Inkjet Printing - What is inkjet printing?
A method of printing; this process creates an image by propelling droplets of ink onto a substrate. The image being printed is sent to the printer, where it is stored in the buffer memory of the printer; rollers draw the substrate through the printer and a stepper motor pulls the printhead back and forth across the substrate. Ink is drawn from a reservoir or cartridge and expelled through a nozzle or nozzles in the printhead before landing on the substrate where it dries in place. There are two main types of inkjet printing, which are categorised based on the method used to deposit ink onto a substrate.
Continuous inkjet printing uses a printhead with a single nozzle to produce a constant stream of liquid ink. This stream is broken up into droplets by a piezoelectric crystal (piezoelectric materials generate an electric charge in response to an applied mechanical force (pressure) and generate a mechanical strain in response to an applied electrical field). Some of these droplets are given a charge by a charging electrode and are then deflected by electrostatic deflection plates to land in a particular place on the substrate. Some droplets are left uncharged; these “guard droplets” prevent electrostatic repulsion between neighbouring droplets and are not deflected by the plates. Instead, they fall into a collection gutter and are sent back to the ink reservoir to be reused. Continuous inkjet printing is commonly used for the marking and coding of products and packaging.
Drop On Demand (DOD) inkjet printing creates and deposits droplets of ink only when and where they are needed. There are two methods of DOD inkjet printing, known as thermal and piezoelectric inkjet printing. Thermal DOD printing (sometimes known as “bubblejet”) uses a thermal printhead and creates droplets of ink using heat. It requires ink cartridges that are made of a series of tiny chambers, each containing a heating element (usually a resistor). A pulse of current is sent to the resistor, which causes the liquid ink to vaporise and form a bubble. As the bubble expands, a droplet of ink is expelled out of that chamber (and either into the next chamber or out of a nozzle in the printhead). The bubble then cools and pops, which creates a vacuum and draws ink into the chamber to replace the ink that has been used (either from the next chamber or from an ink reservoir). This method of DOD printing uses a cheaper printhead but requires more expensive printing inks (they have a volatile component that allows them to vaporise and form a bubble).
Piezoelectric DOD printing uses a printhead that has a piezoelectric material at the back of the ink chamber for each nozzle. An electric current is passed through the piezoelectric material, which causes the material to deform. As the material changes shape, the space in the ink reservoir is reduced and ink is forced out of the nozzle. When the pulse of current passes, the piezoelectric material returns to its original shape, creating a vacuum and pulling ink from the ink reservoir to replace the ink that has been expelled from the chamber. This method of DOD printing uses cheaper printing inks but a more expensive printhead (which contains piezoelectric material).
Inkjet printing is a form of digital printing; the image or document being printed must be in a digital file format before the printing process can begin. A Raster Image Processor converts the document or image being printed into a raster image or "bitmap", so that the document or image is represented by a grid of pixels (points of colour). This bitmap is then sent to the printer for output and droplets of ink are directed (continuous) or deposited (DOD) in place to recreate the grid (and therefore the image) on the substrate.
Multiple droplets of ink are required to recreate the correct colour of a single pixel, and dots may need to be layered and combined to create an accurate colour depth for photo quality images. This means that the resolution of the printer must be higher than the resolution of the image being printed; for example, you may need a printer with a resolution of 1200 dpi (printing dots per inch) to print an image with a resolution of 250 ppi (pixels per inch). The resolution that an inkjet printer is capable of producing is influenced by the number of nozzles in the printhead (DOD) and by the size of the dots that are produced; inkjet printers are capable of printing dots as small as 50-60 microns.
Most desktop inkjet printers use aqueous (or water-based) liquid inks that include water, glycol, and a colourant. Inkjet inks can be made with either pigments or dyes as the colourant; pigment inks tend to hold their colour better but are more expensive, while dye inks are cheaper and offer a wider range of strong and consistent colours but fade much more quickly. Desktop printers tend to use cartridges with the CMYK process colours; the black is usually a separate cartridge, while the CMY colours can be separate or (more commonly) are combined into one cartridge. Cartridges can also be produced with specific spot colours or special purpose inks (such as white or metallic inks).
As the inks as water-based, they are not waterproof, meaning inkjet printers cannot be used to make waterproof print unless special inks are used. Aqueous inks are also subject to a number of issues that result from these inks spreading or drying incorrectly, including bleed, strike-through, feathering, pinholing, blocking, blistering, skips, and set-off. Larger scale inkjet printers, such as wide format printers used in the printing industry, may use solvent inks (pigment inks that are waterproof and dry through the evaporation of the solvent), UV inks (dries through exposure to UV light), or sublimation dyes (traditionally used for printing on fabrics that contain polyester) to overcome some of the more common issues caused by using aqueous inks.
Inkjet printers are low cost and capable of high quality output, including vivid colour and fine details. However, they can be costly in terms of the consumables that need to be replaced (including cartridges and printheads), are subject to problems with ink drying within the printhead or reservoir (often requiring a cleaning cycle during the printing process), and print issues resulting from the use of aqueous liquid inks.Go Back to Glossary