"Inkjet coding" refers to many related technologies. We use these technologies to make very tiny ink droplets eject from the nozzle, the ink droplets pass through the air, and finally fall on the surface of the object to form a printing pattern.
Valve printing
This method is the easiest to implement. In the past 20 years, it has been mainly used for printing in outer packaging boxes. Basically, a valve printing device includes a low-pressure ink system, an electronic control cabinet, and a flexible conduit connected to the cabinet. Sprinkler. The ink in the ink system is sent to the nozzles in the nozzle through a simple opening / closing valve (a nozzle generally has 7 to 18 nozzles with a diameter of 200 microns or more). When an ink droplet needs to be ejected, the electronic component opens the corresponding opening / closing valve, and the ink droplet is ejected.
Due to the simple mechanical structure, the valve-type printing system is easy to establish. Customers generally choose suppliers by comparing the user interface (ie, whether the operation is easy), printing capabilities / printing diversity / quality, and applicable ink series.
The printing quality of valve printing is unstable. This is because the ink stays in the nozzle until it is ejected. If the ink dries in the pipe, it will block. The system uses water-based ink to print on permeable surfaces best. Many valve-type printing system manufacturers produce inks for non-permeable surfaces that dry faster than water-based inks. Blockages will occur at this time. Drying The time is still quite long-about 15 to 30 seconds.
In general, the valve-type printing system can perform well if the print quality requirements are not high and the print head is frequently cleaned. Although the purchase cost is low, the use cost of the valve-type printing system is calculated to be higher after one or two years, so this technology has been gradually replaced by the pulse printing technology. Pulse printing technology is mainly divided into two types: Piezoelectric printing and bubble printing-the realization of these two technologies is very different, the nozzle of pulse printing is developed from the field of office printing-pulse printing Now it is widely accepted in the office printing field and has very good results.
Pulse printing
Although pulse printing is conceptually simple, it is worth noting that no one obtained the original patent until the 1970s, and despite the extensive research done by companies such as Canon and HP, it was not until the 1990s. Cheap and reliable products are put on the market, so pulse printing is not as simple as it seems, from printing on clean paper at a fixed printing distance in the office to inkjet coding in the harsh environment of the factory There are many things to do.
Piezo printing
The first type of pulse printing technology is piezoelectric printing. In short, the ink pressure in the nozzle must be low enough (or negative pressure), because the surface tension of the ink keeps the ink in the nozzle and needs to be printed. When a pulse voltage is applied to the piezoelectric crystal, the piezoelectric crystal deforms, reducing the volume of the nozzle ink chamber. In this way, a drop of ink is ejected from the nozzle, and then the piezoelectric crystal returns to its original shape, and new ink enters the nozzle due to the surface tension. By arranging a large number of nozzles side by side, the ideal printing width and resolution (generally 8-6 dots / mm) can be obtained. Although the print resolution can be improved by tilting the print head (which will sacrifice print height), the print resolution is basically determined by the nozzle pitch. More precise improvements can make each piezoelectric crystal drive more nozzles (let's say 8), 32 piezoelectric crystals can drive the ink in 256 nozzles, so there will be a larger printing range, of course There are only 32 programmable drop points on the printed surface.
Because the system is not continuous, the ink must remain fluid in the nozzle and dry on the surface being printed. The ink used in piezoelectric printing is usually oil-based or paraffin-based. These two types of ink will not dry in the nozzle, but can be absorbed by the printing surface. Piezo printing also uses some quick-drying inks, which still require a considerable amount of time (about 10 seconds) to dry. When the product needs to be processed quickly after printing and the smudge is prohibited, the use of quick-drying ink will cause problems. In order to prevent the ink from drying in the nozzle, we can also apply a lower pulse voltage to the piezoelectric crystal, which will cause a slight disturbance to the ink in the nozzle, and the ink in the nozzle will not dry. This method relies on changes in ink composition or mechanically more sophisticated improvements.
Another way to achieve piezoelectric printing is to heat the print head while using hot-melt ink. In this way, the ink held as fluid in the nozzle will solidify on the surface of the object to be printed which is relatively cold. This piezoelectric printing system can get good results on many printed surfaces, but it is easy to be scraped off during the touch. In addition to the problem of ink drying out in the nozzle, another issue that needs attention is that the nozzle is very sensitive to vibration. Vibration can cause the ink to be shaken out of the nozzle and the ink cavity, so that the surface tension cannot make the ink fill the nozzle, and the system must be restarted. Obviously, when vibration problems are found, the print quality has been affected.
Bubble printing
Bubble jet printing technology is a newer technology and is still widely used in the office field.
