The Ingredients of Quality Printing
Below is an edited version of an article that appeared in the Australian print publication – Proprint (June 2007).
This the third in the three-part series of article by Yves Roussange, internationally recognised expert in colour management and Director of ColourProcess P/L.
Ink – a crucial ingredient
Ink is a crucial component in the printing process, but often receives little attention until problems arise. Ink is also the part of the process over which a printer probably has the most control. Once the paper stock is selected, there is little you can do to change its characteristics. While adjustments can be made to the settings of the press, there are limitations as to how much you can change the physical properties of the machine. There is, however, a huge range of inks on the market, with a similarly huge variance in terms of their quality. The undeniable truth is that in the end, the way ink performs on the press will determine the quality of the printed outcome.
In this article we will look at how inks have evolved and the standards that exist to help us control the colour of inks on press.
The evolution of ink
The colour values specified in the original DIN 16539 colour standard, which achieved worldwide recognition as the “European Colour Scale”, published as ISO 2846:1975, were based on extensive research by European experts in the 1960’s. By the end of the 1980’s it had become clear however, that the colours of process inks had moved a little in relation to the values specified in the original ISO 2846 and DIN 16539 standards, predominately as a result of market demands.
The subsequent revision resulted in the publication of ISO 2846-1:1997, which was assembled using data from Europe, the USA and Japan. Experts from the responsible ISO committee, (ISO Technical Committee 130: Graphic Technology), came to the conclusion that a single set of colour values could represent the proposals from around the world within reasonable tolerances. As a result, the new ink manufacturing specification ISO 2846-1 was born.
According to the German Printing Ink Industry Association, 80% of the process inks supplied in 2004 complied with the new colour standard ISO 2846-1 within the given tolerances. Although the ISO standard brings process colours under one roof worldwide, differences in taste between different regions continue to exist. These are expressed by the preference in the Far East and the USA for colder colours and in Europe for warmer ones. Such differences, however, lie within the tolerances given in the standard. We can therefore work on the assumption that inks meeting the target values of ISO 2846-1 can be supplied on demand worldwide.
Typical ink problems
Even with the ISO 2846-1 standard available printers still encounter serious problems with producing accurate, consistent results – and it is often a function of the inks being used. A couple of examples if ink problems are:
- Dot-gain consistency: one of the most significant issues I have seen occurring on press is inconsistency of ink dot gain. Magenta and yellow are the most sensitive colours open to this issue.
- Customising inks: in some cases, printers customize the primary inks to achieve sign-off, for example by ‘warming up’ a yellow or changing the cast of the magenta or cyan. In mixing the pigment you change the viscosity and the characterisation of the primary colour, so the dot gain will change and shift out of control.
- Different manufacturers: some printers are mixing different types of primary inks from different manufacturers – leading to a very unstable printing run. Ink manufacturers create the ink set to optimise trapping and grey balance.
- Metamerism: some inks contribute more than others to a metamerism effect – the problem where two colours appear to match when viewed under one light source, yet do not match when viewed under a different light source. It’s for this reason that standardised lighting in pre-press and press operation areas is so crucial. Again, a uniform standard for print and proof viewing conditions is explained in the international standard ISO 3664:2000 using D50 lighting.
It’s very confus-ink
There seems to be a large amount of confusion in the market about ink standards and the processes to which they relate. As an example, many people talk about the SWOP and GRACol guidelines when discussing offset printing, without realising that SWOP is purely web-based and uses tolerances designed for that printing condition.
In the end, all printing standards or guidelines have the values of the primary colours described in ISO 2846-1 at their centre.
International Process Standard ISO 12647-2 and ISO 2846-1 Ink Specification
For the 20 years to 2004, ink sets used in production were often ‘matched’ to the colours presented on the proofing materials. We matched the ink set of the press to the delivered proof, and it is also for this reason that inks shifted away from the original ISO 2846 standard. According to the process standard ISO 12647-2, the colours that can be achieved in production printing become the target for proofing.
The process standard ISO 12647-2 and ink specification ISO 2846-1 are both products of a new level of understanding, managing and controlling colour. The process standard, (ISO 12647-2), refers back to the ink specification of ISO 2846-1, while further establishing paper types, their colour, gloss and brightness. It specifies the colours of the primaries and secondaries and, finally, the tone value curves.
In 2003 FOGRA created a process to control and certify the conformity of digital proofs to a chosen reference printing condition. The procedure uses a spectrophotometer to measure the L*a*b values of the colour patches of a digital control strip to be positioned on every proof sheet. Densitometers cannot be used to advantage in this case because the proofing colorant differs appreciably from those of printing. This was a major change in the practice of controlling digital proofs.
Previously we measured the density of the ink only with a densitometer as a means of quality control, but this was not sufficiently accurate. By measuring the colour with a suitable spectrophotometer we are able to determine both the density and the true colour of the ink.
By using the a specifically designed print wedge the printer will be given the same quality control as that used in proofing by monitoring the L*a*b values of the primary and complementary colours, and the dot gain of the press run. The wedge will be placed in the centre of the press sheet, which will give printers the power to certify their press run to the Print Standard Offset ISO 12647-2.
Given the effects of globalisation on the world printing/manufacturing stage, plus the associated international exchange of data and colour management profiles, it is to be expected that the International Standards will also have an educational effect and decrease variability of print results in the long term.