By Olivia Cahoon
Direct printing to pre-manufactured objects like plastic, metal, fabric, or glass require industrial strength printheads. Advancements in printhead technology are designed to enhance graphic quality while also maintaining close proximity to objects during printing.
Direct to object printing is often completed with UV ink sets for its strong adhesion characteristics. To ensure the printhead and ink set is a match, dropwatcher tools are available to provide an organized development process, which involves analyzing ink and printhead combinations.
Above: A tray printed to directly with Xaar 1003 printheads.
Printheads for Direct Printing
Printhead manufacturers are improving their devices to output directly onto more than traditional media. Improvements are designed for close proximity, vertical printing, and the ability to handle more surfaces.
Printhead technology for direct to object printing has existed for some time. Those who led the way in this technology are now joined by an increasing number of OEMs looking to capitalize on it, comments Gary Jenno, senior product manager, Xaar plc. “This rise is in part driven by the OEM’s own customer who is looking to digital technology to help them engage more with the end consumers through personalized messaging onto a range of finished goods from saw blades to footballs and bottle caps to cups, glass, and plastic bottles.”
Traditional printheads are accustomed to printing onto flat surfaces like paper and rigid media. In direct to object printing, it’s essential that the printhead conforms to the object’s shape for accuracy and enhanced print quality. To do so, today’s printheads are now designed with smaller widths and adjacent nozzle rows in close proximity, says John Harman, director sales and strategy, Ricoh Printing Systems America. “In addition, by optimizing both drop size and shape with waveform technology allows wider print gap printing, which also enhances print quality.”
Some of the newer direct to object applications use a robotic arm that moves printheads over pre-manufactured objects for surface printing, shares Jenno. Robotic arms are often integrated into applications for the automotive and aerospace industries where objects are larger and more difficult to move.
Printhead improvements are integral to the success of digital printing’s integration into manufacturing environments. With digital technology, today’s manufacturers are printing on objects like water bottles, USB drives, ceramic tiles, and lunch boxes. “As the direct to object market continues to expand, we see a market need to print on more diverse contoured surfaces, which require greater flexibility capability in digital printing,” comments Harman.
“While there have been machines for printing onto curved surfaces for a few years now, we are starting to see printing onto complex curved surfaces, for example tapered or undulating surfaces,” agrees Jenno.
Successful integration requires printheads that work well with a number of challenges. This includes the ability to print reliably in a vertical printing position rather than jetting downwards, coping with variations in throw distances, and operating well in challenging conditions like an accelerating print station, explains Jenno.
Vertical printing positions are helpful in applications like glass bottles. This requires high flow rates of fluid beyond the nozzle’s back, which prevents sedimentation in inks with high particle loading. It also helps compensate for the pressure experienced in the printhead’s length due to gravity. “The ability of the printhead to print vertically also means that the machines can occupy a smaller footprint than if a cylindrical object has to be laid down before it can be printed on, vertically downwards, and then stood up again,” offers Jenno.
UV Ink Offers Adhesion
Several ink types are available for direct to object printing but the most commonly used is UV. UV ink is broken down into flexible or rigid configurations and can be segmented for adhering to multiple substrates or specially formulated for specific materials like glass.
UV ink offers several performance benefits for direct to object printing, including the ability to provide strong adhesion to a range of substrate types. According to Harman, it’s also easy to dry and offers high surface and scratch resistance. “UV-curable ink is the predominant fluid used in direct to shape printing.”
“UV inks are the most commonly jetted in these types of applications because the ink can be pinned with a UV lamp immediately after jetting. This also ensures a scratch-resistant print,” agrees Jenno.
What to Consider
Before a manufacturer integrates printheads into a full manufacturing process for direct to object printing, it should consider the objects printed, whether it requires adhesion promoters, and ink drop sizes.
The distance between the printhead and objects might vary, especially in manufacturing environments that print on different products. To compensate for this range, it’s important that the printhead works well with variations in the throw distance. “The larger drop sizes cope better with increased throw distances because fundamentally the drop has more mass so it has more momentum over the increased distance to offset the effect of gravity,” explains Jenno.
