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Reading print defects: a field guide from file to finished product

By Piotr Jędrzejczyk, AssembleX Industrial Services

Most print defects have a recognisable shape, and the shape points at the cause. Learn to read where, in which direction and in which colour a defect appears, and you find the source in minutes instead of days.

Start with the pattern, not the cure

The common reflex is to change a parameter and see whether the picture gets better. That works once in a while and costs a lot of scrap the rest of the time. A faster way is to describe the defect first: where it is, how it repeats, and which colours are affected. Those three answers already narrow the cause to one part of the machine.

Figure 1. Six common defects, as they look on a flat tone or on fine lines. 1: a single thin line. 2: bands the width of a head. 3: mottling. 4: colour fringes at the edges. 5: satellite dots around a line. 6: slow colour drift.
Figure 1. Six common defects, as they look on a flat tone or on fine lines. 1: a single thin line. 2: bands the width of a head. 3: mottling. 4: colour fringes at the edges. 5: satellite dots around a line. 6: slow colour drift.

The catalogue

  1. Single thin line along the print direction. A nozzle that is missing or fires off-axis, nearly always in one colour. Run a nozzle check, clean the head, then look at the ink supply and the nozzle plate.
  2. Wider bands, as wide as a head or a module. Different response between heads, or a stitching error at the join. Check the stitching first, then linearisation.
  3. Mottling, a cloudy tone in flat areas. Unstable jetting, an ink amount that does not suit the surface, or uneven absorption of the substrate or glaze. Check the surface before touching the colour settings.
  4. Colour fringes at the edges. Colour-to-colour registration is off: heads of different colours are not aligned, the print gap is wrong, or the transport speed is not tracked correctly.
  5. Satellite dots and misting around lines. The drops split on the way: print gap too large, ink conditions or temperature wrong, and on plastics an electrostatic charge on the surface that deflects the drops.
  6. Slow colour drift. The colour moves over hours or days: ink batch, temperature, and firing or curing conditions. Compare against a reference sample on a schedule.

Reading the pattern

Figure 2. Direction tells the origin. Defects that run along the print direction (left) belong to a nozzle, a head or a join. Bands across the print direction (right) point at timing, transport or the substrate. The arrow shows the print direction.
Figure 2. Direction tells the origin. Defects that run along the print direction (left) belong to a nozzle, a head or a join. Bands across the print direction (right) point at timing, transport or the substrate. The arrow shows the print direction.
  • Direction. A line along the print direction comes from the head side; a band across it comes from timing or transport.
  • Repeat. If the defect repeats at a fixed distance, look for a rotating element with that circumference, such as a roller or an encoder wheel.
  • Colour. One colour only points at its own heads and ink; all colours at once point at the substrate, the transport or the process.
  • Position. A defect that always sits under the same head belongs to that head.

From the file to the finished product

A defect can start before the machine. A low-resolution file stretched to size, a gradient with too few tonal steps, or a colour conversion that clips the saturated colours all look like machine faults. After the print, the same applies to firing or curing: a kiln or a UV lamp can create the defect on a perfect print. That is why the sequence matters.

  1. Describe the defect precisely. Take a photo, and note the position, the direction, the colours and since when it appears.
  2. Isolate the stage. Print a simple test instead of the production image: a solid tone, a gradient and a nozzle check. If the test is clean, the cause is in the file or the RIP.
  3. Check the machine basics. Nozzle check, stitching pattern, print gap and ink conditions.
  4. Compare with the last good result. What changed: ink batch, a head, the substrate, the glaze, the recipe or the firing?
  5. Change one thing at a time and write it down. Several changes at once hide the real cause.
  6. Fix the cause, and re-linearise or re-profile only if the cause really was in the colour setup..

Common mistakes

  • Correcting a mechanical or jetting problem with colour settings in the RIP.
  • Changing several parameters at once.
  • Blaming the ink first, before checking jetting and the file.
  • Judging the print before firing or curing, when the defect appears only after the process.
  • Keeping no photos and no record, so the same defect is analysed again from scratch.

Many of these defects are prevented by a stable base: clean stitching, then linearisation, then a colour profile.