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Piezoelectric Print Heads vs Thermal Print Heads

In the two articles before, we have introduced the thermal foaming and piezoelectric inkjet printing technologies respectively. This issue mainly introduces the differences between piezoelectric print head and thermal print head, including their working principle, ink requirements, service life, and characteristics.
Inkjet printing technology is one of the mainstream printing technologies. It is fundamentally different from laser printers and dot matrix printers at the principle level. The consumables of inkjet printers are ink, the consumables of laser printers are toner, and the consumables of dot matrix printing are ribbons. The biggest advantage of inkjet printing technology is that it can achieve low-cost, high-quality color printing and is widely used to print photos and color documents.
The core technology of inkjet printing is divided into two categories, namely “thermal foaming” and “micro-piezoelectric”.
These two technologies are completely different in principle, but the final result is the same, both of which are to spray ink onto the printing substrate to form an image.

In July 1977, Endo, a member of the 22nd Laboratory of the Canon Product Technology Research Institute in Japan, accidentally placed a heated soldering iron on the attachment of an injection needle and ink flew out of the injection needle. Inspired by this, two years later, in 1979, bubble jet technology appeared. This technology uses a heating component to heat the ink to form water vapor, and then generate pressure to cause the ink to be ejected from the nozzle, and then use the ink cools the nozzle, causing the heating element to cool down the hot spot of the heating element, causing bubbles to subside and thereby controlling the size of the ink droplets.
In 1984, HP also invented inkjet technology based on the same principle as Canon’s bubble jet, named it Thermal ink-jet, and launched HP’s first inkjet printer, Thermal Injet.
In 1985, Canon applied its “thermal foaming inkjet technology” to its inkjet printer Y-80 for the first time, which later became the “thermal print head” and thus created the history of Canon inkjet printers.
In 1987, the HP “print jet” inkjet printer entered the market. It also used thermal foaming technology and had a resolution of 180 dpi. However, this model could not print ordinary paper and mainly printed special coating materials and projection films.
In 1991, HP launched the “HP Deskjet 500c” inkjet printer. This machine has a resolution of 300 dpi. It is a desktop printer and can print on ordinary paper.
In 1994, in order to compete with Canon and HP to seize the inkjet printer market, the Japanese company Epson finally applied micro-piezoelectric inkjet technology to printers after 20 years of research, which later became the “piezoelectric print head“. Since then, the history of piezoelectric inkjet printers has been created.

The basic principle of piezoelectric inkjet printing technology is to place many tiny piezoelectric ceramics near the nozzle of the print head and use the principle of ink to deform under the action of voltage so that the ink is ejected from the nozzle, thereby patterns are formed on the substrate.
Although the core technology of inkjet printers is mastered by HP, Canon, and Epson, the Japanese brother company still has some patents on micro-piezoelectric printing technology. The company also has its own inkjet printers. Perhaps due to technical limitations, there are no high-end six-color, eight-color, or twenty-color inkjet machines, only four-color home machines and commercial machines.

1. Working Principle of Inkjet Print Head

Piezoelectric Print head

The piezoelectric inkjet print head is composed of many fine and firmly divided flow channels. The fine ink droplets are squeezed and ejected from the nozzle through the flow channels onto the printing substrate. Piezoelectric inkjet print heads use special crystal materials that can sensitively respond to voltage. When a voltage is applied, this special crystal undergoes slight deformation, forcing ink droplets to be extruded from the nozzle through a small flow channel. This special crystal quickly returns to its original position when the voltage is interrupted. The unique ink droplet level control technology is used to eject fine ink droplets. The ink droplet level control technology is refined into pull-push-pull. The first pull determines the size of the ink droplets; the push ejects the ink droplets; the second push is to stabilize the liquid level, a piezoelectric crystal area is used to impact and vibrate a diaphragm fixed in the print head during operation, thereby causing the ink in the print head to be ejected. Its operation is a mechanical movement at normal temperature, so its loss is small. , belongs to the long life-span print head.

Thermal Print Head

A small resistor is used to quickly heat the ink to a high temperature of about 400 degrees, and then generate boiling bubbles to eject the ink. Since long-term changes in high and low temperatures can easily lead to rapid aging and wear of the print head, the print head is disposable. If you feel that the above technical description of the print head is a bit complicated, you can understand it this way:

Thermal foaming principle: Similar to the principle of bullet shooting, the ink is equivalent to gunpowder. The ink is heated and the water vapor forms a thrust, and then the ink in the nozzle is sprayed onto the paper to form a pattern. Micro-piezoelectric principle: Piezoelectric ceramics can be understood as sponges. Piezoelectric ceramics deform after charging, thereby squeezing out ink to form patterns on paper.

2. Ink Requirements for Inkjet Print Heads

Piezoelectric Print Head

The piezoelectric print head performs mechanical movement at room temperature and does not produce chemical reactions. The concentration of the ink is not affected by heat. Therefore, the piezoelectric print head has a wider range of ink selections and can choose sensitive ingredients and inks with larger particles, such as some functional organic material system inks and particle inks. In addition, the piezoelectric print head can accurately control the amount of ink output and the chemical composition of the ejected ink, thereby realizing an on-demand ink supply, making it an ideal choice for materials science research. In addition, the thrust generated by the expansion of the piezoelectric crystal can ensure that the nozzle is unobstructed, thereby avoiding plugging and disconnection during the printing process, and truly achieving uninterrupted printing function.

