Analysis of Industrial Inkjet Technology

Although all inkjet technologies can generally be described as the digital control of fluid droplets from the print head to the substrate, this can be achieved through several methods. Industrial inkjet is typically divided into two broad categories: Continuous Inkjet (CIJ) and Drop-On-Demand (DOD), with further subcategories within each type.

★ Continuous Inkjet (CIJ)

As the name suggests, continuous inkjet technology continuously ejects ink droplets. These droplets are either directed onto the substrate or to a collector for recycling and reuse. In contrast, Drop-On-Demand only ejects droplets when required. Continuous inkjet (CIJ) is considered a mature technology. It is primarily used for product and package marking and coding.

In this technology, a pump guides fluid from the ink reservoir to tiny nozzles, where high-frequency vibrations (about 50 kHz to 175 kHz) of a piezoelectric crystal generate a continuous stream of ink droplets. The droplets are charged by an electrostatic field and then deflected by another field to determine where the droplets land on the substrate. Unused droplets are collected and returned for reuse. The high droplet frequency of CIJ translates into high-speed printing capabilities, such as in date coding on beverage cans. Another advantage of CIJ is its high droplet speed (around 25 m/s), allowing for a relatively larger gap between the print head and the substrate (compared to other inkjet technologies), which is particularly useful in industrial environments. Historically, CIJ has an advantage when using volatile solvent-based inks, which allow for quick drying and enhance adhesion to many substrates. However, its drawbacks include relatively low print resolution, well-known high maintenance requirements, and the environmental concerns due to the use of solvent-based fluids. Additionally, there are limitations related to the requirement that the printing fluid must be chargeable.

★ Drop-On-Demand (DOD)

Drop-On-Demand (DOD) is a broad category of inkjet technologies where ink droplets are ejected only when required. Typically, the droplets are formed by generating a pressure pulse inside the ink chamber. The specific methods used to generate these pressure pulses define the subcategories within DOD. The main subcategories are thermal inkjet, piezoelectric inkjet, and electrostatic inkjet. There is also a discussion of an additional category, MEMS (Micro-Electro-Mechanical Systems), but MEMS drop-on-demand print heads generally still rely on piezoelectric or thermal inkjet technology.

① Thermal Inkjet

• Definition of Thermal Inkjet:
  Thermal inkjet is the most commonly used technology in consumer desktop printers and is now expanding into industrial applications. In this technology, ink droplets are formed by rapidly heating a resistor element inside a small chamber containing ink. The temperature of the resistor rises to 350-400°C, causing the ink above the heater to vaporize. This rapid vaporization generates a bubble that creates a pressure pulse, forcing an ink droplet from the nozzle. After the droplet is ejected, a vacuum is created in the chamber, which is then filled by replenishing ink, preparing for the next droplet.
  
  

• Advantages of Thermal Inkjet:
  Thermal inkjet has the advantage of achieving very small droplet sizes, high nozzle density, and compact devices, which also lowers the cost of print heads and products.
  
  

• Disadvantages of Thermal Inkjet:
  The main disadvantage is the limited range of fluids that can be used. The fluid must not only be able to vaporize (usually meaning it's a water-based solution), but it must also withstand extremely high local temperatures. If the fluid is improperly designed, these high temperatures can cause a hard coating to form on the resistor, lowering its efficiency and ultimately shortening the print head's lifespan. If the fluid’s functionality is damaged by high temperatures (such as with certain delicate fluids and polymers), this can also cause problems.
  
  

② Piezoelectric Inkjet

• Definition of Piezoelectric Inkjet:
  Piezoelectric inkjet is the preferred technology for most emerging industrial applications. In this technology, piezoelectric crystals (usually lead zirconate titanate) deform when an electric field is applied, and this deformation is used to mechanically generate a pressure pulse, causing ink droplets to be ejected from the nozzle. There are many structural variants of piezoelectric inkjet, including tube-type, sidewall-type, facewall-type, dynamic-wall-type, and piston-type.
  
  

• Advantages of Piezoelectric Inkjet:
  Piezoelectric inkjet’s advantages include the highest level of freedom in ink development among all inkjet technologies and long print head lifespans.
  
  

• Disadvantages of Piezoelectric Inkjet:
  The main disadvantage is the higher cost of the print head and associated hardware, which limits its cost-effective integration into low-end products.
  
  

③ Electrostatic Inkjet

• Definition of Electrostatic Inkjet:
  Commercial applications of electrostatic inkjet are very few, although they are increasing. Electrostatic inkjet (the most well-known being TTP's Tonejet) is characterized by the fact that ink droplets are pulled from the nozzle under the influence of an electrostatic field. This field acts between electrodes and the nozzle, attracting free charges in the ink (sometimes referred to as liquid developers) to its surface, causing ink droplets to form when the electrostatic force exceeds the surface tension of the ink. Because this technology relies on the attraction of free charges, the ink must be conductive.
  
  

• Advantages of Electrostatic Inkjet:
  The advantages of electrostatic inkjet include the ability to print fluid more concentrated than the actual formulation that passes through the print head, and the achievable resolution is not a function of the nozzle diameter, which may allow for higher resolution than piezoelectric inkjet. Additionally, very small ink droplets can be formed even when using pigments, because the size of the droplets is controlled by the voltage at the ejection point and the characteristics of the particles, not by the nozzle size. Due to the significant concentration of printing materials in the ink droplets, high optical density images can also be achieved.
  
  

• Disadvantages of Electrostatic Inkjet:
  The disadvantages include the limitation to conductive fluids and the high cost of implementing this technology. As the implementation increases, the cost is expected to decrease.
  
  

Thousands of companies are involved in the design and/or delivery of industrial inkjet systems. Some are highly vertically integrated (e.g., HP), providing nearly all parts of a complete solution, including ink, hardware, system integration, and distribution, while others focus on specific links in the value chain (e.g., Xaar). The chart below lists some of the major players in various industrial print head technology variants. It should be noted that while MEMS print heads are typically based on piezoelectric or thermal inkjet configurations, they are shown separately here due to their significant potential impact on the future development of inkjet technology.