IMPROVED POINT BALL FOR BALL POINT PENS

Abstract: The present invention relates to cemented carbide containing tungsten carbide, titanium carbide, nickel, molybdenum and chromium. The composition of the materials provides a good resistance to corrosion as well as a high hardness and wear resistance. These properties are particularly interesting for the manufacture of pen balls, for the ball-point pen balls made with these materials will have steady writing characteristics over a long period of time. This material is particularly suitable when water-based inks are used, because these inks are far more corrosive than oil-based inks.

Description: IMPROVED POINT BALL FOR BALL POINT PENS The present invention relates to a ball-point in which is used a cemented carbide containing tungsten carbide, titanium carbide, nickel, molybdenum and chromium. Some pen balls are used with water based inks. Such inks are corrosive and therefore the pen balls must be resistant to corrosion. The most severe corrosive situation is at hand when the pen is stored and the ball is not in use. In that situation both crevice corrosion as well as galvanic corrosion might occur due to the different metals present in the ball and the seat. The thin space in between the ball and the seat is also unfavourable from corrosion point of view, which is also the difference of oxygen potentials between the ink and the exterior of the pen. The corrosive situation is thus rather complex and if the pen should create a good writing also after some years of storage the ball must have a good corrosion resistance. Furthermore the pen balls must have high hardness in order not to wear when rotating in the seat. The ball material must also be easy to lap during production to make its surface smooth and the diameter within well controlled tolerances. Thus, both strength and toughness is needed for the ball to withstand pressures during lapping. US 3,503,692 discloses C-Co cemented carbide or cermets as a material for penballs. In the former case a substoichiometric carbon content is used. In the latter case one or more carbides of Cr, Ta, Nb, and Ti are bonded together with a nickel or nickel alloy binder phase. US 3,746,456 discloses a pen ball material consisting of C or TiC in a binder of Co, Ni, Cr, Pt and Fe . A cemented carbide, which fulfils the conditions described above, has been prepared and characterized. The cemented carbide contains tungsten carbide, titanium carbide, nickel, molybdenum and chromium. The composition of the cemented carbide provides a good resistance to corrosion as well as a high hardness and wear resistance. These properties are particularly interesting for the manufacture of pen balls, for the ball-point pen balls made with these materials will have steady writing characteristics over a long period of time. This material is particularly suitable when water based inks are used, because these inks are far more corrosive than oil based inks. Fig 1 shows in 200X magnification the microstructure of the cemented carbide according to the invention. Fig 2 shows in 1000X magnification the microstructure of the cemented carbide according to the invention. The cemented carbide according to the invention consists of, in wt-%: 80-90 WC, 5-15 TiC and 7-10 binder phase. The binder phase has the following composition, also in wt-%: 40-60, preferably 45-55 Ni, <20, preferably 10-18 Mo, 15-40, preferably 30-40 Cr. Up to 30 wt-% of Ni can be replaced by Co. The grain size of the WC shall be 1-2 μm. The carbon content shall be low and the cemented carbide shall contain 1-10, preferably 5-7 vol-% η-phase. The η-phase shall be evenly distributed with an average size of about 5 μm. The material shall have a hardness of 1870-2000 HV and less than A02 porosity. The composition of the ball ensures a high resistance to corrosion. At the same time it has been found that the cemented carbide can easily be shaped into balls of the desired size. According to the method of the present invention powders forming the hard constituents and powders forming the binder phase are wet milled together, dried, pressed to bodies of desired shape and sintered. The powder mixture shall have such a carbon content to give a carbon content with an η-phase content of the sintered bodies according to above. The invention also relates to the use of a cemented carbide with the above mentioned composition as balls for ball point pens. Example 1 Three cemented carbide bodies with the composition according to the table below, in wt-%, were prepared and characterised for their corrosion resistance. Sample 1 2 3 invention prior art prior art C 83,3 84,18 85,5 TiC 8,65 0,91 0 Co 0 11,1 12 Ni 4 0 0 Mo 1,15 0 0 Cr 2,9 3,3 2,5 The corrosion resistance of these materials was studied in inks via an electrochemical test intended to simulate the conditions mentioned above. The tests were made at 40 °C in the ink to be tested. The experimental device was composed of three electrodes, and the tip of the rotating working electrode was made with the material tested. As some seats are made with brass, some of the tips were also made with brass in order to study the galvanic corrosion between the ball and the seat. The system was allowed to stabilise up to the free potential, and then the system was polarised at 1 mV/s and the value of the intensity was recorded. The polarisation resistance of the materials in the inks can be measured with this test. This polarisation resistance is conversely proportional to the current density. Hence the higher the polarisation resistance, the higher the corrosion resistance of the material. The values of the polarisation resistance are found in table below R p (Ω * cm 2 ) Grade ink n°l ink n°2 Invention 400 112 Prior art 450 133 Prior art 50 38 From the polarisation curves, it is also possible to measure the galvanic coupling between the ball and its seat. For the brass seat and the grades tested, the brass was always the anode, so it is the brass and not the ball, which will be corroded by the galvanic coupling. The cemented carbide of the invention combines good corrosion resistance with high hardness, which is not seen in prior art.

Claims: Claims 1. A cemented carbide ball-point pen ball c h a r a c t e r i s e d in consisting of in wt-%: 80- 90 WC, 5-15 TiC and 7-10 binder phase of the following composition, in wt-%: 40-60, preferably 45-55 Ni, <20, preferably 10-18 Mo, 15-40, preferably 30-40 Cr. 2. A cemented carbide ball-point pen ball according to claim I c h a r a c t e r i s e d in containing 1-10, preferably 5-7 vol-% η-phase. 3. Use of a cemented carbide with composition of, in wt-%: 80-90 WC, 5-15 TiC and 7-10 binder phase of the following composition, also in wt-%: 40-60, preferably 45-55 Ni, <20, preferably 10-18 Mo, 15-40, preferably 30-40 Cr as balls for ball point pens. 4. Use of a cemented carbide with composition of, in wt-%: 80-90 WC, 5-15 TiC and 7-10 binder phase of the following composition, also in wt-%: 40-60, preferably 45-55 Ni, <20, preferably 10-18 Mo, 15-40, preferably 30-40 Cr and further containing 1-10, preferably 5-7, vol-% η-phase as balls for ball point pens.

Inventors: CHEYNET JEROME, O'DONNELL SYLVIE, UHRENIUS BJOERN, CHEYNET, JEROME, O'DONNELL, SYLVIE, UHRENIUS, BJOERN
Applicants: CHEYNET JEROME, DONNELL SYLVIE O, SANDVIK AB, UHRENIUS BJOERN, CHEYNET, JEROME, O'DONNELL, SYLVIE, SANDVIK AB (PUBL), UHRENIUS, BJOERN
Application year: 1998
Main classification: C22C29
IPC codes: B43K1, C22C29