Unlike some inkjet printers where the cartridge includes the print head, the printhead in these printers is fixed. Over time, parts of the printhead may become permanently clogged, resulting in unsightly streaks, but there are printhead and drum cleaning cycles and jet-substitution options which can resolve most printing issues. There is a filter for the ingested ink implemented at least on the recent ColorQube devices (8570/8870), according to a service manual for those models. Dust caused by the use of cheap paper may cause printhead clogging, which is why Xerox recommends the use of papers that are not prone to giving off dust and fibers during regular usage. Paper dust may also accumulate as debris inside the printer; this could cause abrasions on the drum and may mimic a weak or missing jet. For this reason, the basic built-in drum-cleaning procedure is therefore recommended to be invoked at least three times before initiating printhead-nozzle cleaning cycle. The printhead system also contains a wiper-assembly unit that is used to wipe the printhead from dust, debris and residual ink, in order to protect the nozzles from clogging. Overall, the design of this system is robust: printheads have been known to last one million prints or more.