High-speed two-sided printing is essential in a variety of situations such as offices and schools. However, to achieve high-speed two-sided printing with an inkjet printer, a sheet of paper must be fed back through the printer before the ink from the first print has been fully absorbed. When this happens, ink on the surface of the paper sticks to conventional metallic-particle-coated metal rollers and gets lifted off the sheet. This ink is then re-deposited on the paper, staining it. To solve this problem, Epson developed star-wheel rollers. We employ these star-wheel rollers not only to feed sheets of paper through the printer, but as resist rollers to correct paper warp. Let's take a deeper look at Epson's star-wheel resist rollers, which Minimizes the amount of ink stuck to the rollers, prevents ink from being pulled off the paper, and the sheet of paper from being stained.
Resist rollers are paper-feed rollers whose job is to correct paper angles when feeding sheets of paper to the printhead. A sheet of paper butts up against the resist rollers, which correct its angle and then feeds it to the printhead. The outer diameter of the rollers must be precise to generate a sufficient frictional force against the paper to feed it through the rollers without slipping.
Conventionally, metal rollers whose surfaces are coated with metallic particles have been used for resist rollers. Because these rollers are metal, they feature precise outer diameters and high durability. Moreover, the metal particles form a rough surface that enables a sheet of paper to be fed through the rollers without slipping.
However, when an inkjet printer is used to perform high-speed two-sided printing, each sheet of paper is fed back to the printhead as soon as its first side has finished printing. Because of this, the resist rollers come into contact with sheets of paper that have not yet completely absorbed the ink printed on them. The issue with conventional metallic-particle-coated metal rollers is that ink on the paper surface gets stuck to the roller, and stains the paper when the dirty roller next comes into contact with the paper surface. Star-wheel resist rollers were adopted to solve this problem.
Star-wheel resist rollers have star-wheel projections around their circumference. Hence, only the projections contact the sheet of paper, reducing the contact area. This means that the amount of ink transferred from the paper surface to the roller can be minimized. In addition, there are cleaning rollers opposite the star-wheel resist rollers that wipe off the small quantities of ink that does stick to the rollers. One characteristic of star-wheel rollers is that, because the serrations are uniform, the cleaner does not miss any contact points.
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Star-wheel resist rollers consist of a star-wheel ring of stainless-steel serrations, around a polyacetal resin (POM) core that forms the body of the roller. First, serrations projecting less than 1 mm are etched around the circumference of stainless-steel disks. The rollers require extreme precision to correct the angle of each sheet of paper. Because it is difficult to achieve this kind of precision with presswork, Epson has chosen to employ etching, which allows machining of delicate shapes. The star-wheel section produced through etching is then combined with POM resin through integral molding to form one star-wheel disk. Six of these disks are layered together to form one star-wheel roller. At this time, each of the six disks is offset from the next at a pitch of 0.6°. This brings the end of the star-wheel resist roller to a near perfect circle, increasing paper feed precision. Producing parts to this level of precision requires a high level of manufacturing prowess in addition to etching technology.
By leveraging its manufacturing capabilities, Epson is able to create its own high-precision parts and bring high-speed two-sided printing to inkjet printers.