WIRE-DOT PRINT HEAD AND WIRE THEREFOR

Abstract: In a wire-dot print head comprising a plurality of wires, an arbitrary subset of which can be driven to print a desired pattern of dots. the wire material is a superhard corrosion-resistant tungsten-carbide-cobalt (WC-Co) alloy with a chromium concentration ratio of 0.004 to 0.2

Description: Field of Invention This invention relates to a wire-dot or dot-matrix print head, in particular to a print head with an improved wire material and the wires therefor. Background of the Invention Laid Open Japanese Patent Application No. 56354/1983 has parts shown in cross-section in Figure including an armature 1, a first yoke 2, a magnetic spacer 3, a second yoke 4, a permanent magnet 5, a core 6, a demagnetizing coil 7, a bias spring 8, an armature supporter 9, a wire 10, an ink ribbon 11, a platen roll 12, and a recording media 13. When the demagnetizing coil 7 is not excited, the magnetic flux from the permanent magnets is transmitted through the second yoke 4, the magnetic spacer 3, the first yoke 2, the armature and the core 6, producing a magnetic force that attracts the armature 1 to the core 6. The magnetic force overcomes the bias spring. Next, when the demagnetizing coil 7 is excited, the flux from the demagnetizing coil 7 cancels the flux of the permanent magnet 5, releasing the spring 8, which drives the wire 10 attached to the tip of the armature 1 in the direction of the arrow so that the tip 10a of the wire 10 presses the ink ribbon 11 against the recording media 13 on the platen roll 12 to print on the media. Laid-Open Japanese Patent No.79766/1984 discloses the use of a superhard alloy wire. German Patent Application No.DE-A-1924753 discloses a hard metal composition containing from 0.1 to 2.5% by weight of chromium carbide, 9 to 20% by weight of cobalt and the balance substantially of tungsten carbide, and a method for making such a composition. The chromium carbide is present as a dispersion of fine grains less than 0.2 microns in size and is preferably formed in situ. Summary of the Invention In the invention the wires of the wire-dot print head are made of a superhard corrosion-resistant tungsten-carbide-cobalt (WC-Co) alloy in which the molar ratio of chromium to cobalt is from 0.004 to 0.2. This ratio will be referred to herein as the 'chromium concentration ratio'. The wire portion exposed to ink at least is to be of the above indicated alloy. To explain the above ratio, let m be the weight percentage of the cobalt component of the wire, let n be the weight percentage of the chromium component of the wire, let M be the atomic weight of the cobalt, and let N be the atomic weight of chromium. In a wire weighing W grams, the mass of cobalt is: W x (m/100) (1) Hence in terms of moles, the amount of cobalt is: W x (m/100) x (1/M) (2) Similarly, in terms of moles the amount of chromium is: W x (n/100) x (1/N) (3) From equations (2) and (3) the chromium concentration ratio can be derived: = [W x (n/100) x (1/N)]/[W x (m/100) x (1/M)] = (n/m) x (M/N) (4) Use of a superhard WC-Co alloy having a chromium concentration ratio as described above of from 0.004 to 0.2 mitigates corrosion of the wire by the ribbon without seriously degrading the mechanical strength of the wire. Drawings Figure 1 is a cross-section of the main parts of a wire-dot print head to which the invention may be applied; Figure 2 is a graph illustrating the relationship in a non-accelerated test between time of immersion and corrosion loss; Figure 3 shows the same relationship as in Figure 2 but in an accelerated test; Figure 4 is a graph showing the relationship between dye concentration and corrosion loss; Figure 5 is a graph showing the relationship between chromium concentration ratio and corrosion loss; Figure 6 is a graph showing the relationship between chromium concentration ratio and transverse rupture stength ratio; Figure 7 is a graph showing the relationship between chromium-carbide concentration ratio and corrosion loss; and Figure 8 is a graph showing the relationship between chromium carbide concentration ratio and transverse rupture strength ratio. Description of embodiments of invention Corrosion of a wire made of a superhard tungsten-carbide-cobalt (WC-Co) alloy by ribbon ink as a function of time was investigated. Tests were carried out on wire speciments 0.34mm in diameter, 50mm in