Stitching between printheads: how to make many heads print as one

By Piotr Jędrzejczyk, AssembleX Industrial Services

In a single-pass inkjet line, a wide image is printed by several heads or modules placed side by side. Stitching is the setup that makes the joins between them invisible.

Why stitching exists

One printhead covers only a limited width. To print across a whole slab, sheet or coil, heads are mounted side by side on a beam, usually in two staggered rows, so that the printable areas of neighbouring heads overlap slightly. The result has to look as if the whole width had been printed by one very wide head.

That only works when every head sits in exactly the right place, relative to its neighbours and to the substrate. A join that is off by a fraction of a pixel is visible as a line running along the print direction, which is the first thing the eye finds on a smooth graphic.

Figure 1. Two neighbouring heads with staggered nozzle rows and an overlap zone (top). In the overlap zone the pixel columns are handed over gradually from head A to head B (bottom), so a small residual error is spread out instead of showing as a hard edge.

What has to match

Position across the print width: the last nozzles of one head and the first nozzles of the next must continue the pixel grid without a gap or a double.
Position along the print direction: heads in different rows are physically offset, and the firing timing has to compensate for it exactly.
Angle: a head that is slightly rotated tilts its pixel columns and changes the spacing towards its neighbour.
Height above the substrate: a different print gap changes where the drops land, especially at line speed.
Jetting direction: nozzles that fire off-axis shift the dots, whatever the mechanical position.

To get a feeling for the scale: at 400 dpi one pixel is about 64 µm. A join error of even half a pixel is clearly visible on a large, smooth surface.

Figure 2. Typical join errors at the join between two heads. From the left: a gap (light line), an overlap (dark line), an offset along the print direction (step), and an angle error (tilt).

The procedure

Prepare the heads. Check the print gap, run a nozzle check and make sure the ink supply is stable. Stitching a head with missing nozzles only hides one fault behind another.
Print a stitching pattern. A pattern of fine lines or marks that crosses every join, in every colour, at the real production settings.
Inspect and measure. Read the joins under magnification or with a camera, and note the error in micrometres, separately across and along the print direction.
Correct mechanically. Move or rotate the head with its adjusters until the error is within a fraction of a pixel. Do not try to fix large errors in software.
Fine-tune electronically. Apply the remaining pixel and timing corrections, and set the overlap and the blending zone, as the system allows.
Print again and check every join in every colour. Colour-to-colour registration is part of the same check.
Document the final values, and only then move on to linearisation..

Why stitching comes before linearisation

A join error looks like banding, and so does a head with a different drop volume. If the heads are linearised before they are stitched, the correction curves try to compensate for a mechanical error and end up wrong as soon as the head is touched again. Stitching first, linearisation second.

Keeping it stable

Stitching is not a one-time job. Temperature, vibration and maintenance move heads by a few micrometres, which is already enough to show. After every head replacement, redo the stitching of that head and its neighbours. Between jobs, a short check pattern on a schedule shows drift before it shows on the product.

Common mistakes

Correcting a large mechanical error in software instead of fixing the head position.
Stitching with dirty nozzles or missing nozzles.
Ignoring differences in print gap between heads.
Adjusting one colour only: every colour has its own heads and its own registration.
Skipping the check after a head swap.

Once the joins are clean, linearisation brings the heads to the same tone, and a colour profile can be built on top.