AN OPTICAL METHOD FOR MEASURING DROP FLIGHT STABILITY IN A CONTINUOUSINK-JET

This paper describes an optical method for improved drop velocity stab ility measurement to be used in continuous ink jet printing applicatio ns. Stable drop formation is demanded in continuous ink jet printing a nd 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 veloc ity is to view the drops in stroboscopic light. This method does not p rovide quantified information about the level of drop velocity stabili ty and the roughness of the method makes comparison between different levels of stability subjective and hence difficult. In our method we i lluminate the drop train with a continuous HeNe-laser to create a shad ow image of the drops. This image is magnified through a microscope an d projected onto the light sensitive area of a PIN photodiode-based de tector the output of which is sampled by a digitizing oscilloscope. Th e sampled data is used to calculate the standard deviation of time bet ween drops and this value is used as a measure of drop velocity stabil ity. Our method is primarily developed to measure the stability of dro p velocity of drops with a diameter of 15 mu m at crystal excitation f requencies in the interval of 800 to 1400 kHz. However, the set-up can easily measure drop velocity stability for different sizes of drops b y simply changing the magnification of the microscope. Measurements wi th 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 d eviation for time between drops.