Everything You Wanted To Know About Production Inkjet
Veteran industry specialist and digital printing guru David Zwang takes an incisive look at the state of production inkjet technology and equipment
Production inkjet machines are becoming more and more popular. Their speed, quality and productivity have improved considerably and many commercial printing companies as well as corporates are willing to acquire it. But the sheer variety and applications of production inkjet is mind boggling. In this article David Zwang offers you a clear picture of the technology to help you decide which type of production inkjet suits you most.
It’s interesting how each of the manufacturers has created messaging around imaging. Some focus on resolution, some on halftone screening equivalents, some on raw jetting speed, etc. But what does all that mean to you, or more importantly, to your ability to meet your customer’s expectations?
The best place to start is to understand what the point of reference is. Without a doubt, as was the case with the introduction of electrophotographic (EP) technology, it’s all about looking like offset print. Even with all of the EP in the field today, offset is still the benchmark by which print is judged. Now, that having been said, there are a variety of offset print reference expectations. We have commercial sheetfed, web offset in a variety of types (coldest, heatset), and of course there is also Flexo, etc., each with its own set of output expectations. If you add media type (coated, gloss coated, uncoated, etc.) into the mix, the imaging expectation options increase exponentially. So how different is production inkjet, and what are the controlling factors?
First and foremost, the primary factor is application. What is the print output to be used for? For example, the quality expectations for a transpromo piece are going to be different than those for a direct marketing piece, or of a trade book versus an art book, etc. Another determining factor is speed or run length. While meeting some of these expectations is dependent on the press transport (sheet, web), which I will cover in Part 2, many of them are limitations of the printhead and imaging technologies.
In this series, we will look at the various implementations of inkjet printheads, and discuss things like thermal versus piezo electric, resolution, dot size, variable sized dots, ink characteristics etc. So now I will briefly describe how each of these things affect the imaging.
The main component is, of course, printheads, and whether they are thermal or piezoelectric they really perform the same task. They control the ink expulsion through the many nozzles in a head. In the case of thermal, a tiny heater vaporizes a thin film of ink. A vapor bubble fills the chamber like a piston to force ink through a nozzle. Air bubbles are also forced out on every drop ejection cycle. In the case of piezoelectric, there is a piezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When voltage is applied, the piezoelectric crystal vibrates, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. Both types of printheads create the same effect, at a very basic level. However, there are other competing cost and flexibility factors associated with each of these technologies and their implementations that can have an effect on imaging quality.
You may have noticed that some of the presses I reviewed had different resolutions across the width of the press than they did in the print direction (eg. 600 x 900 or 600 x 1200). That is because the physical resolution of the heads (across the press width) are determined by the physical nozzles in the heads, while the resolution in the print direction can be ‘enhanced’ by slowing down the speed and effectively increasing the number of dots printed beyond the number of nozzles the head has in that direction. So your ‘effective’ resolution can be 1.5 to 2 X the actual printhead resolution in that print direction. Of course your throughput is affected, but the resolution is increased in that direction. In many presses, this is an optional control that can be selected based on the speed or vector image quality requirements of the job. You should be aware of this as you look at meeting your requirements.
The resolution, or the number of drops that can be sprayed in a linear inch (dpi), has an effect, but not necessarily the way some think. Resolution affects vectors (line, shapes, text) differently than it does continuous tone (CT) images. In the case of vectors, which are resolution independent file object descriptions, they are imaged based on the device’s dpi. This means a higher resolution results in achieving smaller and smoother text, curves, etc.
However in the case of CT images, it’s a little more complex. The resolution of the file object (image) is always the first limiting factor. The resolution of a CT image is determined by the number of pixels in the image file in a linear inch (dpi), and every pixel has a color value. If there isn’t enough resolution in the file to handle both the image detail as well as the output imaging requirements, the image will print soft looking. However, the ways in which press systems actually process and print that pixel data can vary. At a basic level, the ratio between the input (file) resolution and the output resolution controls the amount of image pixel data that is used to calculate the value and potentially the size of the printed dot. This affects both color and detail clarity.
