DPI
If you want to understand the printing accuracy of an inkjet printhead, DPI (dots per inch) is a key concept that cannot be avoided.
DPI indicates how many dots (such as ink or pixels) appear within each inch of a printed surface. It encompasses both horizontal and vertical directions, meaning both axes contribute to the overall print resolution.
Take 360×720 DPI as an example: this represents a total of 259,200 dots in a square inch. Similar to how a smartphone with higher screen resolution delivers a sharper display, a greater number of dots per inch results in more precise, detailed prints.
PASS Number of Inkjet Printhead
The PASS number refers to how many times the printhead jets ink over the same area during printing, directly relating to its movement pattern.
For example, "1PASS" means the nozzles pass over and print that area just once. In contrast, "4PASS" indicates that the nozzles print the same section four times (excluding feathering effects). The higher the PASS count, the greater the print’s smoothness, depth, and detail.
Inkjet Printhead Parameters: The Key to Achieving High Printing Precision
To optimize print quality, it’s important to understand three essential inkjet printhead parameters: mechanical resolution, ink droplet volume, and firing frequency.
1) Mechanical resolution: defined by nozzle spacing
A printhead contains hundreds or thousands of nozzles aligned in a fixed layout. The distance between these nozzles determines their mechanical resolution. The smaller the spacing, the finer the output.
However, physical constraints, such as the thickness of piezoelectric ceramics, limit how closely nozzles can be positioned. To overcome this, manufacturers often use staggered nozzle designs. For instance, two interleaved rows of 150 DPI nozzles can simulate 300 DPI resolution.
2) Ink droplet volume: the measure of print detail
Ink drop size refers to the volume of ink released in each ejection, typically measured in μL, nL, pL, or fL. This volume directly influences print clarity.
Different types of printheads offer different droplet volumes. Epson heads often produce 3.5 pL drops, while industrial heads for solvent printing may range from 25 to 60 pL, and UV printheads generally offer 5 to 12 pL. The choice of drop size depends on the ink type, media, and image detail required.
3) Firing frequency: balancing speed and precision
Firing frequency is the number of ink drops ejected per second, and it directly affects printing speed. While it doesn't alter resolution by itself, a low firing rate can limit the speed of high-resolution printing. Choosing the right inkjet printhead often requires balancing the desired speed with precision.
Horizontal and vertical accuracy of Inkjet Printhead
Symphony of X and Y
During the printing process, the horizontal scanning accuracy (X) and the vertical accuracy (Y) jointly determine the overall printing effect. The horizontal scanning accuracy is mainly affected by the ignition frequency and cannot be infinitely enlarged. Because the higher the accuracy of X, the slower the movement speed of the car.
Taking the ignition at 15 KHz as an example, if you want to achieve a speed of 1 m per second for the carriage, then the horizontal scanning accuracy X is limited to about 360 DPI (considering the accuracy value of the grating).
After determining the accuracy of X, we need to determine the accuracy of Y. The accuracy of Y mainly depends on the physical accuracy of the printhead, which is what we often call the number of passes.
Taking the physical accuracy of the printhead as 300 DPI as an example, every time you print 1 PASS, the Y direction has an accuracy of 300. If the physical accuracy of the nozzle is 150 DPI, the accuracy of printing 1PASS is only 180.
But here we also need to consider the coverage issue, that is, how many dots per inch can meet our printing needs. This requires taking the ink drop size into consideration. According to the industry's minimum requirements, an ink drop size of about 25 PL requires at least 200K ink dots to meet the coverage. Therefore, we can deduce the number of PASSes for the longitudinal accuracy Y based on the lateral accuracy X and coverage.
However, it should be noted that some manufacturers may advertise ultra-high accuracy, which can print normally, but the speed is extremely slow and has no practical production significance. Similarly, some manufacturers may advertise ultra-fast speeds, but the accuracy is greatly reduced and cannot really meet the printing requirements.
