Australian print’s top scientist Rod Urquhart says it is important to understand the type of digital printing you are running before you specify the type of paper, as results can vary dramatically if you are not matching them
In almost every printing job, the major component whether measured by cost or by amount is the substrate. As today’s discussion is principally about digital printing, the most likely substrate will be paper or a paper derivative, and thus paper becomes the focus of this article.
Clearly there are many different kinds of papers available for printing, and many of these are tailored for specific purposes or for specific processes, yet there exists a philosophy in many parts of the industry that one size fits all.
The purpose of this discussion is to explain why, for any job, printers should consider carefully the process by which it will be produced (and of course the end use of the product), when considering which grade of substrate to use. Understandably in many cases designers and print commissioners will specify a substrate. However, such people are not always in a position to appreciate how their choice of paper can bring major headaches to a printer in the production of the job.
The term digital printing sounds to the untrained ear as though it was a single printing process as for example, flexography is a printing process, when of course nothing could be further from the truth.
By any measure, the processes known as digital are the fastest growing areas of the printing industry worldwide. However, the term digital applies to the input of data and in part to the control of the printing equipment, and there the description should end. At the risk of labouring the point, the following is an outline of the technical differences in the processes that are described under the digital banner.
A major group of digital presses is the electro-photographic machines; laser printers for short. Laser printers have a common ancestry with photocopiers in that their principal operating component is a central cylinder or drum with a surface conductive to electricity when exposed to light (photoconductive).
In laser presses, the photoconductive cylinder is electro-statically charged to a known potential and then an image is drawn by a low power laser that scans the cylinder surface, causing the charge to decay wherever the light touches. A powdered ‘ink’ (generally referred to as toner), charged to an equivalent level to the surface of the photoconductive cylinder, is then applied. As like charges repel, the toner is able only to adhere where the charge is different; in other words, where the image has been produced by light action. The toner powder is transferred from the cylinder to the intended substrate either by pressure or an opposite electric charge, and finally the loose powder is melted (fused) onto the substrate by a heated fuser roll.
Toner broadly consists of a pigment dispersed into a hydrocarbon resin and then the lot is pulverised to particle size of a few microns across. In order to fuse the toner to the substrate, the resin must reach its melting point, however, with high-speed laser presses, the dwell time under the fuser roll is very short indeed, and so the roll must be heated to very high temperatures in order to provide sufficient heat to melt/fuse the powder in the time available. Actual fuser temperatures can reach 2000C and beyond.
The effect on a sheet of paper of exposing it to a region at around two hundred degrees, even for a very short time, can well be imagined. Printing papers all contain a moisture level, without which the sheet would be brittle, and if the moisture level is severely diminished, the paper will react by shrinking, going wavy, or resembling a piece of corrugated roofing. For this reason, paper manufacturers carefully control (minimise) the moisture level of papers to be used on laser printers, to prevent delivery problems, and to ensure sheet dimensional integrity.
But not all is that easy. Just when electrophotographic based printing seems clear, we come to a different class of machine altogether, and I refer to HP’s Indigo.
The Indigo machine produces an image via the same process as a laser printer, but the similarities end right there. Ink for the Indigo process is a thermoplastic fluid, comprising pigments, resin, plasticiser, and a liquid carrier. The ink is transferred from the imaging cylinder to a heated blanket, and then is offset - just as in a litho press - onto the substrate, where a combination of setting and freezing forms the print. Clearly, the paper requirements for this process are significantly different to those of the laser printers with high temperature fusers.
To further complicate the issue, when the original R&D work on Indigo was done in Israel in the 80’s, a particular grade of paper was used to give the print results sought, however, when HP introduced the machine to the rest of the world, it was found that ink adhesion to most other grades of paper was not optimal. A short-term answer was the use of a prime-coat under the area to be printed to gain maximum adhesion, and indeed this method is still in use, but now paper grades have been developed that no longer require the additional step.
Apart from digital data input, ink jet printing and the requirements surrounding the process are totally different from electro-photographic printing. For a start, ink jet is the only printing process where the print-head does not contact the substrate.
