In this series, I have tried to give you a ‘lay of the land’ as it relates to production inkjet technology and products available today… before drupa. While I have covered a wide range of products, in all fairness I haven’t covered everything that is available out there. Why? There are just too many production inkjet products to investigate, review, and detail, and you can expect more to be introduced before, at, and after drupa—including a highly secretive Digital Nanographic Printing Press that will be announced by Indigo founder Benny Landa on May 2nd, the day before drupa officially begins. However, in all honesty, the products covered offered a fairly complete look at the state of the inkjet technology and how it is being utilized in the production inkjet presses available ‘today’.
So while I have given you lots of facts and figures on feeds and speeds, I thought it was now time to look at how you can assimilate all of that information to figure out how to use it abd to understand what it means to you. This ‘look behind the curtains’ is desgined to help you make purchasing decisions, either before or after drupa.
I am using a similar structure as I used in the product reviews in order to put it all in context. In Part 1 of 2 in this article, I will review inkjet printhead technology, imaging and what you should be looking at to satisfy your company and customer requirements.
A primer on inkjet imaging technology
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 looked at the various implementations of inkjet printheads, and discussed 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.
In the next article, I will continue this pre-drupa educational series by looking at the press transports and digital front ends (DFE). This will further integrate all of the information we have presented to date, and provide guidance about how you can use it to help educate yourself on what production inkjet technology is here today, how people are using it, how you can utilize it in your business, as well as what you can expect to see at drupa.