The device and the principle of operation of an inkjet printer

let's talk about inkjet printing , as the most common in the world, namely about the device and the principle of operation of inkjet printers .

Whatever may be said about the superiority of electronic media over paper, it seems that the age of paper and printed text will not be soon. It has long been known that a printed text is perceived completely differently than its “electronic” copy on the monitor screen. And before that bright day, when the paperless standard of information will triumph and we will no longer have to translate the merry forests into paper. In the meantime, we need to print . Print at home and at work. Print monochrome text, color pictures and even want to print photos not at the photo salon, but at home.

By the principle of operation, inkjet printers resemble matrix ones , but instead of needles hitting the ink ribbon, ink in inkjet printers is applied directly to the paper with drops of paint through very small holes called nozzles. Each drop of paint has a volume of the order of several picoliter with a diameter of the order of several to tenths of a micron (for comparison, the thickness of a human hair is about 100-130 microns). In one cubic mile meter there are about ten thousand such drops. If the image printed on an inkjet printer is viewed under a microscope (Fig. 1), then we will see that the image consists of miniature droplet points.

Figure 1 - View of the paint droplets on paper under the microscope

The main node of an inkjet printer is the printhead (about 80% of the cost of the printer), which actually applies the ink drops to the paper. Paint is applied through small holes called nozzles. The total diameter of one nozzle is about three (at a resolution of 4800 dpi) to several tens of microns. An enlarged view of the nozzle is shown in figure two.

Fig. 2 - Enlarged image of inkjet nozzle

Under the holes are miniature cavity, where the ink comes from the main reservoir of the cartridge. The ink nozzles themselves can not spill out so the hole is very small and the paint in them is held by surface tension. That is, the paint must be forced out. There are two main ways to extrude paint: Piezoelectric and thermal.

Piezoelectric (Piezoelectric Ink Jet) - above the nozzle is a piezocrystal. When electric current is applied to the piezoelectric element, it (depending on the type of the printing head) bends, lengthens or pulls the diaphragm, which creates a local area of ​​increased pressure near the nozzle - a drop is formed, which is subsequently ejected. In some heads, the technology allows you to change the size of the drop

Fig. 3 - The principle of jet printing

Thermal (Thermal Ink Jet) (also called BubbleJet, developed by Canon, in the late 1970s) - a microscopic heating element is placed in the nozzle, which, when passing electric current, instantaneously heats up to a temperature of several hundred degrees, and when heated, ink bubbles form in the ink ( English bubbles - hence the name of the technology), which push droplets of liquid from the nozzle onto the carrier.

Fig. 4 - The principle of thermal printing

Each of these two methods is attractive in its own way, but each of them is not free from shortcomings either. Piezoelectric technology is the cheapest, it is more reliable (because it does not use high temperature). This method of control is less inertial than heat, which allows for faster printing speeds.

Fig. 5 - An enlarged image of an EPSON sandblasting print head.

Thermoelectric technology is associated with high temperature. At high temperatures, the heater over time is covered with a layer of soot; therefore, in printers using this technology, the print head often fails. In such cases, it forms an integral unit along with the ink reservoir.

The main characteristic of the printer on which the optical resolution depends most strongly is the type, number and location of the print heads on the carriage. Photo printers and office printers are rarely equipped with more than one head per color. This is due to low print speed requirements, and besides, the smaller the heads, the simpler and more efficient the system is their calibration and convergence.

The printheads can be structurally integrated with the ink cartridge (Fig.6) and replaced at the same time as it can be permanently installed in the printer (Fig. 7) - only the cartridge is replaced.

Fig. 6 - Printhead with integrated cartridge (circled). The arrow shows the installed system of CISS

Fig. 7- Printer with separate cartridges

Each of these options has its advantages and disadvantages. It would seem that ink capacity without a printhead should cost much less than in combination with a printhead . In fact, this does not happen and there is no noticeable reduction in operation when the print head constantly installed in the printer is not observed. At the same time, an easily replaceable printhead makes it easy to get out of the difficulties associated with drying ink in its channels. It should be remembered that if the ink dries in the head, then, as a rule, it should be changed, if appropriate measures are not taken in a timely manner. In order to reduce the risk of ink drying in the head channels, a special parking position is provided. Most printers have a nozzle cleaning function . Nevertheless, all this does not give complete confidence that during operation the print head will not have to be changed.

The head together with the ink tanks is fixed on the carriage (Fig. 8), which, in a special guide, makes a reciprocating movement across the sheet of paper.

Fig. 8- Carriage Guide for an Inkjet Printer

Although the method of combining the printhead and ink tank is structurally the simplest and because of this is the most widely used, it is not optimal. The fact is that the carriage must move fast enough, and also it is enough to change the direction of movement quickly enough, because the speed of its movement determines the printing speed. For this, the movable carriage must be of a low inertia, i.e. it must have the least possible mass. To this end, reduce the amount of ink tank. Therefore, it is preferable to place the ink tank on the stationary part of the printer, and supply the ink to the printheads using special piping.

