Influence of the Rotation of Inkjet Printing Heads on the Print Quality

In this study, we focus on the design specifications of industrial inkjet printers. In these systems, the printing heads are mounted individually on a carriage. The in-plane print head rotation has been identified as one of the reasons for the ink dots misplacement. This study aims to work out the influence of the angles of CMYK printing heads on the print quality. Ultimately, some mechanical tolerances are given to achieve different levels of print quality. A two-step approach is used: one colour, then the four CMYK colours are considered. In the first step, a layout of the nozzles plate is used in a computer simulation. In parallel, a prototype has been built to validate the simulations. In the second step, a CMYK virtual printer is introduced in place of a four colours experimental system. A criterion on dots superposition is introduced to asses the influence of the heads rotation on the colour reproduction. The nozzle plate layout has been added to a CMYK virtual printer to characterise the printing heads rotation. The model has been tested with XAAR 760 GS8 print head. Experiments are in a fair agreement with simulations for the one-colour approach. A cone angle for one and four colours has been derived. Introduction Nowadays, material deposition by ink-jet printing is used in a lot of applications, either graphical or non graphical. Both applications need to set the drops with a controlled accuracy in order to achieve the desired final quality in the allocated budget. The broad scope of the study is to link mechanical tolerances with print quality, and hence to get a relationship between cost and budget required to achieve a desired level of print quality. Among uses for this result, company may be interested in a better costs control/optimization at the design stage. From the graphical side, the range of accuracy needed for ink deposition seems to extent itself up to 50 μm. If offset lithography print is considered, the lateral registration of the four colours has to less than 80 μm [1]. Burningham [4] studied the effect of the colour misregistration on the print quality, using an offset lithography press. A steep decrease in the print quality appears when the misregistration exceeds 50 μm. Belong 50 μm accuracy, it is expected that line blurriness and colour shift are present. However, in this range, the influence of the dots placement has not been fully quantified. Wencheng & al [8] has defined a model to assess line quality based on perception. However, the blurriness is not taking into account in his model. Svanholm [6] in his dissertation claims that there are strong correlations between both the colour gamut and print sharpness measurements, and the visual experience. Moreover, it is added that the smallest difference in print quality that could be visually discerned was approximately 350 units in gamut area, 1.6 μm in blurriness and 0.002 mm in line width. For non-graphical applications, like printing of micro-lenses [3], the dot positioning has to be as accurate as +/1 μm from centre to centre. In the case of micro-lenses printed over the vertical cavity surface emitting laser (VCSEL) emitters, the drop has to set on a pedestal, conditioning the dot drop placement. From the other hand, from a mechanical design point of view, achieving a level of accuracy has a cost. Curves relating cost with tolerances [2], and machines are commonly used by mechanical designers. Commonly, as several models exist, the most common one is an exponential one. Several techniques are used to studies tolerances, among them Monte Carlo Simulations. There are based on individual assemblies using a random number generator to select values for each manufactured dimension, based on the type of statistical distribution assigned by the designer or determined from production data. These dimensions are combined through the assembly function to determine the value of the assembly variable for each simulated assembly. This set of values is then used to compute the first four moments of the assembly variable. Finally, the moments may be used to determine the system behaviour of the assembly, such as the mean, standard deviation, and percentage of assemblies which fall outside the design specifications. In this case, the output of the model is the print quality. The print quality should be understood in the way defined by Engeldrum [1]: physical image parameters. Some attributes are defined in the standards [7] on print quality like edge raggedness, line width, uniformity, contrast which can be applied to either to a line or to a text. The print head rotation has been identified as one of the reasons for the ink dots misplacement. By example, the print head can rotate when the machined slot where it is mounted, is larger, in both directions, than the head dimensions. Linked to the study of the head rotation is the sensitivity of the nozzles layout to the rotation. As print heads differs by their resolution, nozzles layout, nozzle diameter, the appropriated choice is crucial of the design stage. This study aims to work out the influence of the in-plane angle of printing heads on the accuracy of the ink deposition. The continuous effect of the head rotation, and not just a maximum acceptable angular deviation, is investigated. A link is established between the print quality and the placement. A two-step approach is used: one colour, then the four CMYK colours are considered. In the first step, a layout of the nozzles plate is used in a computer simulation. In parallel, a prototype has been built to validate the simulations. In the second step, a CMYK virtual printer is introduced in place of a four colours experimental system, too expensive to build. A threshold on dots superposition, based on the colour perception, is introduced to asses the influence of the heads rotation when printing with several heads, as in the colour reproduction. NIP25 and Digital Fabrication 2009 Technical Program and Proceedings 103