[viernullvier]

For inkjet technology, it's the print head. Fundamentally, an inkjet printer works similar to a plotter, laser engraver, FDM 3D printer, or CNC router: There's a mechanism that accurately places the tool head at the desired X and Y position in order to do its business [0]. This part is a solved problem, there are many open-source reference designs to draw from.

For a printer, the devil is in the details though - quite literally: A printer needs to place tiny ink dots on the work piece in a fast, accurate, predictable, and repeatable manner. Placing just one single dot at a time would be prohibitively slow, so the print head needs an array of tiny nozzles in order to print multiple rows in one pass. The design also needs to ensure a consistent (and adjustable) dot size across all nozzles regardless of the ink level.

Designing and manufacturing such a print head is extremely complicated (if not impossible) for a hobbyist, and as of today there is no vendor-agnostic third-party design you can just buy off the shelf. The only forays into DIY printers are either rudimentary low-res single-nozzle designs [1], fully repurposing existing printer hardware [2], or reverse-engineering proprietary ink cartridges that come with a built-in print head [3].

[0] Technically, this is not entirely correct because a typical printer's Y axis is a rotating drum that transports the paper instead of a linear axis. Also, a printer's X axis is usually equipped with an encoder ribbon that allows for far more accurate positioning compared to the classic open-loop stepper motor systems used in many open-source designs.

[1] Example: https://hackaday.io/project/167446-diy-inkjet-printer

[2] Example: https://www.youtube.com/watch?v=_VP8ORq2al8

[3] Example: https://ytec3d.com/hp45-inkjet-printhead/