How to choose your digital printhead?
Nozzles, picoliter, thermal printhead, piezo-electric printhead, drop-on-demand(DoD), continuous inkjet, binary, greyscale are specification describing the digital printhead. Understanding the meaning of these terms will help you understand how the printer operates and will help you to select the right digital printhead for the right job. However, it is rare that for the supplier to stop and explain them. So here is where we stop and explain it to you.
Continuous vs. Drop-on-Demand(DoD)
Each printhead has hundred and sometimes thousands of nozzles that are projecting inks onto the media or substrate. Each nozzle is typically around 20-50microns wide. A human hair is roughly 80 microns wide. In a continuous inkjet, the nozzle projects a continuous stream of ink droplets when the printer is running. Ink would be deflected from the substrate through electrostatic plates or air bubbles in areas where ink is not needed. Unwanted ink is collected in a gutter to be returned to the storage tank. This type of inkjet is suitable for printing barcodes and markings that do not have sophisticated graphics. Drop-on-Demand inkjets are the most common type of inkjet on the market. This type of inkjet projects inks onto the substrate only when needed. In other words, dropping ink on demand.
Binary vs. Greyscale
When the nozzles of the printhead projects ink droplets onto the substrate. Binary refers to generating ink droplets of all the same size. This type of process is not good for printing graphic highlights. The amount of ink droplets is measured by picolitres. Binary projects around 30~200 picolitres per droplet. Greyscale can generate droplet of different sizes. Through the usage of the different size droplets, high definition graphics could be produced. A 360dpi greyscale print can generate the same effect as 1000dpi on the binary.
Thermal printhead vs. Piezo-electric printhead
Drop-on-Demand inkjet (DoD), at a high level has two different types of printheads, thermal and piezo-electric. Thermal printhead projects the ink through thermal properties. An element is heated inside the ink chamber, to the point that the liquid ink vaporizes and forms a bubble. The bubble then pushes the ink droplet out of the nozzle. The heating element is turned on and off to control the release of the ink droplets. Thermal printhead is usually binary, meaning each ink droplet is of the same size. Due to the constant heating of the element, the life span of the thermal printhead is shorter. Piezo-electric printhead projects the ink through mechanical properties. An electric current is run through a type of crystal, causing the crystal to bend forcing the ink droplet out of the nozzles. When compared to the thermal printhead, the life span of the piezo-electric is much longer. However, it is also costlier and needs more efforts in its maintenance. Through the mechanical control properties, piezo-electric has more control over its droplet size. It is able to produce a greyscale print. In terms of the printing speed, the thermal printhead is usually faster than piezo-electric as it is able to fit more nozzles onto the printhead. More nozzles usually mean a faster release of the ink, therefore, faster print speed. Thermal printhead usually has around 2,560~5,120 nozzles per color. In some cases, the company Memjet has developed printhead that contains 70,400 nozzles. Piezo-electric usually has around 720 nozzles per color. Thermal printhead usually works only with water-based ink and piezo-electric can work with water, solvent and UV based ink.
When choosing between a thermal or a piezo-electric printhead we should first go back to the type of graphics that is intended and to look into the desired printing speed. If your graphics requires more details and includes highlights, then piezo-electric may be a more suitable choice. If your graphics are less detailed and requires a faster printing speed, then thermal printhead may be the better choice.
Article by Daywey Chen, KYMC
