Solid ink (also known as hot melt ink) is a type of ink used in printing. Solid ink is a waxy, resin-based polymer that must be melted prior to usage, unlike conventional liquid inks. The technology is used most often in graphics and large-format printing environments where color vividness and cost efficiency are important.

Solid ink technology utilizes solid ink sticks, crayons, pearls or granular solid material instead of the fluid ink or toner powder usually used in printers. Some types of solid ink printers use small spheres or pucks of solid ink, which are stored in a hopper before being transferred to the printing head by a worm gear or melted as needed. After the solid ink is loaded into the printer, it is melted and used to print images on paper or any substrate in a process that can be similar to offset printing or standard printing.

Solid ink printers require heated printheads. Continuous Inkjet (CIJ) began the Solid ink industry by using wax and low temperature metal alloy ink in the late 1960s and early 1970s before Drop-On-Demand (DOD) was invented. DOD use piezoelectric devices (polarized ceramics) and heat alters the poling. Howtek broke the barrier of DOD high temperatures with the design of the Howtek style DOD inkjet in 1985. This allowed chemists to expand Solid inks into a new direction and led to a Three-Dimensional printing patent from an ex-Howtek employee who formed the company Visual Impact Corporation.

Due to the way solid ink printers put the ink onto the page, print quality is considered to be precise and accurate, with bright colors. Excellent results can be achieved with low-quality stock, as the Solid ink covers the stock with a glossy, almost opaque, surface. Solid ink printers are able to print on many different types and thicknesses of media. They are much less sensitive to changes in media type than are color laser printers.

Because solid blocks of ink are used, there is less waste generated than is with laser printers or inkjet printers, which produce empty ink or toner cartridges, in addition to packaging and packing materials. A loose ink block does not leave any residual cartridge after it is consumed – only a crushable, thin, plastic packing bag or tray and a recyclable cardboard packaging box.

Solid ink printers have an advantage over ink-jet printers for situations involving intermittent use with long periods of downtime. This is because melted solid ink that has subsequently cooled and re-solidified inside the ink-delivery pathways is a normal part of printer operation. So, this cooled-and-solidified ink does not dry out. And, while the printer is not operating, the solidified wax helps to prevent oxygen and moisture from interacting with many internal parts of the ink-delivery components.

Solid ink blocks can be made non-toxic and safe to handle. In the 1990s, the president of Tektronix ate a piece of solid ink, derived from food-grade processed vegetable oils, to demonstrate their safety. It may also be described like the coating on prescribed pills.

Solid contaminants must be filtered out, or ink can clog printhead nozzles when using original or compatible inks. Clogging can damage the printhead, and replacing it can be costly. For this reason, many third-party ink manufacturers provide a guarantee and will pay for the replacement of a damaged printhead. Xerox also provides its own warranty.