Why stabilize the temperature of inkjet printer ink?

The most important parameters for the ink used by an inkjet printer are density, viscosity and surface tension.
(Article taken from the Durst  ‘Process’ magazine, their monthly newsletter for Screen and Digital Printers.)

On inkjet printers with piezoelectric print heads the mass and speed of the drop produced depend on these parameters. Density and surface tension vary only slightly with differences in temperature and so will not be further considered. The viscosity of all liquids, and therefore of  inks, lessens as temperature rises.

Fig. 1 shows the function of temperature on typical UV inks for inkjet printers.

The ink’s viscosity determines how much the ink resists the print head to prevent the expulsion of a drop. The aforementioned variation in drop mass and velocity therefore takes place depending on ink temperature. Fig. 2 shows how the mass of the drop depends on the temperature of the ink, measured on the printing heads of a Durst Quadro model.

By varying drop mass the thickness of the printed ink layer changes and so does the dot of colour.

Deviations from the dot of colour of ΔE = 10 are not yet perceptible visually. And so the ink temperature should be stabilized within a range of +/- 2.5°C.
Since even drop velocity depends on viscosity and therefore on ink temperature, the ink drops do not fall at the previously calculated point of the support to be printed.
As a consequence, images are grainier in the gray tones with ink coverage from low to medium. Fig. 4 shows the increased graininess of an image in the gray tones when temperature differs +/- 5°C or +/- 10°C from 50°C, with excellent compensation for drop velocity.

For the previously stated reasons, it is important to keep temperature differences as low as possible in a professional inkjet printer, within
and between the print heads, and to stabilize them, making them independent of variations in room temperature. For this purpose you can
use heat regulation systems controlled by a microcomputer. The simplest way to keep room temperature inflow to a minimum is to bring
the ink and the print heads to a temperature just above the highest room temperature there can be.

At temperatures above 40°C some inks can no longer be used with the inkjet printer print heads. In this case the ink and the print heads
have to be heated or cooled by the printer.

The heating elements and temperature sensors of the heat-regulation system must have optimal thermal contact with the print heads and the thermal conduction coefficients of the materials used must be as high as possible.

Thermal conduction coefficients of materials used in inkjet printer print heads.

The modules of the Durst Quadro print heads are made from a special aluminum alloy. The body of the print heads is in graphite. There are materials with thermal conduction coefficients higher that the ones used. However, these are not taken into consideration because of their expense or insufficient chemical resistance.

The measurement values shown in Figs. 5 and 6 show that the difference in temperature within a print head module is at maximum 2.0°C.

Moreover, there is a maximum temperature difference of 3°C between print head modules. This makes it possible to remain in a temperature range of 2.5°C without any problems.

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