Drop Watching

Drop Watching is a video-based method developed for inkjet printing for investigating the properties and flight behavior of ink drops immediately after leaving the print head.

Relevance for inkjet printing technology

In the drop-on-demand technique of inkjet printing, the print head produces the ink droplets using piezo nozzles that are controlled by electrical pulses with a given waveform. The volume, speed, and flight characteristics of the droplets are determined by the geometric properties of the piezo nozzles and the waveform on the one hand, and by the properties of the inks on the other, in particular their surface tension, viscosity and density. The drop watching method is used to correlate the print parameters and the drop behavior in order to optimize the ink formulation and the printing process.

Exact drop image evaluation with double exposure technique

The biggest technical challenge in the analysis of jetted ink droplets is their high speed of up to 40 m/s. In order to capture a drop in the image and measure its speed, camera images are double-exposed with the aid of two flashes that follow shortly after each other. The flashes are triggered after the piezo pulse with a defined delay time. On the corresponding single image of the camera, two images of the drop appear below each other.

In the novel two-color double strobe process, two different-colored light flashes are used instead of white light. By separating the color channels, two images of the drop are obtained without superimposition, so that even with long ligaments or in the case of satellite drops, a clear assignment of the drop images to both flash times is possible.

Illustration of the two-color double strobe method for inkjet analysis

If the magnification of the video image is known, the speed of the drop can be determined on the basis of the specified flash times. Other result parameters, such as the volume of the drop, the ligament length, the number of satellite drops, or the deviation of the trajectory from the vertical, also result from the image analysis. All these data are relevant for the expected quality of the print result.