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Under the Hood

Drop-on-Demand Printheads in Wide Format


By Melissa Donovan


Printheads found in wide format printers advance at a rapid rate, creating faster, higher quality machines. New entries to the market invite competition and raise questions in an otherwise quiet segment that is usually hidden under the hood of the printer.

 

Piezo vs. Thermal

There are two basic printhead technologies—piezo and thermal—used to create large format graphics. Piezo printheads jet a range of chemistries up to hundreds of billions of drops per nozzle over a long lifespan and work well with bulk ink supplies in lieu of cartridges.

Agfa Graphics engineers piezo printheads for UV ink. The fine droplet size that results allows for an accurate lay down of the drop, creating high-quality output.

 

Epson manufactures a unique piezo printhead branded as MicroPiezo, developed in house over the last 30 years. There are various types of MicroPiezo printheads, which are found in Epson printers as well as those from other manufacturers. They are designed to work with a range of ink sets. The MicroPiezo TFP was first used on the Epson Stylus Pro 11880. Today it can be found on just about all of the company’s aqueous wide format printers.

 

“Epson MicroPiezo printheads are well known for two reasons. They are accurate in providing the best print quality possible and are reliable in that they can last the life of the printer,” shares Reed Hecht, product manager, professional imaging, Epson.

 

Hewlett-Packard’s (HP’s) Scitex line of both flatbed and roll-to-roll printers utilize piezo printheads, which run any type of ink—from UV to solvent and aqueous.

 

Xaar plc. develops and manufactures piezo drop-on-demand printheads and did much of the original work on them in the early ’90s. Today’s products run solvent, oil, and UV-curable inks. “We describe our printheads as industrial strength as they are designed to provide robust operation and long life in industrial applications,” says Chris Lynn, VP, sales and marketing, Xaar Americas, Inc.

 

A downside to piezo, according to Eviatar Halevi, director of technology, HP Scitex Industrial Printing, is that it is not easy to pack nozzles at a high density, and thus the cost of the printheads can be high.

 

Replacing piezo printheads is straightforward, depending on the design. “The alignment of the printhead is critical, and manufacturers have techniques to replace printheads in the carriage, aligning them with an accuracy better than one thousandth of an inch,” adds Lynn.

 

HP Scitex X2 printheads are designed to be operator replaceable even in complex systems such as the HP Scitex FB7500 or FB7600 Industrial Printers, which employ more than 300 printheads.

Depending on the printer at hand, replacing a printhead can be a minimal charge or quite costly, ranging from $200 to $2,000. This is something a print provider should take into consideration while researching new equipment.

 

Thermal printheads are traditionally produced in large volumes and available at a low cost. HP Designjet devices utilize thermal inkjet printheads in an off-axis form, which means there is a separated printhead and ink supply. This translates to easy removability for the customer, as they do not need to remove the printhead when replacing the ink tank.

 

HP Designjet thermal inkjet printheads are aqueous and pigment based. “The thermal printheads provide a higher nozzle density for very high image quality at a lower cost compared to piezo technology. The inks provide high quality and detail, from skin tones to graphics and accurate and crisp lines,” explains Jep Tarradas, large format research and development director, HP.

 

At the Creation Level

Developing a printhead involves precise manufacturing. Different technologies require multiple workflows and time to market. The time to build a printhead is based on the skill level of the research and development (R&D) team involved.

 

For example, microelectromechanical systems are part of a sophisticated method that requires fine-featured engraving into silicon wafers. “To get the best accuracy and quality while maintaining a reasonable cost per unit, manufacturers pack many printheads on large wafers, up to eight inches or larger,” explains Halevi.

 

Xaar printheads use a shared wall, shear mode design that allows high nozzle densities—150 to 180 nozzles per inch—with low energy operation. Over ten percent of its sales are spent on R&D to develop next generation printheads.

 

Sometimes it takes years to perfect a new technology. Epson’s MicroPiezo TFP was in development for over 15 years before it was introduced in 2004.

 

The actual design process requires multiple steps. Tarradas outlines how the HP Designjet team fabricates a printhead. First a silicon fab is required. Then the die must be designed, which is a silicon process with inkjet layers to manage the ink flow. Lastly the pen body and ink delivery system must be designed to fit into the printer.

 

Developers don’t just think of the printhead as a basic component in the process. Larry D’Amico, VP digital imaging, Agfa, is quick to point out that “the printhead is more than simply a tool for laying ink on media.” The printhead, printer, and ink are part of a system that collaborate to produce optimum output. “Each part is created to perform with the other, specifically aimed at delivering the best print quality for the specific application requirement,” he continues.

 

Green Heads

Sustainability is important throughout the print process, from the ink used to the substrate printed on, proper outgassing, and printhead disposal. Additionally, what goes into building the device is an essential component for those striving to go completely “green.”

 

The ability of piezo printheads to last for years makes them a consideration for a greener product versus a printhead that lasts a few weeks or months. A printhead that requires a low voltage power supply is also considered more eco-friendly than a high voltage unit. Xaar’s printheads fit both of these descriptions.

 

Covered Under the Hood

Depending on the vendor, printheads are covered under warranty. They can be treated as consumables, a service maintenance call and billed based on time, or as something else entirely.

Epson designs its printheads to last the life of the printer, but warranties do cover a MicroPiezo printhead replacement if the need arises.

 

Often piezo printheads are touted to last for years, and they will, if user intervention is minimized and the correct inks used. Good maintenance and inks are the secret to a long life.

 

According to Lynn, the most frequent causes of failure are not related to printhead manufacture. They are caused by blockages from ink that cures or dries in the nozzle, scratched plates caused by head strikes on the substrate, or from particles on a printhead caused from wiping with a dirty rag.

 

Looking Past Science

Once hardware is placed in a print shop, it is easy to overlook the little nuances associated with its construction. Printheads are one of these items. Many resources are pooled into the R&D of printheads—both thermal and piezo. The right printhead provides efficiency, a wide color gamut, and high-quality output, something every print service provider requires. Respect for everything that goes into the creation of today’s digital large format printers starts with the printhead.

 

Mar2012, Digital Output

 
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