A voltage is applied to the two terminals. Due to the resistance between the terminals, the ink is heated to form a vapor bubble. Due to the expansion of the chlorine bubble, a drop of ink is ejected from the nozzle. When the voltage between the terminals is removed, the bubbles disappear, and due to the surface tension, new ink fills the column nozzles. Like the piezoelectric printing technology, a series of nozzles are arranged together to obtain a larger printing range. The resolution is largely determined by the density of the nozzle arrangement.
Ink characteristics are particularly important for the normal operation of the bubble jet printing system. In the office, the surface of the printed matter can be controlled to match the ink, but it is another matter entirely in the production environment. For this reason, the influence of bubble jet printing technology in the field of product coding has been limited. However, in the occasions where bubble jet printing ink is suitable, we can get excellent printing results.
Continuous inkjet coding has a wide range of applications, and it is perhaps the most diverse technology. From the mid-1970s to the end of the 1970s, early continuous inkjet encoders had complicated operations and frequent failures. This impression still exists, but the situation has changed, just as we no longer leak fuel while driving. The latest continuous inkjet coding system only requires the operator to press the on / off key and perform routine maintenance once a week, and the maintenance requires much less maintenance than some other fateful equipment. There are two continuous inkjet coding technologies that are related but different: deflective printing and binary printing.
Deflection printing
Deflection printing technology has been commercialized since the early 1970s, and it may be the most developed technology among the technologies used in production environments. Although the principle is quite simple, for many years, a large number of control circuits have been combined to ensure reliability and ease of use.
The ink is sent to the nozzle after pressurization, forming an ink flow of about 20M / S. There is a piezoelectric device behind the nozzle. When the voltage is applied, the device will produce displacement. This displacement will disturb the ink flow. If the frequency of the electrical signal applied to the piezoelectric device resonates with the ink jet frequency, the ink flow will break Ink droplets of the same size and the same pitch. There is a charging electrode where the continuous ink flow breaks into a series of ink droplets. If the frequency of the pulse voltage on the charging electrode is the same as the frequency at which the ink flow breaks, each ink droplet will be charged accordingly. The ink droplet continued to move forward, passing a pair of deflection plates. The voltage on the deflection plate is a fixed value (for example, +/- 5KV) to form an electrostatic field. Under the action of the electrostatic field, the charged ink droplets will produce a corresponding direction toward one of the deflection plates according to the difference in the charge amount Amount of deflection. Eventually, the ink droplets pass through the air and land on the surface of the object to be printed passing through the nozzle. The uncharged ink droplets are not deflected and recovered by the recovery tank installed at the bottom of the head, and finally recirculated to the nozzle through an ink reservoir.
Continuous printing
Thus, approximately, the pattern of ink droplet printing corresponds to the pulse voltage applied to the charging electrode. The actual process is not so simple. We must synchronize the breakage of the ink droplets with the charging of the charging plates, and must consider the mutual repulsion between the charged ink droplets, and even the aerodynamic problems of the ink droplets in flight. Users of continuous inkjet encoders will not feel these problems, but it is these problems that make designing continuous inkjet encoders interesting.
Because ink jetting is continuous, continuous inkjet encoders can use many types of ink, especially those that dry very quickly (within 1 second). Therefore, continuous inkjet coding technology is ideal for the printing of products with impermeable surfaces (such as cans and plastics) that need to be processed quickly after printing. In addition, pigment inks with brighter colors can also be used.
Due to the relatively high jetting speed of continuous printing, the printing distance of continuous printing is usually much longer than the printing distance of pulse printing (generally more than 10mm), but the printing quality will not decrease. The placement of the nozzle head can have a greater choice.
Binary printing
The concept of binary-type continuous printing technology is as old as the deflection printing technology. This technology was early developed toward the commercial field of high-speed (high cost) and large-scale printing. As technology advances, binary printing will become practical in a very short time.
The ink is ejected from a series of closely arranged nozzles with a resolution of 4-8 dots / mm. The ink flow is broken by the piezoelectric device into droplets. More ink flow). The ink dots that do not need to be printed are charged, deflected, and then recovered by the recovery tank. The ink dots that need to be printed are not charged or deflected, and directly hit the surface of the object to be printed. In this way, the width of the printing pattern is determined by the number of nozzles or the number of ink flows. Of course, we can also use charged ink droplets to print and recover uncharged ink droplets.
The printing distance of binary printing is smaller than that of deflecting printing, but it is still much larger than that of valve printing. In principle, various inks used in deflection printing can be used in binary printing. In the future, the choice of binary printing or deflection printing will depend on whether the printing focuses on the number of lines of information or speed and cost. When printing more than 3 lines of information at the same time, binary printing is undoubtedly faster than deflection printing. However, binary jet printing is more expensive and requires more manual operations in early applications—especially when using different heterogeneous inks. Basically, binary printing and deflection printing will coexist, because at present, they provide customers with the most diverse and effective solutions in the field of online printing.
