Most consumer inkjet printers, such as those made by Canon, HP, and Lexmark (but not Epson) use a thermal inkjet; inside each partition of the ink reservoir is a heating element with a tiny metal plate or resistor. In response to a signal given by the printer, a tiny current flows through the metal or resistor making it warm, and the ink in contact with the heated resistor is vaporized into a tiny steam bubble inside the nozzle. As a consequence, an ink droplet is forced out of the cartridge nozzle onto the paper. This process takes a fraction of a millisecond.
The printing depends on the smooth flow of ink, which can be hindered if the ink begins to dry at the print head, as can happen when an ink level becomes low. Dried ink can be cleaned from a cartridge print head using 91% denatured isopropyl alcohol (not rubbing alcohol). Isopropyl alcohol does not damage the susceptible sponge that is vital in transferring ink to paper. But it is strong enough to dissolve clogs. It must be administered carefully with a dropper or a syringe. Tap water contains contaminants that may clog the print head, so distilled water and a lint-free cloth is recommended.
The ink also acts as a coolant to protect the metal-plate heating elements − when the ink supply is depleted, and printing is attempted, the heating elements in thermal cartridges often burn out, permanently damaging the print head. When the ink first begins to run low, the cartridge should be refilled or replaced, to avoid overheating damage to the print head.
All Epson printers use a piezoelectric crystal in each nozzle instead of a heating element. When current is applied, the crystal changes shape or size, increasing the pressure in the ink channel and thus forcing a droplet of ink from the nozzle. There are two types of crystals used: those that elongate when subjected to electricity or bi-morphs which bend. The ink channels in a piezoelectric ink jet print head can be formed using a variety of techniques, but one common method is lamination of a stack of metal plates, each of which includes precision micro-fabricated features of various shapes (i.e. containing an ink channel, orifice, reservoir and crystal). This cool environment allows use of inks which react badly when heated. For example, roughly 1/1000 of every ink jet is vaporized due to the intense heat, and ink must be designed not the clog the printer with the products of thermal decomposition. It also can make a smaller ink drop in some situations than thermal inkjet schemes.
Stores the ink of the ink cartridge. Others contain hydrophobic foam that prevents refilling.
Some ink cartridges have printheads installed on them. Printheads of ink cartridges consists of four parts:
- Nozzle plate: the part where ink goes through from ink cartridge to paper.
- Cover plate: covers the nozzles when they are not in use.
- Common ink chamber: where ink is transferred from the body before printing.
- (In piezoelectric printers) piezoelectric substrate: houses the piezoelectric crystal.
- (In thermal printers) metallic plate / resistor: is heated with a weak electrical current which heats the ink.