Fundamentals of Inkjet Printhead Technology
Inkjet printing technology has developed rapidly and is continually expanding into new markets and product areas. As numerous manufacturers compete for future market positions, the development of new printheads, inks, and functional fluids has reached remarkable heights. Having a certain level of background knowledge is crucial, as it will help individuals make informed decisions when purchasing inkjet printers, based on an understanding of the printhead technology chosen by manufacturers.
All inkjet printhead technologies are based on one fundamental principle: electronically controlling the ejection of fluid droplets from the printhead onto the required substrate. However, the ejection of droplets can be achieved using various methods.
Inkjet printhead technology can be divided into continuous inkjet and drop-on-demand (DoD) inkjet categories, with further subcategories under each type.
Continuous Inkjet (CIJ) technology continuously ejects droplets, which are either directed to the substrate or directed toward a recovery system for recycling and reuse.
In contrast, drop-on-demand (DoD) inkjet technology ejects droplets as needed. These droplets are generated by producing pressure pulses within the ink chamber. The subcategories of DoD inkjet are defined by the various methods used to generate pressure pulses. The three main types are thermal inkjet, piezoelectric inkjet, and continuous inkjet (electrostatic).
Thermal Inkjet Technology
The principle of thermal inkjet technology was first discovered in 1977 by Canon engineer Ichiro Endo. Since the early desktop inkjet printer days, thermal inkjet printheads have made significant progress.
While designs for thermal inkjet printheads vary, the basic concept remains the same. This technology provides very small droplet sizes, high nozzle speed, and high nozzle density.
In a small chamber containing ink, a resistive heating element rapidly heats up to several hundred degrees Celsius, causing the ink molecules to vaporize. The boiling ink quickly generates a bubble (pressure pulse), which displaces the ink in the chamber and forces the ink droplet to be ejected from the nozzle. The ejected droplet leaves a void (vacuum) in the chamber, which is subsequently filled with ink from the ink reservoir to prepare for the next droplet formation.
A drawback of this technology is the narrow range of fluids that can be used. New printhead, ink, or functional fluid developments have reached impressive heights, with many manufacturers competing for future market positioning.
Inks used in thermal inkjet must be designed to vaporize and withstand localized high temperatures. Thermal inkjet printheads also experience performance degradation due to a process called cavitation corrosion. Cavitation corrosion occurs when heat-induced bubbles form and implode, causing stress and wear on the heating pads. However, technological advancements have extended the lifespan of thermal inkjet printheads considerably.
To achieve smaller droplet sizes and faster print speeds, high-precision manufacturing technologies are required to arrange more nozzles across a broader area.
-
Canon's FINE printhead has successfully achieved up to 2,560 nozzles per color—that’s 15,360 nozzles per printhead. These printheads are manufactured with multiple nozzle sizes because thermal inkjet printheads cannot generate variable droplet sizes. These printheads include 1pl, 2pl, and 5pl nozzles in a special configuration.
-
HP is another major player, with its Edgeline printhead technology achieving impressive nozzle density. This unit has a 4.25-inch print width and is made up of five silicon printhead chips arranged in an offset pattern.
The native resolution of this printhead is 1200 dpi, and the operating frequency is 48 kHz. Each printhead contains two rows of nozzles (10,560 per chip), which can simultaneously print two colors or print one color with the other row as a backup. Each printhead has five chips, totaling 52,800 nozzles per unit, and can use water-based or latex-based inks.
Edgeline printheads are used in HP’s latex printers and roll-to-roll printing machines. For example, the T300 roll-to-roll printer has a print width of 30 inches, requiring 70 Edgeline printheads per side. For duplex printing mode, this is 140 printheads, totaling 7,392,000 nozzles, with each nozzle ejecting 20,000 ink droplets per second, totaling 148 billion droplets per second, all arranged precisely. It’s important to note that all thermal inkjet printheads are consumables, and their lifespan is tied to the volume of ink throughput.
Other manufacturers, such as Kodak and Lexmark, have also sold thermal inkjet printheads for desktop printers. In the wide-format printing market, HP and Canon are the main competitors for water-based inkjet printers, with HP being the only supplier offering latex printers with thermal inkjet printheads. HP is also the only company using thermal inkjet technology in single-channel roll-to-roll configurations. Despite finding niche markets for thermal inkjet printheads, most large-format and grand-format roll-to-roll and flatbed printers on the market integrate piezoelectric inkjet printheads.
Piezo Drop-on-Demand Inkjet
Piezoelectric printheads, while sharing the common technology of droplet ejection, allow for customization of materials and options, making them particularly suitable for inkjet printer manufacturers.
In piezoelectric drop-on-demand inkjet technology, certain crystals change shape when voltage is applied. This bending is sufficient to deform the ink chamber and generate a pressure pulse inside. There are various designs for piezoelectric printheads from over a dozen manufacturers.
