the intermediate ball 4.

[0045] In Comparative Examples 2 and 14, the diameter of the intermediate ball 4 was 70% or more larger than the diameter of the writing ball 3, but the arithmetic mean height (Sa) of the surface of the intermediate ball 4 was greater than 20 (nm), resulting in a higher initial writing resistance value compared to the Examples, and a larger difference in the writing resistance value between the initial value and the value after long-distance writing. This is presumably because the intermediate ball 4 was less likely to rotate, which made sliding friction more likely to occur between the writing ball 3 and the intermediate ball 4, causing wear of the intermediate ball 4 and resulting in an increase in the writing resistance value after long-distance writing.

[0046] In Comparative Examples 3 to 12, 15, and 16, the diameter of the intermediate ball 4 was less than 70% of the diameter of the writing ball 3, and the arithmetic mean height (Sa) of the surface of the intermediate ball 4 was greater than 20 nm, resulting in a high initial writing resistance and a large difference in the writing resistance between the initial value and the value after long-distance writing. This is presumably because the intermediate ball 4 was difficult to rotate and the frictional resistance between the coil spring 5 and the intermediate ball 4 inhibited the rotation of the intermediate ball 4.

[0047] In Comparative Examples 6, 10, and 15, the inner diameter of the contact portion 13 between the front end of the coil spring 5 and the intermediate ball 4 was less than 70% of the diameter of the intermediate ball 4, resulting in a large initial writing resistance and a large difference in the initial writing resistance and the writing resistance after long-distance writing. This is presumably because the deviation from the contact portion 13 inhibits the rotation of the intermediate ball 4, making it difficult for the intermediate ball 4 to rotate, causing the intermediate ball 4 to slip against the rotation of the writing ball 3 during writing, resulting in sliding friction.

[0048] In Comparative Examples 7, 11, 12, and 16, the inner diameter of the contact portion 13 between the front end of the coil spring 5 and the intermediate ball 4 was larger than 95% of the diameter of the intermediate ball 4, resulting in a large initial writing resistance and a large difference in the writing resistance between the initial value and the value after writing a long distance. This is presumably because the intermediate ball 4 was difficult to rotate and had progressed in wear.

[0049] In Comparative Examples 8, 10, 12, and 14, when writing, with the writing ball 3 seated on the inward protruding portion 10 that serves as the seating surface, the intermediate ball 4 abuts the writing ball 3 and, at the position where it contacts the inner wall of the rear hole 9, the angle θ formed between the line connecting the center 4a of the intermediate ball 4 and the center 3a of the writing ball 3 and the axis of the ball holder 2 is less than 1 degree, resulting in a large initial writing resistance value and a large difference in the writing resistance value between the initial value and the value after long-distance writing. This is presumably because the difference between the curvature of the intermediate ball 4 and the curvature of the rear hole 9 is small, and the intermediate ball 4 comes into circumferential contact with the wall surface of the rear hole 9, increasing the contact area and hindering the rotation of the intermediate ball 4, making it difficult for the intermediate ball 4 to rotate, causing sliding friction as the intermediate ball 4 slips against the rotation of the writing ball 3 during writing.

[0050] In Comparative Examples 9, 11, and 15, when writing, with the writing ball 3 seated on the inward protruding portion 10 that serves as the seating surface, the intermediate ball 4 abuts the writing ball 3 and, at the position where it contacts the inner wall of the rear hole 9, the angle θ formed by the line connecting the center 4a of the intermediate ball 4 and the center 3a of the writing ball 3 and the axis of the ball holder 2 is greater than 10 degrees, resulting in a large initial writing resistance value and a large difference between the initial writing resistance value and the writing resistance value after long-distance writing. This is presumably because the radial movable range of the intermediate ball 4 within the rear hole 9 is larger than necessary, causing greater vibration of the intermediate ball 4 due to sliding friction during writing. [Industrial Applicability]

[0051] The ballpoint pen tip of this embodiment can be used not only for writing letters and drawings, but also as an applicator for cosmetics such as eye shadow and eyeliner by using the pen tip facing upward. [Explanation of symbols]

[0052] 1 ballpoint pen tip 2 ball holders 3 Writing ball 3a Center of the writing ball 4 Medium Ball 4a Center of the middle ball 5 coil springs 5a Large diameter coil section 5b Small diameter coil section 5c Close-fitting coil section 6 Tip opening 7 Writing ball holder 8 medium hole 9 Posterior foramen 9a Inner wall parallel to the axis of the rear hole 9b Additional work 10 Inward protrusion 10a: Rear side of the inward protrusion connected to the bore, a conical tapered wall surface 11 Radial grooves 12 Ball receiving seat 13 Contact part 14 Convex part D Inner diameter of the contact area between the coil spring and the intermediate ball I. Inner diameter of the end of the tightly coiled part of the coil spring M Diameter of intermediate ball S Diameter of steel wire in coil spring W Diameter of writing ball θ is the angle between the line connecting the center of the intermediate ball and the center of the writing ball and the axis of the ball holder.

Claims

1. A ballpoint pen tip has an inward protrusion formed in the middle of the ink flow hole of the ball holder to restrict the backward movement of the writing ball, the writing ball is arranged on the tip side of the inward protrusion, and an intermediate ball is arranged behind the inward protrusion to support the writing ball via a middle hole, the diameter of which is 70% or more of the diameter of the writing ball and the arithmetic mean height (Sa) of the surface is 2 (nm) to 20 (nm).

2. 2. The ballpoint pen tip according to claim 1, further comprising a coil spring disposed behind the intermediate ball for biasing the writing ball forward via the intermediate ball, and the inner diameter of the contact point between the coil spring and the intermediate ball is between 70% and 95% of the diameter of the intermediate ball.

3. A ballpoint pen tip as described in claim 1 or claim 2, wherein, when writing, when the writing ball is seated on the inward protruding portion that serves as the seating surface, the intermediate ball abuts the writing ball and, at the position where it contacts the inner wall of the rear hole, the angle formed by the line connecting the center of the intermediate ball and the center of the writing ball and the axis of the ball holder is greater than or equal to 1 degree and less than 10 degrees.

4. A ballpoint pen tip as described in claim 1, characterized in that the intermediate ball rotates even when the rotational force received from the rotation of the writing ball is small.

5. A ballpoint pen tip as described in claim 2, wherein the pressure load of the coil spring that biases the intermediate ball is 5 gf or more and 30 gf or less.

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