Ink ribbon cassette

Abstract: In a ribbon cassette rich comprises a ribhon storage unit (3) which folds and stores the ink ribbon (1), a driving roller (5) and follow roller (7) which lead the ribbon through the opening of the ribbon storage unit, and an ink reservoir (10) which supplies ink to the inker (9) which touches the follower roller, the viscosity of ink pre­ viously absorhed by the ribbon (1) is higher than that of ink in the ink reservoir (10).

Description: This invention relates to a ribbon cassette, e.g. for a serial printer, according to the peamble of claim 1. Such an ink ribbon cassette is known from EP-A-0 210 755. Some ribbon cassettes store ribbons within a cassette, which is then mounted to the carriage of a printer. Such ribbon cassettes allow easy ink ribbon fitting and prevent ink from smearing on a person's hands or on the device itself. These cassettes also enable the device to be compact. The printing life of the ribbon is generally determined by the amount of ink absorbed by the ribbon and the life of the ribbon's base cloth, which is normally much longer than the printing life. Ideally the life of the ink ribbon material should be nearly equal to that of the ink supply for printing. But this is not the case in conventional ribbon cassettes. A ribbon cassette equipment with a means of dispensing ink in the proper amount to the ribbon and thus extension of the printing life of the ink ribbon is proposed. For example, in systems which do not replenish ink to the ribbon, the viscosity of ink absorbed by the ribbon is set to a small value for graphic printers to increase the supply of ink to the ribbon, since the printing efficiency (number of printed dots/unit ink ribbon area) is high and the ink usage rate is high and the viscosity is set to a higher value for Chinese Character printers to maintain high quality printing. Thus, ink viscosity is an important element of the printing quality. When the viscosity of the ink in the ink replenishing system is higher than that absorbed by the previously described ribbon, the amount of ink replenished is less than what the ribbon can absorb, and the ink runs out the printing quality or density will deteriorate. Namely, the replenishing performance of ink from the ink reservoir depends on the viscosity of the ink. The ink's replenishing properties improve when the ink's viscosity is low. However, when the viscosity of the ink in the ink reservoir is much lower than that of ink absorbed by the ribbon, the reverse condition occurs. Namely, replenishment of ink from the ink reservoir is excessive and the printing is blurred or smudged. Thus, since the relationship between the viscosity of the ink absorbed by the ribbon and that of ink in the ink reservoir is unstable, the printing quality declines and the density or printing is blurred or smeared as mentioned above. The object of this invention is to provide a ribbon cassette with improved printing quality, and with improved ink replenishing ability. This object is solved by the characterizing features of claim 1. With the above configuration, it is possible to transfer ink from the ink reservoir to the ribbon properly and secure the desired printing quality. Other objects and advantages of the present invention will become apparent from the detailed description to follow in conjuction with the accompanying drawings. Fig. 1 shows a partial cutout of the ink ribbon cassette viewed from an angle. Fig. 2 shows the plan of the inside of the ribbon cassette. Fig. 3 is an explanatory drawing of the ink reservoir within the ribbon cassette. Fig. 4 is a graph showing the relationship between ink viscosity and the degree of ink consumption. Fig. 5 is a graph of the number of printed characters versus the degree of ink consumption. Fig. 6 is a graph of the number of printed characters versus the printing density. Fig. 7 is a graph of the number of printed characters versus the amount of ink supply. The following uses an actual example of this invention for description. Fig. 1 shows the ribbon cassette, as viewed from an angle. This ribbon cassette can be used in carriages for wire-dot type printers. Fig. 2 is a plan of the inside of the ribbon cassette. Fig. 3 is an explanatory drawing of the inker. In Fig. 2, the ribbon (1) is endless as it is a möbius strip, and it is stored within the ribbon cassette (2), and is only partially exposed to the outside. The ribbon can be made of nylon® or polyester, for example. In this case, Nylon 66 (Dupon PA66) was used as the ribbon material. This nylon 66 is 0.12 mm thick and has nylon string mesh of 146 lines per 25,4 mm (inch) vertically and 148 lines per 25,4 mm (inch) horizontally. One nylon string consists of a twisted 34 40-Denier string. An ink with a viscosity of 400 to 1500 CP is absorbed by the ribbon. The ribbon (1) is in the ribbon storage unit (3) of the ribbon cassette (2) and the ribbon guiding parts (4a) and (4b) rotate in the direction of arrow. The driving roller (5) is located at the opening of the ribbon storage unit (2) and is supported by the driving roller fixing component (6). This driving roller can be made of polyacetal, for example. The follower roller (7) touches the driving roller (5) and is supported by the follower roller supporter (8). This follower roller (7) is, for example, made of polyacetal. The inker (9) touches the follower roller (7) and the inker (9) is connected to the ink reservoir (10), absorbs ink from the ink reservoir (10) by capillary action, and then dispenses it to the follower roller (7). The spring (11) is a means for pressing the follower roller (7) to the driving roller (5). The inker (9) can be made of felt, for example. The inker (9) can also be made of plastic fiber cotton which absorbes an ink with a viscosity of 200 to 1000 CP. With the above configuration, the ribbon (1) held by the driving roller (5) and the follower roller (7) is wound into the ribbon storage unit (3) by the driving roller (5) and is folded before being stored. On the other hand, the ejected ribbon (1) passes through the ribbon guider (4a), performs its function and then is would into the ribbon storage unit (3) as explained in the above, through the ribbon guider (4b). In this case, ink from the inker (9) which is in contact with the follower roller (7) supplies ink to it (7), which then transfers and replenishes the ribbon (1). The relationship between the ink's viscosity and the amount of ink absorbed by the ribbon is shown in plots (A), (B), and (C) in Fig. 4. It was found that printing efficiency affects the degree of ink absorption. (Number of printed dots/ribbon unit area). The relationship between the number of printed characters and the amount of ink absorbed is shown in Fig. 5. For the ribbon only (i.e. without ink replenishing), this ratio is determined by the amount of ink absorbed by the ribbon and the length of the ribbon, as shown in plots (D), (E), and (F). The amount of ink absorbed depends on the printing efficiency, and the relationship is practically a linear function. However, if the amount of ink used initially is large, when printing efficiency is large, as shown in plot (G), using the ink replenishing method, ink absorbed by the ribbon has already been dipped into ink and the ink reservoir becomes large but the amount of ink on the ribbon and in the Ink reservoir decreases so that the amount of ink available is small later in the printing process, thus reducing the degree of ink consumption. If the amount of ink used initially is small, when the printing efficiency is small, as shown in plot (I), ink absorbed by the ribbon and the ink reservoir decreases and the amount of ink on the ribbon and in the ink reservoir is reserved, it thus conserves ink which can be supplied in the later stages of the printing process and it reduces the drop in the degree of ink absorption. The relationship between the number of printed characters and the printing density with respect to the ink's viscosity in the reservoir is as shown in Fig. 6 and it was found that as the ink's viscosity decreases as shown in plot (J), the printing density deteriorates drastically in the later stages. When the ink's viscosity in the ink reservoir is high, the printing density initially deteriorates drastically with respect to the relationship between the number of printed characters and degree of ink supply as shown in plot (O), Fig. 7. Then, as shown in plot (J), when the ink in the reservoir has a low viscosity, it was found that the printing density maintains its initial value up to a certain number of printed characters (approximately 1.5 million characters). From the above, it was found that no sufficient replenishment of ink can be made even if ink with same viscosity as that absorbed by the ribbon is used. The following describes the viscosity of ink which allows for the ink to be replenished properly in order to ensure the required printing quality. Normally, the printer has various printing patterns such as characters, graphics and ruled lines and the viscosity of ink to be absorbed by the ribbon is determined beforehand to satisfy the printing quality of the resulting print depending on these printing patterns. Then, since ink from the ink reservoir is transferred to the ribbon, the viscosity needs to be lower than that of the ink originally absorbed by the ribbon. As described above, the viscosity of ink originally absorbed by the ribbon should suit the desired printing quality. If the viscosity of ink of the ink reservoir is higher than that of the ink originally absorbed by the ribbon, ink replenishment is inferior to the absorption efficiency of ink on the ribbon, which thus leads to the deterioration of the printing density. On the other hand, if the viscosity of ink in the ink reservoir is lower than that of ink absorbed by the ribbon, too much ink is transferred to the ribbon, thus increasing the printing density, causing blurring and smudges. As described in the above, the viscosity of ink originally absorbed by the ribbon should be greater than that of ink in the ink reservoir and the difference in viscosity should be 100 to 1000 CP. Viscosity values closer to 200 CP were optimum for graphics according to an experiment conducted by the inventor. A viscosity difference of 300 to 500 CP was found appropriate for Chinese characters. It was also found that if the viscosity difference is within 300 CP or the ink's viscosity in the ink reservoir is within 50 % of the ink viscosity of the ribbon, ink from the ink reservoir can be properly transferred to the ribbon and the desired printing quality can be obtained. Printing characteristics of this invention are shown in plot (K) of Fig. 6 and plot (N) of Fig. 7. Thus, it is known that the ribbon cassette of this invention assumes a more constant printing density and conserves the ink supply. Thus, according to this invention described in detail, in an ink ribbon cassette where a ribbon storage unit for folding and storing the ink ribbon is provided, a driving roller and the follower roller pass the ribbon through the opening of the ribbon storage unit, the ink reservoir supplies ink to the inker which touches the follower roller and ink replenishing part, a process of properly transferring ink in the ink reservoir to the ribbon and that for obtaining a desired printing quality can be achieved by setting the viscosity of the ink originally absorbed by the ribbon higher than that of ink in the ink reservoir. Also, the invention is not limited to the above ribbon cassette and this invention can also be applied to an ink ribbon cassette where ink is picked from the ink reservoir by a gear and then supplied to the ribbon.