length, and 64mg in weight by immersing the entire specimen in the ink used in the ribbon and measuring the weight of the specimen before and after immersion. The difference in the weight was defined to be the amount of corrosion loss. Results of these tests are shown in Figure 2. The horizontal axis in Figure 2 represents the time (in months) of immersion of the wire in the ink. The vertical axis represents the corrosion loss (in milligrams). Points marked with X represent wire with a cobalt weight percentage of 30% and the symbols ○ , Δ , and ▭ represent cobalt weight percentages of 25%, 20%, and 15% respectively. The ink used in these tests had a dye content of 25%, and immersion temperature was 25°C. The results of Figure 2 indicate that the corrosion loss increases linearly with the immersion time. Next an accelerated testing method was tried in order to obtain, in a short time, corrosion data equivalent to a year of testing. In the accelerated tests the same ink was used but the immersion temperature was raised to 95°C Figure 3 shows the results obtained. Together, Figures 2 and 3 indicate that the results obtained in 100 hours at an immersion temperature of 95°C approximate the results obtained in one year at an immersion temperature of 25°C. The tests described below were carried out using the accelerated method. Next the differing effects of the components of the ink is causing corrosion loss in superhard WC-Co alloy wire were tested. Ink generally comprises a pigment, a dye, a dispersing agent, and an oil. The pigment, the dispersing agent, and the oil were not observed to cause any corrosion. Figure 4 shows the results of tests of the dye carried out as described above. The corrosion loss can be seen to vary in proportion to the percent of dye in the ink. When this percent is 0, the corrosion loss is 0. The conclusion reached was that the dye is the principal cause of the corrosion described above. The dye is, however, an essential component of the ink because it acts to complement the pigment colour. It came to the inventors' attention that, as reported on page 56 of Funtai oyobe Funmatsu Yakin (a power metallurgy journal published by the Funtai Funmatsu Yakin Kyokai), Vol. 31 No. 2, the addition of the chromium component to a superhard WC-Co alloy improves its corrosion resistance. To form a superhard WC-Co alloy with added chromium. powders of WC, Co and Cr are mixed and sintered in a vacuum furnace, at a temperature of 1350 to 1400°C. Figure shows the relationship between the chromium concentration ratio and the corrosion loss caused by dye in a wire made of superhard WC-Co alloy with a chromium component. The chromium concentration ratio is indicated on the horizontal axis and the corrosion loss (in milligrams) on the vertical axis. The points marked with X represent wires with a cobalt weight percentage of 30% and the symbols ○ , Δ , and ▭ represents cobalt weight percentages of 25%, 20% and 15% respectively. The ink used in these tests had a dye content of 25% the immersion temperature was 95°C, and the immersion time was 100 hours. The results in Figure 5 indicated that at chromium concentration ratios above 0.004, the corrosion loss decreases and corrosion resistance improves markedly as the concentration ratio increases. The mechanical strength of the wire which affects the print head performance is however influenced by the presence of chromium. The mechanical strength of a superhard alloy is usually expressed in terms of transverse-rupture strength. Figure 6 shows the relation between chromium concentration ratio and the transverse-rupture strength ratio. The chromium concentration ratio is indicated on the horizontal axis. The transverse-rupture strength ratio is indicated on the vertical axis. The transverse-rupture strength ratio is defined as the transverse-rupture strength with no chromium added. The closer the ratio is to 1, the less is the degradation of transverse-rupture strength due to the addition of chromium. The data in Figure 6 indicate that up to a chromium concentration ratio of 0.16 there was very little degradation of transverse-rupture strength, and that the transverse-rupture strength value was still satisfactory at a chromium concentration ratio of 0.2, but that at higher concentration ratio values the transverse-rupture strength decreases significantly. It follows that to maintain