Inkjet Technology and Rheology9th Installment

Inkjet Technology and Rheology9th Installment

The image quality achieved by inkjet printers in recent years is remarkable. These printers now enable anyone to print out high quality photo reproductions. Inkjet printing involves the spraying of fine droplets of ink to form characters, images, and color and the method differs greatly from traditional printing processes which transfer ink from photos and plates by direct contact and impact. Certainly the distinctive features of inkjet printing are a reflection of the attention and technical innovation that have been focused on this method of printing. We would like to explore in this column the technology of inkjet printers from a rheological viewpoint.

The Intricacies of Spraying Ink

Inkjet printers propel tiny droplets of ink which form dots on a media (paper) and characters and graphical images are created by clustering these dots in prescribed arrangements. High resolution inkjet prints can provide sharp reproductions of original images. In order to achieve this it is first important that ink droplets be precisely targeted. This may be easily visualized in the example of a simple line drawing which is created by a continuous series of dots. Any misalignment of these dots is readily apparent by discontinuities or unevenness in the line. Therefore it is essential to insure that ink droplets are sprayed onto the paper with accuracy.
Higher ink functionality is another important factor for fine quality prints. Indeed, the optimum control of these two aspects − precise spray placement and the development of the best ink component compositions for ideal performance - may be deemed most critical in inkjet printing technology.
Image sharpness and good tonality as it relates to inkjet print quality are determined to a great extent by dot size. Resolution is the reference used to describe print image definition. Resolution is indicated in ‘dpi’ units or ‘dots per inch’. Even standard model printers of major Japanese manufacturers offer specifications of 4,800 dpi (i.e., 4,800 dots per inch). The volume of ink which forms these dots is equivalent to 2 picoliters (1 pl = 1 trillionth of a liter). High performance models offer resolutions which are twice this or 9,600 dpi! Obviously, with the ‘dpi’ doubled, the ink volume per dot is halved (i.e., 1 picoliter).
Although such resolutions dramatically improve image quality, it becomes increasingly difficult to control spray accuracy of ink droplets as the size of the dots become finer.
With naturally larger sized pigment particles (albeit nanometer sized, i.e., 1 nanometer = 1 billionth of a meter) especially, it becomes increasing difficult to cope with higher resolutions. As a result most home-use inkjet printers employ dye-based inks for color and pigment-based black ink for sharp line creation.

Stable Pigment Dispersion Technology

Let us look at how inks used in inkjet printers are tailored. Good coloring, water resistance, and fade resistance are desirable properties of inks which vie to achieve photographic quality. We take the example of pigment-based inks.
With pigment-based inks, particles of color material are not dissolved but remain suspended in the solvent. Pigment-based inks offer smear and fade resistant sharp images as color pigment particles are not absorbed but instead adhere onto the surface of the paper. However the particle-sized structure of pigment-based inks is also a disadvantage. Although pigment particles are of nanometer size, they tend to precipitate in the ink tank. This tendency accelerates if the particles begin to stick together. As solvent and particles of coloring pigmentation remain basically separate components, it is possible that only the supernatant (clear) portion of the mixture is sprayed, which results in no coloring. Large-sized pigment particles also are more apt to clog print-head spray nozzles.
Therefore in order to utilize pigment-based inks in inkjet printing, it is important to obtain finer pigment particle sizes and achieve uniformity (non-precipitation) in solvent distribution. The technology involved is referred to as pigment dispersion and it is a critical factor which determines the quality of inkjet printers.
Water comprises approx. 80% of volume of water-based inks used in inkjet printers. For good distribution of non-soluble pigments in such liquids, dispersion additives must be used. Particle size and hydrophicity may differ according to the pigment material so it is important to determine the right amount of additive for each type of ink being created.
With such inks, the viscosity of the water solvent may increase depending on the dispersion additive employed. Here viscosity measurement is used to determine the properties of inks containing dispersion agents.

Ink Quality and Relationship to Viscosity

Correct viscosity also correlates with the stability that can be achieved in the spraying of fine ink droplets. Higher viscosities increase friction as the ink passes through the nozzles and this reduces spraying speed. It also results is unevenness in spray volume and shapes of the ‘satellites’ or fine droplets which form behind the main droplet - factors which deteriorate print quality. When large ‘satellites’ form on the paper, they are apt to appear as blemishes or smears.
Perhaps this aspect may be easier visualized if contrasted with ink dots of differing colors. Inkjet printers utilize dots comprised of minimum units of colors - magenta, cyan, and yellow − to express the palette of colors. For example, magenta and cyan dots are used in prescribed proportions to form the color purple (or more accurately, to create the appearance of the color purple). In order to generate such colors accurately, it is prerequisite that the dots of the color units be of uniform size. Otherwise the intended color cannot be obtained even if the unit color dots are of prescribed number. Similar incongruities result when ink droplets are not positioned on the paper correctly.
Thus it is very important that spray placement of the ink on paper be precise and without formation of excessive ‘satellite’ dots. As means to achieve this, ink droplets should be made as spherical as possible, a task which may be difficult as dispersion additives increase ink viscosity while lowering the surface tension of water (that is, of the ink as a whole) due to surface-active behavior which also tends to flatten the ink spheres.
However if ink viscosity is lowered substantially to overcome this complication we may be confronted with the problem of unstable dispersion of pigment particles. Thus difficulties would be encountered in either case and the issue becomes how to strike a proper balance. One method ink manufacturers are turning to is to seek optimum rheologically-based values and utilizing these values in research and development of high quality inks and quality control of manufacturing processes.
Accurate measurement of rheological properties of substances of low, near-water viscosities certainly requires specialized technology and knowhow. As the Japanese pioneer in instruments and equipment for viscosity measurement, Toki Sangyo offers such expertise in contributing to the effort to create inks for inkjet printers that are of the highest quality.
Rotational viscometers employing cone-plate systems are ideal for viscosity measurements of inks for inkjet printers. Generally, viscosity values at low shear speeds (low rpm’s) are used to assess stability of particle dispersion and values at high shear speeds (high rpm’s) are used to evaluate nozzle spray behavior.

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