Claims: A ribbon cassette (2) including a ribbon storage unit (3) for folding and storing a ribbon (1), a driving roller (5) and a follower roller (7) for leading the ribbon through an opening of the ribbon storage unit (3), and an ink reservoir (10) for supplying ink to an inker (9), said inker being arranged between the ink reservoir (10) and the follower roller (7), absorbing ink from the ink reservoir (10) and touching the follower roller (7), characterized in that the viscosity of ink originally absorbed by the ink ribbon (1) is higher than that of the ink in the ink reservoir (10), and in that the difference in ink viscosity ranges from 100 to 1000 CP between ink originally absorbed by the ink ribbon and ink of the ink reservoir. A ribbon cassette according to claim 1 wherein the viscosity of ink originally absorbed by the ink ribbon (1) ranges from 400 to 1500 CP. A ribbon cassette according to claim 1 or 2, wherein the viscosity of ink in the ink reservoir (10) ranges from 200 to 1000 CP. A ribbon cassette according to claim 1, 2 or 3, wherein the viscosity of ink originally absorbed by the ink ribbon (1) exceeds the viscosity of ink in the ink reservoir (10) by about 200 CP. A ribbon cassette according to claim 1, 2 or 3, wherein the viscosity of ink originally absorbed by the ink ribbon (1) exceeds the viscosity of ink in the ink reservoir (10) by an amount in the range of 300 to 500 CP.

Inventors: KIKUCHI HIROSHI C O OKI ELECTR, KIKUCHI, HIROSHI, C/O OKI ELECTRIC.IND.CO.LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1990
Main classification: B41J31
IPC codes: B41J32, B41J31