Evaluating the print quality a specific inkjet printer can deliver is more complicated than it seems. Usually, the first thing that comes to mind is printing resolution. But can we just rely on the number of dots per inch (dpi) or the number of nozzles per inch (NPI) that the print head can print as the main parameter that determines the final printing quality?
When evaluating inkjet image quality, addressable dots per inch are not enough. You have to understand what those dots represent. Each pixel of an image typically requires multiple printing dots, depending on the color. The printed dots themselves can also be composed of many droplets. That’s not all. The printhead can “fire” ink droplets and place them at the same or different locations on the media. With some types of inkjet printers, the ink droplets can be of different sizes, which in turn allows for different-sized print dots. Visible image resolution can be increased by superimposing multiple ink droplets of the same or different colors, or by the calculated combination of closely spaced colored dots. This can mean a lot of things if measured in simple DPI, NPI, or the more similar term LPI (lines per inch).
Physical or native resolution is based on the number of nozzles per inch of the printhead or printhead module. Since we are talking about a production inkjet printer with a fixed inkjet head and media moving under the head, NPI can be thought of as a grid of printed dots whose density is determined by the nozzle spacing along the printhead axis and determined by the frequency and speed of ink droplet generation along the printhead axis. Note that for compact printheads, and thus NPI can be increased by combining multiple printheads to increase the physical resolution.
Since an axis of resolution is controlled by the speed of the medium moving under it, changing the speed will also change the maximum potential point that can be transmitted on this axis. For example, for an inkjet printer with a physical head resolution of 600×600 dpi, the manufacturer can print at 600×1200 dpi by reducing the moving speed of the web.
Physical resolution defines the arrangement of points without overlap, but it does consider the influence of the obtained point size or the use of multiple point sizes. Inkjet printers can produce a variety of droplet sizes, or direct multiple droplets to the same position to achieve similar effects. The resulting variable-size droplets composed of smaller droplets improve the tone range and halftone simulation and improve the clarity of text and vector graphics. Changing the size of water droplets will deceive people to see a smoother image.
The printhead is the heart of any inkjet printer, and its design and construction determine many key parameters, including physical resolution, print speed, and ink compatibility. The printhead’s ability to handle a given amount of ink drops, accuracy of ink drop placement, control of satellite ink drops, and the design of the printhead module itself are also important to printing quality.
If the amount of ink droplets is not maintained accurately, the ink layer thickness will be uneven and streaks will appear in the printing direction. The precise positioning of the droplets on the substrate is also important. If the ink droplets don’t go where they should, print quality will suffer. This can result in uneven text or line edges and inconsistent colors.
Ink and Substrate
Ink properties play an important role in image formation. There are four main types of inkjet inks: water-based, UV, oil-based, and hot-melt, the latter of which is rarely used for production printing. You can drill down into the details of ink colorants and carriers to understand the impact on media compatibility.
The viscosity and surface tension of the ink have a significant impact on the resolution, uniformity, and quality of printed dots and the amount of gain those dots gain on a specific substrate. Uniformity and dot gain particularly affect image sharpness and detail. Ink colorants and absorption levels affect color gamut.
Because inkjet printing performance varies with substrate, especially water-based inks, it’s impossible to evaluate the print quality of an inkjet printer without printing on some objects. The amount of ink used must be adjusted for each medium to avoid printing problems such as uneven solid areas, color bleeding, coalescence, paper curling and warping, and ink drying/setting issues. This makes print quality susceptible to user error.
Some ink and substrate combinations can be pre-treated to increase ink adhesion on non-porous surfaces or to control ink penetration on porous surfaces. This requires further adjustment or analysis to account for additional fluid applications. The variable performance of ink and media combinations further complicates the overall assessment of quality.
In addition to the type of material being printed, the fact that the material moves must also be considered. Controlling the movement of the media as it moves beneath the printhead is just as important as the operation of the printhead itself. Registration accuracy, repeatability, and distance from the print head to the surface being printed can greatly affect drop size and position.
Drying and Curing
For water-based inks, the drying module must quickly remove the moisture from the ink, leaving only the colorant behind, without significantly affecting the moisture content of the paper. This is not a simple task, so it is necessary to pay attention to the processing capacity of the dryer. There are usually a variety of techniques used to dry, cool, and smooth the substrate. Printing quality is determined not just by the shape of the dots on the page, but also by the consistency of the substrate after wetting, handling, and drying.
UV inkjet is much less affected by media changes than water-based inks. UV curing printers use LEDs to fix each color to improve print quality. These “trap” the ink droplets and prevent them from spreading after contact with the substrate, while the final fixation of the print is done by traditional UV lamps.
RIP or Digital Front End (DFE)
Another very important aspect is the Raster Image Processor (RIP). RIP accurately rasterizes and color separates files before sending them to the printer. The print control module controls when and where the printhead should apply ink drops and how much volume each drop should have for optimal quality.
To achieve image depth and color equal to or greater than offset printing, inkjet machines require a complex balance between printhead resolution, color depth, and drop size. All of this requires a lot of RIP processing power.
As you can see, print quality is complex and difficult to quantify. Even broken down into its basic components, there are many nuances to the components here. It’s necessary to pay attention to detail and learn as much as possible about all the factors that may affect it so that you can ask the right questions and test every aspect of your system and media options that are most likely to be challenging.
Additionally, of course, you’ll want to see the printer in action to see how consistent the print quality is and whether the printer can perform at the advertised speeds and resolutions for different types of media and jobs. Finally, it is important to evaluate print quality as objectively as possible by using measurable criteria such as optical density, colorimetric, color gamut, line show, and text sharpness. Print quality isn’t straightforward, but it shouldn’t be viewed as subjective either.