Inkjet inks and offset paper

Last month Canon announced a new iQuarius MX inkset for its VarioPrint i300 series of sheetfed inkjet presses and I thought it would be worth taking a more in-depth look at this.

Canon Oce VarioPrint i300

The fundamental problem with water-based inkjet ink is that it contains a lot of water, which acts as a cheap but effective carrier for the pigments. This water is absorbed into the paper and causes the fibres to swell, leading to dot gain and poor image quality, as well as deforming the paper. The obvious solution is to use inkjet-optimised paper but this is expensive and there’s a limited choice of stocks available.

Most printers would prefer to use the same stocks that they are using with their offset presses, because the paper is cheaper and it simplifies their inventory. The flexibility to pick the right media for each job is even more important with a sheetfed press.

So most press vendors have worked hard to improve the performance of their inks with offset stocks. Typically this has involved tweaking the formulation of the inks, improving the screening and, often, some form of primer.

Canon has done all of these things. The iQuarius inks are part of Canon’s ColorGrip approach, which is essentially a type of primer that’s jetted onto the paper just ahead of the ink. Canon uses a fifth printhead to drop this ColorGrip primer precisely where the ink drops themselves will land, which minimises the total liquids added to the paper surface. This primer keeps the ink pigments close to the surface of the media, while allowing some of the water itself to soak into the paper. This in turn reduces the risk of dot gain or of the ink drops coalescing into each other, or of the water pulling the pigment through the paper, which would lead to less intense colours on one side of the sheet and show-through on the other.

The standard iQuarius ink contains cosolvents to help stabilize the paper structure. This is important because the paper has to cope with a lot of water being jetted onto the surface when the first side is printed, and it also has to deal with the water being pulled out of the paper when it’s dried, and then the paper has to go through this all over again when the second side is printed. This sort of abuse would normally leave the paper suffering from waving and curling so the cosolvents job is to strengthen the fibres to counter this.

The new MX ink uses a combination of two different wax components. This includes a low-melting wax that melts during the drying process to form a thin protective layer on the ink surface and stops the sheets from sticking to each other. The second element is a high-melting wax that creates balls within the ink to protect the ink and prevent it from rubbing off in post-processing machinery.

The MX ink contains low melt wax which melts during the drying process and forms a protective layer.

So far we’ve mostly covered the interaction of the ink and the paper but there are several design decisions on the press that also play a part in persuading offset papers to accept inkjet inks. This includes monitoring the printheads to look for nozzle failures and deals with failed nozzles by either increasing the drop size from surrounding nozzles or replacing those drops with another colours so that there’s no gap in the printing. The i-series machines print a nozzle failure detection sheet after every 100 printed sheets as part of an automated monitoring system that also includes an inline scanner.

Canon also checks the paper and pulls out any sheets with waves, folded corners or other problems before they get to the printing stage. The sheets are transported through the press on a seamless stainless steel belt with a vacuum used to hold the sheets flat to the belt so that the printheads can be positioned close to the media surface. This in turn helps to ensure the accuracy of the print drop placement, which is crucial to good image quality. It also minimises the risk of the ink drops being blown off course by the air currents that are generated by the movement of the paper through the print module.

The VarioPrint uses a drum to dry the sheets.

Finally, there’s also the drying stage, which is absolutely crucial in removing the water from the ink without sucking all the moisture out of the paper. Canon transfers the sheets to a drum, again using a vacuum to hold them flat in place, and gently wafts warm air around to evaporate the water away and counter the risk of cockling. The drying itself only takes two seconds for each sheet at roughly 65°C. The excess moisture is funnelled out of the printer with an external vent to reduce the humidity.

It’s worth noting that other vendors have also introduced inksets that can cope better with offset papers and I’ll come back to this subject and look at some of the vendor’s solutions in due course. But I think that the key message here is that the range of substrates that can be handled by a given press is determined by many different parts of the overall system design though the ink formulation has an important part to play in this.

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