1. Field of the Invention
The present invention relates to ink-jet printers and more particularly to an improved wiper for a printhead on such a printer.
2. Description of the Related Art
An ink-jet printer includes a replaceable printing cartridge having a printhead formed thereon. The cartridge includes a reservoir of ink which is fired through nozzles in the printhead onto a printing medium such as paper. The structure and operation of such printing cartridges is well known to those skilled in the art.
Prior ink-jet printers include a service station at one end of the travel path of a printing carriage upon which the printing cartridge is mounted. The service station includes a wiper for wiping the printhead to remove contaminants, dried ink and the like from the printhead surface containing the nozzle openings. Also provided is a cap which covers the printhead to prevent the ink in the nozzles from drying.
Each time the cartridge travels into or out of the service station, the wiper wipes the printhead. Prior art wipers are molded from an elastomeric material such as ethylene polypropylene diene monomer (EPDM). The wiper is mounted on the printer chassis in the travel path of the cartridge. A pair of wiping edges are on the tip of the wiper on opposite sides thereof. The wiping edges are oriented at a 90.degree. angle relative to the cartridge travel path. One wiping edge is in contact with the printhead surface as it travels into the station thus wiping ink, contaminants, etc. off of the surface. The other wiping edge wipes the surface as the cartridge leaves the station.
Because color ink is more viscous than black ink, effective wiping is harder to achieve on a color cartridge printhead. Although the wiping edge scrapes away particles and pooled ink, viscous ink tends to be spread out by the wiper rather than scraped off. A short time after the viscous ink is spread, it retracts to its former drop or pooled configuration.
Japanese Patent No. 62-251145 deals with this problem by mounting wiper blades on a shaft rotated by a motor. The blades sequentially scrape the printhead surface as the motor rotates the shaft. While this works to clear the printhead, it involves added complexity and expense.
Another problem associated with prior art wipers relates to the rapid rate at which they wear. Wiper wear is proportional to the normal force between the wiper and the printhead surface, all other factors remaining equal. Wiper wear is also proportional to the hardness of the softer of the two surfaces, namely the durometer of the elastomer. Thus, to optimize wiper wear, the hardest elastomer possible should be used with the smallest normal force which is still sufficient to wipe the printhead clean. Increasing the hardness of the elastomer, however, increases the shear forces applied to the printhead and causes undesirable printhead wear.
One possibility for providing a high durometer elastomer wiper with reduced shear forces on the wiped surface in the context of a vehicle windshield wiper is suggested in U.S. Pat. No. 4,638,525 to Sugita et al. The Sugita et al. wiper blade has a cross section which includes a narrowed portion just above the base. When a shear wiping force is applied at the wiping edge opposite the base, the moment of inertia about the base is reduced relative to a blade having a constant cross section. In other words, the wiper tends to bend about the narrowed portion. Because of the reduced stiffness, less shear force is applied to the wiping surface or the same shear force can be applied while using a higher durometer material.
This solution is not suitable for a printhead wiper blade due to the relative dimensions of the blade and to the small scale. Such a wiper may have typical dimensions of 8 mm wide by 7.5 mm tall by 1.25 mm thick. A narrowed portion in the wiper thickness would have to be very carefully and accurately formed to create the appropriate wiper stiffness. Because small changes in the size of the narrowed portion will result in a large change in wiper stiffness, there can be a large variation in wiping force from part to part when the wiper is manufactured in mass quantities.