REDUCTION OF LABEL DIGITAL-PRINTING COSTS VIA AN INNOVATIVE CLEANING METHOD

Ink consumption is one of the variable costs in digital printing. It differs from job to job and often sets the economic limits of the process. The Gallus One digital-printing label machine, incorporates a newly developed, contact-free cleaning system which reduces ink consumption. But is the reduction in ink consumption so significant that the total cost of ownership is less? Dr Burkhard Wolf, Head of Digital Technology, Business Unit Label, at Gallus Ferd. Rüesch AG, believes this to be true. He explains what is behind this innovation and how it can reduce ink costs by up to 20%. Together with his team, Dr Wolf is working on the development of new platforms for digital-printing systems in label printing.

The price of ink in digital printing has a much greater impact on the total cost of ownership than that of analogue printing. Consequently, these costs influence the profitability of every print job. They can also contribute to reducing the margin of a job, to such an extent that analogue production is less expensive than digital, especially for longer runs. It is, therefore, no wonder that converters to digital printing focus on ink as a cost factor and closely analyse the potential affects on print runs. Precise cost recording is an important prerequisite for keeping the total cost of ownership in digital printing under control.

Ink is not only required to achieve ink coverage in printing, it is also used to clean the nozzles when the printheads are flushed. During the printing process, ink particles are deposited on the underside of the print bars and thus on the edge of the nozzles. Scattered light causes the ink particles to cure making them difficult to remove. As a result, even the smallest deposits can cause a nozzle to start ejecting droplets at an angle. This impairs the print quality and makes it necessary to clean the print bar.

The scattered light which hits the print bars is emitted by the pinning modules, which are located directly alongside. Their task is partly to cure the printing ink – using UV energy – before the next droplet of ink is added. This prevents the inks from running into each other, meaning that the ink dots are reproduced sharply and there is no colour shift due to bleeding. UV pinning is therefore an indispensable part of the machine technology for coated papers and plastic films in high-speed, digital-printing machines.

The nozzles are often cleaned after contamination by spraying or rinsing with cleaning agents, followed by vacuuming. Although, this is not enough to clean the nozzles if more stubborn deposits occur. As a result, the printheads must be flushed from the inside by pumping ink through the nozzles. The ink consumption during such a cleaning process should not be underestimated, as up to several hundred millilitres of ink are required per cleaning.

In addition, printheads need to be flushed as the different tendency of the ink shades contaminate them. This can depend on the dyeing components used in the inks. The pigment in violet is particularly difficult to clean, as well as white ink with its titanium dioxide content. This is because the pigments in these inks tend to adhere to the silicon surface of the print bars and cannot be removed by rinsing. Often, they can only be removed by manual cleaning, which carries the risk of damaging the printheads.

“The Gallus One consumes up to 20% less ink than comparable inkjet-printing systems, as there is no need to flush the heads with ink”

Dr Wolf believes that the hazards of manual cleaning are the starting point for the development of a new, efficient and gentle cleaning system. The Gallus One system was developed in cooperation with Heidelberg. An extensive series of tests were carried out, in which the cleaning effect of rinsing – supplemented by ultrasonic cleaning – was investigated.

Ultrasonic cleaning causes the cleaning liquid to vibrate. “When used in this way, ultrasound has an incredible effect on cleaning the inkjet heads,” says Dr Wolf. Once the endurance tests have been completed, this patented cleaning method can be used in digital-printing machinery.

As part of the build concept of the Gallus One, the print bars are fixed. This is a fundamental and patented innovation of the entire printing unit of this digital press. At the start of a cleaning cycle, the arch-shaped print table automatically moves down with the substrate. The cleaning unit then slides out, consisting of a drawer arch on which the cleaning boxes for each print bar are located.

Basic cleaning takes place when the Gallus One is started up and shut down, and at regular intervals in between. Cleaning fluid is rinsed onto the print bars in the cleaning boxes and then extracted.

In extended cleaning with ultrasound, a generator stimulates the cleaning fluid at a frequency, optimised by a long series of tests. The frequency causes vibration and thus gently and thoroughly cleans the nozzle plate of ink components. This includes the polymerised ink residues on the nozzles. Cleaning is carried out exclusively from the outside of the printheads, completely eliminating any ink consumption by flushing the nozzles. This pioneering cleaning process also means that there is no wasted ink.

The degree of contamination of the print bars depends on the scattered light emitted by the pinning modules. Cleaning intervals are based on the power with which they are operated. The performance itself is a retrievable, measurement variable that is used correlatively to control the cleaning intervals. As a result, it is not based – as is so often the case – on assumed cycles with a corresponding safety buffer. The Gallus One, on the other hand, is an intelligent forced cleaning system that – in addition to the basic parameters – takes into account the remaining running meters of an attached roll. This offers a number of advantages that significantly increase the efficiency and productivity of the printing process. For example, the cleaning intervals are controlled so that they occur as infrequently as possible. It is entirely within the intuition of the operator, who combines upcoming cleaning with job or role changes. The example of cleaning intervals also shows the high level of standardisation and reproducibility of the processes. This enables high machine availability and lower down times.

This also has an impact on the total cost of ownership of a digital-printing system. For example, a monthly ink consumption of 200 litres, for a digital-printing system, results in a linear annual ink requirement of 2,400 litres. At a price per litre of €60, this represents costs of €144,000 per year for a UV-printing ink system. 
If these costs can be reduced by 20%, the ink consumption costs are only €115,200 per year. This corresponds to a saving of €28,800 per year in operating costs. Calculated over five years, this amounts to €144,000. This saving becomes an investment-relevant criterion when buying a digital-printing system. This clearly makes the impact of cleaning costs on the total cost of ownership transparent and comprehensible.