Depending on the object’s surface, an adhesion promoter may be necessary if it features low adhesion characteristics. Objects like acrylic, glass, and metal typically require an adhesion promoter that is either in liquid form or jettable.
In addition to adhesion promoters or pretreatments, Harman believes fluid handling, speed and algorithm for printing patterns on the specific contour shape, and optimum printhead design needs to be selected and integrated to deliver the end user’s performance requirements.
Cylindrical objects like mugs, cups, and wine bottles are increasingly popular for direct to object printing technology. However, this shape can pose challenges, so it’s important that manufacturers consider the printheads’ proximity. Depending on the cylinder’s size, Harman says either a small or large width printhead is preferred. “In both cases the close proximity of the parallel nozzle rows is required.”
To avoid quality challenges printheads should be industrial and robust as well as reliable and proven in the specific application. “Whatever technology is chosen, the manufacturer must not face anymore rejects than allowed by the current process,” advises Jenno. More than likely, the manufacturer will need to upskill and consider product decoration as an entirely new process for the production line. “Some knowledge of how to handle the printers as well as maintenance routines required for the printheads is necessary.”
Dropwatcher tools evaluate and optimize printhead and ink combinations to provide precise, repeatable measurements of critical jetting. Also referred to as drop-in-flight analysis solutions, dropwatchers are essential for printhead waveform characterization and development.
Dropwatcher tools are useful in manufacturing environments utilizing digital technologies. While direct to object inkjet printing offers exciting opportunities, it also presents challenges that include an ever-growing catalog of fluids, a wide range of substrates, and a myriad of industrial inkjet printheads, says Matthew Pullen, product manager, DropWatching systems, Meteor Inkjet Ltd. This requires tools for printhead waveform development and characterization to ensure compatibility between printhead and ink combinations.
“Digital printing technology has hundreds of variables that will impact the quality of the end result,” agrees Yair Kipman, president, ImageXpert. “Without an organized development process and tools that provide feedback, it is easy to become overwhelmed by balancing all of these parameters at once.
Evaluating Printheads & Ink
The inkjet development process typically consists of three major steps—ink development, jetting relations, and print analysis. Dropwatcher systems are designed to address each of these steps while allowing users to easily move between them.
The first step in inkjet development is creating an ink that jets adequately out of the printhead. Using ImageXpert’s JetXpert dropwatcher system, Kipman says a developer will visualize and measure the drops leaving the printhead. If the results are not desirable, users can systematically make changes and study how each impacts the results. “As the user is adjusting the formulation, ink temperature waveform, and other jetting variables, they are able to get instant feedback to help understand how to optimize them.”
Once stable jetting is achieved, the next step is to observe how the jetting relates to the actual printing. According to Kipman, this is an important step because it establishes targets for drop volume, velocity, and consistency. “As a new developer, it is not clear what is needed to obtain a quality print until you try printing,” he explains. “Using our system, developers can seamlessly transition between jetting analysis and sample printing, so they can better understand the impact of one or the other.”
The final step is to perform a print analysis. Just like it is hard to make improvements in jetting without seeing and measuring the drops, Kipman believes it’s hard to make improvements to print quality without a thorough analysis. “Our system provides a consistent methodology for analysis, allowing the user to study the impact of the substrate, drying speed, treatments, and more on the print.”
In other systems like Meteor Inkjet’s turnkey solution, Pullen says sophisticated printheads drive electronics and software is synchronized precisely with a GigE camera and high-power LED strobe that characterizes, adjusts, and optimizes inkjet formation, velocity, volume, nozzle crosstalk, and duty cycle for limitless printhead and ink combinations.
Direct Printing Optimized
Advancements in printhead technology allow direct printing to a variety of objects from glass and ceramics to plastic and fabric. To ensure the selected ink and printhead combination is a match, manufacturers can invest in dropwatcher tools, which perform a complete analysis of the inkjet technology working together.
Mar2019, Industrial Print Magazine