Sponsored by Kodak
It’s no surprise that most major production inkjet announcements in the past year have focused on commercial printing and packaging. Since every new inkjet launch touts its technology as a pioneer in delivering “commercial quality” production, it may surprise you to know that continuous inkjet printheads have been producing commercial quality output in predominantly offset segments for years.
Continuous Inkjet (CIJ) is the most widely installed inkjet technology in a commercial print environment with mono or full color arrays mounted on offset, flexographic, and gravure presses. CIJ printheads have demonstrated the ability to keep pace with web and sheet fed offset presses and to deliver output that is virtually indistinguishable from lithography.
The Lettershop Group (TLG) in Leeds, England has been working with 4-color CIJ printheads mounted on offset lithography presses since 2012. TLG’s Managing Director, Simon Cooper says, “Switching our customers from offset to digital, it has to be quality, it has to be the same as litho.” Cooper says TLG can print 90% static litho with 10% personalized color inkjet, post-coat with UV varnish and customers can’t tell the difference. But customers wanted more than 10% flexibility, so TLG evaluated CMYK inkjet presses with high expectations and the requirement to print on offset uncoated, silk and gloss stocks.
Cooper challenged, “With my normal paper stocks, if I precoat and use my normal workflow, have I got something I can sell? Have I got something where, to a customer, it just looks like litho and there’s no expectation to manage?” Again, continuous inkjet was the solution with the purchase of the Kodak Prosper 6000c in 2017 and added pre-coating and post-coating stations. At that time, “Kodak was the only company that could give me the journey of getting off inkjet formulated papers,” said Cooper.
The Challenge is More than Quality
Quality is not the only hurdle that inkjet presses face when complementing or competing with offset presses in high coverage, high quality, markets. There are several key drivers of success and all are important.
- Print quality
- Speed and productivity
- Media compatibility
- Running cost
Often there is confusion among buyers about how different printhead technologies address these challenges, and why it matters.
Printhead Technologies Tackling Top Quality Today
The leading printhead technologies: continuous inkjet (CIJ), piezoelectric and thermal address inkjet quality, efficiency and reliability demands in a different way. First a quick synopsis of how each technology works.
Continuous Inkjet (CIJ) print heads continuously circulate ink through the print head. As ink exits the nozzle, the surface of the ink is heated breaking the stream into droplets which are directed for print or deflected and recirculated.
Piezoelectric print heads are a Drop-On-Demand (DOD) technology where nozzles are fired to create drops of ink only when needed. An electric pulse delivered to a piezo crystal expands and contracts the nozzle ink chamber creating enough pressure to propel a droplet of ink.
Thermal print heads are also drop-on-demand technology. Thermal systems use a heating element in the nozzle (rather than a piezo crystal) to create a pressure pulse, propelling a droplet of ink.
Differences in Printhead Technologies
There are key differences between CIJ and DOD systems related to head positioning, drop formation, drop sizes, and drop placement that impact quality.
CIJ heads are positioned further from the media and have a drop velocity more than double that of DOD heads (over 20 versus 6-10 meters per second). Higher velocity means that CIJ systems have the drop momentum to deliver round, uniform drops to the media, place them more accurately, and the increased distance between media and heads provides protection from splices, heavy media, media shift and contamination from dust.
Accurate drop formation is also critical to quality. Drops should be perfectly round and consistent in size (based on jetted drop volume) with smooth edges and without extra dots, satellites or artifacts. DOD systems, operating closer to the media than CIJ (around 1mm for DOD versus 8 mm for CIJ systems), must adjust for air turbulence. This, along with lower drop velocity, can result in satellites and non-round drops. If drops go off course due to slow speeds, it can show up as banding in production. CIJ systems can place perfectly round drops that are uniform in size very accurately at high speed.
Examples of Drop Formation
“Our drops, because of the way they are generated, have no satellites and the drops become very circular and well formed,” says Randy Vandagriff Senior Vice President, Digital Print with Eastman Kodak. “Others typically have ligaments and tails or satellites which impact drop shape and uniformity of drop volume.”
Small drop volumes are necessary for fine detail in images. According to Vandagriff, “To get to very high or photo image quality, you need drops of 4-6 picoliters or smaller.” Some printheads deliver drop sizes of less than 2 picoliters in volume. A combination of small and larger drop sizes is useful for balancing fine detail with smooth solids and high coverage. The number of drop sizes created and the total drops per inch are both important to print quality, but sometimes buyers confuse dpi resolution with quality.
Many piezo systems have the ability to jet multiple drop volumes to vary the “drops per dot.” The placement of drops with different volumes produces a range of different dot sizes on the media. This ability, often referred to as grayscale, improves edge clarity, tonal range and simulation of continuous tone. The number of gray levels a print head can produce refers to the number of dot sizes that can be jetted plus 1 for no drop at all. So, a head producing 3 dot sizes is said to offer 4 gray levels.
Printheads may also effectively create multiple drops per dot by combining drop volumes with waveform timings to rapid fire multiple drops to the same position. For example, a piezo head that only creates 2 drop volumes, could produce 5 dot sizes (6 gray levels) if it can accurately place each of the single drop sizes, plus the various combinations of those single drop sizes.
Thermal heads are binary and only produce one drop size. HP has compensated for this in their HDNA nozzle architecture by stitching together two sets of nozzles; one producing small drops and one with larger drops. Similar to the piezo example, this will combine drop sizes to enable 3 dot sizes (4 gray levels.) Thermal technology does not have the refill speed to allow two drops from the same nozzle to fire to the same position, so there is not the ability to build 5 dot sizes from 2 drop volumes as seen with piezo heads.
CIJ heads can produce more than one drop volume, but in practical terms, only one is used for printing (the other is recirculated.) However, their increased jetting speed and drop velocity enable some CIJ systems to accurately place multiple drops in the same position, creating multiple dot sizes from a single drop volume. Kodak Stream and ULTRASTREAM CIJ heads can effectively produce 3 dot sizes, plus a null drop (4 gray levels.)
Speed and Productivity
The method of creating and placing drops has an impact on speed. CIJ heads produce drops 5 to 10 times faster than other technologies. DOD technologies compensate for the lower drop generation by increasing the number of modules in the printing system. This can cause confusion about resolution because adding DOD heads can serve speed, or resolution but not both at the same time. This is why it is common to see systems that print at their top speed at half resolution or their top resolution at half the speed. According to Vandagriff, “They use redundancy to try to go faster. We can meet speed requirements without redundancy because we are generating over 400,000 drops per second per nozzle.”
There is also the challenge of downtime for maintenance. DOD systems require frequent cleaning and wiping to avoid nozzle blockage and “jet outs.” CIJ heads have been battle tested as part of hybrid systems. “When talking about commercial presses running huge volumes, the last thing they want to do is stop the press every hour to clean that inkjet head” said Vandagriff. “So, we purpose-built the inkjet to be in that commercial space and be able to run at the productivity levels that they are used to.” Kodak CIJ heads require maintenance only once every 8 – 12 hour shift even in round-the clock environments running 7 days per week.
Continuous inkjet has been driving high-volume applications in commercial environments for over a decade. What is new is the range of applications and media that can be supported. The next post in this series will discuss media compatibility and running costs of CIJ relative to other technologies as well as new advances in CIJ quality that are opening new markets.