Dr. Nicholas Hellmuth & Pablo M. Lee
As print shop owners, print room managers, and printer operators become more savvy, they begin asking questions about which printheads are best for jetting UV-curable ink; then they ask about UV-curing lamps and naturally end-users wish to learn more about UV-cured ink, both the chemistry and the curing.
What counts as innovation in the printing industry are the advances of inks and printheads. The ink is the crucial ingredient and the printhead is the crucial hardware. The printer is primarily the structure; which delivers the ink to the print through the electronics of the print head. Therefore, the release of a new printer has more meaning with novelty in inks and printheads.
It’s the innovation in inks and print heads that leads to innovation in the printing business, we will discuss printheads without delving into patents or excruciatingly technical digressions. If you are an end-user, you mainly need to understand the printheads.
The UV Printhead is one the most important part of the inkjet printer, and it has the function of transporting the ink from the printer to the material through holes called nozzles that can be as small as 10 micrometers in diameter, equivalent to 1/10 of the diameter of a human hair. The number of nozzles can change depending on the manufacturer and model of the printhead.
Drop size: Effect in quality and speed
The unit used to measure an inkjet drop is the picoliter (pl) that it’s an equivalent of a trillionth of a liter.
The Ricoh MH5220 printheads can produce print drops as small as 2.5 PL and printheads like StarFire SG-1024/LA-2Ci can produce drops from 80 PL to 200 PL.
In general, the smaller the drops, the better the quality achieved in print. However, smaller drop sizes can reduce the speed of a printhead system. Likewise, printheads that produce bigger drop sizes do not offer the same print quality, but their print speed tends to be better.
A thermal printhead heats the ink in a fraction of a second. The heat created by the heater boils a thin layer of ink, which generates a bubble of vapor in the ink. This bubble produces an increase in the volume of the ink layer of approximately thousand times. This increase in the volume creates a pressure pulse of the fluid, causing the ink to be ejected.
Thermal printheads are great for water-based inks for wallpaper, giclee, fine art photos, CAD and GIS printers. But to print with solvent or UV-curing inks, you need a piezo printhead.
In most thermal printheads, the ink in the nozzles that is not expelled is suctioned into the heater with a considerable force, which tends to wear out the printhead through time.
This technology is based on a crystal film that has the property of deforming when an electric field passes through them. This type of printheads is designed so that the crystal materials deform one of the walls of the ink channel that end up in each nozzle.
When the voltage is applied to the film it expands. When the crystal material is adapted to the pump chamber connected to each nozzle, this expansion allows the entrance of ink to the pump chamber. When the reaction of expansion ends the ink is pressured and jetted out the nozzle.
MEMS (Micro-Electro-Mechanical-System) Printheads
MEMS technology can be compared to nanotechnology because the dimensions handled range from one micrometer (a millionth of a meter) to one millimeter. These printheads also use the same principal as thermal printheads: the ejection of drops through heat.
Due to the sizes that can be handled with this microscopic technology, one of the advantages of MEMS is the larger and dense groups of nozzles, which raise the resolution and print speed.
The first generation of MEMS printheads a decade ago was a disaster. The next generation that could handle only dye-based inks was not as popular as hoped. But new improved technology has resulted in MEMS printheads nowadays that have a good future before them, especially their advantage in one-pass printing.
Most of the advances in print heads are in smaller picoliter drop sizes. Ten years ago, 80 to 90 picoliters was what you expected from a billboard printer. Since then, industrial printers have reached less than 2.5 PL. Now, even smaller drop sizes are becoming the norm, although you don’t need such quality for a billboard or building wrap.
When you look at a billboard driving down the highway you can’t tell the difference between variations in dpi but when a sales manager goes into a print shop and inspects it, he looks at it just a couple of inches away, so the drop size is still very important.