What is a Waveform?

What is a Waveform?

To help explain the purpose of a waveform, let’s take a look inside a nozzle chamber of a printhead. The jetting process known as fill-and-fire, which is quite commonly applied to many different printheads of varied design. In this case, a stack of PZT (a piezoelectric ceramic material) deforms whenever voltage is applied to it, changing the volume of the ink chamber and causing the ink to move within it and eventually eject. The means of applying this voltage to the PZT is the waveform.

 the PZT is extended only when voltage is applied, so there will be no deformation until the printhead is plugged in. Once powered, the printhead maintains a specific voltage, causing the PZT to extend and remain in its non-jetting position. If the voltage is decreased, the PZT retracts and creates an expansion in the chamber, drawing ink into it. To eject a drop, the voltage is returned to its original value, the chamber shrinks, and the excess ink is forced out. This process repeats thousands of times per second.

It is important to note that printheads can be driven by either positive-going or negative-going pulses, depending on how they are manufactured. Whichever way you're used to looking at it, the important waveform timings are the two sloping parts and the hold time, i.e. how long the voltage is kept at that level (high or low) before it returns to the start position. This pulse timing determines what is happening at the nozzle when the volume of the chamber is returned to a steady state and is the first step to figure out when building a waveform.

If you've stood next to a print head while it is printing, you might have been able to hear it “sing”, depending on what frequency was being used. The reason you can hear it is because the actuators produce sound waves. The most important ones for jetting are the ones that get produced in the ink itself, since they define the pressure variation that gives drop ejection.

Because of the presence of an ink with certain mechanical properties, and the fact the sound waves can lose energy as they bounce around, the pressure in the chamber can be described as a damped resonator. Any change in pressure, such as the PZT deforming, will result in a characteristic pressure variation. As shown here, when the PZT retracts and the chamber increases in volume, the pressure change causes the ink to begin moving back and forth within the chamber.

This energy alone is usually not enough to cause the ink to eject, it just pulls it back to the opposite end of the chamber and bounces off. By using a voltage pulse to reinforce the pressure at the right time, the drop ejection is made more efficient, as shown here. A drop is ejected when the pressure goes over a critical value due to the preferable timing.

The reason the pulse width is so critical is that if it is too short, or too long, then the waves, pressure, and movement of the PZT will be out of sync. If the ink is not moving the right direction at the time more pressure is added, instead of smoothly adding to the momentum, the momentum might be countered. It is similar to pushing a child on a swing. If you push them at the right time, the momentum is increased and they swing higher. If you push them at the wrong time, they will come to a violent stop. Similarly, if the pulse width is wrong, the resulting jetting, repeated over and over, will be inefficient and unstable.

Because the ink needs to move back and forth across the length of the chamber, the maximum frequency that a printhead can produce comes down to the size of the printhead chamber. Printheads that are designed with longer chambers take more time for the acoustic waves to travel back and forth, thus can't eject drops as often.

Since the length of the nozzle chamber is fixed, you might think that the pulse width needed to correctly time the pressure wave is constant for a given printhead. However, the timing is also impacted by the speed of sound for that particular ink. This is why a waveform must be tuned for a particular ink and printhead combination, it is not enough just to have a general waveform for that printhead. Fortunately, if inks are similar in property, then the same waveform can work well for both.