We are all familiar with the ink jet printers attached to our personal computers, and they appear to all be much the same, but even those differ in the delivery of ink, with some using vapour pressure, and others using tiny pumps to jet the ink.
But to go back a little further, ink jet processes are divided into two broad classes; CIJ (continuous ink jet), and DOD (drop-on-demand). The former is like a tiny spray gun that squirts ink in a stream interrupted by deflectors to produce an image. These units are at the lower end of resolution, and find principal use for date marking and barcoding of packaging.
Commercial DOD machines use a series of tiny pumps each in the form of a piezo crystal to force low viscosity ink through nozzles. A piezo crystal is one generating an electric signal when squeezed or deformed, and this property is used in reverse in ink jet pumps, as when an electric field is applied to the crystal it deforms, thus developing a piston-like action to expel the ink. A typical crystal resonates at around 64 KHz, and can pump out drops of around 1.5 nanolitres each.
Although we may logically think that each ink jetted drop is separate, even the best DOD ink jet nozzle squirts out a rain of drops for each impulse, some landing in a puddle on the intended target, while others splash as satellite drops around the intended landing site. Inks can be dyed or pigmented, water based, or solvent based, or be based on ultra violet (UV) curable resins – thus generating a wide range of film properties.
The relationship between inkjet inks and the paper on which they are printed is critical, as low viscosity materials, especially those based on water, can spread significantly along the fibres of uncoated papers, an effect known as wicking, and paper manufacturers have spent a lot of R&D developing additives to reduce the wicking tendency on uncoated stocks. Coated papers are not that simple either, as though the wicking tendency is less, a heavy application of water based ink (and aqueous ink jet inks are around 90 per cent water), can result in dimensional instability of the paper, and it is well known how readily wet paper swells!
UV curable ink jet inks avoid the wicking and wrinkling effects of aqueous inks, but create a new situation where inter-layer adhesion and crease cracking can occur due to the highly cross-linked nature of the ink vehicle.
For a commercial view of the current state of the art with regard to paper I am indebted to a number of leading figures in the industry.
For both conventional lithography and digital printing, the principal trend these days is not for fancy finishes or unusual surface effects. Customers now demand papers containing high levels of recycled fibre from companies with environmental policies which include carbon neutrality and a full life cycle analysis. Brian Longmore of Spicers remarked that the terms environmental and sustainable feature very highly in all areas of the paper industry, for litho, laser, ink-jet, labels, and so on.
When discussing this trend for the use of recycled materials, leading graphics designer Lisa Miniciello of Room 44, commented that recycled white stocks are so good these days it is no longer necessary to carefully pick and choose between various levels of recycled materials to obtain a first class result when environmental specifications are included in a job.
This point was supported by Stephen Hawkins from Australian Paper who confirmed the only technical limitations are mainly with colour and dirt, and paper manufacturers are continually working to improve the separation of dirt and other coloured materials in the recycling process.
Catherine Doggett of KW Doggett Fine Paper further reinforced the trend, remarking that a number of top quality stocks are now available for all processes with up to 100 per cent recycled fibre, which are increasingly meeting client expectations.
Malcolm Cumming of Longbottom Digital Paper added to this message by writing; “Paper manufactured with a low moisture balance minimises, curl, mottling and misfeeds, increasing productivity and helping with cost containment” [for laser printers], continuing “Paper mills are re-developing their paper ranges to adapt to the HP Indigo needs. Simply saying offset paper will run almost never gives a satisfactory outcome. With tightly controlled production and consumable costs, only digital paper can be relied upon for performance, colour accuracy and a quality touch and feel all of which are important to the consumers”.
Thus the recommendations from this discussion are to appreciate the term digital printing to apply across a diverse range of applications, each with its own specific and essential needs; to understand the operation of digital machines, and how best to match the grades of substrate to the requirements of each process.
As all of the industry experts agreed, for best results, the type of stock must be matched to the process, and trial and error leads mainly to error!
