On Development and Characterisation of Continuous Ink Jet Nozzles

Sammanfattning: This thesis concerns nozzles and their impact on droplet formation and droplet flight in a continuous ink jet printer. Methods are suggested to characterise droplet formation and droplet flight stability (D.F.S.). A study about the influence of droplet flight stability on the quality of print out is presented. A method to assess the charging process for the continuous ink jet is described. The development and characterisation of silicon nozzles, with single and multiple orifices, for continuous ink jet printers is presented. The continuous ink jet printer based on the Hertz’ principle produces images of photographic quality by electrostatic deflection of undesired droplets from a continuous flow of droplets. The droplets that form the print must all have the same size and equal inter droplet distances to assure high, reproducible print quality. To attain equally sized droplets at constant inter droplet distances the nozzle is mechanically stimulated at a frequency close to its natural droplet formation frequency. An optical measurement setup is suggested to assess information about the droplet flight stability. Nozzle units are characterised concerning how the choice of stimulation frequency and its amplitude influences droplet flight stability. It was found that the nozzles often had an individual behaviour regarding at which stimulation frequencies the nozzle had its highest droplet flight stability. Nozzles were after characterisation used in a printer to study how the measured droplet flight stability influenced the positioning of droplets on paper during print out. It was found that if the droplet flight stability was high (D.F.S. < 15°) no misplaced droplets that would degrade print quality could be detected in the print. During printing is it essential that the stimulation signal and the charging pulses are correctly synchronised to either fully charge droplets or to leave them uncharged. The charging of droplets was measured while the phase angle between stimulation and charging was varied. This variation showed for which phase angles that the droplets are fully charged. A method is presented to assess the electrical properties of droplet formation. The nozzles used in continuous ink jet printers are traditionally made by glass. The assembling of nozzles introduces great variations in droplet flight stability for different stimulation frequencies. For a given stimulation frequency two glass nozzles might produce droplets at quite different droplet flight stabilities. A method to produce pyramid shaped, truncated silicon nozzles without the drawback of varying droplet flight stability is presented. The nozzles were manufactured during four consecutive etchings. The printing of one A4 page with a continuous ink jet printer is time consuming, since the printer can print approximately 0.5 pages per minute. To increase printing speed a silicon nozzle array with multiple nozzles per colour is developed and characterised. The nozzles were manufactured during three etchings. The silicon processing technique was improved to eliminate orifice size variation due to wafer thickness variation.