The HP DesignJet L65500 printer has been the first water based latex ink printer in the market. Using very high npi TIJ print heads, like many other HP large format printers, the image quality it produces is outstanding; but what really makes the difference is the latex ink it uses. HP
latex inks are specially formulated to work well on vinyl and other plastic surfaces. Like all other HP water based inks its main ingredient is water which is the vehicle that carries everything else. They have pigments and a few other ingredients that keep pigments dispersed in water and
that enables it to wet plastic surfaces. And finally they have little balls of latex. During the printing operation a thin layer of ink is laid down onto the media. This ink contains the required pigments to form the image, and an evenly distributed load of little latex balls. It is
then when the drying system actuates. The drying system evaporates an important fraction of the water to keep pigments in its place, ensuring a good image quality. The drying system is based on infrared emitters which heat up the media, and some blown air which evacuates the water vapor. The
thermal profile the media sees has been optimally designed after long investigations to produce the desired results; but temperature peaks at about 50°C. Then the image enters the curing zone. The curing system raises the media temperature even higher, and its main objective is to finish
evaporation of any remaining water and additives and to form the latex film. When all the water is evaporated, the little balls of latex get in close contact with each other; and when its temperature raise above its glass transition temperature latex macromolecules start to move. This molecule
diffusion movement generates a continuous film of latex which encapsulates the pigments and makes a durable output. Typical curing temperatures are about 90°C. As the film formation is a diffusion process, it takes some time. Time required for the molecules to diffuse and to generate a
continuous latex film is a direct function of the temperature reached. In an HP DesignJet L65500 printer using a typical production print mode the print is held at curing temperature for 30 seconds approximately. The curing system is based on infrared emitters as the drying system. The
drying and the curing systems in the HP DesignJet L65500 printer are basically infrared emitters which behave close to black bodies working at 900 K. The emitters and their working temperature have been chosen to get the maximum matching between the emitted radiation and the absorption of
both, plastic media and water based ink. Water infrared radiation absorption is maximum at 3 μm wavelengths and typical plastics absorb very well at 3.4 μm. A black body at 900 K radiates with a peak of emitted power at around 3.2 μm, very well aligned with both absorption peaks. Finally,
both systems have infrared (non contact) temperature sensors which directly sense media temperature, used to drive a closed loop temperature servo.
Document Type: Research Article
Publication date: January 1, 2012
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