This system allows you to increase the speed of printing and at the same time increase the capacity for ink, but the pipeline system is structurally so complex that this design is used very rarely.

Ink interaction with paper

The cornerstone of high-quality print technology for all printer manufacturers. This process is largely dependent on the type of ink used, which can be divided into water-soluble and pigment inks . Water-soluble inks easily dissolve in water, they are usually used for colored dyes, as they give a wide color coverage. When falling on paper, the ink solution is absorbed into the fibers, dyeing them. Thus, the entire surface of the pattern is painted almost continuous layer. In addition, they provide a sufficient number of shades to ensure a smooth color reproduction. Solvent inks are a water-soluble ink - the most common type of ink. Solvent ink is applied in large format and interior printing. Characterized by very high resistance to water and precipitation. Characterized by viscosity, grain and solvent fraction used. Pigment inks - used for obtaining high quality images in interior and in photo printing.

Most models of inkjet printers use four primary colors , the so-called CMYK color model, where: Cyan is the year of the game, Magenta is pink, Yellow is the yellow color , Key color is black. We will not be aggravated in details of obtaining colors, but it is worth knowing that all colors come from three primary colors, red, green and blue, however this is true only when we perceive color mediocre, for example, from a computer screen, where color formation just happens due to these three colors (the so-called RGB color model ). But in the printed image, we perceive the reflected color, and its perception by the human eye is a little different.

Despite the fact that black can be obtained by mixing in equal proportions of magenta, cyan and yellow dyes, for a number of reasons this approach is usually unsatisfactory. In practice, due to the non-ideality of dyes and errors in the proportions of the components, the real magenta, cyan and yellow colors give a rather dirty brown or dirty gray. Adding black in color separately provides significant savings in ink, since in most cases it is black that is consumed and it is much more profitable to use it separately.

Fig. 9 - The real overlay of colors in the CMYK model, it is clear that when mixing three colors, the “black” color does not work

Modern printers mainly use these four colors, that is, they are four-color. Surprisingly, over the past three years, most manufacturers are on the path to reducing the color palette in home and office printers. This is largely due to the lack of demand for full-color printing at home and small offices. Naturally, these four colors are not enough to get high-quality photos, so in inkjet printers several more bright colors are added to the four primary colors, for example, CMYKLcLm is used for six-color printers (where (Lc is light Cyan, Lm is light Magenta). Color rendering and Saturation when using the expanded palette is much better, so the photo printer should have an expanded color palette.

In office printers, the Continuous Ink Supply System (CISS) is also used to reduce the cost of printing and to improve some of the other printing characteristics, representing a kind of gravity-free ink supply system. The elements of the CISS are ink tanks (ordinary plastic bottles or special “Mariotta vessels”), plastic or silicone tubes connecting the containers with cartridges or capsules installed on the printer’s printhead inlets, automatically resettable microchips similar to those installed on the original cartridges on a separate plate or on cartridges. General view of cartridge CISS is shown in Figure 10.

Fig. 10- General view of cartridge CISS

The principle of work of the CISS is based on the operation of the membranes of the piezoelectric elements of the print head of an inkjet printer; a vacuum is created in the capsule or cartridge of the CISS. In the capsule or cartridge, through their upper part, begin to drop the ink from the external capacitors. The tightness of the continuous ink supply system allows you to maintain a constant level of ink in the capsule / cartridge.

CISS helps to significantly reduce the cost of buying cartridges, but it is especially relevant when you have to print large volumes, in which cases the cost per print is comparable to the cost of printing on a laser printer, and when printing in bulk, the total cost per circle, taking into account the cost of paper, is even lower compared to color "lazernik".

The main difference between continuous ink supply systems is the use of a cartridge or capsule feed system. In the cartridge CISP, instead of the original cartridges, permanent cartridges are used, externally similar to the original with a built-in autochip, which can be reset by itself. The advantage of this type of CISS is in easy installation. In the capsule system, capsules are used instead of cartridges that are mounted directly on the input “needles” of the print head. Capsule CISS (Figure 11) is preferred because of the simpler maintenance, since the capsules are transparent and at any time you can check the level of ink in the capsule.

Fig. 11 - General view of capsular CISS

Installing a CISS usually does not cause problems even for an unprepared user. The package includes all the necessary materials, tools and a detailed algorithm with pictures. The cartridges / capsules of the CISS are installed instead of the originals, the capillary plume is laid in the printer according to the instructions, using clips and stickers included in the package. The containers and capsules are refilled with ink, several head cleanings are performed, and the system is ready for operation. The maintenance of the CISS consists of adding ink in the containers as they are consumed, and monitoring the tightness of the system by checking the level of ink in the capsules.

Here, in brief, we considered the principle of operation of an inkjet printer . In the future we will talk about the correctness of the choice of printer based on their needs.