Printing with less ink

Reducing and using materials more efficiently is a fundamental objective within the 'Reduce, Reuse, and Recycle' principles. In the Print & Sign sector, this can be achieved by, reducing cutting waste, using the substrate more efficiently (e.g., double-sided printing), and reducing ink consumption. For this last point, we went to swissQprint Benelux in Ghent to find answers.

Fig. 1. The swissQprint Nyala 4 large format printer to investigate ink consumption.

Digital Printing

Unlike offset printing, digital printing is not a standardized printing process. The wide variation in digital printing technologies and substrates makes it impossible to define 'best practices' for digital printing. Nonetheless, the 'Coated FOGRA39 (ISO 12647-2:2004)' colour profile is frequently used to provide standardized offset colours for large-format print production. Unlike standardized offset printing, colour conversion is required to convert the provided FOGRA39 offset data into large-format printer data.

Fig. 2. Large-format printing is a non-standardized digital printing process, which requires colour conversion in the print shop.

Maximum Ink Coverage and Gray Replacement

The output colour profile in the digital print shop can help optimize ink usage. When creating that colour profile, you can consider the following ink-saving parameters:

• Setting a limit on the sum of the percentages of process inks (maximum ink coverage). For example, the 'Coated FOGRA39' colour profile from Adobe design software has a limit of 329%.
• Using as much black ink as possible in the separation. In colour construction, black can be considered a degree of freedom: mid-grey (L* = 50) is separated into 52%C + 41%M + 41%Y + 24%K = 158% ink coverage using the 'Coated FOGRA39' colour profile. The same mid-grey can also be achieved using the alternative colour profile 'ISOCoated_VIGC_39L_320', which is also based on the FOGRA39 dataset. The separation then becomes 33%C + 27%M + 27%Y + 46%K = 133% ink coverage. By including more black in the separation and less colour ink, you can further reduce ink coverage (and thus your ink usage).

The technological question is: how can you set both parameters to achieve minimal ink usage without visible compromises to colour quality?

Ink Usage

The test prints are made on a swissQprint Nyala 4 printer. This large-format inkjet printer uses a fixed drop size that is normalized during assembly. When producing a print, you can easily retrieve the consumption (ml) of the process inks. This allows us to directly compare the effect of the separation parameters on ink usage.

Fig. 3. Retrieving the ink usage (ml) of a print job.

Test Plan

For the Fespa Color label, we investigated the colour workflow of 10 large-format printers. Ideally, these printers receive FOGRA39 offset data for large-format print production. As a starting point, test charts were created based on the FOGRA39 colour space to visualize the effects of ink reduction. These test charts were then processed through the print workflow, where they were converted and printed using the rendering intent 'Colorimetric + Black Point Compensation.' You can expect more news about this 'Black Point Compensation' in the next GRACE newsletter. The prints were measured using an X-rite i1PRO3Plus spectrophotometer.

Results

Let's first look at the effect of lowering the limit on maximum ink coverage. A colour chart with dark colours (1140 colour combinations) was printed using a print profile with a high limit of 350%. This is the reference print, where a large amount of ink is allowed. Subsequently, prints were made with limits of 260% and 220%.

Fig. 4. A test image showing the effect of lower maximum ink coverage. Limiting to 260% is almost identical to the original 350%. At 220%, colour deviations can be detected.

The colour similarity of the different prints can be expressed by numbers in a frequency diagram. The y-axis shows the percentage of the dark test colours that are printed below a specific colour difference (x-axis). In analogy with ISO 15311, you can look up the colour difference within 95% of the colour chart is printed (95% dE00 value).

Fig. 5. The impact of the maximum ink coverage on dark colours. At the bottom the 95% dE00 score for a limit of 260% (2.6) and 220% (7.1) is indicated. For comparison: a standardised offset printer achieves a 95% dE00 score between 2 and 3. A 260% ink limit is therefore perfectly usable, without significantly affecting the colour quality.

A lower maximum ink coverage limit, together with a heavy black separation, makes it possible to reduce ink consumption. For a practical print job with images, this led to the following result:

Fig. 6. Ink consumption by a printer profile with high ink limit (left) and the limited version (right), 28% less ink consumption.

Conclusions

How much ink you can save depends primarily on the content of the print job. When reproducing dark and/or non-chromatic colours, the saving can be considerable. In practice, the type of customer will determine how far you can go in terms of ink reduction: for a reproduction for a museum, no concessions on colour quality are feasible, for standard work you can obtain acceptable reproductions with considerably lower ink consumption. More details about the test forms and measurement results will be published in the White Paper at the end of the project.