To compete with color laser printers in small work team environments, HP Officejet Pro X Series printers need a compact, reliable paper transport that produces fast, face-down, correct-order output with built-in duplexing.
HP designed a new paper transport to meet the needs of page-wide array printing. In the figure below it shows a cross-sectional view of the key components. A single sheet of paper, shown by the green arrow, moves from right to left in this view.
HP Officejet Pro X Paper Transport System
A sheet printed on one side (simplex) moves up against the left door guide, crosses the writing system assembly, and exits face down to the output bin. A duplex-printed sheet moves up against the left door guide, then reverses and passes under the duplexing unit (not shown), following the same path taken by sheets coming from the multipurpose tray.
This design efficiently integrates duplexing and multipurpose tray functionality into the paper path. The Officejet Pro X Series paper transport effectively stabilizes and constrains the sheet through the printer from pick to drop. It delivers reliable paper pick, low jam rates, and continuous and accurate movement of the paper in the print zone. Sheets are transported without smearing ink.
The HP Officejet Pro X Series paper transport incorporates a number of innovations that enable cost-effective, precise paper motion control, including the following:
• A gear train with precision-matched pitch diameters
• Precision, low-cost bearings
• Precision platen alignment and positioning
• Servo-controlled overdrive of specific rollers
• Precision roller diameters
• Star wheels
• Drive shaft biasing to prevent backlash
Users have come to expect low rates of pick and jam failures from HP LaserJet solutions. HP adapted paper pick and paper supply tray spring-plate designs from high-end HP LaserJets to give HP Officejet Pro X Series printers pick and jam failure rates measured in single events over several thousand pages — similar to HP LaserJets.
Under steady-state conditions, constant paper velocity in the print zone is relatively easy to produce. In cut-sheet paper handling, though, a sheet’s leading or trailing edge is almost always moving into or out of a set of elastic rollers, and this can disrupt smooth paper motion. If not properly controlled, edge transitions produce paper velocity variations in the print zone that can appear as dark or light bands and irregular lines. The paper transport in HP Officejet Pro X Series printers is designed to effectively handle edge transitions and maintain controlled paper motion through the print zone.
Uncontrolled movement of paper along any axis of motion or rotation translates into dot placement errors on the sheet. Motion in the paper feed direction and movements that affect printhead-to-paper spacing are of particular concern. Multiple hold-down features are incorporated into this design to stabilize and constrain the paper.
A dual reverse-bow is introduced in the paper on the input and output sides of the paper transport, as seen in the figure above. This effectively holds paper against the platen and prevents the paper’s leading and trailing edges from lifting while entering and exiting the print zone.
The high rate of ink application on paper from a page-wide array means that the ink is still wet when it leaves the print zone. Wet paper loses stiffness, so it must be handled carefully to avoid smearing ink. The paper path design addresses issues associated with handling a wet sheet by guiding the paper with star wheels—thin, metal gears that only touch the paper with sharp points, so they can roll over wet areas without leaving ink tracks. Although HP has used star wheels in printers for many years, this design had not been used extensively to drive wet paper around tight corners inside a printer. The paper path for HP Officejet Pro X Series printers uses more than 300 star wheels to control paper motion.
HP Officejet Pro X Series printers have an active flap near the output tray that controls curl as the printer ejects paper. The flap is closed when the printer is not printing. It opens partially when printing with high ink densities in dry environments—when more curl might occur — and opens fully under other conditions to control moderate curl.