All of the printheads can control when a dot prints or not, but some have fixed drop sizes and some can vary the size of the dot. This level of control is where we really begin to see some of the finer quality imaging results. The effect of varying the dot size can be seen in cleaner lighter colors, smoother flesh tones, and gradients, just to mention a few benefits. And this capability in a lower resolution machine can actually improve the CT imaging substantially, beyond what a machine of a higher resolution but fixed dot size can print.
The final area I will cover related to imaging is the ink. In a production inkjet press, ink plays a very important role in imaging speed and quality. In the production inkjet presses that I reviewed, all of the inks are water based, as opposed to oil, solvent or latex based as you may see in other types of inkjet equipment. However, all water-based inks are not created equal. There is a balance between having the ink dry quickly on the paper to allow for a wider range of paper use and possibly increased imaging speed, but not so fast that it dries in the head which can then create print and maintenance issues. Some of the faster drying inks can also reduce the amount of heat required to dry the paper, which reduces energy costs and allows the paper to stay flatter for further finishing, etc.
Of course we can’t really talk about ink without speaking about the paper. Binding the ink to the paper is not only a key factor in ensuring that the print stays on the surface of the paper without wicking into the paper, and it also has a significant effect on the look of the print itself. In offset print, the ink stays high on the sheet surface on a coated sheet versus more absorption on an uncoated sheet, and you can imagine why it is important for the same to occur on an inkjet press.
When production inkjet first started showing up in the marketplace, there wasn’t much inkjet compatible paper. As a result, many of the production inkjet presses offer a pre-coating station to prepare the paper before print. However, the paper manufacturers have finally started to catch up, and there are now inkjet compatible papers readily available to fit most design requirements. As a result, many of the newer production inkjet presses are not including a pre-coating option. In its place, we are starting to see some post-coating stations on new machines. Post-coating performs more of a print surface protection role, and can also add some gloss to the print.
The printhead life and replacement cost also varies by vendor and should be factored into the equation as you are looking at any solution. Additionally, many of the printhead implementations have different ways to streamline head priming, cleaning, etc. Depending on how this is handled, it can result in saved time and reduced paper waste, so it is a factor worth looking at.
Production Inkjet Transports
As we discussed in earlier, the imaging is what allows you to meet your customer application requirements; however, the transport is what facilitates the actual printing in your production workflow. Just as there is a need for both web and sheetfed offset presses to address the various media, quantities, speed and finishing requirements, the same holds true with production inkjet presses. That being the case.
Web fed transports
Web-fed digital presses have been around since the 1970’s, so there has been a lot of time to develop and refine the transport designs. Of course web fed printing has been around for a long time, but inkjet introduces some additional challenges.
Because of the speed of the web traveling through the press, one of the challenges is making sure the ink drops in the right place on the sheet and doesn’t fly along with the draft of the sheet travel. Once you have achieved that, another challenge is keeping the web flat and dry to ensure good duplexing and ultimately finishing. Since the ink is mostly water based, it can play havoc with the paper, causing bleed through, puckers, stretching, etc. Balancing feed, speed, and drying has been a challenge that all of the vendors have used their creativity to address. Since most print jobs need to be printed duplex, developing a system that would do that in a small footprint was also an area in which vendor creativity had to be applied. There are single engine solutions that can be connected to perform duplexing, and there are twin engine solutions that are built as one unit. The configurations can include straight inline, L-shaped and H-shaped to mention just a few. Like the head technology, in many cases, a press vendor may OEM the transport, or even just the engine from other press vendors.
Of course, speed is usually a major factor in choosing a web press. Throughput speeds run from about 200 ft./min to over 1000 ft. min. and increasing. That doesn’t include the inkjet imprint head modules you can add to your offset web press that now top end at 2000 ft./min. The transports use the imaging engines (printhead systems), and the combination of both, really determine which solution will meet your individual quality and production needs. One additional consideration is the ability to field upgrade the press. Some of the vendors offer field upgrades for additional colors, MICR, speed and duplexing capabilities.
While it would be expected that all production inkjet machines support a wide variety of 3rd party paper handling and finishing options, it is always good to see which vendors and devices are supported.
