How HP adjusts its latex ink to add white

Latex inks can use a wide range of materials, which is unusual for water-based inks. Strictly speaking, it is not a true latex ink, but latex has proven to be an easy-to-market concept. Instead, they are resin inks in which the pigment encapsulates in the resin, which is melted and, in the process, the pigment is bonded to the substrate. Other vendors also use this approach, notably Epson's Surepress L-series label printers.

One of the big problems HP encounters when designing latex presses for rigid materials is that latex presses typically use a lot of heat, but most print service providers want to be able to use a lot of heat-sensitive materials. Noelle Peutat, a large format application specialist at HP, said: "The inks we use at lower curing temperatures are designed not to deform the medium because there are a lot of plastics and corrugated paper that are stiff and don't like heating very much. ”

This is not as simple as it sounds, as the heat is only used in part to dry the water from the ink. HP ink hydrologist and ink advocate Thom Brown added: "We had to heat the latex component so that the pigment melted and adhered to the substrate, but this temperature was too high for rigid materials. "This forced HP to reprogram the ink sequentially so they could work with less heat."

Thom Brown, ink expert at HP

To that end, HP made three major changes to the ink to allow it to cure at 20o of heat. First, rigid latex inks use a higher pigment load and therefore require less water. HP has also improved the Optimizer, a separate liquid sprayed with ink that improves color gamut and text clarity. Brown explains: "The pigments are heavier than water, so they want to fall off, but we want them to be evenly dispersed. Therefore, we prevent the pigments from sticking together and combine them together by charging, so that they repel each other and cannot stick together with optimizers with opposite charges.

Standard inks also contain a coating that protects the ink from scratches, but has been removed and instead sprayed in a separate external coating solution. This helps to reduce the amount of liquid placed with the ink, which in turn reduces the amount of drying required.

HP also introduced white ink with R2000, HP's first milky white ink. The problem with white inks is that most suppliers, including HP, use very large and heavy titanium dioxide pigments, so the pigments settle at the bottom of the tank and become precipitate. Most suppliers overcome this by recycling ink from the cartridge through the ink head and back to the cartridge, but HP's thermal printheads aren't really designed for this purpose.

To address this, HP used high-definition printheads originally developed for inkjet web presses with 2400 nozzles per inch, but for rigid latex printers, HP has tweaked them to recycle ink between paired chambers. Peutat says only half of the nozzles are used as micro pumps for recirculating ink: "So there are two nozzles together and the ink moves between them so that one can fire at any time." "That means resolution drops to 1200 nozzles per inch, which is standard for latex printers."

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The white ink of the latex hard printer can be stored in this box when not in use and rotated in a separate chamber to prevent pigment deposits.

The nozzle is smaller, so the ink must be finer, with a droplet size of 10pl instead of 12pl. HP must also change the shape of the nozzle from a simple round to an 8-shaped circle. This is because the new ink formulation with a heavier pigment load is destroyed, but eight shaped circles pinch the droplets as they form, forming a satellite instead of three or four. The satellite merges back into the main droplets on the substrate to produce a single droplet.

This is a very concise solution that allows HP to add recycling without fundamentally redesigning its header. Brown said this doesn't significantly increase the cost of the head because inkjet mesh presses have experienced so many heads that HP has already benefited from economies of scale. The head life is the same as before, and HP guarantees that it will at least be able to pump, although Peutat says 12 liters of ink typically averages 30-40 liters.

A true recirculation system must also include an ink cartridge, so HP also uses a white cartridge. The R2000 uses a 5-liter cartridge as the color, but the white ink is only 3 liters. Inside the cartridge there are two bags where ink is passed from one bag through the head to the other to recycle the ink and ensure that the pigment remains in suspension. In addition, when no white ink is used, the print head can be removed and stored in a rotating chamber built into the side of the printer to prevent ink from depositing inside the print head. There is an analog head printed on an HP 3D printer to fill the gap left when the print head is removed.

The obvious result of all these changes is that HP now meets the long-term customer demand for hybrid latex printers capable of printing to rigid as well as flexible materials. But what's really important is that HP now has a water-based inkjet system capable of printing plastic and other film materials, not just coating materials. There's still a long way to go from printing to the flexible films needed in the packaging industry, but it clearly demonstrates HP's ability to adapt its latex inks to expand the range of applications it should address.

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