Introduction to InkJet Printers

Combining the easy of use like impact printers with graphics capabilities of offset printing has made the inkjet printer the most common printer found in homes and businesses today. Inkjet printers come in many shapes and sizes and offer varied capabilities. What has made them so popular? Color! Color inkjet printers are the most inexpensive printers available and their color photo and presentation quality has made them a high demand item.

Inkjet printers create output similar to dot-matrix printers by laying small dots of ink on the output media. The difference is that inkjet printers do not use pins to impact the paper. Instead ink is fired directly from the printhead in small droplets which are absorbed and held in place by the media substrate. Thus inkjets are known as non-impact printers.

Since inkjet printers are non-impact they can not do certain things that impact printers can such as print multi-part forms. Instead they must print multiples of the same copy on separate pages, This weakness insures that the impact printer will not disappear from the business office. Inkjet printers are much quieter than impact printers and as such make a much more suitable desktop printer than the impact printer.

How The Inkjet Printer Works

To understand how an inkjet printer works you must understand its most important mechanical component, the printhead. An innovation in printing technology, the inkjet printhead acts much the way a dot-matrix printhead does in that it passes back and forth across the paper and places small dots of ink on the paper. What makes the inkjet printhead unique is that it does this without actually touching the paper. Instead of using pins to strike a ribbon or to strike the paper, inkjet printheads fire droplets of ink from small nozzles located on its surface directly onto the paper.
The process is very simple. Ink is stored in a reservoir which is connected to the printhead. These reservoirs can be permanently joined to the printhead or they can independently replaceable components. From the reservoir ink flows through a filter into an area called the pre-chamber, which acts a temporary second storage area. From the pre-chamber the ink flows into the firing chamber. The firing chamber looks like a small flattened box (very flat, in some cases the space between the top and bottom of the firing chamber is smaller than the diameter of human hair.). The bottom of the the firing chamber has a series of holes in it called nozzles. Directly above each nozzle, one or more firing nodes (they can thermal resistors or  piezo crystals) provide the impulse which moves the ink through the nozzles. The surface tension of the ink keeps the ink from escaping the nozzle normally, but when a firing node is activated the ink expands, either due to an increase in temperature from a thermal resistor or from the action of sound waves from piezo crystals, and moves through the nozzle and falls towards the paper. The process is simple and by controlling many variables such as firing temperature, length of firing time, the particular sound waveform and frequency, surface tension of the ink, and the diameter of the nozzle opening, the proper size droplet of ink can be released.

Types Of Printheads

Continuous Thermal Printheads - These printheads maintain a continuously circulating flow of ink through the printhead while the machine is powered up. In all other respects they behave like other thermal printers. Examples of this type of printer include the Canon BJ-300 and BJC-800. The printhead and ink storage reservoir are separate components and the ink storage reservoir also contains and area store waste ink.

Thermal Drop-On-Demand - The most common type of printhead these printheads hold ink in storage in the firing chamber until the printhead is activated (Thus Drop-On-Demand). The firing action is individually controlled for each nozzle by one or more thermal resistors mounted above the nozzles. Activating the resistors heats ink in a localized area cause the ink to expand and fall out of the nozzle in droplets. The printheads are disposable user replaceable components and are relatively inexpensive to manufacture. They do wear out quickly primarily due to a process called "Kogation", which is a formation of non-soluble crystals that coat the inside of the firing chamber and is created by heating of the ink during the firing process. The ink reservoir can be permanently joined to the printhead (integrated cartridges) or fitted as a separate component (non-integrated cartridges). Inks used in these printheads are formulated to minimize Kogation but the process is inescapable. These printheads can use many types of inks such as dye-sublimation and pigmented inks. The best examples of printers using thermal printheads is the HP Deskjet Printers, Lexmark Colorjet Printers, and Canon BubbleJet Printers.

Piezo Drop-On-Demand - Similar in form to Thermal D-O-D printers, Piezo printheads use piezo action instead of heat to create ink droplets. They are more durable than Thermal printheads and as such are usually a fixed component in the printer and are not user replaceable. Ink reservoirs are separate components and is usually referred to as a cartridge or ink tank. Piezo printheads are not subject to Kogation damage like thermal printheads. They are more expensive to produce. Piezo printheads can use a wide variety of inks but work best with low foam formulas. Epson Stylus inkjet printers are the most common piezo printers.

Inks & Cartridges

Inkjet printers use either water-based dye or pigmented inks as the standard colorants for printing. Water based dye inks offer the widest color gamut and the dyes used are the same that used by the textile industry for coloring fabrics. They have poor waterfastness and depending on the type of dye in use have varying resistance to UV and oxidation.
Water based pigmented inks consist of solid micro-sized colorant particles which are permanently and evenly suspended using a dispersant coating. Black pigmented inks are in common use in many thermal drop-on-demand printers. They offer better waterfastness   than water based dyes as well as better UV and oxidation resistance. Color pigments are not standard in any inkjet printer since they offer a poor color gamut. Black tends to look more charcoal in color when compared to waterbased dye inks.

Inkjet Ink Formulas

Inkjet inks are formulated based on the printhead characteristics and the design of the printing engine. The type of printhead (thermal or piezo), diameter of nozzle openings, and print speed are the basic characteristics of the printer considered when formulating inks. The characteristics determine the inks viscosity, drying time, and surface tension. Depending on the resolution and driver capabilities, the inks may require specific types and concentrations of dyes or pigments.

The size of the droplet of ink being fired from a printhead is measured in picoliters. 1 picoliter is equal to 1 billionth of a liter (.000000001 liters). Different printers can produce different droplet sizes ranging from 50 picoliters to as small as 3 picoliters. As such ink characteristics such as surface tension and viscosity as well as the intensity of the dyes become very important in making the printer produce a good image.

Inkjet Cartridges

In general, an inkjet cartridge is defined as a user replaceable component which serves as the main storage reservoir for ink. Inkjet cartridges come in to forms, cartridges with the printhead permanently affixed (integrated cartridges) or cartridges which are separate component from the printhead (non-integrated cartridges). HP printers most commonly use integrated cartridges and Epson printers generally use non-integrated cartridges.
Inkjet cartridges are generally used as the basis for the engine designation of a particular inkjet printer. For example, The first printer released which used the Canon BC-21 cartridge was the Canon BJC-4000 printer. Since then Canon has released 9 additonal machine models which use this cartridge. In addition Apple has two machines it sells which use the same cartridge (branded with the Apple brand name) and Panasonic has a few multi-function printers which also use this cartridge (branded under the Panafax brand name). When referring to the engine designation of these printers they are most commonly described as the "Canon BJC-4000 engine" and the type of color cartridge they use as the Canon BC-21 cartridge. This may seem confusing, but Canon was the original designer of the cartridge and printer design used in these printers while the other companies simply obtained a license from Canon to produce printers with the same design under their own brand name. This doesn't mean that the printers are the same, some offer more features than others, It simply serves as an easy cross-reference for determining the relationship of printer designs.

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