White light, white heat

Some months ago I promised to write a proper explanation of how HP adapted it’s wide format latex ink to work with rigid materials, but have only just got around to doing this.

HP ‘s first latex printer capable of printing to rigid materials is this R2000 hybrid.

The latex ink has always impressed me because of the range of materials it works with, which is unusual for a water-based ink. Strictly speaking, it’s not really latex ink, but latex has proved to be an easy concept to market. Instead these are resin inks, with the pigment encapsulated in resin, which is melted and in the process bonds the pigment to the substrate. Other vendors have also used this approach, notably Epson for its Surepress L-series of label printers.

One of the big problems that HP had in designing a latex printer for rigid materials is that the latex printers typically use a lot of heat but there are a lot of rigid heat sensitive materials that most print service providers would expect to be able to use. Noelle Peutat, Large format applications specialist for HP, says: “We worked on the inks we have at lower curing temperatures in order not to deform the media because there is a lot of plastics and corrugated in rigid which don’t really like heat.”

This is not as simple as it sounds because the heat is only partly used to dry the water out of the ink. Thom Brown, HP’s Inkologist and ink advocate, adds: “We have to heat up the latex components to melt the pigment to adhere to the substrate but this temperature was too high for the rigid materials.” This forced HP to reformulate the inks in order to have them work with less heat.

Thom Brown, HP’s self-styled Inkologist.

To this end HP made three main changes to the ink to allow it to cure with 20º less heat applied. Firstly, the rigid latex inks use a much higher pigment loading so that less water is needed. HP also improved the optimiser, a separate fluid jetted alongside the inks, which improves the colour gamut and the sharpness of text. Brown explains: “The pigments are heavier than water so they want to fall but we want to keep them evenly dispersed. So we prevent the pigment from sticking together and creating clumps that will fit together by charging them so that they repel each other and can’t stick together with the optimiser having an opposite charge.”

The standard inks also contain a coating that protects the ink from scratches but this has been taken out and is instead jetted in a separate Overcoat fluid. This helps to reduce the amount of liquid that is being laid down with the inks, which in turn reduces the amount of drying needed.

HP also introduced a white ink with the R2000, which was HP’s first latex white ink. The problem with white ink is that most vendors, including HP, use titanium dioxide pigments, which are very large and heavy, so that the pigments settle at the bottom of the tanks and turn to sediment. Most vendors overcome this by recirculating the ink from the ink tanks through the heads and back to the tanks but HP’s thermal printheads were not really designed for that.

To get around this problem HP used the high definition heads that were initially developed for the Inkjet Web Press, These heads have 2400 nozzles per inch but for the rigid latex printers HP has adapted them to recirculate the ink between pairs of chambers. Peutat says that only half the nozzles are used with the other half used as a micro pump to recirculate the ink: “So there are two nozzles together and the ink is moved between them so that one is always ready to fire.” It means the resolution drops to the 1200 nozzles per inch that is standard for the latex printers.

The white ink for the latex rigid printers can be stored in this box when not used and rotated in a separate chamber to prevent the pigments settling.

The nozzles are smaller so the ink has to be finer, having a droplet size of 10pl rather than 12pl. HP also had to change the shape of the nozzles from a simple circle to a circle of eight shape. This is because the new ink formulation with its heavier pigment loading was breaking up but the circle of eight shape pinches the droplet as it forms, creating one satellite instead of three or four. This satellite merges back into the main droplet on the substrate to create a single drop.

This is a very neat solution that allows HP to add recirculation without having to fundamentally redesign its heads. Brown says that this does not significantly increase the cost of the heads because the Inkjet Web Presses get through so many heads that HP has benefited from the economy of scale.The head life remains the same as before with HP guaranteeing that it will pump at least 12 litres of ink though Peutat says that they normally average 30-40 litres.

A true recirculation system also has to include the ink tanks so HP has also adapted the white ink cartridges. The R2000 uses 5 litre cartridges for the colours, but the white ink comes in a 3 litre size only. Inside the cartridges there are two bags with the ink passed from one bag through the head to the other bag to recirculate the ink and ensure that the pigments stay in suspension. In addition, when the white ink is not being used the printheads can be taken out and stored in a rotation chamber built into the side of the printer to prevent the ink from settling inside the head. There’s a dummy head, printed on a HP 3D printer, to fill the gap left when the printhead is taken out.

The obvious result of all these changes is that HP has now satisfied a long running customer demand for hybrid latex printers capable of printing to rigid materials as well as flexibles. But the real significance is that HP now has a water-based inkjet system capable of printing to plastics and other filmic material that’s not limited to coated materials. This is still a long way from being able to print to the kind of flexible films that the packaging industry needs, but it clearly demonstrates that HP has the capability to tweak its latex inks to extend the range of applications they can cope with.








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