the necessary concentration ratio must no exeed 0.2. Printing tests were carried out using a wire with a transverse-rupture strength ratio of 0.4 (chromium concentration ratio 0.2) in a wire-dot print head designed to maintain sufficient mechanical strength at these values (wire transverse-rupture strength ratio 0.4, chromium concentration ratio 0.2). No malfunctions were noted over a long period of operation, and resistance to corrosion was excellent. The above results indicate that to improve the corrosion resistance of the wire without seriously degrading its transverse-rupture strength, the chromium concentration ratio should be selected to be from 0.004 to 0.2. In the embodiment described above power of Cr was mixed in WC and Co. Alternatively, powder of chromium carbide (Cr₃C₂) can be mixed in WC and Co and the mixture can be sintered. Similar alloys can be obtained by the use of chromium carbide. Figures 7 and 8 show the results of similar trials using chromium carbide (Cr₃C₂) in place of chromium (Cr). Figure 7 shows the relationship between the ratio of the quantity of moles of chromium carbide to the quantity in moles of cobalt (called the chromium-carbide concentration ratio below) and the amount of corrosion loss. Figure 8 shows the relationship between the chromium-carbide concentration ratio and the transverse-rupture strength ratio. In both Figures 7 and 8 two scales are indicated on the horizontal axis, the lower one giving the chromium-carbide concentration ratio and the upper one giving the chromium carbide concentration ratio converted to a chromium-component (Cr) concentration ratio. The results in Figure 7 and 8 indicate that the addition of chromium carbide has an effect on the transverse-rupture strength and corrosion resistance of the wire similar to the effects of the addition of chromium, and that the chromium-carbide concentration ratio should be in the range from 0.004 to 0.2. As explained in detail above, the wire-dot print head of this invention uses a superheard, corrosion-resistant WC-Co alloy with a chromium concentration ratio of 0.004 to 0.2 which provides adequate mechanical strength and resistance to corrosion, gives long-term, stable printing service, and can therefore greatly improve the reliability of a printer in which it is employed. Such benefits can of course be obtained not only in spring-charged wire-dot print heads but also in other types of print heads such as plunger-type and clapper-type heads. A chromium concentration ratio of greater than 0.01 can give a very significant improvement and at a ratio of 0.05 a substantial amount of the total corrosion prevention effect can be obtained. At a ratio 0f 0.08 or in that region a substantial transverse rupture strength ratio is obtainable. Thus there is a region in which considerable improvements in corrosion resistance are possible at small or negligible losses in transverse rupture strength.

Claims: A wire for a wire dot print head characterised in that it comprises a WC-Co superhard alloy containing chromium, with the molar ratio of tile chromium to the cobalt being from 0.004 to 0.2. A wire according to claim 1, in which the alloy contains cobalt in a concentration substantially from 15 to 30 weight percent. A method of manufacturing a wire according to claim 1 or claim 2, in which the alloy is formed by sintering. A method according to claim 3, in which the alloy is formed by sintering at a temperature of from 1350 to 1400°. A method according to claim 3 or claim 4 in which chromium carbide is added before the sintering. A wire-dot print head comprising a plurality of wires, an arbitrary subset of which can be driven to print a desired pattern of dots, characterised in that the wire material is a superhard corrosion-resistant tungsten carbide-cobalt (WC-Co) alloy with a chromium concentration ratio of from 0.004 to 0.2. A print head according to claim 6, further comprising an ink ribbon, wherein the wire is driven so that its tip presses the corresponding part of the ink ribbon against a recording medium on a platen roll.

Inventors: ANDOU, HIROKAZU OKI ELECTRIC INDUSTRY CO. LTD., HASHIMOTO, IWAO OKI ELECTRIC INDUSTRY CO. LTD., OMORI, YASUO OKI ELECTRIC INDUSTRY CO. LTD., SHINO, MASAKAZU OKI ELECTRIC INDUSTRY CO. LTD.
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1987
Main classification: B41J2
IPC codes: B41J2, C22C29