Drop size and grey levels: how an inkjet head makes tones

By Piotr Jędrzejczyk, AssembleX Industrial Services

An inkjet head can only fire a drop or not fire it. Every smooth tone in a print is built from the number, the size and the placement of those drops. Understanding that is the key to smooth gradients and saturated solid colours.

Dots versus grey levels

The simplest head fires one drop size. To print a lighter or darker tone, the software changes how many dots sit in an area: few dots for a light tone, many for a dark one. This is halftoning, and the dots stay equally large.

A grey-level head can fire several drop sizes from the same nozzle. Each pixel then carries not only a yes or a no, but a small amount of ink between none and the maximum. Tones are built from the size of the dots as well as from their number (Figure 1), which gives smoother gradients at the same resolution.

Figure 1. The same tonal ramp, built two ways. Top: equal dots, tone changes by the number of dots. Bottom: grey levels, tone changes by the size of the dot.

How a drop size is made

A grey-level drop is not a separate kind of drop. The head fires one, two, three or more small droplets in quick succession, and they merge in flight or on the surface into one larger drop. The drive waveform decides how many droplets are fired and how big they are. The volume is measured in picolitres.

Why drop size matters

Smoothness. Small drops give fine detail and smooth gradients; too few levels show up as steps and grain in light areas.
Density. Large drops put more ink into a pixel. That gives saturated solids and deep colours, and on ceramics the amount of pigment decides the colour after firing.
Productivity. At line speed, larger drops are what make it possible to reach the required density in one pass.
Substrate. How a drop spreads depends on the surface, which is why the same setting prints differently on different materials (Figure 2).

Figure 2. The same drop on two surfaces. Left: a porous surface, such as a ceramic body or glaze, takes the ink in. Right: a non-absorbing surface, such as glass, steel or PVC, leaves the drop on top, where it spreads and has to be cured.

What can be set

Drop size is not a free setting. It follows from the head type, the drive waveform and voltage, and the viscosity of the ink, which depends on its temperature. It is established during head setup, and it has to be the same on every head of a colour.

The resolution in both directions.
The grey-level mode of each ink channel.
The drive waveform and voltage per head, which set the drop volume and the drop velocity.
The ink temperature and the print gap.

Tuning procedure

Define the target. What tonal range, smoothness and density does the product need? A large slab with soft graphics and a saturated decor ask for different settings.
Stabilise the jetting. Temperature, circulation and degassing, a nozzle check and clean stitching come first.
Set the drive. Adjust waveform and voltage per head until drop velocity and volume match the target and each other.
Choose resolution and grey-level mode for every ink channel..
Print grey-level wedges. Check that the gradient is smooth, with no steps or grain, and that the density reaches the target.
Linearise, and then build the colour profile..

Common mistakes

Raising the voltage to gain density without checking drop velocity and satellites.
Mixing heads with different drop volumes in one beam.
Using different grey-level modes for the colours of one image.
Changing the ink temperature and not repeating the setup.
Treating grain as a colour problem, when it comes from too few tonal levels.

Drop size sets the raw material of every tone. Linearisation then makes it uniform, and the RIP decides how it is used.