Page Wide Array printing system uses an array of printheads to print at one pass. Printheads are
perpendicular to the media advance axis. In these printing systems, the paper is moving during printing
while printheads are static above the paper. For that reason, there is no chance to do printhead servicing
between passes (like scanning printers) such as flying spit or wipe. Thermal inkjet printheads require
refresh nozzles from time to time to maintain its health. The only way to refresh nozzles while printing in
a PWA system is spitting on the page (SOP).
Usually, customers have various media roll widths depending on the image width. Since media width is a
standard size, most of times images are narrower than the width of the loaded paper.
We used to service (refresh) all nozzles, even those out of the image and not printing anything.
Notice that in a scanning printing system, all nozzles are used almost all‐time while printing, and they are
refreshed while they are firing to print.
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INC, HP, "PWA PRINTHEAD SERVICING BASED ON IMAGE PROCESSING", Technical Disclosure Commons, (May 06, 2019)