This relatively new category of production inkjet is designed to fit the needs of the general commercial print service provider. The web solutions that have existed to date are very successful in environments where there is a lot of volume and minimal paper type requirements. However, the commercial printer uses more than 8 different substrates on a regular basis. Trying to manage this with short runs on a web press is not very practical. Sheetfed inkjet presses, especially with the introduction of B2 and now B1 sized presses fit the sweet spot for general commercial printers.
Feeding a lot of different papers through a press is a lot more difficult than most people realize; just ask an offset pressman. Whether you are using a lightweight uncoated stock or a coated board sheet, getting the paper to feed with precision at a high speed requires significant engineering. Offset presses usually will use a gripper to pull and a side guide to align the sheet into and through the printing system. Many electrophotographic (EP) systems use belts and/or vacuum to transport the sheet. They can get away with that since they don’t need to support the plethora of sheets and board that an offset press may encounter. If you add duplexing to the mix, you have just added many levels of complexity, including drying, flipping heavier stocks, front to back registration, etc.
Since sheetfed production inkjet is a relatively new category of press, we are beginning to see a lot of different variations of design. To date we have seen the offset gripper/side guide model, a vacuum transport, some new patent pending transports, and after drupa the designs will undoubtedly increase. The most important factor here is to look at the work you currently do and plan to do in the future, and see how each of these transports will be able to support your needs. For the most part, paper size and thickness specifications are readily available. However, operational performance in different environmental conditions can present differing results. So pay attention to their facility recommendations. This is especially true in presses that duplex. As we have discussed throughout the series, other issues you need to be aware of are the potential need for, and availability of, pre-coating, and drying that will support your desired paper requirements and ink coverage levels.
Digital Front Ends (DFEs)
This is probably one of the most misunderstood yet most critical parts to any press configuration. The best imaging and transport system can only deliver what gets processed in the DFE. If the DFE isn’t up to the challenge or configured improperly, you are out of luck.
At the base level, file type input is very important and usually dependent on your type of work. PDF/X-4 is the default requirement format, since support of other PDF/X variants would also be available. However, if you are producing variable data documents, you will need to support PDF/VT and AFP/IPDS. Color management support is usually a given, but being able to create color characterized paper libraries is almost a necessity, but not always there or easy to find.
Since most companies don’t have the luxury of starting their company with a clean slate, one of the issues many printers have trouble with is integrating this new press into their existing workflow processes. While many vendors promise ‘JDF/JMF inside’, there are varying levels of implementation. In todays demanding production environment, it is important to look at how your new press will integrate upstream with your MIS system and through dotted line to your existing production systems. Otherwise you are destined to create inefficient and expensive workflow silos. Of course, there are third-party products that can glue some of these together after the fact, but that may not be the best solution. Much better to address these integration issues before you make the purchase decision.
Finally, when working with variable data, both the DFE and your plant infrastructure needs to be designed to handle the volume of data that will flow through it. Many of the included DFE solutions are built with scalable architecture to allow you to grow the solution through the addition of additional CPU’s, RAM, and RIP instances to process the data at a minimum of press speed. This is especially important for the web-fed machines due to their high speeds. If your plant is not currently doing variable print, it is also important to look at your plant data infrastructure to ensure that it can support this new capability.
drupa brought many announcements in both the web fed and sheetfed production inkjet category. We saw announcements from offset press manufacturer KBA about its partnership with RR Donnelley and its entry into web fed production inkjet, and we already know we will see a sheetfed offering from MGI, and from Landa, in both the sheetfed and web category. And I am willing to bet there will be many others as well as we move forward.
This new series is dedicated to looking at production inkjet technology and presses, however we can’t forget that there are other viable, quality, productive print technologies out there. These include offset, EP, and the increasingly growing category of Liquid Toner Sheetfed, once the sole domain of HP Indigo and now the new LID (Liquid Ink) technologies being introduced by Ryobi/Miyakoshi, Canon Océ and Xeikon. However, probably the most exciting news in all of this for commercial printers looking for digital sheetfed technology is that most of the sheetfed solutions will be available in B2 and even B1 sizes offering the production flexibility necessary to support the new on demand and variable print service provider market requirements. And you don’t need to go it alone. There are many resources out there to help you prepare, make the right decision, and work with you to implement any solutions.