Piezoelectric Print Head

Piezoelectric Print Head

Thermal Print Head

Since the thermal foaming inkjet printing technology requires heating the ink to produce a chemical reaction under high-temperature conditions, the chemical reaction components of the ink must accurately match the ink cartridge, otherwise, the ink will easily generate foam and block the nozzle, so the ink compatibility is relatively low. In addition, due to the influence of heat, the water content of the ink is high, generally between 70% and 90%, to keep the nozzle smooth and to cooperate with the effect of heat energy, so that the sprayed image is not easy to dry. In addition, due to the ejection principle of forming bubbles, the ink output cannot be accurately controlled. The ink output is approximately twice that of the piezoelectric print head.

3. Service Life of Inkjet Print Head

Thermal Print Head

Every time you spray ink, you need to heat the nozzle. If the print head works in a high-temperature environment for a long time, its life will inevitably be reduced.

What’s even more fatal is that some brands use “thermal print head” inkjet printers, which are severely reduced in order to reduce costs and are not equipped with “maintenance components”, resulting in the print head being exposed to the air.

Because the ink will be concentrated when heated, and the concentration will become larger. After some pigments are heated, a chemical reaction will occur, making the color “warmer”. When the thermal print head is working, the internal temperature is as high as 200 degrees. If there is no maintenance component, the nozzle will not be cooled in time, eventually causing the water in the nozzle to evaporate quickly, causing the pigment in the ink to clog the nozzle.

Therefore, machines that use thermal print heads are generally of quick-release design. If the print head is broken, you can directly replace it without dismantling the machine. The cost of this kind of machine is very high in the later period. After changing the print head a few times, you can buy a printer The probability of clogging is not low, and the stability and reliability are worrying.

Piezoelectric Print Head

There is no need to heat the nozzle when inkjet, so the power consumption is lower and the risk of clogging is reduced. The core component inside the print head is “piezoelectric ceramics”. The current signal controls the “piezoelectric ceramics” to complete the deformation and change the ink to form a squeeze jet. Since there is no need for heating, the color displayed is the color of the ink itself. Compared with the “thermal print head”, the color will appear “cooler”. Since there is no need for heating, the life of the print head will be longer. Micro-piezoelectric inkjet printers are equipped with a “maintenance component” as standard. When the printer is turned off, the print head will be parked in the maintenance component, which can completely isolate the print head from the air, effectively prevent dust and paper debris from entering, and further reduce the probability of clogging and malfunctions. If you always use original consumables, there will be few problems, and they have strong stability and reliability. You basically don’t need to change the print head in the future, just add ink.

Capping Station Assy

Capping Station Assy

For a machine that uses “original ink”, if the machine is shut down and the power is unplugged before the print head is reset, the head may become clogged after being unused for a long time. However, this can be done through the system’s built-in “cleaning program.” That will solve the problem.
If “compatible ink” is used, it is likely that the nozzle will be completely blocked, and the “cleaning program” will not work. Repeated cleaning will also reduce the life of the “waste ink pad”. In this case, the machine needs to be disassembled and cleaned, or even the print head, after all, there are so many “compatible inks” and different production standards, that it is difficult to ensure that there are no impurities.
As mentioned above, although thermal print heads have a short life, they all adopt a quick-release design. If the print head is broken, users can replace it by themselves without dismantling the machine. For example, HP and Canon inkjet printers have this design.
The micro-piezoelectric print head has a long service life and rarely causes problems when using original ink. However, it does not have a quick-release design. Changing the print head requires disassembly. For example, Brother and Epson inkjet printers require disassembly.
Although the failure rate of micro-piezoelectric print heads is very low, they sometimes break down. Once a problem occurs, users cannot repair it themselves and can only take it to a service station to have it handled by a dedicated person.

4. Printing Effects of Inkjet Print Head

The “thermal print head” needs to heat the ink during the printing process. After the ink is heated, it will be concentrated and the concentration will become larger. When some pigments are heated, a chemical reaction will occur, making the color “warmer”.

Since the “piezoelectric print head” does not require heating, the color displayed is the color of the ink itself. Compared with the “thermal foaming nozzle”, the color will appear “cooler”.

5. Characteristics of Inkjet Print Heads

Piezoelectric Print Head

(1). The print head can have a long service life if used properly;
(2). The ink output is small, only 10-12mL of ink is needed per square meter, which is half of the ink volume of the thermal print head;
(3). Compatible with more inks, including inks of various organic systems;
(4). Not easy to plug or break;
(5). The ink has high concentration, low moisture content, and is easy to dry;
(6). Highest accuracy: 2880dpi

Thermal Print Head

(1). The print head has a short service life, about 20-50㎡, and is a disposable consumable;
(2). The ink output is large, although the ink is 20 yuan/L
(3). It is only suitable for indoor printing, and after 1-2 months, the printed image will easily fade and bloom, and the quality is not guaranteed;
(4). The print head is easy to burn out and the ink suddenly cuts off, causing the entire picture to be scrapped;
(5). The ink contains high moisture and is not easy to dry;
(6). Highest accuracy: 1200dpi

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