Inkjet printheads have a wide range of applications, with one of them being in the graphic arts field. Other applications include: marking and coding, postal and address printing, document processing, textile marking and printing, etching, photovoltaics, material deposition, and precision fluid dispensing.
The characteristics of inkjet printheads can be summarized into the following subcategories:
-
Fluid compatibility (water-based, oil-based, solvent-based, UV, acidic)
-
Operating temperature
-
Number of nozzles
-
Native resolution
-
Print width
-
Structural materials
-
Fixed or variable droplet sizes
-
Minimum droplet size
-
Environmental adaptability
The biggest difference between inkjet printheads is the fixed droplet and variable droplet technology. Fixed droplet technology, also known as binary, produces droplets of a single fixed size, ranging from 1pl to 200pl or larger. The advantage of larger droplets is that they can cover a larger area more quickly. Larger droplets naturally have lower native resolution, making them more suitable for large print jobs or textiles, as well as other applications that don’t require high resolution.
At the smaller droplet scale, such as the Durst Rho P10 series, the Quadro Array printhead uses 10pl droplets with a resolution of up to 1000 dpi. Even 1pl small droplet printheads are designed for fluid deposition and printed electronics rather than graphic applications.
Fixed droplet printheads can also have higher ejection frequencies, typically measured in kHz. Fixed droplet inkjet printers can have four-color or six-color configurations, but keep in mind that for large-scale projects, four-color printing will be faster than six-color, and using multiple printheads per color can significantly increase print speed.
There’s a lot of debate about which is better (fixed droplet vs grayscale) and why, but it ultimately depends on the type of product you’re printing, the printer’s cost, and the production speed needed to remain profitable.
Grayscale or Variable Droplet Printheads
Grayscale or variable droplet printheads have a distinct advantage: they can dynamically change the resolution. This is achieved by using a base droplet size and then combining multiple base droplets in-flight to form a larger droplet. For example, a 6pl base droplet could be combined with additional pulses to form a 12pl droplet.
The key benefit of this technology is the ability to dynamically adjust resolution.
Example with 16-level grayscale printheads:
An 8-level printhead produces 7 droplet sizes, while a 16-level printhead produces 15 droplet sizes. For example, if the base droplet size is 6pl, the droplet sizes grow as follows: 6, 12, 18, 24, 30, 36, and 42.
These are simple multiples of the base droplet size. However, when looking at ejection rates, variable droplet printheads produce droplets at a slower rate, which is reasonable.
For instance, with a 16-level grayscale printhead, the base droplet can be ejected at a rate of 28.0 kHz. When using the 8-level droplet size, the ejection rate drops to 6.2 kHz, and when using all 16 levels, the rate further drops to 2.8 kHz. The droplet count decreases by a factor of 10 from base to 16-level operation. While variable droplet printhead speeds are slower than fixed droplet printheads, their advantage lies in the improved fine text resolution and overall print clarity.
A typical method manufacturers attempt to gain a speed advantage from grayscale printheads is to increase the number of ink channels per color. An ink channel is described as a row of nozzles dedicated to a specific color. It can also be an entire printhead unit dedicated to a single color. This would be a typical scenario for scanning or single-channel printing systems. Scanning printing refers to an inkjet printing method where the printhead carriage moves back and forth across the media surface while the media is delivered intermittently. This can be reversed, as in some flatbed printers, where the substrate reciprocates beneath the printhead array, which moves incrementally along the width of the media.
Continuous Inkjet — High-Speed Inkjet
Continuous Inkjet (CIJ) is a non-contact, high-speed printing technology used to apply variable information onto moving substrates. Initially designed for printing date codes, text, and batch numbers, it has now evolved to support full-color roll-to-roll production.
In this technology, a pump directs liquid ink from the reservoir to multiple micro nozzles, producing a continuous flow of droplets at very high speeds. The piezoelectric crystal vibrates to control the droplet ejection rate, typically from 50 kHz to 175 kHz, generating 50,000 to 175,000 droplets per second per nozzle. The ejected droplets pass through an electrostatic field, which charges them. The charged droplets are then directed to the substrate or diverted to a recovery tank for reuse. Most droplets are recycled, with only a small portion used for actual printing.
Kodak Stream Technology is a hybrid continuous inkjet method. By applying regular pulses to heaters around each nozzle, it breaks the ink into tiny droplets. By changing the pulse size and shape, the dot size and drop rate can be adjusted. Stream technology can produce droplets at 400 kHz, fast enough to match traditional roll-to-roll offset printing presses.
Stream technology is developed using MEMS technology, as is HP's Edgeline printhead. MEMS is an advanced manufacturing method that uses techniques similar to those used in manufacturing integrated circuits to create ultra-miniature inkjet structures within a silicon wafer. Stream uses a nozzle plate that combines mechanical and electronic components on a single shared silicon wafer.