An Optical Method for Measuring Drop Flight Stability in a Continuous Ink Jet

Abstract

This paper describes an optical method for improved drop velocity stability measurement to be used in continuous ink jet printing applications. Stable drop formation is demanded in continuous ink jet printing and it is normally achieved by introducing mechanical vibrations from a piezoelectric crystal onto the jet emerging from a nozzle. The method in use today to obtain information about the stability of drop velocity is to view the drops in stroboscopic light. This method does not provide quantified information about the level of drop velocity stability and the roughness of the method makes comparison between different levels of stability subjective and hence difficult. In our method we illuminate the drop train with a continuous HeNe-laser to create a shadow image of the drops. This image is magnified through a microscope and projected onto the light sensitive area of a PIN photodiode-based detector the output of which is sampled by a digitizing oscilloscope. The sampled data is used to calculate the standard deviation of time between drops and this value is used as a measure of drop velocity stability. Our method is primarily developed to measure the stability of drop velocity of drops with a diameter of 15 mm at crystal excitation frequencies in the interval of 800 to 1400 kHz. However, the set-up can easily measure drop velocity stability for different sizes of drops by simply changing the magnification of the microscope. Measurements with our method show that an increased excitation signal amplitude will result in a higher level of stability. The drop velocity is to a great extent decreased by air resistance as the drops travel. The presence of good and poor stimulation frequencies for nozzle systems is shown, and the frequencies are indicated by low and high levels of standard deviation for time between drops.