Polyester media development for inkjet printers - design of HP DeskJet 1200C and HP DeskJet 1200C/PS printers - Technical

Hewlett-Packard Journal, Feb, 1994 by Daniel L. Briley

A discussion of the mechanisms and ink/printer/media interactions that must be considered in the design of special media for a printer system, and of the methods available for optimizing them.

The full capability of a printer system is best realized by printing on special media designed and developed to work on that specific printer system. HP special media include the CX JetSeries transparency film, the LX JetSeries transparency film, the LX JetSeries glossy paper, and the LX JetSeries single-matte inkjet polyester film. These special media are designed to give the best possible print quality for the printer system for which they were designed. In most cases, the media will operate over a range of products. The LX glossy paper and the LX transparency film both give optimum print quality performance on the HP Paintjet XL300, Deskjet 500C, Deskjet 550C, Deskjet 1200C, and Deskjet Portable printers. To ensure the highest possible print quality on all of these different printer platforms, various ink/printer/media interactions that occur while printing must be understood and accounted for in the printer, ink, and media design.

Polyester Substrates

Among the most common special media are the polyester-based materials. These include overhead transparency film, matte drafting film, and the bright white HP LX JetSeries glossy paper. All of these media are constructed on a base sheet made of biaxially oriented polyethylene terephthalate (PET).

Some of the factors that make polyester film a material of choice include its durability, thermal and hydroscopic stability, clarity, and stiffness. Along with all of these desirable attributes, a coated polyester substrate has several lessfavorable properties that are not found in paper-based media, such as the base material's low absorptive capacity for aqueous inks.

Ink-Receptive Coating The most obvious difference between paper-based materials and polyester film is that in a water-based ink system, the polyester film does not absorb any of the ink applied to the sheet. As shown in Fig. 1, the ink printed on the uncoated polyester sheet simply beads up on the surface. The identical pattern of ink applied to a coated polyester sheet retains the printed image.

The ink-receptive coating on the polyester film functions to absorb the ink, bind the free liquid of the ink vehicle until it can fully dry, cause the ink to spread into well-formed dots on the page, hold the colorant from the ink in place on the film, and provide outstanding image quality for both black and color. These are some of the properties that the development team will optimize during the design of any polyester film-based media.

Drying Time

The drying process on polyester film-based media is a twofold mechanism, a combination of absorption by the inkreceptive coating and evaporation. The evaporation rate of the ink on a printed page can be estimated by printing the ink onto a sheet of an uncoated polyester film. With no inkreceptive coating on the film, all of the drying capacity must come from evaporative forces only. These forces are based on the surface area of the ink exposed to the air and the partial pressure of the ink vehicle.

When an uncoated polyester sheet is printed on a Deskjet 1200C printer and left to dry, an hour later it will still not be dry to the touch. This is because the nonvolatile components in the ink make the evaporative drying rate very low. By contrast, the ink applied to a coated sheet will be absorbed by the ink-receptive coating, and will be dry to the touch within a reasonable amount of time after printing. Achieving fast drying times while still maintaining the desired level of print quality is one of the most difficult aspects of polyester film development.

A typical inkjet printer will have a total ink flux applied to the sheet of approximately 30 to 40 [g/m.sup.2]. The typical inkjet polyester film has an ink-receptive coating weight of 4 to 8 [g/m.sup.2]. The ink-receptive coating must absorb and bind all of the ink applied to the sheet. While a greater amount of coating weight applied to the polyester base sheet would definitely improve the ability of the film coating to absorb the ink, additional problems develop such as image quality, curl, manufacturability of the film, and finished product cost.

Evaporation will eventually remove most of the excess volatile components of the ink. This will leave only the nonvolatile components from the ink to be absorbed by the coating. The inks used in the Deskjet 1200C have approximately 20 to 30 percent nonvolatile components. At full ink densities, there will be 6.9 to 9.2 [g/m.sup.2] of nonvolatile ink components remaining in the coating. Some of these nonvolatile components will act as humectants and hold additional moisture in the coating with them, making the total remaining mass of ink in the coating even higher. Depending on the environmental conditions, the humectant tendencies of the ink may also prevent the inks from fully drying on the film.